DECEMBER, 1936
INDUSTRIAL AND EYGINEERING CHEMISTRY
Nomenclature pressure drop calcd. with fluid properties taken at cold terminal temp. Aph = pressure drop calcd. with fluid properties taken at hot terminal temp. Apo = actual over-all pressure drop h = av. coefficient df heat trans'fer between inner tube surface and fluid, based on arithmeti: mean temp. difference, B. t. u./hr. X sq. ft. X F. = friction factor in Fanning equation = linear velocity of fluid, ft./sec. = density of fluid, lb./cu. ft. = acceleration due to gravity = inside diam. of inner tube, ft. = heated length of inner tube, ft. = weight of fluid passing through inner tube, lb./hr. = mass velocity of fluid passing through inner tube, lb./hr. x sq. ft. = outer pipe as in experimental data tables = inner pipe as in experimental data tables = av65mp. of fluid entering heating section of inner tube, Ap,
=
- P.
av. temp. after mixing of fluid leaving heating section of inner tube, O F. (TI Tz), B. t. u./lb. X ' F. = sp. heat of fluid at = heat given up to or by tube fluid, 1000 B. t. u./hr. = weight of water passing through annular space of heat exchanger, lb./hr. = av. temp. of water entering annulus, F. = av. temp. of water leaving annulus, F. = heat given up by or t o water, 1000 B. t. ud/hr. = av. temp. of inside surface of inner tube, F. = thermal conductivity of fluid inside tub:, evaluated at I/z (TI Tz), B. t. u./hr. X sq. ft. X F./ft. = Prandtl number, evaluated a t I/2 (TI Tz) = viscosity of tube fluid, evaluated a t 1/1 (TI Tg), Ib./hr. X ft. = viscosity of tube fluid, evaluated a t I/z T , (Ti T2)lb./hr. X ft. = viscosity of tube fluid, evaluated a t T,, lb./hr. X ft. =
+
O
+
+
+
+
(E) (E)"'* 114
= 1.1 = 1.02
+
below
DG = 2100 Pa
DG
above - = 2100 Pa
Acknowledgment The authors wish to acknowledge the kindness of W. H. McAdams in making available to them the data of Keevil, of Clapp and Fitzsimmons, and of White.
Literature Cited Clapp and Fitzsimmons, Mass. Inst. Tech., Thesis, 1928. Colburn, A. P., IND. ENO.CHEM.,25, 873 (1933). Colburn, A. P., Trans. Am. Inst. Chem. Engrs., 29, 174 (1933). ENQ.CHEM.,24, 152 (1932). Drew, IND. Drew, Hogan, and McAdams (Holden and White data). Ibid., 23, 936 (1931). (6) Graetz, L., Ann. Physik, 18,79 (1883); 25,337 (1885). (7) Keevil, Mass. Inst. Tech., Thesis, 1930. (8) Keevil and Mcridams, Chem. &. Met. Eng., 36,464 (1929) ENQ.CREM.,23,625 (1931). (9) Kirkbride and McCabe, IND. (10) Kraussold, H., Forsch. Gebiete Ingenieurw. B 2, Forschudngheft 351, 1-20 (1931). (11) Lawrence and Sherwood, IND. ENG.CHEM.,23, 301 (1931). (12) LQvLque, Ann. mines, [I21 13, 201, 305, 381 (1928). (13) Morris and Whitman, IND. ENQ.CHEbr., 20, 234 (1928). (14) Nagle, W. M., Ibid.,25, 604 (1933). (15) Sherwood, Kiley, and Mangsen, Ibid., 24, 273 (1932). (16) Sherwood and Petrie, Ibid.,24, 736 (1932). (17) Underwood, A. J. V., J.Inst. Petroleum Tech., 20, 145 (1934). (1) (2) (3) (4) (5)
RECEIVED June 8,1930. Presented as part of the Heat Transfer Sympasium held under the auspices of the Division of Industrial and Engineering Chemistry of the American Chemical Society a t Yale University, New Haven, Conn., December 30 and 31, 1935.
1435
VAPOR RE=USEPROCESS Separation of Mixtures of Volatile Liquids DONALD P.OTHMER Polytechnic Institute, Brooklyn,N. Y.
