Polymerisation of Butyl acrylate in the two Phase Slug Flow Regime of Parallel Microcapillary Reactors

Summary: The potential and problems of conducting a free radical polymerisation in parallel capillary reactors are presented. By operating in the so-called slug flow regime of immiscible liquid-liquid flow, one can achieve perfectly uniform residence times which are inaccessible using single phase flow. The excellent performance available in microreactors can be exploited for higher throughputs through the simple expedient of numbering-up, i.e. operation of multiple similar reactors in parallel under identical hydrodynamic conditions. In practice this approach often comes to grief on the coupling between hydrodynamics and chemical reaction, for example due to the strong influence of polymerisation on viscosity. Rigorous modeling reveals that the operating conditions sought are actually unstable. Furthermore, the uniformity of flow distribution between parallel capillaries was found to be very sensitive to the manufacturing tolerances of the capillaries used in the presence of polymerisation. Two strategies for resolving such problems are discussed. In the first case, coupling between reaction and the flow distribution is suppressed by a sufficiently high pressure drop upstream of the temperature regulated reactor segments. The pressure drop necessary to achieve this decoupling was estimated by the model. An alternative technique involves an appropriately inexpensive flow control system for each individual capillary. Since commercially available microvalves and flow measurement equipment are too costly for parallelisation purposes, it is necessary to develop new components to fulfill these functions. An optical monitoring technique is presented that meets both the technical and economic criteria, and which can be readily combined with recently developed new micro valves.1