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(I/EqUnit Operations Review
High Temperature Distillation by Thomas J. Walsh, Case Institute of Technology, Cleveland, Ohio
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Digital computers permit precise solution of complex distillation problems Equilibrium constants vary with the total system as well as with pressure, temperature, and component
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H temperature distillation continued to develop without startling changes during 1960. More complex problems are being solved by use of digital computers, and more precise calculations indicate the need for better vapor-liquid equilibria data. A computer is being used on a shared time basis to control a crude still a t the Standard Oil Co. of Indiana. T h e economics of the operation are being studied. Vapor pumps are being incorporated into fractionating equipment to reduce the pressure drop through columns. At least two schemes have been proposed. Laboratory distillation equipment must share its importance with vapor phase chromatography. Data may be obtained more rapidly for several systems with chromatographic analysis. This review covers most journals to Dec. 1, 1960, and selected journals through the December issue.
Commercial Equipment The practical aspects of distillation (9.4) include how to pipe u p the feed, exchangers, and services; how to specify d a t a ; how to select the column controls; and how to determine the costs. A proper manner to specify distillation trays is the subject of a series of articles. General specifications ( I A ) , mechanical specifications ( 7 4 , bubble cap trays ( 7 7 A ) , sieve trays ( 1 3 A ) , valve trays (76A), and staged-opening valve trays (2OA) are separately discussed. Column costs may be estimated by summing the cost items ( 7 4 4 ) . Shell, trays, foundation, and installation costs figure prominently in the calculation. Controls should be considered in terms of the variables to be manipulated and the flexibility required. An optimum control system may be established if the column has a feed with a fixed rate, composition, quality, and pressure ( 7 9 A ) . However, consideration of the distillation equipment as part of a
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process system requires variable capacity and flexibility ( 6 A ) . More plates with lower reflux may be favorable. T h e design should stress maintenance and frequently omit the structural frame of the columns. The transient response of a column to changes in feed composition may be calculated from pseudoequilibrium lines or from plate efficiencies with an accuracy of 20% (27'4). Plate efficiencies have held a position of interest for several years. The final data which were incorporated into the A.1.Ch.E. design manual were reported ( 7 5 A ) . These include the systems: acetone-water, methyl ethyl ketonewater, methanol-toluene, and carbon dioxide-air-water. Multicomponent systems may have plate efficiencies that vary greatly from the efficiency as measured by a binary system ( 7 8 A ) . Furthermore, the efficiency as determined by different pairs of components may be quite different. Study of a ternary system, gas phase controlling, a t a steady state may be used as a first approximation to the multicomponent system. When the plate efficiency of all binary pairs is high, the multicomponent efficiencies will be equal and high. When two components are similar and one is different, the efficiencies of the similar components will be low while the efficiency of the dissimilar component will be close to that of the binary. If one species in a ternary system is dilute in both liquid and gas phases, its efficiency may differ greatly from the binary value with either of the other components. Most still designers use a n empirical relation between cap size and column diameter that produces workable designs based on experience. Greater care should be given to the selection of bubble cap sizes ( 8 A ) ,as this affects the behavior of most tray columns. A trial and error design with skillful selection of dimensions is still necessary for the
INDUSTRIAL AND ENGINEERING CHEMISTRY
optimum tower design. Retraving of dubble cap trays can b e greatly speeded u p by proper planning ( 7 7 A ) ; 22 trays in a 72-inch-diameter still were replaced in 3.9 days after experience was gained doing a similar job in 7 days. Sieve trays have always enjoyed a certain amount of popularity. Data on entrainment ( 5 A ) and efficiency (70A) are available. Hole sizes vary from 0.25 to 0.5 inch. Entrainment increases with weir height, vapor velocity, and hole size and decreases with plate spacing. Efficiency varies from 82 to 105%. The efficiency data confirm the theory of Gerster. Level sieve trays are not an absolute necessity a t high vapor loads (7.24). T h e Turbogrid tray is a modified sieve tray Optimum operation of a Turbogrid tray occurs when ( V / F ) = 6 9 (L/G)-'33 ( 4 A ) . I/ is the optimum superficial vapor velocity (meters per second). F is the percentage free cross section, pa is the density of the gas, po is the density of air at 20' C. and 1 a t m . pressure, L is the liquid flow rate, and G is the vapor flow rate. Packed columns are also used for many distillations. T h e mass transfer efficiency of columns packed with MCMahon packing, Berl saddles, Raschig rings, and Intalox packings may be calculated ( Z A , 3 A ) as a function of liquid properties, column dimensions, and flow rates.
