UNIT O P E R A T I O N S
High Temperature Distillation
THE
past year was marked by a great interest in predicting the efficiency of bubble cap trays. Final reports of the five-year study sponsored by the American Institute of Chemical Engineers were made available as well as the results of some independent studies of the same subject. T h e investigations have illustrated the complexity of the situation but have not shown the path to designing higher efficiency plates.
General A review covering research and practice of Soviet engineers in distillation (7’4) gives many references not available in English. Most interesting is the similarity of problems and proposed solutions in Russia and in the United Statrs. The new crude distillation system installed by Magnolia Petroleum Co. a t Beaumont. Tex , has been described by Wharton and Hardin (4‘4). The operation features three-stage distillation including one atmospheric still and t\vo vacuum stills. Separation of boron isotopes by distillation was conducted by Standard Oil Co. (Indiana) from April 1945 to February 1946. Details of the process and equipment were finally released in 1958. Conn and Wolf (3ii) have described the distillation of the complex of dimethyl ether and boron trifluoride through eight columns in series to produce 95% boron-10 in a system having a n enrichment ratio of 1.016. Reflux ratios and equivalent plates for each column are given. Cost of a distillation system may be estimated from cost of the equipment with curves proposed by Bromberg ( 2 A ) . Cost is figured as the sum of equipment, erection, materials, and office costs. .Each of the latter three is plotted as a function of equipment cost. Column Efficiency, Operation, and Design The Research Committee of the American Institute of Chemical Engineers sponsored between July 1, 1952, and June 30, 1957, a n investigation into tray efficiencies in distillation columns a t the University of Delaware, University of Michigan, and North Carolina State College. From July 1, 1957, to June 30, 1958, the results were correlated and a method of predicting tray efficiencies was developed.
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b Engineers continued to diverge in their approach b y using computers to solve equations without simplifying assumptions and making further simplifying assumptions to aid in solving the equations. Either technique may be preferred for solving equations in a given problem. b The perforated plate received much theoretical attention. b Methods of calculating equilibria, either theoretical or experimental, were common. Several studies (2B> 3B, 72B-74BB; 27B, 228, 25B) based on lvork done during this study !\.ere presented a t the American Institute of Chemical Engineers’ December meeting in Cincinnati. Results of the work are available in a report of the distillation subcommittee (7B) and in the final report from the University of Delaware ( 7 7B). L2’orking essentially with cross-flo\v bubble-cap trays equipped with downcomers? this program developed separate correlations for each of four factors affecting magnitude of tray efficiency: rate of mass transfer in the gas phase, rate of mass transfer in the liquid phase, degree of liquid mixing on the bubble tray, and magnitude of liquid entrainment between trays. The four factors are then combined into a predicted tray efficiency. Unfortunately, the bubble-tray design manual (7B) presents the efficiency prediction method of a calculation form sheet giving the correlating equations with different constants and terms than are used in the discussion. Also, the manual presents the correlating equations without the data justifying the equation. Therefore, the reader cannot judge whether the extra terms are justified.
T. J. WALSH received his B.Ch.E. ( 1 939) and M.Ch.E. ( 1 941) from Rensselaer Polytechnic Institute and Ph.D. (1 949) from Case Institute of Technology where he i s professor of chemical engineering. Walsh is an associate of D. Q. Kern and Associates and a consultant for Glascote Products, Inc., and Thompson Products, Inc. He i s an ACS councilor from the Cleveland section and a member of the AIChE, ASEE, and ASLE.
