GLYCERINE MANUFACTURE AND PURIFICATION.
_Treatment of Lyes--Evaporation to Crude
Glycerine--Distillation--Distilled and Dynamite
Glycerine--Chemically Pure Glycerine--Animal Charcoal for
Decolorisation--Glycerine obtained by other Methods of
Saponification--Yield of Glycerine from Fats and Oils._
As pointed out in Chapter II. the fatty acids, which, combined with soda
or potash, form soap, occur in nature almost invariably in the form of
glycerides, _i.e._, compounds of fatty acids with glycerol, and as the
result of saponification of a fat or oil glycerine is set free.
In Chapter V. processes of soap-making are described in which (1) the
glycerine is retained in the finished soap, and (2) the glycerine is
contained in the lyes, in very dilute solution, contaminated with salt
and other impurities. These lyes, though now constituting the chief
source of profit in the manufacture of cheap soaps, were till early in
last century simply run down the drains as waste liquor.
Much attention has been devoted to the purification and concentration of
glycerine lyes; and elaborate plant of various forms has been devised
for the purpose.
_Treatment of Lyes._--The spent lyes withdrawn from the soap-pans are
cooled, and the soap, which has separated during the cooling, is
carefully removed and returned to the soap-house for utilisation in the
manufacture of brown soap. Spent lyes may vary in their content of
glycerol from 3 to 8 per cent., and this depends not only upon the
system adopted in the working of the soap-pans, but also upon the
materials used. Although, in these days of pure caustic soda, spent lyes
are more free from impurities than formerly, the presence of sulphides
and sulphites should be carefully avoided, if it is desired to produce
good glycerine.
The lyes are transferred to a lead-lined tank of convenient size, and
treated with commercial hydrochloric acid and aluminium sulphate,
sufficient being added of the former to neutralise the free alkali, and
render the liquor faintly acid, and of the latter to completely
precipitate the fatty acids. The acid should be run in slowly, and the
point when enough has been added, is indicated by blue litmus paper
being slightly reddened by the lyes.
The whole is then agitated with air, when a sample taken from the tank
and filtered should give a clear filtrate.
Having obtained this clear solution, agitation is stopped, and the
contents of the tank passed through a filter press. The scum, which
accumulates on the treatment tank, may be transferred to a perforated
box suspended over the tank, and the liquor allowed to drain from it.
The filtered liquor is now rendered slightly alkaline by the addition of
caustic soda or carbonate, and, after filtering, is ready for
evaporation.
The acid and alum salt used in the above treatment must be carefully
examined for the presence of arsenic, and any deliveries of either
article, which contain that impurity, rejected.
Lime, bog ore, and various metallic salts, such as ferric chloride,
barium chloride, and copper sulphate have been suggested, and in some
instances are used instead of aluminium sulphate, but the latter is
generally employed.
_Evaporation to Crude Glycerine._--The clear treated lyes, being now
free from fatty, resinous, and albuminous matter, and consisting
practically of an aqueous solution of common salt (sodium chloride) and
glycerine, is converted into crude glycerine by concentration, which
eliminates the water and causes most of the salt to be deposited.
This concentration was originally performed in open pans heated by fire
or waste combustible gases. In the bottom of each pan was placed a dish
in which the salt deposited, and this dish was lifted out periodically
by the aid of an overhead crane and the contents emptied and washed.
Concentration was continued until the temperature of the liquor was 300°
F. (149° C.), when it was allowed to rest before storing.
This liquor on analysis gave 80 per cent. glycerol and from 9 to 10-1/2
per cent. salts (ash); hence the present standard for crude glycerine.
Concentration in open pans has now been superseded by evaporation _in
vacuo_. The subject of the gradual development of the modern efficient
evaporating plant from the vacuum pan, originated and successfully
applied by Howard in 1813 in the sugar industry, is too lengthy to
detail here, suffice it to say that the multiple effects now in vogue
possess distinct advantages--the greatest of these being increased
efficiency combined with economy.
The present type of evaporator consists of one or more vessels, each
fitted with a steam chamber through which are fixed vertical hollow
tubes. The steam chamber of the first vessel is heated with direct
steam, or with exhaust steam (supplied from the exhaust steam receiver
into which passes the waste steam of the factory); the treated lyes
circulating through the heated tubes is made to boil at a lower
temperature, with the reduced pressure, than is possible by heating in
open pans.
The vapour given off by the boiling liquor is conveyed through large
pipes into the steam chamber of the second vessel, where its latent heat
is utilised in producing evaporation, the pressure being further
reduced, as this second vessel is under a greater vacuum than No. 1.
