Smart micro reactors: carbon nano fibers and pillars in micro channels : reactor design, synthesis, and mass transfer

Abstract

One of the most important tasks of the chemical engineer is to assure both the most efficient and the safest operation of a chemical reactor system. For this reason, there has been a lot of interest in the use of micro reactor systems in the last decades. The biggest difference between these micro systems and conventional large scaled reactors is the increased safety due to smaller reaction volumes present and the increased heat and mass transfer due to short diffusion lengths. However, because of these high gas-liquid and liquid-solid mass transfer rates, most heterogeneously catalyzed gas-liquid reactions in micro channels are chemically/kinetically limited. This motivates the design of systems with a larger surface area, which can be expected to offer higher reaction rates per unit volume of reactor, increasing the productivity and efficiency of the chemical reactor system. This increase in surface area available for catalyst deposition can be realized by using structured micro channels. The project objective of this research was to develop and demonstrate a micro structured multi-phase reactor, with specially modified catalytic coatings on micro-structured internals such as micro pillars and carbon nano fibers. This micro reactor may serve as a tool to develop selective and environmentally benign routes in the production of fine chemicals. The first configuration studied in Chapter 2 and Chapter 3 of this thesis is a rectangular micro channels with a hydraulic diameter of approximately 90 µm containing round pillars. The flow regimes, gas hold-up and pressure drop are determined for different pillar configurations, e.g. pillar hold-up and pillar diameter. It was proven that the gas hold-up was well-described by the Armand correlation. A model was developed for the single and two-phase pressure drop, which better described the experimental data compared with similar models found in literature. The single-phase pressure drop was described within 20-40 % error, whereas the two-phase pressure drop was predicted within 30-50 % error. The open structure and the large surface area available for catalyst deposition make the pillared micro channel an efficient system for performing heterogeneously catalyzed gas-liquid reactions. An alternative larger micro reactor configuration for fast screening and testing of supported catalysts for kinetically fast reactions was designed in Chapter 4. The stainless steel micro reactor consisted of a rectangular channel with a hydraulic diameter of approximately 900 µm, in which three supported catalytic plates could be positioned. The reactor design assured an easy replacement of a catalyst support and the possibility of the ’off board’ production of the catalyst supports. The ’plug & play’ micro channel reactor designed in this chapter proved a practical tool for the rapid screening and direct comparison of the activity of heterogeneous catalysts on different types of supports. In the designed reactor configuration in Chapter 4, carbon nano fibers (CNFs) were used to increase the overall rate of reaction per reactor volume by increasing the catalytic surface area. The micro channel contained three graphite plates on which CNFs with different configurations were deposited. It was shown in Chapter 5 that the carbon nano fibers increased the observed reaction rate of the heterogeneously catalyzed liquid-phase hydrogenation reaction of an alkyne with a maximum factor of 2.8 due to an increase in the surface roughness of the CNF layer in comparison with an empty micro channel. In Chapter 6, it was shown that the gas-liquid flow patterns were not disturbed by the CNF layers. The CNF layer with the most open structure and the largest layer thickness showed an enhancement of the overall reaction rate of the heterogeneously catalyzed multi-phase hydrogenation reaction of the alkyne with a factor of 3.5 to 4 in comparison with an empty micro channel. The most efficient reactor design was found to be a combination of both above mentioned reactor designs. This resulted in a rectangular micro channel containing pillars on which a layer of CNFs is deposited. This reactor system was modeled in Chapter 7. The influence of aspect ratio, pillar hold-up, pillar diameter, CNF layer thickness and volumetric gas and liquid flow rate on the micro channel performance, e.g. conversion, selectivity towards the intermediate product and mass transfer limitations was determined for a slow reaction ( 0.25 mmol g 1 Pt s 1), a moderately fast reaction ( 10 mmol g 1 Pd s 1), and a fast reaction ( 25 mmol g 1 Pd s 1). For all cases, large conversion and electivity towards the intermediate product close to unity were obtained for the (moderately) fast hydrogenation reactions. The CNF pillared micro channel proved to be a useful tool for the chemical engineer for a safe and efficient design of a reactor system for both very fast reactions due to the large mass transfer rates and for slow reactions.