Glycerine Recovery.
The recovery of glycerine is very closely allied with the soap-making
industry, because glycerine is the very valuable by-product obtained in
the saponification of oils and fats. No soap plant is, therefore, fully
equipped unless it has some method whereby the glycerine is recovered
and the importance of recovering this product cannot be too strongly
emphasized.
It has already been pointed out that neutral fats or the glycerides are
a combination of fatty acid with glycerine. These are split apart in the
process of saponification. While by the term _saponification_ as used in
soap making it is inferred that this is the combination of caustic
alkalis with the fatty acids to form soap, this term is by no means
limited to this method of saponification, as there are various other
methods of saponifying a fat. The chemical definition of saponification
is the conversion of an ester, of which glycerides are merely a certain
type, into an alcohol and an acid or a salt of this acid. Thus, if we
use caustic alkali as our saponifying agent for a fat or oil, we obtain
the sodium or potassium salt of the higher fatty acids or soap and the
alcohol, glycerine. On the other hand, if we use a mineral acid as the
saponifying agent, we obtain the fatty acids themselves in addition to
glycerine. While the former is by far the most generally employed for
making soap, other processes consist in saponifying the fats by some
method other than caustic alkalis and then converting the fatty acids
into soap by either neutralizing them with sodium or potassium carbonate
or hydrate.
It is important to again point out here that fats and oils develop free
fatty acid of themselves and that the development of this acid
represents a loss in glycerine. The selection of an oil or fat for soap
making should therefore to a large extent be judged as to its
adaptability by the free fatty acid content, as the higher this content
is, the greater is the loss in the glycerine eventually obtained.
Glycerine often represents the only profit to a soap manufacturer. It is
indeed necessary to determine the percentage of free fatty acid before
purchasing a lot of stock to be made into soap.
In taking up the question of glycerine recovery we will consider the
various methods thus:
1. Where the glycerine is obtained from spent lye by saponifying the
fats or oils with caustic alkali.
2. Where the glycerine is obtained by saponifying the fats or oils by
some other method than the above, of which there are the following:
(a) Twitchell process.
(b) Saponification by lime in autoclave.
(c) Saponification by acid.
(d) Saponification by water in autoclave.
(e) Fermentative (Enzymes).
(f) Krebitz process.
RECOVERY OF GLYCERINE FROM SPENT LYE.
The spent lye obtained from the glycerine changes in making soap varies
greatly, the quality depending upon the stock saponified and the soap
maker's care in handling the operation. No two lyes run exactly alike as
to proportion of the various ingredients, although they are all similar
in containing the same substances either in solution or suspension.
Spent lye is a water solution of mainly glycerine, free alkali either as
caustic alkali or carbonate and salt, including sodium sulfate, but
furthermore contains some soap and albuminous matter either in solution
or suspension. Upon standing in the storage tank the greater part of
the soap usually separates when the lye cools. In order to assure the
greatest economical yield of glycerine by saponifying a fat with caustic
soda it is necessary to obtain a proportion of three parts of water to
every part of fat made into soap. Test runs have shown that this is the
proper proportion and that it is not economical to greatly exceed this
amount, and if a much less proportion is used the full yield of
glycerine is not obtained.
The spent lyes contain varying amounts of glycerine, the first change
being richest in glycerine content, and this being reduced in the
subsequent changes. If the lyes always run high in glycerine it is an
indication that it is not all being obtained. The usual percentage is
from 0.5% to 5% or even more, although the average is somewhere around
2% to 3%. The lye as it comes from the kettle should not contain any
more than 0.5% to 0.6% of free alkali calculated as sodium carbonate,
Na_{2}CO_{3}. If the proportion is higher than this, it shows that the
saponification has been conducted with too high a proportion of alkali,
a condition which should be corrected in the kettle room. An excess of
free alkali does not interfere to any great extent with the successful
recovery of the glycerine, but is a waste of both alkali and the acid
used in neutralizing this. It is, therefore, more economical to run a
strong lye over fresh stock and neutralize the alkali thus, rather than
treating the lye for glycerine recovery.
Before the spent lye can be run into the evaporator it is necessary to
remove the albuminous impurities and soap and to neutralize the excess
alkali to between exactly neutral and 0.02% alkalinity. The lye should
never be fed into the evaporator in the acid condition.
