Steam Deodorization of Edible
Fats and Oils A
Theory and Practice
LTHOUGH a great ~ a r i e t y obtained from steam refining of methods have been tests should be also applicable t o steam deodorization. This proposed for rendering edible oils odorless and tasteless, is important in an investigation A E. BAILEY it is now almost universal pracof the process. for free fatty acid The Cudahy Packing Company, South Omaha, Nebr. tice to accomplish this by a removal from an oil is easily process of steam distillation followed by a simple titration under reduced pressure. Acwith alkali and is therefore well adapted to quantitative study. Oils are also readily available cording to a recent historical review of the subject (6), this with free acids in any desired concentration. process probably originated in France. I n the central and northern European countries it has been used on a large scale Apparatus for Batch Steam Deodorization for many years in the manufacture of margarine fats. ProbThe ordinary type of batch deodorizer is shown in side elevation ably its most extensive and useful application has been in the in Figure 1. It consists essentially of a cylindrical insulated steel United States, however, in converting the large American tank, fitted at the bottom with a pipe for the injection of steam production of cottonseed oil to edible purposes. In the and at the top with a thermocompressor for the maintenance of United States alone the annual production of deodorized fats high vacuum. In the commercial equipment used in the present tests, the injected steam was expanded from the mains (at about is now not less than 1.5 billion pounds. 150 pounds gage pressure) through a throttling valve to 15-75 In spite of the relative importance of the deodorization procpounds, and then passed through a metering nozzle to a disess in oil and fat technology, the published data concerning it tributor at the bottom of the deodorizer where it was divided into have been meager. Theoretical or semitheoretical treatments thirty separate jets. The injected steam was not superheated. The diameter of the deodorizer was 8 feet, so that 20,000 pounds of the subject have been given by Whiton ( I f ? ) ,Brash ( 1 , g), of oil at 460" F. filled it t o a depth of about 8.5 feet. The oil and Singer ( 9 ) , but all of them have neglected factors of imwas heated by Dowtherni (diphenyl and diphenyl oxide) vapor, portance in actual operation-for instance, the several sources condensed in roils in the lower part of the deodorizer body. Presof oil loss in deodorizing. sure within the head space of the deodorizer was measured on a closed-end mercury manometer connected to the deodorizer body Reports on commercial deodorizer operation have also been above the surface of the oil. The injected steam, together wit,h few, although those of Thurman (11), Khiton ( I f ? ) , and Dean the thermocompresaor steam, discharged through a Venturi tube and Chapin (3) may be mentioned. The purpose of this paper to a barometric condenser, so that any substances distilled from is to consider the theory of the process in some detail and to the oil appeared in the water discharged from the condenser. report certain experimental data gained from the operation of Theory- of Distillation by Sleain commercial equipment. The paper ~villbe confined to batch, as distinguished from continuous, deodorization. In the study of petroleum distillation problems, 137. K. Lewis and eo-workers have successfully used a differential Nature of the Process treatment based on the application of Raoult's law. The same method is applicable to the deodorization process. The deodorization process is essentially one of steam disAccording to Dalton's law, in the vapors issuing from the tillation, in which relatively volatile odor-causing substances deodorizer a t any instant, the molar ratio of volatile substance are stripped from the relatively nonvolatile oil. These subto steam will be the same as the corresponding ratio of their stances are probably of aldehydic and ketonic nature. A few partial vapor pressures. Mathematically, definite compounds have been identified in coconut oil (4, 5, 8), but in the oils and fats containing no low-molecularweight fatty acids, the identity of the odoriferous substances is where S = moles of steam still unknown. Hydrogenation, even of a previously odorless 0 = moles of volatile odoriferous substance oil, imparts a characteristic odor and flavor, and the source of p , = actual partial pressure of steam this is also still