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In flow chemistry, a chemical reaction is run in a continuously flowing stream rather than in batch production. In other words, pumps move fluid into a tube, and where tubes join one another, the fluids contact one another. If these fluids are reactive, a reaction takes place. Flow chemistry is a well-established technique for use at a large scale when manufacturing large quantities of a given material. However, it is relatively new to use it in the laboratory environment.
Additional recommended knowledge
Batch vs. flow
Comparing parameters in Batch vs Flow:
Flow Reactor scale
It is possible to have flow reactors operating at large scale; however for use in the laboratory, channel/tube scale is likely to be in the region of 50μm to 500μm. Nonetheless, this channel scale range is sufficiently broad to allow single experiments with approximately 10 mg of starting material per observation point in a similar environment to an annual production rate of several tens of tons of material (Fast transfer from research to production).
In as far as synthetic efficiency is concerned, there are a number of benefits to do with thermal and mass transfer as well as mass transport that allow chemistry to perform efficiently. For a review of synthesis benefits from enhanced physical reaction control see Microflow Synthesis and literature cited therein.
The scale of micro flow reactors can make them ideal for process development experiments. For a review on the types of processes which can be conducted in these and the benefits afforded see Optimised Chemistry Using a Flow Reactor System and literature cited therein.
Continuous flow reactor
A Continuous flow reactor is a device that allows chemical reactions to be performed as a continual process rather than batch-wise. Reagents are continually added to the input of the reactor and product continually collected from the output. The reactor is typically tube like and can be manufactured from a variety of materials including stainless steel, glass and polymers. Mixing methods include diffusion alone (if the diameter of the microreactor is small e.g. <1 mm) and static mixers.
Continuous flow reactors allow good control over reaction conditions including heat transfer, time and mixing.
The residence time of the reagents in the reactor (i.e. the amount of time that the reaction is heated or cooled) is calculated from the volume of the reactor and the flow rate through it.
Residence time = Reactor Volume / Flow Rate
Therefore, to achieve a longer residence time, reagents can be pumped more slowly and/or a larger volume reactor used. Production rates can vary from nano litres to litres per minute.
Examples ... spinning disc reactor (Colin Ramshaw); spinning tube reactor; oscillatory flow reactor; microreactor; hex reactor; 'aspirator reactor' (this has another name ... it is based on a water pump where one reagent is pumped through it which causes a reactant to be sucked up by it. A patent is available (1941???, Nobel companies) that describes it being used to prepare nitroglycerin. From memory, the acid mixture was pumped through and the glycerin was sucked up).
Benefits of flow
Other uses of flow
It is possible to run experiments in flow using more sophisticated techniques, such as solid phase chemistries. Professor Steven Ley's group at the University of Cambridge has pioneered work that has demonstrated how valuable the coupling of flow chemistry and solid supported chemistries can be. http://leygroup.ch.cam.ac.uk
Running flow experiments
The practicalities of running flow experiments in normal chemistry laboratories are not simple. It requires the coupling of a range of equipment that is not commonly used by chemists. Additionally it is beneficial have a software control system to help manage all of the systems that are allowing the experiment to be performed. The chemist needs to have access to specialist microfabricated devices, tubing connectors and tubing as well as the pumps to displace the reagents.
The challenges of controlling flow experiments should not be underestimated however several of the commercial systems available address these well. It is important to consider both the equipment control and the "tracking" of reaction products especially when multiple reactions are being conducted sequentially.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Flow_chemistry". A list of authors is available in Wikipedia.|