High Temperature Distillation an
Unit Operations Review
by Thomas J. Walsh, Department of Chemical Engineering, Case Institute of Technology, Cleveland, Ohio
British chemical engineers seem more interested in distillation than their American brothers, as evidenced by a summer school on distillation, the number of reports in British chemical engineering journals, and several reviews of calculation and design techniques
IN
THE last review (March 1359), the American Institute of Chemical Engineers bubble tray design manual was criticized because the constants and terms of the calculation sheets did not agree with the text of the manual. This criticism revealed the fact that the copy reviewed was one of the first to be issued. The discrepancy was noticed by others also and corrected before many of the manuals were issued. Obviously, this review is the best place to direct attention to the correction of the manual.
General A 2-year review of distillation from the other side of the Atlantic ocean was presented by Jones ( 7 A ) . Readers might be interested in comparing reactions to developments as presented in this review and in one from Europe. It may also be interesting to compare costs of bubble plate towers in Europe with costs in the U.S.A. While precise cost data may not be useful to the American engineer, the method of prcsenting the data in charts may be adopted by anyone. T h e data on weight of trays and number of bubble caps per tray are of course applicable anywhere (7UA). Precesses involving distillation are always important. A plant for the pressure distillation of ammonia liquor was described in detail ( 8 A ) . Flow diagrams, enthalpy concentration data, and calculations of heat loads were included. T h e process flow for the Linde heavy water plant involves distillation designed to make 14 tons per year for the Nangal Fertilizer and Chemicals, Ltd. (2.4). The control of distillation equipment is frequently a moot subject. Several possibilities exist for the control of a column in any situation (6‘4). Different solutions to a given problem are illustrated. One not mentioned in the above article is control by temperature difference. This technique-i.e., controlling the tower by measuring the tem-
peratures on two trays and maintaining a given differential-was shown to be applicable to pure product towers and not to impure products (71A). I t works well on towers separating a pure component from a binary mixture. Stream analyzers, as vapor phase chromatographs, may be adaptable to automation of existing fractionators ( 5 A ) . Cascade control loops are recommended. The advantage of an automated plant is shown by Canada’s continuous tar distillation plant ( 7 A , 3A). Six men operate the plant producing 15 million gallons of product per year compared to 25 men in batch operation. Interaction of variables may complicate experimental studies of distillation columns. Information obtained by application of the Box method of experimentation to study of a perforated plate column has been analyzed ( 4 A ) . An experimental program of 246 points was available for study of four variables; reflux ratio, vapor mass velocity, weir height, and hole diameter. The Box method required 28 points and gave an equally accurate correlation of the data. However, the latter correlation should not be extrapolated, as errors are introduced which lead to unsatisfactory values outside the range of the data, Evaporation of liquids is usually treated as a case of simple distillation. When the less volatile liquid is more dense, the simple calculation may be applied ( 9 A ) . However, if the less volatile liquid is less dense no mixing occurs, and the problem becomes one of unsteady-state molecular diffusion.
Calculations and Theory The theory of distillation has been established. Even so, modifications are possible. It was suggested (8B) that hlcCabe-Thiele plots be modified by plotting temperature us. composition using the 45” axis for T us. x , and a curve for T us. y . Additional information may be obtained from the plot and the operating line. T h e number of plates neces-
sary near either extreme of concentration is often difficult to determine. It is possible to replace the equilibrium curve in these regions with a straight line and solve the equations algebraically. A possible line for this purpose is the tangent to the curve a t the limit x = 1 or x = 0. Solutions to the problem for this condition are given in a modified Fenske equation (2B). I t has been proposed (3B) that a “fractionation index”--thr slope of the characteristic straight line when x d ’ x , is plotted us. p on log-log paper-be usrd as a measure of multicomponent distillations. Interrelations among components in complex fractionations reduce the number of independent composition variables, as all components must fit material balances determined in part by a pair of such characteristic lines for any distillation. Rapid calculations where 10% accuracy is satisfactory may be performed using a log mean composition change in a proposed technique ( 7 B ) . A rigorous trial and error approach was recommended ( 4 B ) . These two studies represent the extremes of simplicity with crror and complexity with rxactness between which the chemical engineer must select his method of solution for a real problem. To aid in solving the complex equations several digital computer programs and advice on programming are now available (7B, 5B, 6 B ) .
