Monolithic catalysts and reactors : high precision with low energy consumption

Structured catalysts and reactors offer high precision in catalysis at all relevant scales of the catalytic process, from that of the catalytic species up to that of the reactor. Monoliths are the prime example of such catalysts because of their wide practical applications. Thus, monoliths are emphasized in this review, but most of the text is also relevant to all structured reactors, including microreactors. Conceptually, monoliths exhibit more degrees of freedom in design than conventional reactors, such as fixed-bed and slurry reactors. The flow in monoliths is laminar, and as a consequence, they are associated with high efficiency and minimum chaotic characteristics. The hydrodynamics of single-phase and multiphase flow reactors are remarkably simple. Under most conditions in multiphase systems, Taylor flow (segmented flow) prevails, associated with high rates of mass transfer notwithstanding low energy consumption, but under other conditions, the film flow regime can be realized either in cocurrent or in countercurrent flow of gas and liquid streams, making the monolith a good structure for novel technologies such as catalytic distillation. Monoliths offer freedom in the design of reactor configuration. Examples are loop reactors for strong exo- and endothermic reactions, which allow a combination with separate heat exchange without the penalty of a large energy consumption, which otherwise is usually unavoidable for the large recycle ratios needed. For applications in fine chemistry