INDUSTRIAL AND ENGINEERING CHEMISTRY
1480 h,
= heat transfer coefficient be&;veen gas and solid, B.t.u. per
h,
=
k
=
= qQ =
L
qs
q.
S
=
= = = =
fb
tg t, = tw = A&,> = p = ni, n, r
hour per square foot per F. heat transfer coefficient between bed and wall, B.t.u. per hour per square foot per a F. thermal conductivity of gas, B.t.u. per hour per square foot per O F. per foot height in bed, inches rate of heat flow to gas, B.t.u. per hour rate of heat flow t o solid, B.t.u. per hour rate of heat flow t o wall, B.t.u. per hour cross-sectional area of tube, square feet temperature of bed, F. temperature of gas, F. temperature of solid, F. temperature of wall, O F. mean temperature difference, F. viscosity of gas, pounds per hour per foot = exponents dimensionless equation LITERATURE CITED
(1) Baerg, A , , Klassen, Jr., and Gishler, P. E., Can.J . Research, 28, 287-307 (1950). (2) Rrinn, M . S., Friedman, S. J . ,Gluokert, F'. A . , and Pigford, R. L., 1x0. ENG.CHEM.,40, 1050-61 (1948). (3) Dalla Valle, J. M . , "Micromeritics," 2nd ed., pp. 331, 555, riew York, Pitman, 1948.
Vol. 44, No. 6
(4) Deitz, V. K., and Robinson, H. E., IND. ENG.CH (1948). (5) Gamson, B. W., Chem. Eng. Progress, 5, 19 (1951). (6) Gamson, B. W., Thodos, G., and Hougen, 0. A , , Tians. Am. Inst. Chenz. Engrs., 39, 1 (1943). (7) Hurt, D. hf., IND. ENG.CHEM.,35, 522 (1943). (8) IND. EKG.CHEM.,41, 1098-1250 (1949). (9) Kettenring, K. N., Manderfield, E. L., and Smith, J. bI., Chem. Eng. Progress, 4, 139 (1950). (10) Leva, M . , and Grummer, M., I X D E N GCHEM., . 40,415-9 (1948). (11) Leva, RI.,Weintraub, M., and Grummer, M., Chem. Eng. Progress, 3, 563-72 (1949). (12) Leva, M . , Weintraub, M., Grummer, M., and Clark, E. L., IND. ENG.CHEM.,40, 747-52 (1948). (13) Levenspiel, O., and Walton, J. S., Am. Sac. Mech. Engrs., Bound Proceedings of Heat Transfer and Fluid Mechanics Institute, Berkeley, Calif., pp. 139-43, May 1949. (14) Lof, G. 0. G., and Hawley, R.W., IND. ENG. CHEM.,40,1061-70 (1948). (15) Luckenbach, F. A., U. S.Patent 210,793, Oficial Gaz. C . S.Pat. Ofice, 14, 890 (1878). (16) Mickley, H. S., and Trilling, C. A., IND. EXG.CHEW,41, 113747 (1949). (17) Simon, R. H., Ph.D. thesis, Oregon State College, 1949. (IS) Singer and Wilhelm, Chem. Eng. Progress, 4 , 343 (1950). (19) Toomey, R. D., and Johnstone, H. F., paper presented at the annual meeting of the Am. Inst Chem. Engrs., Columbus, Ohio, Dec. 6, 1950.
.
RECEIVED for review July 30, 1951.
!lCCEPTED
January 3, 1962
Holdup-Charge Ratio in istillation I
ARTHUR ROSE AND VICTOR J. O'BRIEN, JR.' T H E P E N N S Y L V A N I A S T A T E C O L L E G E , STATE C O L L E G E , P A .
I
T WAS t,he purpose of this investigation to obtain experimental
data indicating the effect of the more important variables in ternary batch distillation as well as to review and study various methods of calculating the course of such distillations. In addition, it was of interest to determine whether the generalizations deduced from prior binary studies were applicable to multicomponent systems. A theoretical analysis was made of possible methods of calculating multicomponent' batch distillations under conditions of both appreciable and negligible holdup. It was concluded from this that, the only practical means available is the numerical procedure of Rose, Johnson, and Williams ( 5 ) , regardless of 15-hether holdup is appreciable or negligible. The experimental studies were limited to t'he system n-heptane-methylcyclohexane-toluene. The normal boiling points of these are 98.4", 100.9", and 110.6" C., respectively, and the relative volatility ranges in the ternary are 1.07 to 1.15 and 1.1to 1.6 for the first and last pair, respectively. Complete vapor-liquid equilibria >\-ereavailable and a convenient method of analysis had been perfected by Kirk (I). The laboratory-sized column used for the distillations had an internal diameter of '/z inch and was packed x+th '/IB-inch 36gage stainless steel helices to a height of 5 feet. The column shoTyed about 80 theoretical plates under total reflux xyith nheptane-methylcyclohexane near the maximum throughput of 400 ml. of methylcyclohexane per hour. The packing holdup 1 Present address, Standard Oil Co. of Indiana, Research Laboratory, Whiting, I n d .
