Narrow-channeled microreactors

Narrow channeled microreactors are good for rapid heterogeneous liquid-liquidreactions. Narrow channels can be manufactured... 

With the same fullerene-method, a quantitative conversion of methionine to the corresponding sulfoxides was achieved in about 40 s, using low power, white LED illumination. The reaction time is considerably shorter when compared to the batch procedure that requires, for the same process, about 1 h illumination and the use of a 300 W tungsten halogen lamp. This may be due to the narrow channels space in a microreactor, which favors singlet oxygen diffusion, thus determining a faster reaction. 

In industry, this reaction is conducted at elevated pressure (10-30 bar) and temperature (80-250 °C). Under such conditions, the content of oxygen must be kept low to avoid the explosive regime. Also, the cyclohexane conversion is kept low (<5%) to avoid the formation of side products. In a microreactor, composition and temperature of the gas phase far above the flammability limit of 4% are possible. A selectivity of 88% was observed. However, it is shown that despite the narrow channels, gas-liquid mass transfer still limits the rate of reaction so that further engineering of a reactor is required that will enable mass transfer free operations and oxygen refilling. 

One of the main aspects of modem chemistry is the safety of the chemical processes. It is easy to see that the volume of a batch reactor must be some orders of magnitude higher than that of the continuous-flow microreactor to reach the identical quantity of final products (using equal amounts of reactants). The small quantity of reactants in the reactor minimizes the potential of thermal explosion by dangerous reactions. Indeed, explosion or depressurization of reaction systems with hazardous substances in the continuous microreactors leads only to insignificant technical problems or to a minimum leakage of chemicals, as opposed to the scales of explosions or leaks in standard reactor volumes. Microreactors, with their narrow channel dimensions, hold such a small quantity of reaction fluid that a mechanical failure in one reactor requires merely a temporary shutdown and subsequent replacement. 

An important advantage of the use of EOF to pump liquids in a micro-channel network is that the velocity over the microchannel cross section is constant, in contrast to pressure-driven (Poisseuille) flow, which exhibits a parabolic velocity profile. EOF-based microreactors therefore are nearly ideal plug-flow reactors, with corresponding narrow residence time distribution, which improves reaction selectivity. 

In conclusion, narrow RTD in multichannel microreactors can only be expected, when the design of gas distributer in front of the microchannel array and the design of the collector behind the channels are optimized. 

Residence time distribution can be an important issue in the selection process. Microreactors usually operate at Reynolds numbers lower than 200. In this regime, laminar flow prevails and mass transfer is dominated by molecular diffusion. An injected substance in the channel will dissipate caused by the flow profile in the channel. Hence the input signal will be broadened until it reaches the exit of the channel (Figure 3.2). The extent of such a distribution depends on the channel design. In microchannels the mixing process can then be described by the Fourier number (no axial diffusion, dominating radial diffusion D ). A high Fourier Po number leads to a narrow residence time distribution ... 

It is known that microchannel reactors, due to their small dimension and well defined structure have many advantages compared to conventional fixed bed reactors. The main ones are an efficient temperature control and well defined flow patterns. As the channel diameters are in the order of micrometers, microreactors operate under laminar flow conditions resulting in a parabolic velocity profile. But, due to the short radial diffiision times the radial concentration profile is flat, resulting in a narrow residence time distribution of the reactant. The latter characteristic is of crucial importance in the actual sbufy. Only reactors with an uniform residence time can be used to get meaningful kinetics information under periodic operation at short cycle periods [9]. 

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