Mass Transfer and Chemical Reactions

In the case of strongly exothermic and endothermic reactions, the reactions may give rise to a temperature profile within the catalytic layer, which is dependent on reaction enthalpy (AHr), activation energy ( ) and the thermal conductivity of the porous catalytic material (2,efr). For quasi-isothermal behavior, the observed rate, reff. 

In general, the thickness of the catalytic layer is kept sufficiently small to avoid the influence of internal mass transfer on the kinetics. In this way, only the transfer of the reactants from the bulk to the catalytic wall must be considered and the reaction rate per unit outer surface area of the catalytic layer. For an irreversible first-order reaction, the rate is given by 

The molar flux from the fluid phase to the surface of the layer is proportional to the concentration gradient between the bulk and the surface  

Under stationary conditions, the molar flux from the fluid phase reaching the catalytic surface and the rate of transformation per surface unit must be identical Ji = Vs- It follows that for a first-order reaction 

Solving Equation (15.19) for the concentration of Ai at the catalyst surface gives 

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Kolbel et al. (K16) examined the conversion of carbon monoxide and hydrogen to methane catalyzed by a nickel-magnesium oxide catalyst suspended in a paraffinic hydrocarbon, as well as the oxidation of carbon monoxide catalyzed by a manganese-cupric oxide catalyst suspended in a silicone oil. The results are interpreted in terms of the theoretical model referred to in Section IV,B, in which gas-liquid mass transfer and chemical reaction are assumed to be rate-determining process steps. Conversion data for technical and pilot-scale reactors are also presented. 

In 1963 and in 1965, Huang and Kuo (H18, H19) applied the film penetration model to the mechanism of simultaneous mass transfer and chemical reaction. 

MASS TRANSFER AND CHEMICAL REACTION IN A CATALYST PELLET... 

In general, the concentration of the reactant will decrease from CAo in the bulk of the fluid to CAi at the surface of the particle, to give a concentration driving force of CAo - CAi)-Thus, within the pellet, the concentration will fall progressively from CAi with distance from the surface. This presupposes that no distinct adsorbed phase is formed in the pores. In this section the combined effects of mass transfer and chemical reaction within the particle are considered, and the effects of external mass transfer are discussed in Section J 0.8.4. 

Figure 10.12. Mass transfer and chemical reaction in a Spherical particle.

Mass transfer and chemical reaction with a mass transfer resistance external to the pellet... 

Biocatalysis in Liquid-Liquid Biphasic Media Coupled Mass Transfer and Chemical Reactions... 

The results above reflect interfacial specificity for mass transfer and chemical reaction. In future studies the advantages of the QELS method can be used to provide details on interfacial specificity for chemical processes by nonperturbative measurements. 

The oscillations observed with artificial membranes, such as thick liquid membranes, lipid-doped filter, or bilayer lipid membranes indicate that the oscillation can occur even in the absence of the channel protein. The oscillations at artificial membranes are expected to provide fundamental information useful in elucidating the oscillation processes in living membrane systems. Since the oscillations may be attributed to the coupling occurring among interfacial charge transfer, interfacial adsorption, mass transfer, and chemical reactions, the processes are presumed to be simpler than the oscillation in biomembranes. Even in artificial oscillation systems, elementary reactions for the oscillation which have been verified experimentally are very few. 

The Effectiveness Factor Analysis in Terms of Effective Diffusivities First-Order Reactions on Spherical Pellets. Useful expressions for catalyst effectiveness factors may also be developed in terms of the concept of effective diffusivities. This approach permits one to write an expression for the mass transfer within the pellet in terms of a form of Fick s first law based on the superficial cross-sectional area of a porous medium. We thereby circumvent the necessity of developing a detailed mathematical model of the pore geometry and size distribution. This subsection is devoted to an analysis of simultaneous mass transfer and chemical reaction in porous catalyst pellets in terms of the effective diffusivity. In order to use the analysis with confidence, the effective diffusivity should be determined experimentally, since it is difficult to obtain accurate estimates of this parameter on an a priori basis. 

Hoftyzer, P. J. and van Krevelen, D. W., The rate of conversion in polycondensation processes as determined by combined mass transfer and chemical reaction, in Proceedings of the 4th European Symposium on Chemical Reactions, Chem. Eng. Sci., 139-146 (1971). 

The problem of spraying, atomization, and bubble formation in agitated systems still need considerable study, though quite a bit of work has already been reported on them. The formation in the above cases has mostly been studied in the absence of heat and mass transfer and chemical reaction, the presence of which can greatly influence the bubble volume. This has to receive considerable attention if the performance of pertinent industrial equipment is to be adequately explained. 

The very important field of bubble and drop formation in non-Newtonian fluids remains virtually untouched, even in the absence of heat and mass transfer and chemical reactions. 

Vanni, M. and Baldi, G., 1991, Mass transfer and chemical reaction with multicomponent diffusion. Chem. Engng Sci. 46, 2465-2472. 

