(_f_) Piping must be thoroughly tested b_Acetylene, the Principles of Its Generation and Use A Practical Handbook on the Production, Purification, and Subsequent Treatment of Acetylene for the Development of Light, Heat, and Power

burners have been installed. It must not show loss in excess of 2 inches

within twelve hours when subjected to a pressure equal to that of 15

inches of mercury.

(_g_) Piping and connexions must be installed by persons experienced

in the installation of acetylene apparatus.

8. CARE AND ATTENDANCE.--In the care of generators designed for a

lighting period of more than five hours always clean and recharge the

generating chambers at regular stated intervals, regardless of the number

of burners actually used.

Where generators are not used throughout the entire year always remove

all water and gas and clean thoroughly at the end of the season during

which they are in service.

It is usually necessary to take the bell portion out and invert it so as

to allow all gas to escape. This should never be done in the presence of

artificial light or fire of any kind.

Always observe a regular time, during daylight hours only, for attending

to and charging the apparatus.

In charging the generating chambers of water-feed machines clean all

residuum carefully from the containers and remove it at once from the

building. Separate from the mass any unslacked carbide remaining and

return it to the containers, adding now carbide as required. Be careful

never to fill the containers over the specified mark, as it is important

to allow for the swelling of the carbide when it comes in contact with

water. The proper action and economy of the machine are dependent on the

arrangement and amount of carbide placed in the generator. Carefully

guard against the escape of gas.

Whenever recharging with carbide always replenish the water-supply.

Never deposit residuum or exhausted material from water-feed machines in

sewer-pipes or near inflammable material.

Always keep water-tanks and water-seals filled with clean water.

Never test the generator or piping for leaks with a flame, and never

apply flame to an outlet from which the burner has been removed.

Never use a lighted match, lamp, candle, lantern or any open light near

the machine.

Failure to observe the above cautions is as liable to endanger life as

property.

9. OUTSIDE GENERATOR HOUSES.--(_a_) Outside generator houses should

not be located within 5 feet of any opening into, nor shall they open

toward any adjacent building, and must be kept under lock and key.

(_b_) The dimensions must be no greater than the apparatus requires

to allow convenient room for recharging and inspection of parts. The

floor must be at least 12 inches above grade and the entire structure

thoroughly weather-proof.

(_c_) Generator houses must be thoroughly ventilated, and any

artificial heating necessary to prevent freezing shall be done by steam

or hot-water systems.

(_d_) Generator houses must not be used for the storage of calcium

carbide except in accordance with the rules relating to that subject

(_vide_ Chapter II.).

_Class B.--Stationary Non-Automatic Apparatus_.

10. FOUNDATIONS.--(_a_) Must be of brick, stone or concrete.

(_b_) Must be so arranged that the machine will be level and so that

strain will not be brought upon the connexions.

11. GAS-HOUSES.--(_a_) Must be constructed entirely of non-

combustible material and must not be lighted by any system of

illumination involving open flames.

(_b_) Must be heated, where artificial heating is necessary to

prevent freezing, by steam or hot-water systems, the heater to be located

in a separate building, and no open flames to be permitted within

generator enclosures.

(_c_) Must be kept closed and locked excepting during daylight

hours.

(_d_) Must be provided with a permanent and effective system of

ventilation which will be operative at all times, regardless of the

periods of operation of the plant.

12. ESCAPE-PIPES.--Each generator must be provided with a vent-pipe of

ample size, substantially installed, without traps. It must be carried to

a suitable point outside the building and terminate in an approved hood

located at least 12 feet above ground and remote from windows.

The hood must be constructed in such a manner that it cannot be

obstructed by rain, snow, ice, insects or birds.

13. CARE AND MAINTENANCE.--All charging and cleaning of apparatus,

generation of gas and execution of repairs must be done during daylight

hours only, and generators must not be manipulated or in any way tampered

with in the presence of artificial light.

This will require gasholders of a capacity sufficient to supply all

lights installed for the maximum lighting period, without the necessity

of generation of gas at night or by artificial light.

In the operation of generators of the carbide-feed type it is important

that only a limited amount of carbide be fed into a given body of water.

An allowance of at least one gallon of generating water per pound of

carbide must be made in every case, and when this limit has been reached

the generator should be drained and flushed, and clean water introduced.

These precautions are necessary to avoid over-heating during generation

and accumulation of hard deposits of residuum in the generating chamber.

(Rule 14, referring to the storage of carbide, has been quoted in Chapter

II. (page 19)).

RULES FOR THE CONSTRUCTION OF GENERATORS.

The following Rules are intended to provide only against the more

hazardous defects usually noted in apparatus of this kind. The Rules do

not cover all details of construction nor the proper proportioning of

parts, and devices which comply with these requirements alone are not

necessarily suitable for listing as permissible for use. These points are

often only developed in the examination required before permission is

given for installation.

_Class A.--Stationary Apparatus for Isolated Installations._

15. GENERAL RULES. GENERATORS.--(_a_) Must be made of iron or steel,

and in a manner and of material to insure stability and durability.

(_b_) Must be automatically regulated and uniform in their action,

producing gas only as immediate consumption demands, and so designed that

gas is generated without producing sufficient heat to cause yellow

discoloration of residuum (which will occur at about 500° F.) or abnormal

pressure at any stage of the process when using carbide of any degree of

fineness.

The presence of excessive heat tends to change the chemical character of

the gas and may even cause its ignition, while in machines of the

carbide-feed type, finely divided carbide will produce excessive pressure

unless provision is made to guard against it.

(_c_) Must be so arranged that during recharging, back flow of gas

from the gasholder will be automatically prevented, or so arranged that

it will be impossible to charge the apparatus without first closing the

supply-pipe to the gasholder, and to the other generating chambers if

several are used.

This is intended to prevent the dangerous escape of gas.

(_d_) The water or carbide supply to the generating chamber must be

so arranged that gas will be generated long enough in advance of the

exhaustion of the supply already in the gasholder to allow the using of

all lights without exhausting such supply.

This provides for the continuous working of the apparatus under all

conditions of water-feed and carbide charge, and it obviates the

extinction of lights through intermittent action of the machine.

(_e_) No valves or pet-cocks opening into the room from the gas-

holding part or parts, the draining of which will allow an escape of gas,

are permitted, and condensation from all parts of the apparatus must be

automatically removed without the use of valves or mechanical working

parts.

Such valves and pet-cocks are not essential; their presence increases the

possibility of leakage. The automatic removal of condensation from the

apparatus is essential to the safe working of the machine.

U-traps opening into the room from the gas-holding parts must not be used

for removal of condensation. All sealed drip connexions must be so

arranged as to discharge gas to the blow-off when blown out, and the

seals must be self-restoring upon relief of abnormal pressure.

(_f_) The apparatus must be capable of withstanding fire from

outside causes.

Sheet-metal joints must be double-seamed or riveted and thoroughly

sweated with solder. Pipes must be attached to sheet-metal with lock-nuts

or riveted flanges.

This prohibits the use of wood or of joints relying entirely upon solder.

(_g_) Gauge glasses, the breakage of which would allow the escape of

gas, must not be used.

(_h_) The use of mercury seals is prohibited.

Mercury has been found unreliable as a seal in acetylene apparatus.(_i_)

Combustible oils must not be used in connexion with the

apparatus.

(_j_) The construction must be such that liquid seals shall not

become thickened by the deposit of lime or other foreign matter.

(_k_) The apparatus must be constructed so that accidental siphoning

of water will be impossible.

(_l_) Flexible tubing, swing joints, unions, springs, mechanical

check-valves, chains, pulleys, stuffing-boxes and lead or fusible piping

must not be used on acetylene apparatus except where failure of such

parts will not vitally affect the working or safety of the machine.

Floats must not be used excepting in cases where failure will result only

in rendering the machine inoperative.

(_m_) Every machine must be plainly marked with the maximum number

of lights it is designed to supply, the amount of carbide necessary for a

single charge, the manufacturer's name and the name of the machine.

16. GENERATING CHAMBERS.--(_a_) Must be constructed of galvanised

iron or steel not less than No. 24 U.S. Standard gauge in thickness for

capacities up to and including 20 gallons, not less than No. 22 U.S.

Standard gauge for capacities between 20 and 75 gallons, and not less

than No. 20 U.S. Standard gauge for capacities in excess of 75 gallons.

(_b_) Must each be connected with the gasholder in such a manner

that they will, at all times, give open connexion either to the gasholder

or to the blow-off pipe to the outer air.

This prevents dangerous pressure within or the escape of gas from the

generating chamber.

(_c_) Must be so constructed that not more than 5 pounds of carbide

can be acted upon at once, in machines which apply water in small

quantities to the carbide.

This tends to reduce the danger of overheating and excessive after-

generation by providing for division of the carbide charges in machines

of this type.

(_d_) Must be provided with covers having secure fastenings to hold

them properly in place and those relying on a water-seal must be

submerged in at least 12 inches of water. Water-seal chambers for covers

depending on a water-seal must be 1-1/2 inches wide and 15 inches deep,

excepting those depending upon the filling of the seal chambers for the

generation of gas, where 9 inches will be sufficient.

(_e_) Must be so designed that the residuum will not clog or affect

the working of the machine and can conveniently be handled and removed.

(_f_) Must be provided with suitable vent connexions to the blow-off

pipe so that residuum may be removed and the generating water replaced

without causing siphoning or introducing air to the gasholder upon

recharging.

This applies to machines of the carbide-feed type.

(_g_) Feed mechanism for machines of the carbide-feed type must be

so designed that the direct fall of carbide from the carbide holder into

the water of the generator is prevented at all positions of the feed

mechanisms; or, when actuated by the rise and fall of a gas-bell, must be

so arranged that the feed-valve will not remain open after the landing of

the bell, and so that the feed valve remains inoperative as long as the

filling opening on the carbide hopper remains open. Feed mechanisms must

always be far enough above the water-level to prevent clogging from the

accumulation of damp lime. For this purpose the distance should be not

less than 10 inches.

17. CARBIDE CHAMBERS.--(_a_) Must be constructed of galvanised iron

or steel not less than No. 24 U.S. Standard gauge in thickness for

capacities up to and including 50 pounds and not less than No. 22 U.S.

Standard gauge for capacities in excess of 50 pounds.

(_b_) Must have sufficient carbide capacity to supply the full

number of burners continuously and automatically during the maximum

lighting period.

This rule removes the necessity of recharging or attending to the machine

at improper hours. Burners almost invariably require more than their

rated consumption of gas, and carbide is not of staple purity, and there

should therefore be an assurance of sufficient quantity to last as long

as light is needed. Another important consideration is that in some

establishments burners are called upon for a much longer period of

lighting than in others, requiring a generator of greater gas-producing

capacity. Machines having several generating chambers must automatically

begin generation in each upon exhaustion of the preceding chamber.

(_c_) Must be arranged so that the carbide holders or charges may be

easily and entirely removed in case of necessity.

18. GASHOLDERS.--(_a_) Must be constructed of galvanised iron or

steel not less than No. 24 U.S. Standard gauge in thickness for

capacities up to and including 20 gallons, not less than No. 22 U.S.

Standard gauge for capacities between 20 and 75 gallons, and not less

than No. 20 U.S. Standard gauge for capacities in excess of 75 gallons.

Gas-bells, if used, may be two gauges lighter than holders.

Condensation chambers, if placed under holders, to be of same gauge as

holders.

(_b_) Must be of sufficient capacity to contain all gas generated

after all lights have been extinguished.

If the holder is too small and blows off frequently after the lights are

extinguished there is a waste of gas. This may suggest improper working

of the apparatus and encourage tampering.

(_c_) Must, when constructed on the gasometer principle, be so

arranged that when the gas-bell is filled to its maximum with gas at

normal pressure its lip or lower edge will extend at least 9 inches below

the inner water-level.

(_d_) Must, when constructed on the gasometer principle, have the

dimensions of the tank portion so related to those of the bell that a

pressure of at least 11 inches will be necessary before gas can be forced

from the holder.

(_e_) The bell portion of a gasholder constructed on the gasometer

principle must be provided with a substantial guide to its upward

movement, preferably in the centre of the holder, carrying a stop acting

to chock the bell 1 inch above the normal blow-off point.

