Fractionating Performance of a Dephlegmating Condenser ROBERT W. TIMMERMAN’ AND CHARLES L. NICKOLLS Oklahoma Agricultural and Mechanical College, Stillwater. Okla.
M
ANY installations of -Measurements of the separating efficiency densate is formed, the liquid of a dephlegmating condenser were made is immediately removed from distillation equipment further interaction with the provide two Or more under different operating conditions. A remaining vapor. Simple concondensers in series, each one condensing part of the vapor. condenser Of engineering laboratory densation will produce a sepaThe usual arrangement is for was used. Binary mixtures of methanol ration equal to that of one the condensate from the first and water were employed in the tests. theoretical plate. Differential A greater separating efficiency was obcondensation can effect separacondenser to be returned to the tions greater than that of one rectifying as reflux and tained with parallel flow of cooling liquid theoretical plate. True diffor that condensed in subseand condensing vapor (both flowing upquent condensers to be withferential condensation is diffidrawn as product. The first ward) than with the countercurrent flow cult to accomplish because of condenser is variously termed (vapor flowing upward). The use of reasonmechanical considerations. ably high vapor velocities gave better fratConditions approaching this the “reflux condenser”, “partial type of achieved condensation arewith not condenser”J Or “dephlegmationation than low velocities. With paralreadily except tor”. A certain amount of fracle1 flow of vapor and liquid, the efficiencyinapparatus of special constructionation takes place in the decreased definitely with increasing reflux tion. phlegmator. Relatively more ratio; with countercurrent flow the effiUnder certain conditions, ciency decreased slightly with increasing reparticularly high vapor veof the less volatile components locity, little or no separation are condensed than are those Of flux ratio. A wide variation in concentramay be obtained. This is higher volatility. The degree of separation effected is pretion Of the entering vapor had but a probably due to the rapidity dieted with uncertainty, howeffect on the theoretical plate equivalent with which the vapors approach the cooling surface and separation obtained. ever, because of the large numcondense on it. No time is her of variables entering into consideration. This paper reallowed. for selective condensation or diffusion of molecules t o take place, and the ports the results of a n investigation of the effect of several of components are condensed substantially in the proportions these variables on such performance. Considerable controversy has existed over the value of that exist in the vapor. dephlegmators as fractionating devices. Some authors (9) have stated that the dephlegmator is an important part of the Experimental Apparatus and Measurements fractionating system; others (1, 4, 6) believe that any separation which occurs in the dephlegmator is incident,al to its The binary mixture used in this study was methanol and water. This mixture was chosen because of ease of analysis and because major function of condensing the vapor and providing reflux. no azeotropes are formed and a fairly large enrichment can be obThe degree of separation of two components effected in a partial condenser is usually referred to as the efficiency of the t n ~ ~ i &t ) ~ ~ ~ ~ ~Figure i The ~ ca- ~ partial condenser. The first expression of partial condenser pacity of the pot Tvas 12 gallons (45 liters). Heat t o boil the solution was provided by steam condensing in a flat spiral coil efficiency as a n equivalent to a fraction of a theoretical plate near the bottom. The two condensers, one used as dephlegwas by Underwood ( 7 ) , stating that this is the only logical mator, and the other as final condenser, were identical. They The vapor leaving a partial condenser may possibly were of copper construction. Cooling water flowed through a be either richer or poorer in the more volatile Component than single-pass bundle of fifty-six l/a-inch (0.635-cm.) inside diameter tubes, The length of the tube bundle was 30 inches (76 em.). that called for by equilibrium conditions. The efficiency can They were uniformly spaced in a shell 6 inches (15.24 cm.) in dithus be greater than, equal to, or less than that of one theeameter. retical plate. Cooling water for the dephlegmator was taken from a 50-gallon Two theoretical mechanisms of’condensation in a dephlegreservoir mounted above the apparatus. This system of supply was used so that the water could be heated to a predetermined mator are possible. One is simple or equilibrium condensatemperature. By the use of warm water for the dephlegmator, tion. This assumes that the vapor is in equilibrium with the control of the amount of vapor condensed was much more reliable condensate before they are separated. The other is differthan would have been possible with cold water. Also this is the entia1 condensation, which assumes that as soon as any concondition which exists in many commercial dephlegmators, the water having first passed through another piece of equipment, 1 Present address, Westvaoo Chlorine Products Corporation, South usually the final condenser. An abundant amount of cold water Charleeton, W. Ve. 1230
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October, 1941
INDUSTRIAL A N D E N Q I N E E R I N E CHEMISTRY
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was passed through the final condenser in these tests t o ensure complete condensation of all vapors. The condensate from each condenser was metered in a calibrated weir box. The flow was through a circular orifice dischargin vertically. Condensates from the two condensers were returne8 continuously to the pot. After e uilibrium had been reached, samples of both partial condensate?.reflux) and product were withdrawn for analysis. Analyses for per c:nt methanol were made by specific gravity measurements. These measurements were made at 15 C. with a Westphal balance capable of indicating specific gravity t o the fourth decimal place. Approximately 3 gallons (11 liters) of solution were placed in the pot for each set of runs. The boiling point of the solutions varied from about 70" to SO" C., depending on the percentage of methanol.
