RTD of Gas Flow in Microchannels

Axial dispersion can be neglected (Bo 100), if the space time is at least two times the radial diffusion time. Accordingly, axial dispersion of gases in microchannels can be neglected, if their diameters are less than 1000 pm and the space time is longer than 0.1 s. This could also be proved experimentally. 

The approach can also be used for multichannel reactors. Because of the small volume of a single channel, many channels have to be used in parallel to obtain sufficient reactor throughput. A uniform distribution of the reaction mixture over thousands of microchannels is necessary to obtain an adequate performance ofthe microstructured reactor. Flow maldistribution will enlarge the RTD in the multitubular reactor and lead to a reduced reactor performance along with reduced product yield and selectivity. Therefore, several authors have presented design studies of flow distribution manifolds [9-13]. 

Besides maldistribution, small deviations in the channel diameter introduced during the manufacturing process cause an enlargement of the RTD. The deviations may also be because of a nonuniform coating of the channel walls with catalytic layers. If the number of parallel channels is large (N 30), a normal distribution of the channel diameters with a standard deviation a can be assumed. The relative standard deviation, influences the pressure drop over the 

The relation (3.76) shows that a variation of the channel diameter leads to a decrease of the pressure drop at a constant overall volumetric flow. As the pressure drop for each channel is identical, the variation of the diameter results in a variation of the individual flow rates, V] and the residence time, r, = VJV.  

Supposing plug flow in each channel Bo oo), the overall dispersion is inversely proportional to the relative standard deviation and can be estimated by Equation 3.77 [11]  

---