RTD in MicroChannel Reactors

The yield of the intermediate product 2= — depends on the space time and the 

The product yield first increases up to a maximum value (1 2, max) down 

Flow in microchannels with diameters between 10 and 1000 pm is mostly laminar and has a parabolic velocity profile. Therefore, the molecular diffusion in axial and radial directions plays an important role in RTD. The diffusion in the radial direction tends to diminish the spreading effect of the parabolic velocity profile, while in the axial direction the molecular diffusion increases the dispersion [7,8]. With the so-called Taylor-Aris correlation the axial dispersion coefficient can be predicted based on the molecular diffusion coefficient D, the mean velocity of the stratified flow, the hydraulic diameter of the microchannel, and the geometry 

The dispersion in tubular reactors can be estimated for stratified flow in microchannels by introducing Equation 3.73 in the o-number. 

The first term in Equation 3.74 corresponds to the ratio between space time and characteristic axial molecular diffusion time (t = I The second term 

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The influence of residence time distribution (RTD) on performance, selectivity and yield is the same in microreactors as in conventional reactors. Therefore, the eflfects are well understood. Nonetheless, the demonstration of this at the microscale has hardly been reported so far. However, some experimental techniques have been developed to measure RTDs in microchannel flows which allow comparison between different types of flows or flows run at different parameters so that at least optimal flow conditions with regard to RTD can be found. 

This approach can also be used for multichannel MSRs. Because of the small volumes of the individual channels, many channels have to be used in parallel to produce substantial quantities of product. A uniform distribution of the reaction mixture over thousands of microchannels is usually necessary to obtain adequate performance of a multichannel MSR [76]. Flow maldistribution will broaden the RTD in such a multitubular reactor and will lead to a reduced reactor performance, meaning reduced product yield and selectivity at a given space velocity [75,77]. Therefore, several authors have presented design studies of flow distribution manifolds [71,78-80]. 

The drawbacks of randomly packed beds in microchannels are the high pressure drop and effects related to the nonuniform packing of the small catalyst particles, namely, channeling and maldistribution of the fluids. A large RTD results, which diminishes the reactor performance and, in the case of sequential reaction networks, the product selectivity. The reactor or the catalyst may be modified such that a structured bed is obtained. 

An RTD study on gas flows in a microchannel reactor specially designed for periodic operation with a y-alumina catalyst deposited on the reactor channels was performed [20]. Argon (in nitrogen) was used as a tracer. The concentration of argon was... 

An alternative to filling or coating with a catalyst layer the microcharmels, with the related problems of avoiding maldistribution, which leads to a broad residence time distribution (RTD), is to create the microchannels between the void space left from a close packing of parallel filaments or wires. This novel MSR concept has been applied for the oxidative steam reforming of methanol [173]. Thin linear metallic wires, with diameters in the millimeter range, were close packed and introduced into a macro tubular reactor. The catalyst layer was grown on the external surface of these wires by thermal treatment. 

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