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Calculations and Theory Calculations involved in distillation may be extremely complex. T h e trend in recent years has been to program the problem for solution on a digital computer. A subroutine for evaluation of vapor-liquid equilibria for nonideal systems (8B) makes use of three-suffix Margules equations, T h e routine requires 17 arithmetic operations and 21 coefficients for a ternary system. Pseudo
an volatilities set by the internal performance of the distillation equipment (9B) between the overhead and the bottom product permit rapid convergence of iterative solutions to multicomponent continuous distillation problems. Batch distillation problems, which are essentially unsteady state, may be studied (3B. 5B) assuming constant reflux ratio and constant hold-up for constant time intervals or by trial and error for successive plates. Equilibrium flash distillation calculations may be solved by machine integration (7B). T h e method is adapted to the evaluation of the entire flash curve and is suitable for interpolation or use as a control model. At the other extreme, simplification of solution is still desired. Charts for ethylene-methane separations (7B) permit ready analysis of this important operation. A general equation for equilibrium stage processes (70B)is an extension of the well-known Kremser equation. Other approaches of interest a r e : Assume total reflux, evaluate the column, and study the deviations that occur with finite reflux ( 6 B ) ; or assume that non-key components approach limiting values in each section of the tower ( 4 B ) . Batch fractionation may also be calculated by determining a separation factor (2B). T h e separation tactor is very similar to the relative volatility.
Azeotropic and Extractive Distillation T h e search for extractive distillation solvents is not always successful. Perfluorotributylamine is not as effective for the separation of aromatics and paraffins as was theoretically predicted (3C). In separating paraffins from olefins. 33 solvents showed none to be particularly effective (7C). However. hydroxy and diamino compounds should be avoided. Selectivity decreased with increasing temperature and decreased most with the most promising solvents. Azeotropic distillation is also not promising as a method of fractionating 2-methylbutanol from 3-methylbutanol (5C). Ten azeotrope formers, including paraffins. aromatics, haloaromatics. ketones. and nitrogen compounds. were studied. Gas chromatography offers a means of evaluating a limiting relative volatility for infinite solvent. The partitioning liquid functions as the solvent (4C). Results are presented for paraffincycloparaffin, paraffin-aromatic. and for cycloparaffin-aromatic systems. Liquid platinum serves a similar function in the vacuum distillation of oxygen from yttrium (ZC). Mild reducing agents. such as NaN02, offer a limiting case in
reducing the volatility of ruthenium during the distillation of “ 0 3 (6C). T h e latter case may actually be a shift in relative volatility caused by chemical reaction.