was approximately 60 ml. and the charge capacity was 5 liters, so t h a t the rat'io of holdup (usually expressed as a per cent and called per cent holdup or simply holdup) could be varied from 1.5p/, to any desired large value by varying the size of the charge. Column holdup was determined under conditions closely approximating the ternary distillations of this n-ork. This vas done by adding nonvolatile diocA3-1phthalate to a ternary charge and determining holdup throughout the-course of a batch distillation, done for the special purpose of determining holdup during actual distillation. Holdup values were calculated by inat'erial balance based on the quantity of charge and the concentration of nonvolatile dioctyl phthalate in the charge, and its concentration in a sample removed from the still at some particular time during the distillation. The results of four of these holdup dist'illations sho\yed that the molal column holdup could be considered to be substantially constant for the column and components of this work. Twent,y-five ternary batch dist,illat'ions and four binary batch distillat,ions were conduct'ed. Details are tabulat'ed in Table I. Five of the ternary runs LTwe duplicates of earlier runs. These check runs sho\+-ed that small discrepancies in distillate mole fraction and break point vr-ere present from run to run. Kone of the discrepancies were large enough to seriously affect the interpretations or the conclusions report,ed herein. All runs were conducted a t total reflux startup and intermittent take-off, Material balances usually showed 1 to 2% loss, largely because of handling and drainage for which corrections were not determined. The two ternary charge compositions were selected for specific
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
June 1952
TABLE I.
SUMMARY ON BATCH DISTILLATIONS Reflux Ratio Mole: Reflux/Mole Distillate
Run NO.
1481
Distillate Throughput G./Hr.
Holdup,
%
Material
Loss, Wt. "/o
TERNARY R U N S , SERIES I"
"
20 30 M O L PERCENT DISTILLED
lo
T-12 T-10 T-13 T-4 T-28 T-5 T-6 T-1
40
7.5/1 7.5/1 7.5/1 15/1 15/1 15/1 15/1 30/1 30/1 30/1 30/1
T-9
Figure 1. Comparison of Binary and Ternary Experimental Distillations
T-27 T-8
-
-.-- Binary run B-5 -.-Holdup, Binary r u n B-1 9 mole %
Ternary run T-10
4.5 9 18 4.5 4.5 9 18 4.5 9 9 18
0 54 0.78 1 36 0 75 0 47 0 84 1 91 0 89 1.05
0.80 1 72
TERNARY R U N S , SERIES T-18 T-24 T-34 T-23 T-21 T-25 T-17 T-26 T-22 T-20 T-19 T-36 T-33 T-35
Reflux ratio, 7.5 to 1 For charge compositions, see Table I
reasons. The one charge composition had a small proportion of n-heptane so that the effect of total reflux startup upon the sharpness of separation of this component was large. Then too, most of the n-heptane was distilled before appreciable toluene appeared in the distillate because of the small proportion of n-heptane present. The second ternary charge composition was selected to reverse the above effects. The effect of total reflux startup was small, while a t the lower reflux ratios appreciable concentrations of all three components were present a t some time during a distillation. The experimental reflux ratios were selected to yield both good and poor separations between components, and charge sizes were chosen to give values of holdup likely to be encountered in the practice of laboratory distillation. COMPARISON OF BINARY AND TERNARY DISTILLATION
One of the purposes of this investigation was to determine the extent to which general conclusions on binary distillations might be extrapolated to predict sharpness of separation between two components in a ternary mixture. This was done by conducting binary distillations and comparing the resulting component curves with the component curves of the corresponding ternary distillations. Binary runs B-5 and B-1 have been plotted together with the corresponding ternary run T-10 in Figure 1. (In the figure, the abscissa for B-5 is the moles distilled over rather than the mole per cent distilled; for B-7 the abscissa is moles distilled over plus 7 . This method of plotting places all the curves on the same absolute basis and makes them directly comparable.) Binary runs B-4 and B-2 have been plotted with their corresponding ternary run,*T-24, in Figure 2. (Abscissa for B-4 is moles distilled over; for B-2 it is moles distilled over plus 30.) In these figures the binary values -of mole per cent distilled have W 2 BO been scaled to a ternary basis; the number of 2 theoretical plates used was 82. The binary runs g60 were conducted under the same reflux ratio and z #with the same absolute number of moles of $40 each component in the charge as for the correD: L sponding ternary. Thus, the charge for binary 20 run B-5 (Figure 1) consisted of 7 parts of nB heptane and 21.7 parts of methylcyclohexane, 0 while for the comparable ternary run, T-IO, the charge contained these quantities of heptane and methylcyclohexane plus 71.3 parts of toluene. Figure 2. It might be expected that the sharpness of separation between methylcyclohexane and toluene would be similar for the binary runs provided there was little n-heptane present in the distillate a t the break (Figure 1).
