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Theoretical Comparison of laboratory Continuous and Batch Distillations ARTHUR ROSE, THEODORE J. WILLIAMS,
AND
HARRY A. KAHN
THE PENNSYLVANIA STATE COLLEGE, STATE COLLEGE, PA.
Development and ready availability of a variety of automatic control devices make it conceivable that continuous distillation could be advantageously substituted for batch distillation in many laboratory and small-scale distillations. A better name for continuous distillation in such instances might well be “batch steady state distillation.” There are but few literature references to such laboratory continuous distillations and even fewer to comparisonsof batch and continuous distillation. This paper makes theoretical calculations and c o m parisons of the quantity and purity of the products of the two types of separation. The comparisons are confined to a binary mixture, and do not extend to consideration of
start-up time, possible decomposition, or any factors other than sharpness of separation. For the particular cases studied, the calculations show that the two types of processes give closely similar results when holdup is negligible. Existence of holdup favors the batch process and is detrimental to continuous distillation. A small proportion of more volatile component in the charge, along with appreciable holdup, increases the relative advantage of batch distillation. Inverted distillation has a corresponding advantage over the continuous process for mixtures with a large proportion of more volatile component. Ordinary batch distillation, however, lacks such advantage for stripping operations.
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sition versus mole per cent distilled. From these graphs the oalculated average product composition when any desired amount of the charge has been distilled is immediately available. In order to specify the conditions of the continuous distillation, the ratio of distillate to bottom product must be set. Although only a narrow range of ratios is of practical interest, the choice of this ratio is unrestricted theoretically. Hence, a complete set of continuous distillations can be picked to compare with the range of the batch distillation cut points. For example, a continuous distillation can be operated in such a fashion that 10% of the feed goes into the distillate, with 90% aa bottoms, The composition of this distillate is comparable with the average compositions from the batch distillation when 10% of the charge has been distilled. Correspondingly, the continuous distillate composition when %%of the feed goes tothedistillate is comparable to the average batch distillate composition when 25% of the charge haa been distilled. By selecting a range of distillate values for the continuous distillation, i t is possible to obtain a curve, analogous to the batch distillation curves, of average distillate composition versus per cent of charge in the distillate; this also gives the amount of product and its composition. From such curves it is evident which mode of operation gives the best separation and over what ranges.
ATCH distillation is widely used on a laboratory and semiworks scale as a purification or analytical procedure under a variety of conditions as to number of components, concentration of desirable and undesirable constituents, relative volatilities, etc. Lloyd (6) has pointed out that many of the applications of batch distillation might be better served by small-scale continuous columns, and gives several examples of operations in which much better results were obtained when the separation was transferred from a batch column to a continuous distillation column. It is of interest, in view of Lloyd’s experiences, to examine the theoretical coursw of the two types of distillation, The comparisons of this paper do not imply that largescale industrial continuOUE distillation is an uneconomic or disadvantageous operation. The over-all advantages of continuous operation on a large scale are well known. The paper does deal with those laboratory, pilot plant, and other situations where the quantity of material to be distilled is small enough that batch distillation would normally be considered, but continuous distillation might be used if there were sufficient advantage in using it, Little attention has been given by other authors to laboratory continuous distillation, so that its theoretical possibilities seemed worthy of comparison. Since a continuous still functions under steady-state conditions, ita operation can be calculated, within the limits of the usual simplifying assumptions, if the number of theoretical plates can be specified. Conditions during a batch distillation, on the other hand, are in continuous flux, and performance lags, due to the column holdup, eauw considerable deviation from the ideal no-holdup case. A series of binary batch distillation calculations has recently been made in this laboratory (7, 9, 11) in which the effect of holdup h a been considered, and confirmed by suitable experimental distillations. The results of these calculations have been used in this paper for comparison with theoretical continuous distillations calculated on the same basis. For comparison purposes the batch distillations have been plotted in Figures 1 and 2 aa cumulative average distillate compo-
METHOD OF CALCULATION FOR CONTINUOUS DISTILLATIONS
The continuous distillations were calculated by a stepwise trial and error procedure using the equilibrium data for a system having a relative volatility of 2.23, and the usual equations of the operating lines, The method amounts to an analytical steppingoff of plates on a McCabe-Thiele diagram. All calculations were made a t a reflux ratio of 4 to 1to correspond to the batch distillations, and for six theoretical stages, five plates plus reboiler For each continuous distillation the ratio of distillate to bottoms product was selected, permitting calculation of the slope of the stripping line; the slope,of the rectifying line is already specified, since the reflux ratio is fixed. The trial and error procedure
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consisted of assuming repeated trial values of xd, the distillate composition, and then calculating six steps down the column. The procedure was continued until the sixth step produced liquid Composition, satisfying the material balance. The change-over from the rectifying line to the s t r i p ping line w a made ~ a t that plate whose liquid composition dropped below the feed composition. It was assumed that the feed entered aa saturated liquid at the boiling point, giving a vertical g line.