T
HE heat requirements are considerable for the separation by distillation of mixtures of two or more liquids in which the less volatile is greatly predominant. By means of the processes described here, it is possible to reduce this amount of heat greatly, to reduce the size of distilling columns required for the separation, and to make possible the operation of some distillation processes without the use of water as a condensing or cooling medium. While the process may be more readily understood by reference to mixtures of two components, it is applicable and shows the greatest merit when used with more involved distillation projects, such as the production of pure grain alcohol or pure absolute alcohol from a distiller's beer.
Theory as Applied to Two Liquids Many examples of pairs of liquids in which the more volatile liquid is present in relatively small amount are met in industrial processes, and these separations are the ones in which the Vulcan vapor re-use process shows the greatest savings. One of these is the beer in alcohol production which usually contains about fifteen times as much water as alcohol. Others are the aqueous solutions of acetone or methyl alcohol (or mixtures of the two) which are obtained by water washing of the vapors in air in various solvent recovery systems. The resulting aqueous solution may have to be as dilute as forty to fifty parts of water for one part of acetone, for example, in order to remove efficiently the vapors of the solvent from the air. The rectification of the alcohol in the first example, or of the acetone in the second, from the water requires the expenditure of considerable heat because of the diluteness of the solution. A distilling and rectifying column may be regarded as a heat engine with poor thermal efficiency when separating a very dilute solution such as these which are in question. It is necessary to distill so much of the mixture even with a theoretically perfect column in order to remove all the solvent. Thus with a 2.5 per cent solution of acetone in water, although the vapors arising are comparatively strong (about 40 per cent acetone), there would necessarily have to be distilled 60 per cent water or one and a &alf times as much water as acetone. As the solution becomes progressively weaker, it is necessary to distill a much larger amount of water to remove the acetone completely. Hambrand' (Principles and Practice of Industrial Distillation) shows that to obtain 1 kg. of acetone as vapors of 99.75 per cent purity from an acetone mixture of 2 per cent requires almost 1000 kg.-cal. This large expenditure of heat shown theoretically by Hausbrand is borne out in actual practice; and in the recovery of acetone from a 2.5 per cent solution in
* "Principles and Praotiae of Industrial Dietillation," New York, John Wiley & Sone, 1928.
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IKDUSTRIAL AND ENGINEERING CHEMISTRY
1436
are in substantial equilibrium with the feed. The vapors arising from a 2.5 per cent acetone solution contain approximately 40 per cent acetone and are passed from this column under a pressure of approximately 50 pounds per square inch, to the heating coils of calandria 6 at the base of column 9. By reason of their high pressure and temperature, they pass heat through the heating surface to the liquid in the base of this column, and in doing so are condensed. The condensate so formed, 40 per cent acetone (containing only 1.5 pounds of water per pound of acetone as compared to 39 pounds of water per pound of acetone in the original feed), is trapped by an ordinary steam trap, 8, and passed through pipe 7 to some intermediary section of column 9; column 9 is fitted with the usual condensers a t the top, and accomplishes the rectification into substantially pure acetone a t the top and substantially pure water a t the bottom in the usual manner. Only about two parts of acetone must be returned as reflux to the head of the column per part of pure acetone drawn off as a product, and the amount of heat required, which is ultimately rejected in the condensers, is considerably less than that available in the vapors passing into 6. This excess heat may be withdrawn from the column base as vapor in equilibrium with the waste withdrawn from pipe 10. Since the exhausting action of the column has produced a nearly perfect separation and the waste is thus substantially pure water, the vapors leaving by pipe 20 are substantially pure steam a t a pressure corresponding to the pressure required to operate the column (in the usual case, approximately 2 to 3 pounds per square inch gage). This low-prewure steam may be used for other purposes, such as the operation of another column accomplishing the same separation in the usual manner. The operation of this system may be clearer from inspection of Figure 4 which shows the vapor composition curve of acet o n e a n d water drawn from the data of Berg-