Data and Calculations Rates of flow of the liquid from the two condensers were converted to mass rate in grams per minute and then to gram moles per minute. Calculation of the theoretical plate equivalent of the dephlegmator for each run mas made graphically by a modification of the method of McCabe and Thiele (2)suggested by Webber and Bridger (8). Since the composition of both reflux and product were known, the composition of the entering vapor could be calculated, and the indirect method as used by Webber and Bridger was unnecessary. Vapor-liquid equilibrium data for the methanol-water system used were those of Bergstrom presented by Hausbrand (1). The molecular weight of methanol used throughout the calculations is a fictitious value, 39.3, which gives equal molal latent heats of vaporization for both components. This assumption is made by the McCabe-Thiele calculation (2). This is essentially the same as using the "equivalent latent heat fraction" as suggested by Peters (6). The theoretical plate equivalent obtained by the method used is analogous to the Murphree plate efficiency (3) as applied to a rectifying column. Representative data are presented in Table I. The mole per cent of methanol in reflux, product, and entering vapor are given for each run, together with rate of flow of each in gram moles per minute. Calculated values of the ratio of
u 5.8 Ly 0 .6
'-0
IO
GRAM
M O L E S PE,R MIN.,
I
~~~
I
2
3
4
5
6
REFLUX RATIO
FIGUREI 2. VARIATION OF DEPHLEGMATOR EFFICIENCY WITH
REFLUXRATIO
Aboue. counterourrent flow of liquid and vapor: liquid and vapor.
below, parallel flow of
reflux to product (reflux ratio), the ratio of reflux to vapor, and the theoretical plate equivalents are also included. Plots of theoretical plate equivalent against the several variables studied are given in Figures 2 and 3. The upper half of Figure 2 and Table I, A , B, C, and D show the variation of efficiency with reflux ratio for countercurrent flow of cooling water and vapor. Points are plotted for four different concentrations of entering vapor. The rate of flow of vapor to the dephlegmator was held Hubstantially constant. The lower half of Figure 2 and Table IE show a set of points of varying reflux ratio with parallel flow of cooling water and vapor (upward) in the dephlegmator. Other variables, including rate of flow and entering concentration were kept as constant as possible. Figure 3 and Table IF show the effect of rate of flow of vapor entering the dephlegmator. The reflux ratio was maintained as constant as possible from run to run. It did vary slightly, but from Figure 2 (upper graph) this should have little effect.
Discussion
.
FIGURE 1. DIAGRAM OF APPARATUS
The points in the upper half of Figure 2 are scattered. This indicates that some influencing variable, probably the rate of flow, could not be controlled closely enough. The maximum deviation from the mean curve is about 0.1 theoretical plate, which is not considered excessive for the set-up of equipment and method used. The general trend is toward a lower efficiency (theoretical plate equivalent) with increasing reflux ratio. The points for the several concentrations of vapor are scattered among each other. The conclusion is that throughout the range studied, the concentration of entering vapor has little effect on the performance of the dephlegmtttor. The actual enrichment of the low-concentration vapor was greater than that of the higher ones (for example, 50 to 84 against 92 to 96), but this is true also in a bubble plate. The results of the aeries of runs in which the cooling water was passed upward through the dephlegmator (Figure 2, lower graph) shows a considerable rise in separating efficiency with increase in reflux ratio. The reason for the difference between this and the previously described series is probably in the
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 33, No. 10
TABLE I. DEPHLEGMATOR TESTDATA Reflux Mole % Mo!es/ CHaOH min.
Run No. A.
Product h,foles % Mo!es/ CHaOH nun.
Vapor Feed IpoaB Mo,les/ min.
Retlux Ratio, R/P
Ratio R/V
Theoretioal Plate Equivalent
Cooling Temp.,
In
"20
C.