This tends to insure the proper action of the bell and decreases the

liability of escaping gas.

(_f_) A space of at least three-quarters of an inch must be allowed

between the sides of the tank and the bell.

(_g_) All water-seals must be so arranged that the water-level may

be readily seen and maintained.

19. WATER-SUPPLY.--(_a_) The supply of water to the generator for

generating purposes must not be taken from the water-seal of any

gasholder constructed on the gasometer principle, unless the feed

mechanism is so arranged that the water-seals provided for in Rules 18,

(_c_), (_d_), and (_e_) may be retained under all

conditions. This provides for the proper level of water in the gasholder.

(_b_) In cases where machines of the carbide-feed type are supplied

with water from city water-mains or house-pipes, the pipe connexion must

discharge into the regularly provided filling trap on the generator and

not through a separate continuous connexion leading into the generating

chamber.

This is to prevent the expulsion of explosive mixtures through the

filling trap in refilling.

20. RELIEFS OR SAFETY BLOW-OFFS.--(_a_) Must in all cases be

provided, and must afford free vent to the outer air for any over-

production of gas, and also afford relief in case of abnormal pressure in

the machine.

Both the above-mentioned vents may be connected, with the same escape-

pipe.

(_b_) Must be of at least 3/4-inch internal diameter and be provided

with suitable means for connecting to the pipe loading outside of the

building.

(_c_) Must be constructed without valves or other mechanical working

parts.

(_d_) Apparatus requiring pressure regulators must be provided with

an additional approved safety blow-off attachment located between the

pressure regulator and the service-pipes and discharging to the outer

air.

This is intended to prevent the possibility of undue pressure in the

service-pipes due to failure of the pressure regulator.

21. PRESSURES.--(_a_) The working pressure at the generator must not

vary more than ten-tenths (1) inch water column under all conditions of

carbide charge and feed, and between the limits of no load and 50 per

cent. overload.

(_b_) Apparatus not requiring pressure regulators must be so

arranged that the gas pressure cannot exceed sixty-tenths (6) inches

water column.

This requires the use of the pressure relief provided for in Rule No. 20

(_a_).

(_c_) Apparatus requiring pressure regulators must be so arranged

that the gas pressure cannot exceed three pounds to the square inch.

The pressure limit of 3 pounds is taken since that is the pressure

corresponding to a water column about 6 feet high, which is about, the

limit in point of convenience for water-sealed reliefs.

22. AIR MIXTURES.--Generators must be so arranged as to contain the

minimum amount of air when first started or recharged, and no device or

attachment facilitating or permitting mixture of air with the gas prior

to consumption, except at the burners, shall be allowed.

Owing to the explosive properties of acetylene mixed with air, machines

must be so designed that such mixtures are impossible.

23. PURIFIERS.--(_a_) Must be constructed of galvanised iron or

steel not less than No. 24 U.S. Standard gauge in thickness.

(_b_) Where installed, purifiers must conform to the general rules

for the construction of other acetylene apparatus and allow the free

passage of gas.

(_c_) Purifiers must contain no carbide for drying purposes.

(_d_) Purifiers must be located inside of gasholders, or, where

necessarily outside, must have no hand-holes which can be opened without

first shutting off the gas-supply.

24. PRESSURE REGULATORS.--(_a_) Must conform to the rules for the

construction of other acetylene apparatus so far as they apply and must

not be subject to sticking or clogging.

(_b_) Must be capable of maintaining a uniform pressure, not varying

more than four-tenths inch water column, at any load within their rating.

(_c_) Must be installed between valves in such a manner as to

facilitate inspection and repairs.

_Class B.--Stationary Apparatus for Central Station Service._

Generators of over 300 lights capacity for central station service are

not required to be automatic in operation. Generators of less than 300

lights capacity must be automatic in operation and must comply in every

respect with the requirements of Class A.

25. GENERAL RULES. GENERATORS.--(_a_) Must be substantially

constructed of iron or steel and be protected against depreciation by an

effective and durable preventive of corrosion.

Galvanising is strongly recommended as a protection against oxidation,

and it may to advantage be reinforced by a thorough coating of asphaltum

or similar material.

(_b_) Must contain no copper or alloy of copper in contact with

acetylene, excepting in valves.

(_c_) Must be so arranged that generation will take place without

overheating; temperatures in excess of 500° F. to be considered

excessive.

(_d_) Must be provided with means for automatic removal of

condensation from gas passages.

(_e_) Must be provided with suitable protection against freezing of

any water contained in the apparatus.

No salt or other corrosive chemical is permissible as a protection

against freezing.

(_f_) Must in general comply with the requirements governing the

construction of apparatus for isolated installations so far as they are

applicable.

(_g_) Must be so arranged as to insure correct procedure in

recharging and cleaning.

(_h_) Generators of the carbide-feed type must be provided with some

form of approved measuring device to enable the attendant to determine

when the maximum allowable quantity of carbide has been fed into the

generating chamber.

In the operation of generators of this type an allowance of at least 1

gallon of clean generating water per pound of carbide should be made, and

the generator should be cleaned after slaking of every full charge. Where

lump carbide is used the lumps may become embedded in the residuum, if

the latter is allowed to accumulate at the bottom of the generating

chamber, causing overheating from slow and restricted generation, and

rendering the mass more liable to form a hard deposit and bring severe

stresses upon the walls of the generator by slow expansion.

26. GENERATING CHAMBERS.--(_a_) Must each be connected with the

gasholder in such a manner that they will, at all times, give open

connexion either to the gasholder or to the blow-off pipe into the outer

air.

(_b_) Must be so arranged as to guard against appreciable escape of

gas to the room at any time during the introduction of the charges.

(_c_) Must be so designed that the residuum will not clog or affect

the operation of the machine and can conveniently be handled and removed.

(_d_) Must be so arranged that during the process of cleaning and

recharging the back-flow of gas from the gasholder or other generating

chambers will be automatically prevented.

27. GASHOLDERS.--(_a_) Must be of sufficient capacity to contain at

least 4 cubic feet of gas per 1/2-foot burner of the rating.

This is to provide for the requisite lighting period without the

necessity of making gas at night, allowance being made for the

enlargement of burners caused by the use of cleaners.

(_b_) Must be provided with suitable guides to direct the movement

of the bell throughout its entire travel.

28. PRESSURE RELIEFS.--Must in all cases be provided, and must be so

arranged as to prevent pressure in excess of 100-tenths (10) inches water

column in the mains.

29. PRESSURES.--Gasholders must be adjusted to maintain a pressure of

approximately 25-tenths (2.5) inches water column in the mains.

CHAPTER V

THE TREATMENT OF ACETYLENE AFTER GENERATION

IMPURITIES IN CALCIUM CARBIDE.--The calcium carbide manufactured at the

present time, even when of the best quality commercially obtainable, is

by no means a chemically pure substance; it contains a large number of

foreign bodies, some of which evolve gas on treatment with water. To a

considerable extent this statement will probably always remain true in

the future; for in order to make absolutely pure carbide it would be

necessary for the manufacturer to obtain and employ perfectly pure lime,

carbon, and electrodes in an electric furnace which did not suffer attack

during the passage of a powerful current, or he would have to devise some

process for simultaneously or subsequently removing from his carbide

those impurities which were derived from his impure raw materials or from

the walls of his furnace--and either of these processes would increase

the cost of the finished article to a degree that could hardly be borne.

Beside the impurities thus inevitably arising from the calcium carbide

decomposed, however, other impurities may be added to acetylene by the

action of a badly designed generator or one working on a wrong system of

construction; and therefore it may be said at once that the crude gas

coming from the generating plant is seldom fit for immediate consumption,

while if it be required for the illumination of occupied rooms, it must

invariably be submitted to a rigorous method of chemical purification.

IMPURITIES OF ACETYLENE.--Combining together what may be termed the

carbide impurities and the generator impurities in crude acetylene, the

foreign bodies are partly gaseous, partly liquid, and partly solid. They

may render the gas dangerous from the point of view of possible

explosions; they, or the products derived from them on combustion, may be

harmful to health if inspired, injurious to the fittings and decorations

of rooms, objectionable at the burner orifices by determining, or

assisting in, the formation of solid growths which distort the flame and

so reduce its illuminating power; they may give trouble in the pipes by

condensing from the state of vapour in bends and dips, or by depositing,

if they are already solid, in angles, &c., and so causing stoppages; or

they may be merely harmful economically by acting as diluents to the

acetylene and, by having little or no illuminating value of themselves,

causing the gas to emit less light than it should per unit of volume

consumed, more particularly, of course, when the acetylene is not burnt

under the mantle. Also, not being acetylene, or isomeric therewith, they

require, even if they are combustible, a different proportion of oxygen

for their perfect combustion; and a good acetylene jet is only calculated

to attract precisely that quantity of air to the flame which a gas having

the constitution C_2H_2 demands. It will be apparent without argument

that a proper system of purification is one that is competent to remove

the carbide impurities from acetylene, so far as that removal is

desirable or necessary; it should not be called upon to extract the

generator impurities, because the proper way of dealing with them is, to

the utmost possible extent, to prevent their formation. The sole

exception to this rule is that of water-vapour, which invariably

accompanies the best acetylene, and must be partially removed as soon as

convenient. Vapour of water almost always accompanies acetylene from the

generator, even when the apparatus does not belong to those systems of

working where liquid water is in excess, this being due to the fact that

in a generator where the carbide is in excess the temperature tends to

rise until part of the water is vapourised and carried out of the

decomposing chamber before it has an opportunity of reacting with the

excess of carbide. The issuing gas is therefore more or less hot, and it

usually comes from the generating chamber saturated with vapour, the

quantity needed so to saturate it rising as the temperature of the gas

increases. Practically speaking, there is little objection to the

presence of water-vapour in acetylene beyond the fear of deposition of

liquid in the pipes, which may accumulate till they are partially or

completely choked, and may even freeze and burst them in very severe

weather. Where the chemical purifiers, too, contain a solid material

which accidentally or intentionally acts as a drier by removing moisture

from the acetylene, it is a waste of such comparatively expensive

material to allow gas to enter the purifier wetter than need be.

EXTRACTION OF MOISTURE.--In all large plants the extraction of the

moisture may take place in two stages. Immediately after the generator,

and before the washer if the generator requires such an apparatus to

follow it, a condenser is placed. Here the gas is made to travel somewhat

slowly through one or more pipes surrounded with cold air or water, or is

made to travel through a space containing pipes in which cold water is

circulating, the precise method of constructing the condenser being

perfectly immaterial so long as the escaping gas has a temperature not

appreciably exceeding that of the atmosphere. So cooled, however, the gas

still contains much water-vapour, for it remains saturated therewith at

the temperature to which it is reduced, and by the inevitable law of

physics a further fall in temperature will be followed by a further

deposition of liquid water from the acetylene. Manifestly, if the

installation is so arranged that the gas can at no part of the service

and on no occasion fall to a lower temperature than that at which it

issues from the condenser, the removal of moisture as effected by such a

condenser will be sufficient for all practical purposes; but at least in

all large plants where a considerable length of main is exposed to the

air, a more complete moisture extractor must be added to the plant, or

water will be deposited in the pipes every cold night in the winter. It

is, however, useless to put a chemical drier, or one more searching in

its action than a water-cooled condenser, at so early a position in the

acetylene plant, because the gas will be subsequently stored in a water-

sealed holder, where it will most probably once again be saturated with

moisture from the seal. When such generators are adopted as require to

have a specific washer placed after them in order to remove the water-

soluble impurities, _e.g._, those in which the gas does not actually

bubble through a considerable quantity of liquid in the generating

chamber itself, it is doubtful whether a separate condenser is altogether

necessary, because, as the water in the washer can easily be kept at the

atmospheric temperature (by means of water circulating in pipes or

otherwise), the gas will be brought to the atmospheric temperature in the

washer, and at that temperature it cannot carry with it more than a

certain fixed proportion of moisture. The notion of partially drying a

gas by causing it to pass through water may appear paradoxical, but a

comprehension of physical laws will show that it is possible, and will

prove efficient in practice, when due attention is given to the facts

that the gas entering the washer is hot, and that it is subsequently to

be stored over water in a holder.