water coming from solvent recovery operations, a n amount of liquid is returned as wash to the head of the column equivalent to as much as nine times the product obtained. The reflux wash in a n efficient column separating practically pure acetone from a 40 per cent feed may, on the contrary, be less than one part for each part of product. Various methods of re-use of a part of the heat required for this separation-by multiple-effect stills, by thermocompression, and by various ways of utilizing the heat passed to the condenser-have been proposed; but all of them have disadvantages which militate against the desired economies. The present process partakes to some extent of the features of these other methods but is based on an entirely different and novel principle which allows it to accomplish its economies
EXCESS STEAM
S
k
‘SLOP q
WATER
EXCESS STEAM
FIGURE1. DISTILLATION Sy8TEM FOR SEPARATION OF Two COMPONENTS
FIQURE2. DISTILLATION SYSSEPARATION OF THREE COMPONENTS FIGURE% DISTILLATION SYSTEM FOR SEPARATIOS OF FOUR COMPONENTS TEM FOR
W STEAM
PUMP
FIGURE 2 {LOP
WATER
in a simple, practical manner. In no case is it to be confused as being a “multiple-effect” system similar to the familiar multiole-effect evaoorator. Figure 1 illustrates the separation of acetone from water as an example of the features of the process: A solution of 2.5 per cent acetone in 97.5 per cent water is run from supply tank 1 through valve line 2 to the top plate of column 3 which is maintained under a pressure of 50 pounds per square inch. This column is supplied with steam a t the base in the usual manner and in such quantity as to exhaust completely all of the acetone from the liquid as it descends from plate to plate. The spent water is discharged from 4. The large amount of water present supplies all of the reflux wash required in this exhausting column, and the vapors a t the top
BEER FEED
1
I
I
I
II
W BUTANOL
S
T
E
A
#SLOP WATER
FIGURE
3
DECEMBER, 1936
INDUSTRIAL AND ENGINEERING CHEMISTRY
strom reprinted by Hausbrand.’ The relative ease of separation of acetone and water is clearly indicated by the wide divergence of the curve from the 45” line. However, this ease of separation indicates only that a small number of plates of a distilling column is required, and takes no account of the fact that considerable heat is necessary and a large cross section of column is required to handle the large amounts of vapors produced in separating low concentrations of acetone in water. I n Figure 4 the pressure feed line represents the concentration of 2.5 per cent acetone which is supplied to the pressure exhausting column a t the top plate. The composition of the vapors in equilibrium with the liquid which is fed to the top plate is 41.6 per cent acetone. This is the concentration of the acetone-water vapor mixture which leaves the top of the pressure exhausting column in equilibrium with the feed. These vapors are then condensed in the calandria supplying heat to the atmospheric column. The condensation is indicated on Figure 4 by the horizontal line; and the feed to the atmospheric column is indicated by the second vertical feed line. Thus, by the use of the pressure exhausting column the feed concentration of the atmospheric rectifying and exhausting column is changed from 2.5 t o 41.6 per cent with no heat expense since all of the heat so used in the pressure colun~n is available for re-use. As the new feed concentration has a niuch higher strength of acetone, i t requires a correspondingly lower amount of reflux for the separation and thus reduces the amount of heat actually used in the separation The amount of heat supplied in the pressure exhausting column is exactly the same as the amount of heat which would be supplied in an ordinary exhausting and rectifying column, accomplishing the desired separation of acetone and water under the superatmospheric pressure used. (This is almost the same amount as would be required in an ordinary column making the same separation under atmospheric pressure.) The new feed, with a concentration of 41.6 per cent, however, requires a smaller amount of heat for its separation; and this amount of heat is equivalent to that required for any other liquid feed in an atmospheric column at this concentration of approximately 41.6 per cent acetone. The difference in the heat requirements of two atmospheric pressure columns operating on these two different feeds represents substantially the difference between the heat requirements of the pressure and the atmospheric columns. It is equal to the amount of heat exhausted in the low-pressure steam available from the base of the atmospheric column. In the case under consideration, with radiation neglected or largely prevented by adequate insulation and sensible heat not considered, owing to the fact that efficient heat interchange would minimize this factor, the difference or saving by the vapor re-use method would amount to the latent heat of the difference in the amount of reflux required per pound of acetone recovered. The difference of approximately 9 pounds of reflux with the standard method and 1 pound with the vapor re-use method is 8 pounds per pound of product or, in steam requirements, almost 2 pounds of steam per pound of acetone. Because of the large amount of water to be heated before passage to the pressure column, it will entirely absorb the heat of condensation if passed through the condenser, and no cooling water will be required, as opposed to the large amount required in the usual operation.
