This review evaluates the present state of the art on backmixing in gas-liquid and gas-liquid-solid reactors. A brief outline of numerous techniques for measuring residence time distribution (RTD) of various phases in a multiphase reactor is presented. This is followed by a brief description of differential and stagewise models for characterizing backmixing from RTD measurements. Both simple (that is, single-parameter axial dispersion model) and more complex (that is, two-, three-, or four-parameter models) models are evaluated. Backmixing characteristics of various gas-liquid columns such as trickle beds, spray columns, mechanically agitated columns, plate columns, fluidized bed columns, etc., are subsequently evaluated. The performance of a bubble column under various reaction conditions is analyzed. Criteria for the elimination of backmixing in packed-bed reactors are presented, and the effect of backmixing on the multiple steady states in a gas-liquid reactor is briefly reviewed. Finally, the scale-up problems associated with gas-liquid reactors with various degrees of backmixing and the recommendations for the future work in RTD and macromixing models are outlined.
SCOPEIt is well known that backmixing is detrimental to the performance of a gas-liquid or a gas-liquid-solid reactor. This review outlines various aspects of backmixing in gas-liquid reactors.The review first outlines the methods for measuring residence time distributions for various phases in a gasliquid and a gas-liquid-solid reactor. The data for the residence time distributions (RTD) are evaluated by a variety of simple (for example, one-parameter axial dispersion model) and complex (for example, two-, three-, or four-parameter) differential or stagewise macromixing models. These models are briefly evaluated.From the point of view of backmixing, gay-liquid and gas-liquid-solid reactors can be classified into three broad categories: film reactors where the liquid is the dispersed phase and flows as a film, for example, trickle-bed reactors; gas dispersed as bubbles in a continuous liquid phase, for example, bubble column and slurry reactors; and liquid dispersed as droplets in a continuous gas phase, for example, spray column reactors. Backmixing characteristics of each of these types of reactors are briefly evaluated. The backmixing in these reactors has been largely characterized on the basis of axial dispersion model. Models for gas-liquid bubble column reactors are briefly reviewed. Slow, fast, and instantaneous gas-liquid reactions are considered. Criteria for eliminating the effect of backmixing on the performance of isothermal and adiabatic trickle-bed reactors are outlined. Unlike a singlephase reactor, an adiabatic gas-liquid reactor can lead to five steady states, and the literature on this unique feature of the gas-liquid reactor is briefly reviewed.Finally, there is a brief outline of the scale-up problems associated with RTD, and some recommendations for the future work on RTD and macromixing models are presented.
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