7.5/1
7.6/1 7.5/1 7.5/1 15/1 15/1 15/1 15/1 30/1 30/1 30/1 60/1
60/1
60/1
IIb
4.5 9 9 18 4.5 9 18 18 4.5 9 18 4.5 18 18
0.75 0.87 0.72 1.66 0.65 1.3 1.72 2.4 1.04 1.08 2.3 1.0
2.41 2.5
BINARY RUNS, SERIES IIIc
B-5
7.5/1 7.5/1 7.5/1 7.5/1
B-1 B-4 B-2
30 10 18 13
212 310 295 278
2.96 0.71 1.78 0.56
a n-Heptane, 7 inole %; methycyolohexane, 21.7 mole % ; and toluene, 71.3 mole %. b %-Heptane,30 mole % : methylcyolohexane, 20 mole % ; and toluene, 50 mole %. 0 B-5: 7 parts (moles) n-heptane, and 21.7 parts (moles) methylcyclohexane B-1: 21.7 arts methyloyolohexane and 71.3 parts toluene. B-4: 30 par& heptane a n a 2 0 parts methyloyolohexane. B-2: 20 parts methylcyclohexane and 50 parts toluene.
IYhen appreciable n-heptane is still present in the distillate at the break between methylcyclohexane and toluene as in Figure 2, the component curves cannot be expected to agree. But it is found that the ratio of methylcyclohexane to toluene is close for the two cases. The distillation of n-heptane and methylcyclohexane during the first portion of these ternary batch distillations was conducted in the presence of appreciable toluene in the pot. Evidently, the effect of this third component was not too great, a t least in the absence of appreciable toluene in the distillate, for the binary n-heptane-methylcyclohexane curves agree with t h e ternary curves.
_1
ID
20
30
40
50
MOL PERCENT DISTILLED
Comparison of Binary and Ternary Experimental Distillations
-
-. -.-- BinaryrunB-4 Binaryarun B-2 9 mole $40
Ternary r u n T-24
Holdup, Reflux ratio, 7.5 to 1 For charge eompositione, see Table I
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
1482
M O L FERCENT DISTILLED
Figure 3.
---
Effect of Holdup on Sharpness of Separation
Ternary run T-12
T-10 , -.Reflux .- ratio, Ternary run T-13 7.5 t o 1 Ternarv run _ __. . .
-.
For charge compositions and mole per cent holdup, see Table 1
Vol. 44, No. 6
section show that the n-heptane-methylcyclohexane component curves are very different when the holdups were 4.5, 9, and 18% on a ternary basis. Thus, the sharpness of separation between two components was apparently governed by the effective holdup-Le., the per cent of holdup based upon the absolute amounts of the two Components under consideration in the initial charge. The n-heptane-methylcyclohexane curves of Figure 1 also illustrate this point. The per cent of holdup to charge capacity of several commercial fractionators was estimated by Pigford and coworkers ( 3 )to be about 1%. Such a value of holdup might appear to be negligible, but on the basis of the above discussion this holdup would be quite important in certain multicomponent fractionations. For instance, if two components were present in a multicomponent mixture t o the extent of lo%, the effective holdup governing the sharpness of separation between these components would be 10% even if the relation between column holdup and total charge were only 1%. If the ratio of column holdup to still capacity or total charge were 2% (still an apparently negligible value), the effective holdup for the above two components would be 20%. For this reason, high effective holdups can be expected to be regularly present and t o have a pronounced influence upon the sharpness of separation between components in a multicomponent distillation. Since it was indicated that sharpness of separation between components of a multicomponent mixture can often be predicted from binary data, it follows that such binary data should be obtained under conditions of appreciable as well as small holdup.
