INDUSTRIAL AND ENGINEERING CHEMISTRY Meunier, G., Chimie & industrie, 21, Special No., 553 (Feb., 1 QZR).
Mirlis, I., and Gorokhohnskaya, N. S., J . Chem. I n d . (U. S . S. R.),12,601 (1935). Ost, H., and Wilkening, L., Chem.-Ztg., 34, 461 (1910). Owen, TV. L., Facts About Sugar, 31, 431, 435 (1936). PeldLn, H., Suomen Kemistilehti, 10B, 13 (1937). Peterson, W. H., Fred, E. B., Langlykke, A. F., and Sjolander, N. 0.. Wis. Agr. Expt. Sta., Bull. 439 (1937). Ritter, G. I., IND. ENG.CHEM.,ANAL.ED.,4, 202 (1932). Schoen, M., Ann. combustibles Ziquides, 11, 591 (1936).
(33) (34) (35) (36) (37) (38) (39) (40) (41)
Vol. 34, No. 3
Scholler, H., private communication, 1935. Scholler. H.. French Patent 777,824 (March 1, 1935). Schwalbe, C. G., 2. angew. Chem., 37, 218 (1924). Sherrard, E. C., Chem. A g e (X, Y ~ 29, ) , 76 (1921). Sherrard, E. C., and Ganger, W. H., IND. EKG.CHEM.,15,1164 (1923). Skogh, C. G. C., C. S. Patent 2,096,353 (Oct. 19, 1937). Compt. rend., 150, 783 (1910). T'ille, J., and hlestrezat, W., Virtanen, A. I., and Kirkomaki, T., Suomen Kemistilehti, 9 3 19 (1936). Zimmerman, il., Chem. Trade J., 51,588 (1915).
High-Temperature Saponification AN ANHYDROUS SYSTEM Joseph J. Jacobs, Jr. Polytechnic Institute, Brooklyn,
everal of the variables for the process of saponification of fats w i t h anhydrous alkali, i n the presence of a hydrocarbon diluent, have been analyzed in the laboratory. A small p i l o t unit has been built from data obtained. The possibilities for the development of a continuous process are shown.
S
NTIL recently the manufacture of soap was considered U an outstanding example of an industry in which successful operation was almost wholly dependent upon familiarity with the art rather than upon modern scientific practice. While the old soap boiler is slowly disappearing from the field, the basic methods for the production of most soap today are the same as they were twenty years ago. The greater technical training of the responsible men in the industry has manifested itself in certain improvements which have decreased manufacturing costs and improved yields. Until recently, however, there have been no basic changes in methods of saponification. The process used now consists essentially of heating tallow or other fats with dilute aqueous alkali in large open kettles, until all of the fat has reacted with the alkali t o form soap, salting out the soap, allowing the layers to settle, and washing out the glycerol. Three to seven days are required to finish a soap boil, depending upon the size of the kettle and the extent of saponification and purification desired. Live steam is used for heating and agitation. After saponification is complete, salt is added to throw the soap out of solution. When separation is effected, the upper layer is withdrawn, cooled, dried, and processed further. The lower aqueous layer, containing the salt, glycerol, and organic impurities, is withdrawn and treated chemically. The glycerol solution is evaporated and the salt is precipitated. Further evaporation gives the 80 per cent soap lye crude. This concentrated glycerol solution is then distilled in the presence of superheated steam and partially condensed to give anhydrous glycerol.