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Charge In DMIlIate, %
RESULTS
Figure 1. Comparison of Cdlculated Yields and Purities of Products from Ordinary Batch and Laboratory Continuous Distillation Thecurves dotted in Fiaures 1 and 2 give the calculated yieids obtainCharge or feed containing 25 mole 9% more volatile aomponent of mixture with ~1 2.23 o Batah distillation with 15% holdup able from a continuous column with 0 Batah dktillation with 7.546 holdup A Batah distillation with 0%holdup negligible holdup, and those which 0 Continuous distillation could be o b t a i n e d f r o m i d e n t i c a l columns operated in batch fashion with ever, be preferable to inverted batch distillation, in which the 0, 7.5, and 1570 holdup, in which holdup ie given as mole per charge is placed in a specially designed condenser or equivalent cent of original charge. Figure 1 gives the curves for an initial reservoir at the top of the column and product is withdrawn from charge or feed composition of 25 mole 70 of the more volatile component. Figure 2 is for a charge composition of 9.6 mole 70. a still pot or reboiler with minimum volume ( 4 ) . This little used procedure would be expected to have the same advantages over The batch distillations were not calculated beyond the break continuous distillation in stripping operations that ordinary point in the usual type of batch distillation curve-a plot of inbatch distillation has in rectifying operations. stantaneous distillate composition, zd,versus per cent of charge distilled. This is the useful range of a binary batch distillation. As far as they were carried, the batch distillation calculations inEFFECT OF HOLDUP dicated superior results over identical continuous columns in The effect of column holdup has been neglected in the calculaevery case, except one in which identical curves were obtained. tion of the continuous distillations. Under all conditions where By extrapolation of the curves in Figure 1, an apparent point of the average distillate composition of the batch distillation is suintersection between the batch curves and the continuous curve is perior to the head product of the continuous distillation, the still observed. This occum a t about 3oy0 distilled, where the average residue, which could be obtained by stopping the distillation a t distillate composition is approximately 0.70 mole fraction of the that point and allowing the column to drain back into the pot, more volatile component, At this point [ ‘ 0 * 3 ~ $ ) ’ 7 0 ’(100) would in turn be superior to the bottom product obtained from the continuous distillation because it would necessarily have a 84% of the more volatile component is in the distillate fraction. lower concentration of the more volatile component. Holdup in Thus, it is indicated that under the specified conditions, if less the continuous column, which would be an intermediate fraction than S4% of the desired product in the charge is required, it would present a t the end of the distillation, acts to the detriment of the be obtained in better purity from a batch distillation than from a continuous-type separation. If the holdup fraction of the concontinuous distillation. If a yield better than 84% is desired and tinuous distillation is discarded, the percentages of charge appearthe corresponding low purity of 0.70 is acceptable, the continuous ing in the distillate and bottoms are both decreased, moving the still would appear to have a slight advantage. However, this adcontinuous still curves to the left in Figures 1 and 2 and indivantage is of only academic interest because if such low purity rating an added superiority of the batch distillations. If the were desired, the operation would be carried out with lower reflux holdup fraction is added to either the distillate or bottoms prodor fewer plates. For the conditions of the present comparison, it ucts, it acts as a diluent and makes the fraction correspondingly would appear more advantageous to use batch distillation with less desirable, again leaving the batch separation with an added somewhat lower yield and higher purity and then ailute the prodadvantage. If the holdup fraction is discarded, and in order to uct with undistilled charge to obtain the desired yield and purity, equalize the material balances, a correspondingly shed intermedirather than to use continuous distillation with high yield. ate cut is taken during the batch distillat?on and discarded, the Figure 2 indicates that decreasing the charge composition, esbatch distillation products would both be improved over what is pressed as mole fraction of more volatile component, throws the