Still Design and Capacity Many of the investigations of trays are in some manner concerned with the relative behavior of bubble trays and sieve trays. A translation (13C) of a Russian study (22C) ofrers evidence that at high gas velocities (1.5 feet per second superficial and above) bubble trays have the higher mass transfer coefficients. At lower gas velocities, the sieve trays have the greater coefficient. Liquid mixing on both types of trays was followed by electrical conductivity measurements and studied by frequency response VOL. 52,
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Unit Operations Review liquid. This aids in understanding the wall effect. Pall rings should have better performance characteristics than Raschig rings because of the perforated wall and the internal ribs. Loading of tower packings was found (72C) to be due to the formation of standing waves on the liquid film a t a point of maximum vapor velocity. In packings characterized by high void volume, loading starts at the bottom.
laboratory Stills
COURTESY
U . S . STONEWARE
CO.
Pall rings are available in either metal (shown) or plastic
methods (8C). The data were correlated by a modified Peclet number which may be used to relate the over-all Murphree efficiency to the point efficiency. However, it has been shown (79C) that pulsations on a tray decrease back mixing and thus improve efficiency. T h e source of the pulsations has not been determined. They are acoustic in nature and are not observed at low gas velocities. Sieve trays have efficiencies between 30 and SOY6 over a wide range of operating conditions according to one study (77C) ; others (5C) presented data graphically. I t was observed (7C) that the entrainment from a bubble cap tray changed abruptly at a liquid viscosity of 5 cp. After studying slot submergence, it was recommended (24C) that caps be operated with 3.5 inches of clear liquid above the slots. T h e Rogers-Thiele equations for slot capacities were found to be 14% low for trapezoidal slot and 50% low for triangular slots (2K’). Triangular slots have maximum flexibility, while trapezoidal slots are better than rectangular slots in both capacity and flexibility. Reviews of design equations for several types of trays are available in the British literature (3C, 4C, 77C). I n the “ballast” tray, a valved sieve tray appears to have a wide operating range (26C), as does the Uniflux tray (2OC). T h e capacity of the latter is limited by a sharp increase in entrainment as vapor velocity is increased be)-ond the desirable maximum. More significant than tray type may be the method of evaluating the performance. I t was shown (9C) that the same experimental data may be interpreted to mean that column trays are 77y0 or 118% efficient, depending upon the source of the equilibria data used. The example used is a de-ethanizer column in which ideal equilibrium constants could be markedly in error.
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Foot valves for downcomers were proposed (28C) as a means of preventing vapor blowback. Level trays may not be worth the cost of leveling. Sieve trays out of level by 0.5 inch in a 66-inch diameter column did not affect the performance of the column (76C). Heat transfer to the liquid in a bubble column may be correlated by equations resembling the usual heat transfer relations between Nusselt number, Reynolds number, and Prandtl number. T h e exponents and coefficients determined (75C) are unusual. A horizontal fractionating device featuring a low pressure drop was studied (78C). The liquid is pumped over a series of expanded metal bats through which the vapors flow. The device may be useful when vapor pressure drop is critical. Packed towers are an alternate to plate towers as a means of fractionating. T h e selection of a packing has been reviewed (25C). Gauze packing (6C), Intalox (7C), and Pall rings (70C) were all recommended. I n general, the efficiency and mass transfer coefficients are affected by the ratio of vapor to liquid volume (7-IC)and the composition of the flowing streams. I t was suggested (27C) that the low efficiency frequently reported for columns operating near the limits of concentration for either component may be due to heat transfer lags. If the heat needed to change the temperature of one stream cannot be supplied by the other, an unequal situation may develop which limits the approach to equilibrium. Other studies of mass transfer coefficients were presented (ZC, 27C). Random packings do not always assume random distributions in a column. Raschig rings tend to lie on their sides (23C), a position which favors liquid distribution. HoLvever, next to a wall 7576 of the rings will not redistribute a
INDUSTRIAL AND ENGINEERING CHEMISTRY