~
0 lo
Figure 4.
--
20 M O L PERCENT DISTILLED
SHARPNESS OF SEPARATIOR IY TERNARY EXPERIWEYTS
40
30
Effect of Holdup on Sharpness of Separation
Ternary run T-5 -.-Reflux .- ratio, Ternary run T-6 15 to 1
Ternary run T-4
For charge compositions and mole per cent holdup, see Table I
1
TOLUENE
0
10
20
30
40
M O L PERCENT DISTILLED
Figure 5.
---- -
Effect of Holdup on Sharpness of Separation
Ternary run T-1 Ternary run T-9 Ternary run T-8 Reflux ratio, 30 t o 1 For charge compositions and mole per cent holdup, see Table I
-. .-
ABSOLUTE AND EFFECTIVE IIOLDU-P
Apparently the sharpness of separation between any two components of the ternary mixture was influenced by the ratio of the holdup t o the absolute quantities of these two components in the mixture rather than the ratio of holdup to total multicomponent charge. For example, the moles of total holdup and the absolute quantities of n-heptane and methylcyclohexane were the same for the binary and ternary runs of Figure 2. The amount of holdup to total ternary charge was 9 % for T-24, while to the binary charge for B-4 it was 18%. Yet the component curves are almost identical. The ternary runs of Figure 6 discussed in the following
EFFECTOF HOLDUP.The ternary runs were conducted a t holdups of approximately 4.5,9, and 18% based upon the initial ternary charge. I n general, a t reflux ratios of 15 to 1 or less an increase in holdup increased the sharpness of scpai ation between components. ilbove a reflux ratio of 30 to 1 an iriciease in the per cent of holdup decreased the sharpness of separation. However, the magnitude of the effect was different foi the two pairs of components, and it was also influenced by the piesenc'r of total reflux startup. The effect of holdup can be studied by superimposing a number of batch distillation curves on one graph. This has been done in Figures 3 to 9. (In each of these figures, the number of thcoretical plates was 82 and the distillation v a s operated a t total reflux startup.) Considering the break between methylcyclohexane and toluene for the distillations in which the initial charges mere 30y0 nheptane, 20% methylcyclohexane, and 50% toluene (Figures 6 to 9), it is seen t h a t there was a definite increase of sharpness of separation with an increasing per cent of holdup a t a reflux ratio of 7.5 to 1 (Figure 6). At a reflux ratio of 15 t o 1 (Figure 7) increasing the per cent of holdup was beneficial, but the over-all effect was small. When the reflux ratio was 30 to 1 there %as no appreciable effect of holdup (Figure 8 ) , while a t a 60 to 1 reflux ratio (Figure 9) the 18% holdup break appeared to be slightly less sharp than that of 4.5%. Figures 3 through 5 show effects of holdup and reflux ratio for 7% n-heptane-21.7% methylcyclohexane-71.3 % toluene mixtures similar t o those shown for 3020-50% mixtures in Figures 6 through 9. The effect of holdup on the sharpness of separation between the components n-heptane-methglcyclohexanecan be noted by again referring to Figures 6 to 9. Increasing the per cent of holdup was beneficial in these runs up t o a reflux ratio of 30 t o 1. At a reflux ratio of 60 to 1 there seemed t o be a small but definite decrease in the sharpness of separation a t the larger holdup. The sharpness of separation between n-heptane and methylcyclohexane was complicated by the presence of an effect due to total reflux startup. Such an effect occurs whenever the distillate composition of the most volatile component of a batch distil-
June 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
1483
