Performance of Commercial Perf oratedZlate Distillation Column R. C. GUNNESS’ AND J. G. BAKERP School of Chemical Engineering Practice, Massachusetts Institute of Technology, Cambridge, Mass.
In two tests upon a commercial perforated-plate column used for the stripping of alcohol from fermented molasses mash or “beer,” an overall plate efficiency of 40 per cent was determined. With a plate spacing of 18 inches, the column operated with a superficial vapor velocity of 2.6 feet per second and an average vapor velocity through the inch diameter perforations of 17 feet per second. Bubble-cap plates in a similar installation would probably have had a greater efficiency at the start of operations. However, the ease of fouling and difficulty of cleaning make their use in such an installation inadvisable.
HE perforated-plate column finds its chief use in commercial operations as a substitute for the bubble tower when the stocks to be handled would rapidly foul and clog the latter. The simple construction of the perforated plate, consisting of sheet metal with numerous small holes, allows low investment cost, relative freedom from clogging, and easy cleaning. In its efficient operating range the perforated plate, or sieve plate, operates as a bubbling device, the vapor passing up through the holes a t sufficient velocity to hold a body of liquid on the plate above. This liquid drains to the plate below through an overflow similar to that used on bubble trays. Efficient operation is restricted, however, to a rather narrow range of vapor velocities, since insufficient velocity will permit the plates to drain through the vapor holes and excess velocities will cause
T
entrainment. Furthermore, the plate must be accurately leveled or drainage will occur through the low side and vapors will go through the high side. The fermentation alcohol industry has for many years used perforated plates in “beer stills” for distilling alcohol from mash, because of the clogging tendencies of the feed. Modifications have been successfully used commercially in hydrocarbon a n d o t h e r ‘;absorption towers. With the recent d e v e l o p m e n t s of s a t i s factory automatic control systems it is likely that an increased interest will be shown in perforated-plate columns, and that their use will be extended beyond the present restricted field. At present few data are available on the rectification performance of commercial perforated plates. Brown (I) reported the performance of commercial beer stills, presenting no data and no description of PERFORATED-PLATE TEST COLUMN
1 Present address, Standard Oil Company (Indiana), Whiting, Indiana. 2 Present address, Monsanto Chemiaal Company, St. Louis, Mo.
Courtesy, The Lurnmurr Company
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DECEMBER, 1938
INDUSTRIAL AND ENGINEERING CHEMISTRY
the column. Peters (16)presented data on the performance of sieve plates with 0.2-inch holes and 1-inch liquid depth in rectifying acetic acid-water mixtures. Campbell ( 2 )described his experiments with a modification of the perforated plate which he applied to natural gasoline absorbers. Kirschbaum and Andrews (9)reported the results of a series of laboratory, tests, in which the performance of sieve plates in the rectification of alcohol-water mixtures was studied. The diameter of the perforations in the experimental plates was varied from 0.04 to 0.1 inch. With perforations of such small diameters, it is questionable if sieve plates are adaptable to the commercial rectification of dirty stocks. The purpose of this paper is to present the results of tests upon the commercial perforated-plate column used for the separation of alcohol from a molasses mash. Two tests, each consisting of two runs, were made upon the column under normal operating conditions to obtain data on the rectification performance of perforated plates, primarily for use in design. Since the column was a part of an operating system, i t was impractical to study the effect of the rectification variables upon its performance. However, two tests were conducted a t different times after cleaning, so that the effect of column cleanliness upon plate efficiency might be ascertained. The column upon which the performance tests were conducted is the beer still of the New England Alcohol Company at Everett, Mass. I n the manufacture of industrial alcohol from molasses it is desirable to separate alcohol and other volatile constituents from nonvolatile matter contained in the fermented mash or "beer" before the alcohol is concentrated. The mash, consisting of a n aqueous solution of about 5 per cent ethyl alcohol and containing approximately 3 per cent nonvolatile matter and traces of aldehydes and fusel oil, is fed to the beer column, which functions principally as a stripper. The overhead product contains about 50 per cent alcohol and the bottoms, or "slops," leave the column containing less than 0.1 per cent alcohol. The feed to the column is preheated to about 170' F. by heat exchange with the bottoms. The overhead vapors from the column pass to a partial condenser, the condensate returning to the column as cooled reflux and the uncondensed vapors going on as feed to the rectifying columns. Heat is supplied to the base of the column in the form of open steam.