undetermined. In the case of the common p , = actual partial pressure of volatile substance animal and vegetable fats, such as cottonseed oil, soybean oil, But p , is very small in relation to p,, so p , approaches palm oil, tallow, lard, etc., the vapor pressures and probably closely the total pressure. Consequently as a close approxialso the molecular weights of the unkno\\n substances are mation of Equation 1we may write: comparable t o those of the CI6and Cl8 fatty acids, since in practice flavor and odor removal is observed to parallel free dS P fatty acid removal very closely. a = g The concentration in which the odoriferous substances exist where P = total pressure in these oils is likewise unknown, although it is ordinarily less From Raoult's law, than about 0.20 per cent, since the above-mentioned fats, if 0 well refined, can be deodorized with losses not greater than (3) P: = Po . . , this. where p: = equilibrium pressure of odoriferous substance Closely related to the process of steam deodorization is that Po = vapor pressure of pure odoriferous substance of steam refining. The two are similar in execution although F = moles of fat or oil different in purpose. I n steam refining the object is not the 0 is very small in relation to F , however, so that (0 F ) removal of odoriferous substances but of free fatty acids. As approaches closely the value of F . Therefore, as an approximentioned above, the odoriferous substances and the free mation of Equatior, 3 we may write: fatty acids of the oil are comparable in volatility; hence data
(mF)
+
404
March, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY p: = P,O/F
(4)
It is convenient at this point to introduce a new factor, E , the so-called efficiency of vaporization. By definition,
E = P~/P:
(5)
Combining Equations 4 and 5, po = EP,O/F Finally, from Equatjons 2 and 6, dS PF
a=m
Integrating Equation 7,
s=
g
(7)
( 1 n0 g)
where 0, = initial concentration of odoriferous substance O2 = final concentration of odoriferous substance Thus, the amount of steam required for deodorization is: (a) directly proportional to the amount of fat or oil, (a) directly proportional to the total pressure in the deodorizer, (c) inversely proportional to the vapor pressure of the pure volatile or odoriferous substance a t the temperature of operation, and (d) inversely proportional to the vaporieation efficiency. The vaporization efficiency is, practically speaking, a measure of the completeness with which the steam bubble becomes saturated with volatile substance during its passage through the oil. From the two-film theory of gas absorption (7) we have the condition, that a t any instant the rate of transfer of volatile substance from the oil into the steam bubble is equal to the difference between saturation pressure in the bubble and actual pressure, times the surface area of the bubble, times a constant characteristic of the oil and steam. Mathematically. d p o / d t = IcA(pA - po) (9) where t = time of contact between steam bubble and oil A = surface area of bubble k = gas diffusion constant Integrating Equation 9,
r-l mitic acid a t 460" F. is about 33 mm., of oleic and other C I S acids, about 18 mm. If a value of 33 mm. is taken, Equation 8 gives 0.64 for E. If 18 mm. is taken, E becomes 1.18. The mean of these two valuesis0.91. The freefatty acid content of the oil was originally 4.6 per cent, and this had been reduced to 2.6 per cent by distillation before the experiment was started. This preliminary distillation, if effecFIGURE1. DIAGRAM OF RATCH DEODORIZER t i v e a t all i n separating the GO and Cls fatty acids, could have served only to decrease the original proportion of the former, so from this standpoint the above figure may reasonably be regarded as a minimum one. However, such experiments as the one outlined above are subject to some uncertainties unavoidable in the use of ordinary commercial equipment and thus are by no means of a high order of precision. Other experiments with palm oil have generally indicated vaporization efficiencies of 0.7 to 0.9. It seems certain that vaporization efficiency in deodorization is comparatively high, and probable that it is greater than about 0.7. The progressive removal of free fatty acids with distillation time in the above experiment is shown graphically in curve 1 of Figure 2. The rate of fatty acid removal slows appreciably as the concentration of the acid falls to the lower levels. It is
';6
RUN NO. 1
(12) 1
=
In
405
- 19.WO LBS. O I L - 4 0 0 L b I STEAn