lation initially at total reflux is much greater than if the column were operating at the finite reflux ratio to be used throughout the distillation. I n such a case the distillate composition tends t o drop more or less sharply as soon as product removal is begun in order to attain a distillate composition consistent with still composition and the reflux ratio employed. This occurred in the runs of Figures 3 to 5 . Increasing the holdup from4.5 to 9% (ternary basis) noticeably MOL PERCENT DISTILLED improved the sharpness of separation between n-heptane and methylcycloFigure 6. Effect of Holdup on Sharpness of Separation hexane, even a t a reflux ratio of 30 t o 1. Ternary run T-18 Ternary run T-24 When the holdup was increased from 9 to Ternary run T-23 1S%, there was little improvement at For charge compositions, reflux ratios, and mole per cent holdup, see Table I reflux ratios of 15 and 30 to 1. This was probably the result of depletion of the n-heptane still pot composition at t h e highest per cent of holdup (actually a n effective holdup of 60%) so t h a t the initial distillate mole fraction of nheptane was lower than at lesser values of holdup. I n the absence of such a n effect-Le., at valuee of holdup of 9% or less-the data indicate t h a t increasing t h e per cent of holdup will be beneficial t o sharpness of separation a t much higher reflux ratios than 30 t o 1. These data show t h a t the general MOL PERCENT DISTILLED conclusions regarding the effect of holdup Figure 7. Effect of Holdup o n Sharpness of Separation in binary mixtures are applicable to ternary mixtures. Colburn a n d Ternary run T-21 TernaryrunT-27 Stearns (I), Pigford, Tepe, and Garrahan Ternary run T-26 For charge compositions, reflux ratios, and mole per cent holdup, see Table I (S), and Rose et al. (5-7) have indicated or shown experimentally that increasing the per cent of holdup could have either 100 a beneficial, detrimental, or little effect w upon the sharpness of separation between !jeo components of a binary system. The I data of this investigation substantiate 55 6 0 this for a ternary system and again show B t h a t the effect of holdup may be large in k batch distillation. EFFECTOF REFLUXRATIO. In the absence of appreciable effect of total reJ 20 8 flux startup increasing the per cent of holdup had little influence on the sharp0 10 20 30 ' 40 60 ness of separation between components MOL PERCENT DISTILLED at a reflux ratio of 30 to 1. Above this Figure 8. Effect of Holdup on Sharpness of Separation value of reflux ratio, increasing holdup Ternary run T-22 was detrimental, while below this reflux TernaryrunT-20 ratio increasing holdup was beneficial. Ternary run T-19 Reflux ratio, 30 t o 1 Prevost ( 4 ) first observed this effect of For charge compositions and mole per cent holdup, see Table I reflux ratio while studying the binary batch distillation of methylcgclohexaneferent. I n addition, no effect of charge composition was obtoluene in columns of 20 and 40 theoretical plates. He defined the critical reflux ratio as the ratio where holdup had no served. If further experimental data should substantiate t h a t effect on, the sharpness of separation. critical reflux ratio depends only on the number of theoretical Prevost believed t h a t the critical reflux ratio would depend not plates, then the concept of a critical reflux ratio would become a only on the number of theoretical plates of the column, but also powerful tool for predicting the effect of increasing t h e per cent of on the charge composition and relative volatility of the mixture holdup upon the course of a batch distillation. The critical being distilled. The data reported herein indicate t h a t the critireflux ratios obtained by Prevost were roughly 24 to 1 and 9 t o 1 cal reflux ratio, R,, is independent of the relative volatility. I n in 40- and 20-plate columns, respectively. The data of this paper the absence of effects caused by total reflux startup, it was obshow a critical reflux ratio of approximately 30 t o 1 in an SO-plate served that R, was the same for both pairs of components, ncolumn. This would indicate t h a t R, for a given column might heptane-methylcyclohexane and methylcyclohexane-toluene, albe expected t o be about one half of the number of theoretical plates at total reflux of the column. though the relative volatilities of the pairs were considerably dif-
--.-.--
---. .-
8"
---. .-
INDUSTRIAL AND ENGINEERING CHEMISTRY
1484
VOl. 44, No. 6
TERNARY EXPERIMENTAL vs. BIKARY CALCULATED CCRVES
100 W