In order to treat the spent lyes for evaporation, they are first allowed
to cool in the storage tank, after which any soap which may have
separated is skimmed off and returned to the soap kettle. This lye is
then pumped to the treatment tank, an ordinary tank equipped with some
method of agitating the liquor, either by a mechanical stirrer, steam
blower or compressed air, until it is about two feet from the top.
After the lye has been skimmed off it is thoroughly agitated and a
sample taken. The amount of lye in the tank is then calculated. Spent
lye is about 1.09 times heavier than water, or weighs about 9 pounds to
the gallon. While the sample is being tested for alkalinity it is
advisable to add sulfate of alumina, which may be dissolving while the
sample is being titrated. This substance should be added in the
proportion of anywhere from 6 to 14 pounds per thousand pounds of lye,
depending upon the amount of impurities contained therein. For a clean
lye six pounds per thousand is sufficient, but for an impure lye a
greater quantity is necessary. The sulfate of alumina used should be
free from arsenic and sulfides and should contain a minimum amount of
grit (silica), as grit reduces the life of the pump valves. This may be
estimated with sufficient accuracy by rubbing the filtered-off portions,
insoluble in water between the fingers and a plate of glass. The object
of adding the sulfate of alumina is to transform the soap contained in
the lye into the insoluble aluminum soaps, and at the same time to
coagulate the albuminous impurities. It must be remembered that the
sulfate of alumina is added only for the fresh lye put into the tank.
Thus if there were 10,000 pounds of lye in the treating tank when the
fresh lye was run in, and 50,000 pounds when the tank is filled, adding
nine pounds of sulfate of alumina per thousand of lye, only 360 pounds
would be added or enough for 40,000 pounds. Sulfate of alumina
neutralizes one-third of its weight of caustic.
To determine the alkali in the sample, 10 cubic centimeters are pipetted
into a beaker, a little distilled water added, then 3 or 4 drops of
phenolphthalein indicator. From a burette, quarter normal (N/4) sulfuric
acid is added until the pink color is just discharged. When this point
is reached 4 to 5 c. c. more of acid are added and the solution is
boiled to expel the carbon dioxide. Should the solution turn pink, it is
necessary to add more acid. After having boiled for 3 to 4 minutes, N/4
caustic soda is added until the pink color just returns and the amount
of caustic soda used is read on the burette. The difference between the
number of cubic centimeters of N/4 sulfuric acid and N/4 caustic soda
gives the amount of alkali in the sample. By using a 10 c. c. sample and
N/4 sulfuric acid and N/4 caustic soda each c. c. obtained by the
difference of these two solutions is equal to one-tenth of one per cent.
(0.1%) of the total alkali in the lye. As an example, say we first used
7.7 c. c. of N/4 sulfuric acid to just discharge the pink, then added 4
c. c. more, or 11.7 c. c. in total. After boiling it required 5.3 c. c.
to bring back a slight pink, the total alkalinity would be 11.7 c. c. -
5.3 c. c. = 6.4 c. c., or 0.64% total alkali in the lye in terms of
caustic soda. If there were 40,000 pounds of lye to be treated then we
should have to neutralize:
40,000 x .0064 = 256 lbs. alkali. Since sulfate of alumina neutralizes
one-third of its weight in caustic, and there are say 9 lbs. of this
added per thousand pounds of lye we would add
40,000 x 9 = 360 lbs. of sulfate of alumina. This would neutralize 360 x
1/3 = 120 lbs of alkali. There are then 256 - 120 = 136 lbs. of alkali
still to be neutralized. If 60deg. B. sulfuric acid is used it requires
about 1.54 lbs. of acid to one pound of caustic. Therefore to neutralize
the caustic soda remaining it requires:
136 x 1.54 = 209.44 lbs. 60deg. B. sulfuric acid to neutralize the total
alkali in the 40,000 pounds of spent lye.
The acid is added and the lye well stirred, after which another sample
is taken and again titrated as before. From this titration the amount of
acid to be added is again calculated and more acid is added if
necessary. Should too much acid have been added, caustic soda solution
is added until the lye is between exactly neutral and 0.02% alkaline.
The filtered lyes at this stage have a slight yellowish cast.