Perforated Plate Column The column contains sixteen plates, all but the top plate being of the perforated type. The column diameter is 51/2 feet, and the plate spacing is 18 inches. Feed is introduced a t the center of the fifteenth or uppermost perforated plate, reflux enters through a distributor ring around the perimeter of the top plate, and open steam enters through a submerged sparger coil a t the column bottom. Overflow arrangement within the column is of the rim and center type; there are three central down pipes and six smaller rim down pipes on alternating plates. The tops of the down pipes are 21/2 inches above the plate level. The perforated plates are inch thick and contain l/Z-inch holes, located on l'/s-inch triangular centers and totaling about twenty-five hundred per plate. The top plate, located above the point of feed and contacting only distilled liquids, is of conventional bubblecap construction. Constructional details may be obtained from Figures 1 and 2. The feed rate to the column, which is largely determined by the fermentation cycle, is maintained constant by a head tank above the column. Adjustment in feed rate is made by a valve whose setting is normally unchanged over long periodft of time. The reflux rate is controlled by regulating the cooling water rate; thia adjustment is by hand. Steam
1395
m
-00
. ..... .. e-.
'LATE'/z"HOLES ON 2500 I '/a TRIANGULAR PER *'***
CENTERS
FIGURE1. CONSTRUCTIONAL DETAILS OF
THE
COLUMN
flow to the column is measured by a recording orifice meter and is automatically controlled to maintain a constant pressure at the base of the column. As the column gradually becomes fouled, it is necessary to increase this control pressure.
Test Methods Each test lasted for over 3 hours of steady operation. Data were taken throughout the entire test periods, which were divided into two runs for purposes of studying constancy of operation during the course of a test. Test I was conducted 34 days after cleaning, test I1 was made after 77 days of operation. The normal period between cleanings, as dictated by decrease in column capacity, was 120 days. I n running a test upon the column, the first step was to establish the unit in a state of dynamic equilibrium. Uniformity of feed stock and constancy in the recorded variables of steam flow, feed rate, and column pressure were of value in determining when this condition had been reached. The column was operated with substantially no change in operating conditions for a period of a t least an hour before tests were started. Direct measurements made during a test were: (a) rates of flow of the feed, cooling water, and steam; ( b ) temperatures of the feed into and out of the preheater, of slops into and out of preheater, of vapors into and out of partial condenser, of cooling water into and out of the partial condenser, of reflux, and of steam; (c) pressures of entering steam and a t column bottom, and pressure differential between pairs of plates; (d) composition of the feed, slops, reflux, vapors to and from partial condenser, and liquid leaving plates 3, 5, 7, 9, 11, 13, and 15. Plates are numbered from the bottom up. The flow of feed stock was determined by measuring the change in level in the fermenter tank from which the feed stock was pumped to the head tank. The flow thus measured was estimated t o be accurate within one per cent. Cooling water
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,RT
I
-
LOCATION OF MEASUREMENTS C COMPOSITION P - PRESSURE R = RATE OF FLOW T = TEMPERATURE
jJ IL-=J. _.
