ROBERT D. BEATTIE' and DONALD F. OTHMER Polytechnic Institute o f Brooklyn, Brooklyn, N.
Y.
Vacuum-Compression Distillation Column Fractionuting Efficiencies
THE
PRECEDING ARTICLE described a three-stage vertical column with a vapor compressing impeller in each stage and a system for spraying reflux liquid countercurrent to the vapor stream. These impellers were on a common axial shaft. A static diffuser, with six helical vanes generated a t a 10' angle, was shown to be the most nearly optimum design for changing the direction of vapors with minimum friction loss (73). Other design factors and fluid dynamics were discussed. Using this column with appropriate auxiliary equipment, mass transfer studies were made in absorption, vacuum rectification, and atmospheric rectification. T h e system: n-butyl ether-dichloroethyl ether was used for testing rectification efficiencies at pressures of 20,30, and 50 mm. of H g absolute. The chloroform-benzene system was used a t atmospheric pressure. Pressure losses, liquid and vapor composition, and power consumption were measured for several rotation rates, degrees of vacua, superficial vapor velocities, and liquor rates. Murphree efficiencies were determined and found to increase with rotation rate, other variables being constant. Performance with respect to gas rate, liquor rate, operating pressure, and reflux ratio is analogous to conventional distillation columns, although Murphree stage efficiencies are generally higher under similar conditions and often are greater than 100yodue to the countercurrent vapor-liquid contacting in each stage. T h e earlier paper (73) describes the Present address, 320 Lexington S t . , Watertown 72, Mass.
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Figure 1. Murphree efficiency for distillation at 30 rnm. of Hg and total reflux of ether system is higher for 1009 r.p.m. (Curve 2 ) than for 300 r.p.m. (Curve 1 )
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multibladed impeller, which compresses the vapor in each stage, so that pressure loss is usually less than 10% of that in conventional vacuum distillation towers, and may even be reduced to a zero or even negative value. Investment and operating costs may be slightly higher, but only a small improvement in yields of heat sensitive products is economic justification for its use. Absorption and distillation efficiencies a t atmospheric pressure and under several degrees of vacuums have been determined. T h e present paper reports Murphree efficiencies during distillation using different vapor-liquid systems. Distillation performance has been defined in terms of vapor and liquid rates, rotation rate, operating pressure, and reflux ratio. Experimental Variables Stages: 10.5 inches I.D. X 8.3 inches high with a 7.5-inch diameter compressor impeller. Static diffuser section: six helical vanes a t 10 O angle of generation. Liquor distributor: four equally spaced vertical weir troughs, each 2.25 inches long with multiple notching on both edges. Rotation rates: 294 to 1552 r.p.ni. Systems: n-butyl ether-dichloroethyl ether and chloroform-benzene. Pressure: 20 mm. of Hg absolute to 1 atm. Vapor rate: 0.33 to 2.36 1b.-moles per hr.-sq. ft. Liquor rate: 0.44 to 2.36 1b.-moles per hr.-sq. ft. Reflux ratio: 1.18 to total. Operation T h e operation of this column was previously discussed ( 7 3 ) for determination of pressure drops and other factors of hydromechanics, power consumption and design, also including details of operation for vacuum and atmospheric fractionation, and the taking of samples used for the determination of vapor and liquid compositions. In order to calculate the Llurphree efficiency, all the analyses by refractive index were converted to mole per cent, and for the case of total reflux operation, using the equilibrium vapor concentration from the X-Ydiagram ( 7 4 , the Murphree efficiency was calculated by direct subctitution in the following relation.
INDUSTRIAL AND ENGINEERING CHEMISTRY
However. at finite reflux ratios, vapor concentrations were determined by material ba!ance, using rotameter readings and induced reflux quantities to establish the actual reflux ratio. The basic material balance is expressed in terms of the vapor concentrations for the n*h plate.