To be sure that the lyes are treated correctly the precipitation test is
advisable. To carry this out filter about 50 c. c. of the treated lye
and divide into two portions in a test tube. To one portion add ammonia
drop by drop. If a cloudiness develops upon shaking, more alkali is
added to the lye in the tank. To the other portion add a few drops of 1
to 5 sulfuric acid and shake the test tube. If a precipitate develops or
the solution clouds, more acid is needed. When the lyes are treated
right no cloudiness should develop either upon adding ammonia or the
dilute acid.
The properly treated lye is then run through the filter press while
slightly warm and the filtered lye is fed to the evaporator from the
filtered lye tank. The lye coming from the filter press should be clear
and have a slight yellowish cast. As the pressure increases it is
necessary to clean the press or some of the press cake will pass through
the cloths. Where sodium silicate is used as a filler, the silicate
scrap should never be returned to the soap kettle until the glycerine
lyes have been withdrawn. This practice of some soapmakers is to be
strongly censured, as it causes decided difficulty in filtering the lye,
since during the treatment of the lye, free silicic acid in colloidal
form is produced by the decomposition of the sodium silicate by acid.
This often prevents filtering the treated lye even at excess pressure
and at its best retards the filtering.
As to the filter press cake, this may be best thrown away in a small
factory. Where, however, the output of glycerine is very large it pays
to recover both the fatty acids and alumina in the press cakes.
In some cases, especially when the lyes are very dirty and the total
residue in the crude glycerine runs high, for which there is a penalty
usually attached, a double filtration of the lye is advisable. This is
carried out by first making the lye slightly acid in reaction by the
addition of alum and acid, then filtering. This filtered lye is then
neutralized to the proper point with caustic, as already described, and
passed through the filter press again.
While in the method of treating the lyes as given sulfuric acid is used
for neutralizing, some operators prefer to use hydrochloric acid, as
this forms sodium chloride or common salt, whereas sulfuric acid forms
sodium sulfate, having 3/5 the graining power of salt, which eventually
renders the salt useless for graining the soap, as the percentage of
sodium sulfate increases in the salt. When the salt contains 25 per
cent. sodium sulfate it is advisable to throw it away. Sulfuric acid,
however, is considerably cheaper than hydrochloric and this more than
compensates the necessity of having to eventually reject the recovered
salt. It may here also be mentioned that recovered salt contains 5-7 per
cent. glycerine which should be washed out in the evaporator before it
is thrown away. The following tables give the approximate theoretical
amounts of acids of various strengths required to neutralize one pound
of caustic soda:
For 1 pound of caustic soda--
3.25 lbs. 18deg. B. hydrochloric (muriatic) acid are required.
2.92 " 20deg. B. " " " " "
2.58 " 22deg. B. " " " " "
For 1 pound of caustic soda--
1.93 lbs. 50deg. B. sulphuric acid are required.
1.54 " 60deg. B. " " " "
1.28 " 66deg. B. " " " "
It is, of course, feasible to neutralize the spent lye without first
determining the causticity by titrating a sample and this is often the
case. The operator under such conditions first adds the sulfate of
alumina, then the acid, using litmus paper as his indicator.
Comparatively, this method of treatment is much slower and not as
positive, as the amount of acid or alkali to be added is at all times
uncertain, for in the foaming of the lyes their action on litmus is
misleading.
After the lye has been filtered to the filtered lye tank it is fed to
the evaporator, the method of operation of which varies somewhat with
different styles or makes. When it first enters the evaporator the lye
is about 11deg.-12deg. B. After boiling the density will gradually rise
to 27deg. B. and remain at this gravity for some time and during which
time most of the salt is dropped out in the salt filter. As the lye
concentrates the gravity gradually rises to 28deg.-30deg. B., which is
half crude glycerine and contains about 60 per cent. glycerine. Some
operators carry the evaporation to this point and accumulate a quantity
of half crude before going on to crude. After half crude is obtained the
temperature on the evaporator increases, the vacuum increases and the
pressure on the condensation drain goes up (using the same amount of
live steam). As the liquor grows heavier the amount of evaporation is
less, and less steam is required necessitating the regulation of the
steam pressure on the drum. When a temperature of 210deg. F. on the
evaporator, with 26 or more inches vacuum on the pump is arrived at, the
crude stage has been reached and the liquor now contains about 80 per
cent. glycerine in which shape it is usually sold by soap manufacturers.
A greater concentration requires more intricate apparatus. After
settling a day in the crude tank it is drummed.
