Three-phase fixed–bed reactor has been conventionally operated under the gravitational force with the gas and liquid phases flowing through a bed of randomly packed granular catalyst pellets. Frequently, the hydrodynamic or reaction conditions attained in this natural way may not be the desired ones. Actually, the reactor is usually subject to liquid maldistribution, incompelete catalyst wetting, surplus liquid residue holdup, and the slow reaction rate, etc. To overcome these limitations, alternative methods are need to be developed. In this review, examples of unconventional methods such as the forced pulsing flow operation, monolithic reactor, rotating packed bed, and reaction under catalyst partial wetting condition or even under phase transition are illustrated.
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