Two equilibrium flash distillation stills were described in the past years’ literature. T h e first ( 2 0 ) , a package unit, operates at vacuum, while the second ( 5 0 ) operates at pressures up to 1000 p.s.i.a. T h e stills, charging 1 to 20 ml. per minute, may be brought to equilibrium in 20 to 40 minutes. Spinning element stills have become popular for high efficiency-low pressure drop units. -4 column 120 cm. long with a 1.3-cm. diameter and fitted with a Teflon spinner shows a throughput range from 100 to 1500 ml. per hour with the equivalent of 75 to 100 theoretical plates ( 6 0 ) . .4 spinning glass cylinder inside a stationary glass cylinder was studied as a means of separating the resistances of the phases ( 7 0 ) . T h e Oldershaw column performance as a function of reduced pressure was studied (30). T h e column showed a reduction in fractionating efficiency with decreased pressure, but the amount of reduction depended upon the foaming tendencies of the test system. Still heads were described ( 4 0 , 8 0 ) . The former study also noted that 1hexadecene isomerized to 2-hexadecene during a run. T h e isomerization makes it impossible to use refractive index as an analytical tool for systems containing this component. T h e effect of still head hold-up on reflux ratios for columns with intermittent take-off was analyzed ( 7 0 ) .
Vapor-liquid Equilibria Data on the equilibria that exist in distilling systems are fundamental to any analysis of the operation or design of equipment to perform the separation which is the purpose of the distillation. S o attempt has been made here to include all of the articles which may be interpreted as relating to basic data for distillation. Rather, the review covers certain equilibria that may be of particular interest and studies that help to illuminate the calculation of data from generalizations. A new text (8E),in three parts, which considers thermodynamics, laboratory technique, and sources of data, is excellent.
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an T h e theory of equilibria is not always simple. I t was suggested (76E) that a cluster theory may permit simpler relations for expressing the equilibria involved. A cluster is a molecular association which causes a. deviation from ideal behavior. Calculation then becomes a matter of an ideal calculation plus an empirical correction. Another proposal (27E) involves checking data for agreement with the Gibbs-Duhem equation through a “composition resolution test.’’ T h e test is based upon the prediction of a set of partial values from an excess property 0s. composition curve. T h e data are then compared Lvith calculated data from the predicted partial properties. I t would be most helpful if multicomponent system data could be predicted from the data of binary systems, and means for doing this have been proposed (3E: ZZE). T h e petroleum industry uses a n equilibrium constant K to calculate equilibria in multicomponent mixtures. Values of K are often used blindly with little regard to the applicability under the circumstances involved in the problem. T h e proper selection of I; values was discussed ( 7 E ) ,followed by a method (ZE) for the numerical determination of the constants. I; data were reported for aromatics (9E) and for acetylene (7029. T h e calculation of ideal K values was proposed (5E) as an extrapolation of a plot of In z (compressibility) us. pressure. Discrepancies in data calculated using K factors are frequently attributed to using the wrong “convergence pressure.” An empirical prediction of convergence pressures for closeboiling nonideal systems was offered ( 75E). Light hydrocarbon behavior was studied experimentally (78E, 79E): and partial enthalpy data were offered ( 7 E ) for saturated paraffins through the butanes. Correlations of azeotrope data are valuable in calculating many systems. O n e empirical correlation ( 7 X ) is an extension of the Meissner-Greenfield correlation. T h e new relation has a wider application and is simpler to use but is not as accurate as the original. Using a method based upon hlargules binary and ternary equations, a saddle point azeotrope was predicted (77E) and found in the system toluenepyridine-lbutanol. Other azeotropes in systems of ammonia and hydrocarbons \rere reported and correlated (73E, 74E). Changes in activity coefficient Lvith temperature may be related to the heat of mixing of the system. .4n equation for this was presented ( 6 E ) , and it \vas also taken into account with a modified Redlich and Kister equation ( 4 E ) . Heats of solution may be correlated using a
reference substance plot by another method (77E); extension to an enthalpycomposition diagram was demonstrated. Equilibrium with chemical reaction requires short heating time. Neither the \’an Laar nor the Redlich-Kister equations apply for systems of acetic acid and alcohols (ZOE).
