Chemical kinetics, conversion factors

For reversible chemical reactions in which 100% conversion of reactants to products cannot be achieved, the upper integration limit is XequiBbrium and the factor of 3 in (15-19) must be replaced by 3/[l — (1 — Xequmbnum) ]- Equation (15-19) is evaluated for irreversible nth-order chemical kinetics when the rate law is only a function of the molar density of the key-limiting reactant. Under these conditions. 

Quantitative results in Table 30-1 reveal that one achieves maximum conversion of reactants to products in ideal (i.e., 30%) and non-ideal (i.e., 25%) packed catalytic tubular reactors when the mass transfer Peclet number is approximately 6 for second-order irreversible chemical kinetics with an interpeUet porosity of 50%. Specific values for PeMT and the corresponding maximum conversion are sensitive to the simple mass transfer Peclet number and the chemical reaction coefficient, where the latter is defined by the product of the effectiveness factor, the interpeUet Damkohler number, and the catalyst filling factor. For example, when Pesimpie is 50 and the chemical reaction coefficient is 5 for second-order irreversible chemical kinetics, the critical value of PeMT [i e., (Re Sc)criacai] is approximately 30, whereas maximum conversion is obtained when PeMT is only 6. Hence, one concludes that the ideal simulations in Table 30-1 with a 0,... 

The efficiency factor for an immobilized enzyme. In general the conversion rate of an immobilized enzyme is lower than that of an equal amount of the free en me. This decreased activity is caused by diffusional limitations to the rate at which the subtra-te is transported to the site of reaction in the immobilized enzyme particles. In chemical engineering the subject of the interplay between diffusional limitations and chemical kinetics in heterogeneous catalysis has been extensively studied. The state of the art on this subject is described by Satterfield (). 

Chemical thermodynamics and kinetics provide the formalism to describe the observed dependencies of chemical-conformational reactions on the external physical state variables temperature, pressure, electric and magnetic fields. In the present account the theoretical foundations for the analysis of electrical-chemical processes are developed on an elementary level. It should be remarked that in most treatments of electric field effects on chemical processes the theoretical expressions are based on the homogeneous-field approximation of the continuum relationship between the total polarization and the electric field strength (Maxwell field). When, however, conversion factors that account for the molecular (inhomogeneous) nature of real systems are given, they are usually only applicable for nonpolar solvents and thus exclude aqueous solutions. Therefore, in the present study, particular emphasis is placed on expressions which relate experimentally observable system properties (such as optical or electrical quantities) with the applied (measured) electric field, and which include applications to aqueous solutions. 

Dihydroxyindole blocking factor blocks the indolization of quinone imine derivatives. Dihydroxyindole conversion factor catalyzes the dehydrogenation of 5,6-dihydroxyindole to indole-5,6-qui-none. Dopachrome oxidoreductase converts dopachrome to 5,6-dihydroxyindole and also may block 5,6-dihydroxyindole oxidation and subsequent melanogenic reactions. Relatively little information is available about the physical, chemical and kinetic properties of these proteinaceous factors in mammals. Controversy about melanin-related regulatory factors has focused on whether activity is due to unique individual proteins or is only an expression of activities of a multicatalytic enzyme (61.62). For example, dihydroxyindole conversion activity in mice melanoma is apparently due to tyrosinase, not a unique factor (56). 

For the epoxy resins studied, the mobility factor based on heat capacity coincides very well with the diffusion factor, calculated from the nonreversing heat flow via chemical kinetics modelling, and describing the effects of diffusion control on the rate of conversion of the cure reaction. Although the two resins behave quite differently, this coincidence between the mobility factor and diffusion factor is valid for both systems. Therefore, the mobility factor can be used for a quantitative description of then-altered rate of conversion in the (partially) vitrified state for the decrease in rate during vitrification, the increase in rate during devitrification and the diffusion-controlled rate in the (partially) vitrified region in between both processes. 

Our previous discussion of chemical equilibria and chemical thermodynamics allows us to assess whether or not a chemical reaction will proceed in a certain direction, and what the concentrations of the reactants and products will be when a system is in chemical equilibrium. In this chapter we are concerned with how fast reactants are converted into products, some of the factors upon which the rate of conversion depends, and the sequence of steps by which the conversion occurs. These subjects are the province of chemical kinetics. 

It is considered to carry out the same reaction in an upflow column, with a catalyst of the same material, but with a diameter of 4 mm. Estimate the required reaction volume, for a mean residence time of the liquid phase of 30 minutes (it is expected that the conversion will be the same as in the batch reactor). The Thiele modulus is proportional to the particle diameter, so for the larger catalyst particles is approximately 16, and the effectiveness factor is 1/16 (see eqs. (5.48) and (5.50)). This means that the required catalyst volume is 16 times larger, that is 16 x 0.1 x 10 = 16 m. When the bed has a void fraction of 0.5, die total effective reactor volume has to be 32 m. But this is only correct if the process rate is still determined by chemical kinetics. The gas/liquid mass transfer would be a possible limiting factor. One can make the following estimate The bubble hold-up in a stirred tmik and in an upflow column will both be on the order of 0.2. The bubble diameter will be on the order of 1 mm in the stirred tank, and 2 mm in the upflow column (with particles of 4 mm). 

The diffusivity, or diffusion coefficient, /) is a property of the system dependent upon temperature, pressure, and nature of the components. An advanced kinetic theory [12] predicts that in binary mixtures there will be only a small effect of composition. The dimensions of diffusivity can be established from its definition, Eq. (2.1), and are length /time. Most of the values for D reported in the literature are expressed as cm /s the SI dimensions are m /s. Conversion factors are listed in Table 1.5. A few typical data are listed in Table 2.1 a longer list is available in The Chemical Engineers Handbook [18]. For a complete review, see Ref. 17. 

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