shown in Figures 1 and 2, while the continuous compositions calculated advantage heavily to the batch still, and that as the would remain unchanged. purification probIem approaches the case of removing a small Although the calculations are presented only for the distillate amount of light impurity, the batch still appeam far superior to compositions, conclusions drawn are equally valid if the bottom the continuous type of operation. This is largely due to the total product is the desired one. The only limitation is that the comreflux start-up which is classically used in batch distillation, and positions be as considered here. Thus, the calculations indicate the consequent concentration of a large fraction of the more volathat whether the top or bottom product is the desired one, as the tile material in the column during the initial Stabilization period. amount of the more volatile component in the charge is decreased, The intersection of the batch distillation curves with the conthe batch still becomes more and more advantageous. tinuous curve points to the stripping operation as the most adIn both cases for which calculated values were available, a comvantageous function of the continuous column. In this type of parison of the no-holdup batch and continuous distillation curves distillation almost all of the desired product present in the charge gives small reason to choose one type of distillation over the other. appears in the distillate, and the point of comparison on Figures 1 Theoretically, there seems to be little difference between the and 2 is to the right of the indicated intersection of the continuous separation achieved with a column operated as a batch still or and batch curves. Such continuous operation would not, how-
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as a continuous still, if the effect of holdup is ignored. In each of the cases presented, however, it is apparent that increasing the holdup has increased the separation of the batch process and decreased the desirability of the continuous operation. This tendency is accentuated as the mole fraction of the more volatile constituent is decreased. The improvement in batch distillation separation compared with the theoretical no-holdup batch distillation curve has been thoroughly substantiated by experimental data obtained in this laboratory on a variety of columns and with sevcral mixtures.
Charge in Distillate,
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Figure 2. Comparison of Calculated Yields and Purities of Products from Ordinary Batch and Laboratory Continuous Distillation
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Charge or feed containing 9.6 mole % more volatile component of mimture with (I 2.23 o Batch distillation with 15% holdup 0 Batch distillation with 7.5Yo holdup A Batch distillation with 0 % holdv;, 0 Continuous distillation
Houston ( d ) and Prevost (6, 10)found that their experimental batch distillation curves were invariably above the theoretical no-holdup curves except when the reflux ratio used was so low that assumption of the same number of theoretical plates aa that obtained at total reflux was no longer valid for the no-holdup calculations (3). They reported on two 13-mm. diameter columns packed with jack chain and glass helices and having 20 and 40 theoretical plates, respectively, when tested a t total reflux. The mixture used consisted of toluene and methylcyclohexane. Rose, Johnson, and Williams (7,8) verified the calculated batch distillation curves presented herein, using a 1-inch diameter column packed with glass Raschig rings, testing a t five theoretical platea plus still pot, and using a test mixture of ethylene dichloride and toluene. Kahn (3)made 13 batch distillations in a Cinch column having 50 theoretical plates and packed with protruded stainless steel packing ( I ) , using n-heptane and methylcyclohexane in the test mixture. Although all 13 runs were made a t a reflux ratio of 30 to 3, the experimental curves closely duplicate the theoretical no-holdup curves for reflux ratios between 40 and 50. Thus, in the moderate to small holdup range-that is, where holdup is under 20% of the charge-the separation obtained with a batch-type distillation will be better than that predicted by a calculated, Rayleigh-type curve, ignoring holdup. If the situation with regard to the comparison of no-holdup curves for binary batch and continuous distillations is typical as shown in the presented figures, then it would be reasonable to say that theoretically a batch distillation will give a better separation than a continuous distillation in an identical column, up to the break point in the batch distillation curve. As the amount of light constituent in the charge increases, the batch distillation curves will draw
