A continuous-flow recirculation mode, generally called a recycle mode, is known to be practically meaningless except when the reactant is separated from the product at the reactor exit or when the reaction is autocatalytic, because when simply circulating a small amount of the fluid containing a reactant, the reactant concentration in this mode is lowered due to mixing of the fluid at the reactor entrance, leading to a decrease in the conversion at the reactor exit. This mode may, however, be meaningful in photocatalytic reactions with very large film-diffusional resistance. To indicate the validity of this estimation, therefore, characteristics of a continuous-flow recirculation reactor have been investigated both theoretically and experimentally. As a result, it is found that by increasing the circulation flow rate the conversion and productivity in this reactor is higher than that in a continuous-flow reactor because the film-diffusional resistance is remarkably reduced.
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