laboratory Equipment and Techniques Most laboratory studies involve small or moderate size distillation equipment. Equilibrium stills are frequently of this type. .4 “hypodermic still” which depends upon the establishment of equilibrium between liquid and vapor a t a boiling interface permits taking small samples of vapor and liquid within 1 cm. of each other (5B). Eight to 10 points may be determined per day. A unit with interchangeable accessories, pot heater, Cottrell heater, external vaporizer, and aids for handling partially miscible systems has been described ( 9 0 ) . T h e internal heater performed better than the Cottrell or the external vaporizer. Data obtained with this still for the systems of formic acid, acetic acid, water, and chloroform as binary pairs are available ( 7 0 ) . A vapor recirculation equilibrium still for miscible and immiscible systems uses a pump to keep the condensate mixed (80). T h e unit is constructed from Schedule 40 pipe and tested with alcohol-water systems. A liquid recirculating equilibrium still uses a stirrer in the boiler to achieve mixing (30). This still will be subject to error if the recirculated liquid enters the stirrer directly. Data on three of the four systems reported for testing the still show this type of shift from other data for the same system. Micro columns frequently achieve intimate contact of vapor and liquid through spinning bands in the column. Band wobble may be eliminated by use of a single spider a t the bottom of the column ( 7 0 0 ) . Leaks may be eliminated by special fittings for the column head and flash (70). Polymer trays, either sieve type or bubble cap tvpe, which expand against the column walls (60) can simplify the construction of small columns. The trays are assembled as a cartridge and inserted into the column body. Either pressure or chemical reaction can cause the trays to expand to the Ivalls, restricting leakage. IVire mesh trays are reported for the separation of hydrogen isotopes ( 4 0 ) . This is not exactlv high temperature, but otherwise the operation is similar to normal distillation. T h e tray efficiency, Eo, was correlated with vapor velocity. u (feet per second), as In Eo = - 0 . 1 0 0 ~
+ 4.122.
Unit Operations Review
Chromatography may eventually make analytical distillations obsolete. A column packed with silicone on insulating brick with a programmed temperature drive (20) makes the equivalent of a TBP distillation on compounds boiling u p to 400’ C. in 30 minutes.
Vapor-liquid Equilibria With the application of computers to the solution of distillation problems arises the problem of evaluating vaporliquid equilibria data in a form suitable for the computer interpretation. T h e equilibrium constant, K , is a convenient manner of expressing the data for light hydrocarbons. Unfortunately, the simple concept of equilibrium constant is unsatisfactory in that the constant is a function of composition of the entire system rather than of the component with attending temperature and pressure. Machine evaluation of K may be done using the Edmister-Ruby correlation ( 7 E ) , the concept of solubility parameter (77E). or the Benedict-WebbRubin (BWR) equations (74E, 78E). Results are poorest when hydrogen is present and equilibrium data are required a t low temperature (20E). Correlation can be improved if y in the BWR equation is considered a function of the temperature (74E). Methane introduces errors u p to 95% in calculated equilibria of propane a t -200’ F. (78E). Special values of K to be used in the presence of hydrogen may help solve