Chapter 6. Isothermal Reactor Design: Molar Flow Rates

6.1 The Molar Flow Rate Balance Algorithm

There are many instances when it is much more convenient to work in terms of the number of moles (N_A, N_B) or molar flow rates (F_A, F_B, etc.) rather than conversion. Membrane reactors and multiple reactions taking place in the gas phase are two such cases where molar flow rates are necessary rather than conversion. We now modify our algorithm by using concentrations for liquids and molar flow rates for gases as our dependent variables. The main difference between the conversion algorithm and the molar flow rate/concentration algorithm is that, in the conversion algorithm, we needed to write a mole balance on only one species, whereas in the molar flow rate and concentration algorithm, we must write a mole balance on each and every species. This algorithm is shown in Figure 6-1. First we write the mole balances on all species present, as shown in Step . Next we write the rate law, Step , and then we relate the mole balances to one another through the relative rates of reaction, as shown in Step . Steps and are used to relate the concentrations in the rate law to the molar flow rates. In Step , all the steps are combined by the ODE solver (e.g., Polymath).

Figure 6-1. Isothermal reaction design algorithm for mole balances.

6.2 Mole Balances on CSTRs, PFRs, PBRs, and Batch Reactors