I
R. A. GASKA' and
M. R. CANNON2
The Pennsylvania State University, University Park, Pa.
Controlled Cycling Distillation
. . . in
Sieve a n d Screen Plate Towers This new method of controlled distillation can lead to new types of equipment for conventional processes
C o N m o u m CYCLING (7) is a new method of operating various types of existing equipment including distillation towers and permits new types of equipment to be designed for many conventional processes with some important advantages. For example, no downcomers are needed on plates that are operated with controlled cycling, and capacity is greater than that attainable with conventional operation. I t is the purpose of this paper to report the results of applying controlled cycling to several types of plates in distillation towers.
Plate Type No. 1 was a '/*-inch thick brass plate with 1 9 holes l / 8 inch in diameter spaced on equilateral triangles. 'The free area was 7.470. Plate Type No. 2 was similar to Type No. 1 but it had 61 holes with a free area of 23.8%. Plate Type KO. 3 is simply a 10-mesh screen of 0.025-inch diameter wire with a free area of 56%. The rate of boilup in the steam heated still-pot was controlled automatically by use of column pressure drop. A simple manometer circuit contained one fixed electrode immersed in the manometer liquid and one movable electrode that could be set for any desired column pressure drop. FVhen the set pressure drop was reached a small electric current flowed through the manometer circuit to a simple electronic relay which operated the electric valve in the steam line to the still. Excellent control was attained. The vapor line leading from the still to the base of the tower contained an electric valve which was controlled by a cycle timer. This permitted the control of the time for each of the two periods of the cycle, the vapor flow period and the liquid flow period. The base of the tower was 4 feet above the liquid level
Equipment a n d Operation
The test towers were made of 2-inch inside diameter flanged, glass pipe, one with nine plates spaced 18l/2 inches and the other with 17 plares spaced g1/2 inches apart. The test mixture employed was benzene-toluene at atmospheric pressure and was analyzed by boiling point measurements. All runs were made a t total reflux. 1 Present address. Dow Chemical Co., Midland, Mich. 2 Present address, Cannon Instrument Co., State College, Pa.
in the boiler and a LTbend in the liquid line prevented back flow into the tower when the vapor valve was closed. The condenser was directly attached to the top plate, and during the vapor flow period the condensate caused the liquid level on the top plate to increase but caused no trouble in operation. Results
The results with plate Type N o . 1 were of little importance. When the column was operated without cycling, flooding occurred at such a low boilup rate it could not be measured with any accuracy. With cycling the flood rate was 26 liters of liquid per hour and an over-all plate efficiency of 60% was obtained for the nine plates spaced a t 18l/g inches. At flooding, the column F-factor was 1.4 and the total column pressure drop was 108 mm ol Hg. I n all of the data and figures: the Efactors and vapor velocities are average values that were computed over the time for the complete cycle and not just for the vapor flo~v-period. Such average values are the true measure of column capacity and must be used for comparison with other columns. Results with plate Type S o . 2 are
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Cycling increases column capacity for type-two plates (left) and produces highest efficiencies at highest rates (right) The tower contains 9 plates spaced 18.5 inches with a 5.4-secoid vapor flow and 2.0-second liquid flow
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
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Figure 2. Cycling increases the column capacity for typethree plates (top); and condenser capacity was exceeded before maximum rate was attained (bottom)
Figure 3. The maximum rate of operation for the same pressure drop is different for each cycle (top); and efficiency of plates varies with cycle times (bottom)
Same specifications as in Figure 1 except, at bottom, the liquid flow was 1.1
The tower contained 17 plates of type three with 9.5-inch spacing
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shown in Figure 1. Note that when a cycle of 7.4 seconds was used, the maximum rate was increased from 5.6 feet per second without cycling to 8.6 feet per second with cycling; the latter figure corresponds to an F-factor of 3.8. Note that a column pressure drop of 15 mm. of Hg. corresponds to a vapor velocity of 5.4 feet per second without cycling and 8.0 feet per second with cycling. This amounts to a 48% increase in total vapor load a t a fixed pressure drop. The data illustrate an interesting fact, namely, that the maximum rate of phase flow is not dictated by physical dimensions of the equipment and the properties of system only, but it is also a function of the method of operation employed. Thus, for example, the authors believe that the capacity of existing bubble-cap plate towers as well as other types can be increased by use of controlled cycling. One can operate at two different pressure drops for the same vapor velocity. This is easily explained. Boilup is increased by increasing the distance between the electrodes in the manometer. When the maximum vapor velocity point is passed, the froth height on the
plate increases rapidly as one increases the pressure drop by increasing the distance between the electrodes. Thus one can operate a t different liquid depths on the plate. The more torturous path for the vapor a t high liquid depth produces a high pressure drop that causes the steam valve to close sooner than when the liquid depth is that which exists at maximum vapor velocity. T h e same phenomena have also been obtained in packed towers when this type of boilup control is used. I n this case, the higher pressure drop corresponds to a higher liquid holdup in the packing. I t is also of interest to note in Figure 2 that the maximum plate efficiency occurs in the area of high vapor velocities. Figures 2 and 3 present the data on the plates made of screen with 5670 free area. As one might expect, the capacity is high but the efficiency is low. Increasing the liquid flow time for a fixed vapor flow time decreases the efficiency but markedly increases the capacity. The following article on the performance of packed plates ( 2 ) gives additional information on other cycle times. An open 2-inch diameter tube 160
inches high was also tested. I t flooded a t an F-factor of 5.1 without cycling and in excess of 6.6 with cycling. T h e maximum rate could not be determined because of limited condenser capacity. Two theoretical plates were measured in the range of 4 to 11 feet per second vapor velocity and approximately 4 when operated close to the flood point. Acknowledgment
Data contained in this paper are from the thesis “Control of Vapor and Liquid Phase Flow in Packed and Plate Fractionation Towers” submitted by R . A. Gaska as part of requirement for the Ph.D. degree, June 1959, T h e Pennsylvania State University, University Park, Pa. Literature Cited (1) Cannon, M. R., IND.END.CHEM.53, 629 (1961). (2) McWhirter. J. R.. Cannon. M. R., ’
’Zbid., 53, 632’(1961).‘
RECEIVED for review March 20, 1961 ACCEPTEDJune 2, 1961 VOL. 53, NO. 8
AUGUST 1961
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