Crude glycerine (about 80 per cent. glycerol) free from salt is 33deg.
B., or has a specific gravity of 1.3. A sample boiled in an open dish
boils at a temperature of 155deg. C. or over.
TWITCHELL PROCESS.
The Twitchell process of saponification consists of causing an almost
complete cleavage of fats and oils by the use of the Twitchell reagent
or saponifier, a sulfo-aromatic compound. This is made by the action of
concentrated sulfuric acid upon a solution of oleic acid or stearic acid
in an aromatic hydrocarbon. From 0.5 per cent. to 3 per cent. of the
reagent is added and saponification takes place from 12-48 hours by
heating in a current of live steam. The reaction is usually accelerated
by the presence of a few per cent. of free fatty acids as a starter.
Recently the Twitchell double reagent has been introduced through which
it is claimed that better colored fatty acids are obtained and the
glycerine is free from ash.
The advantages claimed for the Twitchell process as outlined by
Joslin[12] are as follows:
1. All the glycerine is separated from the stock before entering the
kettle, preventing loss of glycerine in the soap and removing glycerine
from spent lye.
2. The liquors contain 15-20 per cent. glycerine whereas spent lyes
contain but 3-5 per cent. necessitating less evaporation and
consequently being more economical in steam, labor and time.
3. No salt is obtained in the liquors which makes the evaporation
cheaper and removes the cause of corrosion of the evaporator; also
saves the glycerine retained by the salt.
4. The glycerine liquors are purer and thus the treatment of the lyes is
cheaper and simpler and the evaporation less difficult.
5. The glycerine can readily be evaporated to 90 per cent. crude rather
than 80 per cent. crude, thus saving drums, labor in handling and
freight. The glycerine furthermore receives a higher rating and price,
being known as saponification crude which develops no glycols in
refining it.
6. The fatty acids obtained by the Twitchell saponifier may be converted
into soap by carbonates, thus saving cost in alkali.
7. There is a decrease in the odor of many strong smelling stocks.
8. The glycerine may be obtained from half boiled and cold made soaps as
well as soft (potash) soaps.
While the advantages thus outlined are of decided value in the
employment of the Twitchell process, the one great disadvantage is that
the fatty acids obtained are rather dark in color and are not
satisfactorily employed for the making of a soap where whiteness of
color is desired.
To carry out the process the previously heated oil or fat to be
saponified is run into a lead lined tank. As greases and tallow often
contain impurities a preliminary treatment with sulfuric acid is
necessary. For a grease 1.25 per cent. of half water and half 66deg. B.
sulfuric acid is the approximate amount. The undiluted 66deg. B. acid
should never be added directly, as the grease would be charred by this.
The grease should be agitated by steam after the required percentage of
acid, calculated on the weight of the grease, has been added. The wash
lye coming off should be 7deg.-10deg. B. on a good clean grease or
15deg.-22deg. B. on cotton oil or a poor grease. As has been stated the
grease is heated before the acid is added or the condensation of the
steam necessitates the addition of more acid. After having boiled for
1-2 hours the grease is allowed to settle for 12 hours and run off
through a swivel pipe.
After the grease has been washed, as just explained, and settled, it is
pumped into a covered wooden tank containing an open brass coil. Some of
the second lye from a previous run is usually left in this tank and the
grease pumped into this. The amount of this lye should be about
one-third to one-half the weight of the grease so that there is about 60
per cent. by weight of grease in the tank after 24 hours boiling. Where
occasions arise when there is no second lye about 50 per cent. by weight
of distilled water to the amount of grease is run into the tank to
replace the lye. The saponifier is then added through a glass or granite
ware funnel after the contents of the tank have been brought to a boil.
If the boiling is to be continued 48 hours, 1 per cent. of saponifier is
added. For 24 hours boiling add 1.5 per cent. The boiling is continued
for 24-48 hours allowing 18 inches for boiling room or the grease will
boil over.
After boiling has continued the required length of time the mass is
settled and the glycerine water is drawn off to the treatment tank.
Should a permanent emulsion have formed, due to adding too great an
amount of saponifier, a little sulfuric acid (0.1 per cent.-0.3 per
cent.) will readily break this. During the time this is being done the
space between the grease and the cover on the tank is kept filled with
steam as contact with the air darkens the fatty acids.
