Analysis of mass transfer effects

The initial rate data were also analyzed to check the significance of liquid-solid and intraparticle mass transfer effects under the conditions used in this work. For this purpose factors a2 and ( )exp were evaluated as described earlier (Section 2.4.3.2) and are presented in Table 4.3. 

Reaction Condition Temp = 318 K, Maleic acid-MeOH molar ratio = 1 4, d = 4.58 x 10m 

K dp 5 mm Catalyst loading, kg/in Initial Maleic acid cone., kmol/m Initial MeOH cone., kmol/m Initial rate X 10 kmol/ m /sec X 10  

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The sections which follow outline the general multidimensional distribution theory. Applications of the theory are discussed to describe droplet size distributions and mixing frequencies in chemically equilibrated systems, the effects of droplet mixing on the extent of reaction, the analysis of mass transfer with and without chemical reaction, hydrocarbon fermentation, and emulsion polymerization. 

The analysis of initial rate data is useful in understanding the dependency of the reaction rate on individual reaction parameters and also in the evaluation of mass transfer effects. Initial rates of hydrogenation were calculated from the experimentally observed H2 consumption in the reservoir vessel vs time data. The initial rate data observed are presented in Table 3. The effects of individual parameters on the initial rate are dicussed below. 

Bhattacharjee C., Datta S. (1996), Analysis of mass transfer during ultrafilttation of PEG-6000 in a continuous stirred cell effect of back transport, J oumal of Membrane Science, 119, 39-46. 

In many studies of oxidation reactions in our laboratory, dating back to the early 80s, and more recently in our smdy of the role of Cl on Pt supported catalysts, we have carefully measured reaction rates independent of mass transfer effects. In these studies, we have observed that the conversion and the reaction rate per unit mass were independent of Pt dispersion. Consequently, when reaction rates are calculated per Pt area, i.e., as turnover frequency, the catalysts with the larger Pt area (high dispersion) have the lowest TOP. The presence of oxygen makes it difficult to determine uniquely the cause of this result because it is not clear if the surface is metallic, oxidized, or a mixture of oxide and metallic phases. Whereas this is true for supported Pd and Rh catalysts, in the case of Pt catalysts, we found that, as suggested by Burch and Loader, - platinum particles seem to have a strong memory of previous reduction pretreatments, as they remain in metallic state under oxidizing conditions. Because most surface analysis techniques are conducted under vacuum or ex situ, it is not always clear if the same oxidation state is valid under reaction conditions at atmospheric pressure. Controlled atmosphere EXAFS and... 

Quantitative analytical treatments of the effects of mass transfer and reaction within a porous structure were apparently first carried out by Thiele (20) in the United States, Dam-kohler (21) in Germany, and Zeldovitch (22) in Russia, all working independently and reporting their results between 1937 and 1939. Since these early publications, a number of different research groups have extended and further developed the analysis. Of particular note are the efforts of Wheeler (23-24), Weisz (25-28), Wicke (29-32), and Aris (33-36). In recent years, several individuals have also extended the treatment to include enzymes immobilized in porous media or within permselective membranes. The important consequence of these analyses is the development of a technique that can be used to analyze quantitatively the factors that determine the effectiveness with which the surface area of a porous catalyst is used. For this purpose we define an effectiveness factor rj for a catalyst particle as... 

The effect of the cell density was studied in biodesulfurization of diesel oil by P. delafieldii R-8 [259], An optimum was reported to exist for this biocatalyst as well. Above 25g/L cell density, the specific desulfurization rate decreased. In this case a statistical analysis was not performed to identify the point of mass transfer limitation. 

Traditionally, CVD reaction data have been reported in terms of growth rates and their dependence on temperature. The data are often confounded by mass-transfer effects and are not suitable for reactor analysis and design. Moreover, CVD reaction data provide little insight, if any, into impurity incorporation pathways. Therefore, the replacement of traditional macroscopic deposition studies with detailed mechanistic investigations of CVD reactions is an area of considerable interest. A recent, excellent review of CVD mechanistic studies, particularly of Si CVD, is available (98), and the present discussion will be limited to highlighting mechanisms of Si CVD and of GaAs deposition by MOVCD as characteristic examples of the combined gas-phase and surface reaction mechanisms underlying CVD. 

For a resolution of question (3), either MASC or the simpler SSHTZ program was run under both isothermal and adiabatic conditions, with effective mass transfer coefficients chosen to simulate the stable portion of the sorption fronts. Fortunately, in most cases described below, the programs predicted that the steady-state MTZ lengths did not change by more than 10Z or so between the two extremes. Thus, an extensive analysis of the wall effects in the various columns was not required for proper interpretation of MTZ data. 

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. 

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. 

E will be different from 1 only if R4 is small relative to / 2, resulting in a bulk concentration of c — 0 and in a real parallel mechanism of the enhancement. The advantage of the concept of the enhancement factor as defined by eq 33 is the separation of the influence of hydrodynamic effects on gas-liquid mass transfer (incorporated in Al) and of the effects induced by the presence of a solid surface (incorporated in E ), indeed in a similar way as is common in mass transfer with homogeneous reactions. The above analysis shows that an adequate description of mass transfer with chemical reaction in slurry reactors needs reliable data on ... 

Experiments in which catalyst wafers are used may suffer from reactant mass transfer problems, which limit the validity of the data or complicate their analysis. To determine the reaction kinetics and activation energies, mass transfer effects have to be understood (Burcham and Wachs, 1999). These difficulties can be avoided if a conventional fixed-bed reactor is mimicked closely and the catalyst is used in powder form and the reactant gases flow through the bed. It is also important to prevent homogeneous gas-phase reactions by reducing the dead volume. 

If recirculation rates are 10 to 15 times the feed rate, the reactor would tend to operate nearly isothermally. High velocities past the bed of particles could eliminate almost completely any external mass-transfer influence on the reactor performance. By varying the circulation rates, the reaction condition for which the mass transfer effect is negligible can be established. Except for the rapidly-decaying catalyst system, steady state can be achieved effectively. Sampling and product analysis can be obtained as effectively as in the fixed-bed reactor. Residence-time distributions for the fluid phases can be measured easily. High fluid velocities would cause less flow-maldistribution problems. 

Sampling and analysis of product composition are good - only normal problems are encountered. The reactor is well mixed and isothermal conditions in the reactor can be maintained. Due to well-mixed conditions, the extraneous heat- and mass-transfer effects are at a minimum. 

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