GENERATOR IMPURITIES.--The generator impurities present in the crudest

acetylene consist of oxygen and nitrogen, _i.e._, the main

constituents of air, the various gaseous, liquid, and semi-solid bodies

described in Chapter II., which are produced by the polymerising and

decomposing action of heat upon the carbide, water, and acetylene in the

apparatus, and, whenever the carbide is in excess in the generator, some

lime in the form of a very fine dust. In all types of water-to-carbide

plant, and in some automatic carbide-feed apparatus, the carbide chamber

must be disconnected and opened each time a fresh charge has to be

inserted; and since only about one-third of the space in the container

can be filled with carbide, the remaining two-thirds are left full of

air. It is easy to imagine that the carbide container of a small

generator might be so large, or loaded with so small a quantity of

carbide, or that the apparatus might in other respects be so badly

designed, that the gas evolved might contain a sufficient proportion of

air to render it liable to explode in presence of a naked light, or of a

temperature superior to its inflaming-point. Were a cock, however, which

should have been shut, to be carelessly left open, an escape of gas from,

rather than an introduction of air into, the apparatus would follow,

because the pressure in the generator is above that of the atmosphere. As

is well known, roughly four-fifths by volume of the air consist of

nitrogen, which is non-inflammable and accordingly devoid of danger-

conferring properties; but in all flames the presence of nitrogen is

harmful by absorbing much of the heat liberated, thus lowering the

temperature of that flame, and reducing its illuminating power far more

seriously. On the other hand, a certain quantity of air in acetylene

helps to prevent burner troubles by acting as a mere diluent (albeit an

inferior one to methane or marsh-gas), and therefore it has been proposed

intentionally to add air to the gas before consumption, such a process

being in regular use on the large scale in some places abroad. As Eitner

has shown (Chapter VI.) that in a 3/4-inch pipe acetylene ceases to be

explosive when mixed with less than 47.7 per cent. of air, an amount of,

say, 40 per cent. or less may in theory be safely added to acetylene; but

in practice the amount of air added, if any, would have to be much

smaller, because the upper limit of explosibility of acetylene-air

mixtures is not rigidly fixed, varying from about 50 per cent. of air

when the mixture is in a small vessel, and fired electrically to about 25

per cent. of air in a large vessel approached with a flame. Moreover,

safely to prepare such mixtures, after the proportion of air had been

decided upon, would require the employment of some additional perfectly

trustworthy automatic mechanism to the plant to draw into the apparatus a

quantity of air strictly in accordance with the volume of acetylene made

--a pair of meters geared together, one for the gas, the other for the

air--and this would introduce extra complexity and extra expense. On the

whole the idea cannot be recommended, and the action of the British Home

Office in prohibiting the use of all such mixtures except those

unavoidably produced in otherwise good generators, or in burners of the

ordinary injector type, is perfectly justifiable. The derivation and

effect of the other gaseous and liquid generator impurities in acetylene

were described in Chapter II. Besides these, very hot gas has been found

to contain notable amounts of hydrogen and carbon monoxide, both of which

burn with non-luminous flames. The most plausible explanation of their

origin has been given by Lewes, who suggests that they may be formed by

the action of water-vapour upon very hot carbide or upon carbon separated

therefrom as the result of previous dissociation among the gases present;

the steam and the carbon reacting together at a temperature of 500° C. or

thereabouts in a manner resembling that of the production of water-gas.

The last generator impurity is lime dust, which is calcium oxide or

hydroxide carried forward by the stream of gas in a state of extremely

fine subdivision, and is liable to be produced whenever water acts

rapidly upon an excess of calcium carbide. This lime occasionally appears

in the alternative form of a froth in the pipes leading directly from the

generating chamber; for some types of carbide-to-water apparatus,

decomposing certain kinds of carbide, foam persistently when the liquid

in them becomes saturated with lime, and this foam or froth is remarkably

difficult to break up.

FILTERS.--It has just been stated that the purifying system added to an

acetylene installation should not be called upon to remove these

generator impurities; because their appearance in quantity indicates a

faulty generator, which should be replaced by one of better action. On

the contrary, with the exception of the gases which are permanent at

atmospheric temperature--hydrogen, carbon monoxide, nitrogen, and oxygen--

and which, once produced, must remain in the acetylene (lowering its

illuminating value, but giving no further trouble), extraction of these

generator impurities is quite simple. The dust or froth of lime will be

removed in the washer where the acetylene bubbles through water--the dust

itself can be extracted by merely filtering the gas through cotton-wool,

felt, or the like. The least volatile liquid impurities will be removed

partly in the condenser, partly in the washer, and partly by the

mechanical dry-scrubbing action of the solid purifying material in the

chemical purifier. To some extent the more volatile liquid bodies will be

removed similarly; but a complete extraction of them demands the

employment of some special washing apparatus in which the crude acetylene

is compelled to bubble (in finely divided streams) through a layer of

some non-volatile oil, heavy mineral lubricating oil, &c.; for though

soluble in such oil, the liquid impurities are not soluble in, nor do

they mix with, water; and since they are held in the acetylene as

vapours, a simple passage through water, or through water-cooled pipes,

does not suffice for their recovery. It will be seen that a sufficient

removal of these generator impurities need throw no appreciable extra

labour upon the consumer of acetylene, for he can readily select a type

of generator in which their production is reduced to a minimum; while a

cotton-wool or coke filter for the gas, a water washer, which is always

useful in the plant if only employed as a non-return valve between the

generator and the holder, and the indispensable chemical purifiers, will

take out of the acetylene all the remaining generator impurities which

need, and can, be extracted.

CARBIDE IMPURITIES.--Neglecting very minute amounts of carbon monoxide

and hydrogen (which may perhaps come from cavities in the calcium carbide

itself), as being utterly insignificant from the practical point of view,

the carbide impurities of the gas fall into four main categories: those

containing phosphorus, those containing sulphur, those containing

silicon, and those containing gaseous ammonia. The phosphorus in the gas

comes from calcium phosphide in the calcium carbide, which is attacked by

water, and yields phosphoretted hydrogen (or phosphine, as it will be

termed hereafter). The calcium phosphide, in its turn, is produced in the

electric furnace by the action of the coke upon the phosphorus in

phosphatic lime--all commercially procurable lime and some varieties of

coke (or charcoal) containing phosphates to a larger or smaller extent.

The sulphur in the gas comes from aluminium sulphide in the carbide,

which is produced in the electric furnace by the interaction of

impurities containing aluminium and sulphur (clay-like bodies, &c.)

present in the lime and coke; this aluminium sulphide is attacked by

water and yields sulphuretted hydrogen. Even in the absence of aluminium

compounds, sulphuretted hydrogen may be found in the gases of an

acetylene generator; here it probably arises from calcium sulphide, for

although the latter is not decomposed by water, it gradually changes in

water into calcium sulphydrate, which appears to suffer decomposition.

When it exists in the gas the silicon is derived from certain silicides

in the carbide; but this impurity will be dealt with by itself in a later

paragraph. The ammonia arises from the action of the water upon

magnesium, aluminium, or possibly calcium nitride in the calcium carbide,

which are bodies also produced in the electric furnace or as the carbide

is cooling. In the gas itself the ammonia exists as such; the phosphorus

exists mainly as phosphine, partly as certain organic compounds

containing phosphorus, the exact chemical nature of which has not yet

been fully ascertained; the sulphur exists partly as sulphuretted

hydrogen and partly as organic compounds analogous, in all probability,

to those of phosphorus, among which Caro has found oil of mustard, and

certain bodies that he regards as mercaptans. [Footnote: It will be

convenient to borrow the phrase used in the coal-gas industry, calling

the compounds of phosphorus other than phosphine "phosphorus compounds,"

and the compounds of sulphur other than sulphuretted hydrogen "sulphur

compounds." The "sulphur compounds" of coal-gas, however, consist mainly

of carbon bisulphide, which is certainly not the chief "sulphur compound"

in acetylene, even if present to any appreciable extent.] The precise way

in which these organic bodies are formed from the phosphides and

sulphides of calcium carbide is not thoroughly understood; but the system

of generation employed, and the temperature obtaining in the apparatus,

have much to do with their production; for the proportion of the total

phosphorus and sulphur found in the crude gas which exists as "compounds"

tends to be greater as the generating plant yields a higher temperature.

It should be noted that ammonia and sulphuretted hydrogen have one

property in common which sharply distinguishes them from the sulphur

"compounds," and from all the phosphorus compounds, including phosphine.

Ammonia and sulphuretted hydrogen are both very soluble in water, the

latter more particularly in the lime-water of an active acetylene

generator; while all the other bodies referred to are completely

insoluble. It follows, therefore, that a proper washing of the crude gas

in water should suffice to remove all the ammonia and sulphuretted

hydrogen from the acetylene; and as a matter of fact those generators in

which the gas is evolved in presence of a large excess of water, and in

which it has to bubble through such water, yield an acetylene practically

free from ammonia, and containing nearly all the sulphur which it does

contain in the state of "compounds." It must also be remembered that

chemical processes which are perfectly suited to the extraction of

sulphuretted hydrogen and phosphine are not necessarily adapted for the

removal of the other phosphorus and sulphur compounds.

WASHERS.--In designing a washer for the extraction of ammonia and

sulphuretted hydrogen it is necessary to see that the gas is brought into

most intimate contact with the liquid, while yet no more pressure than

can possibly be avoided is lost. Subdivision of the gas stream may be

effected by fitting the mouth of the inlet-pipe with a rose having a

large number of very small holes some appreciable distance apart, or by

bending the pipe to a horizontal position and drilling it on its upper

surface with numbers of small holes. Another method is to force the gas

to travel under a series of partitions extending just below the water-

level, forming the lower edges of those partitions either perfectly

horizontal or with small notches like the teeth of a saw. One volume of

pure water only absorbs about three volumes of sulphuretted hydrogen at

atmospheric temperatures, but takes up some 600 volumes of gaseous

ammonia; and as ammonia always accompanies the sulphuretted hydrogen, the

latter may be said to be absorbed in the washer by a solution of ammonia,

a liquid in which sulphuretted hydrogen is much more soluble. Therefore,

since water only dissolves about an equal volume of acetylene, the liquid

in the washer will continue to extract ammonia and sulphuretted hydrogen

long after it is saturated with the hydrocarbon. For this reason,

_i.e._, to avoid waste of acetylene by dissolution in the clean

water of the washer, the plan is sometimes adopted of introducing water

to the generator through the washer, so that practically the carbide is

always attacked by a liquid saturated with acetylene. Provided the liquid

in the generator does not become seriously heated, there is no objection

to this arrangement; but if the water is heated strongly in the generator

it loses much or all of its solvent properties, and the impurities may be

driven back again into the washer. Clearly if the waste lime of the

generator occurs as a dry or damp powder, the plan mentioned is not to be

recommended; but when the waste lime is a thin cream--water being in

large excess--it may be adopted. If the generator produces lime dust

among the gas, and if the acetylene enters the washer through minute

holes, a mechanical filter to remove the dust must be inserted between

the generator and the washer, or the orifices of the leading pipe will be

choked. Whenever a water-cooled condenser is employed after the

generator, in which the gas does not come in contact with the water, that

liquid may always be used to charge the generator. For compactness and

simplicity of parts the water of the holder seal is occasionally used as

the washing liquid, but unless the liquid of the seal is constantly

renewed it will thus become offensive, especially if the holder is under

cover, and it will also act corrosively upon the metal of the tank and

bell. The water-soluble impurities in acetylene will not be removed

completely by merely standing over the holder seal for a short time, and

it is not good practice to pass unnecessarily impure gas into a holder.

[Footnote: This is not a contradiction of what has been said in Chapter

III. about the relative position of holder and chemical purifiers,

because reference is now being made to ammonia and sulphuretted hydrogen

only.]

HARMFULNESS OF IMPURITIES.--The reasons why the carbide impurities must

be removed from acetylene before it is burned have now to be explained.

