Rapid, effective separation of phases is an essential step in many hydrocarbon conversion gas-solid processes. In heavy oil upgrading, fluid catalytic cracking (FCC) and biomass pyrolysis, for example, very fast separation of reacting gases from catalytic or heat-bearing solids is required to inhibit further reaction. The primary functions of a rapid separator in the context of a gas-solid flow are, first, to terminate contact between the reacting gases and the catalytic or hot solid particles and, second, to recover the isolated product vapors. Uninterrupted vapor-solid contact results in degraded, overcracked and generally less valuable products. Increasing interest in rapid separation has led in the past 25 years to a wide variety of proposed separator designs in the literature.

Centrifugal separation is the fundamental principle underlying cyclone operation and most rapid gas-solid separation processes. Traditional cyclones rely on centrifugal separation and are very widely applicable. They are used for the removal of solid particulate matter from gas or liquid streams, for gas demisting, and in hydrocyclones. Most FCC processes employ traditional cyclone configurations for secondary and tertiary separation. The review briefly summarizes important topics studied in reverse flow cyclone operation. For initial separation, however, most fast gas-solid separators in the literature feature only partial cyclone layouts. Separation typically occurs more quickly in these devices relative to traditional cyclones, likely at the expense of separation efficiency.

The current review places emphasis on experimental results and novel separation techniques in the context of fast gas-solid separation. Several recent methods of rapid gas-solid separation employ co-current or "uniflow" centrifugal arrangements. Hence, particular attention is directed to co-current centrifugal separators. These devices have been studied much less intensively than their reverse flow counterparts. Fast co-current separators, with their novel, partial cyclone layouts, are interesting for their performance characteristics in relation to popular commercial designs.