(83q) Micro Structured Reactors for Heterogenously Catalyzed Gas Phase Reactions – a Concept Study

Introduction The use of micro structured reactors in large scale processes like the production of vinyl acetate from ethylene, acetic acid and oxygen using a Pd/Au catalyst poses several challenges. Processes performed in conventional reactors like multi tubular reactors are well known and in case of the vinyl acetate production high selectivities can already be achieved. Nevertheless the increase of conversion can lead to reduced recycle flow rates and thus to energy cost savings. In the following the already known concept of the DEMiS^® reactor shall be evaluated in terms of productivity and heat respectively mass transport limitations. Results The space-time-yield related to the reactor volume is directly dependent on the thickness of the catalyst coating. As shown in Fig. 1 the productivity achieved in a conventional fixed-bed reactor [1] can be obtained when using a catalyst thickness of about 500 µm. This is valid for a catalyst with the same activity and selectivity as the one in the conventional reactor [2]. The rate constant for an improved catalyst is shown as line a) in Fig. 1 [3]. Here the activity is increased by a factor of 3. However due to design effects this leads to an increased productivity of only 2.3 for a wall coated micro structured reactor with a catalyst thickness of 500 µm. Further increases in productivity can be achieved by further improvements of catalyst activities or by increasing the catalyst coating thickness. First, the focus will be on increasing catalyst thickness: although the maximum thickness obtainable is dependent on both, mechanical stability and beginning mass transfer limitations. From the mechanical point of view a catalyst coating with a thickness of 1000 µm is still stable [4]. The mass transport limitation derived by a degree of efficiency for the catalyst of 95% using the Weisz-Prater criterion starts for the given reaction conditions with a rate constant of approximately 0.5 m³_VAM/kg_cath. The rate constants actually achieved allows an increase in productivity up to 260 kg_VAM/m³_reactorh. Compared to data from conventional reactors this means 4.7 times higher productivity. The other option for increasing productivity is the increase of catalyst activity. Due to high heat transfer abilities of micro structured reactors the catalyst used as a wall catalysts is able to have a higher activity while the reactor maintains a high selectivity towards the wanted product. This was shown in [3] for a wall catalyst with 2.5 wt% Pd and 1.1 wt% Au. Summary An evaluation of the DEMiS^® reactor concept for the acetoxylation of ethylene to form vinyl acetate was performed. Mass transport limitations only take place for higher activities than the ones being achieved until now and permit the use of catalyst thicknesses up to 1000 µm for achieving increased reactor productivities. Higher reactor productivities enable the reduction of reactor sizes for the production of same amounts of vinyl acetate as in multi tubular reactors. Literature [1] G. Roscher in: Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2005. [2] E. Klemm, H. Doering, A. Geisselmann, S. Schirrmeister, Chemie Ingenieur Technik 79 (2007) 697-706. [3] WO 2006/042659 A1, 2006. [4] DE 10 2005 019 000 A1, 2006. Fig. 1: Space-time-yield for a wall catalyst as a function of reaction rate for the formation of vinyl acetate