Pore production rate, effect

Figure 2a. Experimental data on the effect of operating pressure, average pore size on membrane surface, and feed concentration on solute separation and product rate for the reverse osmosis system cellulose acetate membrane-sodium chloride-...

For liquid-phase catalytic or enzymatic reactions, catalysts or enzymes are used as homogeneous solutes in the hquid, or as sohd particles suspended in the hquid phase. In the latter case, (i) the particles per se may be catalysts (ii) the catalysts or enzymes are uniformly distributed within inert particles or (hi) the catalysts or enzymes exist at the surface of pores, inside the particles. In such heterogeneous catalytic or enzymatic systems, a variety of factors that include the mass transfer of reactants and products, heat effects accompanying the reactions, and/or some surface phenomena, may affect the apparent reaction rates. For example, in situation (iii) above, the reactants must move to the catalytic reaction sites within catalyst particles by various mechanisms of diffusion through the pores. In general, the apparent rates of reactions with catalyst or enzymatic particles are lower than the intrinsic reaction rates this is due to the various mass transfer resistances, as is discussed below. 

The major results of this study are consistent with a simple picture of mordenite catalysts. An increase in effective pore diameter, whether by extraction or exchange, will increase the rate of transport of reactant and product molecules to and from the active sites. However, aluminum ions are necessary for catalytic activity as aluminum is progressively removed by acid extraction, the number of active sites and the initial activity decrease. Coke deposition is harmful in two ways coke formation as the reaction proceeds will cause a decrease in effective pore diameter and effective diffusivity, and coke deposited on active sites will result in a chemical deactivation as well. 

During the calcination, the gaseous product CC>2 diffuses out through the pores. The rate of CO2 evolution depends upon whether diffusion or surface reaction is the controlling mechanism. Since calcination reaction is reversible concentration profile of CO2 within the pores would strongly effect the apparent rate of... 

They considered deactivation to occur by either pore-mouth (shell-progressive) or uniform (homogeneous) poisoning and examined the effect these types of deactivation had on overall activity and production rates for a single catalyst pellet. Analytical solutions were obtained for the production per pore by considering the time dependence of activity. Their results will be used here as the basis for the development of models for deactivation in fixed bed reactors. 

As most of the acid sites are located in pores of molecular size the rate and the selectivity of catalytic reactions depend not only on the intrinsic properties of the sites but also on the pore structure. A zeolite catalyst selects the reactant or the product by their ability to diffuse to and from the active sites (reactant and product selectivity). Steric constraints in the environment of the sites limit or inhibit the formation of intermediates or transition states (restricted transition state selectivity) [24,25]. The strong polarizing interaction between zeolite crystallites and adsorbed molecules leads to an unusually high concentration of the reactants in the pores. This concentration effect causes an enhancement of the rates of bimolecular reaction steps over monomolecular reaction steps [26]. 

Fig. 36] and other measurements shows that the flux is limited predominately, but not entirely, by the rate of reaction or production rather than transport effects (Aller, 1980). Essentially all the NH/ being produced is escaping the sediments at an equivalent rate independent of biogenic reworking or burrow formation. This would not be true of an element such as Si whose production rate depends on the Si pore-water concentration and is therefore highly sensitive to transport. 

These changes apparently result from a combination of lower production rates of Fe " and Mn ", together with the relatively increased importance of biogenic transport at this time compared to summer. Production rates decrease because of decreased microbial activity associated with lower water temperatures and depleted food sources [e.g., Eqs. (6.1) and (6.2)]. This allows the transport effects of biogenic reworking and irrigation to dominate the form of the fall solute profiles relative to those of the summer period. Similar changes are also observed for other pore-water constituents (Part I). 

Fig. 19. Comparison of the one- and two-dimensional models for Mn distribution in the top 0-18 cm of sediment at NWC. The production rate in both cases is that found for core NWC-4. The anoxic precipitation rate is assumed to be zero. The effective cylinder geometry used in the two-dimensional model is that determined for NH4 in Part I r, = 0.14 cm, rj = 4.5 cm. The basal gradient is constrained to be zero. The diffusion geometry created by irrigated burrows results in a vertical pore-water solute profile exhibiting apparent precipitation.

In order to test the effect of pore size on the cell penetration and collogen II production rates, five types of styrene/2EHA PHP were produced with average pore sizes (D) of 8, 17, 24, 31, 45, and 89 itm. The corresponding average interconnect sizes ( f) in these... 

More recently, methods have been developed for reducing the effective pore and channel dimensions. These techniques employ both physical treatments and chemical reagents. They have provided the basis for para-selective alkylation catalysts (18). These modified zeolites permit discrimination between molecules of slightly different dimensions. As a result, the para-isomers of the xylene or ethyltoluene products with the smallest minimum dimensions (Table 4) are able to diffuse out of the catalyst pores at rates about three orders of magnitude greater than those for the corresponding ortho- and meta-isomers (20). This discrimination capability is schematically represented in Figure 1, where the effective size of a para-selective catalyst pore is shown by the dashed line. 

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