N. Y
The concentration of glycerol in the aqueous layer is about 5 per cent, although this may be raised considerably by countercurrent operation-that is, by using the strong lye with no glycerol in it to finish the saponification, washing out the last traces of glycerol from the soap, and using the weak lye containing large amounts of glycerol to start saponification of the fresh tallow. The concentration of glycerol in the final effluent depends upon the number of these so-called changes that are made. Several disadvantages in the present methods of saponification are evident. Despite the fact that the use of large kettles and countercurrent operation has allowed a close approach t o a truly continuous process, it is, nevertheless, a batch process, and there are sizable heat losses due to heating and cooling of the batches. These disadvantages, combined with the use of open steam for agitation and heavy evaporator duty, make the over-all heat costs evcessively high. The recovery of dynamite-grade glycerol from dilute soap lyes is an expensive, if not a difficult, operation. These and other minor disadvantages have prompted research with a view toward the development of a continuous, rapid method of saponification. One of the large soap companies recently built a plant for the production of soap by the continuous saponification of fatty acids (4). The fatty glycerides are split continuously in a pressure tower. Hydrolysis is accomplished under superpressures and high temperatures in order t o decrease the time required for splitting. It was stated that the concentration of glycerol recovered from the botton of the fat-splitting tower is approximately proportional to the height of the tower, and that with a column 50 feet high the concentration of glycerol recovered is 20-25 per cent (4). Another proposed continuous saponification method is the Clayton process (1). This involves the continuous saponification of fats with a relatively concentrated alkali (25-50 per cent) in a coil reaction tube. Fat and alkali are proportioned to a pressure pump which mixes the materials and forces them through the reaction tube, and saponification takes place at approximately 425" F. The semifluid mass of soap and water is forced through a spray nozzle into a vacuum chamber where
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INDUSTRIAL AND ENGINEERING CHEMISTRY
The recovered glycerol generally contained less than one per cent water. This factor was variable, however, and was dependent upon the fatty acid content of the tallow. The fatty acid combined with the alkali to produce soap and water which tended to dilute the glycerol.
Un b
REFLUX
TIME
C ON DEN SATE RECEIVER
Figure
I.
323
Laboratory Apparatus for Saponification and Recovery of Products
the glycerol and water are flashed off and the soap remains behind. If the spray chamber is heated, the plastic soap will act as the lubricant and will seal the screw conveyor which is used to remove the soap continuously. Kokatnur (2) proposed a unique method for saponification with anhydrous alkali which offers possibilities for continuous operation. The basic process of saponification involves the reaction at high temperatures of anhydrous powdered caustic with fat which has been dissolved in kerosene. As saponification proceeds, the essentially anhydrous glycerol produced is distilled out with the kerosene, and an anhydrous soap dissolved in kerosene is left behind. After as much kerosene is distilled off as possible without burning the soap, the remaining gel is washed with a solvent to remove residual kerosene and glycerol, and the soap is dried. While this process has the advantage of rapid saponification, i t also has some disadvantages as such. It was felt that the development of this process in the laboratory with a view to ultimate continuous operation would enhance its value.
*
FOR SAPONIFICATION
TWOof the variables of the process arc tho time required for saponification nnd the completeness of reaction. To determine the optimum conditions, the following procedure was carried out: 200 grams of kerosene and 200 grams of tallow were charged into an agitated three-neck flask fitted with a condenser and sampling thief, and maintained at 220' C. in an oil bath. The exact stoichiometric quantity of finely divided caustic (as determined by the saponification number) was added, and 10-gram samples were withdrawn and titrated in alcoholic solution with standard acid, in order to determine the free alkali. The results of this test are shown in curve A , Figure 2. It had been observed qualitatively that anhydrous crystalline caustic produced by the partial pressure method (3) gave more rapid saponification. Accordingly, a duplicate run was made on a sample of this caustic, of approximately the same screen analysis as the powdered caustic. The results are shown in curve B, Figure 2. More rapid saponification is ob-
A-POWOERED CAUSTIC E-CRYSTALLINE CAUSTI
1 I
SAPONIFICATION APPARATUS
The laboratory setup shown in Figure 1 was devised, and the majority of the laboratory tests mentioned in this paper were carried out in it. The operation was started by charging the fat mixed with kerosene of a suitable boiling range into the agitated and heated three-neck flask, A , fitted with an air condenser open t o the atmosphere. The kerosene and fat were heated t o 180" C. This was necessary because the soap, which was formed almost immediately upon addition of the alkali, would tend t o thicken and gel below this temperature. Above 180" C., however, the solution of soap in kerosene was a fluid of relatively low viscosity, so that any saponification which took place did not affect the agitation. After the mixture of kerosene and tallow was heated to 180' C., the stoichiometric quantity of powdered anhydrous alkali was added and the mass was agitated while the temperature was raised t o approximately 220" C. as quickly as possible. In approximately 15 minutes saponification was complete and the fluid gel of soap and kerosene with the glycerol suspended in it was brought through the glass spray nozzle, C, into the heated vacuum flask, D. Here, under reduced pressure (approximately 10 mm. mercury) the kerosene and glycerol flashed off, and left a powdered anhydrous soap of high porosity Since glycerol and kerosene are immiscible, they will exert thc same vapor pressures as if each existed alone. The mixture, therefore, will distill off at a lower temperature than either comonent. The removal of glycerol from the soap is considerably facilitated by this partial pressure dist'illation which is analagous t o the commonly used steam distillation methods. The glycerol and kerosene which flashed off were condensed in air condenser E and dropped into receiving flask F. The kerosene was separated by decantation from the glycerol for re-use.