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closer together and closer to the no-holdup curve, particularly through the first portion of the distillation where the effect of the material in the holdup fraction is less significant. ADVANTAGES OF BATCH-TWE OPERATION
Many binary batch distillations do not require completion to achieve the desired separation; therefore time and power requirements are appreciably reduced when compared with continuous distillation. This factor carries the greatest weight in the region of composition where the operational advantages of a batch distillation are greatest-namely, in the lower composition ranges of charge where the separation is more advantageous and the fraction to be distilled is smaller. Available information on time required for a batch column to reach equilibrium is too meager to estimate whether or not this factor would outweigh the other time advantages of a batch operation. Some time would also be required for start-up of laboratory or small-scale continuous distillation. There are inherent advantages in the batch type of operation where the separation would be classified essentially as a rectifying operation. The first, which is more important in purification from traces of light impurities, is the total reflux start-up which loads the column a t a higher efficiency than will be used in the actual distillation. Another advantage is that the initial portion of the distillate is obtained with the total number of plates above the feed composition. ,This has some effect until the head composition of the batch distillation drops below the corresponding composition for the continuous distillation. This is the point at which the decreasing pot composition and the corresponding downward movement of the operating line nullify the advantage of having the total number of plates above the pot. In addition, experimental evidence indicates that the effect of holdup is to modify the shape of the operating line in a batch distillation in such a way as to improve materially the separation over what would be predicted with a straight McCabe-Thiele operating line (7, f 0). MODIFICATIONS OF THE CONTINUOUS STILL
The continuous rectifying column-that is, one in which the feed is directly into the still pot-is really nothing more than a continuous still with the feed on the wrong plate. As such it is less efficient than a column being fed on the correct plate. A continuous stripping column is also bound to be less efficient than a properly operated continuous column. However, the feed compositions involved in stripping are in the range in which a continuous column seems to show substantial equality with the standard batch distillation. Thus, & continuous stripping column might be more satisfactory than the corresponding batch distillation even though it did not quite match the optimum performance of a continuous column with proper feed. Inverted batch distillation would be markedly superior to continuous stripping for the same reasons that ordinary batch distillation is superior for enriching operations. EFFECT O F MULTIPLE C O M W N M T S
No comparison is made for laboratory and other small-scale multicomponent distillations as the use of batch operation is inherently advantageous. Continuous operation requires either a complex arrangement and technique for withdrawing side streams or repeated continuous distillation with removal of a single component a t either top or bottom in each cycle. LIMrrATIONS OF COMPARISON
The comparison made here is not intended to depreciate the use of small-scale continuous operation for special situations such as the collection of data for purposes of designing large-scale continuous equipment or the distillation of heat-sensitive materials.
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With reference to distillation of heabscnsitive materials, however, the advantages of inverted batch operation have apparently not been recognized. No comparison is made in this paper with respect to time required to reach equilibrium or to relative ease of operation of the two processes on a laboratory and pilot plant scale.
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start-up. Batch operation has some advantages because of more complete use of column capacity throughout ita entire length and also because it is not always necessary t o distill the complete charge of a binary mixture in batch operation. UTERATLJREW E D
(1) Cannon, M. R., IND. ENQ.CHEM.,41, 1953-5 (1949).
