Challenges to implementing Flow Chemistry
Handling solids in flow reactors
Flow reactors are now common in many R&D laboratories. Despite their growing popularity however, their use has been largely restricted to processes with clean fluids. This has more to do with the types of flow reactor in common use than user need.
Many useful industrial processes operate with a mixture of liquids and solids. Even where process fluids are nominally free of solids, problems can still arise as a result of fouling or the accumulation of debris. In this article we look at some of the practical solutions for handling slurries in flow reactors and specifically the use of AM Technology’s Coflore reactors.
Problems with solids
Four common problems are associated with handling solids in flow systems: bridging, settling, accumulation and fouling. Each presents different challenges.
Bridging occurs when particles get trapped in the flow channel and then serve to trap more particles, ultimately leading to bridging and blockage. The problem of bridging reduces as the channel size increases relative to the size of the particles. As a rule of thumb, a channel diameter which is greater than ten times the particle diameter has a low risk of bridging.
Settling is a common problem and 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 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 a 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, 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 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 it will always limit the cycle time of the reactor. Processes with a strong propensity to foul are generally better handled in a batch reactor.
Reactors for solids handling
Whilst all flow reactors have some solids handling capabilities, the concentration, size and density of solids they can tolerate are closely linked to the reactor design, size and properties of the process material. Microreactors represent an extreme case and can only handle very small solids in low concentrations.
Larger, statically mixed flow reactors – simple tubes, baffled tubes, and static mixers – have better solids handling characteristics but their mixing performance, which is necessary for keeping solids uniformly dispersed, is directly related to axial velocity in the tube. Problems occur where there are localised restrictions, stagnant zones or a transient loss of flow. Statically mixed reactors can handle slurries with varying degrees of success but their capabilities are generally limited to high throughputs and solids with neutral or near neutral buoyancy.
A dynamically mixed reactor relies on mechanical agitation and therefore mixing is not dependent on axial velocity in the channel. This ensures good mixing independently of fluid velocity and permits the use of short large diameter channels. These characteristics make such systems inherently better suited to solids handling than statically mixed systems.
A continuously stirred tank reactor (CSTR) with continuous feed and discharge is an example of a dynamically mixed flow reactor. A single CSTR, however, suffers from severe back mixing and provides very poor residence time control. This reduces the working capacity and performance of the reactor, since reacted and unreacted materials are not discharged in a time ordered manner. For many types of reaction, back mixing and poor residence time control have a serious and detrimental impact on yield, quality and reactor size. By using multiple CSTRs in series however, the problems of back mixing and poor residence time control can be overcome.
Whilst dynamically mixed flow reactors have inherently better solids handling capabilities, the engineering problems of mounting rotating mixers in multi stage systems or horizontal tubes are formidable and add substantial complexity and cost to the design. In the case of tubular systems, horizontal mounting is necessary to minimise back mixing and phase separation. Rotational mixing also has a tendency to promote centrifugal separation which is undesirable when materials of more than one density are present.
Coflore reactors are dynamically mixed systems which employ an alternative method of mixing to the traditional rotating agitator. This design eliminates the need for mechanical seals, rotating shafts and baffles and avoids the problem of centrifugal separation. Coflore reactors have successfully handled a variety of process materials that have hitherto been considered impractical for operation in flow systems.
Delivering slurries to a flow reactor at a measured rate has proved more difficult than handling solids which form within the reactor. This problem is most acute with small systems operating at low flow rates. Whilst syringe pumps are a common method for accurate dosing, their use is primarily limited to clean fluids, as any solids present will tend to settle in the syringe body and this frequently causes the pump to seize.
Other types of positive displacement pump such as diaphragm pumps, progressive cavity pumps and gear pumps can be used for accurate metering but are generally only suitable with slurries at high throughputs. At low throughputs, the displacement volumes in these pumps are small making them vulnerable to blockage in the pump chamber. Where non-return valves are used in the pumps, solids can either block in the valve or cause flow error by preventing the valves from sealing properly.
Coflore reactors have low pressure drops and therefore peristaltic pumps can be used at low or high flow rates with solids having low to moderate settling velocities. Where the pump materials are incompatible with the process fluid, gravity transfer can be used as an alternative. Gravity transfer methods however require more care for setup and calibration.
The most effective way to handle fast settling solids is to employ oversized pumps, large transfer tubes and high fluid velocities. This clearly conflicts with process needs when the required feed rate is low. From our experience, these conflicting needs can only be overcome by using a recycle loop and pulsing fluid into the reactor with a diverter valve or valves. The transfer line between the recycle line and the reactor inlet needs to be as short as possible and with minimum bends. It also helps to slope the transfer pipe to match the settling direction of the solids.