INDUSTRIAL AND ENGINEERING CHEMISTRY
A better presentation of the experimental program related to operating the tray design variables for gas phasecontrolling and liquid phase-controlling systems is given in the University of Delaware report. The equations used are those given in the text of the design manual but not in the recommended efficiency prediction method. Results of the prediction technique are compared with industrial data on five columns. Several other studies have also been directed a t the solution of this or related problems. Among these, Johnson and Bowman (76B) were concerned with mass transfer in a bubble column. The authors found that the liquid film coefficient varies directly with gas rate, is independent of liquid rate. and almost independent of the seal height. Zuiderweg and Harmens (27B)studied the influence of surface phenomena on the performance of distillation columns. LVhen contact occurred bet\veen films, mass transfer increased if surface tension increased down the column. When contact occurred in a droplet dispersion, little effect of surface tension was noted. The entrainment effect was studied by Eduljee (8B) and Fair and Matthews (9B). Eduljee relates entrainment to clear height (foam free) between plates, viscosity, surface tension, slot velocity density of vapor, and pressure drop. Fair and Matthew’ relation concerns velocity of both liquid and vapor and density of each phase. Classically, more interest has been devoted to pressure drop of bubble trays than to efficiency. A new correlation for pressure drop was presented by Welch (26B) with graphs to evaluate the constants in pressure drop equations. The behavior of perforated trays has been studied in several details. Foss, Gerster. and Pigford (7OB) have reported the effect of liquid mixing on the performance of sieve trays having constant hole diameter. The conductivity of liquid containing salt was used to study the liquid mixing. Johnson and Marangozis (77B) concluded that mixing on a perforated tray is due mainly to splashing of the liquid. Because their performance results are similar to those obtained with bubble cap trays, they suggest that a similar splashing causes the bulk of liquid mixing. Performance of the perforated tray was shown by McAllister, McGinnis,
Five-year study on tray efficiencies is completed at three universities
and Plank (79B) as pressure drop cs. gas rate for various values of liquid rate, plate thickness, and hole diameter. Plate behavior may be considered in four zones: raining, weeping, stable, and oscillating, as the gas rate increases. General design of plates is also of interest. Roach and Swanton (238) directed attention to problems connected with selection of plates for good column operation, \vhile Robin (24B) discussed mechanical design problems. Sieve type columns may be designed using standard column relations, but nomographs presented by Huang and Hodson (75B) will simplify the calculations. Tray selection is based upon building of liquid in the downcomer. General considerations for the design of sieve trays tvere reviewed by Eduljee (7B). Myers (2OB) reported on a pilot plant perforated plate column having plates 1 inch in diameter. Leva (48,788) has offered a new style of tray particularly suited for low liquid rates. The unit, a horizontal surface contact tray with downcomers, is recommended for vacuum distillations with low liquid rates. The West distillation plate reported by Dummett (6B) is self adjusting to vapor load. .4 packed column design correlating vapor and liquid rates, vapor and liquid densities, liquid viscosity, packing free space, and packing surface has been offered by Czermann, Gyokhegyi, and Hay (5B). Calculations
The availability of computers to reduce the effort necessary to solve a distillation problem has led to several studies on utilizing these operational tools. Application of the IBM 650 by relaxaLion techniques was suggested by Rose: Sweeny, and Schrodt (73C). The IBM 704, using Sewton’s method for 20 components and any number of plates, \vas studied by Greenstadt, Bard? and X-lorse (6C). Three specifications must be fixed and these must not conflict. Equilibria data may be expressed as polynomials or in the Benedict-LVebb-Rubin equations. The Univac scientific computer Model 1103 with a matrix solution approach was proposed by Amundson and Pontinen (2C). In the successive examples shown, calculations are made for constant molal overflow with five components, for nonconstant molal overflow with five components in a 15-tray column, and finally for a 51-tray column. Calculation of transient column behavior may be performed by Laplace transform, perturbation methods, graphical analysis! analog computer, or digital