From the strictly chemical point of view there are three compounds of

phosphorus, all termed phosphoretted hydrogen or phosphine: a gas, PH_3;

a liquid, P_2H_4; and a solid, P_4H_2. The liquid is spontaneously

inflammable in presence of air; that is to say, it catches fire of itself

without the assistance of spark or flame immediately it comes in contact

with atmospheric oxygen; being very volatile, it is easily carried as

vapour by any permanent gas. The gaseous phosphine is not actually

spontaneously inflammable at temperatures below 100° C.; but it oxidises

so rapidly in air, even when somewhat diluted, that the temperature may

quickly rise to the point of inflammation. In the earliest days of the

acetylene industry, directly it was recognised that phosphine always

accompanies crude acetylene from the generator, it was believed that

unless the proportion were strictly limited by decomposing only a carbide

practically free from phosphides, the crude acetylene might exhibit

spontaneously inflammable properties. Lewes, indeed, has found that a

sample of carbide containing 1 per cent of calcium phosphide gave

(probably by local decomposition--the bulk of the phosphide suffering

attack first) a spontaneously inflammable gas; but when examining

specimens of commercial carbide the highest amount of phosphine he

discovered in the acetylene was 2.3 per cent, and this gas was not

capable of self-inflammation. According to Bullier, however, acetylene

must contain 80 per cent of phosphine to render it spontaneously

inflammable. Berdenich has reported a case of a parcel of carbide which

yielded on the average 5.1 cubic foot of acetylene per lb., producing gas

which contained only 0.398 gramme of phosphorus in the form of phosphine

per cubic metre (or 0.028 per cent. of phosphine) and was spontaneously

inflammable. But on examination the carbide in question was found to be

very irregular in composition, and some lumps produced acetylene

containing a very high proportion of phosphorus and silicon compounds. No

doubt the spontaneous inflammability was due to the exceptional richness

of these lumps in phosphorus. As manufactured at the present day, calcium

carbide ordinarily never contains an amount of phosphide sufficient to

render the gas dangerous on the score of spontaneous inflammability; but

should inferior material ever be put on the markets, this danger might

have to be guarded against by submitting the gas evolved from it to

chemical analysis. Another risk has been suggested as attending the use

of acetylene contaminated with phosphine (and to a minor degree with

sulphuretted hydrogen), viz., that being highly toxic, as they

undoubtedly are, the gas containing them might be extremely dangerous to

breathe if it escaped from the service, or from a portable lamp,

unconsumed. Anticipating what will be said in a later paragraph, the

worst kind of calcium carbide now manufactured will not yield a gas

containing more than 0.1 per cent. by volume of sulphuretted hydrogen and

0.05 per cent. of phosphine. According to Haldane, air containing 0.07

per cent. of sulphuretted hydrogen produces fatal results on man if it is

breathed for some hours, while an amount of 0.2 per cent. is fatal in 1-

1/2 minutes. Similar figures for phosphine cannot be given, because

poisoning therewith is very rare or quite unknown: the cases of "phossy-

jaw" in match factories being caused either by actual contact with yellow

phosphorus or by inhalation of its vapour in the elemental state.

However, assuming phosphine to be twice as toxic as sulphuretted

hydrogen, its effect in crude acetylene of the above-mentioned

composition will be equal to that of the sulphuretted hydrogen, so that

in the present connexion the gas may be said to be equally toxic with a

sample of air containing 0.2 per cent. of sulphuretted hydrogen, which

kills in less than two minutes. But this refers only to crude acetylene

undiluted with air; and being a hydrocarbon--being in fact neither oxygen

nor common air--acetylene is irrespirable of itself though largely devoid

of specific toxic action. Numerous investigations have been made of the

amount of acetylene (apart from its impurities) which can be breathed in

safety; but although these point to a probable recovery after a fairly

long-continued respiration of an atmosphere charged with 30 per cent. of

acetylene, the figure is not trustworthy, because toxicological

experiments upon animals seldom agree with similar tests upon man. If

crude acetylene were diluted with a sufficient proportion of air to

remove its suffocating qualities, the percentage of specifically toxic

ingredients would be reduced to a point where their action might be

neglected; and short of such dilution the acetylene itself would in all

probability determine pathological effects long before its impurities

could set up symptoms of sulphur and phosphorus poisoning.

Ammonia is objectionable in acetylene because it corrodes brass fittings

and pipes, and because it is partially converted (to what extent is

uncertain) into nitrous and nitric acids as it passes through the flame.

Sulphur is objectionable in acetylene because it is converted into

sulphurous and sulphuric anhydrides, or their respective acids, as it

passes through the flame. Phosphorus is objectionable because in similar

circumstances it produces phosphoric anhydride and phosphoric acid. Each

of these acids is harmful in an occupied room because they injure the

decorations, helping to rot book-bindings, [Footnote: It is only fair to

state that the destruction of leather bindings is commonly due to traces

of sulphuric acid remaining in the leather from the production employed

in preparing it, and is but seldom caused directly by the products of

combustion coming from gas or oil.] tarnishing "gold-leaf" ornaments, and

spoiling the colours of dyed fabrics. Each is harmful to the human

system, sulphuric and phosphoric anhydrides (SO_3, and P_4O_10) acting as

specific irritants to the lungs of persons predisposed to affections of

the bronchial organs. Phosphorus, however, has a further harmful action:

sulphuric anhydride is an invisible gas, but phosphoric anhydride is a

solid body, and is produced as an extremely fine, light, white voluminous

dust which causes a haze, more or less opaque, in the apartment.

[Footnote: Lewes suggests that ammonia in the gas burnt may assist in the

production of this haze, owing to the formation of solid ammonium salts

in the state of line dust.] Immediately it comes in contact with

atmospheric moisture phosphoric anhydride is converted into phosphoric

acid, but this also occurs at first as a solid substance. The solidity

and visibility of the phosphoric anhydride and acid are beneficial in

preventing highly impure acetylene being unwittingly burnt in a room;

but, on the other hand, being merely solids in suspension in the air, the

combustion products of phosphorus are not so easily carried away from the

room by the means provided for ventilation as are the products of the

combustion of sulphur. Phosphoric anhydride is also partly deposited in

the solid state at the burner orifices, perhaps actually corroding the

steatite jets, and always assisting in the deposition of carbon from any

polymerised hydrocarbons in the acetylene; thus helping the carbon to

block up or distort those orifices. Whenever the acetylene is to be burnt

on the incandescent system under a mantle of the Welsbach or other type,

phosphorus, and possibly sulphur, become additionally objectionable, and

rigorous extraction is necessary. As is well known, the mantle is

composed of the oxides of certain "rare earths" which owe their practical

value to the fact that they are non-volatile at the temperature of the

gas-flame. When a gas containing phosphorus is burnt beneath such a

mantle, the phosphoric anhydride attacks those oxides, partially

converting them into the respective phosphates, and these bodies are less

refractory. A mantle exposed to the combustion products of crude

acetylene soon becomes brittle and begins to fall to pieces, occasionally

showing a yellowish colour when cold. The actual advantage of burning

acetylene on the incandescent system is not yet thoroughly established--

in this country at all events; but it is clear that the process will not

exhibit any economy (rather the reverse) unless the plant is provided

with most capable chemical purifiers. Phosphorus, sulphur, and ammonia

are not objectionable in crude acetylene because they confer upon the gas

a nauseous odour. From a well-constructed installation no acetylene

escapes unconsumed: the gas remains wholly within the pipes until it is

burnt, and whatever odour it may have fails to reach the human nostrils.

A house properly piped for acetylene will be no more conspicuous by its

odour than a house properly piped for coal-gas. On the contrary, the fact

that the carbide impurities of acetylene, which, in the absolutely pure

state, is a gas of somewhat faint, hardly disagreeable, odour, do confer

upon that gas a persistent and unpleasant smell, is distinctly

advantageous; for, owing to that odour, a leak in the pipes, an unclosed

tap, or a fault in the generating plant is instantly brought to the

consumer's attention. A gas wholly devoid of odour would be extremely

dangerous in a house, and would have to be scented, as is done in the

case of non-carburetted water-gas when it is required for domestic

purposes.

AMOUNTS OF IMPURITIES AND SCOPE OF PURIFICATION.--Partly for the reason

which has just been given, and partly on the ground of expense, a

complete removal of the impurities from crude acetylene is not desirable.

All that need be done is to extract sufficient to deprive the gas of its

injurious effects upon lungs, decorations, and burners. As it stands,

however, such a statement is not sufficiently precise to be useful either

to consumers of acetylene or to manufacturers of plant, and some more or

less arbitrary standard must be set up in order to define the composition

of "commercially pure" acetylene, as well as to gauge the efficiency of

any process of purification. In all probability such limit may be

reasonably taken at 0.1 milligramme of either sulphur or phosphorus

(calculated as elementary bodies) per 1 litre of acetylene, _i.e._,

0.0-1.1 grain per cubic foot; a quantity which happens to correspond

almost exactly with a percentage by weight of 0.01. Owing to the atomic

weights of these substances, and the very small quantities being

considered, the same limit hardly differs from that of 0.01 per cent. by

weight of sulphuretted hydrogen or of phosphine--it being always

recollected that the sulphur and phosphorus do not necessarily exist in

the gas as simple hydrides. Keppeler, however, has suggested the higher

figure of 0.15 milligramme of either sulphur or phosphorus per litre of

acetylene (=0.066 grain per cubic foot) for the maximum amount of these

impurities permissible in purified acetylene. He adopts this standard on

the basis of the results of observations of the amounts of sulphur and

phosphorus present in the gas issuing from a purifier charged with

heratol at the moment when the last layer of the heratol is beginning to

change colour. No limit has been given for the removal of the ammonia,

partly because that impurity can more easily, and without concomitant

disadvantage, be extracted entirely; and partly because it is usually

removed in the washer and not in the true chemical purifier.

According to Lewes, the maximum amount of ammonia found in the acetylene

coming from a dripping generator is 0.95 gramme per litre, while in

carbide-to-water gas it is 0.16 gramme: 417 and 70.2 grains per cubic

foot respectively. Rossel and Landriset have found 4 milligrammes (1.756

grains [Footnote: Milligrammes per litre; grains per cubic foot. It is

convenient to remember that since 1 cubic foot of water weighs 62.321 x

16 - 997.14 avoirdupois ounces, grammes per litre are approximately equal

to oz. per cubic foot; and grammes per cubic metre to oz. per 1000 cubic

feet.]) to be the maximum in water-to-carbide gas, and none to occur in

carbide-to-water acetylene. Rossel and Landriset return the minimum

proportion of sulphur, calculated as H_2S, found in the gaseous state in

acetylene when the carbide has not been completely flooded with water at

1.18 milligrammes per litre, or 0.52 grain per cubic foot; and the

corresponding maxima at 1.9 milligrammes, or 0.84 grain. In carbide-to-

water gas, the similar maxima are 0.23 milligramme or 0.1 grain. As

already stated, the highest proportion of phosphine yet found in

acetylene is 2.3 per cent. (Lewes), which is equal to 32.2 milligrammes

of PH_3 per litre or 14.13 grains per cubic foot (Polis); but this sample

dated from 1897. Eitner and Keppeler record the minimum proportion of

phosphorus, calculated as PH_3, found in crude acetylene, as 0.45

milligramme per litre, and the maximum as 0.89 milligramme per litre; in

English terms these figures are 0.2 and 0.4 grain per cubic foot. On an

average, however, British and Continental carbide of the present day may

be said to give a gas containing 0.61 milligramme of phosphorus

calculated as PH_3 per litre and 0.75 milligramme of sulphur calculated

as H_2S. In other units these figures are equal to 0.27 grain of PH_3 and

0.33 grain of H_2S per 1 cubic foot, or to 0.041 per cent. by volume of

PH_3 and 0.052 per cent. of H_2S. Yields of phosphorus and sulphur much

higher than these will be found in the journals and books, but such

analytical data were usually obtained in the years 1896-99, before the

manufacture of calcium carbide had reached its present degree of

systematic control. A commercial specimen of carbide was seen by one of

the authors as late as 1900 which gave an acetylene containing 1.12

milligramme of elementary sulphur per litre, i.e., 0.096 per cent, by

volume, or 0.102 per cent, by volume of H_2S; but the phosphorus showed

the low figure of 0.36 milligramme per litre (0.031 per cent, of P or

0.034 per cent, of PH_3 by volume).