1
Figure
2
3
(
,
(
,
4 5 6 7 8 TIME * MINUTES
2. Effect of Type OF Caustic on Saponification Time
tained when crystalline caustic is used, because the caustic produced by the partial pressure method is crystallized in flat plates and more surface is available for reaction. It should be noted that all the caustic added in the soap flask appears in the final Boap, so that any errors in weighing may cause high free-alkali contents. The criterion, therefore, of completeness of saponification is a constancy of free alkali rather than a prescribed value. Since caustic soda is insoluble in kerosene or soap, in commercial operation the installation of a strainer in the soap feed line would remove mechanically any free alkali that might be added. Since this is a heterogeneous system, in which the time of reaction is dependent upon the completeness of contact between the caustic and the fat solution, the results obtained are not directly translatable t o larger batches. However, they are indicative of times of reaction t o be obtained in continuous operation where the net mass of material to be considered in the reaction tube would be of the same order of magnitude.
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DECOMPOSITION OF GLYCEROL
I n actual operation it mas not possible to recover the glycerol flashed from the soap efficiently enough to make any direct estimation of yields, since it tended to stick to the sides of the flask, the vapor neck. and the condenser. It was thought to be far simpler to analyze the soap produced for residual glycerol. However, in order t o conclude that the difference between this value and the theoretical glycerol yield was the amount actually flashed off, it had to be shown that there was no appreciable decomposition of glycerol a t the high temperatures involved. The same setup and conditions were used as in the time runs, and 10-gram samples were withdran-n a t various intervals. These samples were then treated with mineral acid to decompose any soap present; the kerosene layer containing the liberated fatty acids and any unreacted tallow was decanted and washed thoroughly with water to remove all suspended glycerol. The washings were combined with the original acid water and extracted with petroleum ether. The solution was then made up to 260 cc., and the glycerol determined by the standard dichromate method.
temperature. This temperature was somewhat lower than the initial boiling point of the kerosene, owing to the presence of an immiscible liquid (glycerol). The temperature of the spray flask was maintained a t 190' C. and the vacuum at 10 mm. in all runs. After saponification for 15minutes, the stopcock was opened wide and approximately 100 grams of the gel mere sprayed over. The remaining soap gel was necessary in order to seal the delivery tube and prevent air from rushing through and charring the soap a t the high temperature in the spray flask. After cooling, the soap mas removed from the spray flask and analyzed for glycerol by the standard dichromate method and for kerosene by determining the weight of unsaponifiable material which volatilized with steam. The following data are graphed in Figure 4 : Boiling R s n g e of Kerosene. C.