To the grease remaining in the tank distilled water (condensed water
from steam coils) to one-half its volume is added and the boiling
continued 12-24 hours. The grease is then settled and the clear grease
run off through a swivel pipe. A layer of emulsion usually forms between
the clear grease and lye so that it may easily be determined when the
grease has all been run off. To prevent discoloration of the fatty acids
it is necessary to neutralize the lye with barium carbonate. The amount
of this to be added depends upon the percentage of saponifier used.
About 1/10 the weight of saponifier is the right amount. The barium
carbonate is added through the funnel at the top of the tank mixed with
a little water and the lye tested until it is neutral to methyl orange
indicator. When the fatty acids are thus treated they will not darken
upon exposure to the air when run off.
Fresh grease is now pumped into the lye or water remaining in the tank
and the process repeated.
The glycerine water or first lye is run to the treatment tank, the fat
skimmed off and neutralized with lime until it shows pink with
phenolphthalein, after having been thoroughly boiled with steam. About
0.25 per cent. lime is the proper amount to add. The mixture is then
allowed to settle and the supernatant mixture drawn off and run to the
glycerine evaporator feed tank. The lime which holds considerable
glycerine is filtered and the liquor added to the other. The evaporation
is carried out in two stages. The glycerine water is first evaporated to
about 60 per cent. glycerol, then dropped into a settling tank to settle
out the calcium sulfate. The clear liquor is then evaporated to crude
(about 90 per cent. glycerine) and the sediment filtered and also
evaporated to crude.
As to the amount of saponifier to use on various stocks, this is best
determined by experiment as to how high a percentage gives dark colored
fatty acids. For good stock such as clean tallow, prime cottonseed oil,
corn oil, cocoanut oil and stock of this kind 0.75 per cent. saponifier
is sufficient. For poorer grades of tallow, house grease, poor
cottonseed oil, etc., 1 per cent. saponifier is required and for poorer
grade greases higher percentages. The percentage of fatty acids
developed varies in various stocks, and also varies with the care that
the operation is carried out, but is usually between 85 per cent.-95 per
cent. Due to the water taken up in the saponification process there is a
yield of about 103 pounds of fatty acids and glycerine for 100 pounds of
fat.
The Twitchell reagent has undoubtedly caused a decided advance in the
saponification of fats and oils and has been of great value to the soap
manufacturer, because with a small expenditure it is possible to compete
with the much more expensive equipment necessary for autoclave
saponification. The drawback, however, has been that the reagent
imparted a dark color to the fatty acids obtained, due to decomposition
products forming when the reagent is made, and hence is not suitable for
use in soaps where whiteness of color is desired.
There have recently been two new reagents introduced which act as
catalyzers in splitting fats, just as the Twitchell reagent acts, but
the fatty acids produced by the cleavage are of good color. The
saponification, furthermore, takes place more rapidly. These are the
Pfeilring reagent and Kontact reagent.
The Pfeilring reagent is very similar to the Twitchell reagent, being
made from hydrogenated castor oil and naphthalene by sulfonation with
concentrated sulfuric acid. It is manufactured in Germany and is being
extensively used in that country with good success.
The Kontact or Petroff reagent, discovered by Petroff in Russia, is made
from sulfonated mineral oils. Until very recently it has only been
manufactured in Europe, but now that it has been found possible to
obtain the proper mineral constituent from American petroleum, it is
being manufactured in this country, and it is very probable that it will
replace the Twitchell reagent because of the advantages derived by using
it, as compared to the old Twitchell reagent.
The method and equipment necessary for employing either the Pfeilring or
Kontact reagents is exactly the same as in using the Twitchell process.
AUTOCLAVE SAPONIFICATION.
While the introduction of the Twitchell process to a great extent
replaced the autoclave method of saponification for obtaining fatty
acids for soap making, the autoclave method is also used. This process
consists in heating the previously purified fat or oil in the presence
of lime and water, or water only, for several hours, which causes a
splitting of the glycerides into fatty acids and glycerine. The
advantage of autoclave saponification over the Twitchell process is that
a greater cleavage of the fats and oils results in less time and at a
slightly less expense. The glycerine thus obtained is also purer and of
better color than that obtained by Twitchelling the fats.