The British Acetylene Association's regulations relating to carbide of

calcium (_vide_ Chap. XIV.) contain a clause to the effect that

"carbide which, when properly decomposed, yields acetylene containing

from all phosphorus compounds therein more than 0.05 per cent, by volume

of phosphoretted hydrogen, may be refused by the buyer." This limit is

equivalent to 0.74 milligramme of phosphorus calculated as PH_3 per

litre. A latitude of 0.01 per cent, is, however, allowed for the

analysis, so that the ultimate limit on which carbide could be rejected

is: 0.06 volume per cent. of PH_3, or 0.89 milligramme of phosphorus per

litre.

The existence in appreciable quantity of combined silicon as a normal

impurity in acetylene seems still open to doubt. Calcium carbide

frequently contains notable quantities of iron and other silicides; but

although these bodies are decomposed by acids, yielding hydrogen

silicide, or siliciuretted hydrogen, they are not attacked by plain

water. Nevertheless Wolff and Gerard have found hydrogen silicide in

crude acetylene, and Lewes looks upon it as a common impurity in small

amounts. When it occurs, it is probably derived, as Vigouroux has

suggested, from "alloys" of silicon with calcium, magnesium, and

aluminium in the carbide. The metallic constituents of these substances

would naturally be attacked by water, evolving hydrogen; and the

hydrogen, in its nascent state, would probably unite with the liberated

silicon to form hydrogen silicide. Many authorities, including Keppeler,

have virtually denied that silicon compounds exist in crude acetylene,

while the proportion 0.01 per cent. has been given by other writers as

the maximum. Caro, however, has stated that the crude gas almost

invariably contains silicon, sometimes in very small quantities, but

often up to the limit of 0.8 per cent.; the failure of previous

investigators to discover it being due to faulty analytical methods. Caro

has seen one specimen of (bad) carbide which gave a spontaneously

inflammable gas although it contained only traces of phosphine; its

inflammability being caused by 2.1 per cent. of hydrogen silicide.

Practically speaking, all the foregoing remarks made about phosphine

apply equally to hydrogen silicide: it burns to solid silicon oxide

(silica) at the burners, is insoluble in water, and is spontaneously

inflammable when alone or only slightly diluted, but never occurs in good

carbide in sufficient proportion to render the acetylene itself

inflammable. According to Caro the silicon may be present both as

hydrogen silicide and as silicon "compounds." A high temperature in the

generator will favour the production of the latter; an apparatus in which

the gas is washed well in lime-water will remove the bulk of the former.

Fraenkel has found that magnesium silicide is not decomposed by water or

an alkaline solution, but that dilute hydrochloric acid acts upon it and

spontaneously inflammable hydrogen silicide results. If it may be assumed

that the other silicides in commercial calcium carbide also behave in

this manner it is plain that hydrogen silicide cannot occur in crude

acetylene unless the gas is supposed to be hurried out of the generator

before the alkaline water therein has had time to decompose any traces of

the hydrogen silicide which is produced in the favouring conditions of

high temperature sometimes prevailing. Mauricheau-Beaupré has failed to

find silica in the products of combustion of acetylene from carbide of

varying degrees of purity. He found, however, that a mixture of strong

nitric and hydrochloric acids (_aqua regia_), if contaminated with

traces of phosphoric acid, dissolved silica from the glass of laboratory

vessels. Consequently, since phosphoric acid results from the phosphine

in crude acetylene when the gas is passed through aqua regia, silica may

be found on subsequently evaporating the latter. But this, silica, he

found, was derived from the glass and not through the oxidation of

silicon compounds in the acetylene. It is possible that some of the

earlier observers of the occurrence of silicon compounds in crude

acetylene may have been misled by the solution of silica from the glass

vessels used in their investigations. The improbability of recognisable

quantities of silicon compounds occurring in acetylene in any ordinary

conditions of generation is demonstrated by a recent study by Fraenkel of

the composition of the deposit produced on reflectors exposed to the

products of combustion of a sample of acetylene which afforded a haze

when burnt. The deposit contained 51.07 per cent. of phosphoric acid, but

no silica. The gas itself contained from 0.0672 to 0.0837 per cent. by

volume of phosphine.

PURIFYING MATERIALS.--When acetylene first began to be used as a domestic

illuminant, most generator builders denied that there was any need for

the removal of these carbide impurities from the gas, some going so far

as to assert that their apparatus yielded so much purer an acetylene than

other plant, where purification might be desirable, that an addition of a

special purifier was wholly unnecessary. Later on the more responsible

members of the trade took another view, but they attacked the problem of

purification in a perfectly empirical way, either employing some purely

mechanical scrubber filled with some moist or dry porous medium, or

perhaps with coke or the like wetted with dilute acid, or they simply

borrowed the processes adopted in the purification of coal-gas. At first

sight it might appear that the more simple methods of treating coal-gas

should be suitable for acetylene; since the former contains two of the

impurities--sulphuretted hydrogen and ammonia--characteristic of crude

acetylene. After removing the ammonia by washing with water, therefore,

it was proposed to extract the sulphur by passing the acetylene through

that variety of ferric hydroxide (hydrated oxide of iron) which is so

serviceable in the case of coal-gas. The idea, however, was quite

unsound: first, because it altogether ignores the phosphorus, which is

the most objectionable impurity in acetylene, but is not present in coal-

gas; secondly, because ferric hydroxide is used on gasworks to extract in

a marketable form the sulphur which occurs as sulphuretted hydrogen, and

true sulphuretted hydrogen need not exist in well-generated and well-

washed acetylene to any appreciable extent; thirdly, because ferric

hydroxide is not employed by gasmakers to remove sulphur compounds (this

is done with lime), being quite incapable of extracting them, or the

analogous sulphur compounds of crude acetylene.

About the same time three other processes based on somewhat better

chemical knowledge were put forward. Pictet proposed leading the gas

through a strong solution of calcium chloride and then through strong

sulphuric acid, both maintained at a temperature of -20° to -40° C.,

finally washing the gas in a solution of some lead salt. Proof that such

treatment would remove phosphorus to a sufficient degree is not

altogether satisfactory; but apart from this the necessity of maintaining

such low temperatures, far below that of the coldest winter's night,

renders the idea wholly inadmissible for all domestic installations.

Willgerodt suggested removing sulphuretted hydrogen by means of potassium

hydroxide (caustic potash), then absorbing the phosphine in bromine

water. For many reasons this process is only practicable in the

laboratory. Bergé and Reychler proposed extracting both sulphuretted

hydrogen and phosphine in an acid solution of mercuric chloride

(corrosive sublimate). The poisonousness of this latter salt, apart from

all other objections, rules such a method out.

BLEACHING POWDER.--The next idea, first patented by Smith of Aberdeen,

but fully elaborated by Lunge and Cedercreutz, was to employ bleaching-

powder [Footnote: Bleaching-powder is very usually called chloride of

lime; but owing to the confusion which is constantly arising in the minds

of persons imperfectly acquainted with chemistry between chloride of lime

and chloride of calcium--two perfectly distinct bodies--the less

ambiguous expression "bleaching-powder" will be adopted here.] either in

the solid state or as a liquid extract. The essential constituent of

bleaching-powder from the present aspect is calcium hypochlorite, which

readily oxidises sulphuretted hydrogen, and more particularly phosphine,

converting them into sulphuric and phosphoric acids, while the acetylene

is practically unattacked. In simple purifying action the material proved

satisfactory; but since high-grade commercial bleaching-powder contains

some free chlorine, or some is set free from it in the purifier under the

influence of the passing gas, the issuing acetylene was found to contain

chlorine, free or combined; and this, burning eventually to hydrochloric

acid, is hardly less harmful than the original sulphur compounds.

Moreover, a mixture of acetylene, chlorine, and air is liable to catch

fire of itself when exposed to bright sunlight; and therefore the use of

a bleaching-powder purifier, or rather the recharging thereof, was not

unattended by danger in the early days. To overcome these defects, the

very natural process was adopted of diluting the bleaching-powder, such

diluent also serving to increase the porosity of the material. A very

unsuitable substance, however, was selected for the purpose, viz.,

sawdust, which is hygroscopic organic, and combustible. Owing to the

exothermic chemical action between the impurities of the acetylene and

the bleaching-powder, the purifying mass became heated; and thus not only

were the phenomena found in a bad generator repeated in the purifying

vessel, but in presence of air and light (as in emptying the purifier),

the reaction proceeded so rapidly that the heat caused inflammation of

the sawdust and the gas, at least on one occasion an actual fire taking

place which created much alarm and did some little damage. For a time,

naturally, bleaching-powder was regarded as too dangerous a material to

be used for the purification of crude acetylene; but it was soon

discovered that danger could be avoided by employing the substance in a

proper way.

HERATOL, FRANKOLINE, ACAGINE AND PURATYLENE.--Setting aside as unworthy

of attention certain compositions offered as acetylene purifying

materials whose constitution has not been divulged or whose action has

not been certified by respectable authority, there are now three

principal chemical reagents in regular use. Those are chromic acid,

cuprous chloride (sub- or proto-chloride of copper), and bleaching-

powder. Chromic acid is employed in the form of a solution acidified with

acetic or hydrochloric acid, which, in order to obtain the advantages

(_see_ below) attendant upon the use of a solid purifying material,

is absorbed in that highly porous and inert description of silica known

as infusorial earth or "kieselguhr." This substance was first recommended

by Ullmann, and is termed commercially "heratol" As sold it contains

somewhere about 136 grammes of chromic acid per kilo. Cuprous chloride is

used as a solution in strong hydrochloric acid mixed with ferric

chloride, and similarly absorbed in kieselguhr. From the name of its

proposer, this composition is called "frankoline." It will be shown in

Chapter VI. that the use of metallic copper in the construction of

acetylene apparatus is not permissible or judicious, because the gas is

liable to form therewith an explosive compound known as copper acetylide;

it might seem, therefore, that the employment of a copper salt for

purification courts accident. The objection is not sound, because the

acetylide is not likely to be produced except in the presence of ammonia;

and since frankoline is a highly acid product, the ammonia is converted

into its chloride before any copper acetylide can be produced. As a

special acetylene purifier, bleaching-powder exists in at least two chief

modifications. In one, known as "acagine," it is mixed with 15 per cent.

of lead chromate, and sometimes with about the same quantity of barium

sulphate; the function of the latter being simply that of a diluent,

while to the lead chromate is ascribed by its inventor (Wolff) the power

of retaining any chlorine that may be set free from the bleaching-powder

by the reduction of the chromic acid. The utility of the lead chromate in

this direction has always appeared doubtful; and recently Keppeler has

argued that it can have no effect upon the chlorine, inasmuch as in the

spent purifying material the lead chromate may be found in its original

condition unchanged. The second modification of bleaching-powder is

designated "puratylene," and contains calcium chloride and quick or

slaked lime. It is prepared by evaporating to dryness under diminished

pressure solutions of its three ingredients, whereby the finished

material is given a particularly porous nature.

It will be observed that both heratol and frankoline are powerfully acid,

whence it follows they are capable of extracting any ammonia that may

enter the purifier; but for the same reason they are liable to act

corrosively upon any metallic vessel in which they are placed, and they

therefore require to be held in earthenware or enamelled receivers. But

since they are not liquid, the casing of the purifier can be safely

constructed of steel or cast iron. Puratylene also removes ammonia by

virtue of the calcium chloride in it. Acagine would probably pass the

ammonia; but this is no real objection, as the latter can be extracted by

a preliminary washing in water. Heratol changes, somewhat obscurely, in

colour as it becomes spent, its original orange tint, due to the chromic

acid, altering to a dirty green, characteristic of the reduced salts of

chromium oxide. Frankoline has been asserted to be capable of

regeneration or revivification, _i.e._, that when spent it may be

rendered fit for further service by being exposed to the air for a time,

as is done with gas oxide; this, however, may be true to some extent with

the essential constituents of frankoline, but the process is not

available with the commercial solid product. Of all these materials,

heratol is the most complete purifier of acetylene, removing phosphorus

and sulphur most rapidly and thoroughly, and not appreciably diminishing

in speed or efficiency until its chromic acid is practically quite used

up. On the other hand, heratol does act upon pure acetylene to some

extent; so that purifiers containing it should be small in size and

frequently recharged. In one of his experiments Keppeler found that 13

per cent. of the chromic acid in heratol was wasted by reacting with

acetylene. As this waste of chromic acid involves also a corresponding

loss of gas, small purifiers are preferable, because at any moment they

only contain a small quantity of material capable of attacking the

acetylene itself. Frankoline is very efficacious as regards the

phosphorus, but it does not wholly extract the sulphur, leaving,

according to Keppeler, from 0.13 to 0.20 gramme of the latter in every

cubic metre of the gas. It does not attack acetylene itself; and if,

owing to its free hydrochloric acid, it adds any acid vapours to the

purified gas, these vapours may be easily removed by a subsequent passage

through a vessel containing lime or a carbide drier. Both being

essentially bleaching-powder, acagine and puratylene are alike in

removing phosphorus to a satisfactory degree; but they leave some sulphur

behind. Acagine evidently attacks acetylene to a slight extent, as

Keppeler has found 0.2 gramme of chlorine per cubic metre in the issuing

gas.

