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Micro process engineering is the science of conducting chemical or physical processes (unit operations) inside small volumina,typically inside channels with diameters of less than 1 mm(microchannels) or other structures with sub-millimeter dimensions. These processes are usually carried out in continuous flow mode, as opposed tobatch production, allowing a throughput high enough to make micro process engineering a tool for chemical production. Micro process engineering is therefore not to be confused withmicrochemistry, which deals with very small overall quantities of matter.
The subfield of micro process engineering that deals with chemicalreactions, carried out in microstructured reactors or"microreactors", is also known asmicroreaction technology.
The unique advantages of microstructured reactors ormicroreactors areenhanced heat transfer due to the large surfacearea-to-volume ratio, and enhancedmass transfer. For example, the length scale ofdiffusion processes is comparable to that of microchannels or even shorter, and efficient mixing ofreactants can be achieved during very short times (typically milliseconds). The good heat transfer properties allow a precise temperature control of reactions. For example, highlyexothermic reactions can be conducted almostisothermally when the microstructured reactor contains a second set of microchannels ("cooling passage"), fluidically separated from the reaction channels ("reactionpassage"), through which a flow of cold fluid with sufficiently highheat capacity is maintained. It is also possible to change the temperature of microstructured reactors very rapidly to intentionally achieve a non-isothermal behaviour.
While the dimensions of the individual channels are small, a micro process engineering device ("microstructured reactor") can contain many thousands of such channels, and the overall size of a microstructured reactor can be on the scale of meters. The objective of micro process engineering is not primarily to miniaturize production plants, but to increaseyields and selectivities of chemical reactions, thus reducing the cost of chemical production. This goal can be achieved by either using chemical reactions that cannot be conducted in larger volumina, or by running chemical reactions at parameters (temperatures, pressures, concentrations) that are inaccessible in larger volumina dueto safety constraints. For example, the detonation of thestoichiometric mixture of two volume unit ofhydrogen gas andone volume unit ofoxygen gas does not propagate in microchannelswith a sufficiently small diameter. This property is referred to as the"intrinsic safety" of microstructured reactors. The improvement of yields and selectivities by using novel reactions or running reactions at more extreme parameters is known as "process intensification".
Historically, micro process engineering originated around the 1980s, when mechanical micromachining methods developed for the fabrication ofuraniumisotope separation nozzles were first applied to the manufacturing of compactheat exchangers at theKarlsruhe (Nuclear) Research Center.