or by the gas jetting before the bed was able to reach the screen, or fines could be carried back up with the gas because of the shorter off-cycle. Based on these results a cycle of 0.13-second on-0.13-second off was chosen for all further studies. Bulk Density. Studies on diatomaceous earth and powdered Teflon showed the versatility of the Cycle-Screener. Previous screening of these materials was found to be impossible on many different commercial screeners. The problems that occurred were blinding of the screen and floating of the particles above the screen. Setting the ratio to zero gave a very rapid separation in both cases. Mechanism. A photographic study of the bed movement at 64 frames per second and a frame-by-frame study of the film showed that two mechanisms were occurring during CycleScreening: a mixing and swirling action which renews the surface at the screen and on the top of the bed, and the vertical bouncing which occurs in many commercial screens. At a gas pressure of 30 p.s.i.g. and an internal pressure of 3 p.s.i.g. the bed movement was turbulent and erratic, with a rapid renewal of the bed surfaces. T h e average bed expansion was 4 inches with a maximum for some particles of 8 inches. Increasing the internal pressure to 5 p.s.i.g. greatly reduced the swirling and mixing of the bed. T h e bed tended to expand in sections rather than uniformly. A bed expansion of 3 inches resulted, with a maximum of 7 inches. Reducing the gas pressure to 24 p.s.i.g. a t a n internal pressure of 3 p.s.i.g. drastically reduced the turbulence of the bed. T h e surface of the bed was intact for a t least half of the gas flow cycle. Turbulence started a t the screen and then progressed through the bed as it expanded. The bed expansion was 2 to 3 inches, with a maximum for some particles of 6 inches. These results show that increasing the gas pressure increased the screening rate by a more rapid renewal of the bed surface in contact wjth the screen, and the turnover in the bed allowed the fines to reach the screen. Increasing the internal chamber pressure reduces the screening rate by decreasing this surface renewal and turnover. T o study segregation in the bed, all the fines used in this study were dyed black. However, there was no sign of segregation of the coarse and fines. This adds further evidence against the argument of the gas separating the fines by accelerating them faster than the coarse particles.
those of low bulk density which present great difficulties with other screening techniques. Scaling up of the apparatus to commercial size and operating it on a semicontinuous basis appear to present no serious difficulty, although some experimental work will undoubtedly be required on a specific system of solids in order to operate the Cycler in an optimal manner. Besides speed and efficiency, the Cycle-Screener offers the additional advantages of low noise and vibration levels, absence of blinding, and relatively low initial and operating costs. Nomenclature
B B’
solids on screen a t 70% fines passed, ml. ( B - 390)/65 C feed to screen, ml. C’ (C - 600)/100 F = feed particle distribution I = internal chamber pressure, p.s.i.g. I’ = I - 4.0 N = fines passing, ml. No = fines in feed, ml. P = air pressure to jet, p.s.i.g. P’ = ( P - 2 7 ) / 3 Q = coarse in feed, ml. R = volume ratio, air out top of apparatus/air out bottom of apparatus R’ = ( R - 1.0)/2 t = time of screening, seconds t, = time for 50% fines passed, seconds X = screening rate, ml./sec. Y = yo fines pass/100 sec. (efficiency) ugt = time geometric standard deviation = = = =
Literature Cited
(1) Cannon, M. R., Znd. Eng. Chem. 53, 629 (1961). ( 2 ) Cannon, M. R., Oil Gas J . 51, 268 (July 28, 1952). (3) Ibid., 5 5 , 68 (Jan. 23, 1956). (4) Gaska, R. A , , Cannon, M. R., Ind. Eng. Chem. 53, 630 (1961). (5) MciVhirter, J. R., Cannon, M. R., Ibtd., 53, 632 (1961). (6) Perry, J. H., ed., “Chemical Engineers’ Handbook,” 3rd ed., McGraw-Hill, New York, 1950. (7) Robertson, D. C., master’s dissertation, Dept. Chem. Eng., Pennsylvania State University, 1960. ( 8 ) Speaker S. M., doctoral dissertation, Dept. Chem. Eng., Pennsylvania State University, 1957. (9) Szabo, T. T., doctoral dissertation, Dept. Chem. Eng., Pennsylvania State University, 1958. (10) Taggart, A. F., “Handbook of Mineral Dressing,” Wiley, New York, 1945. RECEIVED for review May 2, 1966 A4CCEPTED October 26, 1966
Conclusions
T h e results show the great potential of the Cycle-Screener for separating near-mesh mixtures of particles, particularly
Division of Industrial and Engineering Chemistry, 151st Meeting, ACS, Pittsburgh, Pa., March 1966.
