Rate-selective adsorption

New Adsorbent Materials. SihcaUte and other hydrophobic molecular sieves, the new family of AlPO molecular sieves, and steadily increasing families of other new molecular sieves (including stmctures with much larger pores than those now commercially available), as well as new carbon molecular sieves and pillared interlayer clays (PILCS), will become more available for commercial appHcations, including adsorption. Adsorbents with enhanced performance, both highly selective physical adsorbents and easily regenerated, weak chemisorbents will be developed, as will new rate-selective adsorbents. 

As conclusion, bifunctional fibers having both phosphonic acid and sulfonic acid groups exhibit the characteristic metal ion selectivity and high breakthrough capacities in addition to the extremely fast adsorption rates. Studies on behavior of FPS-f in adsorption of other heavy metals like Fe(III) are now in progress. The bifunctional fiber developed in this work is attractive to application to the protection of the environment because of its extremely rapid adsorption rates and characteristic metal ion selectivity. 

When developing a liquid phase adsorptive separation process, a laboratory pulse test is typically used as a tool to search for a suitable adsorbent and desorbent combination for a particular separation. The properties of the suitable adsorbent, such as type of zeolite, exchange cation and adsorbent water content, are a critical part of the study. The desorbent, temperature and liquid flow circulation are also critical parameters that can be obtained from the pulse test. The pulse test is not only a critical tool for developing the equilibrium-selective adsorption process it is also an essential tool for other separation process developments such as rate-selective adsorption, shape-selective adsorption, ion exchange and reactive adsorption. 

Although most of the commercial adsorptive separation processes are operated under the selective-equilibrium adsorption mechanism, adsorptive separation may also be based on diffusion rates through a permeable barrier which are designated as rate-selective adsorption processes. In some instances there may be a combination of equilibriums as well as rate selective adsorption. A rate-selective adsorption process yields good separation when the diffusion rates of the feed components through the permeable barrier differ by a wide margin. 

Examples of rate-selective adsorption are demonstrated using silicalite adsorbent for separation of Ciq-Cm n-paraffins from non- -paraffins [40, 41] and Ciq-Ch mono-methyl-paraffins from non-n-paraffins [42-45]. Silicalite is a ten-ringed zeolite with a pore opening of 5.4A x 5.7 A [22]. In the case of -paraffins/non-n-paraffins separation [40, 41], n-paraffins enter the pores of silicalite freely, but non-n-paraffins such as aromatics, naphthenes and iso-paraffins diffuse into the pores more slowly. However, the diffusion rates of both normal -paraffins and non-n-paraffins increase with temperature. So, one would expect to see minimal separation of n-paraffins from non-n-paraffins at high temperatures but high separation at lower temperature. 

Another example of rate-selective adsorption is the separahon of diisopropylbenzene isomers using a silicalite adsorbent. Figure 6.12 shows the adsorption rates of 1,3-diispropylbenzene and 1.4-di-isopropylbenzene into silicalite adsorbent. In particular, it illustrates the more rapid adsorption of 1,4-di-isopropylbenzene compared to 1,3-di-isopropylbenzene. 

Shape-selective adsorption, also known as molecular sieving, is a process that separates molecules based on inclusion or exclusion from specific zeolite pores. In contrast, the equilibrium- and rate-selective mechanisms are based on adsorb-... 

Figure 6.12 Rate-selective adsorption of diisopropylbenzene isomers on Silicalite adsorbent.

Removal of suspended solids from a liquid may be required, either to recover valuable products or to remove obnoxious particulates. Clarification of dilute aqueous suspensions is frequently performed in deep-bed granular filters, where the grains provide a surface on which the particles may adsorb. If two types of particles differ in their adsorption rate, separation of them may be accomplished by selectively adsorbing one species on the grains. Furthermore, reversibility of adsorption would... 

Another problem in model applications is the adsorption on a nonuniform adsorbent. The immobilization of polyclonal antibodies will lead to different populations of binding sites, and the measurements will only give an apparent adsorption rate constant [22], The properties of the adsorbent surface are also greatly affected by the procedure used for protein immobilization. It may be important to select coupling methods that orient the covalently attached protein... 

Effects of coke on adsorption, diffusion, selectivities of catalytic reactions, and rates of catalytic reactions. 

The added surfactant molecules intended for CMP slurry stabilization can adsorb not only onto the abrasive particle but also onto the surface of the wafer to be polished. Depending on the extent of such adsorption, the added surfactant may influence the CMP process in several ways such as change in friction behavior of the slurry, modification of removal rate and selectivity, alteration of defectivity level, and shift in post-CMP profile. In this section the impact of surfactant adsorption on the removal rate, selectivity, and post-CMP cleaning characteristics will be discussed. 

Comparison of the adsorption rate constants derived from field studies with values for " Kj and "32 selected (a) in this paper and (b) by Schindler (29). 

Erionite has been synthesized at i00°-I50°C from a (Na,K) aluminosilicate gel with Si02/AUOs = 10. X-ray and electron diffraction results on the product show intergrowths of the related offretite structure, which is a large-pore zeolite. Adsorption capacity for n-hexane is consistent with the density but adsorption rates are far slower than for zeolite A. Adsorption rates for n-octane are even slower but still better than for natural erionite. Hydrocracking tests on a C /Cq naphtha show strong selectivity for converting normal paraffins to Cf gas, particularly propane. As temperature is increased, other components of the naphtha feed are cracked and selectivity decreases. 

The performance of immobilized and freely suspended affinity adsorbents was compared by calculating adsorption rates and selectivities for four different bead geometries. Simulation results indicate that the performance of finely ground adsorbent particles immobilized in hydrogel beads is superior compared to freely suspended adsorbents. The mathematical model was further used for simulation studies to investigate the effect of bead design parameters on product adsorption. 

Figure 7 shows the variation of selectivity with respect to time for three types of affinity beads (Cases (a), (b) and (c)). In all three cases, selectivity decreases from the initial maximum value as time progresses. Due to identical diffusivities, the two products have very similar concentration profiles within the immobilized adsorbent bead at initial time. Thus the initial selectivity is just the ratio of their adsorption rate constants. However, since product... 

It was found that the decline in selectivity was least in case (c) because of a smaller overall diffusional resistance of the bead. Figure 8 shows the variation of product purity (Pu) as a function of time for these three cases. The product purity curves show the same general trend as the selectivity curves. Final product purity was also found to be highest for case (c). By virtue of their lower overall mass transfer resistance case (c) immobilized adsorbent beads not only display a higher adsorption rate but also offer a higher selectivity for the desired product. 

The type 3 of adsorbent is selected based on considerations of the selectivity, the equilibrium capacity, the dynamic adsorption rate, case of regeneration, compatibility with the stream, coadsorption effects, type of process cycle and economics. Many of these have been discussed at some length above. 

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