Thus we get a "double effect," as the plant consisting of two pans is
termed. The vapours discharged from the second vessel during boiling are
passed through pipes to the steam chamber of the third vessel (in a
"triple effect"), and there being condensed, create a partial vacuum in
the second vessel. The third vessel may also be heated by means of live
steam. The vapours arising from the last vessel of the evaporating
plant, or in the case of a "single effect" from the vessel, are conveyed
into a condenser and condensed by injection water, which is drawn off by
means of the pump employed for maintaining a vacuum of 28 inches in the
vessel.
In the most recent designs of large evaporative installations, the
vapours generated from the last vessel are drawn through a device
consisting of a number of tubes enclosed in a casing, and the latent
heat raises the temperature of the treated lyes proceeding through the
tubes to supply the evaporator.
It will thus be observed that the object of multiple effects is to
utilise all the available heat in performing the greatest possible
amount of work. Special devices are attached to the plant for
automatically removing the condensed water from the steam chambers
without the loss of useful heat, and as a precaution against splashing
over and subsequent loss of glycerine through conveyance to the steam
chamber, dash plates and "catch-alls" or "save-alls" of various designs
are fitted on each vessel.
In working the plant, the liquor in each vessel is kept at a fairly
constant level by judicious feeding from one to the other; the first
vessel is, of course, charged with treated lyes. As the liquor acquires
a density of 42° Tw. (25° B.) salt begins to deposit, and may be
withdrawn into one of the many patented appliances, in which it is freed
from glycerine, washed and dried ready for use at the soap pans.
Difficulty is sometimes experienced with the tubes becoming choked with
salt, thereby diminishing and retarding evaporation. It may be necessary
to dissolve the encrusted salt with lyes or water, but with careful
working the difficulty can be obviated by washing out with weak lyes
after each batch of crude glycerine has been run away, or by increasing
the circulation.
It is claimed that by the use of the revolving heater designed by
Lewkowitsch, the salting up of tubes is prevented.
The salt having been precipitated and removed, evaporation is continued
until a sample taken from the last vessel has a density of 60° Tw. (33.3
B.) at 60° F. (15.5° C.). When this point is reached, the crude
glycerine is ready to be withdrawn into a tank, and, after allowing the
excess of salt to deposit, may be transferred to the storage tank.
The colour of crude glycerine varies from light brown to dark brown,
almost black, and depends largely on the materials used for soap-making.
The organic matter present in good crude glycerine is small in amount,
often less than 1 per cent.; arsenic, sulphides and sulphites should be
absent. Crude glycerine is refined in some cases by the producers
themselves; others sell it to firms engaged more particularly in the
refined glycerine trade.
_Distillation._--Crude glycerine is distilled under vacuum with the aid
of superheated steam. The still is heated directly with a coal or coke
fire, and in this fire space is the superheater, which consists of a
coil of pipes through which high pressure steam from the boiler is
superheated.
The distillation is conducted at a temperature of 356°F. (180° C.). To
prevent the deposition and burning of salt on the still-bottom during
the distillation, a false bottom is supported about 1 foot from the base
of the still. With the same object in view, it has been suggested to
rotate the contents with an agitator fixed in the still.
Every care is taken that the still does not become overheated; this
precaution not only prevents loss of glycerine through carbonisation,
but also obviates the production of tarry and other bodies which might
affect the colour, taste, and odour of the distilled glycerine. The
vacuum to be used will, of course, depend upon the heat of the fire and
still, but as a general rule good results are obtained with an 18 inch
vacuum.
There are quite a large number of designs for still heads, and
"catch-alls," having for their object the prevention of loss of
glycerine.
The distillate passes into a row of condensers, to each of which is
attached a receptacle or receiver. It is needless to state that the
condensing capacity should be in excess of theoretical requirements. The
fractions are of varying strengths and quality; that portion, with a
density less than 14° Tw. (19.4° B.), is returned to the treated-lyes
tank. The other portion of the distillate is concentrated by means of a
dry steam coil in a suitable vessel under a 28 inch vacuum.
When sufficiently concentrated the glycerine may be decolorised, if
necessary, by treating with 1 per cent. animal charcoal and passing
through a filter press, from which it issues as "dynamite glycerine".
The residue in the still, consisting of 50-60 per cent. glycerine and
varying proportions of various sodium salts--_e.g._ acetate, chloride,
sulphate, and combinations with non-volatile organic acids--is generally
boiled with water and treated with acid.
The tar, which is separated, floats on the surface as the liquor is
cooling, and may be removed by ladles, or the whole mixed with waste
charcoal, and filtered.
The filtrate is then evaporated, when the volatile organic acids are
driven off; the concentrated liquor is finally mixed with crude
glycerine which is ready for distillation, or it may be distilled
separately.