1
I
I
I
I
I
I
I
I
Figure 3. Effect of Time of Heating on Decomposition of Glycerol d t H i g h Temperatures
The results of these tests are shown in Figure 3. It is evident that there is no appreciable decomposition of glycerol within the time ordinarily required for saponification by this method. An additional check on the time required for saponification was noted from the fact that the maximum amount of glycerol was liberated in approximately 10 minutes. While it can be shown that the theoretical latent heat necessary to vaporize all of the glycerol and kerosene is available as sensible heat in the temperature drop between the saponification flask (220' C.) and the calculated temperature of flash in the vacuum flask (about 125' C.), it was found necessary in the laboratory runs to heat the vacuum flask in a n oil bath. This procedure was adopted to counteract the radiation losses in this laboratory unit and to provide an additional safety factor to ensure complete vaporization. BOILING RANGE OF KEROSENE
Several other variables influence this process. One is the boiling range of the kerosene, which affects not only the vaporization characteristics in the flash chamber but also the boiling point in the saponification flask and, therefore, the available temperature drop. Another variable is the temperature in the spray flask which determines the amount of heat transfer there. Still another is the pressure in the spray flask which affects the boiling points. Several less tangible factors, such as design of spray nozzle, liquid and vapor rates, etc., also have to be considered. To test the effect of boiling range, various cuts of highly purified water-white kerosene were used in the laboratory setup. I n each case 100 grams of tallow were charged with 100 grams of fresh kerosene into the agitated flask a t 180" C. Caustic was added and the batch brought t o the maximum
Ternp. of S a a p Flask, C. 214 225 245 250 220
220-230 2 30-2 50 250-290 290-310 290-3 10" a
101
Vol. 34, No. 3
70 Glycerol on soap 3.36 2.46 2.06 1.43 1.47
% Kerosene on Soav Trace 0.40 1.06 1.76 0.60
Recycled, 107, make-up, n,t plotted
Evidently the higher boiling range allows a more complete recovery of glycerol. With fresh kerosene the residual kerosene is higher alw. However, recycling the kerosene reduced the residual kerosene to almost one third of this amount. Another series of runs was made on a 230-275" C. wide cut of kerosene, in which the temperature in the soap flask was maintained constant a t 220" C. and the temperature of the spray flask was varied. The following results are graphed in Figure 5 : T e m p of S p r a y Flask, 140 150 170 190 200
C.
% ' Glycerol
5% Kerosene
on Soap 3.70 3.10 2.80 2.60 2.45
on S s a p 4.5
4.0 2.9 1.2
0.4
EFFECT OF PRESSURE
A series of runs was made with a commercially available kerosene boiling between 260" and 290" C. in which the pressure in the spray flask was varied. The temperature of the soap flask was 220' C. and of the spray flask 190' C. throughout the runs. The following results are graphed in Figure 6 : Pressure in Spray Flask, M m 1.0 3.0 6.5 16.0 22.0
7"Glycerol
% Kerosene
2.64 2.60 2.05 2.42 2.78
l,85
o n Soap
on Soap
1.60 1.40 1.70 2.93
Ordinarily it would not be expected that the pressure-recovery curve would go through a minimum. The only explanation available is that the increased liquid velocities due to the greater pressure drop and the larger vapor volumes did not allow enough time for the diffusion of the kerosene and glycerol to the surface of the particles for vaporization. While it was impractical to recover the flashed glycerol a t the end of each run, all the glycerol was collected from a series of runs using tallow containing 4.2 per cent free fatty acid. This was analyzed and showed a concentration of 98.6 per cent glycerol. The glycerol was a clear viscous liquid, slightly off water-white in color. The soap produced varied in color, depending mainly upon
INDUSTRIAL AND ENGINEERING CHEMISTRY
March, 1942
3.0-
2.5
-
$2.00 u)
z 1.50 (3
z 1.0-
5
E0.5Iz
bp
0.
I
210
I
220
I
I
230
I
I
240
I
250
TEMPERATURE OF SOAP FLASK, 'C.