I
I lit I
I
P RT
a-STEAM
14
RECORDING ORIFICE
flow was determined by closing the water discharge line and noting the rate of rise of level in the condenser shell. Steam flow was recorded by a commercial orifice meter for which corrections were made for deviations of the steam from calibration conditions. Condenser water and steam flows were probably accurate to 3 per cent. A11 temperatures, with the exception of that of the steam, were taken with calibrated mercury thermometer8 capable of being read to 0.5 O F. The thermometers were inserted directly into the flowing streams. Steam temperature was measured by a thermocouple peened into the pipe wall which was heavily insulated. Steam pressure was determined with a calibrated Bourdon gage at the point of temperature measurement. Column pressure and pressure differential were measured with manometers. During the course of a test, rates of flow, temperatures, and pressures were noted a t intervals of about 15 minutes. The data, thus obtained, were divided into two runs, for each of which the data were averaged to give values representative of operations over each period. Vapor samples were withdrawn from the 14- and 12-inch vapor lines leading to and from the partial condenser. Sample lines from the pipes carried vapors t o condensers which reduced condensate to a temperature well below the boiling point. Samples of feed, reflux, and slops were withdrawn from convenient cocks in the respective lines a t temperatures below 100" F. Intracolumn liquid samples were drawn off plates 3, 5, 7,9, 11, 13, and 15 through specially designed tubes which were installed through the manhole covers between the plates. A '/4-inch copper tube extended down to within inch of the plate level. The end of the tube was
FIGURE 2. DIAGRAM OF THE COLUMN
shielded by a ll/s-inch cup which prevented vapors rising through the plate from entering the sampling tubes. The top surface of the cup was about 1 inch below the nominal liquid level on the plate. The sampling tubes were located a t a point 4 ipches from the column wall on plates where the liquid overflowed through six rim down pipes. The location was midway between two down pipes (Figure 1). As they were withdrawn, the liquid samples were cooled below the atmospheric boiling point in order to prevent flashing. Samples of all streams were taken at half-hour intervals and composited for each run.
Analytical Methods Liquid samples below the feed plate were analyzed for their content of alcohol, water, and nonvolatile matter; vapor samples and reflux, for alcohol content only. In addition to alcohol, water, and nonvolatile matter the feed stock contains a very small amount (about 0.05 weight per cent) of volatile compounds, both higher and lower boiling than alcohol. The predominant volatile components were fusel oils and aldehydes, both of which tended to go overhead, one as a result of an enhanced volatility in dilute aqueous solution and the other as a result of lower boiling points than alcohol. The result was that substantially none of these components were present below the feed plate. Volatile components other than alcohol and water were not considered in the analyses. Samples containing nonvolatile matter were first distilled, and the distillate was then analyzed. The fraction of non-
DECEMBER, 1938
INDUSTRIAL AND ENGINEERING CHEMISTRY
TABLE I. OBSERVED DATA -Test Run A
-I Run B
-Test RunA
11RunB
Rate of flow, Ib./min.: Feed 720 720 689 668 972 402 402 Condenser water (sea) 972 Steam 157 132 162 132 Temperature, O F.: Feed to preheater 103 103 101 101 Feed from preheater 17f.5 17E.O 170 170 Slops t o preheater 219 219 Slops from preheater 154.5 154.5 168 158 Vapors to condenser 205 206 Vapors from condenser 20 1 201 202 203 Reflux from condenser 175 174 169 169 Cooling nater t o condenser 67.7 72.1 61.5 61 143.3 182 Cooling water from condenser 145.1 183.5 Steam 297 300 315 315 Pressure, lb./sq. in. abs.: Below plate 1 16.8 17.3 28.1 32.2 Steam Difference across plates, in. of H20: 2.0 4and 5 2.7 6 arid 7 2.0 2.0 and 1 9 3.1 1.7 8 and 10 1 2.8 1.5 12 and 13 ' 2.0 1.5 14 and 15 2.0 1.3 Composition, weight %: Alcoh 01: Feed 4.82 o,032 4.93 o,038 Slopr1 Reflux 14.7 14.7 17.1 15.9 Vapors t o condenser 37.8 35.6 47.2 45.5 Vapors from condenser 52.0 50.9 49.9 51.6 Liquid plate 3 0.07 0.07 7:'$ ;:;O,l Liquid from from plate 5 0.12 0.12 Liquid from plate 7 0.19 0.20 0.50 0.42 Liquid from plate 9 0.62 Liquid from plate 11 0.32 0.94 0.36 1.12 1.33 Liquid from plate 13 3.7 3.8 4.3 3.6 Liquid from plate 15 4.8 4.9 5.2 4.6 Nonvolatile matter: Feed 2.7 2.5 2.5 2.5 2.7 Slops 3.0 2.9 Liquid on plates (av.) 3.0 2.6 a Temperature measured in test incorrect because of flashing after flow controller.
...
of cubic centimeters of dichromate originally added, less the number of cubic centimeters of thiosulfate, corrected for the normality of the thiosulfate as determined by standardization in a blank run, gives the number of cubic centimeters of dichromate consumed in oxidizing the alcohol. One cubic centimeter of dichromate is equivalent to 0.0115 gram of alcohol.