in xvhich standard nomenclature is used. Results a n d Correlation of Data
The experimental results were obtained on several variables in the mass transfer studies. Vapor rate, !iquor rate, rotation rate? and total system pressure caused major changes in Murphree efficiency, although other factors were also studied-e.g., reflux ratio. The nbutyl ether-dichloroethyl ether system (74) (hereinafter called the ether system) was u s d at absolute pressures of 20, 30, and 50 mm. of Hg to determine the fractionation performance of a threestage vacuum - compression column. There are not sufficient data to reveal the effects of concentration and viscosity on Murphree effiriency, and it was found most convenient 10 present data using superficial vapor velocity as the independent variable. Rotation Rate. Curve 2, in coniparison with Curve 1 in Figure 1, shows that the Murphree efficiency curve increased for a change in rotation rate from 300 to 1009 r.p.m. at 30 mm. of H g absolute pressure, other conditions being constant. A similar trend occurred in efficiency data at other rotation rates. T h e trend of increasing hfurphree efficiency with rotation rate is confirmed a t other pressures, 20 and 50 mm. of Hg absolute, using the ether system; and also a t atmospheric pressure, using the chloroform-benzene sysrem. I n Figures 2A through 2 0 , Murphrce stage efficiencies were plotted against rotation rate at several constant molar gas rates, GAM;and the data are all a t total reflux 30 that G,Ti = L41, the molar liquor rate. Also each plot is a t constanr. total operating pressure. Furthermore, the data plotted were selected from calculated results with the previously stated conditions in mind. The curves in Figures 2-4 through 2 0 have been plotted against semilogarithmic scales and confirm t h e trend that Murphree efficiency incream with rotation rate. As pressure increases in Figures 2'4 through 2D, the range of molar gas rates used increased over wider !imits. I n addition, the general slope of the sheafs of curves in Fiqures 2il through 2C for the ether system increases as the pressure increases. In each case, the sheaf of curves begins in the 50 to 6070 efficiency
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B. For the ether system a t 30.0 mm. of Hg pressure. A. For the ether system at 20.2 mm. of Hg pressure. Hg pressure. D. For the chloroform-benzene system a t atmospheric pressure
range and then increases with rotation rate. At 30 mm. of Hg the efficiency range is 85 to 125% a t 1600 r.p.m. while a t 50 mm. of Hg upper efficiency range is 95 to 150% a t 1600 r.p.m. Besides the increasing slope of Murphree efficiency curves with pressure, there seems to be a leveling off tendency at higher rotation rates. This tendency does not appear in Figure 2 A a t 20 mm. of Hg, but a t 30 and 50 mm. of Hg in Figures 2B and 2C (all for the ether system) leveling off occurs a t 1400 to 1600 r.p.m. I n Figure 2 0 for the chloroformbenzene system a t atmospheric pressure, a series of maximum points seems to be developing in the Murphree efficiency curves, Best defined is the maximum point a t 1100 r.p.m. in the Murphree efficiency curve a t GM = 1.70 lb. moles per hr.-sq. ft., while the other curves exhibit leveling off as seen in Figures 2B and 2C for the ether system. Vapor Rate and Liquor Rate. Figure 1 shows the typical effect of vapor and liquor rate on Murphree efficiency. Change in liquor rate offers an explanation of the minima-maxima behavior of the Murphree efficiency curves (Figure 1). At relatively low liquor rates (less than 3 feet per second vapor velocity a t 30 mm. of Hg) the run-down troughs beneath impellers (73,Figure 1) had sufficient capacity to drain the contacting stages, so that liquid spray from the vertical weirs (73,Figure 1)furnished most of the vapor-liquid contacting surface. However, at higher liquor rates (corresponding to those above 3 feet per second vapor velocity a t total reflux and 30 mm. of Hg), a head of liquid collected in the contacting stages to force liquor through the run-down troughs. l h c n the spin-
