I
J. R. McWHlRTER and M. R. CANNON' The Pennsylvania State University, University Park, Pa.
Controlled Cycling DistiIIatio n
. . . in
a Packed-Plate C o l u m n
It is possible to produce maximum tower efficiency or maximum tower capacity with controlled cycling
1,
THE two previous articles (2, 3): the principle of controlled cycling was explained and performance data were given for sieve and screen plate distillation towers. This article presents data on the performance of packed plate. distillation towers with controlled cycling. By using controlled cycling in a packed plate tower it is possible to set the controls to produce maximum tower efficiency- or maximum tower capacity, or anywhere in between these limits. With 9]/2-inch spacing of eight 2-inch packed plates, the maximum capacity corresponded to an F-factor of 1.85 and an average plate efficiency of 60Yo with the system normal heptanemethylcyclohexane. When the controls were set to produce maximum efficiency, these same plates had an average efficiency of 13070 near the flood rate which in this case corresponded to an F-factor of 1.1. Controlled cycling is a new method of operating different types of equipment, and applications studied to date show its maximum advantage to be in liquidliquid extraction towers (7, 8) where both capacity and efficiency are increased by its use. It is a different process than pulsing only; controlled cycle extractors have higher capacities than pulsed extractors.
standard 2-inch diameter borosilicate glass pipe. The base of the brass tube was covered by brazing a 5 X 5 mesh wire screen (0,027-inch diameter wire). The tube which was I-inch high was then filled with packing. and the top of the tube was covered Tvith a second screen of the same mesh. The second and third types of plates were like the
first except that the second held a 2-inch depth of packing and the third a 3-inch depth of packing. A methylcyclohexane normal heptane test mixture was used with all runs made at atmospheric pressure and total reflux. A relative volatility of 1.074 was used. Boilup rate was controlled by a pressure regulator on the steam used to
Equipment and Operation Three different types of packed plates were tested. The first was made by force-fitting a 1.81-inch inside diameter thin-walled brass tube into a hole in a 0.25-inch thick brass plate that was machined to fit between the flanges of 1
Present address, Cannon Instrument
Co., State College, Pa.
632
This is the appearance of a 6-inch diameter packed plate during the vapor flow period (left) and the liquid flow period (right)
INDUSTRIAL A N D ENGINEERING CHEMISlRY
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Figure 1. For a fixed vapor flow period of 3.3 seconds, the tower has a different flood rate for each liquid flow period (left); the plate efficiency varies with liquid flow period and boilup rate (middle); and the pressure drop per plate varies with liquid flow period and boilup rate (right) The tower has eight plates containing a 2-inch depth of 0.24-inch protruded packing spaced
heat the still, and controlled cycling was obtained by an electric valve in the vapor line between the still-pot and column. The valve was controlled by a cycle timer. I n other details, the equipment was the same as that employed previously (3).
Results The fact that controlled cycling permits unusual flexibility in tower operation is unique. For example, each cycle produces in the tower a different flood-rate and plate efficiency. As illustrated by the data in the table and many of the figures, it is possible to operate a tower for maximum efficiency or for maximum capacity. When operating for maximum efficiency, the chief characteristic of the cycle will be that the vapor flow time will be a high per cent of the total time, whereas when operating for maximum capacity, the liquid flow period will exceed 50% of the cycle time. As shown in the table a 4.4-second cycle, in which the vapor is flowing 75% of the time, results in a plate efficiency near the flood rate of 130%. T h e same tower, when operated on a 7.9-second cycle, where the vapor is flowing 43.4% of the time, has a plate efficiency of 60% but a significantly higher flood rate. Thus, it is possible to more than double the number of theoretical plates by changing the cycle for high capacity where the column F-factor at flooding is 1.85 to a cycle for high efficiency where the Ffactor is 1.10. Gaska (4) obtained the first data on packed plates in a 6-inch diameter column. The plates contained 2 inches of packing and were supported in the glass column on rods. There were seven plates spaced 8 inches apart. The appearance of these vapor and
liquid flow periods is shown (p. 632). A tight fit was not obtained between the plates and the column wall and some leakage occurred. Because of this, extensive data were not taken. However, plate efficiencies of 110% were obtained at a column F-factor of 1.1 with a high efficiency cycle. In all of the data and figures, the Ffactors and vapor velocities are average values that are computed over the time for the complete cycle and not just for the vapor flow period. Such average values are the true measure of column capacity and must be used for comparison with other coliImns. No leakage could occur around the 2inch diameter packed plates and they were studied extensively. The results are summarized in Figures 1 to 5. Figure 1, left, shows that for a fixed vapor flow period the flood rate is different for every liquid flow period selected. Thus, when the liquid flow period is 1.0 second (total cycle 4.3 seconds), flooding occurs at a boilup of 33 liters of liquid per hour and, for a liquid-flow period of 4 seconds, the flood rate was increased to 44 liters of liquid per hour. In order to obtain these data, it is only necessary to increase the steam pressure in the jacket of the still since this increases the steam temperature and rate of boilup. The steam pressure regulator is then set to maintain this rate, and the column is operated until product composition becomes constant with time, and then a run under a new set of conditions can be started. Efficiencies and pressure drop data for plates containing a 2-inch depth of packing are given in the table and Figure 1, middle and right. I n all cases, high efficiency corresponds to a cycle period where the vapor flow contact time is a high per cent of the cycle time. O n the other hand, highest capacity is obtained