Vaporization Efficiency in Practice A rough idea of the magnitude of E in actual practice has been gained from observations of the rate of free fatty acid removal from palm oil under controlled conditions of vacuum, temperature, and injected steam. The following experiment is typical: Oil used, pounds Steam, pounds per hour Tem erature O F Absoyute pre;sure; mm. Time. hours Average mol wei ht of oil Free fatty aiid re&ction, %
19,000 400
460 10 0.50 848
2.6-0.90
The chief difficulty here is in assigning an average value for the vapor pressure of the fatty acids. Palm oil contains palmitic and oleic (and other 18-carbon fatty acids) in a p proximately equal proportions. The vapor pressure of pal-
0
1
2
3
4
HOURS DCODORIZLD
FIGURE 2. REMOVAL OF FREB FATTY ACIDS FROM PALM OILAT Two DIFFERENT PRESSURES
406
INDUSTRIAL AND ENGINEERING CHEMISTRY
believed that this slowing is apparent only, since there is a slight tendency for the steam t o split the fat with the freeing of fatty acids even a t the low pressures employed. This splitting action is constantly in operation, of course, but is apparent only toward the end of the process when its magnitude approaches that of the reverse action of fatty acid removal by distillation. The range of free fatty acid concentration chosen in the experiment (2.6-0.9 per cent) would appear to be somewhere near the optimum for purposes of calculation; it is low enough so that the conditions assumed in Equation 8 should apply, yet high enough so that slight hydrolyzation of the fat should not vitiate the results. All of the foregoing has been developed on the assumption that Raoult's law applies to the mixture of oil with fatty acids or other volatile material. It is realized that this may not be entirely justified, and that the factor Po in Equation 8 might be more accurately represented by the Henry's law constant. Even should this be true, however, it is certain that the variation of this constant with temperature, within the range in which we are interested, cannot be greatly different from the variation of Po. Any deviation from Raoult's law would affect the above calculations of vaporization efficiency. At present there are apparently no data available on the mixtures in question, and it can only be said that the method outlined has proved useful in predicting actual deodorizer performance.
Losses from the Deodorizer Although the neutral triglycerides are among the least volatile of all organic substances, their vapor pressures are high enough so that distillation losses become appreciable in deodorizing a t elevated temperatures. Figure 3 shows the results of a typical loss test on a hydrogenated cottonseed oil shortening deodorized a t varying temperatures. I n this test deodorization was carried out in stepwise fashion a t temperature levels of from 300' t o 490" F., while the Dressure on the deodorizer was held a t 10 mm. and the steam flow a t 350 pounds per hour throughout t h e run. Loss figures were obtained by determining the neutral fat in the condenser water, the flow of which was measured each time a sample was taken. The distillation loss increases a t a rate r50 300 330 400 450 500 approaching exTCMPLRATURE - or. ponential, doublingfor eachincreFIQURE3. DISTILLATION Loss FROM DEODORIZER AT 10 MM. PRESSUFLE ment in temperaAND 350 POUNDS PER HOUROF INture of about 40" JECTED STEAM F. (22" C.). The data of Figure 3 also provide the basis for a rough calculation of the vapor pressure of the glycerides. The loss a t 10 mm. pressure with 350 pounds of injected steam per hour is about 55 pounds per hour a t 482" F. (250"C,). Assuming an average oil molecular weight of 860 and a vaporization efficiency of 0.8, the vapor pressure of the glycerides so calculated is 0.04 mm. The question of loss by entrainment is also of considerable practical importance, since the batch will ordinarily be
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steamed a t a rate just short of that a t which significant quantities of oil begin to be blown out of the deodorizer. The entrainment losses (total loss corrected for distillation loss) in deodorizing at two different pressures are shown graphically in Figure 4. Appreciable entrainment becomes evident a t a steam Row of about 400 pounds (181 kg.) per hour at 10 mm. pressure and at about 600 pounds (272 kg.) per hour a t 25 mm. pressure. Over the range covered by the experiments, 35 to 65 per cent more steam is required for a given loss at 25 mm. than at 10 mm. This is in reasonably good accord with theory. Entrainment presum5 so