(83f) Micro Reactor Application for the Beckmann Rearrangement to Epsilon-Caprolactam

Mass and heat transfer can put significant technological limitations to reactor and process design, which leads to far from ideal solutions for bulk processes. Scaling down to centimeter, millimeter, or even micrometer scale can increase mass and heat transfer in such cases tremendously to allow for a much improved process design with respect to conversion, selectivity, reactor volume, and, in case of strongly poisonous and dangerous chemicals and runaway conditions, safety. When several of these conditions are present, a strong impetus exists to replace the present bulk scale technology by micro reactor technology. The goal of this paper is to report on the design, construction, and testing of a single channel micro reactor integrated with a micro mixer to demonstrate the viability of a miniaturized reactor system for an industrial bulk process, based on micro reaction technological considerations. As model reaction is chosen the Beckmann rearrangement of cyclohexanone oxime to epsilon-caprolactam with oleum as a catalyst and solvent. Epsilon-caprolactam is the main precursor for nylon-6 and is produced with an annual production of 4.4 million tons and may therefore be considered as one of the main bulk chemicals. The admixing of molten cyclohexanone oxime (1 mPas) and the reaction mixture (50-300 mPas), consisting of caprolactam and oleum, is the key step in the industrial process. In current processes, a jet mixer, operated in the turbulent regime (Re > 5000), is used with a considerable recycle of the reaction mixture. However, the integration of the reaction and the heat exchange sections by the application of micro technology will certainly decrease residence time and may therefore increase selectivity and safety. Furthermore, the window of operation in the Beckmann reactor is enlarged, which is beneficial for process flexibility. Already in the 1980s BASF described a process for the continuous preparation of caprolactam by Beckmann rearrangement of cyclohexanone oxime, dissolved in a solvent which is inert towards oleum and immiscible with reaction mixture. This is beneficial for the process. Useful solvents are cycloalkanes, which in addition remove water from the cyclohexanone oxime feed. For the reason that two phase flow in micro mixers creates a large interfacial surface area by the formation of small alternating fluid zones, the hydrodynamics and flow patterns of relevant viscous liquids and immiscible cycloalkanes have been investigated by microscopic high speed imaging. A split and recombine mixer shows various flow patterns as a function of different flow rates (Figure 1). At low flow rate, Taylor flow patterns can be distinguished. Higher flow rates result in the formation of a large number of small droplets in liquid slugs in different fashions. Six different patterns can be distinguished. Other mixer designs, including the Y-mixer and the interdigital mixer, give rise to various other flow patterns which will be shown in the paper. The formation of large numbers of small droplets is beneficial for increased mass transfer in micro channels, as the interfacial mass transfer area will increase per volume of reactor. Also fluid viscosity and surface tension effect the observed flow patterns. Image analysis tools have been used to determine droplet size (ca. 30 ìm) distributions, which are critical parameters in mass transport models. At 100% conversion, the Beckmann rearrangement in a stainless steel single channel micro reactor, with a residence time of approximately 4 sec, shows a strong dependency of the selectivity on the mixing intensity and the way of mixing. In Y-mixer reactors (250 ìm channel diameter), used as a benchmark, an increase in reactor length from 20 cm to 50 cm increases the selectivity from 84 to 94%. Increasing the mixing intensity and mass transfer by the application of two phase flow, reaction mixture and inert solvent, cyclooctane, for cyclohexanone oxime, the selectivity ranges from 60 to 99+%. Despite the simple design, a Y-mixer reactor already demonstrates selectivities up to 99.4% to caprolactam, with 100% conversion. However, the selectivity is very susceptible to process conditions, especially temperature. By the application of more sophisticated designs, such as the well known IMM high pressure interdigital mixer, the selectivity is much more stable towards reaction conditions (Figure 2) and, within experimental error, results in 100% selectivity with 100% conversion for high oleum concentrations. To reach high selectivities at low oleum concentration (higher viscosity), further intensified mixing is needed. To this end, the split and recombine mixer has been used. This mixer offers a very elegant method for creation of multi micro zones of oxime and oleum rich phases and shows excellent performance.