computer. After studying each method, Rosenbrock (74C) has recommended the digital computer. Hoirever, Maddox (77C) noted that the time and cost for a computer solution are equivalent to the time and cost of a manual solution for a given distillation problem. He recommended that judgment be used before putting a problem on a computer and that short-cut methods be employed to rough out areas of interest rather than feed the computer with a general problem. The successful use of a computer in selecting a preferred column design was described by ,4lbright ( I C ) . The computer was programmed to investigate cost breakdoivn for varied design combinations, and the most economical was selected. Happel (7C) presented plots for solution of similar problems Lvithout a computer. Based on a n evaluated cost factor, xrhich includes investment costs, steam costs, hours of operation and plate efficiency, the ratio of optimum trays to minimum theoretical trays and optimum reflux to minimum reflux was determined graphically. The correlation calls for more trays and a lower reflux ratio than has been customary in the past. The use of a computer to predict vapor-liquid equilibria data based on deviations from the molal average boiling point was suggested by Canjar, Ford, and Sebulsky (5C). The method relates the activity coefficient of a component to the spread benveen molal average boiling point of the system and boiling point of the component. Most other calculation technique proposals relate to modification of existing techniques. Horvath and Schubert (8C) recommended that a log-log plot equivalent to the McCabe-Thiele approach be inverted lvhen dealing \vith a n almost pure overhead product. \Vim (17C) proposed that the relative volatility relation be replaced by a t\vo-parameter expression of the type 0 = ‘xI)/ ( y / x 2 ) b where p and 6 are arbitrary constants while x and j are the mole fractions of components 1 and 2 in the liquid and vapor phases, respectively. Using this form of the expression, he has derived the equivalent of the Fenske equation for minimum number of theoretical plates and applied the relation to nonkey components of a multicomponent mixture. Vapor-liquid relationships of the zero reflux equilibrium flash separation as performed industrially in a pipe still are usually calculated by trial and error in a sequence solution that does not always lend itself to ready adjustment of values. Several studies each year
suggest modifications of the calculation aimed a t reducing the number of trials. Batchelor (3C) has predicted that limiting series solutions will converge rapidly near the dew and bubble points of a system. Lockhart and McHenry (IOC) proposed that a niulticomponent mixture be handled as a binary composed of one component consisting of all material Lvhich tends to the vapor state and another component consisting of everything that tends to remain in the liquid. Salmon (75C) has offered a new approach for the particular situation involving either a gas make-up stream or a vapor phase recycle. Batch distillation separations for ternary mixtures may be calculated as two successive Rayleigh distillations provided the still is capable of separating each component with a purity of 9570 in a binary split. Rose and Sweeny (72C) reported this calculation. Husain (9C) suggested that a desired fraction be considered as a pure component and the McCabe-Thiele approach be used to calculate the column needed. Buiten ( 4 C ) , studying the removal of a light component by batch distillation, found that high reflux ratios are required and noted that batch rectification is economically inferior to continuous rectification when high purity is required for a heavy component. Van Lt‘ijk and Hruijn (76C) studied heat input to a column containing infinite plates. Reflux varied from plate to plate but “generalized liquid and vapor rates” remained constant. Instrumentation and Control
An instrumentation and control system for columns, recornmended by TVilliams ( d o ) , is based on making a specification overhead product. Essentially the feed flow is controlled, and steam to the reboiler is proportioned to the feed. Reflux rate is flow-controlled Lvith the control point reset by product composition. This system is reported to give maximum operating stability, and the need for analytical F,ensitivity and rapid analysis is stressed. T o meet the analytical requirements of column controllers, L\.’herry and Berger (2D) reported on the use of infrared for analysis of isobutane in the presence of ethane, propane, and nbutane and the use of gaseous refractive index for the analysis of a pentane cut. The column had 5 0 tray-s each 13 feet in diameter. The time for response to a change in control setting \vas 35 minutes after a 4-minute dead time. Manipulation of steam flow rate irith cascade controls was rccommendcd.
VOL. 51, NO. 3, PART II
0
MARCH 1959
371
UNIT OPERATIONS Hull ( 7 0 ) proposed that total gamma count can be used as a n analytical tool in distillation around a refinery. T e n millicuries of cobalt-60 are added to a lubricating oil still. Entrainment of nonvolatile material in the overhead and side streams may be followed by the radiation from these streams. Wilkinson and Armstrong (30) discussed the transient response of a plate column to change in feed composition. An approximate solution of a simplified equation was suggested. T h e solution follows the column for the initial stages of the response for a two-component system.