Three-Component Systems The Vulcan vapor re-use principle of distillation may be applied not only in the simple case where a mixture of liquids is to be separated into only two constituents, but is even more advantageous for the separation of three or more fractions of components of more complicated liquid mixtures. This
1437
operation of withdrawing condensate from a pressure calandria of a column and feeding it midway of the column itself may be regarded as an advantageous unitary process which may be combined in various relations with other distillation equipment in various processes. The production of 190-proof spirits is an example of a system in which substantially three liquids are separated by rectification. Besides the water and alcohol, there are small quantities of aldehyde bodies and other materials which are more volatile than the alcohol itself, and which in Figure 1 would be discharged with the alcohol from the final condenser. It is necessary to separate these material.. from the alcohol if pure spirit is to be obtained, and, since all of these liquids have a lower boila m ing point than that $: of alcohol, they may E be removed together before the alcohol 5 itself is separated :: from the w a t e r . 2: ( T h e f u s e l oil or similar material is ; not considered here S b u t w i l l be men2 tioned later.) OO IO 20 30 4 0 5 0 6 0 7 0 0 0 90 100 Figure 2 is repreW E I G H T P E R C E N T ACETONE IN LIQUID sentative of a distillation system for FIGURE4. VAPORCOMPOSITION CURVE OF ACETONE AND WATER the s e p a r a t i o n of three componentsin this caqe, alcohol, light-boihng materials or “heads,” and water. The beer is fed into pressure column 3, and vapors are withdrawn from the top containing water, all of the alcohol (in about a 33 per cent mixture), and all of the heads. This mixture is passed to the two calandrias, 6 and 26, which are connected, respectively, to the rectifying and exhausting column, 9, and the purifying column, 29. The condensate in both of these calandrias is, therefore, the same and is trapped off and passed through lines 7 and 27 to purifying column 29 which separates the heads in line 35. Heat for the operation of column 29 is supplied by the vapor from the dilute alcohol (still approximately 33 per cent water and with substantially no heads) which is boiling in calandria 26. The dilute alcohol stripped of heads is then passed by pipe 47 to concentrating column 9, separating the alcohol which is discharged as pure spirits a t 15 and as water a t 10. If some heads remain in the feed to column 9, they may be removed a t 15 and the pure alcohol drawn from one of the upper plates of the column, as is standard practice; and higher boiling alcohols or fusel oil may be drawn from one of the lower plates and separated as is common practice; both of these details are standard and not primarily concerned with the fundamental process. Even after accomplishing the separation of pure 190-proof spirits and heads from each other and from the water present in the beer, there is an excess of heat available in the vapors coming from the beer still and this may be drawn off a t 20, as before, aslow-pressure steam for use outside of the process. In other words, the heat supplied to the beer still, whichissubstantially that which would be required by the atmospheric-pressure beer still in the usual operation, accomplishes the separation and purification of 190-proof spirits, and has a considerable excess left for other uses. If absolute alcohol is desired, i t may be obtained by an auxiliary distillation (without any heat being required except that furnished to the beer still). This auxiliary process will not be described here, but it is substantially different from those now employed for the purpose,
E
c,
1138
INDUSTRIAL AND ENGINEERING CHEMISTRY
Another use of the excess heat in the production of neutral spirits is for the separation of fusel oil and the operation of a distilling column for that purpose.
Butyl Alcohol-Acetone-Ethyl Alcohol Fermentation Liquors Many other industrial distilling operations involve dilute aqueous solutions of two or more solvent materials and may be separated by means of the principles described. One of these is the separation of acetone, ethyl alcohol, and butyl alcohol from the water of the familiar butyl alcohol fermentation mash and from one another. It is given merely as an illustrative example of the application of this principle. Figure 3 shows the combination of standard distilling units for accomplishing this separation. The vapors from pressure beer still 3 are used to supply heat to the three atmospheric pressure columns, and the condensate formed in the respective calandrias (or heating coils of auxiliary kettles) is trapped, collected, and passed to a middle plate of the column 9. The balance of the water is discharged from the base, and vapors from the top of this column contain substantially all of the ethyl alcohol (as 190 proof) and acetone, and no water other than that with the alcohol. After passing through a suitable dephlegmator condenser, these vapors enter column 29 a t the proper intermediate plate as a vapor feed and are rectified to give acetone a t the top and 190-proof ethyl alcohol a t the bottom. The butyl alcohol forms, with water, a constant-boiling mixture which, under the conditions of operation of column 9 is prevented by its boiling point from being discharged with the lower boiling vapors at the top or with the water a t the bottom. Therefore an oily layer of butyl alcohol collects in the middle part of column 9, and