ably occurs when E ,, oildroplets, thrown up above the sur0 face of the oil, are 2 more powerfully 2 70 impelled upward $ by the vapor stream than they $ are downward by t h e f o r c e of ZOO 400 6W em 1WO 12w gravity. The rLOW OP INJECTCD STCAM-POUNDS R R HOUR kinetic energy of the oil droplets as FIGURE4. ENTRAINMENT LOSSESIN they are thrown up DEODORIZING AT Two DIFFERENT PRESSURES is known to be a factor in entrainment in certain classes of equipment. I n view of the great distances from the surface of the oil to the top of the deodorizer and also from the surface of the oil down to the steam jets, it is not believed to be important in the present case, however. Under a given set of conditions, then, there is a critical velocity of the steam required for carrying droplets of a given size. This velocity is given by the expression (IO):
A,, 0
where u
linear velocity of steam diameter of droplet dl = density of droplet dp = density of steam K = a constant
D
= =
At a given temperature dl is constant and hence may be incorporated in a new constant, K'. The magnitude of dz is insignificant with respect to that of dl. As an approximation of Equation 11 we may therefore write:
It will be remembered that each value of v has corresponding to it a definite degree of entrainment and a definite rate of loss. A consideration of Equation 12 will reveal that equal losses should theoretically result when the steam flow a t 25 mm. is about 58 per cent greater than a t 10 mm. It would seem reasonable to expect the number of small droplets thrown up into the vapor stream, and hence the entrainment, to bear some relation t o the vapor velocity alone without regard to its density; but it is evident from the experimental data that if this is a factor to be considered, it is a t least not a major one. Figure 4 also shows that entrainment losses increase with great rapidity as the steam flow increases t o the higher levels. This is also t o be expected. The steam velocity required to entrain oil droplets varies with the square root of the diameter of the droplet, whereas the weight of the droplets varies with
March, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY
the third power of the diameter. The weight of the largest droplets of oil entrained varies, therefore, with the sixth power of the steam velocity or steam flow. No attempt will be made here to correlate the weight of the largest particles of oil entrained with the actual loss in pounds of oil per hour, but it is obvious that the one will be more or less inclined to follow the other. A third source of loss in deodorizing is in the tendency of steam a t elevated temperatures to split neutral glycerides into their component fatty acids and glycerol, which readily distill off. This splitting tendency of the steam seems to bear a direct relation to its total pressure and is largely avoided in practice by keeping the pressure very low. Determination of free fatty acids in the total water-insoluble distillate from the deodorizer indicates a loss from this source of about 3 pounds per hour a t 460" F. (238" C.),and 10 mm. pressure, with a steam flow of 350 pounds (150 kg.) per hour. At pressures of 25 mm. and higher, with the steaming correspondingly increased, the loss may amount to several times this figure. It is particularly important that the pressure be kept low if a very low free fatty acid content is desired in the finished oil; otherwise the building up of acids from fat splitting may nearly approach the rate a t which they are removed by distillation, toward the end of the deodorization period. Figure 2 shows how much more rapidly a low fatty acid in palm oil is attained a t a pressure of 10 mm. than a t 25 mm.
407
has been found satisfactory as a guide for operation in the deodorization of hydrogenated cottonseed oil products. It is based upon a steam flow of 300 pounds (136 kg.) per hour at 6 mm. pressure, and batches of 20,000 pounds of oil. The remaining temperature to be considered is that of the injected steam. If the vacuum is high and the requirement of steam correspondingh low, there is no evidence that any benefit results from superheating the latter. This is only reasonable in view of the relative heat capacities of the oil and steam. A batch of oil weighing 20,000 pounds gains approximately 1,000,000 B. t. u. as it is heated from 350" to 450"E'. (177"to 232" C.). The steam required for an hour's blowing of this oil (300 pounds) gains only about 15,000 B. t. u. as i t is superheated from saturation at 350" to 450' F.