4 s stated, the shapes of the coniponent' curves for each of two experimental ternary distillations were closely equivag60 P lent to the corresponding curves of two texperimental binary distillations performed under identical experimental conditions. In these ternary distillations .J eo the holdup on a ternary basis \vas 9%. I It, n-as of interest, therefore, to compare 0 10 PO 30 40 W some of the other experimental ternary MOL PERCENT DISTILLED distillations for which the per cent of Figure 9. Effect of Holdup on Sharpness of Separation holdup was the smallest with binary Ternary ruu T-36 curves calculated on the assumption of .. Ternarj r u n T-35 Reflux ratio, 60 to 1 negligible holdup. Such binary calculaFor charge compositions and mole per cent holdup, see Table I tions \\-ere made for both the n-heptanemet,hylcyclohexane and methylcyclo100 hexane-toluene systems. In these binary calculations the relative volatility of the w system n-heptane-methyl c y cl o h e x a n e 580 -I was taken as 1.075. The relative vola!bo tility of the syst,em methylcyclohexaneE toluene varied and the calculations were Imade with the aid of an equilibrium dia2" gram plotted from the binary data of Kirk (2). Aictually, the relative volaeo I tility of the system n-heptane-methylcyclohexane was not constant in the pres0 10 20 30 40 50 of toluene, but varied from 1.075 ence MOL PERCENT DISTILLED t,o 1.15. The relative volatility of methylFigure 10. Effect of Reflux Ratio at High Percentage of Holdup cyclohexane-toluene in the ternary mixT-23 ture v a s close to that of the binary and T-26 varied from 1.1 to 1.6. The binary cal-. .- T-19 . . T-35 culation procedure was that described by Holdup, 18 m o l e % For reflux ratios and charge rompositions, see Table I Smoker and Rose ( 8 )and the calculations Tvere made using the same mole rat,ios and reflux ratios a8 for the corresponding IO0 t,ernary distillations. w n-€Iep t , a n e - m e t hylcyclohexane no$80 holdup binaries were calculated for comi F parison x-ith the 4.5% holdup ternary E60 runs at reflux ratios of 15, 30, and 60 to 1 P Iand are plotted on a ternary basis in 540 Figures 12, 13, and 14, together with a the experimental ternary runs, using 82 If -1 20 theoretical plates. (In order to make 8 the binary curves directly comparable _~___ with t,he ternary curves, the abscissa for 0 10 30 40 50 the n-heptane-methylcyclohexane binary MOL PERCENT DISTILLED curve has been plot,t,ed as moles disFigure 11. Comparison of Calculated Binary and Experimental Ternary Batch tilled orer, and that for the methylcyDistillation Curves clohexane-toluene binary as moles disExperimental T-18 . Calculated for toluene from methylcs clohexane-toluene binary, assuming n o holdup tilled over plus 30.) Agreement is Binary charge composition. 20.0 moles of methylcyclohevane and 50.0 moles of toluene surprisingly good c o n s i d e r i n g t h a t For ternary charge composition, reflux rntio, and mole per cent holdup, see Table I t h e e x p e r i m e n t a l effective holdups were 9%. Both Prevost ( 4 ) and Pigford and coxorkers ( 5 ) noted for The methylcyclohexane-toluene binary batch distillation binary systems that when holdup %-aslarge the effect of reflux curves m r e calculated for the reflux ratios of 7.5 and.15 to 1 . These have been plotted with the corresponding 4.5% holdup ratio on the sharpness of separation between components TTas ternary runs in Figures 11 and 12. ( I n Figure 11, in order t o much less pronounced than RThen holdup was small. This was make the binary curve directly comparable with the ternary also indicated by the ternary data of this thesis. However, it curves, the abscissa for the binary is plotted as moles distilled shouId not be inferred that reflux ratio can be neglected under over plus 30, instead of mole per cent distilled.) I n these t'iT-0 conditions of holdup of perhaps 20 to 30%. Thus, the runs of cases there is almost no resemblance between the calculated and Figure 10, using 82 theoretical plates and operating a t total reflux experimental curves. The effective holdups of the ternary runs startup, show a marked effect of reflux ratio on the sharpness of were 6.491;. Although it might appear that some error mas made separation between n-heptane-methylcyclohexane when the in the calculations, none could be found. These results, then, holdup was 18% on a ternary basis (effective holdup 36%).
f
80
_I
F
5" E
--
i _I
--. ..
