Propylene, effective diffusivity

They used this model to predict the effective diffusivity in the partially impervious deposit (zone 4). They developed and solved the coupled mass balance equations for spherical pellets in a fixed bed reactor. To compare the model with experimental data, they measured the conversion of propane and propylene over partially poisoned pellets in a small isothermal packed bed reactor. For intrinsic kinetic rates of propane and propylene they employed the empirical rate equations proposed by Volts et al. and Hiam et al. The Hegedus model is briefly described below ... 

Equilibrium Uptake (mg gas/g sample) CO] Propylene Isobutylene Effective Diffusivity (cmVsec) Propylene Isobutylene I nop sobttt... 

Du 1986). This reflects the importance of smaU pores in order to apply effectively capillary condensation as a separation mechanism. Uhlhom (1990) demonstrated the effect of multilayer diffusion of propylene through a modified y-alumina membrane at 0°C. The separation factor for the N2/CjHg mixture was 27, where propylene is the preferentially permeating component, while the permeability increased to 7 times the Knudsen diffusion permeability. Although this mechanism appears to be very effective because of a high separation factor and a high permeability, it is limited by the obvious need for a condensable component. This in turn restricts the applicability range, due to limits set by temperature and pressure, needed for formation of multilayers or capillary condensation. 

Cymene or isopropyltoluene is produced via alkylation of toluene with propylene. Cymene is an important intermediate in the production of cresol, and it is also used as an industrial solvent. Again, for both environmental and economic reasons, the use of zeolitic materials for this conversion has been studied. For example, Flockhart et al. have used zeolite Y to effect this reaction (7). They observed that the state of the zeolite, including its degree of ion-exchange and the temperature at which it was calcined, strongly affected the distribution of cymene isomers obtained. In order to enhance the selectivity to para-cymene, the direct precursor to para-cresol, various studies have focused on the use of surface modified zeolites, for example, ZSM-5 materials, including those produced by chemical vapor deposition (CVD) of silicate esters. These species serve to reduce surface acidity and change limit diffusion within the crystal. 

Figure 3.41. Structure of carbon paper (left) and carbon cloth (right) used for gas diffusion layers in PEM fuel cells. A coating of 20% (by weight) fluorinated ethylene propylene has been applied. (From C. Lim and C-Y. Wang (2004). Effects of hydro-phobic polymer content in GDL on power performance of a PEM fuel cell. Electro chimica Acta 49, 4149-4156 G. Lu and C-Y. Wang (2004). Electrochemical and flow characterization of a direct methanol fuel cell.. Power Sources 134, 33-40. Used with permission from Elsevier.)...

In accordance with the equation for the activation energy of diffusion proposed by Meares [31], cohesion energy density (CED) of the polymer has a significant effect on diffusion coefficients of lower hydrocarbons. This is especially typical of mbbery polymers an increase in CED results in reduction of diffusion coefficients. Similar dependencies also apply to glassy polymers [9,32]. For example, Figure 9.8a shows the dependence of diffusion coefficient of propylene on CED of both glassy and mbbery polymers. 

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