VAPOR PULSING IN A SIEVE PLATE LABORATORY DISTILLATION COLUMN G . V. McGURL, J R . , ’ A N D
R. N. MADDOX
School of Chemical Enginering, Oklahoma State University, Stillwater, Okla.
past 20 years the use of pulsed extraction in industrial T h e development of this technique is credited to V a n Dijck (70). T h e catalyst for pushing this invention toward practical use was the need for purification of uranium for use in the Manhattan Project N THE
I separations has become commonplace,
(.3). \-,1 Present address, l/Lt. Gilbert v. McGurl, Jr., 05214337, H H C 45th Engr. Gp (Const.), APO 96240 San Francisco, Calif.
6
I & E C PROCESS DESIGN A N D DEVELOPMENT
I n extraction, pulsation improves the efficiency in two ways: I t increases the contact area between the phases, and it decreases the thickness of the boundary layer which controls the interphase mass transfer because of increased turbulence. I n a few instances natural pulsation in distillation columns has been reported ( 5 ) . These natural pulsations occur only a t high liquid and gas flow rates and increase the efficiency of the distillation column. T h e great success of pulsed extrac-
Substantial increases in the efficiency of a sieve plate distillation column can be obtained by amplitude and pulse frequency at different internal flow rates. A qualitative mechanism for explaining this increase in efficiency is given. The effect of frequency-amplitude product on efficiency varies with flow rate, but maximizes at each flow rate. Under certain conditions of loading, pulsing decreases column efficiency.
tion combined with the report of natural pulsations in distillations prompted this study into the pulsed distillation column. A recent report indicates a n increase in the efficiency of a packed distillation column where the vapor phase is pulsed ( 7 7). These data fit in nicely with the results obtained in the study reported here and extend them to packed as well as plate-type distillation columns. Equipment
T h e distillation column used in this study was a batch-type column designed and first described by Oldershaw (8). I t was a perforated plate column of glass, 32 mm. in inside diameter and with plate spacing of 30 mm. Each plate contained 82 perforations, 0.85 mm. in diameter, ?nd had a weir 1 mm. high. To ensure adiabatic operation, the column was surrounded by a silvered vacuum jacket, provided with slits to allow visual observations of the column. T h e section of column employed contained five plates. T h e column was equipped with a condenser with a manual take-off device for collecting samples. After condensation, the condensate was slightly subcooled and either returned to the column as reflux or taken off as a sample. T h e lead lines for the sample were kept short and small, and the sample size was kept to a minimum to avoid disturbing equilibrium a t the top of the column. A reflux rate and holdup apparatus similar to that described by Oldershaw (8) was used to determine the reflux rate in the column. This is essentially a n annulus in which vapor flows up the inner tube, which is covered with a dome to prevent the returning reflux from running down it. The reflux runs down the outer tube, which is connected to the reboiler by a tube with a stopcock. With the stopcock closed, the reflux collected in the outer tube of the annulus. This tube was calibrated a t the 50-ml. mark. By using a stopwatch. the length of time necessary to collect a 50-ml. sample was measured. T h e stopcock allowed removal of the reflux stream for analysis and collection of a vapor sample from the reboiler. T h e reboiler was a 1000-ml. three-necked glass flask with 29/42 ground glass joints. One of the necks was used for a thermometer, and another was used to introduce the vapor pulse. T h e pulse was introduced through a side arm on the reboiler. I t was produced by the pulsation of a column of liquid in a glass tube, achieved through the use of a model CPS-1 Lapp Pulsafeeder driven by a Graham variable speed drive. T h e liquid was of the same composition as that in the reboiler. T h e frequency available was continuously variable from 0 to 23.5 cycles per minute. 'The pump was arranged to provide pulsation without net liquid movement. T h e pulse volume was variable from 0 to 12.5 ml. per pulse. T h e Pulsafeeder was constructed of stainless steel and Teflon. T h e reagent head was constructed of stainless steel, and the diaphragm, 6 inches in diameter, was constructed of Teflon. A schematic view of the experimental apparatus is shown in Figure 1.