-. -
INDUSTRIAL AND ENGINEERING CHEMISTRY
June 1952
cast doubt on the advisability of attempting to predict sharpness of separation from no-holdup calculations when the effective holdup is anything but negligible. Further experiments a t successively lower valuea of holdup should be made to confirm the large difference between calculated no-holdup curves and experimental curves a t 6% holdup. Or alternately, the procedure of Rose, Johnson, and Williams (6) should be used to calculate batch distillation curves under holdup conditions of less than 6y0. CONCLUSIONS
1485
100 W
5 eo
di j m E IE40
!? 20 -I
I 0
lum 1-1-11 IO
YflmLCVCLOWfXAMf
PO
1
30
[Tl__ 1
1 I r";?'ry,/
40
~
60
1
MOL PERCENT DISTILLED
Figure 12. Comparison of Calculated Binary and Experimental Ternary Batch Distillation Curves
--. -.--
Experimental T-21 Calculated n-heptane-methylcyclohexane Calculated methylcyclohexane-toluene Binary charge oompositions. 30.0 moles of n-heptane and 20.0 moles of methylcyclohexane: 20.0 moles of methylcyclohexane and 50.0 moles of toluene For ternary charge compositions, reflux ratio, and mole per cent holdup, see Table I
The general effects of holdup in ternary batch distillation were the same as observed for binary systems (3, 4,7). The effect of increasing the per cent of holdup was found to be beneficial, detrimental, or of no effect upon the sharpness of separation between components of a ternary batch distillation, depending on the reflux ratio. In the absence of total reflux startup effects, it was found that holdup was beneficial a t reflux ratios of 7.5 and 15 t o 1,detrimental at reflux ratio of 60 to 1, and exerted no effect when the reflux ratio was 30 to 1. This was true of the separation of both n-heptane-methylcycloMOL PERCENT DISTILLED hexane (relative volatility, 01 = 1.08 t o Comparison of Calculated Binary and Experimental Ternary Batch Figure 13. 1.15) and methylcyclohexane-toluene (a Distillation Curves = 1.10 to 1.60). The reflux ratio of 30 Experimental T-22 t o 1, then, was the critical reflux ratio Calculated n-heptane-methylcyclohexane Binary charge composition. 30.0 moles of n-heptane and 20.0 moles of methylcyclohexane as defined by Prevost (4) for this column For ternary charge compositions, mole per cent holdup, and reflux ratio, see Table I of 82 theoretical plates. The effect of holdup was complicated 100 by total reflux startup. When total reflux startup effects are present, the disW tillate composition is much higher in the 80 most volatile component a t the start of d L t h e batch distillation than it would be if ijw the column were operating a t the reflux z Iratio t o be used throughout the distillaE40 tion. I n such a case the mole fraction of the most volatile component tends to J eo drop sharply, and the per cent of holdup I has a pronounced effect upon the shape of the batch distillation curve. In the 0 IO 20 30 40 so MOL PERCENT DISTILLED presence of this effect it was found that increasing holdup was beneficial t o the Figure 14. Comparison of Calculated Binary and Experimental Ternary Batch Distillation Curves sharpness of separation between nheptane and methylcyclohexane in the Ex erimentalT-36 Cafculated n-heptanemethylcyclohexane ternary mixture at reflux ratios of 7.5, Binary charge composition. 30.0 moles of n-heptane and 20.0 moles of methylcyclohexane 15, and 30 to 1. For ternary charge composition, mole per cent holdup, and reflux ratio, see Table I A comparison of the batch distillation curves of the ternarv and binarv compared with the corresponding experimental ternary cases in runs conducted under identical conditions of reflux ratio and which all variables were the same except the per cent of holdup. absolute charge of each component showed that the separaWhen the effective holdup of n-heptane-methylcyclohexane in the tion between the two components of the binary was similar t o experimental ternary distillations was 9%, the experimental curves the separation of these two components in the ternary, proagreed well with the binary curves calculated a t reflux ratios of 15, vided that only these two components were present in the ternary 30, and 60 t o 1. However, the experimental curves of methyldistillate in appreciable quantity. cyclohexane-toluene in which the effective holdup was 6% did not A number of binary batch distillation curves were calculated agree with the binary calculated curves a t reflux ratios of 7.5 for the systems n-heptane-methylcyclohexane and methylcycloand 15 to 1. hexane-toluene assuming negligible holdup. These curves were
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I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
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The binary-ternary comparisons also indicated that the sharpness of separation between two components must be governed by the effective holdup-that is, the per cent of holdup based on the absolute amount of the two components under consideration in the init,ial ternary charge. Since tsvo components may constitute a small portion of a multicomponent charge, large effective holdups and, consequently, large effects on separation can be expected t o be regularly present in multicomponent batch distillation. ACKNOWLEDGMENT
Grateful acknowledgment is made to the Shell Oil Co. and Research Corp. for financial support that made this work possible.
Engrnyring
Vol. 44, No. 6
LITERATLRE CITED
(1) Colburn, A. P., and Steams, R. F., T r a n s . Am. Inst. Chem. Engrs., 37, 291 (1941).
