Transfer and Chemical Reaction

The general discussion of the reaction in gas-liquid-solid systems is based on the simple film model. In addition, we consider an irreversible reaction between a gaseous reactant (Aj ) and a reactant in the liquid phase A-, which is in large excess  

If the thickness of the catalytic layer on the reactor wall is sufficiently small, internal mass transfer resistances can be neglected and only external resistances in the fluid phases are considered. The reaction rate per unit of the outer surface of the catalytic layer is described by a pseudo first order reaction (mol m s )  

The reaction rate per unit volume of the liquid (continuous) phase is given as 

The mass transfer rate (r ) from the liquid phase to the surface of the catalytic layer is proportional to the concentration gradient between the two phases  

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In addition to the Burke and Schumann model (34) and the Displacement Distance theory, a comprehensive laminar diffusion flame theory can be written using the equations of conservation of species, energy, and momentum, including diffusion, heat transfer, and chemical reaction. 

The numerical solution of the energy balance and momentum balance equations can be combined with flow equations to describe heat transfer and chemical reactions in flow situations. The simulation results can be in various forms numerical, graphical, or pictorial. CFD codes are structured around the numerical algorithms and, to provide easy assess to their solving power, CFD commercial packages incorporate user interfaces to input parameters and observe the results. CFD... 

The conserved quantities that are of utmost importance to a chemical engineer are mass, energy, and momentum. It is the objective of this text to teach you how to utilize the conservation of mass in the analysis of units and processes that involve mass flow and transfer and chemical reaction. For each conserved quantity the principle is the same—conserved quantities are... 

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... 

Fluid flow and reaction engineering problems represent a rich spectrum of examples of multiple and disparate scales. In chemical kinetics such problems involve high values of Thiele modulus (diffusion-reaction problems), Damkohler and Peclet numbers (diffusion-convection-reaction problems). For fluid flow problems a large value of the Mach number, which represents the ratio of flow velocity to the speed of sound, indicates the possibility of shock waves a large value of the Reynolds number causes boundary layers to be formed near solid walls and a large value of the Prandtl number gives rise to thermal boundary layers. Evidently, the inherently disparate scales for fluid flow, heat transfer and chemical reaction are responsible for the presence of thin regions or "fronts in the solution. 

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. 

Irreversibility versus reversibility inpolarography. Previously in this chapter we dealt only with reversible redox systems, i.e., with truly Nemstian behaviour and merely diffusion control. This also applies to combined processess of electron transfer and chemical reaction (e.g., complexation) provided that both take place instantly. For instance, in EC such as... 

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. 

Exact analytical solutions of the coupled equations for simultaneous mass transfer, heat transfer, and chemical reaction cannot be obtained. However, various authors have employed linear approximations (56-57), perturbation techniques (58), or asymptotic approaches (59) to obtain approximate analytical solutions to these equations. Numerical solutions have also been obtained (60-61). Once the solution for the concentration profile has been determined, equation 12.3.98 may be used to determine the temperature profile. The effectiveness factor may also be determined from the concentration profile, using the approach we have... 

O. Kajimoto, Solvation in Supercritical Fluids Its Effects on Energy Transfer and Chemical Reactions , Chem. Rev. 1999, 99, 355-389, and references cited therein. 

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. 

Relaxation times, MT ratios, and diffusion properties allow insight into the microstructure of various tissues. Determination of these parameters is possible by recording and analysing of a series of volume selective spectra, even for metabolites with relatively low concentrations in vivo. For recording series of spectra usually one parameter is changeable (e.g., inversion time TI for Ti measurements, echo time TE for T2 measurements, MT preparation for assessment of spin transfer and chemical reaction rates, or diffusion sensitizing gradients for assessment of apparent diffusion coefficients or even diffusion... 

Many industrial processes involve mass transfer processes between a gas/vapour and a liquid. Usually, these transfer processes are described on the basis of Pick s law, but the Maxwell-Stefan theory finds increasing application. Especially for reactive distillation it can be anticipated that the Maxwell-Stefan theory should be used for describing the mass transfer processes. Moreover, with reactive distillation there is a need to take heat transfer and chemical reaction into account. The model developed in this study will be formulated on a generalized basis and as a consequence it can be used for many other gas-liquid and vapour-liquid transfer processes. However, reactive distillation has recently received considerable attention in literature. With reactive distillation reaction and separation are carried out simultaneously in one apparatus, usually a distillation column. This kind of processing can be advantageous for equilibrium reactions. By removing one of the products from the reactive zone by evaporation, the equilibrium is shifted to the product side and consequently higher conversions can be obtained. Commercial applications of reactive distillation are the production of methyl-... 

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

Electrocatalysis is, in the majority of cases, due to the chemical catalysis of the chemical steps in an electrochemical multi-electron reaction composed of a sequence of charge transfers and chemical reactions. Two factors determine the effective catalytic activity of a technical electrocatalysts its chemical nature, which decisively determines its absorptive and fundamental catalytic properties and its morphology, which determines mainly its utilization. A third issue of practical importance is long-term stability, for which catalytic properties and utilization must occasionally be sacrificed. 

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 goal of the electrochemical studies is the elucidation of the sequence of electron transfer and chemical reactions that occur at or near the electrode surface. Although no single experimental plan can be devised that will suffice for all studies, answers to the following questions provide much of the information needed to describe a redox pathway ... 

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. 

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