Microreactors are small devices used for chemical reactions, typically less than an inch in size with tiny channels or chambers for reactants and products. They offer advantages such as rapid reactions, safe handling of toxic compounds, and the ability to stack many units for mass production. However, challenges include the wall effect and difficulty with solid reactions. Microreactors have potential in organic synthesis and biodiesel production.
A microreactor is a very small-scale device in which chemical reactions can take place. Typically, it measures less than an inch (2.54 cm) in length and width and perhaps less than a sixteenth of an inch (1.56 mm) in thickness, although sizes vary. It will usually have inlet and outlet tubes, with tiny channels or chambers inside, where the reactions take place. Usually the reactants and the product are fluids – liquids or gases – which can be introduced using tiny pumps or electro-osmosis. As of 2011, microreactors are only used for experimental and prototyping purposes, but there is a real prospect of deploying them in large numbers for the mass production of useful chemicals.
The device is normally constructed by etching tiny channels into a suitable material in a similar way to manufacturing integrated circuits. They can be made with silicon wafers, glass, metal or ceramic materials. The channels cannot be wider than a human hair. The engraving can be done by laser, electric discharge or by chemical means. Often the microreactor consists of two etched plates sandwiched together.
Microreactors offer some significant advantages over more traditional, large-scale means of carrying out chemical reactions. The high surface area to volume ratio allows reactions to proceed more rapidly and often at a lower temperature than is possible on larger scales. Highly exothermic reactions that would normally be potentially dangerous or harmful to equipment can be performed safely; any heat generated dissipates quickly due to the much smaller volumes of reactants. A failure in one part of a traditional chemical plant could lead to the release of large quantities of hazardous chemicals or a complete halt in production. Conversely, a plant consisting of a large array of microreactors would not be significantly affected by the failure of one part.
Typically, microreactors operate with a continuous flow of reactants. Although the production rate of a single microreactor is obviously very small, it can still be considered a small factory. The potential exists to employ very large numbers of mass-produced microreactors stacked together to deliver products on an economically feasible scale, and several possibilities are being explored.
The use of microreactors in organic synthesis is a very promising area. They offer rapid mixing of reagents, fast reaction times, higher yields and safe handling of toxic and explosive compounds. The transition from laboratory to industrial production does not involve any modification of the procedures to obtain optimal yields: it would simply be a matter of adding more microreactor units.
Another potential commercial use is in the production of biodiesel, an alternative to fossil fuels. Current production methods require the main raw materials, vegetable oil and methanol, to be mixed with a catalyst and left for several hours to complete the reaction. In a biodiesel microreactor, the reaction is almost immediate and, again, scaling the process to produce useful quantities would simply involve combining a large number of microreactors.
There are, however, a number of problems that must be overcome to achieve economical large-scale production of chemicals using microreactors. One of these is the wall effect: reactants and products tend to stick to the walls of the reaction chamber. This is generally insignificant for traditional chemical manufacturing using large reaction vessels, but at the micro scale, a significant percentage of the potential yield could be lost. Another problem is that it is difficult to perform reactions involving solids, either as reactants or products, in a microreactor as they tend to clog the channels.
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