An autoclave or digestor consists of a strongly constructed, closed
cylindrical tank, usually made of copper, and is so built as to resist
internal pressure. The digestor is usually 3 to 5 feet in diameter and
from 18 to 25 feet high. It may be set up horizontally or vertically and
is covered with an asbestos jacket to retain the heat. Various inlets
and outlets for the fats, steam, etc., as well as a pressure gauge and
safety valve are also a necessary part of the equipment.
LIME SAPONIFICATION.
The saponification in an autoclave is usually carried out by introducing
the fats into the autoclave with a percentage of lime, magnesia or zinc
oxide, together with water. If the fats contain any great amount of
impurities, it is first necessary to purify them either by a treatment
with weak sulfuric acid, as described under the Twitchell process, or by
boiling them up with brine and settling out the impurities from the hot
fat.
To charge the autoclave a partial vacuum is created therein by
condensation of steam just before running the purified oil in from an
elevated tank. The required quantity of unslaked lime, 2 to 4 per cent.
of the weight of the fat, is run in with the molten fat, together with
30 per cent. to 50 per cent. of water. While 8.7 per cent. lime is
theoretically required, practice has shown that 2 per cent. to 4 per
cent. is sufficient. The digestor, having been charged and adjusted,
steam is turned on and a pressure of 8 to 10 atmospheres maintained
thereon for a period of six to ten hours. Samples of the fat are taken
at various intervals and the percentage of free fatty acids determined.
When the saponification is completed the contents of the autoclave are
removed, usually by blowing out the digestor into a wooden settling
tank, or by first running off the glycerine water and then blowing out
the lime, soap and fatty acids. The mass discharged from the digestor
separates into two layers, the upper consisting of a mixture of lime
soap or "rock" and fatty acids, and the lower layer contains the
glycerine or "sweet" water. The glycerine water is first run off through
a clearing tank or oil separator, if this has not been done directly
from the autoclave, and the mass remaining washed once or twice more
with water to remove any glycerine still retained by the lime soap. The
calculated amount of sulfuric acid to decompose the lime "rock" is then
added, and the mass agitated until the fatty acids contained therein are
entirely set free. Another small wash is then given and the wash water
added to the glycerine water already run off. The glycerine water is
neutralized with lime, filtered and concentrated as in the Twitchell
process.
Due to the difficulties of working the autoclave saponification with
lime, decomposing the large amount of lime soap obtained and dealing
with much gypsum formed thereby which collects as a sediment and
necessitates cleaning the tanks, other substances are used to replace
lime. Magnesia, about 2 per cent. of the weight of the fat, is used and
gives better results than lime. One-half to 1 per cent. of zinc oxide of
the weight of the fat is even better adapted and is now being
extensively employed for this purpose. In using zinc oxide it is
possible to recover the zinc salts and use them over again in the
digestor, which makes the process as cheap to work as with lime, with
far more satisfactory results.
ACID SAPONIFICATION.
While it is possible to saponify fats and oils in an autoclave with the
addition of acid to the fat, unless a specially-constructed digestor is
built, the action of the acid on the metal from which the autoclave is
constructed prohibits its use. The acid saponification is therefore
carried out by another method.
The method of procedure for acid saponification, therefore, is to first
purify the fats with dilute acid as already described. The purified, hot
or warm, dry fat is then run to a specially-built acidifier or a
lead-lined tank and from 4 per cent. to 6 per cent. of concentrated
sulfuric acid added to the fat, depending upon its character, the degree
of saponification required, temperature and time of saponification. A
temperature of 110 degrees C. is maintained and the mass mixed from four
to six hours. The tank is then allowed to settle out the tar formed
during the saponification, and the fatty acids run off to another tank
and boiled up about three times with one-third the amount of water. The
water thus obtained contains the glycerine, and after neutralization is
concentrated.
AQUEOUS SAPONIFICATION.
While lime or a similar substance is ordinarily used to aid in splitting
fats in an autoclave, the old water process is still used. This is a
convenient, though slower and more dangerous method, of producing the
hydrolysis of the glyceride, as well as the simplest in that fatty acids
and glycerine in a water solution are obtained. The method consists in
merely charging the autoclave with fats and adding about 30 per cent. to
40 per cent. of their weight of water, depending on the amount of free
fatty acid and subjecting the charge to a pressure of 150 to 300 pounds,
until the splitting has taken place. This is a much higher pressure than
when lime is used and therefore a very strong autoclave is required.