Influence of Vacuum
One effect of vacuum has been noted-viz., with other factors constant, the steam requirements are directly proportional to the absolute pressure. With ordinary three-stage thermocompressor equipment operating with a barometric condenser, the maximum vacuum easily obtainable is about 0.25 inch (6 mm.) of mercury. If this vacuum drops to 0.50 inch the steam requirements would be expected to be doubled; if it drops to 1.00 inch, they would be expected to be quadrupled. Actual observation seems to c o d i n n the above expectations, a t least within the somewhat wide limits of error Influence of Temperature of our methods of measurement, and within the range of 6 to 25 mm. of pressure. Thus in the two experiments of Figure 2 It was noted above that the amount of steam required for a t equivalent temperatures and with approximately equal deodorization is in inverse proportion to the vapor pressure of amounts of oil, the total pressures in the two runs stand in the the volatile compounds of the oil. As the temperature range inverse ratio of 1to 2.5, whereas the corresponding steam rewith which we are concerned is only about 300" to 500" F. quirements for equivalent fatty acid distillation are about 1 to (149" to 260' C.), and within this range there is practically a 8.3. Furthermore, in the series of experiments on the steam linear relation between the temperature and the logarithm of refining of palm oil mentioned in a previous paragraph, there the vapor pressure, the importance of operating at the highest was no correlation between the pressures which varied from 6 possible temperature is immediately apparent. to 25 mm. and the vaporization efficiencies which varied from Just what constitutes the highest possible temperature may 0.7 to 0.9. Theoretically the pressure should affect the well depend upon the facilities a t hand for heating the deodorvaporization efficiency considerably unless the latter is so high izer rather than on the possibility of doing harm to the oil. as to be very close to unity at all pressures. Eight and one If steam a t the pressures available in most plants is depended half feet of oil a t 450"F. exert a hydraulic pressure of some 190 upon for heating, a top temperature of about 375'F. (191"C.) mm. of mercury. Each bubble of steam during its passage may be reached. Boilers employing a high-boiling-point through the oil is therefore subjected to a constantly varying organic compound instead of water have recently come into external pressure. Both its internal pressure and its surface extensive use, and with these, temperatures in excess of 500' F. area vary continuously, and under these conditions Equations (260" C.) are easily attainable. High temperatures may 9 and 10 are not applicable without modification, since they also be obtained by a flame applied either directly to the oil comprehend no such variation. A complete mathematical or to a liquid heat-transfer medium. treatment of this phase of the subject appears to offer some difficulties and will not be attempted here. It is apparent, however, that a high vacuum serves not only to make the REQUIRED FOR DEODORIZATION OF HYDROGENTABLE I. TIME average steam bubb!e comparatively large, thus making its ATED COTTONSEED OIL surface area small in proportion to its volume, but also makes Temp., Vapor Pressure of Time its average volume during its passage through the oil small in F. Oleio Acid, Mm. Hour; 1.4 16.0 proportion to its final volume. Vaporization efficiency is 350 2.5 9.0 375 based upon this final volume at the surface of the oil; there4.6 4.0 400 8.3 a.7 425 fore it should decrease as the pressure falls. That it appar15 1.5 450 26 ently does not may be evidence that saturation of the steam 0.9 475 43 0.5 500 bubble a t high temperatures is so rapid as to be substantially attained at any pressure down to at least 6 mm. Another highly important consideration with respect to If due care is exercised in other respects, the temperature vacuum is its effect on the maximum permissible rate of of deodorization may be surprisingly high without apparent steaming. If steam can be injected at a maximum rate of 350 pounds per hour at a vacuum of 0.25 inch without excessive injury to the oil. The writer has seen cottonseed oil products successfully deodorized a t temperatures as high as 525" F. entrainment, it can be shown from Equation 12 that the maximum rate of steaming a t a vacuum of 1.00 inch is twice as (274" C,). Sufficiently rapid deodorization may usually be obtained a t 425' to 475" F. (218"to 246" C.), however. In great, or 700 pounds per hour. Since the weight of steam required a t 1.00 inch is four times that a t 0.25 inch, then twice practice good results have been obtained by adjusting the as long a time will be required to deodorize a t the lower time of deodorization on the basis of the vapor pressures of the common fatty acids a t the various temperatures. Table I vacuum. O
