Barrier-based micro/milli channels reactor

Abstract

Gas-liquid processing in microreactors remains mostly restricted to the laboratory scale dueto the complexity and expenditure needed for an adequate numbering-up with a uniform flowdistribution. The barrier-based distributor is a multiphase flow distributor which assures flowuniformity and prevents channeling between the two phases.Experimentally, design criteria for the barrier-based distributor are obtained in a setupmade of capillary and tube fittings. The design criteria are illustrated for the case of anitrogen-water Taylor flow in four parallel microchannels. Gas-liquid channeling is preventedat equal pressures in the gas and liquid manifolds. An optimal operational window is realizedwhen the gas to liquid flow ratio is kept constant and the ratio of the maximum over the minimumflow rates remain less than 15. The effect of variations in the inner diameters due to thefabrication tolerance of the barrier channels and the microchannels on the flow distribution isdemonstrated.Numerically, the flow distribution is studied using a method based on hydraulic resistivenetworks (RN). The single phase hydraulic RN model is extended to account for two phasesgas-liquid Taylor flow. For ReGL <30, the accuracy of the model was above 90% for thecapillary and tubing setup. The model was used to study the effect of fabrication tolerance onflow distribution and barrier channel dimensions. A design methodology has been proposedto determine the required hydraulic resistance in the barrier channels and cut-off values ofthe maximum allowed fabrication tolerance.Based on the design methodology, the barrier-based micro/milli reactor (BMMR) is designedand fabricated to deliver flow nonuniformity of less than 10%. The BMMR consistsof eight parallel channels operated in the Taylor flow regime with a liquid flow rate up to 150mL/min which is suitable for a production capacity in the order of kg/h. The quality of theflow distribution is considered by studying two aspects. The first aspect is the influence ofdifferent viscosities, surface tensions and flow rates. The second aspect is the influence ofmodularity by testing three different reaction channels types: (1) square channels fabricatedin a stainless steel plate, (2) square channels fabricated in a glass plate, and (3) circular channels(capillaries) made of stainless steel. Additionally, the BMMR is compared to a singlechannel and shows the same performance regarding the slug and bubble lengths and bubblegeneration frequency.Under cold flow conditions, the influence of temperature is studied in the BMMR. Themethodology provides a cut-off value of the maximum allowed temperature deviation in eachpart of the BMMR. Temperature deviation effect on flow distribution is quantified using ahydraulic resistive network model. Flow rate effect on the temperature deviation is demonstratedusing a one dimensional energy balance model. Experiments in the BMMR wereconducted to validate these models. Temperature deviation in the barrier channels affectsflow non-uniformity by ten times more than in the mixer and reaction channels. Above a certaincritical liquid residence time, the flow rate had no significant effect on the temperaturedeviation. The critical liquid residence time depends on the liquid used, BMMR material ofconstruction, and its geometrical dimensions. The design methodology provides engineeringsteps to give a first estimation on the effect of temperature on flow distribution.Under reaction conditions, the BMMR is tested using the hydrogenation reaction forphenylacetylene to styrene and ethylbenzene using a homogenous cationic rhodium catalysts[Rh(NBD)(PPh3)2]BF4. In a semi-continuous batch reactor, a parametric study wasperformed by changing the hydrogen pressure, catalyst concentration, initial concentrationof phenylacetylene and styrene. The kinetic parameters were estimated by fitting the kineticmodel to the experiments. Catalyst deactivation was observed and incorporated in thekinetic model. The kinetic model predicts the experimental result within an accuracy of20%. Preliminary results for performing the hydrogenation reaction in the BMMR reactorare demonstrated. The reaction was conducted in the BMMR and the reactant and productconcentrations of a single channel were compared to that of eight parallel channels in theBMMR. For 95% of the obtained results, the difference in concentrations between the singleand the eight channels remains within ± 10% and depended on the gas and liquid flow rates.As a proof of concept, the numbering-up concept of gas-liquid Taylor flow in the BMMR hasbeen successfully realized.As a case study, the oxidation of ethylbenzene was used as a relevant industrial applicationfor process intensification using the micro/milli channel reactor. The BMMR was usedto scale-up ethylbenzene oxidation via numbering-up to total flow capacity of 10 m3/h whichequals to 80000 tons/y. The size and operational aspects of the BMMR were compared tocurrently used industrial reactor, the horizontal bubble column reactor HBCR. The size ofBMMR is five to ten times smaller than the HBCR if the residence time is less than 2 minutes.In principle, the BMMR shows its capability to scale-up multiphase flow applicationsto reach industrial bulk scale capacities. This opens a wide variety of opportunities to furtherinvest in this field and conduct research to fully exploit this technology.