Data
...
i:$o 8:;i3
z
y:::
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The original data taken during the tests are presented in Table I. I n order to establish the accuracy of the experimental results, material and heat balances were struck around the column and preheater. These balances are given in Tables I1 and 111. Since no direct measurements were made of the flow of product (vapors leaving the partial condenser) or of slops, these rates were determined by calculation. The product flow was calculated by alcohol balance, using the product composition and the feed composition and flow, and allowance was made for the small amount of alcohol leaving in the slops. Slops rate was calculated by a heat balance on the slops-feed exchanger. Specific heats, determined experimentally by the method of mixing, were used in these calculations. Measured temperatures were used except for the slops leaving the column in test I for which the temperature was calculated as the boiling point at column pressure. The fact that material balances checked to within 3 per cent supports the validity of the experimental data. In striking over-all heat balances the flows presented in Table I1 were used. The heat contents of feed and slops were calculated from measured specific heats, the heat of mixing of alcohol and water being negligible a t all concentrations encountered in the column. The heat content of the product vapors was computed from the heat content of water (8) and alcohol (7) vapor, assuming additivity of heat contents. Although the heat balances for test I1 are quite satisfactory, those for test I show considerable discrepancy. The ma-
volatile matter in liquid samples was determined by weighing the residue remaining from evaporation overnig11t a t 105" c. Two methods were used for determining the alcohol content of distilled liquids. Samples containing greater than one per cent by weight of alcohol were analyzed by measurement of liquid density a t 25" C. with a pycnometer. The COF AROUND COLUMN AND TABLE 11. MATERIALBALANCE responding alcohol concentration was then read from standPREHEATER (POUNDS PER MINUTE) ard tables (6). Samples containing less than one per cent --Test -I Test IIby weight were analyzed by the controlled oxidation of Input: Run A Run B Run A Run B alcohol to acetic acid by dichromate in acid solution. The Feed 720 720 689 668 162 132 157 132 -. method used was a modification of that described by LiverTotal 882 877 821 800 sedge (11) and was found to be very satisfactory, checking Output: Product" 66 69 74 70 the pycnometer method and synthetic samples even a t ex746 Slopsb 803 795 770 tremely low dilution (0.01 weight per cent alcohol). A deTotal 869 864 844 816 Unaccounted scription of the modified method follows: +13 (1.5%) +13 (1.5%) -23 (2.7%) -16 (2.0%) for: A sample is measured out which contains approximately 0.0345 gram (3 milliequivalents) of alcohol. TO this are added ~ L ~ ~ ~ ~ h ~~ feed-slops ~ ~~ heat ~ exchanger. ~ ~ ~ o ~u 9 cc. (three times in excess) of 1 N potassium dichromate solution from a buret and then 15 cc. AROUND COLUMN AND PREHEATER IN B. T. u. TABLE 111. HEATBALANCE of concentrated sulfuric acid from a graduated PER MINUTEABOVE 32' F. (LIQUID) cylinder. The volume is made up to 100 cc. by Test I Test 1 adding distilled water, and care is taken not to Run A Run B Run A Run B heat the mixture to near its boiling point. The Input: solution is placed in a drying oven a t 80" C. for Feeda 48,200 48,200 44,800 43,500 Steam 193,000 187,600 157,800 157,800 30 minutes, taken out, and cooled to room temCondenser waterb 32,200 37,500 11,400 11,200 perature. Now 50 cc. of 10 per cent potassium Total 273,400 273,300 214,000 212,500 iodide solution are added all at once, and the O?f)oudt& 52,500 54,500 59,700 56,100 93,500 92,800 92,500 80,500 mixture is shaken and titrated with standardized, SlOPS~ 106,000 104,200 58,000 58,600 Condenser water approximately 1 N, sodium thiosulfate solution Surface losses, est. 1,000 1,000 1,000 3,000 253,000 252,500 211,200 205,200 until the brown color fades to a brownish green. Total Two cubic centimeters of starch solution are Unaccounted for: +20,400 (7.5%) +20,800 (7.6%) +2,800 (1.3%) +7,300 (3.4%) a Specific heat determined as 0.944. added, and the solution is titrated with thiosulb'Sea water; specific heat, 0.963: specific gravity, 1.024. fate to the end point. The murky bluish green Specific heat determined as 0.952. changes to a clear light green. The number
'
--
--
n
~
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INDUSTRIAL AND ENGINEERING CHEMISTRY
1398
terial balances for this test are likewise off in the same direction, and an 8 per cent error in the steam rate could have caused the discrepancies in both heat and material balances. The source of the discrepancy is not directly discernible, however.
