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M. R. CANNON' The Pennsylvania State University, University Park, Pa.
Controlled Cycling Improves Various Processes Controlled cycling means that a cyclic process in which the time for each part of the cycle or amount of material moved in each part of the cycle, or both, is controlled by automatic equipment
CONTROLLED
CYCLING was advanced as a new method of operations for various types of processes in the fall of 1952. Since that time its soundness and practical use have been proved by laboratory experiments in the fields of distillation, liquid-liquid extraction, and particle separation. It can be used to advantage also in other processes such as absorption, filtration, and many others. The ideas involved are easily understood by explaining briefly their application in a few fields. When applied to a sieve or screen plate distillation column, for example, the cycle consists of two parts. a vapor flow period and a liquid flow period. The time for each period is easily imposed on the equipment by means of a cycle timer and one automatic valve in the vapor line leading to the distillation tower. If the timer is so set that the valve is open for 3 seconds and closed for 1 second, the total cycle period will be 4 seconds. During the time the valve is open. vapor will flow to the tower. At low vapor velocities, some leakage of liquid occurs through the holes in the plate. The amount of such leakage depends upon the hole size and the number of holes per square inch. However, the general idea is to have operation in the range where the thrust of the rising vapor prevents liquid downflow during the vapor flow period. When the vapor valve closes there is no vapor thrust and liquid flows down the column. Thus, the cycle consists of a vapor flow period followed by a liquid flow period. With this type of operation, no downcomers on plates are needed since the vapor and liquid phases use the same avenues of travel during their particular part of the cycle. To date, 2-inch diameter and 6-inch diameter plate towers have been operated in this manner. There should be a vapor space in the still or line u p to the control valve equal 1
Present Address, Cannon Instrument
Co., State College, Pa.
to three or more volumes of one plate space. No additional controls on the heat flow to the still are needed. This means that still pressure fluctuates to Some extent and that some flashing occurs when the vapor valve is first opened but causes no trouble. A vertical U tube may be installed across the vapor control valve. This will fill with condensate, but if the control valve fails closed, it offers a bypass to prevent pressure build up in the still. Applications
Advantages of controlled cycling in distillation are : 0 Higher capacity 0 Simpler and cheaper plate design e High flexibility due to a choice of operating conditions dependent upon cycle times Data on the performance of sieve and screen plate distillation columns with controlled cycling were obtained by Gaska (2); data on the performance of packed plates in distillation were obtained by McWhirter ( 3 ) . Packed plates are made by enclosing a depth of efficient packing between screens. Such designs result in high plate efficiency packing and high capacity ing because of the large vapor space between plates. When applied to liquid-liquid extraction through various types of plates the cycle may be of four parts: Denser phase flows down through plates Pause period during which time neither phase is flowing but coalesing of droplets occurs between plates Lighter phase flows upward Pause period for coalesing In this type of extractor, each phase is continuous for part of the cycle and is discontinuous for part of the cycle. Each phase is broken into streams of droplets between plates and coalesces into a continuous pause period-between plates. The rate of
flow of the phases through the extractor is a function of the impact pressure of the entering phase, and thus in this vital respect the process is differect from processes which depend upon density differences of the phases as the driving force for phase flow. Details of this type of extractor have been published ( 7 ) . Speaker ( 5 ) and Szabo (6) have tested such extractors and found them to have high efficiencies and high capacities. Some are now in industrial use. The application of controlled cycling to particle size separation in screening has been studied by Robertson ( 4 ) . In this apparatus the pressure of the gas (usually air) on the downstream and upstream side of the screen is varied in such a way that the bed of mixed particles is blown off the screen into the space between screens, whereupon, the pressure is changed so that the cloud of mixed particles is sucked or forced against the screen. The cycle consists of two parts, a low pressure period followed by a higher pressure period on each side of the screen. The cycle is easily imposed on the equipment by means of a timer, electric valves, and two surge tanks maintained at different pressures. literature
Cited
(1) Cannon, M. R., Oil Gas J., (Jan. 23, 1956). (2) Gaska, R. A., M.S. thesis, 1957; Ph.D. thesis, 1959; The Pennsylvania State University, University Park, Pa. (3) McWhirter, J. R., M.S. thesis, The Pennsylvania State University, University Park, Pa., 1961. (4) Robertson, D. C., M.S. thesis, The Pennsylvania State University, University Park, Pa., 1960. (5) heaker. S. M.. Ph.D. thesis. The Pen'nsylvinia Stat; University, university Park, Pa., 1957. (6) Szabo, T. T., Ph.D. thesis, The Pennsylvania State University, University Park, Pa., 1958. \
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RECEIVED for review March 20, 1961 ACCEPTEDJune 2, 1961 VOL. 53,
NO. 8
0
AUGUST 1961
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