Vapor-liquid Equilibria X‘ecessary for distillation calculations are the vapor-liquid equilibria data relating to the system under consideration. These data may be determined experimentally or they may be calculated. Two new equilibrium cells ivere proposed during 1958. Rigas. Mason, and Thodos (9E) described a variable volume cell that handles between 18 and 120 ml. of solution. T h e cell reaches equilibrium in 2 to 4 hours, and 5 hours are sufficient for a n equilibrium run. Rock (70E)described a new recirculation type still. I n calculations of equilibria. equilibrium constants (K’s) are frequently used. Black (7E) has suggested that they can be evaluated from a modified Berthelot vapor pressure equation. T h e correlation is good u p to the point of a minimum K but diverges from experimental values above this point. Lenoir and White (6E) have predicted convergence pressure (used in correlating value of K) by assuming a fictitious light and heavy component. thus treating a multicomponent mixture as a binary system. The ternary Margules equations seem to be more popular than the van Laar equations for correlating activity coefficients of ternary systems. Tierney ( 7 7E) recommended equations for calculating either set of constants. Dakshinamurty, Rao, Acharya. and Rao ( 4 E )found that the system benzene-cyclohexane-methyl isobutyl ketone correlates with the threesuffix Margules equations. No ternarv azeotrope was found. Myers ( 8 E ) used three-suffix Margules equations to fit data of the toluene-heptanecvclohexane system. and Murti and Van Winkle ( 7 E ) used the same equations for the n-octaneethylbenzene-Cellosolve system. In the latter system also, no ternary azeotrope was found, although all the binary systems were nonideal. Ewanchyna and Ambridge ( 5 E ) used three-suffix Margules and two-suffix van Laar equations in correlating data for
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systems of Cq hydrocarbons with acetone and water. Black ( 2 E ) recommended a modified V a n Laar equation with theoretical coefficients based on binary, systems while Chao and Hougen (3E) correlated activity coefficients from Redlick-Kister equations modified to fit the Gibbs-Duhem law in the systems ethyl acetate-benzene-cyclohexane.
literature Cited General (IA) Aerov, M. E., Brit. Chem. Eng. 3, 486 (1958). (2A) Bomberg, I., Petrol. Rejner 37, No. 12, 141 (1958). (3Ai Conn. A. L.. Wolf, J. E.. IND.END. CLEM.50,1231 ’(1958): (4A) Wharton, G. W., Hardin, E. P., Petrol. Refiner 37, No. 10, 105 (1958). ~
Column Efficiency, Operation, and Design
(1B) Am. Inst. Chem. Engrs., New York, N. Y . , Distillation Subcommittee, A.1.Ch.E. Research Committee, “Bubble Tray Design Manual-Prediction of Fractionation Efficiency,” 1958. (2B) .4shby, B. B., Begley, J., Gordon, K. F., Williams, G. B., Am. Inst. Chem. Engrs. Ann. Meeting, Cincinnati, Ohio, December 1958. (3B) Begley, J., Williams, G. B., Gordon, K. F., Zbid. (4B) Chem. Bng. News 36, 30 (Dec. 22, 1958). (5B) Czermann, J. J., Gyokhegyi, S. L., Hay, J. J., Petrol. Refiner 37, No. 4, 165 (1958). (6B) Dummett, G. A , , Brit. Chem. Eng. 3, 563 11958’1. (7Bj-Eduljee; H. E., Ibid., 15 (1958). (8B) Zbid., p. 474. (9B) Fair, J . R., Matthews, R. L., Petrol. Refiner 37, No. 4, 153 (1958). (10B) Foss, A. S., Gerster, J. A., Pigford, R. L.. A.Z.Ch.E. Journal 4. 231 (1958). ( l l B ) Gerster, J. A., Hill, A. B., Hochgraf, N. N., Robinson, D. G., “Tray Efficiencies in Distillation Columns,” Report to Research Committee, Am. Inst. Chem. Engrs., New York, N. Y., 1958. (12B) Gerster, J. A., Hochgraf, N. N., Schiven, L. E., D’Evadio, J. E., Rosenhouse, H., Am. Inst. Chem. Engrs. Ann. Meeting, Cincinnati, Ohio, December 1958. (13B) Gerster, J. A., Laverty, A. G., King, M., Am. Inst. Chem. Engrs., Cincinnati, Ohio, 1958. (14B) Hochgraf, N. N., Hill, A. B., Dillon, B., Hachamack, R., Wallis, F., Gerster, J. A., Zbid. (15B) Huang, C. J., Hodson, J. R., Petro!. Rt$ner 37, No. 2, 105 (1958). (16B) Johnson, A. l . , Bowman, C. W., Can. J . Chem. Eng. 36, 253 (1958). (17B) Johnson, A . I., Marangozis, J., Zbid., 161 (1958). (18B) Leva, M., Am. Inst. Chem. Engrs. Ann, Meeting, Cincinnati, Ohio, December 1958. (19B) McAllister, R. A,, McGinnis, P. H., Plank, C. A., Chem. Ene. Sci. 9, 25 (1958). (20B) Myers, H. S., IND. END. CHEM. 50, 1671 (1958). (21B) Plank, C. A , , Schoenborn, E. M., Am. Inst. Chem. Engrs. Ann. Meeting, Cincinnati, Ohio, December 1958.
