Solids and Slurries Handling in Flow Reactors
An Overview of Solids and Slurries Handling in Flow Reactors
Suspended solids (intentional or otherwise) are a common feature of many chemical processes. Examples include reactions with catalysts, live cells, slurry feeds, impurities and precipitated products. The absence of low throughput reactors which can handle slurries under flow conditions has severely limited the progress of flow technologies in this field.
Solid particles are present in many chemical process operations, and in a continuous reactor these materials need to flow through the system without blocking or settling. Standard flow methods used for handling homogeneous fluids in flow rarely work well when solids are present within the system.
Handling solids under flow conditions requires reactors with a simple internal geometry, as well as channel diameters which are large enough to prevent bridging. Good mixing is also essential to maintain a uniform solids-liquid dispersion. In horizontal transfer lines, good mixing is required to keep solids suspended and moving, and for which high axial velocity with turbulent flow is required. Stagnant pockets where solids can settle out need to be eliminated by keeping the diameter of the channels uniform, and by using long-radius bends.
Coflore reactors have been designed to work well with solids by the use of free moving agitators within the process channels, which generate efficient radial mixing via rapid transverse movement of the flow reactor block. Good mixing, combined with a simple cell geometry, and comparatively large inter-stage channels, provide an effective means of keeping slurries flowing through the reactor.
Solids Handling in Flow
Three common problems are associated with handling solids in flow: Bridging, Settling, and Fouling. Each presents different challenges, and can lead to Blocking.
Bridging occurs when stationary particles in the system trap more particles to form a bridge across the face of the channel. This rapidly leads to a complete blockage. The primary means for preventing bridging relies on adequate channel sizing. A channel with a diameter which is more than ten times greater than the particle diameter is not as susceptible to bridging. This minimum diameter rule applies to all line elements including valves, pumps and instruments. Small pumps, flow control valves and reduced bore instruments such as mass flow meters are generally unsuitable where solids are present.
Settling is a common problem and is most likely to occur where the solids have high settling velocities. The settling velocity of a particle is related to its difference in density compared to the process fluid, its shape and its size. Small particles and those with a similar density to the process fluid have low settling velocities and are therefore easier to handle. Settling is counteracted by efficient and uniform mixing throughout the reactor. Solids usually travel through the flow reactor at different velocities to the process fluid. Where they travel at a lower velocity than the fluid, accumulation occurs. In serious cases, this can impair mixing, Handling solids in flow reactors leading to settling and complete blockage. The problem can be mitigated by reducing the solids concentration or employing a sloping flow path in the reactor which matches the settling direction of the solids.
Fouling occurs when solids deposit on the surfaces of the flow channel. Fluids with a tendency to foul are a serious problem for flow reactors, since the fouled material accumulates over time. This can affect the flow pattern, working volume and heat transfer characteristics of the reactor and can ultimately lead to complete blockage. Where fouling is a problem, the effects can be reduced with good mixing but fouling will always limit the cycle time of the reactor. Processes with a strong propensity to foul are generally better handled in a batch reactor.
Blocking of the process lines can occur as a result of excessive Bridging, Settling and Fouling, leading to a complete breakdown of the flow system and the prevention of material flowing through the reactor. This requires the entire process to be stopped, and the blockages removed before the process can resume.
“One of today's most important tools for modernizing the pharmaceutical industry is a process known as continuous manufacturing”
Director of the FDA’s Center for Drug Evaluation and Research, Janet Woodcock M.D.