out
33 40 34 41 35 36 44 38 37
Countercurrent Flow of Cooling Water and Vapor; Composition of Entering Vapor Approximately 93 Mole % CHaOH: Reflux Ratio Varied 3.45 89.2 96.3 5.00 93.3 8.45 0.690 0.408 1.03 50 59 3.60 89.3 95.9 4.36 92.9 7.96 0.827 0.452 0.90 50 59 4.64 90.4 96.3 3.99 92.8 8.63 1.16 0.638 0.91 48 56 4.78 89.5 96.1 3.25 92.0 8.03 1.47 0.596 0.96 45 55 5.83 91.5 96.4 2.95 93.0 8.78 1.9s 0.664 0.87 44 53 6.39 91.2 96.5 2.89 92.8 9.28 2.21 0.689 0.94 45 54 5.91 89.6 95.8 2.21 91.3 8.12 2.68 0.728 0.90 45 54 92.1 7.31 96.5 1.96 92.8 9.27 3.73 0.788 0.91 41 52 7.63 92.1 96.5 1.48 92.8 9.11 5.15 0.839 0.83 40 51
3 17 5 14 21 13 12 23 25
68.9 69.5 71.1 73.5 74.7 76.7 79.1 78.9 79.1
2.63 3.67 3.73 4.51 5.67 5.73 6.02 6.73 7.13
55 51 52 56 53 58
61.6 60.1 62.4 67.3 68.8 68.1
4.82 5.01 5.30 6.74 7.68 7.57
75 84 76 85 86 77 78
32.9 31.8 40.9 38.8 43.8 46.7 46.1
3.23 5.27 5.72 7.16 8.34 7.52 8.20
87 91 88 92 93 90 94
32.2 31.3 35.0 34.7 39.4 39.5 43.1
B . Same as A , but Composition of Entering Vapor Approximately 81 Mole % CHaOH 87.8 88.9 89.3 88.9 89.5 89.8 91.3 90.2 90.1
5.74 5.00 4.75 4.02 3.17 2.94 2.76 1.50 1.28
81.5 80.4 81.2 81.0 80.5 80.5 83.2 80.6 80.5
8.37 8.67 8.48 8.53 8.84 8.69 8.78 8.23 8.41
0.448 0.734 0.785 1.12 1.79 1.95 2.18 4.49 5.57
0.314 0.423 0.440 0.529 0.641 0.661 0.685 0.816 0.849
0.99 1.15 1.10 0.90 0.91 0.89 0.91 0.90 0.83
55 53 54 51 47 48 48 46 _. 49
62 61 61 58 56 57 57
0.95 1.02 1.01 0.75 0.93 0.73
52 49 49 50 42 45
59 59 58 59 55 55
0.91 0.88 0.87 0.83 0.74 0.74 0.73
55 52 48 47 46
64 62 59 59 59
s4 .~
54
C. Same a8 A , but Composition of Entering Vapor Approximately 72 Mole % CHsOH 83.9 85.0 85.0 83.3 87.2 83.0
4.45 4.49 4.25 2.55 2.03 1.97
72.2 72.2 73.0 71.7 72.5 71.4
9.27 9.50 9.55 9.29 9.71 9.54
1.08
1.11
1.25 2.64 3.78 3.84
0.520 5.270.50a 0.725 0.791 0.793
D. Same as A , but Composition of Entering Vapor Approximately 52 Mole % CHsOH
E. 4.44 4.76 6.80
7.37 8.68 8.40 10.19
67.6 66.0
71.1 84.3 84.4 70.4 70.5
5.82 6.00 3.85 4.40 2.97 2.35 1.30
55.0 50.0 52.9 50.0 50.3 51.7 51.7
Parallel Flow of Liquid and Vapor; Entering 61.8 6.05 50.3 61.3 6.41 48.5 71.0 4.10 48.5 70.8 4.15 47.6 76.3 3.08 49.1 ,77.0 2.48 47.9 80.4 1.32 47.5
9.05 11.27 9.57 11.56 11.31 9.87 9.50
0.555 0.878 1.49 1.63 2.81 3.20 6.31
0.357 0.467 0.598 0.619 0.738 0.762 0.864
Vapor 50 Mole % ' CHaOH; Reflux Ratio Varied 10.49 0.735 0.424 0.61 11.17 0.742 0.426 0.64 10.90 1.66 0.624 1.02 11.52 1.77 0.638 1.03 11.76 2.82 0.738 1.20 10.88 3.38 0.762 1.25 11.47 7.70 0.885 1.34
F. Countercurrent Flow of Liquid and Vapor; Entering Vapor 71 Mole % CHaOH; Reflux Ratio 1.4 65.5 64.4 63.2 63.2 59.2 60.8
2.40 3.24 3.58 4.34 5.55 8.75
79.5 83.5 82.9 82.8 83.7 84.7
1.62 2.56 2.66 3.67 4.56 4.92
71.1 72.8 71.5 72.3 70.3 69.4
temperature gradient along the condenser tubes. With the water passing upward, the coldest part of the tubes is a t the lower end. Condensate runs down the surface of the tubes as a film of increasing thickness toward the bottom. The film of liquid becomes cooled to below its boiling point and consequently does not interact readily with the ascending vapors. This should permit a nearer approach to ideal differential condensation. The heat transfer coefficient a t the bottom of the tubes is poorer than i t is near the top because of the thicker liquid film. With cooling water passing upward, the coldest part and consequently the largest over-all temperature difference will be a t the bottom. This higher temperature difference tends to counterbalance the lower value of conductivity and results in a more uniform condensation over the entire length of the tubes. This probably will influence the efficiency obtained. The conclusion can be made that the mechanism of condensation in all the tests reported is a combination of equilibrium and differential condensation, together with a certain amount of condensation without separation. Even though the theoretical plate equivalent is exactly 1, it is unlikely that simple condensation is taking place. The flow of entering vapor in the series plotted in Figure 3
4.02 5.80 6.24 8.01 10.11 13.67
1.48 1.27 1.35 1.18 1.22 1.78
0,597 0.559 0.574 0.541 0.548 0.640
No data No data
39 39 41 40 40 40 40
76 76 71 70 67 68 65
a 0.3, Vapor Velooity Varied
0.56 0.82 0.83 0.82 1.02 1.06
53 50 50 48 49 43
61 59 59 58 59 56
was over threefold. The separating efficiency rises markedly from about 0.6 to a little over 1.0 plate. It is believed that with a further increase in vapor velocity the efficiency would reach a maximum and then possibly decline. Better interaction of vapors due to more rapid convection makes a nearer approach to equilibrium possible, but extremely high velocity
ENTERING V A W R , G R A M MOLES PER MIN.