INDUSTRIAL AND ENGINEERING CHEMISTRY
(22B) Plank, C. A,, Window, C. E., Schoenborn, E. M., Am. Inst. Chem. Engrs., Cincinnati, Ohio, 1958. (23B) Roach, M. L., Swanton, W. F., IND.ENC.CHEM.50. 57.4 IJulv 1958). (24B) Robin, B. J., Brit. Chem. Eng. 3, 480 (1958). (25B) Robinson, D. G., Dickens, E. S., Gerster, J. A., Hill, A. B., Am. Inst. Chem. Eners. Ann. Meeting. -. Cincinnati, Ohio, December 1958. f26B) Welch. H. T.. Petrol. Refiner 37, No. 8, 127’(1958). ’ (27B) Zuiderweg, F. J., Harmens, A., Chem. Eng. Sci. 9, 89 (1958).
Calculations (1C) Albright, M. A , , Petrol. Refiner 37, No. 12, 111 (1958). (2C) Amundson, N. R., Pontinen, A . J., IND.ENG.CHEM.50, 730 (1958). (3C) Batchelor, J. B., Petrol. Refiner 36, No. 10, 113 (1957). (4C) Buiten, J., Chem. Eng. Sci. 9, 104 (1958). (5C) Canjar, L. N., Ford, H. B., Sebulsky, R. T., Petrol. Refiner 36, No. 10, 135 (I 957). (6C) Greenstadt, J., Bard, Y., Morse, B., IND.ENG.CHEM.50, 1644 (1958). (7C) Happel, J., Chem. Eng. 6 5 , No. 14, 144 (1958). (8C) Horvath, P. J., Schubert, R. F., Zbid., No. 3, 129 (1958). (9C) Husain, A,, Brit. Chem. Eng. 3, 668 (1958). (IOC) Lockhart, F. J., McHenry, R. J., Petrol. Refiner 37, No. 3, 209 (1958). (11C) Maddox, R. N., Petrol. Engineer 30, c-15 (April 1958). Sweeny, R. F., IND.ENG. (12C) Rose, .4., CHEM.50, 1687 (1958). (13C) Rose, .4., Sweeny, R . F., Schrodt, V. N., Ibid., 50, 737 (1958). (14C) Rosenbrock, H. H., Brit. Chern. Eng. 3, 364 (1958). (15C) Salmon, R., Petrol. Refiner 36, NO. 12, 133 (1958). (16C) Van Wijk, W. R., Bruijn, P. J., Chem. Eng. Sci. 8, 225 (1958). (17C) Winn, F. W., Petrol. Refiner 37, No. 5, 217 (1958). Instrumentation and Control (1D) Hull, D. E., IND. ENG. CHEM.50,
Vapor-Liquid Equilibria (1E) Black, C., IND. ENG. CHEM. 50, 191 11958’1. (2E) Zbzd., p. 403. (3E) Chao, K. C., Hougen, 0. A , , Chem. Eng. Scz. 7, 246 (1957). (4E) Dakshinamurty, P., Rao, G. J., Acharya, M. V. R., Rao, C. V., Zbtd., 9, 69 (1958). (5E) Ewanchyna, J. E., Ambridge, C., Can. J . Chem. Eng. 36, 19 (1958). (6E) Lenoir, J. M., White, G. A , , Petrol. Refiner 37, No. 3, 173 (1958). (7E) Murti, P. S., Van Winkle, M., A.1.Ch.E. Journal 3. 517 (1957). .... (8E) Myers, H. S., Zbid., 467 (1957). (9E) Rigas, T. J., Mason, D. F., Thodos, G., IND.ENG.CHEM.50, 1297 (1958). (10E) Rock, H., Brit. Chem. Eng. 3, 20 (1958). (IIE) Tierney, J . W.,IND. ENG. CHEM. 50, 707 (1958). ~
