Effectiveness factor selectivity considerations

Microlevel. The starting point in multiphase reactor selection is the determination of the best particle size (catalyst particles, bubbles, and droplets). The size of catalyst particles should be such that utilization of the catalyst is as high as possible. A measure of catalyst utilization is the effectiveness factor q (see Sections 3.4.1 and 5.4.3) that is inversely related to the Thiele modulus (Eqn. 5.4-78). Generally, the effectiveness factor for Thiele moduli less than 0.5 are sufficiently high, exceeding 0.9. For the reaction under consideration, the particles size should be so small that these limits are met. 

The degree of conversion has to be considerably higher than 50% to obtain the desired product in sufficiently high enantiomeric purity. The incomplete enantio-selectivity of PPL (E = 18) was countered by continuation of the reaction to 67% conversion. The productivity in a multi-phase reactor (Figure 13.24), in which the racemic ester was circulated in the lumen, was 17.6 g (h m2)-1 or 28.4 mmol (h mg enzyme)-1. The main problem of the multi-phase reactor is the low catalyst effectiveness factor, which is normally found to lie between 30 and 50%. 

As the polarities of the solute and solvent molecules increase, the interactions of these molecules become much stronger with the adsorbent, and they adsorb with localization. The net result is that the fundamental equation for adsorption chromatography with relatively nonpolar solutes and solvents has to be modified. Several localization effects have been elucidated, and the modified equations that take these factors into consideration are rather complex [7,8,10]. Nevertheless, the equations provide a very important framework in understanding the complexities of adsorption chromatography and in selecting mobile phases and stationary phases for the separation of solutes. 

In this chapter, general considerations are presented in an attempt to advance the understanding of the LM science at facilitated, coupled transport which allows the optimization of solutes separations. Factors that influence the effectiveness and selectivity of separation are analyzed. 

Figure 1 shows the dependence effectiveness factor tj = roi/ri on Thiele modulus for the selected values of parameter p and for = 2, v = 2 (sphere), yt = 7, yi = 12, 5 = 1 (endothermic reactions) and xa(1) = 1. One can find that for Peffectiveness factor rj assumes in the certain ranges of Thiele modulus values much higher than unity. This means that in the cases discussed the internal diffusion, in contrast to the classical isothermal or endothermic catalytic reactions, may considerably increase the rate of the heterogeneous autocatalytic reactions. 

For rapid reactions the Thiele modulus is > 3, and the effectiveness factor is equal to /<(>, for all particle shapes. When 9 3, the reaction proceeds only in a thin shell close to the outer surface of the catalyst particle. For such rapid reactions catalyst particles may be used that consist of a massive core, coat with a thin layer of porous catalyst material. When the catalyst is expensive this could result in considerable savings, but it can also be desirable for obtaining higher selectivities (see below). 

The opinion offered is to add additional focus, mainly the potential therapeutic benefit that can be derived from cascade regulation of the inducible nitric oxide synthase (iNOS), NO, cyclooxygenase (COX-2), and hypoxia-inducible factor la (HIF-la) by non-toxic but a biologically effective and selective small multi-targeted molecule, selenium, with considerable antioxidant properties. 

The choice of variables remaining with the operator, as stated before, is restricted and is usually confined to the selection of the phase system. Preliminary experiments must be carried out to identify the best phase system to be used for the particular analysis under consideration. The best phase system will be that which provides the greatest separation ratio for the critical pair of solutes and, at the same time, ensures a minimum value for the capacity factor of the last eluted solute. Unfortunately, at this time, theories that predict the optimum solvent system that will effect a particular separation are largely empirical and those that are available can be very approximate, to say the least. Nevertheless, there are commercially available experimental routines that help in the selection of the best phase system for LC analyses, the results from which can be evaluated by supporting computer software. The program may then suggest further routines based on the initial results and, by an iterative procedure, eventually provides an optimum phase system as defined by the computer software. 

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