Most of the parameters that influence the rates of mass transfer and chemical reaction, and therefore the efficiency of the system, have already been discussed in Chapter B 3, however, in addition to the resistances to mass transfer found in gas/water systems, two more resistances can be found in three-phase systems ... 

The rate at which the overall process of mass transfer and chemical reaction occurs may be found by substituting for CAi in equation 3.61 to give ... 

The development and application of a rigorous model for a chemically reactive system typically involves four steps (1) development of a thermodynamic model to describe the physical and chemical equilibrium (2) adoption and use of a modeling framework to describe the mass transfer and chemical reactions (3) parameterization of the mass-transfer and kinetic models based upon laboratory, pilot-plant, or commercial-plant data and (4) use of the integrated model to optimize the process and perform equipment design. 

Complexity in multiphase processes arises predominantly from the coupling of chemical reaction rates to mass transfer rates. Only in special circumstances does the overall reaction rate bear a simple relationship to the limiting chemical reaction rate. Thus, for studies of the chemical reaction mechanism, for which true chemical rates are required allied to known reactant concentrations at the reaction site, the study technique must properly differentiate the mass transfer and chemical reaction components of the overall rate. The coupling can be influenced by several physical factors, and may differently affect the desired process and undesired competing processes. Process selectivities, which are determined by relative chemical reaction rates (see Chapter 2), can thenbe modulated by the physical characteristics of the reaction system. These physical characteristics can be equilibrium related, in particular to reactant and product solubilities or distribution coefficients, or maybe related to the mass transfer properties imposed on the reaction by the flow properties of the system. 

Atherton, J.H. (1994) Mechanism in two-phase reaction systems coupled mass transfer and chemical reaction. Research in Chemical Kinetics, 2, 193. 

Astarita, G. (1966) Mass Transfer and Chemical Reaction. Elsevier, Amsterdam. 

High-intensity inline devices are often used to mix fluids in the process industries. Such devices include simple pipes, baffled pipes, tees, motionless mixers, dynamic mixers, centrifugal pumps, ejectors, and rotor/stator mixers. In addition to their traditional application in physical processes such as mixing and dispersion, such devices can provide very effective environments for mass transfer and chemical reaction to take place. Furthermore, combining effective inline mixing with heat transfer is the basis of combined heat exchanger reactors (HEX reactors). 

The modeling of RD processes is illustrated with the heterogenously catalyzed synthesis of methyl acetate and MTBE. The complex character of reactive distillation processes requires a detailed mathematical description of the interaction of mass transfer and chemical reaction and the dynamic column behavior. The most detailed model is based on a rigorous dynamic rate-based approach that takes into account diffusional interactions via the Maxwell-Stefan equations and overall reaction kinetics for the determination of the total conversion. All major influences of the column internals and the periphery can be considered by this approach. 

Gas-liquid reactors present a number of interesting problems in reactor analysis and design which arise from the coupling of mass transfer and chemical reaction processes. Thus, the difficulty of resolving the relative contributions of filmwise and bulkwise reaction remains unsolved for all but the simplest kinetics. Such difficulties are compounded when thermal effects and significant heat release accompany the absorption and reaction. 

Although the reaction is of obvious economic importance, very little has been published about the inter-relationships between mass transfer and chemical reaction. The great bulk of the literature available (mainly in patents) describes product distribution obtained by subjecting air and cyclohexane to a wide variety of pressure, temperature, catalyst and reaction time conditions. Measurements of the chemical rate constants are rare. Most available kinetic data seem to be, at least to some extent, obscured by mass transfer effects. 

The overall process rate involves the addition of resistances for mass transfer and chemical reaction. With respect to chemical rate, Orejas [11] proposes the following equations obtained by the regression of industrial data ... 

The contents of the present contribution may be outlined as follows. Section 6.2.2 introduces the basic principles of coupled heat and mass transfer and chemical reaction. Section 6.2.3 covers the classical mathematical treatment of the problem by example of simple reactions and some of the analytical solutions which can be derived for different experimental situations. Section 6.2.4 is devoted to the point that heat and mass transfer may alter the characteristic dependence of the overall reaction rate on the operating conditions. Section 6.2.S contains a collection of useful diagnostic criteria available to estimate the influence of transport effects on the apparent kinetics of single reactions. Section 6.2.6 deals with the effects of heat and mass transfer on the selectivity of basic types of multiple reactions. Finally, Section 6.2.7 focuses on a practical example, namely the control of selectivity by utilizing mass transfer effects in zeolite catalyzed reactions. 

The mathematical description of simultaneous heat and mass transfer and chemical reaction is based on the general conservation laws valid for the mass of each species involved in the reacting system and the enthalpy effects related to the chemical transformation. The basic equations may be derived by balancing the amount of mass or heat transported per unit of time into and out of a given differential volume element (the control volume) together with the generation or consumption of the respective quantity within the control volume over the same period of time. The sum of these terms is equivalent to the rate of accumulation within the control volume ... 

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