Rectification Performance The analysis of the performance of a distillation column involves calculations in which the basic assumption is made that consistent heat and material balances may be struck around any points in the system. Accordingly, the flow of steam and slops was computed so as to give completely compatible heat and material balances throughout. As has been demonstrated in Tables I1 and 111, the size of the inconsistencies of the original data is small. The compatible flows are given in Table IV. By the use of heat and material balances the molal flows of liquid and vapor in the stripping section of the column were computed. I n these calculations the specific heat of liquids was taken as 0.95, and the heat content of vapors was computed by the method described above for the vapor product. The values of molal flows remained substantially constant throughout the entire stripping section of the column, and average values are presented in Table IV. The maximum deviations from these averages were considerably less than one per cent.
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I n making all rectification calculations for run 11, the observed slops compositions were not used. Doubt was cast upon the accuracy of the measured values, both by the failure of the figures to check those obtained by the company's slop tester and by the closeness of the observed slops compositions to those of samples withdrawn from the third plate. A more likely value was computed by assuming that the two plates and sparging section below the third plate had a rectification efficiency of 15 per cent (based on performance in test I) and that the observed third-plate compositions were correct. The computed slops mole fractions for test I1 were 0.018 and 0.019 for runs A and B. These values are to be compared with the observed analyses of 0.036 and 0.029. It is again to be
TABLE V.
OBSERVED VAPORAND LIQUIDCOMPOSITIONS (MOLE PERCENTALCOHOL)
Feed Product Slops Liquid from plate:
-
5 7 9 11 13 15 a
-Test Run A 2.00 29.8 0.013 0.028 0.048 0.077 0.13 0.38 1.53 1.99
IRun B 2.05 29.0 0.015
---Test RunA 2.28 28.0 0.036s
11Run B 2.29 29.4 0.029"
0.028 0.048 0.081 0.15 0.46 1.57 2.03
0.039 0.11 0.20 0.25 0.54 1.75 2.14
0.041 0.11 0.17 0.27 0.47 1.46 1.90
Slope composition used in rectification oaloulations is given i n the text.
TABLE IV. COMPATIBLE MOLALFLOWS (POUND MOLESPER
MINUTE)
-Test I--Test IIRun A Run B Run A Run B Input: 37.5 37.5 36.0 34.9 Feeda 8.00 7.89 7.08 6.96 Steam output: Product 2.52 2.66 2.90 2.69 SloDsa 43.0 42.7 40.2 39.2 Intracolumn, below feed: 43.2 42.9 40.5 Liquid, 0 39.5 8.24 8.09 7.35 7.23 Vapor, V a Nonvolatile matter was assumed to have.such a high moleoular weight that molal flows could be computed on the basis of alcohol and water content only.
For purposes of rectification calculations, compositions on a molal basis were required. I n making the conversion from weight per cent to mole per cent, the assumption was made that the moles of dissolved nonvolatile matter in the liquid phase were so small as to be negligible. This nonvolatile matter was the residue from the molasses fermentation, and it is questionable if any of it existed in a state of true solution. The total amount of nunvolatile matter present was so small as to make any possible errors from the assumption inconsequential. The alcohol concentration of the several streams, expressed on a mole per cent basis, are presented in Table V. Since the objective of the tests was the study of the performance of perforated plates, rectification calculations were limited to the section of the column between the third and fifteenth plates. This choice was made to eliminate the effect of the uncertain performance of the sparger section and of the bubble-plate and partial condensers. Rectification calculations around the condensers revealed inconsistencies in data and cast doubt upon the validity of the measured composition of the vapors entering the condenser system. Difficulty in obtaining a truly representative vapor sample might well explain this apparent discrepancy. It is to be noted, however, that incompatibilities in the experimental data a t this p'oint in the system could have had substantially no effect on the computed efficiency of the perforated plates below the point of feed.