ning impellers churned the collected liquor in each stage into froth, and the froth height increased with liquor rate. Consequently, above the "minimum" point vapor velocity, additional vaporliquid contacting surface was available because of the above described frothing mechanism. The additional contacting surface caused improved Murphree efficiencies until the "maximum" point in the efficiency curve was reached. Then entrainment and froth blown through to the stage above caused the Murphree efficiency curves to resume the expected trend-Le., decreasing with increasing vapor rate. Presumably the minimummaximum point phenomenon in the efficiency curves would be eliminated or transposed to a much higher vapor velocity (total reflux) if larger run-down troughs had been milled into the floors of the contacting stages (73,Figure 1). The effect of vapor rate on Murphree stage efficiency has been shown by selection of efficiency values using the ether system a t constant liquor rates, rotation rate (1000 r.p.m.), and pressure (30 mm. of Hg). I n Figure 3, Murphree effi-
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ciency is plotted against molar vapor rate on arithmetic scales with curves of constant molar liquor rate. The Murphree efficiency curves a t constant liquor rates between 0.7 and 1.2 lb. moles per hr.-sq. ft. increase parabolically with vapor rate to maximum points and then decrease with further increase in gas rate. At constant liquor rate a n increase in vapor rate initially causes improvement in Murphree efficiency because: Higher gas velocity reduces the gas film resistance and higher gas velocity increases turbulence so that a larger number of collisions of liquid drops occurs, increasing the area for interphase mass transfer. As the gas rate was increased to values more than 1.3 to 1.7 times liquor rate, increasing turbulence caused a fine enough spray such that entrainment counterbalanced any further improvement in Murphree efficiency. At gas rates above 1.7 times the liquor rate, entrainment was controlling so that Murphree efficiency decreased with further increases in gas rate. The effect of liquor rate on Murphree stage efficiency has been shown by
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in liquid film resistance above LA[ = 1 .Olb. moles per hr.-sq. ft. Reflux Ratio. The effect of reflux ratio o n Murphree stage efficiency has been shown by selection of efficiency values using the ether system at constant rotation rate (1000 r.p.m.), constant pressure (30 mm. of Hg), and alternately constant molar gas or liquor rates. This has been done in Figures 5 and 6. I n Figure 5> Murphree efficiency has been plotted against the quanrity (R I)/& in which the reflux ratio, R is defined as L:. D . .Also ( R 1)/R is equivalent to G,,JL,,,. Consequently Figure 5 shows same results as Figure 3, because both are plotted a t constant liquor rates, since (R 1)/R reduces to G&f. Coniparison of Figures 3 and 3 shows the same parabolic shape in the Murphree efficiency curves. Similarly in Figure 6, where hlurphree efficiencies are plotted at constant molar gas rate, (K 1)/R reduces to l/L.,f. This reciprocal reverses the slopes of Murphree efficiency curves, as found in Figure 4, but the same effect of liquor rate on Murphree efficiency is found. Hence reflux ratio changes hfurphree efficiency only to the extent that it changes either the gas or liquor rates. Pressure. i2lthough pressure is not a primary variable affecting Murphree efficiency (15): it does affect viscosity and surface tension through its effect on the distilling temperature within a given contacting stage and can be considered a secondary variable. Viscosity and surface tension are recognized as primary variables (among others) affecting Murphree efficiency. The effect of pressure on Mu-rphree stage efficiency has been shown by piotting efficiency values selected for the ether system a t constant gas and liquor rates (total reflux) and rotation rate (1000 r.p.m.) in Figures 7 A through 7 0 where Murphree eficiency is plotted against reciprocal pressure on semilogarithmic graph paper with 1 / P on the log scale as abscissa. Figures 7 A through 71) indicate that Murphree efficiency decreases as pressure decreases. Although the results a t 300 r.p.m. in Figure 7 A seem anomalous, results correlated a t higher rotation rates in Figures 7B, C, and D confirm the trend quite well that Eniv decreases Jvith P. At rotation rates of 1015 and 1541 r.p.m. in Figures 7C and 7 0 , the sheafs of E,,fv curves a t constant GJi and total reflux appear to approach asymptotes of about 50 and 70’%, respectively. T h e Murphree eficirncy may not be lower than 50% for rotation rates of 1000 r.p.m. and higher when the fractionator tested is operated a t pressures approaching 1 mm. of Hg. Gas and liquor rates of a t least 1.0 Ib. mole per hr.-sq. fr. w o d d be required for the design tested. Atmospheric Fractionation Studies. Because it was desired to work at an