Kirk, N., Ph.D. thesis, The Pennsylvania State College, 1946. (3) Pigford, R. L., Tepe, J. B., and Garrahan, C. J., IND.ESG. CHEM., 43, 2592 (1981). (4) Prevost, C., M.S. thesis, The Pennsylvania State College, 1948. (5) Rose, Arthur, Johnson, R. C., and Williams, T. J., IND.ENG. CHEM.,42, 2494 (1950). (2)
(6) Ibid., 43, 2459 (1951). (7)Rose, Arthur, Williams, T. J., and Prevost, Charles, Ibid., 42, 1876 (1960) (8) Smoker, E. H., and Rose, Arthur, Trans. Am. Inst. Chem. Engrs., 36, 285 (1940).
A C C ~ P T EJanuary D 22, 1952. Presented before the X I I t h International Congress of P u r e and Applied Chemistry, S e w York, September 1961.
RECEIVED for revieiv August 9, 1951.
Extraction of Mercaptans from
Process development
Distillate R.
L. YAHNME, J. H.KRAUSE, AND G. H. WEISEMANN
RESEARCH DEPARTMENT, STANDARD OIL CO. (INDIANA), WHITING, IND.
V
IRGIN light distillate fuel obtained from high-sulfur crudes has a high mercaptan content and consequently an unpleasa n t odor. This oil boils in the range of 330" to 580" F. and contains 0.08% mercaptan sulfur. T o make this oil satisfactory for use as a domestic heating fuel, it is necessary to remove the mercaptans or convert them t o compounds having less offensive odor. Extraction processes employing sodium hydroxide and methanol have been reported by other investigators for removing mercaptans from gasoline ( I ) and for improving the color stability of cracked distillate fuels ( 2 ) . Since the mercaptans present in virgin light distillate fuels are higher in molecular weight and more complex structurally than those found in gasoline, they behave more Iike hydrocarbons and are much less susceptible to extraction with a basic solvent. Nevertheless, the effectiveness of caustic and methanol as a solvent for mercaptans \Tarranted a detailed investigation of the extraction of high boiling distillate with solvents of this type. In order to overcome the odor problem, special emphasis had to be placed on removal of the maximum amount of difficultly extractable mercaptans. Extensive bench-scale and pilot plant studies have led to the development of a continuous process in which operating conditions and solvcnt composition are markedly different from those used in previous processes. BENCH-SCALE STUDIES
Bench-scale experiments were conducted to explore such fundamental variables as temperature, solvent composition, and solvent-oil ratio. In viay of the high cost of methanol recovery, a major objective was to obtain efficient extraction mith the minimum amount of methanol in the solvent. The bench-scale studies were performed batchwise in glassware. Generally, 1 liter of the oil and the desired quantity of solvent were mechanically agitated for 15 minutes in a 2-liter flask equipped with a thermometer, a heating mantle, and a stopcock a t the bottom to facilitate draining; a stream of nitrogen was bubbled through the mixture to prevent oxidation. Except when the solvent-oil ratio was being investigated, the oil was twice extracted with 20y0 by volume of fresh solvent.
Properties of the virgin light distillate employed in the present studies are shown in Table I. On the basis of mercaptan sulfur content and an average mercaptan molecular weight of 170, this stock contained about 0.4% mercaptans by volume and had a mercaptan number of 67. The term "mercaptan number" denotes mercaptan sulfur content in milligrams per 100 ml. of sample.
TABLE I. FEEDSTOCK INSPECTION DATA (Virgin light distillate fraction from high-sulfur crude, prewashed mith dilute caustic) mg./100 ml. 67 0.081
+3170.862 9.7 329 379
413 443 476
552 579
Mercaptan sulfur contents of the oil before and after treatment were obtained by titration with copper sulfate. Total sulfur was determined by the lamp method (ASTBI D90-41T). Color intensities were measured u ith the Saybolt chromometer (ASTM D156-49).
EFFECTOF TEMPERATURE. The effect of temperature on mercaptan removal is illustrated in Figure l a . Mercaptan extraction efficiency decreases mith rising temperature, so that Ion-er temperatures are preferred. However, 90" F. appears to be a practical lower limit, because the solvent begins to solidify a t slightly loxver temperatures. The optimum range of 90" t o 100" F. was employed in the subsequent studies. SOLVEST COMPOSITION.To prevent solidification of the solvent a t 90" F., 5970 aqueous potassium hydroxide requires the addition of 19% by volume of methanol, whereas a 48% solution of sodium hydroxide requires 28% by volume of methanol. Of these two solvents, the potassium hydroxide not only requires less methanol but also is much more effective for the extraction of
