The evaporation of the weak gelatine sols (3-9 per cent.) obtained by
the processes described in previous sections into sols of such
concentration (20-55 per cent.) that they readily set to a stiff gel on
cooling, is now an essential feature of gelatine manufacture, and is one
of the most important processes.
In the early days of this industry, manufacturers aimed at obtaining a
concentrated sol, as this saved time in drying, and so reduced the
possibilities of putrefaction. The advent of evaporation has reduced
these possibilities to a minimum, and has also enormously reduced the
space required and the capital outlay needed in the drying sheds. It
has, in addition, given the practical advantages involved in dealing up
to the last minute with a much less viscous liquor. As the liquors
extracted are weaker, the extraction is more complete and the
decolorization more easily effected.
The earliest attempts at evaporation were not very successful, partly on
account of the prolonged "stewing" which ruined the setting power, and
partly because of the poor economy of heat. Thus in the open evaporators
the sol was maintained at a high temperature for a long period, and this
process only proved suitable for low-grade products.
A great stride forward was made by Howard's invention of the Vacuum Pan.
This made it possible to undertake concentration at much lower
temperatures, a most important improvement in the case of gelatine and
other organic matters easily damaged by heat. The process, however, was
still slow, and the sol exposed to heat for a long time, as must be the
case when evaporation takes place in bulk. These disadvantages were
still fatal to the production of the highest-grade gelatine. There were
also the practical difficulties of entrainment ("blowing over"), in
which parts of the sol were carried away by the escaping vapour, and
also of "incrustation" which so rapidly reduces the heating efficiency
and evaporative capacity of the machine. The vacuum pan, however,
presented two decided advantages--evaporation at a low temperature, and,
as a corollary, the possibility of utilizing exhaust steam to attain
this temperature.
Whilst the vacuum pan was a satisfactory machine for many branches of
chemical engineering, the problem of evaporation was still unsolved for
gelatine liquor because of the "stewing" involved, until the advent of
the "film evaporator," which dealt with the liquor not in bulk, but in a
continuous stream. In this way the product was only exposed to heat for
a comparatively short time. Many evaporators of this type came into
being, and rapid improvement was made in the constructional details. The
film evaporators retained usually the advantage of evaporation _in
vacuo_, so that it was now possible to evaporate gelatine sols by
exposure for a short time to a comparatively low temperature. Of this
type of evaporator, the Lillie, Yaryan, Schwager, Claassen, Greiner,
Blair Campbell, and the Kestner machines are well-known examples.
A further advance in solving this problem was the application of the
principle of multiple-effect evaporation. The vapour driven off during
evaporation possesses of course many heat units, and is of very
considerable volume. In multiple-effect evaporators this vapour is used
to work a similar evaporator, and the evaporated liquor passes
immediately into what is practically a second machine, and is further
evaporated by the heat from the vapour just driven from it. Such an
arrangement would be termed a double-effect evaporator. The vapour from
the second effect may of course be similarly used to operate a third
effect, and the vapour from this to work a fourth effect, and so on.
Thus, we may have triple effect, quadruple effect, etc., even up to
octuple effect. The great advantage of multiple-effect evaporation is in
the saving of costly steam. Reavell gives the following figures to
illustrate the economy thus obtained:--
WATER EVAPORATED PER 100 UNITS STEAM.
-----------+-----------+-----------+--------------
Single. | Double. | Triple. | Quadruple.
-----------+-----------+-----------+--------------
95 | 150 | 220 | 300
-----------+-----------+-----------+--------------
There is naturally a limit beyond which the capital cost of the machine
neutralizes the advantage of steam economy, and it is seldom that
octuple effects are used. There are probably more triple effects in use
than any other machine.
An essential and important part of the modern evaporator is the
"condenser," in which the vapour from the last effect is conducted into
water (jet condensers) or over cooled surfaces (surface condensers),
with a view to producing and maintaining the vacuum.