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selection of efficiency values calculated from experimental results using the ether system a t constant gas rates, rotation rate (1000 r.p.m.), and pressure (30 mm. of Hg). This was done in Figure 4 ; Murphree efficiency is plotted against molar liquor rate on arithmetic scales with lines of constant molar gas rate. As shown in Figure 4, the Murphree efficiency curves a t constant molar gas rates of 1.2 and about 2.3 Ib. moles per hr.-sq. ft. exhibit no significant change with liquor rate. At G2+{ = 1.2 Ib. moles per hr.-sq. ft.: hlurphree eficiencies increase only slightly \vith liquor rate. ,4t a slightly higher gas rate (G.xI = 1.62 1b.-moles per hr.-sq. ft.); Murphree efficiency decreases markedly with increasing liquor rate. Again at GJf = 1.9 lb. moles per hr.-sq. ft. and higher, Murphree efficiency decreases slightly with increasing liquor rate. T h e gas rate line a t 1.62 lb. moles per hr.-sq. rt. is in the minimum-maximum region of the Murphree efficiency us. superficial vapor velocity relations. At GiTf = 1.62 lb. moles per hr.-sq. ft., there was evidently sufficient gas velocity, as LM increased, to entrain liquid droplets. As the liquor rate increased, a greater depth of froth collected in each contacting stage so that at higher liquor rates the gas stream carried away more entrainment, T h e depth of froth varied with the liquor rate, and entrainmenr varied
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with froth depth (all at a constant G.M = 1.62 lb. moles perhr.-sq. ft.). Further. as entrainment increased, Murphree efficiency decreased, as has been shown (9). Higher molar gap rate lines seem to fall on a common trend line in Figure 4 because of the nechanics of the contacting stage. T h e vertical weirs (73, Figure 1) throw out a continuous volume of liquid spray in counterflow to the gas stream. Beyond the periphery of the impeller. the spray broke through froth, rising from the annular region around the impeller, so that any entrainment, which is related to froth depth, is linited by the geometry of the contacting stage. Also as liquor rate increased, coarser spray was thrown from the vertical weirs so that entrainment was further retarded. T h e results of correlation in Figure 4 indicate no significant decrease in Murphree e& ciency between liquor rates of 1.0 and 2 4 lb. moles per hr.-sq. ft. a t lines of constant gas rate between 1.9 and 2.3 lb. rnoles per hr.-sq. ft. I n addition, Figure 4 indicates that liquor rate is not controlling a t GAIf greater than 1.9 to 2.3 Ib. moles per hr.sq. ft. in the vacuum-conipression fractionator tested, because a dual action comprising frothing and spray provides relatively infinite interphare contacting area, so that liquid film resistance is minimized. For the conditions in Figure 4 (30 mm. of Hg, 1000 r.p.m., ether system, and G U == 1.9 lb. moles per hr.-sq. ft.). there seem to be no further changes
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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VACUUM COMPRESSION atmospheric boiling point near the boiling point of dichloroethyl ether under vacuum, the chloroform-benzene system was used to determine the fractionation performance of the same three-stage column a t atmospheric pressure. As before, the fractionation performance was evaluated in terms of the Murphree plate efficiency, EMv, and the variables studied were vapor rate and rotation rate a t total reflux and atmospheric pressure. Like the results from the studies using the ether system under vacuum, Murphree efficiencies taken on the chloroform-benzene system a t atmospheric pressure decrease with increasing vapor rate (Figure 8), and no minima-maxima behavior was found in the Murphree efficiency curves. Assuming this behavior is caused by collection of liquor in the contacting stages, the dip in the Murphree efficiency curves a t atmos-
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Figure 7. Murphree efficiency at total reflux and constant gas rate generally decreases with increase of reciprocal pressure for: A.
300 r.p.m. B. 710 r.p.m. D. 1541 r.p.rn.
r.p.m.
C.
1015
pheric pressure would occur between zero and 0.02 feet per second. Consequently, no minimum-maximum behavior would be expected for the range of vapor velocities studied. Similarly, Murphree efficiency results from the chloroform-benzene system a t atmospheric pressure increase as rotation rate increases. Pressure drop and power consumption results were also obtained during fractionation, and the effects of vapor rate, rotation rate, total pressure, and liquor rate on these variables have already been reported (73).
