Adsorbent working capacity

In solvent recovery plants, temperature-swing processes are most frequently used. The loaded adsorbent is direct heated by steam or hot inert gas, which at the same time serves as a transport medium to discharge the desorbed vapor and reduce the partial pressure of the gas-phase desorpt. As complete desorption of the adsorpt cannot be accomplished in a reasonable time in commercial-scale systems, there is always heel remaining which reduces the adsorbent working capacity. 

Adsorbent drying systems are typicaHy operated in a regenerative mode with an adsorption half-cycle to remove water from the process stream and a desorption half-cycle to remove water from the adsorbent and to prepare it for another adsorption half-cycle (8,30,31). UsuaHy, two beds are employed to aHow for continuous processing. In most cases, some residual water remains on the adsorbent after the desorption half-cycle because complete removal is not economically practical. The difference between the amount of water removed during the adsorption and desorption half-cycle is termed the differential loading, which is the working capacity available for dehydration. 

Gasoline working capacity (GWC) also shows a strong relationship with the pore volume in the mesopores. Similar to BWC, GWC is a measure of adsorption capacity in which actual gasoline vapors are used as the adsorbate. The relationship between the BWC and GWC is shown in Fig. 12. The data shows a strong relationship between the BWC and GWC. The relationship would be expected since both the BWC and GWC have excellent linear correlations with the pore volume in the small mesopores. 

The adsorbent particles are normally used as beads, extrudates, or granules (-0.1 -0.3 cm equivalent diameters) in conventional H2 PSA processes. The particle diameters can be further reduced to increase the feed gas impurity mass transfer rates into the adsorbent at the cost of increased column pressure drop, which adversely affects the separation performance. The particle diameters, however, cannot be reduced indefinitely and adsorption kinetics can become limiting for very fast cycles48 New adsorbent configurations that offer (i) substantially less resistance to gas flow inside an adsorber and, thus, less pressure drop (ii) exhibit very fast impurity mass transfer coefficients and (iii) minimize channeling are the preferred materials for RPSA systems. At the same time, the working capacity of the material must be high and the void volume must be small in order to minimize the adsorber size and maximize the product recovery. Various materials satisfy many of the requirements fisted above, but not all of them simultaneously. 

The regenerability of an adsorbent determines the fraction of capacity, or working capacity, that is recovered for future use. In most cases, a constant decrease in working capacity occurs after the first cycle and is maintained for up to approximately 50-100 cycles. Eventually, however, slow aging or gradual poisoning causes the working capacity to be reduced to the point that the adsorbent needs to be replaced. 

Attrition can cause a host of problems, from increased pressure drop due to the presence of fines in the intersticies between whole particles to total failure of the adsorber when adsorbent dust is conveyed into the pipes and fittings downstream. In addition, a short-term loss of working capacity (mentioned earlier) commonly occurs during the first few cycles of operation of fresh adsorbent, followed by gradual decay, perhaps over hundreds of cycles, due to aging (partial collapse of the adsorbent s pore structure), chemisorption (poisoning), or other causes. It is that decay plus attrition losses that essentially govern the life of the adsorbent. 

Data for typical systems are provided in Table 14.8. There we find, for -heptane, the working capacity is 6 kg/100 kg loading, and the steam requirement is 4.3 kg steam/kg solvent. A mass balance yields an estimate for the required amount of adsorbent = 6250 kg/bed, and the required steam flow rate = 537.5 kg/hr. 

With these purely thermodynamic questions disposed of, one can hope that the future will bring a number of really complete experimental studies (both isotherm and calorimetric measurements, including heats of immersion and heat capacities in some cases) of the simplest possible systems—for example, argon or krypton adsorbed on nonpolar, non-porous adsorbents. Work along these lines is already in progress in the laboratories of J. A. Morrison and G. Jura. Aside from intrinsic interest, a backlog of detailed thermodynamic data of this type should prove invaluable for future theoretical attempts to understand the nature of adsorbed films. 

The selection of a suitable zeolite adsorbent for CO2 removal from flue gas (mixture of CO2 and N2) has been carried out. The limiting heats of adsorption, Henry s Law constants for CO2 and N2, CO2 pure component adsorption isotherms and expected working capacity curves for Pressure Swing Adsorption (PSA) separation application were determined. The results show that the most promising adsorbent characteristics are a near linear CO2 isotherm and a low Si02/Al203 ratio with a cation in the zeolite structure that has strong electrostatic interaction. 

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