Product selectivity mass transfer effect

Mass transfer effects are very important for the selectivity in the Fischer-Tropsch synthesis. Even though the reactants are in the gas phase, the catalyst pores will be filled with liquid products. Diffusion in the liquid phase is about 3 orders of magnitude slower than in the gas phase and even slow reactions may become diffusion limited. Diffusion limitations may occur through limitation on the arrival of CO to the active points or through the limited removal of reactive products.8,9... 

As demonstrated by means of residue curve analysis, selective mass transfer through a membrane has a significant effect on the location of the singular points of a batch reactive separation process. The singular points are shifted, and thereby the topology of the residue curve maps can change dramatically. Depending on the structure of the matrix of effective membrane mass transfer coefficients, the attainable product compositions are shifted to a desired or to an undesired direction. 

Basically, reactant and product selectivities are mass transfer effects, where the diffusivities of the various species in practice frequently do not differ that extremely as indicated above. Instead, in most cases only a preferred diffusion of certain species is observed, a fact which often hinders a clear understanding of product shape selectivity. This is because the various products, during their way through the pore system, may be reacted when contacting the catalytically active surface of the wall. This combined effect of diffusion and reaction will be discussed in detail in the following, as it is of great importance for the product distribution in zeolite-catalyzed reactions. 

Selectivity may be determined in the integral or differential mode. Integral selectivity depends on the overall extent of the reaction (degree of conversion) and on the type of reactor used even if heat and mass transfer effects are eliminated. It may be called reactor selectivity for the formation of product P, from the set of reactants B when it is calculated as the mole fraction of P, in the products (exluding unconverted feed) at the exit of the reactor ... 

An acid treated zeolite beta with a 6 M HNO3 acid solution is a shape-selective catalyst for the alkylation of biphenyl with propene. This is however not the case for the alkylation of naphthalene. The reaction is highly mass transfer limited and the selectivity mechanism is attributed to a product selectivity. The major effect of the acid treatment is to deactivate the external surface area so that the intrinsic micropore properties can come out. Contrary to zeolite mordenite, the acid treatment does not reduce deactivation and the formation of highly aromatic coke remains important at high temperatures. 

When consecutive or parallel reactions are carried out between a gas and a liquid, the concentration gradients near the interface may influence the selectivity as well as the overall rate of reaction. For chlorination or partial oxidation of hydrocarbons, several workers have reported that the yield of intermediate products was influenced by agitation variables [6,7] and was less than predicted from the kinetic constants. Rigorous analysis of multiple reactions is complex, but film theory can be used to show when mass transfer effects are likely to change the selectivity [8]. 

This reaction provides a wide variety of hydrocarbons (Ci to 50-200) from synthesis gas, becoming a very attractive route for the production of clean fuels (LPG, gasoline, kerosene, and diesel) and a source of chemicals (namely, linear olefins) [42]. However, it is important to ensure an appropriate size and type of reactor due to its exothermicity (Af/R = -165kJ/mol). Mass transfer effects are also significant since catalyst pores are filled with liquid products (waxes and water) limiting the diffusion rate. Despite the gas phase of reactants, selectivity to C5+ seems to decrease notably for diffusion lengths exceeding 0.1-0.3mm. Therefore, MSRs have been proposed as a suitable alternative. 

The high surface-to-volume ratio can also significantly improve both thermal and mass transfer conditions within micro-channels in two ways firstly, the convective heat and mass transfers, which take place at the multi-phase interface, are improved via a significant increase in heat and mass transfer area per unit volume. Secondly, heat and mass transfers within a small volume of fluid take a relatively short time to occur, enabling a thermally and diffusively homogeneous state to be reached quickly. The improvement in heat and mass transfer can certainly influence overall reaction rates and, in some cases, product selectivity. Perhaps one of the more profound effects of the efficient heat and mass transfer property of micro-reactors is the ability to carry potentially explosive or highly exothermic reactions in a safe way, due to the relatively small thermal mass and rapid dissipation of heat. 

Sonophotocatalysis is photocatalysis with ultrasonic irradiation or the simultaneous irradiation of ultrasound and light with photocatalyst. Tnis method includes irradiation with alternating ultrasound and light. Ultrasound effects on heterogeneous photocatalytic reaction systems have been demonstrated by Mason,1 Sawada et al.,2) Kado et al.,3) and Suzuki et al.4) In these papers, not only acceleration of photocatalytic reactions but increase in product selectivity by ultrasonic irradiation has also been reported. It was postulated that ultrasound effects, such as surface cleaning, particle size reduction and increased mass transfer, were the result of the mechanical effects of ultrasound.1,5) Lindley reviewed these and other effects.5)... 

NF is used when high molecular weight solutes have to be separated from a solvent. It is effective in the production of drinking water, especially in the case of water softening. Compared to RO, a lower retention is found for monovalent ions. But very recently [9], it has been found that NF separates the ions of the same valency for a selective defluorination of brackish water. RO and UF have shown, respectively, solution-diffusion and convection mass transfers. In NF, a synergism between both can be observed but strongly depends on the operational conditions (pH, ionic strength, flow rate, transmembrane pressure) and on the membrane material used. 

In analogous manner, residue curve maps of the reactive membrane separation process can be predicted. First, a diagonal [/e]-matrix is considered with xcc = 5 and xbb = 1 - that is, the undesired byproduct C permeates preferentially through the membrane, while A and B are assumed to have the same mass transfer coefficients. Figure 4.28(a) illustrates the effect of the membrane at nonreactive conditions. The trajectories move from pure C to pure A, while in nonreactive distillation (Fig. 4.27(a)) they move from pure B to pure A. Thus, by application of a C-selective membrane, the C vertex becomes an unstable node, while the B vertex becomes a saddle point This is due to the fact that the membrane changes the effective volatilities (i.e., the products xn a/a) of the reaction system such that xcc a. ca > xbbO-ba-... 

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