Discussion Fractionation efficiencies in the vacuum-compression distillation column increased substantially with increasing rotation rate, while increasing gas rate and decreasing pressure caused lower Murphree efficiencies. At molar gas rates above 2 lb. moles per hr.-sq. ft., liquor rate seemed to have little effect on Murphree efficiency. Comparison of performance with other equipment for mass transfer is desirable. Murphree efficiency comparisons would be made best with similar systems a t comparable pressures. Many reported determinations have been made using the ethanol-water system a t atmospheric pressure, both for bubble cap rolumns and sieve plate columns (2). Next in frequency of use have been the benzenetoluene and methanol-water systems (2). In recent years, Gerster and coworkers (3, 6) have favored use of the air-water or oxygen-water systems a t atmospheric pressure. Most of the data on hydrocarbon fractionations are not comparable because large diameter columns (4 to 42 fcet in diameter) were used; and the effects of back-mixing and concentration gradient on larger diameter trays cause much higher Murphree plate efficiencies than in 1- to 2-foot diameter towers, as has been shown by Lewis (IO). Accordingly, comparison can be made best with efficiency results on equipment representative of the latest tray designs from the recent literature on binary systems with similar temperature ranges and gas and liquid film resistances. Jones and Pyle (g), compared 18-inch diameter sieve and bubble cap tray efficiencies using the acetic acid-water system at total reflux and atmospheric pressure, and more recently Gerster and others (7) worked on the acetone-benzene system in a 2-foot diameter bubble cap column at 20 p.s.i.a. In Figure 9, the Murphree efficiency results from the present work are compared with those reported by Jones and Pyle (Q), Gerster, Bonnet, and Hess (6), and Garner, Ellis, and Luxon (5). All the efficiency data plotted were determined at total reflux except those of
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Gerster and others (7) on the acetonebenzene system which were taken at L/V equal to 1.72 (average). However, the operating pressures varied from superatmospheric (7) to 30 mm. of Hg absolute in the present work. Different systems were used in each case. although the operating temperatures for the acetone-benzene system at 20 p.s.i.a. are similar to those for the chloroformbenzene system used in this work at atmospheric pressure. The operating temperatures for the acetic acid-water system at 1 atm. were only about 50" F. higher than for the n-butyl ether-dichloroethyl ether system used in this work at 30 mm. of Hg absolute operating pressure. Also because the Murphree efficiencies in Figure 9 have been plotted against F-factor, the comparisons are on a nearly similar basis. In addition the efficiencies determined by Garner and others ( 5 ) for Kaskade trays a t total reflux, compare the effect of coarse spray on mass transfer. It would have been desirable also to compare the resultr of the present work with those of Benenati ( I ) , because the contacting mechanism-counterflowing liquid spray and vapor-was the same in both studies. Although it was possible to correlate the average Murphree point efficiencies of Benenati ( I ) by plotting against the molar L / V ratio a t parameters of constant F-factor, only fcur F-factor parameters were obtained so that comparison in Figure 9 was not convenient. As shown by Figure 9, the fractionation efficiencies obtained using the vacuum-compression fractionator compare favorably with those obtained with conventional equipment. The Murphree plate efficiencies determined in this work using the ether system a t 30 mm. of Hg, total reflux, and 1000 r.p.m. (Curve 3, Figure 9) are in the same range as the VOL. 53, NO. 10
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Literature Cited
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(1) Benenati, R. F., “Construction and Operation of a Spray Column for Distillation,” B.Ch.E. thesis, Polytechnic Institute of Brooklyn, Brooklyn, N. Y . , June 1942. (2) Chemical Engineers Handbook, J. H. Perry, ed., 3rd ed.: p. 597, McGraw-Hill, New York. 1950. (3) Foss, A.’S.. Gerster, J. 4.. Chem. Enq. Progr. 52, 28-5 (1956). (4) Garner. F. H., Ellis, S. R. M., Hugill, A. J.. Trans. Inst. Chem. Eners. (London) 31, 13 (1953). (5) Garner, F. H., Ellis, S. R. M., Luxon, E. S., J . Inst. Pefrol. 43, 86 (1957). ( 6 ) Gerster. J. A.: Bonnet, W.E., Hess, I., Chem. Eng. Pray. 47, 523, 621 (1951). (7) Gerster, J. A., Hill, A. B., Hockgraf, N. N., Robinson, D. G., “Tray Efficiencies in Distillation Columns,” Final Report of A.1.Ch.E. Research ComI
