Multiphasic reactions under flow
Inherent to laboratory based and industrial manufacture is multiphasic systems to, for example, transfer reaction products between aqueous-organic phases, in crystallisation, catalysis or gas-liquid reactions. The mainstay of batch processes is a stirred tank reactor. Here active mixing is provided through agitators capable of suspending solids, creating and maintaining droplet suspensions or in dispersing gas as small bubbles. The energy input effects contact between the phases.
There are different ways of effecting contact between two or more phases - one strategy is to flow separate phases into the reactor and promote interaction between these through mixing; active mixing includes stirrers whilst passive mixing includes static mixing elements which promotes mixing through twisting flow elements together at the expense of a larger pressure drop.
| Laminar pipe flow | Cascade CSTR | |
|---|---|---|
| Gas-Liquid | The flow is segregated consisting of continuous slugs of gas and liquid. Unlike laminar pipeflow reactors, the liquid slugs can give almost plug flow characteristics, with slow mixing within the liquid phase. The gas-liquid transport is relatively low due to a small surface area. | Continuous mixing creates a population of gas bubbles within the liquid phase. The residence time characteristics are similar a single phase CSTR with good mixing of the liquid phase. The mass transfer is efficient due to the area of contact. |
| Liquid-Liquid | Stable emulsions created outside of the reactor will flow through the pipe. Emulsions that separate will cause minimal interfacial contact. Good for stable emulsions but these require generation prior to the reactor, low interfacial area for systems that segregate. |
Active mixing refreshes surface so can deal with emulsions that separate; capable of creating emulsions within the reactor. Mixing of both phases creates good mass transfer between phases. |
| Liquid-Solid | At the low velocities found within most laminar flow pipe reactors, particles will settle onto the base of the tube. This bed can move through the reactor at a slow speed or can cause excessive pressure drop and blockage. | Continual stirring suspends particles within the flow. Keeping short sections of pipe between reactors can minimise settling, but this remains a potential site of blockage. |
Where one of the phases is solid, an alternative strategy is to fix a solid phase (e.g. a catalyst) within the reactor and flow liquid or gas-liquid systems through this matrix – these are packed bed or trickle bed reactors, respectively. This does rely on the solid having a high number of turnovers (substrate converted per catalyst used - or life within the reactor). Alternatively, the solid phase can flow through the reactor as a suspended particle, before downstream separation.
There are more specialised multiphasic reactors that can be used to create the interfaces between phases. These include spinning disc reactors, falling film reactors, acoustic (sono-) reactors, oscillating baffle reactors, shaken reactors and bubble column reactors. Understanding your requirements around residence time and energy transfer into the system to maintain the conditions for mixing (mass transport) and safe reaction conditions (including thermal conditions) is something you ideally establish through laboratory scale flow reactors before more completely optimising your system.