Characteristics of Micro-Reactors

The unique operating characteristics of micro-reactors, compared to conventional batch reactors, include a high surface-area-to-volume ratio, enhanced heat transfer diffusion-dominated mass transfer, spatial and temporal control of reagents and products, the generation of concentration gradients and the opportunity to integrate processes and measurement systems in an automated manner. Some of characteristics are discussed here  

When scaling a conventional centimeter-sized reactor down to the micron scale, the surface-to-volume ratio significantly increases to the point where the container walls can effectively become an active or influential part of the reaction or process occurring in the fluidic channel. Clearly, this attribute of micro-reactors can be viewed in a positive way and leads to the opportunity to exploit the surface-dependent performance. A relatively simple but important example of this effect is where the surface charge of the capillary is neutralised by the solution contained within it to form a charged double layer, which under the influence of an applied electric field leads to the electro-osmotic mobilisation of the solution. In more chemical applications the surfaces could represent reagents, catalysts or even physical molecular imprinted structures. 

The high surface-to-volume ratio can also significantly improve thermal transfer conditions within micro-channels in two ways firstly, the convective heat transfer which takes place at the solid/fluid interface is improved via an increase in heat transfer area per unit volume and, secondly, heat transfer wifhin a small volume of fluid takes a relatively short time period to occur, enabling a thermally homogeneous state to be reached quickly. The improvement in heat transfer can certainly influence overall reaction rates and, in some cases, product selectivity. Perhaps one of the more profound effects of the efficient heat-transfer property of micro-reactors is the ability to carry potentially explosive or highly exothermic reactions in a safe way, due to the relatively small thermal mass and rapid dissipation of heat. 

It is well known that the flow within micro-channels is restricted to diffusive mixing under laminar flow conditions. Based on Pick s law, the relationship between the travel distance (L) of a molecule by diffusion and time (t) can be simplified as L = (2D tf where D is the diffusion coefficient. 

From the above equation, it can be seen that by scaling down the dimension in which diffusive mixing occurs, a significant reduction in the time taken to achieve complete 

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