A lasting vacuum cannot be maintained without an air-pump, as air is
often introduced (1) with the steam, having entered the boiler dissolved
in the feed water; (2) by leakage from the atmosphere into the condenser
and the connected vacuous spaces; and (3) in jet condensers, in solution
with the circulating condenser water. That from the first two sources
may be reduced, but the third is beyond control: hence if high vacua are
necessary, surface condensers are to be preferred. Dissolved air is
usually 5-20 per cent. of the water volume, and is least for sea-water.
It should be noted that water leaving a surface condenser is in a very
air-free state, and therefore particularly suitable for boiler supply.
Apart from the capital cost of a condenser the chief cost of maintaining
a vacuum is in pumping the circulating water, of which up to 70 lbs. is
usual per lb. of steam condensed.
If W = weight of steam condensed (lbs. per hour);
Q = weight of cooling water circulated (lbs. per hour)
T{i} = inlet temperature (° F.) of cooling water;
T{o} = outlet temperature (° F.) of cooling water;
then
T{o} = T{i} + 1050(W/Q)
It will be understood that for high vacua, low temperature of cooling
water (T{i}) is more important than copious supply (Q/W). It is
advantageous, however, to choose a site yielding plenty of cold water,
such as a river or canal side. Otherwise it is often necessary to use
cooling towers or spray nozzles. The cooling is by evaporation (= 60 to
80 per cent. of W), cold water replacing that evaporated, and yielding
water 75° to 80° F. If T{i} = 80° F. and Q/W = 70°, a vacuum of
28.34" is possible, but the 0.34" should be allowed for the partial
pressure of the air, determined exactly by the air entering and by the
displacement of the air-pump.
Another feature of the modern evaporator is the "heater" or
"calorifier," by which the liquor to be evaporated is led in a
continuous rapid stream through heated tubes immediately prior to its
entry into the first effect. It is the aim of the heater to raise the
temperature of the liquor to the temperature of evaporation, and so to
avoid this being necessary in the first effect. The heater thus further
avoids stewing, ensures steady running, and effectively increases the
capacity of a machine.
It is noteworthy that superheated steam is not desirable for working an
evaporator. The principle of evaporation by steam is not merely that the
temperature of the liquor is raised to boiling point; it is that in the
condensation of the heating steam its latent heat is yielded to the
liquor being evaporated. To evaporate quickly, therefore, the heating
steam must condense rapidly. Hence, as superheated steam has a rate of
condensation 20-30 times slower than saturated steam, the latter is much
to be preferred. A slight superheating, however, may be justifiable
where the steam has any distance to travel before use. It is the fact
that it is the latent heat of steam which is mainly utilized which gives
steam its great practical advantage over hot non-condensable gases.
Steam in condensing yields an enormously greater number of heat units
per lb. than hot waste gases. Steam has also the advantage of more
constant temperature.
The capacity and efficiency of an evaporator depends upon a good many
factors, some of which are worthy of discussion at this point.
The transference of heat and the amount of evaporation are directly
proportional to the mean temperature difference between the heating
steam and the liquor being evaporated. These temperatures, however, both
vary somewhat, the steam losing part of its pressure and temperature as
it passes along the heating surface; the liquid generally increases in
temperature. The mean difference in temperature, moreover, is not the
arithmetic mean between the smallest and largest temperature
differences, but is given by the following expressions, which yield
results not wide apart:--
If [theta]{a} = temperature difference at commencement;
[theta]{e} = " " " end;
and [theta]{m} = mean temperature difference;
then
[theta]{m} = ([theta]{a} - [theta]{e}) /
log([theta]{a} / [theta]{e})
or = ([theta]{a} - [theta]{e}) /
[ n(1 - [nth root of]([theta]{e} / [theta]{a})) ]
This mean temperature difference is in practice usually spoken of as the
"temperature head" or "heat drop." It will be clear that this
temperature head is increased by using steam at higher pressure
(temperature), and by evaporating under reduced pressure. Since most
liquids have their boiling points reduced about 40° C. by operating _in
vacuo_, the advantage of the vacuum is apparent. It should be remembered
that the temperature head has not the same value in any part of the
scale: it has more value higher up the scale, because the steam is
denser and more heat units come in contact with a given area in a given
time. It must also be remembered that whilst the pressure gauge is a
most useful indicator of steam temperature, it is not necessarily
accurate. The pressure in the hot space is the _sum_ of the pressures of
air and steam, and since the temperature (the important condition) of
the hot space depends upon the pressure of the _steam_, and not on the
sum of the pressures, the temperature in a steam space is always rather
lower than would be supposed from the pressure indicated by the gauge.