this may be drawn off in a mixture with an aqueous phase, separated in a gravity separator or decanter from the water, and passed to column 39 for dehydration. The water layer is returned to column 9 a t a lower plate. This is identical with the customary fusel oil separation in the manufacture of neutral spirits. Column 39 dehydrates the butyl alcohol by bringing the water over in a constant-boiling mixture with the butyl alcohol, condensing the vapors into a butyl alcohol phase and an aqueous phase, passing the two phases to the decanter where the water is separated and passed to column 9, and the butyl alcohol is returned t o column 39. Dry butyl alcohol is discharged from the base of 39. A small amount of acetone or ethyl alcohol which is unavoidably brought into the decanter with the two phases from column 9 does not interfere with the final, complete separation unless they are present in such an amount as to make the two layers completely miscible. Smaller amounts are continually washed back by the aqueous phase into column 9, and even though part of these heads are passed to the top of column 3 in the butanol layer, they are immediately flashed off and passed to the condenser and decanter where a considerable fraction separates in the aqueous phase for return to column 9. The balance merely cycles around the top plates of column 39, the condenser, and the decanter. Proper location of the draw-off from 9 to the decanter and proper operation of column 9 prevent the carrying into the decanter of sufficient ethyl alcohol and acetone t o prevent separation of two phases entirely. Even after supplying heat for the separation of the three solvents from the water and from one another, the vapors from the beer still contain an excess amount of heat which is discharged in the form of steam a t a pressure of 2 or 3 pounds per square inch gage from the lower part of column 9. This heat might be used instead to vaporize the two alcohols as finally discharged. These vapors would be condensed, to give redistilled products. The excess heat may be used instead
VOL.28, NO. 12
in an additional distillation to remove any impurities from the acetone. In this regard, chemical treating plates may be installed in column 29 if necessary or desirable. STEAM. It is eatimated that the total amount of heat required will be supplied by between 2 and 2.5 pounds of steam per pound of solvent (acetone-ethyl alcohol-butyl alcohol) produced, starting with the cold beer. These figures are only estimates, since sufficient data on the compositions and the vapor-liquid relations of these solutions as encountered in the plant are not available. WATER. Full advantage will be taken of feed heating arrangements for using the beer for a cooling medium in the condensers, and for cooling the slops from the pressure beer still. These arrangements are not shown in the figures. Because of the low heat requirements and the large amount of water in the beer, all cooling will be done by the beer itself in preheating; no cooling water will be required. EQUIPMENT. Standard and existing equipment may be used throughout with the exception of the beer still which must be of sufficiently healy construction to withstand 30 to 50 pounds per square inch pressure. This column could be considerably smaller in size than the usual atmospheric one, because of the greater density of the pressure vapors. CAPACITY.The capacity of existing columns in an operating plant which would be rearranged to conform with the needs of the process would be several times the capacity under batch operation, first because of the advantages inherent in continuous operation and, of more importance, because of the considerable decrease of the amount of reflux liquid necessary and the consequent decrease of the amount of vapors passed through the column.
Advantages of Vapor Re-use Process Obviously a tremendous saving in steam and cooling water is possible by the use of the vapor re-use process as compared to standard methods, and the equipment sizes will be much smaller because of the smaller amount of vapors to be handled. I n the simple two-component system given in Figure 1, the excess steam, for example, may be recompressed by a thermorecompression system so that it may be re-used in the pressure exhausting column to secure full advantage of the system. The theoretical laws and heat balances on which the vapor re-use process is based have been subjected to thorough laboratory and industrial size distilling unit investigations. The results of the experimental determinations and of the heat balances in industrial plants will be presented a t a later time. I
Acknowledgment Full credit for the development of this method of distillation is due to the Research and Development Staff of the Vulcan Copper and Supply Company of Cincinnati, which company has fully covered this process and related subject matter by either existing patents or patent applications. RECEIVED Ootober 15, 1936.
Metallic Phthalates SIR: William Blum of the National Bureau of Standards has brought t o my attention the word “metallates” which appeared as the title of my article in the September number (page 1020). The word “metallates” is one which I coined as a descriptive and brief term. However, Blum suggests that it is reasonable t o assume that a metallate is a salt in which a customarily metallic element is in the anion-for example, a plumbate or a ferrate. For this reason, it might he desirable in the future to refer to the products I have produced as ‘‘metallic phthalates” rather than metallates. HENRYA. GARDNER
