A microreactor is a very small-scale device in which chemical reactions can take place. Typically, it measures less than inch (2.54 cm) in length and breadth and perhaps less than one sixteenth of an inch (1.56 mm) in thickness, although dimensions vary. It will normally have input and output tubes, with tiny channels or chambers inside, in which the reactions take place. Usually the reactants and the product are fluids — liquids or gases — which may be introduced using tiny pumps or electro-osmosis. As of 2011, microreactors are used only for experimental and prototyping purposes, but there is a real prospect of employing them in large numbers for the mass production of useful chemicals.
The device is normally constructed by etching tiny channels onto a suitable material in a similar way to the manufacture of integrated circuits. They can be made from silicon wafers, glass, metal or ceramic materials. The channels may be no broader than a human hair. The etching may be performed by laser, electrical discharge or by chemical means. Often the microreactor is made from two etched plates sandwiched together.
Microreactors offer some significant advantages over more traditional, larger scale means of carrying out chemical reactions. The high surface-area-to-volume ratio enables reactions to proceed more quickly and often at a lower temperature than is possible at larger scales. Highly exothermic reactions that would normally be potentially dangerous or damaging to equipment can be carried out safely; any heat generated dissipates quickly due to the much smaller volumes of reactants. A failure in some part of a traditional chemical plant could result in the release of large amounts of hazardous chemicals or completely shut down production. In contrast, a plant consisting of a large array of microreactors would not be significantly affected by the failure of one part.
Usually, microreactors operate with a continuous flow of reactants. Although the rate of output from an individual microreactor is obviously very small, it can nevertheless be regarded as a tiny factory. There is the potential to employ very large numbers of mass-produced microreactors stacked together to provide products on an economically viable scale, and a number of possibilities are under investigation.
The use of microreactors in organic synthesis is one very promising area. They offer rapid mixing of reactants, fast reaction times, increased yields and safe handling of toxic and explosive compounds. The scaling up from laboratory to industrial level production does not involve any change to the procedures to achieve optimum 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 methods of production 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 up 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 cling to the walls of the reaction chamber. This is generally insignificant for traditional chemical manufacture using large reaction vessels, but on the micro-scale, a significant proportion of the potential yield may be lost. Another problem is that it is difficult to perform reactions involving solids, either as reactants or as products, in a microreactor as they tend to clog up the channels.