INDUSTRIAL AND ENGINEERING CHEMISTRY
408
Design of Deodorizing Vessel The optimum size and shape of the deodorizing vessel have always been a matter of some discussion and conjecture. From the preceding results i t would seem possible that these are, within wide limits, comparatively unimportant. It is obvious that increasing the depth of the vessel will tend to raise the vaporization efficiency, since factor t in Equation 10 is Drouortional to the oil deDth. As mentioned above. however, there is some evidence that good efficiency is attained even under adverse conditions of pressure. If this is the case, similarly good efficiency should be obtained with shallow bodies of oil. The use of shallow vessels will permit a comparatively greater rate of steaming, since the maximum rate is more or less a function of the surface area of the oil body. For the same reason deodorization may be conducted more rapidly in small vessels than in large ones. Distribution of the steam in the form of fine bubbles is indicated, since factor A in Equation 10 is thereby increased. I n view of the high vaporization efficiencies obtained with
Vol. 33, No. 3
ordinary steam distribution, however, the value of elaborate distributing or diffusing devices would appear to be questionable. Acknowledgment The author is indebted to R. H. McKinney and R. F. Krage for preparation of the figures in this article, and to A. R. Diehl and Tales Newby for assistance in conducting the plant tests and experiments. Literature Cited Brash, J. Sac. Chem. I n d . , 45, 73T (1936). I b i d . , 45, 331T (1926). Dean and Chapin, Oil & S o a p , 15, 200 (1938). Haller and Lassieur, Compt. rend., 151, 697 (1911). Haller and Lassieur, J. SOC.Chem. I n d . , 28, 719 (1909). Lee and King, Oil & S o a p , 14, 263 (1937). ENG.CHEM.,16, 1215 (1924). Lewis and Whitman, IND. Salway, J . Chem. Soc., 111, 407 (1907). Singer, Seifensieder-Ztg.,65, 487, 507 (1938). ENG.CHEX.,26, 98 (1934). Souders and Brown, IND. Thurman, Ibid., 15, 395 (1923). Whiton. Cotton Oil Press, 7. No. 10, 32 (1924).
Calculating Beattie-Bridgeman Constants from Critical Data SAMUEL H. MARON AND DAVID TURNBULL Case School of Applied Science, Cleveland, Ohio NE of the most accurate and useful equations of state for gases is that of Beattie and Bridgeman (2) :
0
where p,
yB, and 6
are defined by:
where A , B , C, D, E , F , and G are given in terms of the Beattie-Bridgeman constants and the critical constants P, and T,by: A =- Bop, 2.303RT, -AJ'o 2.303 (ET,) =- -cP, 2.303RTC4 -BobPo' = 2 X 2.303(RT0)Z AoaPC2 = 2 X 2.303(RTJ8 -BocPO2 = 2 X 2.303R*TC5 BobcPO3 = 3 X 2.303R3TC6
B =
c Ao, Bo,a, b, and c are constants independent of temperature and pressure, which must be evaluated empirically for any given gas. The purpose of this paper is to show how, on the basis of certain assumptions, these five constants may be evaluated for any gas from its critical constants and the Beattie-Bridgeman and critical constants of some reference gas, and also to test the calculated constants for each gas with known compressibility data.
Derivations of Equations In the preceding paper ( I O ) the authors derived a n expression relating log,, y , where y is the activity coefficient of a gas, to the Beattie-Bridgeman constants and the reduced pressure and temperature, P, and T,. The derived expression was as follows:
D E
F G
Newton (11) found that when values of y obtained graphically were plotted against P, a t constant T,, a n average curve could be drawn through the points from which the activity coefficients of most of the gases studied deviated by no more than 2 per cent. This finding of Newton's may be generalized to state that a t the same values of T,and P, all gases, with possibly a few exceptions, have the same activity coefficient. Applying this condition to Equation 2, i t must follow that