recognized that the discrepancies between observed and computed slops compositions could have had but a minor effect upon the computed efficiency of the perforated plates. Had calculations been based upon the observed slops composition, the computed perforated plate efficiency would have been only 3 per cent lower than that actually found, using the more probable calculated slops analysis. The rectification performance of a distillation plate may be expressed by several methods, the most common of which are the over-all efficiency and the Murphree efficiency. The performance of the section of the column between the third and fifteenth plates was analyzed in terms of both methods. The over-all efficiency, which is the one most simply adapted to purposes of design, is defined as the ratio of the number of theoretical plates necessary to produce a given separation to the number of actual plates used to give the same separation under similar reflux conditions. The vapors from a theoretical plate are in equilibrium with the liquid leaving the plate, by definition. Using the familiar McCabe and Thiele graphical method (1.2) and the vapor-liquid equilibrium data of Carey and Lewis ( 4 ) and of Cornell and Montonna ( 5 ) , the number of theoretical plates required to produce the observed separations was calculated. From this, the over-all efficiency was computed. The results are presented in Table VI. Tests I and I1 gave substantially the same efficiency-namely, 42 per cent. From a consideration of sources of error in the measurements and calculations, it is concluded that the computed efficiency is accurate to one part in ten. Test I1 was made 43 days after test I ; there were no cleanings between tests. It is apparent that there was no significant loss in fractionation ability caused by the fouling of the column during this period of operation. The Murphree efficiency (IS), which is of greater technical significance than the over-all efficiency, is defined by the ratio of the actual change in composition which vapor undergoes in passing through a plate, to the change in composition which the vapor would undergo if it left the plate in equilibrium
INDUSTRIAL AND ENGINEERING CHEMISTRY
DECEMBER, 1938
OF PERFORATED PLATES (IN PERCENT) TABLE VI. EFFICIENCY
Over-all, E Average Murphree, E ~ s v . Individual Mur hree, EM: Plates 3 an$ 4 Plates 5 and 6 Plates 7 and 8 Plates 9 and 10 Plates 1 1 and 12 Plates 13 and 14 Column load: Va or velqcity. ft./sec.: 8u:oerfimal
--Test RunA 42 36 19 18 22 53 87 18
IRun B 43 36
17
22 23 5s
78 19
with the liquid actually leaving the plate. matically,
-Test RunA 42 36 42 27 10
39 80 17
II-
Run B 40 34 41 19 21 27 74 20
Expressed mathe-
where yr, yn-l = mole fractions of component in vapor leaving plates n and n-1, respectively y,* = mole fraction of component in equilibrium vapor of liquid leaving nth plate Using fa,miliar graphical technique, the Murphree efficiency of a plate may be calculated, provided the molal rates of flow within the column are known and provided the compositions of the liquids entering and leaving the plate are known. If the compositions of the liquid entering one plate and of the liquid leaving the plate below are known, then the average Murphree efficiency of the pair of plates may be computed even though the efficiency of each plate may not be separately calculated. The results of the determination of the average Murphree efficiencies of pairs of plates are presented in Table VI. Also given is the average Murphree efficiency of the entire section between the third and fifteenth plates and the column load. As in the case of the over-all efficiency, tests I and I1 gave similar results. The average of the Murphree efficiencies of the column section is somewhat smaller than the corresponding over-all efficiency. As was shown by Lewis ( I O ) , the relation between the two types of efficiencies depends upon the ratio of the slope of the equilibrium line to that of the operating line. For sections in which.the ratio is greater than unity, as in the test section, the over-all is necessarily greater than the Murphree efficiency. The results for the pairs of plates show wide variation in certain regions which are most probably due to irregularities in the character of the liquid flow across the plates. Furthermore, slight deviations in the relation of the sampling point to the overflow pipes might well be a contributing factor. Upon distillation plates in which there is a directional flow of liquid, concentration gradients across the plate are developed. I n such cases rectification efficiencies in excess of that of a “perfect plate” may be obtained, even though vapor rising through the plate