Reactor concentration single reversible reactions

At a fixed temperature, a single, reversible reaction has no interior optimum with respect to reaction time. If the inlet product concentration is less than the equilibrium concentration, a very large flow reactor or a very long batch reaction is best since it will give a close approach to equilibrium. If the inlet product concentration is above the equilibrium concentration, no reaction is desired so the optimal time is zero. In contrast, there will always be an interior optimum with respect to reaction time at a fixed temperature when an intermediate product in a set of consecutive reactions is desired. (Ignore the trivial exception where the feed concentration of the desired product is already so high that any reaction would lower it.) For the normal case of bin i , a very small reactor forms no B and a very large reactor destroys whatever B is formed. Thus, there will be an interior optimum with respect to reaction time. 

Mass transport in an electrochemical reactor occurs by three mechanisms migration in the electrical field, film diffusion, and convection. The first of these is a special feature of electrochemical reactions, whereas the other two are common to all reactions that have a solid phase. However, where an inertsupporting electrolyte is used, the effect of migration can be neglected. With this assumption, let us consider a single electrode reaction given by reaction 21.3. When a finite current is passed through the cell and conditions are perfectly reversible, the concentration overpotential can be expressed as (Pickett, 1979)... 

E17.5 One desires to process a reversible reaction 2A< P + Q at constant volume in a CSTR reactor or several reactors in series. Pure reactant A is fed under flow of 3.5 m /h and initial concentration of 48 kmol/m. The forward reaction rate constant is equal to 0.75 m /(kmol h) and the equilibrium constant K" = 16. If the final conversion is desired to be 85% of the equilibrium conversion, calculate the volume of a single CSTR. If the capacity of the available reactors is only 5% of the calculated volume, how many CSTR reactors in series would be needed ... 

Table VIII demonstrates the inverse relationship of conversion to S02 concentration in the feed that is a consequence of applying flow reversal to S02 oxidation using a single reactor. As the S02 concentration in the table moves from 0.8 to over 8 vol%, the conversion drops from 96-97% down to 85%. At the same time, the maximum bed temperature changes from 450 to 610°C. For an equilibrium-limited, exothermic reaction, this behavior is explained by variation of the equilibrium conversion with temperature.

Because the parameter of the axial dispersion model, as observed from numerous experimental studies (58), has been so extensively correlated with Peclet number, designers consider the model useful for scaleup and use it for reactor calculations. The model gives a nice analytical expression for prediction of conversion of a single, irreversible first-order reaction (E(s) in Table 1 with Da replacing s). The expressions for exit concentrations for a system of reversible first-order reactions with the same axial dispersion coefficient (turbulent flow) are much more complex and their evaluation is computationally demanding. 

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