A Greener, Leaner, Cleaner Alternative to Large-Scale Batch Chemical Production
In flow chemistry, a chemical reaction is set up as a continuously flowing stream. This feature is the main differentiator between flow and batch chemical production; the other widely-used method of chemical production.
The reaction pathways of flow reactors are typically tube-like in design and are manufactured from non-reactive materials. Mixing methods in flow reactors can include diffusion alone, passive (or static) mixing, and more recently active mixing.
Flow reactors allow efficient control over reaction parameters such as heat-transfer (heating and cooling), mixing (and mass-transfer) and the time of reaction (residence time). This level of control affords better potential outcomes of reaction processes, including percentage yields, purities, selectivities, waste, safety and emissions, whilst accommodating lower capital costs, smaller reactor footprints, efficient energy use, reduced solvent use and improved safety management.
Active vs Passive Mixing in Flow
For the majority of flow reactors, mixing of reagents in flow is achieved via diffusion within the reaction medium through a microchannel, through the use of patterns or objects within a microchannel, or a combination of the two. The latter is known as static mixing wherein the velocity of the reaction fluid is responsible for mixing efficiency. For these methods to be of practicable use for mixing in flow, the diameter of the flow channel tends to be narrow (typically on the order of a few mm or less).
For passive mixing, the mixing efficiency of a reaction medium and its reaction time (or more accurately residence time) are intrinsically coupled; to increase the residence time, the length of the microchannel will need to be increased, the flow rate reduced, or a combination of the two. This inevitably poses restrictions upon the process in terms of reaction time, mixing efficiency, reactor length and flow rate. Another limitation inherent with passive mixing regards the properties of the reaction mixtures that can be used in the flow reactor. Due to the small-sized nature of the micro-channel, they are ideally, but only, suited to liquid-liquid reaction media; liquid-gas, liquid-solid (slurries) and liquid-gas-solid forms, can pose issues within the reactor such as gas build-up, blocking and fouling, greatly affecting the viability and efficacy of a given process.
Active mixing in flow uses an agitator to mechanically mix the reaction mixture, permitting the use of larger channels within the reactor (which would perform poorly under passive mixing conditions), whilst maintaining excellent heat- and mass- transfer properties of the flow reactor itself. Such a mixing design decouples mixing efficiency from residence time and greatly increases the versatility of the flow reactor by allowing adjustments to be made to the flow rate or residence time, without the need to alter the reactor's configuration; and a single reactor setup can be used for many different process applications. Larger reaction channels increase a reactor's flexibility and facilitates the compatibility with a wider range of reaction processes; liquid-gas, liquid-solid and liquid-solid-gas reaction media can be routinely readily used without blocking, fouling or gas build-up. A widening of the flow channels within the flow reactor also facilitates a higher throughput of product, greatly increasing the productivity of the flow reactor in terms of chemical output for a given reaction process.
Active mixing in flow allows the use of larger reaction channels within the flow reactor. The use of larger channels within the flow reactor facilitates the production of greater amounts of material, a feature until now only considered possible with batch chemical manufacture.
In essence, the benefits of flow reactor and batch reactor technology have been combined and the advent of active mixing within flow facilitates a merging of benefits of flow and batch reactor technology, all within a single flow reactor.
An Introduction to Flow Reactors
Batch processing and continuous processing are two different ways to manufacture chemicals. The top 30 petrochemicals and most of the top 300 organic chemicals are manufactured at large scale using dedicated, continuous processing plants which run for many years without shutdown. As bulk chemicals, there is significant pressure on the price and so the cost of manufacture of the chemical must be minimised. As it is expected to be produced for many years, a significant amount of development work to fully understand the chemistry and chemical engineering requirements can be justified. This information informs the design and installation of a bespoke chemical plant highly tuned to the desired chemistry process with minimal waste of energy and materials. The need to provide as efficient a plant as possible leads to the selection of continuous processing technologies.
The feed of raw materials into the continuous reactor are tightly controlled, as is the residence time within the reactor and the temperature profile that the reacting materials are exposed to. As a result, the product flowing from the reactor tends to be more consistent as the reaction parameters are better controlled than in a batch operation whose reaction parameters must change over time. The capital costs per tonne of product of a continuous reactor are significantly lower that for a batch reactor system.
Batch reactors are, however, very process and product flexible. A batch reactor can be reconfigured from operating one process to operating another. The process is inherently inefficient but for low volume, high value chemicals, this has historically been an acceptable compromise.
To fully understand the impact on the process and economics of running a batch reactor, consider the following typical scenario. For a process A + B -> C, the reaction time may be only 5 minutes, but when running at pilot plant scale, say 1000L, the following sequence of process steps is not uncommon:
Begin preparing reactor for use t = 0
Seal and inert reactor 30 mins
Introduce solvent 30-60 mins
Add reagent A 30 mins
Heat to near reaction temp 2 hours
Add reagent B 30 mins
Reaction complete 5 mins
Cool to room temperature 2 hours
Discharge product 2 hours
Clean reactor 12 hours
For a 5 minute reaction time, using a batch reactor results in a process cycle time of anywhere from 8 to 24 hours.
For a continuous reactor, if we want to process the same quantity of material in 12 hours with a 5 minute residence time, the reactor would be 7 L in size:
7 L processed every 5 minutes = 1.4 L per min = 84 L per hour = 1008 L in 12 hours