Although some of these materials attack acetylene slightly, and some

leave sulphur in the purified gas, they may be all considered reasonably

efficient from the practical point of view; for the loss of true

acetylene is too small to be noticeable, and the quantity of sulphur not

extracted too trifling to be harmful or inconvenient. They may be valued,

accordingly, mainly by their price, proper allowance being made for the

quantity of gas purified per unit weight of substance taken. This

quantity of gas must naturally vary with the proportion of phosphorus and

sulphur in the crude acetylene; but on an average the composition of

unpurified gas is what has already been given above, and so the figures

obtained by Keppeler in his investigation of the subject may be accepted.

In the annexed table these are given in two forms: (1) the number of

litres of gas purified by 1 kilogramme of the substance, (2) the number

of cubic feet purified per lb. It should be noted that the volumes of gas

refer to a laboratory degree of purification; in practice they may all be

increased by 10 or possibly 20 per cent.

_________________________________________________

| | | |

| | Litres | Cubic Feet |

| | per Kilogramme. | per Lb. |

|______________|___________________|______________|

| | | |

| Heratol | 5,000 | 80 |

| Frankoline | 9,000 | 144 |

| Puratylene | 10,000 | 160 |

| Acagine | 13,000 | 208 |

|______________|___________________|______________|

Another method of using dry bleaching-powder has been proposed by

Pfeiffer. He suggests incorporating it with a solution of some lead salt,

so that the latter may increase the capacity of the calcium hypochlorite

to remove sulphur. Analytical details as to the efficiency of this

process have not been given. During 1901 and 1902 Bullier and Maquenne

patented a substance made by mixing bleaching-powder with sodium

sulphate, whereby a double decomposition occurs, sodium hypochlorite,

which is equally efficient with calcium hypochlorite as a purifying

material, being produced together with calcium sulphate, which, being

identical with plaster of Paris, sets into a solid mass with the excess

of water present, and is claimed to render the whole more porous. This

process seemed open to objection, because Blagden had shown that a

solution of sodium hypochlorite was not a suitable purifying reagent in

practice, since it was much more liable to add chlorine to the gas than

calcium hypochlorite. The question how a solidified modification of

sodium hypochlorite would behave in this respect has been investigated by

Keppeler, who found that the Bullier and Maquenne material imparted more

chlorine to the gas which had traversed it than other hypochlorite

purifying agents, and that the partly foul material was liable to cause

violent explosions. About the same time Rossel and Landriset pointed out

that purification might be easily effected in all generators of the

carbide-to-water pattern by adding to the water of the generator itself a

quantity of bleaching-powder equivalent to 5 to 20 grammes for every 1

kilogramme of carbide decomposed, claiming that owing to the large amount

of liquid present, which is usually some 4 litres per kilogramme of

carbide (0.4 gallon per lb.), no nitrogen chloride could be produced, and

that owing to the dissolved lime in the generator, chlorine could not be

added to the gas. The process is characterised by extreme simplicity, no

separate purifier being needed, but it has been found that an

introduction of bleaching-powder in the solid condition is liable to

cause an explosive combination of acetylene and chlorine, while the use

of a solution is attended by certain disadvantages. Granjon has proposed

impregnating a suitable variety of wood charcoal with chlorine, with or

without an addition of bleaching-powder; then grinding the product to

powder, and converting it into a solid porous mass by the aid of cement.

The material is claimed to last longer than ordinary hypochlorite

mixtures, and not to add chlorine to the acetylene.

SUBSIDIARY PURIFYING MATERIALS.--Among minor reagents suggested as

purifying substances for acetylene may be mentioned potassium

permanganate, barium peroxide, potassium bichromate, sodium plumbate and

arsenious oxide. According to Benz the first two do not remove the

sulphuretted hydrogen completely, and oxidise the acetylene to some

extent; while potassium bichromate leaves some sulphur and phosphorus

behind in the gas. Sodium plumbate has been suggested by Morel, but it is

a question whether its action on the impurities would not be too violent

and whether it would be free from action on the acetylene itself. The use

of arsenious oxide dissolved in a strong acid, and the solution absorbed

in pumice or kieselguhr has been protected by G. F. Jaubert. The

phosphine is said to combine with the arsenic to form an insoluble

brownish compound. In 1902 Javal patented a mixture of 1 part of

potassium permanganate, 5 of "sulphuric acid," and 1 of water absorbed in

4 parts of infusorial earth. The acid constantly neutralised by the

ammonia of the crude gas is as constantly replaced by fresh acid formed

by the oxidation of the sulphuretted hydrogen; and this free acid, acting

upon the permanganate, liberates manganese peroxide, which is claimed to

destroy the phosphorus and sulphur compounds present in the crude

acetylene.

ÉPURÈNE.--A purifying material to which the name of épurène has been

given has been described, by Mauricheau-Beaupré, as consisting of a

mixture of ferric chloride and ferric oxide in the proportion of 2

molecules, or 650 parts, of the former with one molecule, or 160 parts,

of the latter, together with a suitable quantity of infusorial earth. In

the course of preparation, however, 0.1 to 0.2 per cent. of mercuric

chloride is introduced into the material. This mercuric chloride is said

to form an additive compound with the phosphine of the crude acetylene,

which compound is decomposed by the ferric chloride, and the mercuric

chloride recovered. The latter therefore is supposed to act only as a

carrier of the phosphine to the ferric chloride and oxide, by which it is

oxidised according to the equation:

8Fe_2Cl_6 + 4Fe_2O_3 + 3PH_3 = 12Fe_2Cl_4 + 3H_3PO_4.

Thus the ultimate products are phosphoric acid and ferrous chloride,

which on exposure to air is oxidised to ferric chloride and oxide. It is

said that this revivification of the fouled or spent épurène takes place

in from 20 to 48 hours when it is spread in the open in thin layers, or

it may be partially or wholly revivified _in situ_ by adding a small

proportion of air to the crude acetylene as it enters the purifier. The

addition of 1 to 2 per cent. of air, according to Mauricheau-Beaupré,

suffices to double the purifying capacity of one charge of the material,

while a larger proportion would achieve its continuous revivification.

Épurène is said to purify 10,000 to 11,000 litres of crude acetylene per

kilogramme, or, say, 160 to 176 cubic feet per pound, when the acetylene

contains on the average 0.05 per cent, by volume of phosphine.

For employment in all acetylene installations smaller than those which

serve complete villages, a solid purifying material is preferable to a

liquid one. This is partly due to the extreme difficulty of subdividing a

stream of gas so that it shall pass through a single mass of liquid in

small enough bubbles for the impurities to be removed by the time the gas

arrives at the surface. This time cannot be prolonged without increasing

the depth of liquid in the vessel, and the greater the depth of liquid,

the more pressure is consumed in forcing the gas through it. Perfect

purification by means of fluid reagents unattended by too great a

consumption of pressure is only to be effected by a mechanical scrubber

such as is used on coal-gas works, wherein, by the agency of external

power, the gas comes in contact with large numbers of solid surfaces kept

constantly wetted; or by the adoption of a tall tower filled with porous

matter or hollow balls over which a continuous or intermittent stream of

the liquid purifying reagent is made to trickle, and neither of these

devices is exactly suited to the requirements of a domestic acetylene

installation. When a solid material having a proper degree of porosity or

aggregation is selected, the stream of gas passing through it is broken

up most thoroughly, and by employing several separate layers of such

material, every portion of the gas is exposed equally to the action of

the chemical reagent by the time the gas emerges from the vessel. The

amount of pressure so consumed is less than that in a liquid purifier

where much fluid is present; but, on the other hand, the loss of pressure

is absolutely constant at all times in a liquid purifier, provided the

head of liquid is maintained at the same point. A badly chosen solid

purifying agent may exhibit excessive pressure absorption as it becomes

partly spent. A solid purifier, moreover, has the advantage that it may

simultaneously act as a drier for the gas; a liquid purifier, in which

the fluid is mainly water, obviously cannot behave in a similar fashion

For thorough purification it is necessary that the gas shall actually

stream through the solid material; a mere passage over its surface is

neither efficient nor economical of material.

DISPOSITION OF PURIFYING MATERIAL.--Although much has been written, and

some exaggerated claims made, about the maximum, volume of acetylene a

certain variety of purifying material will treat, little has been said

about the method in which such a material should be employed to obtain

the best results. If 1 lb. of a certain substance will purify 200 cubic

feet of normal crude acetylene, that weight is sufficient to treat the

gas evolved from 40 lb. of carbide; but it will only do so provided it is

so disposed in the purifier that the gas does not pass through it at too

high a speed, and that it is capable of complete exhaustion. In the coal-

gas industry it is usually assumed that four layers of purifying

material, each having a superficial area of 1 square foot, are the

minimum necessary for the treatment of 100 cubic feet of gas per hour,

irrespective of the nature of the purifying material and of the impurity

it is intended to extract. If there is any sound basis for this

generalization, it should apply equally to the purification of acetylene,

because there is no particular reason to imagine that the removal of

phosphine by a proper substance should occur at an appreciably different

speed from the removal of carbon dioxide, sulphuretted hydrogen, and

carbon bisulphide by lime, ferric oxide, and sulphided lime respectively,

Using the coal gas figures, then, for every 10 cubic feet of acetylene

generated per hour, a superficial area of (4 x 144 / 10) 57.6 square

inches of purifying material is required. In the course of Keppeler's

research upon different purifying materials it is shown that 400 grammes

of heratol, 360 grammes of frankoline, 250 grammes of acagine, and 230

grammes of puratylene each occupy a space of 500 cubic centimetres when

loosely loaded into a purifying vessel, and from these data, the

following table has been calculated:

__________________________________________________________

| | | | |

|_____________|____________|________________|______________|

| | | | |

| Water | 10.0 | 62.321 | 27.73 |

| Heratol | 8.0 | 49.86 | 31.63 |

| Frankoline | 7.2 | 41.87 | 38.21 |

| Acagine | 6.0 | 31.16 | 55.16 |

| Puratylene | 4.6 | 28.67 | 60.28 |

|_____________|____________|________________|______________|

As regards the minimum weight of material required, data have been given

by Pfleger for use with puratylene. He states that 1 Kilogramme of that

substance should be present for every 100 litres of crude acetylene

evolved per hour, 4 kilogrammes being the smallest quantity put into the

purifier. In English units these figures are 1 lb. per 1.5 cubic feet per

hour, with 9 lb. as a minimum, which is competent to treat 1.1 cubic feet

of gas per hour. Thus it appears that for the purification of the gas

coming from any generator evolving up to 14 cubic feet of acetylene per

hour a weight of 9 lb of puratylene must be charged into the purifier,

which will occupy (60.28 / 9) 542 cubic inches of space; and it must be

so spread out as to present a total superficial area of (4 x 144 x 14 /

100) 80.6 square inches to the passing gas. It follows, therefore, that

the material should be piled to a depth of (542 / 80.6) 6.7 inches on a

support having an area of 80.6 square inches; but inasmuch as such a

depth is somewhat large for a small vessel, and as several layers are

better than one, it would be preferable to spread out these 540 cubic

inches of substance on several supports in such a fashion that a total

surface of 80.6 square inches or upwards should be exhibited. These

figures may obviously be manipulated in a variety of ways for the design

of a purifying vessel; but, to give an example, if the ordinary

cylindrical shape be adopted with four circular grids, each having a

clear diameter of 8 inches (_i.e._, an area of 50.3 square inches),

and if the material is loaded to a depth of 3 inches on each, there would

be a total volume of (50.3 x 3 x 4) = 604 cubic inches of puratylene in

the vessel, and it would present a total area of (50.3 x 4) = 201 square

inches to the acetylene. At Keppeler's estimation such an amount of

puratylene should weigh roughly 10 lb., and should suffice for the

purification of the gas obtained from 320 lb. of ordinary carbide; while,

applying the coal-gas rule, the total area of 201 square inches should

render such a vessel equal to the purification of acetylene passing

through it at a speed not exceeding (201 / 5.76) = 35 cubic feet per

hour. Remembering that it is minimum area in square inches of purifying

material that must govern the speed at which acetylene may be passed

through a purifier, irrespective probably of the composition of the

material; while it is the weight of material which governs the ultimate

capacity of the vessel in terms of cubic feet of acetylene or pounds of

carbide capable of purification, these data, coupled with Keppeler's

efficiency table, afford means for calculating the dimensions of the

purifying vessel to be affixed to an installation of any desired number

of burners. There is but little to say about the design of the vessel

from the mechanical aspect. A circular horizontal section is more likely

to make for thorough exhaustion of the material. The grids should be

capable of being lifted out for cleaning. The lid may be made tight

either by a clamp and rubber or leather washer, or by a liquid seal. If

the purifying material is not hygroscopic, water, calcium chloride

solution, or dilute glycerin may be used for sealing purposes; but if the

material, or any part of it, does absorb water, the liquid in the seal

should be some non-aqueous fluid like lubricating oil. Clamped lids are

more suitable for small purifiers, sealed lids for large vessels. Care

must be taken that condensation products cannot collect in the purifying

vessel. If a separate drying material is employed in the same purifier

the space it takes must be considered separately from that needed by the

active chemical reagent. When emptying a foul purifier it should be

recollected that the material may be corrosive, and being saturated with

acetylene is likely to catch fire in presence of a light.

Purifiers charged with heratol are stated, however, to admit of a more

rapid flow of the gas through them than that stated above for puratylene.

The ordinary allowance is 1 lb. of heratol for every cubic foot per hour

of acetylene passing, with a minimum charge of 7 lb. of the material. As

the quantity of material in the purifier is increased, however, the flow

of gas per hour may be proportionately increased, _e.g._, a purifier

charged with 132 lb. of heratol should purify 144 cubic feet of acetylene

per hour.

In the systematic purification of acetylene, the practical question

arises as to how the attendant is to tell when his purifiers approach

exhaustion and need recharging; for if it is undesirable to pass crude

gas into the service, it is equally undesirable to waste so comparatively

expensive a material as a purifying reagent. In Chapter XIV. it will be

shown that there are chemical methods of testing for the presence, or

determining the proportion, of phosphorus and sulphur in acetylene; but

these are not suitable for employment by the ordinary gas-maker. Heil has

stated that the purity of the gas may be judged by an inspection of its

atmospheric flame as given by a Bunsen burner. Pure acetylene gives a

perfectly transparent moderately dark blue flame, which has an inner cone

of a pale yellowish green colour; while the impure gas yields a longer

flame of an opaque orange-red tint with a bluish red inner zone. It

should be noted, however, that particles of lime dust in the gas may

cause the atmospheric flame to be reddish or yellowish (by presence of

calcium or sodium) quite apart from ordinary impurities; and for various

other reasons this appearance of the non-luminous flame is scarcely to be

relied upon. The simplest means of ascertaining definitely whether a

purifier is sufficiently active consists in the use of the test-papers

prepared by E. Merck of Darmstadt according to G. Keppeler's

prescription. These papers, cut to a convenient size, are put up in small

books from which they may be torn one at a time. In order to test whether

gas is sufficiently purified, one of the papers is moistened with

hydrochloric acid of 10 per cent. strength, and the gas issuing from a

pet-cock or burner orifice is allowed to impinge on the moistened part.

The original black or dark grey colour of the paper is changed to white

if the gas contains a notable amount of impurity, but remains unchanged

if the gas is adequately purified. The paper consists of a specially

prepared black porous paper which has been dipped in a solution of

mercuric chloride (corrosive sublimate) and dried. Moistening the paper

with hydrochloric acid provides in a convenient form for application

Bergé's solution for the detection of phosphine (_vide_ Chapter

XIV.). The Keppeler test-papers turn white when the gas contains either

ammonia, phosphine, siliciuretted hydrogen, sulphuretted hydrogen or

organic sulphur compounds, but with carbon disulphide the change is slow.

Thus the paper serves as a test for all the impurities likely to occur in

acetylene. The sensitiveness of the test is such that gas containing

about 0.15 milligramme of sulphur, and the same amount of phosphorus, per

litre (= 0.0655 grain per cubic foot) imparts in five minutes a distinct

white mark to the moistened part of the paper, while gas containing 0.05

milligramme of sulphur per litre (= 0.022 grain per cubic foot) gives in

two minutes a dull white mark visible only by careful inspection. If,

therefore, a distinct white mark appears on moistened Keppeler paper when

it is exposed for five minutes to a jet of acetylene, the latter is

inadequately purified. If the gas has passed through a purifier, this

test indicates that the material is not efficient, and that the purifier

needs recharging. The moistening of the Keppeler paper with hydrochloric

acid before use is essential, because if not acidified the paper is

marked by acetylene itself. The books of Keppeler papers are put up in a

case which also contains a bottle of acid for moistening them as required

and are obtainable wholesale of E. Merek, 16 Jewry Street, London, E.C.,

and retail of the usual dealers in chemicals. If Keppeler's test-papers

are not available, the purifier should be recharged as a matter of

routine as soon as a given quantity of carbide--proportioned to the

purifying capacity of the charge of purifying material--has been used

since the last recharging. Thus the purifier may conveniently contain

enough material to purify the gas evolved from two drums of carbide, in

which case it would need recharging when every second drum of carbide is

opened.

REGULATIONS AS TO PURIFICATION.--The British Acetylene

Association has issued the following set of regulations as to purifying

material and purifiers for acetylene:

Efficient purifying material and purifiers shall comply with the

following requirements:

(1) The purifying material shall remove phosphorus and sulphur compounds

to a commercially satisfactory degree; _i.e._, not to a greater

degree than will allow easy detection of escaping gas through its odour.

(2) The purifying material shall not yield any products capable of

corroding the gas-mains or fittings.

(3) The purifying material shall, if possible, be efficient as a drying

agent, but the Association does not consider this an absolute necessity.

(4) The purifying material shall not, under working conditions, be

capable of forming explosive compounds or mixtures. It is understood,

naturally, that this condition does not apply to the unavoidable mixture

of acetylene and air formed when recharging the purifier.

(5) The apparatus containing the purifying material shall be simple in

construction, and capable of being recharged by an inexperienced person

without trouble. It shall be so designed as to bring the gas into proper

contact with the material.

(6) The containers in purifiers shall be made of such materials as are

not dangerously affected by the respective purifying materials used.

(7) No purifier shall be sold without a card of instructions suitable or

hanging up in some convenient place. Such instructions shall be of the

most detailed nature, and shall not presuppose any expert knowledge

whatever on the part of the operator.

Reference also to the abstracts of the official regulations as to

acetylene installations in foreign countries given in Chapter IV. will

show that they contain brief rules as to purifiers.

DRYING.--It has been stated in Chapter III. that the proper position for

the chemical purifiers of an acetylene plant is after the holder; and

they therefore form the last items in the installation unless a "station"

governor and meter are fitted. It is therefore possible to use them also

to remove the moisture in the gas, if a material hygroscopic in nature is

employed to charge them. This should be true more particularly with

puratylene, which contains a notable proportion of the very hygroscopic

body calcium chloride. If a separate drier is desirable, there are two

methods of charging it. It may be filled either with some hygroscopic

substance such as porous calcium chloride or quicklime in very coarse

powder, which retains the water by combining with it; or the gas may be

led through a vessel loaded with calcium carbide, which will manifestly

hold all the moisture, replacing it by an equivalent quantity of

(unpurified) acetylene. The objection is sometimes urged against this

latter method, that it restores to the gas the nauseous odour and the

otherwise harmful impurities it had more or less completely lost in the

purifiers; but as regards the first point, a nauseous odour is not, as

has previously been shown, objectionable in itself, and as regards the

second, the amount of impurities added by a carbide drier, being strictly

limited by the proportion of moisture in the damp gas, is too small to be

noticeable at the burners or elsewhere. As is the case with purification,

absolute removal of moisture is not called for; all that is needed is to

extract so much that the gas shall never reach its saturation-point in

the inaccessible parts of the service during the coldest winter's night.

Any accessible length of main specially exposed to cold may be

safeguarded by itself; being given a steady fall to a certain point

(preferably in a frost-free situation), and there provided with a

collecting-box from which the deposited liquid can be removed

periodically with a pump or otherwise.

FILTRATION.--The gas issuing from the purifier or drier is very liable to

hold in suspension fine dust derived from the purifying or drying

material used. It is essential that thin dust should be abstracted before

the gas reaches the burners, otherwise it will choke the orifices and

prevent them functioning properly. Consequently the gas should pass

through a sufficient layer of filtering material after it has traversed

the purifying material (and drier if one is used). This filtering

material may be put either as a final layer in the purifier (or drier),

or in a separate vessel known as a filter. Among filtering materials in

common use may be named cotton-wool, fine canvas or gauze, felt and

asbestos-wool. The gas must be fairly well dried before it enters the

filter, otherwise the latter will become choked with deposited moisture,

and obstruct the passage of the gas.

Having now described the various items which go to form a well-designed

acetylene installation, it may be useful to recapitulate briefly, with

the object of showing the order in which they should be placed. From the

generator the gas passes into a condenser to cool it and to remove any

tarry products and large quantities of water. Next it enters a washing

apparatus filled with water to extract water-soluble impurities. If the

generator is of the carbide-to-water pattern, the condenser may be

omitted, and the washer is only required to retain any lime froth and to

act as a water-seal or non-return valve. If the generator does not wash

the gas, the washer must be large enough to act efficiently as such, and

between it and the condenser should be put a mechanical filter to extract

any dust. From the washer the acetylene travels to the holder. From the

holder it passes through one or two purifiers, and from there travels to

the drier and filter. If the holder does not throw a constant pressure,

or if the purifier and drier are liable to cause irregularities, a

governor or pressure regulator must be added after the drier. The

acetylene is then ready to enter the service; but a station meter (the

last item in the plant) is useful as giving a means of detecting any leak

in the delivery-pipes and in checking the make of gas from the amount of

carbide consumed. If the gas is required for the supply of a district, a

station meter becomes quite necessary, because the public lamps will be

fed with gas at a contract rate, and without the meter there would be no

control over the volume of acetylene they consume. Where the gas finally

leaves the generating-house, or where it enters the residence, a full-way

stopcock should be put on the main.

GENERATOR RESIDUES.--According to the type of generator employed the

waste product removed therefrom may vary from a dry or moist powder to a

thin cream or milk of lime. Any waste product which is quite liquid in

its consistency must be completely decomposed and free from particles of

calcium carbide of sensible magnitude; in the case of more solid

residues, the less fluid they are the greater is the improbability (or

the less is the evidence) that the carbide has been wholly spent within

the apparatus. Imperfect decomposition of the carbide inside the

generator not only means an obvious loss of economy, but its presence

among the residues makes a careful handling of them essential to avoid

accident owing to a subsequent liberation of acetylene in some

unsuitable, and perhaps closed, situation. A residue which is not

conspicuously saturated with water must be taken out of the generator-

house into the open air and there flooded with water, being left in some

uncovered receptacle for a sufficient time to ensure all the acetylene

being given off. A residue which is liquid enough to flow should be run

directly from the draw-off cock of the generator through a closed pipe to

the outside; where, if it does not discharge into an open conduit, the

waste-pipe must be trapped, and a ventilating shaft provided so that no

gas can blow back into the generator-house.