Total reflux operation was used a t all times, because the only true steady state in a batch distillation column is a t total reflux. After the column had reached steady state, samples were taken of the reflux and of the liquid draining from the bottom plate. T h e bottom plate samples were taken to avoid having to make any assumptions about the operation of the reboiler. T h e reflux samples were withdrawn slowly through a stopcock to avoid disturbing column operation. T h e maximum rate was one drop per 3 seconds. Two samples were taken; and if their relative indices differed by more than 0.0002, other samples were taken until this criterion was reached on two consecutive samples. T h e bottom samples were always identical, and thus only one was taken. Analysis of the samples was by Abbt refractometer. T h e reproducibility of this instrument is 0.0002 and readings were taken to that level. T h e refractometer was kept a t 25.0' f 0.2' C. by the use of a constant temperature water bath. The carbon tetrachloride-benzene system was used because it is a standard for determining the efficiency of distillation columns and because Oldershaw used it in his original work with this type of column ( 5 ) . The refractive indices of these compounds differ sufficiently to make refractive index analysis fast and accurate. Results and Discussion
T h e McCabe-Thiele ( 6 ) method was used to determine the efficiency of the column. T h e compositions of the samples were taken and the McCabe-Thiele method was used to determine the theoretical number of plates necessary to effect the indicated separation. This was then divided by the actual number of plates in the column to give the efficiency. T h e main assumptions of the McCabe-Thiele method as applied here are that constant molal overflow is obtained and that the column is adiabatic. For these experiments, calculations show
Procedure
Before beginning a series of runs, the reboiler was filled with a solution of carbon tetrachloride and benzene of known composition. T h e heating mantle was turned on, and the column allowed to reach steady state. T h e pulsing unit, previously set for the desired amplitude and frequency, was turned on. Sufficient time was allowed for the column again to come to steady state. After the second steady state was obtained, the time necessary to measure 50 ml. of reflux was measured. T h e column was then allowed to return to steady state.
Figure 1.
Pulsed distillation apparatus VOL. 6
NO. 1
JANUARY 1967
7
that the molal overflow could not have changed by more than 0,33470 over the length of the column. T h e Dewar type construction of the column ensured near-adiabaticity. Necessary equilibrium data were obtained from Chu ( 2 ) . T h e efficiency of the Oldershaw column without pulsing was first determined as a function of flow rate (Figure 2). As expected, efficiency decreased with increasing flow rate above a certain minimum which was necessary for stable operation of the column. T h e minimum vapor rate for stability of the column was between 800 and 1000 ml. per hour. Above 1000 ml. per hour the column was definitely stable and below 800 it was definitely unstable. In the range between 800 and 1000, its operation was unpredictable. At a vapor rate in excess of 2500 ml. per hour, the column continued to operate satisfactorily but the condenser flooded. No results are reported a t vapor rates in excess of 2800 ml. per hour. T h e first series of pulsed runs was made a t the maximum amplitude of 12.5 ml. per pulse. Column efficiency a t different flow rates was determined a t frequencies of 2, 4, 8, 15, 30, 60, 120, and 220 cycles per minute. Because of the small effect of frequency on efficiency, runs a t 6 and 1 ml. per pulse were made only a t frequencies of 4, 30, and 220 cycles per minute. The minimum reproducible pulse amplitude was 1 ml. per pulse. T h e curves of efficiency us. vapor rate were drawn for all combinations of frequency and pulse amplitude. I n each of these curves, the nonpulsed efficiency curve was shown as a broken line. A typical efficiency curve is shown in Figure 3 (7). With the 12.5-ml. pulse amplitude, the efficiency of the column shows a n increase a t all vapor rates for most frequencies. T h e maximum increase in efficiency is noted a t 15 cycles per minute. Eight and 30 cycles per minute gave lesser increases a t all flow rates and 4 and 60 cycles per minute showed even smaller but still definite increases in efficiency. A frequency of 2 cycles per minute a t 12.5 ml. gave erratic results, probably because of the slowness of the pulse. At 120 cycles per minute the efficiency of the column was reduced below nonpulsing values for vapor rates up to 1300 ml. per hour. Above this point the efficiency was improved in the normal manner. At 200 cycles per minute this same diminution in efficiency was present up to a vapor rate of 2200 ml. per hour. Only a t the highest vapor rate of 2600 ml. per hour was the efficiency with pulsing higher than without. At a pulse amplitude of 6 ml., the efficiency was improved a t all vapor rates for all frequencies. T h e maximum increase in efficiency was noted a t 30 cycles per minute. Lesser but still significant increases were observed a t 4 and 220 cycles
1
F
I
75
+
75-
z u Y 0:
70-
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i
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Figure 2. 8
A
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---_ ---
-RUN N 156
I100
1400 1700 2000 VAPOR R A T E , r n l / h r
2300
2600
2900
Efficiency of Oldershaw column without pulsing
I & E C PROCESS D E S I G N A N D DEVELOPMENT
500
- 12 5
mllp"ls0
800
1100
Figure 3.
1400 1700 2000 VAPOR R A T E , m l / h r
2300
Efficiency with pulsing
2600
I
2900
A
:6
ml/pulra
n 70 j51 i
z W
2
65-
LL
IY L
60-
w U
>
I
500
I100
BOO
1400
1700
2000
2300
2600
I
29M1
VAPOR R A T E , m l / h r
Figure 4.
Effect o f frequency on efficiency
1
f :2 2 0 / m i n
65 7
?
