Conversion of a reactant

Catalyst Activity. Of utmost importance in the design of most catalysts is activity, which is a measure of the ability of a catalyst to effect conversion of the reactant(s) to the desired product(s) under specified conditions. In industrial applications, catalyst activity is usually discussed in terms of the percent conversion of a reactant under given conditions of temperature, pressure, and contact time. 

The term V/F for flow reactors is used to evaluate the size of the reactor required to achieve the conversion of a reactant feed rate F. It is the numher of reactor volumes of feed at specified conditions that can he treated in unit time, where F is the molar flowrate (uC q) and V is the volume of the reactor. 

Figure 1. Selectivity is determined by the relative difference in activation energy between two possible products, while the rates of reaction to product 1 or 2 are determined by the absolute activation barriers, AgJ and AG. Curve calculated assuming AG = 18 kcal moF and a temperature of 300 K. Inset is a simplified potential energy diagram for the conversion of a reactant into two parallel products [10]. (Reprinted from Ref [10], 2002, with permission from American Chemical Society.)...

Kinetics and thermodynamics address different kinds of questions about a reacting system. The methods of thermodynamics, together with certain experimental information, are used to answer questions such as (1) what is the maximum possible conversion of a reactant, and the resulting equilibrium composition of the reacting system at given conditions of T and P, and (2) at given T and P, how far is a particular reacting... 

Fractional conversion of a reactant, /A for reactant A, say, is the ratio of the amount of A reacted at some point (time or position) to the amount introduced into the system, and is a measure of consumption of the reactant. It is defined in equation 2.2-3 for a batch system, and in equation 2.3-5 for a flow system. The definition is the same whether the system is simple or complex. 

The withdrawal of a product (continuously, as in Figure 12.3(b), or intermittently) can result in higher conversion of a reactant, particularly if the reaction is equilibrium-limited. 

When the density varies, we need to find another variable to express the progress of a reaction. Earlier we defined the fractional conversion X for a single reaction, and in this chapter we defined the conversion of a reactant species for reactant A and Xj for reaction j. For the conversion in a reaction we need a different variable, and we shall use Xj (bold type), with the index i describing the reaction. We will first work our series and parallel reactions with these variables and then consider a variable-density problem. 

Figure 1.107 Relation between yield of a product R and conversion of a reactant A for different rate constants and lamination widths for one selected scenario of elemental reaction (two reactants A + B form R, while B can react with R as well in a consecutive reaction to the consecutive product S). W lamellae width k rate constant cf> ratio of reaction rate to diffusion rate [129] (by courtesy of Elsevier Ltd.).

It is often convenient to work with fractional conversion of a reactant species. Let i = A, a reactant, then... 

Conversion of a Reactant. The conversion of a reactant A (XA) is the fraction of reactant A converted/transformed to products. 

Fig. 7.2. Conversion of a reactant vs. temperature.The concentration of reactants [R] within the porous catalyst structure. Concentration of R is (a) uniform for kinetic control, (b) decreasing within the catalyst for pore diffusion control, and (c) zero immediately at the surface of the catalyst for bulk mass transfer.

Analogous to the batch reactor, a fractional conversion of a reactant A can be defined as ... 

Fractional conversion of a reactant is defined as the ratio of the amount consumed to that charged. In this book, the following definitions of yield, yield ratio, and selectivity are used The yield of a product is the ratio of the amount of reactant (or reactants) converted to the product to the total amount of reactant (or reactants) charged. The cumulative yield ratio of two products is the ratios of their yields. The instantaneous yield ratio is the ratio of the momentary rates of conversion to these products. The cumulative selectivity to a product is the ratio of the amount of reactant (or reactants) converted to that product to the amount consumed. The instantaneous selectivity is the ratio of the momentary rate of reactant conversion to the product to that of reactant consumption. Not used in this book is the extent of reaction, defined as the number of moles consumed or formed, divided by the stoichiometric coefficient of the respective participant. 

Example 8.5. Ping-pong transfer reactions. Some enzymatic transfer reactions proceed by so-called ping-pong mechanisms [40,41]. In these, the conversion of a reactant to a product leaves the enzyme in a different form. The modified enzyme then converts a second reactant to another product while itself being restored to its original form. Enzymatic transaminase reactions interconverting amino and keto acids provide a typical example [40] ... 

Chemical reactions do not take place instantaneously, and indeed often proceed rather slowly. In such cases, it is not practical to design the reactor for complete conversion of the limiting reactant instead, the reactor effluent emerges with some of the limiting reactant still present and is then usually subjected to a separation process to remove the unconverted reactant from the product. The separated reactant is then recycled to the reactor inlet. The fractional conversion of a reactant is the ratio... 

The fractional conversion of a reactant is the ratio of amount reacted to amount fed. The fractional conversions of different reactants are generally different unless the reactants are fed in stoichiometric proportion. 

We can think of this as a reaction in which the conversion of a reactant species A to an intermediate or product B is assisted or catalyzed by B... 

Conversion of a reactant is the number of moles converted per initial or feed moles of a reactant. Thus for component A... 

If the compoimd of interest has a very low reaction rate (<1% of the activity of a known standard substrate), further evaluations with concentrated enzyme are recommended. There are examples of traces of reactive substrates remaining at these low levels in inactive compoimds. Since 0.1% conversion of a reactant to product can easily be detected in radiochemical glycosyltransferase assays, confirmation that the low rate of conversion is not due to the presence of contaminants is essential. This level of contamination in modified synthetic substrates is sometimes difficult to detect even by high-field NMR spectroscopy. Therefore before the actual preparative synthesis is performed, 10% conversion of substrate to product should be verified by assay to ensure that the low rate of reaction is not due to traces of substrate in a largely inactive compound. 

When a single chemical reaction takes place, the conversion of a reactant relates to the extent of the reaction. For batch reactors, from Eq. 2.3.5,... 

Conversion of a reactant, dimensionless (Eqs. 2.6.1a and 2.6.1b) Friction factor, dimensionless Mass velocity, mass/time area... 

Where complex reactions are concerned, conversion of a reactant is much less important than the actual moles converted in each step. This has the advantage of reducing the number of equations... 

A macromolecular crystal usually consists of between 30% and 80% solvent of crystallisation. Hence, the enzyme active site, for example, can be accessible to these solvent channels and able therefore to catalyse conversion of a reactant to product. Because of this observation alone one is able to say that the crystal structure is directly relevant in helping to determine the macromolecule s functional state. Of course, the results themselves, defining the structure, do make chemical sense. 

Oxidation of SO2 into SO3 is a classic example of exothermic reversible reaction. Optimal temperature regime for such a reaction requires starting at as high temperatures as a catalyst can handle, then the temperature decrease along with progressing conversion of a reactant. It is traditionally performed in multi-bed adiabatic reactors with intermediate cooling. Temperature profile in an RFR (Fig. 4) has lower temperatures at both ends of the catalyst bed, suggesting that an RFR would perform close to the theoretical optimum. 

However, the adsorption and adsorption kinetics of binary or even more complex adsorbate mixtures are of paramount importance in gas separation and catalysis. Thus, even in the simplest case of the conversion of a reactant A into a product M inside the pore system of a zeolite catalyst, the... 

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