literature Cited General (1A) Can. Chem. Processing 43, 141 (June
1959). (2.A) Chem. Eng. 66, 68 (Feh. 23, 1959). (3A) Chem. & Process Eng. 40, 239 (1959). (4A) Dechman, D. A., Van Winkle, M., IND.ENG. CHEM.51, 1015 (1959). (5.4) Harris, H. R., Petrol. Eng. 31, No. 5, ‘2-48 (1959). (6A) Hengst, K., Chem. f n g . Tech. 31, 425 11959). (7.4) Jones,’ H. H. M., Chem. 3 Process Eng. 40, 168 (1959). (8A) h-iebergall, W., Chem. Ing. Tech. 31, 155 (1959). (9A) Richardson, J. F., Chem. Eng. Sei. 10, 234 11959). (1o.Al ‘Stoop, M. L., IND. ENG. CHEM. 51, 71 A 1SeDtember 1959. Pt. I ) . (11.4) Wedber, W. O., ’Petrol. Refiner 38, No. 5, 187 (1959). Calculations and Theory
(1B) Amundson, N. R., Pontinen, A . J., Tierney, J. W., A.I.Ch.E. Journal 5, 295 11959). (2B) &it. Chem. Eng. 4, 327 (1959). (3B) Geddes, R . L., A.1.Ch.E. Journal 4, 389 (1958). (4B) Graven, R. G., Petrol. Refiner 38, No. 5, 209 (1959). (5B) Lvster, W. N.. Sullivan. S. L.. others, Ibid., 38, No.’ 6, 221; KO. 7, 151 No. 10. 139 11959). (6B) Maddox,‘ R . N., Erhar, J. H., Petrol. Engr. 31, No. 10, C-35 (1959). (7B) Surowiec, A . J., Petrol. Refiner 38, KO.5, 247 (1959). (8B) Wing, R . H., Chem. Eng. 6 6 , 185 (October 19, 1959). Still Design and Capacity
( I C ) Barker, P. E., Choudhury, M. H., Brit. Chem. Eng. 4 , 348 (1959). (2C) Dorweiler, V. P., Fahien, R . W., A.Z.CI1.E. Journal 5 , 139 (1959). (3C) Eduljee, H. E., Brit. Chem. Eng, 4, 24 (1959). (4C) Ibid., p. 320. (5C) Ellis, S. R . M., Moyade, H. K., Zbid.. 4, 342 (1959). (6Cl Ellis, S. R . M., Varjavandi, J., Chem. Process Eng. 39, 239 (1958). (7C) Field, J. A., Ibid., 40, 310 (1959). (8C) Gilbert, T. J., Chem. Eng. Sci. 10, 243 (1959). (9C) Gul
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(13C) ‘Jackson, J., Brit. Chem. Eng. 4 , 544 (1959). (14C) Klingenspor, H., Chem. Zng. Tech. 31. 598 11959’1. ( I j C i Kobe], H., Borchers, E., Muller, K.,Ibid., 30, 729 (1958). (16C) Lockwood, D. C., Glausser, W. E., Petrol. Rffiner 38, No. 9, 281 (1959). (17C) Majeweski, J., Brit. Chem. Eng. 4, 336 (1959).
