Adsorbent utilization

Both batch and continuous adsorption processes are used. In a batch process, the adsorbent bed is allowed to become saturated with adsorbed material and is subsequently regenerated in a cyclic manner. In a continuous process, usually the counter-current mode is adopted for adsorption and desorption, either in time form or in simulated mode. Continuous operation offers many advantages with respect to the efficiency of adsorbent utilization. Thus, for... 

The total pore volume of the voids was determined from the amount of adsorbed water at saturation assuming that the water is present as a normal liquid with an average density assumed to be that of liquid water. Other adsorbates utilized in this study include the gases oxygen and nitrogen at their respective boiling points and n-butane and neopentane at room temperature. 

The integrated sequencing provides the opportunities for more efficient hydrogen recovery through extra steps compared to traditional PSA cycles. Improvements of better than ten percentage points have been realized (2). Adsorbent utilization is improved so that productivity, i.e., hydrogen produced per mass unit adsorbent, increased by 10 to 20%. 

The greater part of the modification studies of silica with boron compounds has been directed towards achieving an understanding of the surface structure of silica and silica based adsorbents, utilizing the quantitative reactions with boron containing probe molecules. Because hydroxyl type specificity occurred in some reactions, boron compounds were used to make a distinction between isolated and vicinal surface hydroxyls. Diborane was even utilized as a probe to distinguish surface silanols from hydration water.6,7... 

Of the many other methods we can mention the preparation of biospecific adsorbents utilizing trichloro-5-triazine [120], coupling via thiol-disulfide interchange with activated thiolated support [121] or via azide formation [122]. The attachment of affinants containing the aromatic residues can be carried out by means of diazonium groups [87,123]. The coupling of the affinant onto the silinized glass after the reactions with p-nitrobenzoylchloride followed by reduction with sodium dithionite was described by Weetall and Filbert [124]. 

As with temperature, the most convenient pressure for adsorption is that of the feed. When a range of pressure is available, the highest pressure should be chosen so that the adsorptive loading is maximized. The comments above on condensation are applicable to pressure selection also. For pressure-swing cycles, the adsoiption pressure must be arrived at frm economic trade-offs, since lar r swings in pressure yield better adsorbent utilization but require more compression power. 

The performance of a multiple-column periodic countercurrent system has been analyzed theoretically by Svedberg, Neretnieks, Rhee, Rip-pin, 4. 5) and others. The variation of adsorbent utilization with bed length is shown in Figure 12.10 for a single fixed bed, an ideal continuous countercurrent system, and a periodic countercurrent system containing 2,4, and 6 beds. 

FIGURE 12.10. Comparison of continuous countercurrent, periodic countercurrent, and cyclic batch adsorption systems showing effectiveness of adsorbent utilization as a function of bed length. (From ref. 15, reprinted with permission.)... 

Figure 15.24 shows expanded upflow columns operating in parallel with a single downflow column. These are staggered when put into operation. Downflow operation is used on large-volume plants, and the breakthrough curve should be steep to maximize adsorbent utilization. 

When PAC is used, not only is its adsorbability utilized but its surface charge may also cooperate as a coagulant for colloidal fractions in the liquid phase. In this case, however, the regeneration of PAC may become rather difficult. This is one of the reasons why spent PAC is in most cases dumped rather than regenerated for repeated use. 

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