Microreactors (microscale reactors) 

Chemical reactions can happen in very small confinements, if this confinement has lateral dimensions below 1 mm, the device is called a Microreactor. In a microreactor reaction happens in microchannels/microtubes and the reactants may flow or may be stagnated. Microreactors like micro heat exchangers (MicroHX) are utilized by different sections of industry like process engineering, bio engineering, and energy engineering.
Microreactors have several advantages over conventional scale reactors. Some of the advantages can be listed as:

High surface to volume ratio increases the ability of the device to transfer heat to the surroundings. It can be helpful for two reasons: First exothermic reactions* can be used to provide heat to another fluid or to a system that needs heat (Microscale reactor-heat exchanger, MicroCHX). On the other hand, heat can be easily transferred to the inside media of the reactor for endothermic reactions to happen (Steam reforming reactors). Second, very high temperature reactions can occur relatively safe because the rate of heat transfer is very fast and the device will not get as hot (Nitrogen oxidation reactors).

High diffusion times** is a problem in some conventional reactors and mixing should be considered even in the reactor. However, in microreactors since the channel dimension are very small, the diffusion of the species (reactants) happens almost instantly. And typical diffusion times changes from fractions of a second (conventional reactors) to microseconds.

Much more surface is available in a microreactor for catalyst deposition; therefore reactants access to the catalyst is higher. Combining that and small diffusion times means higher conversions and efficiencies.

In several cases, when the reactants are at higher pressures, the reaction rate and efficiency would be higher. Microreactors can be pressurized easier than conventional reactors because of their very small geometry.

Besides all these advantages, microreactors have some down points too. The major problems are: 

The channel size in a microreactor might be too small for some particulate flow reactions, which causes clogging and agglomeration of particles. In those sorts of reactions, either the channel size should be increases or the size of particles. Nanoparticles are available for many particulate reactions nowadays.

High surface to area ratios in microscale geometry can be a problem depending on the application. Sometimes high heat loss is desired but in some cases for high efficiencies all heat generated should be transferred to the desired direction. Therefore heat loss should be controlled.


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