Parametric pumping

Parametric pumping is a cyclic fixed-bed process in which the axial fluid flow direction and the driving force direction (perpendicular to the axial flow direction) are changed periodically and synchronously throughout the bed in the presence of reflux of the fluid at one or both ends of the bed (Sweed, 1972). The most common driving force is -Vpi created by a difference in temperature between the mobile phase and the stationary phase. Other driving forces used include a pH difference or a partial pressure difference. Some of the earliest descriptions of parametric pumping appear in Wilhelm et al. (1966, 1968). 

During the next half of the square wave cycle, the fluid from the top reservoir is pushed downward into the packed bed, which is suddenly cooled to Tcoia the colder fluid is now pushed into the bottom reservoir with the same speed. Since the adsorbent particles are cold, their adsorption capacity is much higher, thus purifying the fluid of solute i. The fluid coming into the bottom reservoir is substantially purified of species i. This cycle is ended when the volume of fluid introduced into the bottom reservoir during the second half of the cycle is equal to that removed from the same reservoir during the first half of the cycle. A new cycle is then initiated. Note that the top reservoir now has a solute concentration more than that in the initial 

Employing assumptions (3) and (5) in equation (7.1.3), the mass balance equation for species i is obtained as (/ = 1, solid phase, j = 2, mobile phase)  

If Pj is the density of the solid particles (mass/volume) and qii represents the moles of species i per unit mass of solid particles, then 

If yi (instead of for phase 2) represents the moles of species i per mole of fluid, and pj, stands for the number of moles per unit fluid volume, then 

TABLE 19.5. Specifications of Spiral and Tubular Equipment for Reverse Osmosis and Ultrafiltration  

The goal of a commercial sublimation is the separation of a valuable material from nonvolatile ones at temperatures low enough to avoid thermal degradation. The preservation of cell structure (and taste) is a deciding factor in the choice of freeze drying, a special instance of sublimation, foods, pharmaceuticals, and medical products. 

Only a few solids have vapor pressures near atmospheric at safe temperatures, among them COz, UF , ZrCL(, and about 30 organics. Ammonium chloride sublimes at 1 atm and 350°C with decomposition into NH3 and HC1, but these recombine into pure NH4C1 upon cooling. Iodine has a triple point 113.5°C and 90.5 Torr it can be sublimed out of aqueous salt solutions at atmospheric pressure because of the entraining effect of vaporized water. 

Among substances that are sublimed under vacuum are anthranilic acid, hydroxyanthraquinone, naphthalene, and fl-naphthoi. Pyrogallol and d-camphor distill from the liquid state but condense as solids. Several metals are purified by sublimation, for instance, magnesium at 600°C and 0.01-0.15 Torr. 

The common carrier gases are air or nitrogen or steam. Condensate from a carrier usually is finely divided, snowlike in character, which is sometimes undesirable. Substances which are sublimed in the presence of a carrier gas include anthracene, 

3 vessels, each 4ftdia by 12.5 ft TT and 100 cuft of dessicant j on an 8 hour I cycle. 

Representative values and ranges of operating parameters are summarized in Table 15.3. Cycle times for some adsorptions are adjusted to work shift length, usually multiples of 8 hr. When cycle times are short, as for solvent recovery, automatic opening and closing of valves is necessary. 

Steam rates for regeneration of a particular adsorbent carbon are shown in Table 15.3(b). Steam/solvent ratios as high as 8 sometimes are necessary. 

The capacity of regeneration furnaces is selected so that they operate 80-90% of the time. In multiple-hearth furnaces the loading is 70-80 lb/(sqft)(day). In countercurrent direct fired rotary kilns, a 6% volumetric loading is used with 45 min at activation temperature. 

Details of the design and performance of other commercial liquid phase adsorptions are proprietary. 

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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. 

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. 

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. 

As described by Wilhelm et alS55 an alternative operating procedure has been developed in order to improve the separation obtained, where separation is defined as the ratio of concentrations in the upper and lower reservoirs, or in a reservoir and the feed. The technique has become known as parametric pumping because changing an operating parameter, such as temperature, may be considered as pumping the adsorbate into a reservoir at one end of a bed and, by difference, depleting the adsorbate in a reservoir at the other end. 

Figure 17.34. Parametric pumping, batch operation (a) Direct thermal mode (b) Recuperative thermal mode...

Figure 1134b shows an alternative method of supplying heat in thermal parametric-pumping. Heat is supplied in upwards flow by passing feed from the lower reservoir... 

It may be shown that the ratio of the concentration in a hot zone to that in a cold zone for recuperative parametric pumping is given by ... 

Many workers have demonstrated the effectiveness of parametric pumping in order to achieve separations in laboratory-scale equipment. It is mainly liquid systems that have been studied, using either temperature or pH as the control variable. Pressure parametricpumping is described in a US patent and is discussed by Yang(3). 

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. 

A schematic of a batch parametric pumped adsorption process is sketched in Figure 19.12(a), whereas Figure 19.12(b) shows the synchronized temperature levels and flow directions. At the start, the interstices of the bed and the lower reservoir are filled with liquid of the initial composition and with the same amount in both. The upper reservoir is empty. The bed is kept cold while the liquid is displaced from the interstices into the upper reservoir by liquid pumped from the lower reservoir. Then the temperature of the bed is raised and liquid is pumped down through the bed. Adsorption... 

Although parametric pumping appeared on the academic scene in 1966, no commercial installations appear to have been made, at least no widely publicized ones. Periodic heating and cooling of beds of granules or even of periodic compression and decompression of the process gas appear to be serious economic obstacles. 

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.

Static mixers Vibratmg-plate extractors Ultrasonic extractors Parametric pumping extractors... 

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. 

This section outlines the basic pressure-swing parametric pumping process, which will be described below in the context of oxygen production from air. The preferred adsorbent for this separation can be either 5A or 13X zeolite. 

The key feature of pressure-swing parametric pumping is the unusual pressure versus distance and time feature of the bed. 

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... 

Figure 3. Schematic of pressure-swing parametric pump...

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