Figure 4. Residual Glycerol and Kerosene on the Soap as a Function of Temperature of Soap Flask (or Boiling Range of Kerosene)
the grade of tallow used. I n general, however, the soap was slightly darker than that produced by the usual method from the same tallow. It must be borne in mind that the foregoing data are true only for the laboratory setup shown and that the results are by no means indicative of yields to be obtained in actual plant operation. However, they do show trends and enable a more intelligent design of pilot plant equipment. A pilot unit was built, translating the glass laboratory setup into steel; two 25-gallon steel kettles were used as soap and spray flasks, respectively, and a gear pump was included to ensure forcing the liquid soap gel through the spray nozzle. Preliminary runs on this unit showed that the design of the spray nozzle, the pressure drop across it, and the consequent effect on particle size have a pronounced effect upon yields. Soap has been produced containing 0.75 per cent glycerol and only a trace of kerosene, using the commercially available 280-290' C. cut. While all the work to date has been in batch units, continuous operation can be projected. As visualized now, the process could be carried out by proportioning tallow and kerosene (both preheated by the vapors from the flash chamber) and caustic into a heated reaction tube. After allowing sufficient time for saponification in the tube, the mass could be flashed and the vapors removed continuously. The soap would have to be removed through a soap-sealed screw conveyor or a motor-operated pocket valve. This projection presupposes the solution of the mechanical difficultieswhich are bound to arise.
325
Because of the possibilities of heat interchange and the elimination of water, which has an abnormally high latent heat, from the system, the heat requirements for this process should be very low. Based upon the laboratory work, these have been calculated to be approximately 900 pounds of steam per ton of dry soap, which is about one tenth the heat required for soap boiling and glycerol recovery, as given by Webb (6). Several evident disadvantages may be overcome by further experiments now being carried on. For one thing, there is no refining during the saponification. I n the full boiled process, an intermediate layer of impure soap called "nigre" is separated from the so-called neat soap. This is, in effect, a refining operation and permits the use of a lower grade of tallow to produce a soap of comparable quality. However, in industrial soaps this may not be important. If it is important, the effect could be reduced somewhat by caustic refining the tallow before use in the pr"ocess.
0 1401
,
160 1 1
1 180 1
1 200
TEMPERATURE OF SPRAY FLASK,
OC.
Figure 5. Effect of Temperature of Spray Chamber on Amount of Residual Glycerol and Kerosene on the Soap
ADVANTAGES AND DISADVANTAGES
The operation of this process on a plant scale would seem to offer several advantages. If operated continuously or even semicontinuously, the time of saponification is so short a t the high temperatures that considerable savings in time, labor, and plant investment will result. While it is entirely feasible to carry on this process without a diluent, use of kerosene p r e vents local overheating and consequent destruction of glycerol and charring of the soap. The vapors of kerosene create an inert atmosphere which excludes air from the soap. The mass is a fluid of low viscosity a t the temperature of operation, and consequently any pumping difficulties encountered with plastic masses of soap are avoided. The system is anhydrous and glycerol recovered can be used as dynamite grade with little further treatment.
I n some cases the anhydrous condition of the soap is a disadvantage. To make a cake soap, 12 to 15 per cent water has to be added. T o do this homogeneously is sametimes a difficult operation, owing to the formation of an impervious layer of soap gel around each particle after the first addition of water. This layer prevents diffusion of further water into the particle and causes hard grits of anhydrous soap in the final cake. This again is not an important objection in industrial powdered soaps. The traces of kerosene present on the soap may be undesirable, but there is no detectable odor, probably because of the deodorizing effect of caustic upon kerosene. I n any case the amount remaining on the soap can be kept below the generally accepted limits for unsaponifiable matter in soap without sacrificing too much of the glycerol yield.
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326
OTHER USES FOR THE PROCESS
SUMMARY
GREASES. Sodium soaps are used to a great extent to produce high-temperature fibrous greases. These soaps are usually produced by saponification of fatty acids. If tallow is used, the glycerol is not recovered. Since this process offers a simple method for glycerol recovery, the manufacture of greases by this method is advantageous. Greases are essentially a gel or solution of soap in lubricating oil; and after
P R E S S U R E - MM.