CONCLUSIONS
The theoretical considerations substantiate the conclusions presented by Lloyd (6)to the extent that the advantage leans to the batch distillation aa operation approaches a rectification and swings toward the continuous still as the procedure becomes that of stripping. However, the calculated advantage of continuous distillation disappears altogether if inverted batch distillation is considered the appropriate batch operation for stripping. The theoretical calculations indicate that the batch type of distillation is superior over a much wider range than is indicated by Lloyd if the distillations are performed in identical columns. The batch process is indicated as having its greatest advantage for charges With proportions Of the two components and this is enhanced by the presence of appreciable holdup and total reflux
Houston, R., M.S.thesis, The Pennsylvania State College, 1947. Kahn, H. A., Ibict., 1949. Langdon, W. M., IND. ENQ.CEEM.,ANAL.ED.,17,590-2 (1946). Lloyd, L. E., Petroleum Refiner, 29, 135 (February 1950). (6) Prevost, C. F.. M.S. thesis, The Pennsylvania State College, 1948. (7) Rose, Arthur, Johnson, R. C., and Williams, T.J., IND. ENO. CEEM.,42, 2145-9 (1950). (8) I b X , in press. (9) Rose,Arthur, and Williams, T. J., Ibid., 42, 2494-7 (1950). (10) Rose, Arthur, Williams, T. J., and Prevost, C. F., Ibid., 42,1876(2) (3) (4) (5)
9 (1950).
(11) Williams, T. J., M.S. thesis, The Pennsyivania State College, 1950.
RBCBIVBD August 80, 1960. Presented before the Division of Induetrjal and Engineering Chemistry at the 118th Meeting of the AMERICAN C~EMI~AI, SOCIETY,
Chiaapo. 111.
Performance of Packed Columns during Batch I
J. ERSKINE H A W K I N S
AND 1. A L L E N BRENT, UNIVERSITY OF FLORIDA, GAINESVILLE, FLA.
T h e studies were made because no single test mixture was known for testing columns operating at 20 to 760 mm, of mercury, no data were available on the effect of reduced pressure on column Performance, and no satisfactory method of comparing column performances at different pressures and finite reflux ratios was available. These studies have produced the test mixture, ethylbenzene-chlorobenzene, which can be used to determine the number of theoretical plates over the pressure range indicated above, and have shown that at total reflux the
JR.
columns tested have the same number of theoretical plates regardless of the pressure, and have resulted in a simplified method of comparing column performance under different conditions when the reflux ratio is finite. I t should now be possible to evaluate columns with 60 to 70 theoretical plates at various pressures and over a wide range of operating conditions. Columns having 100 to 200 theoretical plates may be evaluated by using the mixture rc-heptane-methylcyclohexane at a pressure which will give an appropriate value of the relative volatility.
MIXTURES FOR TESTING DISTILLATION COLUMNS AT ATMOSPHERIC AND REDUCED PRESSURES
AS
PART of the program of determining the effect of reduced pressure on packed column performance, it was desired that a test mixture be available for testing columns at both atmospheric and rcduced pressures. Since it has been shown that different mixtures sometimes give different results (39),it was desired t o have a mixture that could be used a t both atmospheric and reduced pressures. To investigate the effect of change of relative volatility with pressure on column performance it was desired to have two mixtures available, one of which would show an increase and the other a decrease in relative volatility as the pressure was lowered. A number of mixtures are available for testing columns at at-
mospheric pressure. Some of the most frequently used are ethylene dichloride and benzene (19),benzene and carbon tetrachloride (39),benzene and toluene (fO),n-heptane and toluene (39),nheptane and methylcyclohexane (8Q and rnethylcyclohexane and toluene (27). However, for determining the number of plates a t reduced preasures only a few mixtures have been described, and these do not have all the properties desired. The data for ethylene dichloride and benzene have been determined a t pressures from 760 t o 100 mm. of mercury by Bragg and Richards (7). These data show R deviation from ideality, with the relative volatility changing with concentration. Furthermore, because ethylene diahloride is