DISPOSAL OF RESIDUES.--These residues have now to be disposed of. In some

circumstances they can be put to a useful purpose, as will be explained

in Chapter XII.; otherwise, and always perhaps on the small scale--

certainly always if the generator overheats the gas and yields tar among

the spent lime--they must be thrown into a convenient place. It should be

remembered that although methods of precipitating sewage by adding lime,

or lime water, to it have frequently been used, they have not proved

satisfactory, partly because the sludge so obtained is peculiarly

objectionable in odour, and partly because an excess of lime yields an

effluent containing dissolved lime, which among other disadvantages is

harmful to fish. The plan of running the liquid residues of acetylene

manufacture into any local sewerage system which may be found in the

neighbourhood of the consumer's premises, therefore, is very convenient

to the consumer; but is liable to produce complaints if the sewage is

afterwards treated chemically, or if its effluent is passed untreated

into a highly preserved river; and the same remark applies in a lesser

degree if the residues are run into a private cesspool the liquid

contents of which automatically flow away into a stream. If, however, the

cesspool empties itself of liquid matter by filtration or percolation

through earth, there can be no objection to using it to hold the lime

sludge, except in so far as it will require more frequent emptying. On

the whole, perhaps the best method of disposing of these residues is to

run them into some open pit, allowing the liquid to disappear by

evaporation and percolation, finally burying the solid in some spot where

it will be out of the way. When a large carbide-to-water generator is

worked systematically so as to avoid more loss of acetylene by solution

in the excess of liquid than is absolutely necessary, the liquid residues

coming from it will be collected in some ventilated closed tank where

they can settle quietly. The clear lime-water will then be pumped back

into the generator for further use, and the almost solid sludge will be

ready to be carried to the pit where it is to be buried. Special care

must be taken in disposing of the residues from a generator in which oil

is used to control evolution of gas. Such oil floats on the aqueous

liquid; and a very few drops spread for an incredible distance as an

exceedingly thin film, causing those brilliant rainbow-like colours which

are sometimes imagined to be a sign of decomposing organic matter. The

liquid portions of these residues must be led through a pit fitted with a

depending partition projecting below the level at which the water is

constantly maintained; all the oil then collects on the first side of the

partition, only water passing underneath, and the oil may be withdrawn

and thrown away at intervals.

CHAPTER VI

THE CHEMICAL AND PHYSICAL PROPERTIES OF ACETYLENE

It will only be necessary for the purpose of this book to indicate the

more important chemical and physical properties of acetylene, and, in

particular, those which have any bearing on the application of acetylene

for lighting purposes. Moreover, it has been found convenient to discuss

fully in other chapters certain properties of acetylene, and in regard to

such properties the reader is referred to the chapters mentioned.

PHYSICAL PROPERTIES.--Acetylene is a gas at ordinary temperatures,

colourless, and, when pure, having a not unpleasant, so-called "ethereal"

odour. Its density, or specific gravity, referred to air as unity, has

been found experimentally by Leduc to be 0.9056. It is customary to adopt

the value 0.91 for calculations into which the density of the gas enters

(_vide_ Chapter VII.). The density of a gas is important not only

for the determination of the size of mains needed to convey it at a given

rate of flow under a given pressure, as explained in Chapter VII., but

also because the volume of gas which will pass through small orifices in

a given time depends on its density. According to Graham's well-known law

of the effusion of gases, the velocity with which a gas effuses varies

directly as the square root of the difference of pressure on the two

sides of the opening, and inversely as the square root of the density of

the gas. Hence it follows that the volume of gas which escapes through a

porous pipe, an imperfect joint, or a burner orifice is, provided the

pressure in the gas-pipe is the same, a function of the square root of

the density of the gas. Hence this density has to be taken into

consideration in the construction of burners, i.e., a burner required to

pass a gas of high density must have a larger orifice than one for a gas

of low density, if the rate of flow of gas is to be the same under the

same pressure. This, however, is a question for the burner manufacturers,

who already make special burners for gases of different densities, and it

need not trouble the consumer of acetylene, who should always use burners

devised for the consumption of that gas. But the Law of effusion

indicates that the volume of acetylene which can escape from a leaky

supply-pipe will be less than the volume of a gas of lower density,

_e.g._, coal-gas, if the pressure in the pipe is the same for both.

This implies that on an extensive distributing system, in which for

practical reasons leakage is not wholly avoidable, the loss of gas

through leakage will be less for acetylene than for coal-gas, given the

same distributing pressure. If _v_ = the loss of acetylene from a

distributing system and _v'_ = the loss of coal-gas from a similar

system worked at the same pressure, both losses being expressed in

volumes (cubic feet) per hour, and the coal-gas being assumed to have a

density of 0.04, then

(1) (_v_/_v'_) = (0.40 / 0.91)^(1/2) = 0.663

or, _v_ = 0.663_v'_,

which signifies that the loss of acetylene by leakage under the same

conditions of pressure, &c., will be only 0.663 times that of the loss of

coal-gas. In practice, however, the pressures at which the gases are

usually sent through mains are not identical, being greater in the case

of acetylene than in that of coal-gas. Formula (1) therefore requires

correction whenever the pressures are different, and calling the pressure

at which the acetylene exists in the main _p_, and the corresponding

pressure of the coal-gas _p'_, the relative losses by leakage are--

(2) (_v_/_v'_) = (0.40 / 0.91)^(1/2) x (_p_/_p'_)^(1/2)

_v_ = 0.663_v'_ x (_p_/_p'_)^(1/2)

It will be evident that whenever the value of the fraction

(_p_/_p'_)^(1/2), is less than 1.5, _i.e._, whenever the pressure of

the acetylene does not exceed double that of the coal-gas present in

pipes of given porosity or unsoundness, the loss of acetylene will be

less than that of coal-gas. This is important, especially in the case of

large village acetylene installations, where after a time it would be

impossible to avoid some imperfect joints, fractured pipes, &c.,

throughout the extensive distributing mains. The same loss of gas by

leakage would represent a far higher pecuniary value with acetylene than

with coal-gas, because the former must always be more costly per unit of

volume than the latter. Hence it is important to recognise that the rate

of leakage, _cteris paribus_, is less with acetylene, and it is

also important to observe the economical advantage, at least in terms of

gas or calcium carbide, of sending the acetylene into the mains at as low

a pressure as is compatible with the length of those mains and the

character of the consumers' burners. As follows from what will be said in

Chapter VII., a high initial pressure makes for economy in the prime cost

of, and in the expense of laying, the mains, by enabling the diameter of

those mains to be diminished; but the purchase and erection of the

distributing system are capital expenses, while a constant expenditure

upon carbide to meet loss by leakage falls upon revenue.

The critical temperature of acetylene, _i.e._, the temperature below

which an abrupt change from the gaseous to the liquid state takes place

if the pressure is sufficiently high, is 37° C., and the critical

pressure, _i.e._, the pressure under which that change takes place

at that temperature, is nearly 68 atmospheres. Below the critical

temperature, a lower pressure than this effects liquefaction of the gas,

_i.e._, at 13.5° C. a pressure of 32.77 atmospheres, at 0° C., 21.53

atmospheres (Ansdell, _cf._ Chapter XI.). These data are of

comparatively little practical importance, owing to the fact that, as

explained in Chapter XI., liquefied acetylene cannot be safely utilised.

The mean coefficient of expansion of gaseous acetylene between 0° C. and

100° C., is, under constant pressure, 0.003738; under constant volume,

0.003724. This means that, if the pressure is constant, 0.003738

represents the increase in volume of a given mass of gaseous acetylene

when its temperature is raised one degree (C.), divided by the volume of

the same mass at 0° C. The coefficients of expansion of air are: under

constant pressure, 0.003671; under constant volume, 0.003665; and those

of the simple gases (nitrogen, hydrogen, oxygen) are very nearly the

same. Strictly speaking the table given in Chapter XIV., for facilitating

the correction of the volume of gas measured over water, is not quite

correct for acetylene, owing to the difference in the coefficients of

expansion of acetylene and the simple gases for which the table was drawn

up, but practically no appreciable error can ensue from its use. It is,

however, for the correction of volumes of gases measured at different

temperatures to one (normal) temperature, and, broadly, for determining

the change of volume which a given mass of the gas will undergo with

change of temperature, that the coefficient of expansion of a gas becomes

an important factor industrially.

Ansdell has found the density of liquid acetylene to range from 0.460 at

-7° C. to 0.364 at +35.8° C., being 0.451 at 0° C. Taking the volume of

the liquid at -7° as unity, it becomes 1.264 at 35.8°, and thence Ansdell

infers that the mean coefficient of expansion per degree is 0.00489° for

the total range of pressure." Assuming that the liquid was under the same

pressure at the two temperatures, the coefficient of expansion per degree

Centigrade would be 0.00605, which agrees more nearly with the figure

0.007 which is quoted, by Fouché As mentioned before, data referring to

liquid (_i.e._, liquefied) acetylene are of no practical importance,

because the substance is too dangerous to use. They are, however,

interesting in so far as they indicate the differences in properties

between acetylene converted into the liquid state by great pressure, and

acetylene dissolved in acetone under less pressure; which differences

make the solution fit for employment. It may be observed that as the

solution of acetylene in acetone is a liquid, the acetylene must exist

therein as a liquid; it is, in fact, liquid acetylene in a state of

dilution, the diluent being an exothermic and comparatively stable body.

The specific heat of acetylene is given by M. A. Morel at 0.310, though

he has not stated by whom the value was determined. For the purpose of a

calculation in Chapter III. the specific heat at constant pressure was

assumed to be 0.25, which, in the absence of precise information, appears

somewhat more probable as an approximation to the truth. The ratio

(_k_ or C_p/C_v ) of the specific heat at constant pressure to that

at constant volume has been found by Maneuvrier and Fournier to be 1.26;

but they did not measure the specific heat itself. [Footnote: The ratio

1.26 _k_ or (C_p/C_v) has been given in many text-books as the value

of the specific heat of acetylene, whereas this value should obviously be

only about one-fourth or one-fifth of 1.26.

By employing the ordinary gas laws it is possible approximately to

calculate the specific heat of acetylene from Maneuvrier and Fournier's

ratio. Taking the molecular weight of acetylene as 26, we have

26 C_p - 26 C_v = 2 cal.,

and

C_p = 1.26 C_v.

From this it follows that C_p, _i.e._, the specific heat at constant

pressure of acetylene, should be 0.373.] It will be seen that this value

for _k_ differs considerably from the corresponding ratio in the

case of air and many common gases, where it is usually 1.41; the figure

approaches more closely that given for nitrous oxide. For the specific

heat of calcium carbide Carlson quotes the following figures:

0° 1000° 1500° 2000° 2500° 3000° 3500°

0.247 0.271 0.296 0.325 0.344 0.363 0.381

The molecular volume of acetylene is 0.8132 (oxygen = 1).

According to the international atomic weights adopted in 1908, the

molecular weight of acetylene is 26.016 if O = 16; in round numbers, as

ordinarily used, it is 26. Employing the latest data for the weight of 1

litre of dry hydrogen and of dry normal air containing 0.04 per cent. of

carbon dioxide at a temperature of 0° C. and a barometric pressure of 760

mm. in the latitude of London, viz., 0.089916 and 1.29395 grammes

respectively (Castell-Evans), it now becomes possible to give the weight

of a known volume of dry or moist acetylene as measured under stated

conditions with some degree of accuracy. Using 26.016 as the molecular

weight of the gas (O = 16), 1 litre of dry acetylene at 0° C. and 760 mm.

weighs 1.16963 grammes, or 1 gramme measures 0.854973 litre. From this it

follows that the theoretical specific gravity of the gas at 0°/0° C. is

0.9039 (air = 1), a figure which may be compared with Leduc's

experimental value of 0.9056. Taking as the coefficient of expansion at

constant pressure the figure already given, viz., 0.003738, the weights

and measures of dry and moist acetylene observed under British conditions

(60° F. and 30 inches of mercury) become approximately:

Dry. Saturated.

1 litre . . . 1.108 grm. . . 1.102 grm.

1 gramme . . . 0.902 litre. . . 0.907 litre.

1000 cubic feet . 69.18 lb. . . . 68.83 lb.

Acetylene, the Principles of Its Generation and Use A Practical Handbook on the Production, Purification, and Subsequent Treatment of Acetylene for the Development of Light, Heat, and Power · 第 17 / 22 章

(_f_) Piping must be thoroughly tested both before and after the