Figure 6. on Amount
Val. 34, No. 3
Effect of Pressure in Spray Chamber of Residual Glycerol and Kerosene
on the Soalo the kerosene and glycerol had been flashed off, the addition of a suitable oil t o the soap flask would result in a grease. While the multitude of different greases used in industry would prevent any large-scale continuous operation, the preparation of concentrated base greases, to be diluted with lubricating oil for the desired consistency, would permit smoother operation. Such base greases have been prepared and compare favorably with those made by present batch methods. METALLIC SOAPS. Present methods for the preparation of calcium soaps involve the initial formation of sodium soap and then precipitation of the calcium soap from aqueous solution by the addition of a calcium salt. Calcium hydrate can be reacted directly with fatty acids also; but the physical properties of the final product are not comparable with the finely divided, powdered product obtained by the double precipitation method. The most straightforward way would be t o react calcium hydroxide with tallow directly, but this reaction proceeds only with great difficulty a t the temperature of boiling water. The temperatures of operation in the kerosene method are sufficiently high to ensure complete saponification of tallow with calcium hydroxide in one or two hours. The product delivered from the spray nozzle is a finely divided white powder comparable to the calcium soap produced by the precipitation method. Other metallic soaps have been prepared.
The basic process of saponification of fats a t high temperature in the presence of kerosene as disclosed by Kokatnur ($) was investigated. Several of the operating difficulties, such as the necessity for washing with solvents and discoloration of the soap, were eliminated. The process mas carried out in the laboratory as follows: Tallow or any other suitable fat was dissolved in kerosene and heated to 180" C. in an agitated flask. Powdered anhydrous caustic was added and the temperature brought to 220" C. Saponification was completed in the laboratory equipment in less than 15 minutes. The use of crystalline anhydrous caustic completes the saponification in less than 8 minutes. dfter the saponification, the soap flask will contain a gel of soap in kerosene (which is liquid above 180" C.) and glycerol in suspension. This liquid is then sprayed into a vacuum flask (1 to 10 mm. mercury) through a glass spray nozzle. In the vacuum flask the kerosene and glycerol flash off together, leaving behind a finely divided powdered soap. Since kerosene and glycerol are immiscible, they will flash off at a temperature lower than the boiling point of either. After condensation, the kerosene can be decanted from the glycerol and recycled. Since the system is essentially anhydrous, the glycerol recovered will contain 1t o 6 per cent mater, depending upon the fatty acid content of the tallow, and is only slightly off-color. The highest recovery obtained in the laboratory to date has been 86 per cent. Since the unrecovered glycerol is present in the soap, proper design of spray nozzle and adjustment of feed rates, etc., will probably increase this yield. The minimum residual kerosene on the soap, using fresh kerosene, was less than 0.4 per cent. This can be reduced t o a trace by recycling the kerosene and consequently eliminating the high boilers. The odor of kerosene was noticeably absent in most cases. This is probably due to the deodorizing effect of caustic and soap on kerosene a t high temperatures. A small pilot unit was built to study ~e effects of design of the spray nozzle and other factors on recovery. The process is applicable t o continuous operation. ACKNOWLEDGMENT
Thanks are due Autoxygen, Inc., for permission to publish this material, and also to Robert Ward and Frank Lipinski for the use of some of their data. LITERATURE CITED (1) Clayton, Benjamin, Kerrick, W. B., and Stadt, H. M. (to Refining, Inc.), U. s. Patent 1,968,626(July 31, 1934). (2) Kokatnur, V. R., Ibid., 1,813,454(July 7, 1931). (3) Othmer and Jacobs, IND. ENQ.CHEM., 32, 154 (1940). (4) Soap, 15,No. 5,25 (1940). (5) Webb, C.T.,Chem. Trade J . , 77,367 (1928).
End of Symposium
