Diffusion configurational

Synthesis ofp-Ethyltoluene. j )i7n7-Ethyltoluene, the feedstock for j )-methylstyrene, is difficult to separate from the products of toluene alkylation with ethane using conventional acidic catalysts. The unique configurational diffusion effect of ZSM-5 permits -dialkylbenzenes to be produced in one step. In the alkylation of toluene with ethene over a chemically modified ZSM-5, -ethyltoluene is obtained at 97% purity (58). 

Knudsen diffusion is generally encountered in heterogeneous catalysis. In the smallest pores configurational diffusion takes place with diffusion coefficient ranging from 10 to 10 m /s. 

For high-quality MFI-type zeolite membranes either Knudsen or configurational diffusion is representative of the flow. Knudsen diffusion occurs in pores where... 

REGULATORYAGENCIES - CHEMICAL PROCESS INDUSTRY] (Vol21) Configurational diffusion... 

Spry and Sawyer (1975) developed a model using the principles of configurational diffusion to describe the rates of demetallation of a Venezuelan heavy crude for a variety of CoMo/A1203 catalysts with pores up to 1000 A. This model assumes that intraparticle diffusion is rate limiting. Catalyst performance was related through an effectiveness factor to the intrinsic activity. Asphaltene metal compound diffusivity as a function of pore size was represented by... 

Fig. 56. Model computation for the demetallation rate as a function of pore size and time on stream for first-order kinetics and configurational diffusion of a 12.5-A-diameter metal-bearing molecule (Rajagopalan and Luss, 1979).

Haynes apd Leung (1983) formulated a similar configurational diffusion model combining the effects of active site poisoning as well as pore plugging on the HDM reaction. In this case the reaction form in the conservation equation is multiplied by a deactivation function which accounts for the loss of intrinsic activity, (1 - ) is frequently chosen, where x is the fractional coverage of the sites. Other forms of the site deactivation function have been discussed by Froment and Bischoff (1979). The deactivation was found to depend on a dimensionless parameter given by... 

Vertical Y- and T-type Configuration Diffusive Mixing Most Relevant Citations... 

Ansari, A., Kuznetsov, S. V., and Shen, Y. (2001). Configurational diffusion down a folding funnel describes the dynamics of DNA hairpins. Proc. Natl. Acad. Sci. USA 98, 7771-7776. 

When aerosols are in a flow configuration, diffusion by Brownian motion can take place, causing deposition to surfaces, independent of inertial forces. The rate of deposition depends on the flow rate, the particle diffusivity, the gradient in particle concentration, and the geometry of the collecting obstacle. The diffusion processes are the key to the effectiveness of gas filters, as we shall see later. 

FIGURE 5.27 Transport mechanisms in porous media molecular or gaseous flow, (a) Knudsen flow, (b) surface diffusion, (c) multilayer diffusion, (d) capillary condensation, and (e) configurational diffusion. 

Restricted transport or, by a different name, configurational diffusion [13] occurs when the diffusing molecules are comparable in size to the pores within which they diffuse. This happens, for example, in hydrodemetallation over alumina-supported Co-Ni catalysts [14]. The observation that the effective diffusivity depends on the fourth power of the molecule-to-pore size ratio is important, but it is not yet evident how to correlate complex pore size distributions with effective diffusivities in the configurational... 

Extensive manifestations of configurational diffusion can be seen in catalytic zeolites. The landmark measurements by Gorring [16] of the diffusion coefficients of alkanes (in potassium T zeolites) as a function of their carbon number are shown in Fig. 4, indicating over two orders of magnitude of change in diffusivity, with a minimum at C-8 and a maximum at C-12 for unexplained reasons. Similarly, spectacular effects for more complex molecules have been observed by Haag and Chen [17] and are shown in Fig. 5. Although we do not yet have a workable correlation between zeo-... 

Spry, J. C., and Sawyer, W. H., Configurational Diffusion Effects in Catalytic Demetallization of Petroleum Feedstocks. AIChE 68th Annual Meeting, Los Angeles, Nov. 16-20, 1975. 

In zeolitic materials, where the molecules are of pore size dimensions, activated configurational diffusion... 

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