The transference of heat is influenced by the velocity of both the
heating fluid and the fluid being heated over the heating surface. The
more rapidly each fluid moves, the more rapid is the transference of
heat, because a greater number of particles of both fluids are brought
to the heating surface in any given time. This is popularly known as the
effect of "circulation," and is illustrated by the advantage of stirring
a liquid being heated in bulk. In the film evaporators the circulation
is through tubes at high speed (up to 2 miles a minute), and the maximum
effect in this sense is thus obtained. The increase in heat transference
is not directly proportional to the increase in velocity, but in a lower
ratio, sometimes approximately the square root of the velocity. In such
a case, if either velocity be quadrupled, the heat transference is
doubled. Other advantages of high velocity are that the heating steam
more readily sweeps away condensed steam from the heating surface, and
the high-speed film similarly "scours" away "incrustations" on the
interior of the tubes.
The transference of heat is also proportional to the conductivity of the
metal forming the heating surface. For gelatine liquors, copper tubes
are almost invariably employed, the advantage being great even when
price is taken into consideration. The following conductivity
coefficients illustrate this point (calories per hour through 1 sq.
metre of metal 1 metre thick, with a temperature difference of 1° C.):--
Copper...330
Iron.....56
Steel....22-40
Tin......54
Zinc.....105
Lead.....28
The coefficient of heat transmission decreases the more with increasing
thickness of wall, the worse conductor is the metal. For copper tubes,
however, this decrease is usually unimportant.
The transference of heat is also much influenced by the viscosity of
the liquor being evaporated; the greater the viscosity, the lower the
coefficient of heat transmission. Unfortunately for this process of
evaporation, gelatine sols are exceedingly viscous, and thus the
difficulty in obtaining a concentrated sol is thus greatly enhanced.
The transference of heat is often greatly hindered by incrustations of
the tubes, which incrustations generally conduct heat very badly. Thus
the relative heat conductivities of copper and chalk are as 1000:5.
The amount of heat transferred is of course determined also by the area
of the heating surface. The amount of evaporation needed thus determines
the number of tubes (of standard size) in the evaporator, and thus the
capacity of the machine. An evaporator should have its heating surface
area chosen with a view to the duty required of it.
In practice the working of an evaporator is often not a very difficult
matter, and large numbers of machines are operated by unskilled labour.
Troubles generally arise from inconstant steam pressure, incrustation,
leakages of air, which reduce the vacuum, the temperature head, and
hinder heat transmission. For the evaporation of gelatine liquors the
Yaryan, the Kestner, and the Blair-Campbell film evaporators are the
most widely used. The velocity of the liquor through some of these
machines is so great that occasionally no vacuum is used. The
temperature obtained is high (200° F.), but the time is very short, if
rapid cooling of the evaporated liquor is arranged.
REFERENCES.
"Evaporating, Condensing and Cooling Apparatus," by E. Hausbrand.
Scott, Greenwood & Son (1916 Ed.).
"Evaporation," by E. Kappeschaar. Norman Rodger (1914).
"Evaporation in the Chemical Industry," by J.A. Reavell,
M.I.Mech.E., _J.S.C.I._, 1918, April 11th.
"Glue and Glue Testing," S. Rideal, D.Sc., pp. 56-59.
"Gelatine, Glue, and their Allied Products," T. Lambert, pp. 26-29.
"Notes on Condensing Plant," J.M. Newton, B.Sc., _J. Junior Inst._
Engineers, Aug., 1912.