the problem (76E). At higher temperatures, the evaluation of equilibria data may be aided by charts giving K for 58 compounds as a function of temperaturr, pressure, and convergence pressure ( 2 E ) . T h e zygograph is again presented as a handy method of evaluating equilibrium compositions (22E). This semicircular nomograph now has a square grid to extend its range. T h e thermodynamic consistency of reported equilibria data is not always good, especially when one component is present above its critical temperature. T h e consistency may be checked using the Gibbs-Duhem relationship in a n integrated form (7E). The usual relation amounts to two terms of the series integration, whereas the third term may be significant. Correlation of data is frequently done through integrated equations of other forms. The Van Laar equations correlate the data on toluene-p-cresol a t 760 m m . of mercury pressure (72E); whereas the Margules equations are used to correlate the data for ethylbenzene - ethylcyclohexane - hexylene glycol a t 400 mm. of mercury (79E), and four-su& Margules equations are VOL. 53, NO. 3
MARCH 1961
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required a t 100 m m . of mercury for cyclohexanol - phenol - cychhexanone combination (3E). T h e gas chromatograph may be csed to measure the equilibrium constant (7315). Data may also be obtained by bringing vapor and liquid to equilibrium through partial condensation in a packed column ( 9 E ) . Samples are taken simultaneously of both liquid and vapor. Data may be calculated frcm constants of the -4ntoine equation (8E).or ternary data may be predicted from binary data ( 7 0 E ) . T h e minimum data necessary to predict a ternary azeotrope are the boiling points of the three pure components. two azeotrope points, a n d one point on the third binary. T h e salt effect is similar to the use of a n extractive distillation solvent. For saturated systems with no chemical effect and salt-free ideality. the salt eflect may be e5timated with an Othmer-type plot ( 7 7 E ) . An excellent theoretical and expcrimrntal analysis of the salt effrct shoivs it to be effective in alcohol-nater separations ( 6 E ) . T h e salt should dissolve in both ingredients, but to a varying amount. T h e system diethylene gl>-col-divinyl ether-diethylene glycol diethyl ether ma!- be used for testing columns having an expected 1 5 to 20 theoretical plates a t 10 m m . of mercur); absolute pressure (5.E). T h e boiling point is about 73’ C.. the relative volatility about 1.38. and analysis is by refractive index. T h e azeotrope composition of the system water-sec-butvl alcch-I-benzene (8.63. 3.82, 85.557,. respxtively) is n c t shifted greatly by pressure variation betlveen 200 and 760 m m . of mercury absolute pressure ( Y E ) . Other data a t subatomospheric pressurcs are reported for the ethyl alcohol-benzene-n-heptane system 175E), and related data a t 1 atm. are reported for the ethyl alcoholbenzene - n-hexane - methylcyclopentane system (.?I@. literature Cited Commercial Equipment (1A) Atkins. G. T., Petrol. R@ner 39, No. 8. 86 (1960). (2A) Cornell, D.. Knapp, W. G., Fair, J. R.. Chem. Eng. Progr. 56, No. 7, 68 (1 960).
(3A) 16;d.,No. 8: 49 (1960). (4A) Foldes, P., Brit. Chem. Eng. 5, 498 (1960). (5.4) Friend, L.. Lemieux, E. J.. Schreiner, W. C.. Chem. Eng. 67, No. 22, 101 (1960).
