External resistance to mass transfer

To begin our discussion on the diffusion of reactants from the bulk fluid to the external smface of a catalyst, we shall focus attention on the flow past a single catalyst pellet. Reaction takes place only on the catalyst and not in the fluid surroimding it. The fluid velocity in the vicinity of the spherical pellet will vaiy with position aroimd the sphere. The hydrodynamic boundary layer is usually defined as the distance from a solid object to where the fluid velocity is 99% of the bulk velocity U. Similarly, the mass transfer boundary layer thickness, 8, is defined as the distance from a solid object to where the concentration of the diffusing species reaches 99% of the bulk concentration. 

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The membrane is usually dense but sometimes micro-porous. If the external resistances to mass transfer are neglected in Figure 10.10, then pFj = p F l and pi, = p P i and Equation 10.20 can be written in terms of the volumetric flux as ... 

Consider an nth-order surface reaction, represented by A(g) — product(s), occurring in a catalyst particle, with negligible external resistance to mass transfer so that cAs cAg- Then the observed rate of reaction is... 

External Resistance to Mass Transfer 699 11.3.1 Mass Transfer Coefficient 699... 

A mathematical analysis of a crossflow magnetically stabilized fluidized bed chromatograph has been presented (14). The geometry of this system is similar to the rotating annular chromatograph and therefore the modeling approach is quite similar to that reported here. A parametric sensitivity study was conducted and the results indicated that the extent of band broadening was most sensitive to two factors. These factors were the external resistance to mass transfer and the width of the feed band. 

Modeling Diffusion with Chemical Reaction External Resistance to Mass Transfer 771... 

Estimate the external resistance to mass transfer by invoking continuity of the normal component of intrapellet fluxes at the gas/porous-solid interface. Then use interphase mass transfer coefficients within the gas-phase boundary layer surrounding the pellets to evaluate interfacial molar fluxes. 

Ideal Isothermal Packed Catalytic Tubular Reactors with First-Order Irreversible Chemical Kinetics When the External Resistance to Mass Transfer Cannot Be Neglected... 

Separation of variables provides the analytical solution to this first-order ODE given by (30-60). When the external resistance to mass transfer is significant, the following result allows one to predict reactant conversion in the exit stream as a function of important design parameters based on isolated pellets as well as the entire packed catalytic tubular reactor ... 

The highest conversion of reactants to products is achieved in an ideal PFR with no external mass transfer resistance. However, all simulations in Table 30-1 for ideal tubular reactors are not justified because one is operating at mass transfer Peclet numbers that are three- to sixty-fold smaller than (Re Scjcnticai- The only valid simulations in Table 30-1 are those which include interpellet axial dispersion and significant external mass transfer resistance, because Pcmt < (Re Sc)criticai and reactant molar densities near the external surface of the catalytic pellets are less than those in the bulk fluid phase. In general, external resistance to mass transfer reduces reactant molar densities on the catalytic surface, decreases the rate of conversion of reactants to products, and requires longer PFRs to achieve the same final conversion relative to the case where o = 0. 

If the external resistance to mass transfer is large, then the molar density of reactant A near the external surface of the catalyst is much smaller than its bulk gas-phase molar density. Equation (30-57) for first-order irreversible kinetics in an isothermal packed catalytic tubular reactor predicts that ... 

The above picture of the mechanism of osmotic dehydration suggests that the plasma lemma resistance to mass transfer affects the process to only a small extent. The process will be rather dependent on the internal resistance to osmotic flow and apoplast dewatering and, to some extent, on external resistance to mass transfer. 

In the earlier sections of this chapter attention has been focused on the sorption of a single component, with the implicit assumption that if a second (inert) component is present in the system it does not affect the sorption rate. Such an assumption requires further examination since whenever a second component is present, there is in principle a possibility of external resistance to mass transfer. Furthermjure, if the second component is adsorbed it may also affect the intraparticle (hffusion rate. 

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