60
+
+
2
K y
froth above the plate and only a minor portion takes place in the clear liquid (4). T h e A.1.Ch.E. tray efficiency study (7) showed that the froth height is proportional to the square of linear velocity for a gas of constant density. Since the pressure drop in this column is low, the vapor may be assumed to possess constant density. In the nonpulsed case the average froth height will be proportional to the vapor velocity squared, F a V z . If the flow rate with pulsing is V A sin C T , then the average froth height F a ( V z ) (AZsin2 C T ) , which is greater. Since the average foam height is increased, the average residence time of the gas in the froth will be increased and thus the efficiency of the column will be higher. This argument does not apply to a packed column. T h e pulsation will cause a constant breaking down of the boundary layers which control mass transfer, and this will increase the mass transfer. Ziolkowski (77) contends that this is the entire reason for the increase in efficiency noted in the packed column. I n the plate column there is great turbulence even a t low flow rates, and this effect should be small. Visual observations of sieve plate columns (7) have indicated that in steady state the gas tends to flow from the sieve plates almost in a column and not to mix intimately with the liquid. This is effectively a channeling through the liquid. With pulsation the vapor flow rate changes constantly and there is little tendency to channel. This effect, combined with the increased residence time, probably accounts for the major portion of the increase in efficiency which was noted.
\
,
,
,
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500
800
Figure
L
1100
Y
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2600
I
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O
Summary
I t is possible to increase the efficiency of a plate-type distillation column by pulsing the vapor phase. I n general, there will be a n optimum combination of pulse amplitude and frequency for any given system, although this optimum may not be within the parameters available. There is no general criterion on which to predict the conditions to elicit the optimum increase in efficiency. T h e causes of increased efficiency in plate-tray columns are the increased residence time of vapor in the froth, the continuous disruption of the boundary layers, and a decrease in the tendency of the vapor to form channels in the liquid.
5. Effect of amplitude on efficiency
i u
1400 1700 2000 VAPOR R A T E , m l / h r
b
2000 m l l h r
2600mlfhr
Literature Cited 500
1000
FREQUENCY-AMPLITUDE
I500
ZWJ
PRODUCT, mllrnin
Figure 6. Efficiency vs. frequency-amplitude product a t various flow rates
(1 ) American Institute of Chemical Engineers, New York, “Bubble Tray Design Manual,” 1958. ( 2 ) Chu, J. C., et al., “Vapor-Liquid Equilibrium Data,” J. W. Edwards, Inc., Ann Arbor, Mich., 1956. ( 3 ) Claybaugh, W. E., “Limiting Flow Capacities in a Pulsed Extraction Column.” M.S. thesis, Oklahoma State University, 1959. (4).Garner, F. H., Porter, K. E., “Mass Transfer Stages in Distillation,” Proceedings of the International Symposium on Distillation, Institution of Chemical Engineers, London, 1960. (5) McAllister, R. A., Plank, C. A., A.Z.Ch.E. J . 4, 282 (1958). (6) McCabe, W. L., Smith, J. C., “Unit Operations of Chemical Engineering,” McGraw-Hill, New York, 1955. ( 7 ) McGurl, G. V., “Pulsed Laboratory Distillation Column,” M.S. thesis, Oklahoma State University, 1962. (8) Oldershaw, C. F., Znd. Eng. Chem., Anal. Ed. 13,265 (1941). (9) Prince, R. G. H., “Characteristics and Design of Bubble Plate Columns,” Proceedings of the International Symposium on Distillation, Institution of Chemical Engineers, London, 1960. (10) van Dijck, W. J. D., U. S. Patent 2,011,186 (Aug. 13, 1935). (11) Ziolkowski, Z., Filip, S., Intern. Chem. Eng. 5 , 40 (1965). I
Ziolkowski ( 7 7) noted no decrease in efficiency a t low flow rates with pulsation in a packed column, because the minimum flow rates in his experiments were well above the minimum flow rate necessary for stability in a nonpulsed column. H e notes a phenomenon which he calls overloading a t large pulse amplitudes. This caused a great decrease in the efficiency of his column. This corresponds to the entrainment noted in this study, and he interprets it to be caused by a n excessive rush of liquid toward the top of the column. Explaining the increase in efficiency noted with pulsation is more complicated than explaining the decreases. A distillation column will be 100% efficient if, a t every plate, there is equilibrium or in the case of packed columns there is equilibrium a t every point. T h e two ways of approaching this ideal are to increase the contact time between the phases and to decrease the resistance to mass transfer. I n plate-type distillation columns most of the mass transfer takes place in the
.
RECEIVED for review May 2, 1966 ACCEPTED November 7 , 1966 Division of Industrial and Engineering Chemistry, 151st Meeting, ACS, Pittsburgh, Pa., March 1966. VOL 6
NO. 1
JANUARY 1967
9