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(6A) Fryback, M. G., Hufnagel, J. A , , IND. ENG.CHEM.52, 654 (1960). (7A) Glitsch. H. C.: Petrol. ReJiner 39, No. 8, 91 (1960). (EA) Gyokhegyi, L. S., Hay, J. J., Czermann, J. J.. Ibid.:39, No. 5, 201 (1960). (9.A) Haines, H. W., Jr., IKD.ENG.CHEM. 52, 662 (1960). (1OA) Hay, .J. M.. .Johnson, A. I.. A.I.Ch.E. Journal 6, 373 (1960). (11A) McClain. R. \l’.* Petrol. Refiner 39, No. 8, 92 (1960). (12A) Melichar, B., Brit. Chem. E q . 5, 723 (1960). (13A) Pat&. B. A , . Pritchard. B. L., Jr., Petrol. Rpjner 39, No. 8, 95 (1960). (14.4) Prater, N. H.. Antonacci. D. W.. Ibid.,3 9 , N o . 7 . 119 (1960). (15-4) Schoenborn. E. M., Plank, C. A , , Winslokv. C. E.. A.I. Ch.E. Research Comm.. Final Report. from North Carolina State College. 1959. (164) Thrift. G. C.. Pdrol. Refiner 39, No. 8. 93 (1960). (17.4) Tipton. J.. Chem. Eng. 6 7 , No. 15, 134 (1960). (18.4) Tour. H. L.. Burchard. .I. K., ‘4.I.Ch.E. Journal 6, 202 (1960). (19.4) Wil!iams. T. J., Chm. C i g . 6 7 , No. 15. 119 (1960). (20.4) Winn. F. tV,; Pe/rol. Refiner 39, No. 10, 145 (1960). (21.4) tl’ood. R. M.. Armstrong. LV. D.. Chpm. En,?. Sci. 12, 272 (1960). Calculations and Theory (1B) Brnnrtt. C . -4..Brasket. C. .J.. Tierney. .I. b’..A.I.Ch.B. Journal 6 , 67 (1960). (2B) Briggs. D. R. H.. Ll’addinyton. t V , . McNeil. D.. IND.EXG. CHEM.52, 145 (1960). (3B) Danly. D. I:.. Huckaba. E. C.. Chrm. Erif. Progr. 56, No. 3. 86 (1960). (4B) Henqstebeck. R. .J.. Chein. En?. Progr. S’mposiiim Ser. 5 5 , h-0. 21, 9 (1959). (5B) Huckaba, C . E.. Danly. D. E.. A.I.Ch.E. Journal 6, 335 (1960). (6B) Lyster. !\’. K., Sullivan. S . I... .Jr.. others. Petrol. Rejrfinrr 39, No. 8. 121 (1960). (7B) Mason. LV, 4..Ibid.. 39, S o . 5. 233 (1960). (8B) O’Erien. Y. G.. Turnrr. R. L., Chem. Eng. Progr. Symposiuni ,Ser. 5 6 , No. 31. 28 (1960). (OB) Peiscr. A. M.. Chcm. En?. 6 7 , No. 14, 129 (1960). (10B) Smith. B. D.. Brinkley. \V. K., A.I.Ch.E. Journal 6, 446 (1960). Azeotropic and Extractive Distillation (1C) Gerster. J. X.. Gorton. J . .A,> Eklund. R. B., J . Cheni. €3 Eng. Data 5, 423 (1960). (2C) Horrigan. V. M.. Fassel. V. L4., Goetzinger, .I. t\’., Anal. Chem. 32, 787 (\ I- .960) -
~
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(3C) Kyle, B. G.. Tetlow. N. J.. J . Chem. 3 En?. Data 5, 275 (1960). (4C) Porter, R . S.. Johnson. J. F., IND. ESG. CHEM.52, 691 (1960).
INDUSTRIAL AND ENGINEERING CHEMISTRY
(5‘2) Terry. T. D.. Kepner, R . E.. \Vebb, A. D.. J . Chem. t 3 Eng. Data 5, 403 (1960). (6C) Wilson, .4. S., Ibid., 5, 521 (1960). Laboratory Equipment a n d Techniques (ID) Conti. J. J., Othmer, D. F., Gilmont, R.. J . Chem. t Y Ene. Data 5. 301 11960). (2D)’ Eggerstsen, F? T., G;oennlnes. ’S., Holst. J. J., Anal. Chem. 32, 904 (1960). (3D) Ellis, S. R. M.. Garbrtt, R. D., IND.EKG.CHEM.52, 38fi (1960). (4D) Flynn, T. M., Chem. En,?. Protr. 56, No. 3, 37 (1960). (5D) Howath. P. J., Chem. En!. 67, No. 15, 142 (19601. (6D) Kilgalion: J. D.. Streight. H. R. L., Can. J . Chem. En,p. 38, 89 (1960). (7D) Nerheim, A. G.! Anal. Chetn. 31, 2114 (1960). (8D) Orr, V.. Coates, J., IND.Exc. CHEM. 52, 27 (1960). (OD) Othmer. D. F., Gilmont. R., Conti, J. J., Ibid..52, 625 (1960). (10D) Pease, W. F.: Gilbert. A . H.. Cahn. A.. .4naI. Chem. 32, 894 (1960). Vapor Liquid Equilibria (lE) Frif (1E) Adlrr. Adler. S. S. R.. B., Friend. L.. othrrs. ‘ A‘.I.Ch.k. A.1.Ch.E. Journa? Journal 6, 104 (1960). (2E) Cajande, B. C.. Hipkin, H. G.. Lenoir. J. M.. J . Chem. €3 Ene. En?. Data 5, 251 (loAn\ (1960). (3E) Cova, D. R., Ibid.. 5, 282 (1960). (4E) Davis, J. R . . Evans. I.. R.. Ibid.. 5 . 401 (1960). (5E) Delgenne. A . 0.. Ibid.,5 , 413 (1960). (6E) Furtes, W. F.. Johnson. A. I.. Can. J . Chem. En€. 38, 78 (1960). (7E) Gordon, E.. Goodwill. M . J . . Paylor, J. W.. Chem. En