Figure 9. Murphree efficiency of the vacuum-compression distillation column compares favorably with values reported for various other distillation equipment on various systems when plotted versus F-factor
efficiencies obtained by Jones and Pvle (9) in bubble cap and sieve plate columns (Curves 5 and 6: Figure 9) for the acetic acid-water system at atmospheric pressure and total reflux. Considering the difference in pressures (30 us. 760 mm. of Hg), there is considerable advantage in using the vacuum-compression fractionator, especially where small quantities of liquor are available in relation to the vapor rate. This efficiency advantage is confirmed also by the data from this work using the chloroform-benzene system at total reflux: 1 atm. pressure, and 1000 r.p.m. (Curve 1> Figure 9). ilnd the 30 mm. of Hg absolute efficiency data. obtained from the vacuum-compression fractionator using the ether sysrem a t total reflux and 1520 r.p.m. (Curve 2, Figure 9), are more favorable by a wide margin than an! conventional fractionation equipment, particularly in the region below 0.7 F-factor. Previously (73), liquor rate had little observable effect on pressure drop. but power consumed during distillation of the ether and chloroform-benzene systems increased negligibly with vapor rate, while liquor rate caused sharp increases in power drawn during liquid seal formation to values of about 0.1, 0.17. a r d 0.30 hp. per stage (including drive friction losses) a t respective rotation rates of 700, 1000, and 1520 r.p.m. Pressure drops observed for a vacuum-compression stage were less than 10% of the minimum pressure drop reported for conventional
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fractionators (2. 4, 7-9, 77, 74); and proper design of the impeller should allow the reduction of these pressure losses to zero or even negative values. A special bubble cap to\\ er ar 56 mm. of Hg on an ethyl benzene-styrene separation (77) gave a pressure drop of 2.0 mm. of H g per plate. whereas the vacuum-compression unit at 30 mm. of H g and about 1500 r.p.m. gave an observable pressure drop of 0.045 inm. of Hg per stage. Acknowledgment
The help of Hoffmann-LaRoche, Inc., and Cabot Corp. is gratefully appreciated, Nomenclature
D
moles of distillate withdrawn per hour EA\,, = Murphree plate or stage efficiency based on vapor in F-factor = G, = molar gas or vapor velocity, lb. moles per hr.-sq. ft. L = moles of liquor returned per hour = molar liquor velocity, lb. moles L, per hr.-sq. ft. = total operating pressure, mm. P of HE R = L/D = vapor density, lb. per cu. ft. pc = superficial gas or vapor rate, U ft. per sec. xD = mole fraction of the more volatile component in the distillate withdrawn
INDUSTRIAL AND ENGINEERING CHEMISTRY
=
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mittee. Dec. 1. 1958.
(8) Hutchinson.’M. H., Baddour, K. F., Chem. En,?. Progr. 52, 503 (1956). (9) Jones. .J. B., Pyle: C., Ibid., 51, 424 / I nrrr \17JJ,.
(10) Lewis, W. K., IND.ENG.CHEM.28, 399 (1936). (11) May, J. A , , Frank, J. C., Chem. En,q. Progr. 51, 189 (1955). (12) Muller, H. M., Othnier, D. F.; INI). ENG.CHEM.51, 625 (1959). (13) Othmer, D. F., Beattie, K. I>.. IKD.ENG.CHEM.53, 779 (1961). (14) Othmer, D. F., Scheibel, E. G., Trans. Am. Inst. Chem. Engrs. 37, 211
(1941). (15) Othmer, D. F.? Beattie, R. D.. “Vapor-Liquid Equilibria for n-Butyl Ether-Dichloroethyl Ether System.” unpublished data, (16) Perry, J. H., ed., “Chemical l h q i neers’ Handbook,” 3rd ed., p. 597, McGraw-Hill, New York, 1950. (17) Rogers. M. C., Thiele, E. W.! ISIJ. END.CHEM.26. 524 (1934). (18) Stepanoff, A. J.: “Turboblowers,” Wiley, New York, 1955. (19) White. R . E., Othmcr. D. F.. 7 i a n r . A.Z.Ch.E. 38, 1067 (1942). RECEIVED for review July 25. 1960 ‘ ~ C C E P T E D June 6. 1961 Division of Industrial and Engineering Chemistry, 138th Meeting, ACS. New York, September 1960. Patent protection is being secured for this vacuum-compression distillation column. Microfilm copies (L. C. Card No. Mic 59-3617) of the doctoral dissertation of Robert D. Beattie, 1959 (Polytechnic Institute of Brooklyn), from which this is an abstract, may be obtained from the firm of University Microfilms, 313 N. First St. Ann Arbor, Mich.
