Diffusion enhancement

Selected entries from Methods in Enzymology [vol, page(s)] Analysis of GTP-binding/GTPase cycle of G protein, 237, 411-412 applications, 240, 216-217, 247 246, 301-302 [diffusion rates, 246, 303 distance of closest approach, 246, 303 DNA (Holliday junctions, 246, 325-326 hybridization, 246, 324 structure, 246, 322-324) dye development, 246, 303, 328 reaction kinetics, 246, 18, 302-303, 322] computer programs for testing, 240, 243-247 conformational distribution determination, 240, 247-253 decay evaluation [donor fluorescence decay, 240, 230-234, 249-250, 252 exponential approximation of exact theoretical decay, 240, 222-229 linked systems, 240, 234-237, 249-253 randomly distributed fluorophores, 240, 237-243] diffusion coefficient determination, 240, 248, 250-251 diffusion-enhanced FRET, 246, 326-328 distance measurement [accuracy, 246, 330 effect of dye orientation, 246, 305, 312-313 limitations, 246,... 

The importance of diffusion enhancement to heavy oil cracking is further illustrated by the alumina-montmorillonite complexes which crack heavier feeds, i.e. Wilmington fraction No. 6, more effectively than REY. When used as matrices for REY, the alumina-montmorillonites results in considerably more active catalysts, at the same zeolite content, compared with a catalyst having a kaolin-binder matrix, while the selectivity properties differs very little between the two types of catalysts (Sterte, 3. Otterstedt, 3-E. Submitted to Appl.Catal.). 

Fig. 6.14. Label-free chemical imaging of the penetration pathways for the topically applied drug diffusion enhancer dimethyl sulfoxide (DMSO) into mouse skin tissue Dual-frequency SRS imaging tuned into the characteristic vibration of DMSO at 670 cm-1 (bright gray regions) and the CH2 vibration of lipid-rich adipocytes at 2845 cm-1 (dark gray regions) at a depth of 65pm into the tissue. DMSO is hydrophilic and hence avoids lipid structures such as adipocytes (Image courtesy of Brian Saar, Chris Freudiger, and Wei Min [12])...

Hydrocarbon distributions in the Fischer-Tropsch (FT) synthesis on Ru, Co, and Fe catalysts often do not obey simple Flory kinetics. Flory plots are curved and the chain growth parameter a increases with increasing carbon number until it reaches an asymptotic value. a-Olefin/n-paraffin ratios on all three types of catalysts decrease asymptotically to zero as carbon number increases. These data are consistent with diffusion-enhanced readsorption of a-olefins within catalyst particles. Diffusion limitations within liquid-filled catalyst particles slow down the removal of a-olefins. This increases the residence time and the fugacity of a-olefins within catalyst pores, enhances their probability of readsorption and chain initiation, and leads to the formation of heavier and more paraffinic products. Structural catalyst properties, such as pellet size, porosity, and site density, and the kinetics of readsorption, chain termination and growth, determine the extent of a-olefin readsorption within catalyst particles and control FT selectivity. 

Recently, we reported detailed descriptions of hydrocarbon chain growth on supported Ru catalysts (7,8) we showed that product distributions do not follow simple polymerization kinetics and proposed a diffusion-enhanced olefin readsorption model in order to account for such deviations (7,8). In this paper, we describe this model and show that it also applies to Co and Fe catalysts. Finally, we use this model to discuss a few examples from the literature where catalyst physical structure and reaction conditions markedly influence hydrocarbon product distributions. 

The trends in carbon number distribution and in a-olefin/paraffin ratio on Ru, Fe, and Co, three very different catalytic surfaces, are remarkably similar. All catalysts show a curved Flory plot and an a-olefin/paraffin ratio that decreases with increasing carbon number until only paraffins are observed at high carbon numbers. In each case, diffusion-enhanced olefin readsorption accounts for such trends. Its contribution depends on the catalytic surface, its physical structure, and reaction conditions. 

Results for the Synthol entrained-bed process (16) are plotted in Figure 8. The available C to C15 data follow the conventional Flory plot with a equal to 0.7. The Synthol process uses a fused Fe catalyst of low surface area and porosity and operates at high temperatures ( 590K). The products in the reactor are mainly gaseous, wax formation is minimal, and the pellet pore structure remains free of liquid products therefore, diffusion-enhanced a-olefin readsorption is much less likely than in the ARGE process. Whereas the product selectivity in the ARGE process is altered by diffusion-enhanced a-olefin readsorption, that in the Synthol process is not. 

Delayed CT scan after Patchy wedge-shaped Diffuse enhancement... 

There is a basis17 to assume that in the studied samples1 both the oxygen diffusion decrease, and the copper diffusion enhancement are caused by the influence of the co-segregation at dislocations. This is also consistent with the fact pointed in study15 that twin boundaries in the 123 matrix do not contribute apparently to the effective bulk oxygen diffusion. 

A suggestion was made that A add of the 2-hydroxy-2-propyl radicals of DAR to MA (Schemes 12.1 and 12.3) decreases with the viscosity increase,which would be a sign of a diffusion-controlled or a diffusion-enhanced reaction.In fact, an increase in viscosity 1200 times leads to a decrease in A add only 5 times and not by orders of magnitude, which does not allow the classification of the addition as almost a diffusion-controlled (diffusion-enhanced) reaction. 

Stryer F, Thomas D, Meares C. Diffusion-enhanced fluorescence energy transfer. Annu. Rev. Biophys. Bioeng. 1982 11 203-222. 

As expected from the key role of diffusion-enhanced olefin readsorption, C5+ selectivity increases with increasing site density (Fig. 13a curve A, 1.0... 

The initial increase in C5+ selectivity as x increases arises from diffusion-enhanced readsorption of a-olefins. At higher values of CO transport restrictions lead to a decrease in C5+ selectivity. Because CO diffuses much faster than C3+ a-olefins through liquid hydrocarbons, the onset of reactant transport limitations occurs at larger and more reactive pellets (higher Ro, 0m) than for a-olefin readsorption reactions. CO transport limitations lead to low local CO concentrations and to high H2/CO ratios at catalytic sites. These conditions favor an increase in the chain termination probability (jSr, /Sh) and in the rate of secondary hydrogenation of a-olefins (j8s) and lead to lighter and more paraffinic products. 

The diffusion-enhanced olefin readsorption model described in Section III,C was used to predict the effect of carbon number on chain growth probability and paraffin selectivity. The model requires only one adjustable parameter the exponent c in a hydrocarbon diffusivity equation that depends on molecular size ( ), but that is identical for paraffins and olefins of equal size ... 

B. Diffusion-Enhanced Bifunctional Catalysis and THE Control of Carbon Number Distribution AND Product Functionality... 

Fig. 27. Effect of diffusion-enhanced a-olefin cracking catalytic function on carbon number distribution (simulations experimental/model parameters as in Fig. 15, 10% CO conversion). (A) FT synthesis without cracking function (B) with intrapellet cracking function, jS = 1.2 (C) with extra pellet cracking function, jS = 1.2. (a) Carbon selectivity vs. carbon number (b) Flory plots.

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