Reaction-mass transport

The HTE characteristics that apply for gas-phase reactions (i.e., measurement under nondiffusion-limited conditions, equal distribution of gas flows and temperature, avoidance of crosscontamination, etc.) also apply for catalytic reactions in the liquid-phase. In addition, in liquid phase reactions mass-transport phenomena of the reactants are a vital point, especially if one of the reactants is a gas. It is worth spending some time to reflect on the topic of mass transfer related to liquid-gas-phase reactions. As we discussed before, for gas-phase catalysis, a crucial point is the measurement of catalysts under conditions where mass transport is not limiting the reaction and yields true microkinetic data. As an additional factor for mass transport in liquid-gas-phase reactions, the rate of reaction gas saturation of the liquid can also determine the kinetics of the reaction [81], In order to avoid mass-transport limitations with regard to gas/liquid mass transport, the transfer rate of the gas into the liquid (saturation of the liquid with gas) must be higher than the consumption of the reactant gas by the reaction. Otherwise, it is not possible to obtain true kinetic data of the catalytic reaction, which allow a comparison of the different catalyst candidates on a microkinetic basis, as only the gas uptake of the liquid will govern the result of the experiment (see Figure 11.32a). In three-phase reactions (gas-liquid-solid), the transport of the reactants to the surface of the solid (and the transport from the resulting products from this surface) will also... 

Reactions in which one reactant is gaseous, the other is in a liquid phase, and the catalyst is dispersed in the liquid phase, constitute a special but not unusual case, for example, the hydrogenation of a liquid alkene catalysed by platinum. A batch reactor is most commonly employed for laboratory scale studies of such reactions. Mass transport from the gaseous to the liquid phase may reduce the rate of such a catalytic reaction unless the contact between the gas and the liquid is excellent (see 1.6). 

Transport simultaneous with reaction Mass transport (a) Effective diffusivity... 

Dewers, T. and Ortoleva, P.J. 1990. Interaction of reaction, mass transport, and rock deformation during diagenesis mathematical modelling of integranular pressure solution, stylolites, and differential compaction/cementation. In l.D. Meshri and P.J. Ortoleva (Editors), Prediction of Reservoir Quality through Chemical Modelling, Memoir, 49. Am. Assoc. Pet. Geol. Tulsa, OK. 

FIGURE 12.10 Examination of mass transport limitation in the S(IV)-03 reaction. Mass transport limitation is absent for points below and to the left of the indicated bounds (Schwartz 1988). 

As discussed in more detail in Sect. 1.1.5, this volume of the Encyclopedia is divided into three broad sections. The first section, of which this chapter is an element, is concerned with introducing some of the basic concepts of electroanalytical chemistry, instrumentation - particularly electronic circuits for control and measurements with electrochemical cells - and an overview of numerical methods. Computational techniques are of considerable importance in treating electrochemical systems quantitatively, so that experimental data can be analyzed appropriately under realistic conditions [8]. Although analytical solutions are available for many common electrochemical techniques and processes, extensions to more complex chemical systems and experimental configurations requires the availability of computational methods to treat coupled reaction-mass transport problems. 

I 7 Cels Coupled to Oscillatory Reactions Mass transport... 

As mentioned before, nonequilibrium thermodynamics could be used to study the entropy generated by an irreversible process (Prigogine, 1945, 1947). The concept ofhnear nonequilibrium thermodynamics is that when the system is close to equilibrium, the hnear relationship can be obtained between the flux and the driving force (Demirel and Sandler, 2004 Lu et al, 2011). Based on our previous linear nonequihbrium thermodynamic studies on the dissolution and crystallization kinetics of potassium inorganic compounds (Ji et al, 2010 Liu et al, 2009 Lu et al, 2011), the nonequihbrium thermodynamic model of CO2 absorption and desorption kinetics by ILs could be studied. Figure 17 shows the schematic diagram of CO2 absorption kinetic process by ILs. In our work, the surface reaction mass transport rate and diffusion mass transport rate were described using the Hnear nonequihbrium thermodynamic theory. 

In Eqs. (1) and (2), J was the CO2 absorption or desorption rate (or flux) by ILs and Xa were the surface reaction mass transport rate constant and diffusion mass transport rate constant, respectively and, , and were the fugacities of CO2 in ILs at equflibrium, at the vapor—Hquid interface, and at the bulk phase of the ILs, respectively. 

If one of these requirements is not realized by the interface, then the interface and the reacting phases could not be in a state of local equilibrium. Additional constraints must be considered and therefore additional state variables must be introduced to describe and quantify the properties of contacting phases and the evolution of the system. These constraints can be local stresses or constraints related to interface reactions, mass transport and interface movement. 

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