Since fatty acids and pure glycerine water are obtained no subsequent
treatment of the finished charge is necessary except separating the
glycerine water and giving the fatty acids a wash with water to remove
all the glycerine from them.
SPLITTING FATS WITH FERMENTS.
In discussing the causes of rancidity of oils and fats it was pointed
out that the initial splitting of these is due to enzymes, organized
ferments. In the seeds of the castor oil plant, especially in the
protoplasm of the seed, the enzyme which has the property of causing
hydrolysis of the glycerides is found. The ferment from the seeds of the
castor oil plant is now extracted and used upon a commercial basis for
splitting fats.
The equipment necessary to carry out this method of saponification is a
round, iron, lead-lined tank with a conical bottom, preferably about
twice as long as it is wide. Open and closed steam coils are also
necessary in the tank.
The oils are first heated and run into this tank. The right temperature
to heat these to is about 1 degree to 2 degrees above their
solidification point. For liquid oils 23 degrees C. is the proper heat
as under 20 degrees C. the cleavage takes place slowly. Fats titering 44
degrees C. or above must be brought down in titer by mixing with them
oils of a lower titer as the ferment or enzyme is killed at about 45
degrees C. and thus loses its power of splitting. It is also necessary
to have the fat in the liquid state or the ferment does not act. The
proper temperature must be maintained with dry steam.
It is, of course, necessary to add water, which may be any kind desired,
condensed, water from steam coils, well, city, etc. From 30 per cent. to
40 per cent., on the average 35 per cent. of water is added, as the
amount necessary is regulated so as to not dilute the glycerine water
unnecessarily. To increase the hydrolysis a catalyzer, some neutral
salt, usually manganese sulfate is added in the proportion of 0.15 per
cent. appears to vary directly as the saponification number of the fat
or oil. The approximate percentages of fermentive substance to be added
to various oils and fats follow:
Cocoanut oil 8 %
Palm Kernel oil 8 %
Cottonseed oil 6-7 %
Linseed oil 4-5 %
Tallow oil 8-10%
The oil, water, manganese sulfate and ferment having been placed in the
tank in the order named, the mixture is agitated with air for about a
quarter of an hour to form an even emulsion, in which state the mass is
kept by stirring occasionally with air while the saponification is
taking place. A temperature is maintained a degree or two above the
titer point of the fat with closed steam which may be aided by covering
the tank for a period of 24 to 48 hours. The splitting takes place
rapidly at first, then proceeds more slowly. In 24 hours 80 per cent. of
the fats are split and in 48 hours 85 per cent. to 90 per cent.
When the cleavage has reached the desired point the mass is heated to 80
degrees-85 degrees C. with live or indirect steam while stirring with
air. Then 0.1 per cent.-0.15 per cent of concentrated sulfuric acid
diluted with water is added to break the emulsion. When the emulsion is
broken the glycerine water is allowed to settle out and drawn off. The
glycerine water contains 12 per cent. to 25 per cent. glycerine and
contains manganese sulfate, sulfuric acid and albuminous matter. Through
neutralization with lime at boiling temperature and filtration the
impurities can almost all be removed after which the glycerine water may
be fed to the evaporator. Should it be desired to overcome the trouble
due to the gypsum formed in the glycerine, the lime treatment may be
combined with a previous treatment of the glycerine water with barium
hydrate to remove the sulfuric acid, then later oxalic acid to
precipitate the lime.
The fatty acids obtained by splitting with ferments are of very good
color and adaptable for soap making.
KREBITZ PROCESS.
The Krebitz process which has been used to some extent in Europe is
based upon the conversion of the fat or oil into lime soap which is
transformed into the soda soap by the addition of sodium carbonate. To
carry out the process a convenient batch of, say, 10,000 pounds of fat
or oil, is run into a shallow kettle containing 1,200 to 1,400 pounds of
lime previously slaked with 3,700 to 4,500 pounds of water. The mass is
slowly heated with live steam to almost boiling until an emulsion is
obtained. The tank is then covered and allowed to stand about 12 hours.
