Modeling, Simulation, and Design Procedure Development of Micro-channel FT Reactor using Computational Fluid Dynamics

Abstract

Fischer ̶ Tropsch (FT) synthesis is the main step in Gas-to-Liquid (GTL), coal-to-liquid (CTL) and biomass-to-liquid (BTL) processes. In GTL, natural gas is used as feedstock to produce syn-gas (a mixture of carbon-monoxide-CO and hydrogen-H2) needed for FT reaction where the reaction then produce hydrocarbon fuels (Fischer and Tropsch). In CTL and BTL, syngas is produced from coal and biomass through coal and biomass gasification. GTL is particularly of interest to oil and gas industry today, partly due to volatile fuel price, and partly due to environmental restrictions on flaring offshore stranded and associated gas, and the quest for monetizing these unusual resources. Commercial reactors in GTL are generally classified as high-temperature FT (593 ̶ 623 K) and low-temperature FT (493 ̶ 523 K) reactors depending on the product specifications and operating requirements. The reaction is characterized by high exothermicity (heat of reaction = 165 kJ/mol CO reacted) with both product selectivity and catalyst deactivation showing high sensitivity to temperature. This demands adequate heat removal and temperature control of the FT reactors for high reactor yield Low-Temperature FT synthesis in commercial GTL plants use conventional fixed bed and slurry bubble column reactors. However, fixed bed reactor has associated problem of high pressure drop and diffusion limitations, in addition to insufficient heat removal capacity. And, slurry bubble column has a major issue regarding liquid products-catalyst separation. In the recent years, microchannel reactors have attracted attention among researchers, as they are said to shorten the diffusion distance, and lower heat and mass transfer resistance, thus making it as an emerging technology for FT synthesis applications. Reduced mass and heat transfer distances provides enhanced process intensification, making it suitable for highly active FT catalyst. Moreover, many applications such as offshore and remote production facilities require compact and modular conversion technology. And, microchannel reactor blocks are considered to be highly integrated, compact, portable and safe technology making it ideal for those applications. Additionally, the small-scale source for syngas like municipal waste and biomass waste can utilize small scale microchannel technology to produce liquid fuels. However, the high exothermic nature of FT reaction and short residence time of microchannel reactor demands an active coolant having high heat removal capacity, for instance, saturated water. A microchannel reactor block, with reaction and coolant channel planes arranged in alternate manner and cross flow configuration was considered for Fischer ̶ Tropsch (FT) synthesis. Since past few years, using Computational Fluid Dynamics (CFD) tool to study microreactor or microchannel reactor simulation to either supplement or even replace expensive and difficult experiments have become a common trend. CFD simulation of heat transfer in the microchannel block was carried out to see the effect of wall boiling condition in coolant channel on reactor temperature. First, reaction inside a catalyst packed representative single channel was simulated to obtain typical heat generation profile along the channel length considering different operating conditions (GHSV 5000 hr-1

30,000 hr-1

catalyst loading or activity 60 % - 300 %, where 100 % loading equals 1060 kg/m3 of Cobalt based catalyst from Oxford Catalyst Ltd. and corresponding catalyst activity as 100%). Validity of single channel reaction model was checked by simulating an experimental single channel reactor model and comparing model prediction with the experimental data. Heat generation profiles of practical interest were then imported into multichannel block as time constant heat source input to carry out heat transfer simulation. Cooling oil (Merlotherm SHTM), subcooled water and saturated water (saturated at the reactor operating condition) were chosen as coolants. In one simulated case, temperature difference between hottest spot and coldest spot was found to be 32 K, 17 K and 12 K for cooling oil, subcooled water and saturated water, respectively, indicating highest heat transfer across channel wall for saturated water. Saturated water flow rate of 3 - 6 g/min per channel predicted high wall heat flux above 8900 W/m2-K. At intensified process condition (GHSV 30,000 hr-1

catalyst loading 300 %), mean FT temperature obtained was 510 K for saturated water and 519 K for subcooled water. Different candidate design of coolant and process channels geometry are evaluated to get insight into the effect of channel geometry on reactor heat transfer performance. A modified reactor block with an additional coolant layer gave improved thermal performance predicting noticeable heat transfer enhancement under wall boiling condition even at very low exit vapor fraction. Accordingly modified reactor block is tested for intensified process condition (GHSV = 30,000 hr-1 and super active catalyst condition). Strong correlation exists between temperature, reaction conversion and product selectivity. For low temperature FT synthesis, it is preferred to maintain reaction channel temperature below 523K. Based on the predicted result, temperature of 517 K could be an optimum value. However, in general, as the intrinsic activity of the catalyst declines, reactor is operated at slightly higher temperature to achieve the same level of CO conversion. From studying effect of syngas ratio and reactor operating pressure, syngas ratio of 2 and operating pressure of 20 – 22 bar predicts more desired product selectivity compared to other set of values. Method of catalyst bed zone division and loading different % of catalyst in different zone is evaluated. Study indicates noticeable advantage of the method. Also, the method can be optimized to obtain optimum number of zone division, zone length and strategic loading % in each zone. Further, on evaluating heat transfer performance of cross-current flow and co-current flow configuration of syngas and coolant flows, there is clear indication that co-current configuration gives better heat transfer performance compared to cross-current flow configuration in microchannel reactor block operation. A systematic microchannel FT reactor design procedure is also formulated for future microchannel reactor simulation and design process.