9I/a inches
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Figure 2. For a fixed vapor flow period of 3.3 seconds, maximum boilup rate varies with liquid flow period Tower has eight plates, with 0.24-inch protruded packing, spaced 9*/2 inches. Same for Figure 3
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Figure 3. For a fixed cycle of 3.3 seconds vapor flow and 2.2 seconds liquid flow, the plate efficiency varies with depth of packing VOL. 53, NO. 8
AUGUST 1961
633
when the vapor flow is a relatively low per cent of the cycle time. I n the four cycle times presented, a plate efficiency of 130% of an F-factor of 1.1 for a 4.4second cycle where vapor is flowing 75YGof the time can be obtained, or a
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Figure 4. The efficiency of 2-inch packed plates containing 0.24-inch protruded packing exceeds that of 0.25-inch Raschig rings in all tests The vapor flow period was constant at 4.3 seconds, and the tower has eight plates spaced 9>/2inches
plate efficiency of 607G can be obtained by selecting a 7.9-second cycle, but the flood rate may be increased to that corresponding to a column F-factor of 1.85. Thus one has a choice of setting the controls to operate the tower for maximum capacity or maximum efficiency or anywhere between these limits. Figure 1, right, shows that a given pressure drop per plate does not fix the boilup rate as it does in conventional columns. A different boilup rate for a fixed pressure drop will be obtained for each cycle. Thus, a pressure drop of 4 mm. of H g per plate corresponds to a boilup rate of 26 liters of liquid per hour for one cycle period but also to a boilup rate of 40 liters per hour for a different cycle period. Figures 2 and 3 present data on the performance of packed plates containing packing depths of 1, 2, and 3 inches. As one would expect, the greater the packed depth the higher is the efficiency and the lower is the capacity. Figure 2 also shows the effect of changing the liquid flow time for a fixed vapor flow time. The optimum cycle periods for maximum capacity and the optimum cycle periods for maximum efficiency have not been pin-pointed to this date, but experience indicates that the cycle times presented here are close to the optimums. Figures 4 and 5 compare the performance of 0.24-inch protruded packing ( 6 ) with standard 0.25-inch diameter ceramic raschig rings. Comparing the best curves for each shows protruded packing to have an efficiency about 45y0 greater than the Raschig rings, the area of protruded packing per cubic foot exceeds that of Raschig rings by approximately 6070. Comparison of Figure 4 with Figure 5 shows that the pressure drop per theoretical plate for the plate with protruded packing is about two thirds of that for a plate packed with Raschig rings. Comparison with Other Plates
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Figure 5. The pressure drop for eight 3-inch deep packed plates, spaced 9’/2 inches varies with boilup rate for a fixed cycle of 4.3 seconds vapor flow and 2.2 seconds liquid flow
Application of controlled cycling to to plate towers eliminates the need for downcomers and gives added flexibility. The packed plate is of simplified design and it should scale u p well since the problems caused by a varying liquid head across plates that require down-
The Variation of Column Flood Rate and Efficiency with Cycle Times Eight packed plates spaced 91/2inches, containing a 2-inch depth of protruded packing of 0.24-inch size
Liquid Flow Flow Time, Sec. Time, Sec. Vapor
3.3 3.3 3.3 3.3
634
1.1 1.8 3.3 4.6
Per
Minute
Vapor Flow, %
Time for Liquid Flow, %
Rate F-Factor
Av. Plate Eff. Near Flood Rate, %
13.6 11.8 9.1 7.6
75.0 64.8 50.0 43.4
25.0 35.2 50.0 56.6
1.10 1.50 1.75 1.85
130 104 70 60
Cycles
Time for
INDUSTRIAL AND ENGINEERING CHEMISTRY
Flood
comers are not present. Valid efficiency comparisons with present industrial plates cannot be made since the packed plate has been run in only 2and 6-inch diameter towers. However, various types of industrial plates have been studied by several laboratories under the direction of a n American Institute of Chemical Engineers Research Committee and a report on tray efficiencies is available (7). Fractionation Research Inc. has reported (5) on bubble tray efficiencies in a 4-foot diameter column. An excellent summary of all work in this field u p to 1960 has been made by Gerster (5). H e reports the following tray efficiencies in a 4-fOOt diameter tower at an F-factor of 1.5: which is the rate that is close to the maximum efficiency in each case. Ballast tray 9570 ; fractionation research bubble tray with 6-inch weir 90% and 88Y0 with a 2-inch weir; and a Robin bubble cap tray 807,. All of these efficiencies drop to approximately 60YG when operation is close to 90% of flooding. Acknowledgment The data in this paper are from a thesis by J. R. McWhirter, submitted as partial requirement for the degree of Master of Science in Chemical Engineering, The Pennsylvania State University, University Park, Pa., January 1961 . Nomenclature F-factor =
U
U(p)llZ
= Superficial average velocity
P
in the column, feet per second = Vapor density in pounds per cubic foot
literature Cited (1) American Institute of Chemical Engineers, Final Report, Research Committee, “Tray Efficiencies in Distillation Columns.’’ (2) Cannon, M. R., IND.ENG.CHEY. 53, 620 (1961). ( 3 ) Gaska, R. A., Cannon, M. R., Ibzd., 53, 630 (1961). (4) Gaska, R. A,, “Control of Vapor and Liquid Phase Flow in Packed and Plate Fractionating Towers,” Ph.D. thesis, The Pennsylvania State University, University Park, Pa., June 1959. (5) Gerster, J. A., IND.ENG. CHEM.52, 645 (1960). (6) Scientific Development Co., State College, Pa. Bull. 12-A on Protruded Packing. (7) Speaker, S. M., Jr., “The Development of a New Liquid-Liquid Extractor Using Controlled Cycling,” Ph.D. thesis, The Pennsylvania State University, University Park, Pa. (8) Szabo, T. T., “A Study of Perforated Plate and Packed Liquid Extractors U-5ing Controlled Cycling,” Ph.D. thesis. The Pennsylvania State University, University Park, Pa. RECEIVED for review March 20, 1961 ACCEPTEDJune 2, 1961
