Shell-progressive mechanism

For these and similar systems the original source, resin or solution, of the counter-ion being chemically consumed and the nature of the co-ion greatly influence the observed kinetics. The association-dissociation of weakly functional resins is of particular practical interest since in these instances a reactive and non-reactive core respectively forms within the resin which shrinks towards the bead centre as exchange proceeds. This Shrinking Core or Shell Progressive mechanism is usually particle diffusion controlled and explains why exchange on weakly functional resins is invariably flow-rate sensitive under column operation. 

Reactor Model A variation of the pseudo-homogeneous 1-D plug flow model with shell progressive mechanism of poisoning (SPM) is proposed. This model accounts for the intraparticle-diffusional resistance using a pore mouth poisoning mechanism. The... 

The shell progressive mechanism can be applied to some ion-exchenge processes. Any of the threa subsequant steps, that is. film diffusion, ash-layer diffusion, and chemical reaction control, can be rate determining, depending on the prevailing conditions. The following relationships have been obtained for the indicated conditions. 

The silica penetrates the catalyst bead via a shell progressive mechanism and deposits in the micropores of the catalyst, deposition is non-selective and silica masks the noble metal active sites... 

The amount of poison deposited is given as a function of the dimensionless process time by Fig. 5.2a -l. Also, the deactivation function for given poison levels is in Fig. 5.2.C-2. Combine these in a figure for the deactivation function as a function of dimensionless time for the shell progressive mechanism. 

The kinetics of the chlorine displacement from PVC by the ether thiolate group in the presence of phase transfer catalyst obeys the shell-progressive mechanism. The rate at which an individual spherical particle reacts depends on the diffusion through the reacted layer. 

Pore Mouth (or Shell Progressive) Poisoning This mechanism occurs when the poisoning of a pore surface begins at the mouth of the pore and moves gradmuly inward. This is a moving boundary problem, and the pseudo-steady-state assumption is made that the boundary moves slowly compared with diffusion of poison and reactants and reaction on the active surface. P is the fraction of the pore that is deactivated. The poison diffuses through the dead zone and deposits at the interface between the dead and active zones. The reactants diffuse across the dead zone without reaction, followed by diffusion-reaction in the active zone. 

Deactivation of large-pore slab catalysts where intraparticle convection, diffusion and first order reaction are competing mechanisms was analyzed by uniform and shell-progressive models. For each situation, analytical solutions for concentration profiles, effectiveness factor and enhancement factor due to convection were developed thus providing a sound basis for steady-state reactor design. 

Distribution of Active Sites. If the chemical reaction that functionalizes the polymer proceeds slower than diffusion of the reagent into the polymo, functional groups will be distributed uniformly throughout the particle. If chemical reaction proceeds faster than diffusion, the polymer be is functionalized first at its surface, and the reaction proceeds into the bead 1 a shell (Ufhisive mechanism 1241. Partial conversion, shell diffusive reactions give particles functionalized predonunantly near the surface. The progress of shell diffusive rations in microporous polymers can be observed with beads under a microscope. 

Despite this progress, the evidence linking cysts to shellfish toxicity remains circumstantial and care should be exercised before attributing toxin increases to this mechanism. The major problem is that it has yet to be demonstrated that shellfish can remove toxin from cysts. The feeding studies mentioned earlier (which do not yet include scallops 35) indicate that many viable G. tamarensis cysts can be isolated from the fecal pellets of mussels and soft-shelled clams fed cyst suspensions. There is certainly some cyst mortality as well (Figure 5), but whether this is also associated with toxin retention by the shellfish has yet to be demonstrated. It is reasonable to expect that the assimilation of toxin from cysts will not be a highly efficient process. 

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