The lime soap thus formed is dropped from the tank into the hopper of a
mill, finely ground and conveyed to a leeching tank. The glycerine is
washed out and the glycerine water run to a tank for evaporation. The
soap is then further washed and these washings are run to other tanks to
be used over again to wash a fresh batch of soap. About 150,000 pounds
of water will wash the soap made from 10,000 pounds of fat which makes
between 15,000 and 16,000 pounds of soap. The first wash contains
approximately 10 per cent. glycerine and under ordinary circumstances
this only need be evaporated for glycerine recovery.
After extracting the glycerine the soap is slowly introduced into a
boiling solution of sodium carbonate or soda ash and boiled until the
soda has replaced the lime. This is indicated by the disappearance of
the small lumps of lime soap. Caustic soda is then added to saponify the
fat not converted by the lime saponification. The soap is then salted
out and allowed to settle out the calcium carbonate. This drops to the
bottom of the kettle as a heavy sludge entangling about 10 per cent. of
the soap. A portion of this soap may be recovered by agitating the
sludge with heat and water, pumping the soap off the top and filtering
the remaining sludge.
While the soap thus obtained is very good, the percentage of glycerine
recovered is greatly increased and the cost of alkali as carbonate is
less. The disadvantages are many. Large quantities of lime are required;
it is difficult to recover the soap from the lime sludge; the operations
are numerous prior to the soap making proper and rather complicated
apparatus is required.
DISTILLATION OF FATTY ACIDS.
The fatty acids obtained by various methods of saponification may be
further improved by distillation.
In order to carry out this distillation, two methods may be pursued,
first, the continuous method, whereby the fatty acids are continually
distilled for five to six days, and, second, the two phase method,
whereby the distillation continues for 16 to 20 hours, after which the
residue is drawn off, treated with acid, and its distillate added to a
fresh charge of fatty acids. The latter method is by far the best, since
the advantages derived by thus proceeding more than compensate the
necessity of cleaning the still. Better colored fatty acids are
obtained; less unsaponifiable matter is contained therein; there is no
accumulation of impurities; the amount of neutral fat is lessened
because the treatment of the tar with acid causes a cleavage of the
neutral fat and the candle tar or pitch obtained is harder and better
and thus more valuable.
The stills are usually built of copper, which are heated by both direct
fire and superheated steam. Distillation under vacuum is advisable. To
begin the distilling operation, the still is first filled with dry hot
fatty acids to the proper level. Superheated steam is then admitted and
the condenser is first heated to prevent the freezing of the fatty
acids, passing over into same. When the temperature reaches 230 deg. C.
the distillation begins. At the beginning, the fatty acids flow from the
condenser, an intense green color, due to the formation of copper soaps
produced by the action of the fatty acids on the copper still. This
color may easily be removed by treating with dilute acid to decompose
the copper soaps.
In vacuum distillation, the operation is begun without the use of
vacuum. Vacuum is introduced only when the distillation has proceeded
for a time and the introduction of this must be carefully regulated,
else the rapid influence of vacuum will cause the contents of the still
to overflow. When distillation has begun a constant level of fatty acids
is retained therein by opening the feeding valve to same, and the heat
is so regulated as to produce the desired rate of distillation. As soon
as the distillate flows darker and slower, the feeding valve to the
still is shut off and the distillation continued until most of the
contents of the still are distilled off, which is indicated by a rise in
the temperature. Distillation is then discontinued, the still shut down,
and in about an hour the contents are sufficiently cool to be emptied.
The residue is run off into a proper receiving vessel, treated with
dilute acid and used in the distillation of tar.
In the distillation of tar the same method as the above is followed,
only distillation proceeds at a higher temperature. The first portion
and last portion of the distillate from tar are so dark that it is
necessary to add them to a fresh charge of fatty acids. By a well
conducted distillation of tar about 50 per cent. of the fatty acids from
the tar can be used to mix with the distilled fatty acids. The residue
of this operation called stearine pitch or candle tar consists of a
hard, brittle, dark substance. Elastic pitch only results where
distillation has been kept constant for several days without
interrupting the process, and re-distilling the tar. In a good
distillation the distillation loss is 0.5 to 1.5% and loss in pitch
1.5%. Fatty acids which are not acidified deliver about 3% of pitch.
Very impure fats yield even a higher percentage in spite of acidifying.
For a long time it was found impossible to find any use for stearine
pitch, but in recent years a use has been found for same in the
electrical installation of cables.
FOOTNOTES:
[12] Journ. Ind. Eng. Chem. (1909), I, p. 654.