may not come fully to equilibrium with the liquid with which it is in contact. Lewis (IO) derived the relation between the efficiency of an entire plate and the “local” efficiency of vapor rising through liquid. Although the plate efficiency may be in excess of 100 per cent, i t is obvious that the local efficiency must always be less than 100 per cent. Using the method of Lewis and assuming substantially no mixing of liquid upon the plates, the local Murphree efficiency for the perforated plates was computed to be about 25 per cent. In order to ascertain if plate efficiency showed a consistent trend with plate number or liquid composition, variations caused by experimental technique were ironed out by plotting
1399
the intracolumn analyses against plate number on semilogarithmic paper. Compositions on the several plates were then read from a smooth curve drawn through the experimental data. Uaing these points, the individual Murphree efficiencies of the plates were calculated and plotted against plate number in Figure 3. The efficiencies of the upper plates in the column are substantially higher than those of the lower plates. The conditions of operation of the plates are apparently almost constant throughout the test section, and this considerable change in efficiency would hardly be anticipated. The alcohol concentration varied from 0.03 mole per cent on the third plate to 1.5 mole per cent on the fourteenth plate. Although no change in the diffusional characteristics of the system would be predicted from this small change in concentration (on an absolute basis), it is possible to explain the increased efficiency on the upper plates by an increased interfacial area caused by greater frothing. Even small concentrations of alcohol are known to reduce surface tension materially. Thomson ( l e ) ,in studying the performance of bubble-cap plates, noted a n increased frothing tendency with increasing alcohol concentration. Frothing, particularly when associated with small plate spacing and high vapor velocities, is known to lead to low efficiencies because of the production of excessive entrainment. However, in a column with a large plate spacing it is possible that a slight increase in frothing might increase plate efficiency. Further indications of a difference in operating conditions for the upper and lower plates are found in the character of the scale deposited within the column. It is found upon cleaning that the scale on the upper plates is hard and brittle whereas that in the lower section is somewhat less dense and more readily removed. From the data available it cannot be said whether the efficiency a t the top of the column is representative of normal plate operation and that in the bottom section is low because of excessive fouling, or whether the efficiency
FIGURE3. INDIVIDUAL MURPHREE EFFICIENCIES OF THE PLATESvs. PLATE NUMBER
at the bottom of the column is normal and that a t the top is unusually high as a result of increased interfacial area, due to greater frothing. For purposes of design it may be concluded that for similar perforated plates, operating under the conditions of the test with a superficial vapor velocity of 2.4 feet per secend and an average vapor velocity of 17 feet per second through the perforations, the over-all efficiency is 40 per cent, corresponding to a Murphree plate efficiency of 35 per cent.
Discussion The rectification performance of the commercial perforated plates, with a n efficiency of 42 per cent, is substantially lower than that reported for other perforated plates. Peters (26) found an efficiency of over 50 per cent for plates containing 0.2-inch holes and carrying liquid to a depth of 1 inch. This efficiency was determined for the rectification of
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INDUSTRIAL AND ENGINEERING CHEMISTRY
an acetic acid-water mixture which has been shown to yield lower efficiencies than the alcohol-water system. Kirschbaum and Andrews (0), in their tests on sieve plates containing 0-1-inch holes located on 0.275-inch centers and operating with an overflow height of 1.2 inches, found a local efficiency of about 65 per cent when rectifying alcohol-water mixtures at similar vapor velocities and reflux ratios. This value of 65 per cent is to be compared with 25 per cent found in the present tests for this type of efficiency. Brown ( I ) states efficiencies in excess of 100 per cent for commercial beer columns. However, neither data nor column description is presented. Probably the essential difference between the several perforated plates whose performances have been tested lies in the size of the individual perforations. The tests of Kirschbaum and