System, description experimental models

Chemisorption is a phenomenon of importance in catalysis which may be treated by MO theory. Experimental studies have been carried out for a variety of systems, but theoretical descriptions of the electronic features of chemisorption beyond simple considerations are in a primitive stage. There are several factors responsible for this state of affairs. One is, of course, the complexity of the substrate system to be modeled, which has forced theorists to work with a small-size representation for the surface, as implied by the surface molecule concept of localized interactions. Although some early work has been done by... 

The computing problem is concerned with calculating the maximum number of unknown parameters of a proposed reaction system from available experimental data. This data can be any combination of values for constant parameters (rate and equilibrium constants) and variable parameters (concentration versus time data). Moreover, data for different variable parameters need not have the same time scale. When the unknown parameters are calculated, it is important that the mathematical validity of the proposed model be determined in terms of the experimental accuracy of the data. Also, if it is impossible to solve for all unknown parameters, then the model must be automatically reduced to a form that contains only solvable parameters. Thus, the input to CRAMS consists of 1) a description of a proposed reaction system model and, 2) experimental data for those parameters that were measured or previously determined. The output of CRAMS is 1) information concerning the mathematical validity of the model and 2) values for the maximum number of computable unknown parameters and, if possible, the associated reliabilities. The system checks for model validity only in those reactions with unknown rate constants. Thus a simulation-only problem does not invoke any model validation procedures. 

Substrate limitations have been documented and quantitatively described ( U, 2, 17 ). Dooley et al. (11) present an excellent description of modeling a reaction in macroreticular resin under conditions where diffusion coefficients are not constant. Their study was complicated by the fact that not all the intrinsic variables could be measured independently several intrinsic parameters were found by fitting the substrate transport with reaction model to the experimental data. Roucls and Ekerdt (16) studied olefin hydrogenation in a gel-form resin. They were able to measure the intrinsic kinetic parameters and the diffusion coefficient independently and demonstrate that the substrate transport with reaction model presented earlier is applicable to polymer-immobilized catalysts. Finally, Marconi and Ford (17) employed the same formalism discussed here to an immobilized phase transfer catalyst. The reaction was first-order and their study presents a very readable application of the principles as well as presents techniques for interpreting substrate limitations in trlphase systems. 

The application of suitable models to various systems must be determined on a case-by-case basis. This could be judged from the behavior of experimental mass transfer coefficient with respect to the contact time of two phases. For dynamic systems, the penetration model is physically more realistic than the stagnant film model. Flowever, the mixing in different phases is important to describe the overall mass transfer performance, and, therefore, the above models are usually combined with fluid flow models, which includes detailed flow description. 

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