Unit Operations Review
(18C) Markels, M., Jr., Drew, T. B., IND.ENG.CHEM.51, 619 (1959). (1%) McAllister, R. A , Plank, C. .4., A.I.Ch.E. Journal 4,282 (1958). (20‘2) Muller, H. M., Othmer, D. F., IND.ENG.CHEM.51, 625 (1959). (21C) Onda, K., Sada, E., Murase, Y . , A.I.Ch.E. Journal 5 , 235 (1959). (22C) Planovski, A. N., Matrosov, V. I., others, Khim. i Tekhnol. Tofiliv i ,Wasel 3, No. 3. 11958). (23C) Porter, K . E., Chem. 3 P~ocessEng. 40, 303 (1959). (24‘2) Prater, N. H.. Petrol. Refiner 38, No. 9, 251 (19591. (25C) Randall, D. G.. Chem. 3 Process Eng. 40, 306 (1959’3. (26C) Robin, B. J., Brit. Chem. Eng. 4, 351 11959’1. (27Ci ‘Sawistowski, H.. Smith. W.,IND. ENG.CHEM.51, 915 (1959). (28C) Sun, H. H., Chern. Eng. 66,162 (July 13. 1959). (29C) Ting, A. P., A.I.Ch.E. Journal 5, 271 (1959). Laboratory Stills
(1D) Chang, Y . C.. Fan, L. T., h a / . Chem. 31, 1121 (1959). (2D) Chem. Eng. 66, 92 (May 18, 1959). (3D) Ellis, S. R. M.. Contractor. R. M., J . Inst. Petrol. 45, 147 (1959). (4D) Hall, R. A , . .4nal. Chem. 31, 437 11959). (5D) Jentoft, R E.. Johnqon. J. F., IXD. ENG.CHEW51, 519 (1950). (6D) Jones, F. S.. Nerheim. A . G., Anal. Chem. 31, 1929 (1959’1. (7D) Macleod, K.,Matterson, K. J., Chem. Eng. Sci. 10, 254 (1959). (8D) Nerheim, A . G.. Anal. Chem. 31, 2114 (1959). Vapor-Liquid Equilibria
(1E) Adler, S. B., Palazzo, D. F., Chem. Eng. 6 6 , 95 (June 29, 1959). (2E) Zbid., 123 (July 27, 1959). (3E) Black. C., IND. EN(:. CHEM. 51, 211 (1959). (4E) Chao, K . C.. Zbid., 51, 93 (1959). (5E) Ehrett, U‘. E., TVeber, J. H., Hoffman. D. S.! Ibid., 51, 711 (1959). (6E’i Ellis, S. R. M., Razavipour, M., Chem. Eng. Sci. 11, 99 (1959). (7E)Gullv, A. J., Petrol. Engr. 31, No. 7, C-3 (19591. (8E) Hala, E., Pick, J.. others, “VapourLiquid Equilibrium,” Pergamon Press, New York, (1958). (9E) Hoffman: D. S.. ‘IVeber. J. H., Petrol. Re)ner 38, No. 7 . 137 (1959). (10E1 Ibid.,No. 9, 285 (1959). (11E) Hollo. J., Lengyel, T.. IND.LSG. CHEM.51, 957 (1959). (12E) Johvson, ’4. J., Madonis, J. A , , Can. J . Chem. Eng. 37, 71 (1959’3. (13E) Kav, W. B., Frish, H. A , , A.I.Ch.E. Journal 4, 293 (1958). (14E) Kav, W. B., Warzel: F. M., Zbid., 4 , 296 (1958). (15E) Leroir. J. M.. Zbid.. 4, 263 (19583. (16E) Lu, B. C. Y . , Li. J. C. M., Ting, T. I V . . IND.Eric. CHFM.51, 219 (1959). (17E’I Othmer, D. F.. Kowalski, R. C., Naphtali, L. M., Ibid.. 51, 89 (1959). (18E) Price, A . K.. Kobayshi, R., J . Chem. Fng. Data 4, 40 (1959). (19E) Rigas, T. J . . Mason, D. I:,, Thodos, G., Jbid.,4, 201 (1959). (20E) Rius, 4..Otero. J . L., Macarron, X.,C‘hem. Fn,p.Sci. 10, 105 (1959). (21E) Van Ness, H. C.. Zbid., 11, 118 ( 1 9 5 9 ) . (22E) Van Wijk. u’. R.. Bruijn, P. J., Goedkoop, H., Brit. Chrm. Eng. 4 , 328 (1959). VOL. 52,
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