Andrews upon plates with 0.1-inch holes gave higher efficiencies than were found by Peters for plates with 0.2inch holes. I n turn the present tests upon plates with 0.5inch perforations gave efficiencies considerably below those of the other investigators. It would be anticipated that a n increased perforation size would decrease the efficiency of vapor-liquid contact since the resulting increase in bubble size would yield a smaller interfacial area per unit of vapor rising through the plate, this area being inversely proportional t o diameter for spherical bubbles. Excepting the efficiency reported by Brown, the present results are in line with those of previous work on perforated plates. A comparison of the performance of the perforated plate with what might be expected of a bubble-cap plate under similar conditions of vapor velocity and liquid seal leads to the conclusion that the bubble-cap plate is more efficient. At a vapor velocity through the slots equal to that through the perforations of the test plates (17 feet per second), Carey, Griswold, Lewis, and McAdams (3) found an efficiency of about 85 per cent for a bubble cap with slots l / 4 inch wide, rectifying an alcohol-water mixture in a test column. Uchida and Matsumoto ( I 7 ) , for a 10-inch bubble-cap column rectifying alcohol and water, found efficiencies of 70 per cent a t a slot velocity of 45 feet per second and a superficial vapor velocity of 1.7 feet per second. Kirschbaum and Andrews (9) obtained efficiencies of 75 per cent for a single bubble cap, distilling alcohol and water with a superficial vapor velocity of 2.6 feet per second. Thomson (16)reported efficiencies of about 60 per cent for dilute alcohol and water mixtures, rectifying a t a superficial vapor velocity in excess of 2 feet per second in a 61/2-inchbubble-cap column with a 6-inch plate spacing. Peavy and Baker ( I 4 ) , working with an 18-inch column having a plate spacing of 18 inches, found efficiencies in the rectification of alcohol-water mixtures in excess of 100 per cent a t a vapor velocity of 2.5 feet per second and a liquid
VOL. 30, NO. 12
depth on the plates of 1 inch. The corresponding value of the local Murphree efficiency was computed t o be about 85 per cent. Although these performance data show conclusively that an efficiency considerably in excess of 40 per cent would be anticipated for bubble-cap plates operating under similar conditions of vapor velocity and liquid level, a n allowance must be made for the fouling effect of the stock used in the perforated plate column. Although bubble-cap plates installed in a commercial beer still would undoubtedly yield an increased alcohol recovery a t the start of a run, the rate of loss of capacity and the expense of cleaning would be so great as to make the installation of bubble-cap plates in such service economically inadvisable.
Acknowledgment The data here presented were obtained by students of the School of Chemical Engineering Practice who carried out all test work. The care and thoroughness of their work are here acknowledged. Thanks are due to the New England Alcohol Company for permission to conduct the tests and to publish the results. The active cobperation of E. W. Haywood, plant superintendent, during the course of the tests was particularly valuable.
Literature Cited (1) Brown, G. G., Trans. Am. Inst. Chem. Engrs., 32,349 (1936). ( 2 ) Campbell, J. A , , Refiner NaturalGasoZineMfr., 15,127-34 (1936). (3) Carey, J. S.,Griswold, J., Lewis, W. K., and McAdams, W. H., Trans. Am. Inst. Chem. Engrs., 30, 504-19 (1933-34). (4) Carey, J. S., and Lewis, W. K., IND. ENQ. CHEM.,24, 882-3 (1932). (5) Cornell, L. W., and Montonna, R. E., Ibid., 25, 1331-5 (1933). (6) Handbook of Chemistry and Physics, p. 1171,21st ed., Cleveland, Chemical Rubber Pub. Co., 1936. (7) International Critical Tables, Vol. V, pp. 80, 81, 114, 138, New York, McGraw-Hill Book Co., 1929. (8) Keenan, J. H., Steam Tables, Am. SOC. Mech. Engrs., 1933. (9) Kirschbaum, E.,and Andraws, C. A., J . Inst. Petroleum Tech., 22, 803-20 (1936). (10) Lewis, W. K., Jr., IND.ENG.CHEM.,28, 399-402 (1936). (11) Liversedge, S. G.,Analust, 56,595 (Jan., 1931). (12) McCabe, W.L., and Thiele, E. W., IND.ENQ.CHEW,17, 605-11 (1925). (13) Murphree, E. V.,Ibid., 14, 476-9 (1922). (14) Peavy, C. C., and Baker, E. M., Ibid., 29, 1056-64 (1937). (15) Peters, W.A., Jr., Ibid., 14, 476-9 (1922). (16) Thomson, A. K. G., Trans. Inst Chem. Engrs. (London), 14, 119-28 (1936). (17) Uohida, S.,and Matsumoto, K , J. Soc. Chem. Ind. Japan, 39, 226-7 (1936).
RECEIVED October 3, 1938. Presented before the meeting
of the American Institute of Chemical Engineers, Philadelphia, Pa., November 9 to 11, 1938.