_Distilled Glycerine._--This class of commercial glycerine, although of
limited use in various other branches of industry, finds its chief
outlet in the manufacture of explosives.
Specifications are usually given in contracts drawn up between buyers
and sellers, to which the product must conform.
The chief stipulation for dynamite glycerine is its behaviour in the
nitration test. When glycerine is gradually added to a cold mixture of
strong nitric and sulphuric acids, it is converted into nitro-glycerine,
which separates as an oily layer on the surface of the acid. The more
definite and rapid the separation, the more suitable is the glycerine
for dynamite-making.
Dynamite glycerine should be free from arsenic, lime, chlorides, and
fatty acids, the inorganic matter should not amount to more than 0.1 per
cent., and a portion diluted and treated with nitrate of silver solution
should give no turbidity or discoloration in ten minutes. The specific
gravity should be 1.262 at 15° C. (59° F.) and the colour somewhat
yellow.
_Chemically pure glycerine_ or double distilled glycerine is produced by
redistilling "once distilled" glycerine. Every care is taken to avoid
all fractions which do not withstand the nitrate of silver test. The
distillation is very carefully performed under strict supervision.
The distillate is concentrated and after treatment with animal charcoal
and filtration should conform to the requirements of the British
Pharmacopoeia. These are specified as follows: Specific gravity at
15.5° C., 1.260. It should yield no characteristic reaction with the
tests for lead, copper, arsenium, iron, calcium, potassium, sodium,
ammonium, chlorides, or sulphates. It should contain no sugars and leave
no residue on burning.
_Animal Charcoal for Decolorisation._--The application of animal
charcoal for decolorising purposes dates back a century, and various are
the views that have been propounded to explain its action. Some
observers base it upon the physical condition of the so-called carbon
present, and no doubt this is an important factor, coupled with the
porosity. Others consider that the nitrogen, which is present in all
animal charcoal and extremely difficult to remove, is essential to the
action. Animal charcoal should be freed from gypsum (sulphate of lime),
lest in the burning, sulphur compounds be formed which would pass into
the glycerine and contaminate it.
The "char" should be well boiled with water, then carbonate of soda or
caustic soda added in sufficient quantity to give an alkaline reaction,
and again well boiled. The liquor is withdrawn and the charcoal washed
until the washings are no longer alkaline. The charcoal is then
separated from the liquor and treated with hydrochloric acid; opinions
differ as to the amount of acid to be used. Some contend that phosphate
of lime plays such an important part in decolorising that it should not
be removed, but it has, however, been demonstrated that this substance
after exposure to heat has very little decolorising power.
Animal charcoal boiled with four times its weight of a mixture
consisting of equal parts of commercial hydrochloric acid (free from
arsenic) and water for twelve hours, then washed free from acid, dried,
and burned in closed vessels gives a product possessed of great
decolorising power for use with glycerines.
A good animal charcoal will have a dull appearance, and be of a deep
colour; it should be used in fine grains and not in the form of a
powder.
The charcoal from the filter presses is washed free from glycerine
(which is returned to the treated lyes), cleansed from foreign
substances by the above treatment and revivified by carefully heating in
closed vessels for twelve hours.
_Glycerine obtained by other Methods of Saponification._--French
saponification or "candle crude" glycerine is the result of
concentration of "sweet water" produced in the manufacture of stearine
and by the autoclave process. It contains 85-90 per cent. glycerol,
possesses a specific gravity of 1.240-1.242, and may be readily
distinguished from the soap-crude glycerine by the absence of salt
(sodium chloride). This glycerine is easily refined by treatment with
charcoal.
The glycerine water resulting from acid saponification methods requires
to be rendered alkaline by the addition of lime--the sludge is
separated, and the liquor evaporated to crude. The concentration may be
performed in two stages--first to a density of 32° Tw. (20° B.), when
the calcium sulphate is allowed to deposit, and the separated liquor
concentrated to 48° Tw. (28° B.) glycerine, testing 85 per cent.
glycerol and upwards.
_Yield of Glycerine from Fats and Oils._--The following represent
practicable results which should be obtained from the various
materials:--
Tallow 9 per cent. of 80 per cent. Glycerol.
Cotton-seed oil 10 "
Cocoa-nut oil 12 "
Palm-kernel oil 18 "
Olive oil 10 "
Palm oil 6 "
Greases (Bone fats) 6-8 "
The materials vary in glycerol content with the methods of preparation;
especially is this the case with tallows and greases.
Every care should be taken that the raw materials are fresh and they
should be carefully examined to ascertain if any decomposition has taken
place in the glycerides--this would be denoted by the presence of an
excess of free acidity, and the amount of glycerol obtainable from such
a fat would be correspondingly reduced.