(235e) A Transparent Micro-Structured Reactor for the Investigation of Gas-Liquid Reactions at Elevated Pressures and Temperatures

At the laboratory scale, micro-structured reactors can bring several advantages such as: to lower inventory of reacting material, to use dangerous/toxic reagents, to perform reactions under non-conventional operating windows, to decrease heat and mass transfer limitations, etc. Most of these capabilities have been demonstrated for monophasic systems, i.e. when only one fluid, either a gas or a liquid, is involved, a solid catalyst being generally coated on the microstructure wall. However, some of these potential applications have not been demonstrated with multiphase reactions since it is much more difficult to control gas-liquid flow while performing fast and exothermic reactions such as hydrogenations, oxidations, fluorinations and others. In fact, most of the micro-structured reactors designed for gas-liquid or gas-liquid-solid applications cannot meet the multiple challenge of high temperature, high pressure and controlled flow. In this report, a micro-structured reactor answering these challenges is reported and applied to the investigation of reactions under non-usual pressure and temperature ranges. It is well known that Taylor flow can be obtained easily under standard conditions and using components as simple as capillary tubes. Many reports actually deals with such simple and efficient reactors. While experimentation using capillaries are easy to set and quite inexpensive they cannot lead to compact systems to allow visual inspection of the all channel which precludes the study of the reaction conversion and solid formation and the mapping of the flow regime. Also, the arrangement of the capillary is ?space demanding?, in particular when heating is required (oil bath, oven etc.). last, the design of the gas-liquid injector, which is determining for the perfect control of bubble size and liquid slug length, cannot be easily changed nor precisely machined. Silicon made micro-reactors offer the possibility to conduct Taylor flow in compact systems. The microreactor used here is made from silicon by well known lithography and etching techniques. The channels (300x300 µm, 1 m long) are covered by bonding a Pyrex glass wafer to the oxidized silicon device which provides visual access to the reaction medium. This is particularly useful for the monitoring of gas-liquid reactions. The test reaction was the exothermic (> 200 kJ.mol-1) oxidation of an alkane into a mixture of the corresponding ketone (K), alcohol (A) and hydroperoxide (HP). Working with pure oxygen is very tempting to increase the gas-liquid mass transfer rate, to speed-up the reaction rate and to decrease side products formation. Because of the safety issues raised by the alkane/oxygen mixture in the gas phase, such investigations are very scarce. In order to check for the possible intensification of the process, the reaction was investigated in a large range of pressure and temperature including pure oxygen. At 200°C and ca. 12 bar O2 pressure, a conversion of more than 4 % is measured at residence time as short as 2 min in the micro-reactor. The selectivity in the desired products HP, K and A is also very good (88%). The productivity in the desired products is increased fourfold compared to a similar experiment performed with air. Attempts to perform the reaction at higher temperature and/or residence time resulted in over-oxidation and by-product formation. Such limits are easily identified by visual observation using a microscope (See Figure). A very nice Taylor flow is generated at the injector (a) and no significant bubble shrinking due to oxygen consumption by the reaction is observed throughout ca. 60% of the channel (b). At some point however, a serious shrinking of the bubbles is evidenced (c) which ultimately leads to a liquid monophasic flow (d). Interestingly, new bubbles are formed close to the outlet of the channel which likely reveals over-oxidation driving to formic acid, ethylene and propylene and ultimately to CO2. Thus, the Pyrex capped silicon component allows the direct visualisation of the reaction progress under high pressure and temperature. However, further experiments have revealed that gas-liquid mass transfer still limits the rate of reaction despite the low dimensions of the channel (300 µm). Further studies are in progress to design a more efficient tool able to provide both mass transfer free operations and oxygen refilling.