Parametric pumping cycles

Temperature Two modes of temperature parametric-pumping cycles have been defined—direct and recuperative. In direct mode, an adsorbent column is heated and cooled while the fluid feed is pumped forward and backward through the bed from reservoirs at each end. When the feed is a binary fluid, one component will concentrate in one reservoir and one in the other. In recuperative mode, the heating and cooling takes place outside the adsorbent column. Parametric pumping, thermal and pH modes, have been widely studied for separation of liquid mixtures. However, the primary success for separating gas mixtures in thermal mode has been the separation of propane/ethane on activated carbon [Jencziewski and Myers, Ind. Eng. Chem. Fundam., 9, 216-221 (1970)] and of air/S02 on silica gel... 

Patrick, Schrodt, and Kermode, Sep. Sci, 7, 331-343 (1972)]. The difficulty with applying the thermal mode to gas separation is that in a fixed volume gas pressure increases during the hot step, which defeats the desorption purpose of this step. No thermal parametric-pumping cycle has yet been practiced commercially. 

Cyclic adsorption processes such as parametric pumping, cycling zone adsorption and heatless adsorption have received much attention both theoretically and experimentally in the past several years (3j. Skarstrom (4J has reviewed applications of heatless adsorption (synonymous with pressure swing adsorption) to air drying, hydrogen purification and air fractionation. 

The term parametric pumping was coined by Wilhelm et al. [Wilhelm, Rice, and Bendelius, Ind. Eng. Chem. Fundam., 5,141-144 (1966)] to describe a liquid-phase adsorption process in which separation is achieved by periodically reversing not only flow but also an intensive thermodynamic property such as temperature, which influences adsorptivity. Moreover, they considered the concurrent cycling of pressure, pH, and electrical and magnetic fields. A lot of research and development has been conducted on thermal, pressure, and pH driven cycles, but to date only gas-phase pressure-swing parametric pumping has found much commercial acceptance. 

RPSA [Rapid pressure swing adsorption] A version of the PSA process which uses fast pressure-cycles known as parametric pumping. The molecular sieve adsorbent for this duty has to be of a smaller grain size than that for PSA. Developed by the Union Carbide Corporation. 

Further discussions of pressure swing adsorption, parametric pumping and cycling-zone adsorption have been presented by Yang 3-1, Schweitzer 7 1 and Wankat(58). 

Wankat, P. C. Cyclic Separations—Parametric Pumping, Pressure Swing Adsorption and Cycling Zone Adsorption (A.I.Ch.E. Modular Instmction Series, Module B6.ll, 1986). 

A class of operations has been devised in which the process fluid is pumped through a particular kind of packed bed in one direction for a while, then in the reverse direction. Each flow direction is at a different level of an operating condition such as temperature, pressure, or pH to which the transfer process is sensitive. Such a periodic and synchronized variation of the flow direction and some operating parameter was given the name of parametric pumping by Wilhelm (1966). A difference in concentrations of an adsorbable-desorbable component, for instance, may develop at the two ends of the equipment as the number of cycles progresses. 

Figure 19.12. Batch parametric processing of solid-liquid interactions such as adsorption or ion exchange. The bottom reservoir and the bed interstices are filled with the initial concentration before pumping is started, (a) Arrangement of adsorbent bed and upper and lower reservoirs for batch separation, (b) Synchronization of temperature levels and directions of flow (positive upward), (c) Experimental separation of a toluene and n-hcptane liquid mixture with silica gel adsorbent using a batch parametric pump. (Reprinted from Wilhelm, 1968, with permission of the American Chemical Society), (d) Effect of cycle time t on reservoir concentrations of a closed system for an NaCl-H20 solution with an ion retardation resin adsorbent. The column is initially at equilibrium with 0.05M NaCl at 25°C and a = 0.8. The system operates at 5° and 55°C. [Sweed and Gregory, AIChE J. 17, 171 (1971)J.

In PSA, pressure drop is purposely minimized, so that, although the pressure within a bed changes substantially during a cycle, the pressures at various points in a bed at a given time are virtually the same. In pressure-swing parametric pumping, large... 

The adsorbent particles in pressure-swing parametric pumping are substantially smaller than those in PSA and produce rapidly fluctuating pressure profiles in the bed during a cycle. These profiles and the cycle times which influence them are responsible for the continuous product flow, the high productivities and the high product purities which can be attained in the process. 

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