FIQURE3. VARIATION OF DEPHLEGMATOR EFFICIENCY WITH VAPORVELOCITY(COUNTERCURRENT FLOW OF LIQUIDAND VAPOR)
October, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY
does not allow sufficient time for complete interaction. Thus these two influences tend to nullify each other. All of the tests in this investigation were made with the same binary mixture and in the same apparatus. Therefore the results can yield no information as to the effect of other combinations of substances and apparatus construction. It is known that the efficiency of a bubble plate varies with the properties of the mixture being distilled, so it is likely that the efficiency of a dephlegmator varies in an analogous fasyion. Although numerous dephlegmators of unusual design have appeared in the past, most modern equipment of this type has the same general design-a tube bundle and shell. The arrangement, size, and spacing of tubes and baffles, as well as proportion of length to number of tubes for the same surface area, might be expected to influence the performance of the dephlegmator
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Literature Cited (1) Hauabrand, E., (tr. by E. H. Tripp), “Principles a n d Practice of Industrial Distillation”, London, C h a p m a n a n d Hall,1925. (2) McCabe, W. L., and Thiele, E. W., IND. E N G . CHEM., 17, 605
(1925). (3) Murphree, E. V., I b i d . , 17,960 (1925). (4) Perry, J. H., Chemical Engineers Handbook, New York, McGraw-Hill Book Co., 1934. ( 5 ) Peters, W. A., J. IND. ENG.CHEM.,14, 476 (1922). (6) Robinson, C. S., and Gillilrtnd, E. R., “Elements of Fractional Distillation”, New York, McGraw-Hill Book Co., 1939. (7) Underwood, A. J. V., Trans. Inst. C h e w Engrs., 10, 141 (1932). (8) Webber, H. A., and Bridger, G. L., IND.ENQ.CHEM., 30, 315 (1938). (9) Young, S., “Distillation Principles and Processes”, London, Macmillan a n d Co., 1922. PR~DSENTED before the Division of Industrial and Engineering Chemirtry at the lOlat Meeting of the American Chemical Society, St. Louis, M o .
EVALUATING FILTER AIDS.
..
RICHARD N. COGGER AND
HARVEY M. MERKER Parke, Davis & Company, Detroit, Mich.
EXPERIMENTAL APPARATUS
T
HE ordinary run of commercial work in the field of filtration as a chemical engineering unit process can be placed in one of two categories. One class of industrial filtra-
tion is carried out with the main purpose of recovering the solid material or the sludge and discarding the filtrate. Another class is concerned solely with clarification, in which the sludge is the waste product and a clear filtrate is the desired object. Dewatering or sludge filtrations are usually characterized by the formation of filter cakes which are homogeneous and relatively incompressible, with a tendency toward being reproducible from batch to batch. It is seldom advisable or even permissible to add foreign materials as aids to filtration. On the other hand, the fluid products which require Eltration to produce clarity are usually complex mixtures or vegetable extracts and have formed organic
sediments which are nonhomogeneous, decidedly oompressible, and seldom reproducible from batch to batch. The usual prerogative of good practice in these cases is to add inert materials to aid in filtration. The difficulties which are met in attempting to analyze a given filtration problem are multiplied by the lack of uniformity of conditions in clarification filtration as compared with the characteristics of sludge filtration. Dictates of practice and utility, combined with the many variable individual operating conditions of processing work, have resulted in the use of a great variety of filters throughout the field of filtration engineering. Individual designs of leaf and drum filters, operating under pressure and vacuum, have been numerous and excellent and are continually undergoing improvement and change. It is still
