(450f) 2-D Modeling of Fischer Tropsch Packed Bed Reactor: First Step Towards Scale-up

Authors: 
Challiwala, M. S., Texas A&M University
Wilhite, B., Texas A&M University
Ghouri, M. M., Texas A&M University at Qatar
Elbashir, N., Texas A&M University at Qatar
Fischer Tropsch (FT) synthesis enables conversion of natural gas, shale gas or bio gas to liquid hydrocarbons, and is a boon for gas rich countries like the United States and Qatar to monetize their vast natural gas wealth. This process is an exothermic chemical reaction that converts synthesis gas or ‘syngas’ (a mixture of hydrogen and carbon monoxide produced by reforming of methane) into a variety of value-added chemicals and ultra clean fuels. Commercial FT reactor designs are the multi tubular fixed bed reactor and the slurry bubble column reactor. The slurry bubble column reactor provides excellent heat transfer, as the reaction takes place in a liquid slurry phase. However, it faces non-trivial process challenges including product recovery and catalyst attrition. On the other hand, the multi tubular reactor comprises of several tubes containing fixed catalyst bed, and enjoys advantages of good product selectivity, recovery and ease of maintenance. However, it faces challenges related to heat and mass transfer resistances that greatly limit its scale up and operation at high capacities. This modeling work aims at providing detailed assessment of the effect of mass and heat transfer in a fixed bed reactor to assess new fixed bed materials aimed at alleviating radial heat transfer limitations.

In order to understand higher resolution picture of such effects, a multi-scale CFD model is developed in COMSOL®, which facilitates observation both at the catalyst pellet level and in bulk fluid conditions. The reactor bed model is developed using different kinetic models which represent FT mechanisms on different cobalt-based catalysts, and thus facilitates comparison under a variety of operational conditions. In particular, a novel Micro Fibrous Entrapped Cobalt Catalyst (MFECC) developed by our collaborator (Sheng et al. 2012 , Tatarchuk et al. 2013) has been simulated in a bed scale model, which demonstrates orders of magnitude improvement in bed thermal conductivity. Additionally, the model investigates the potentials of scaling up the diametrical geometry of the reactor to improve throughput by utilizing the MFECC bed, as it could help in minimizing the impact of hot spot formations on the product profile and undesired reaction routes that result in formation of methane and light hydrocarbons. Current study results for a catalyst at 20 bar and at a gas hourly space velocity of 5000 1/h in a reactor tube of 0.59 inch ID show hotspot formation of about at the tube center. In contrast to this, the temperature rise in MFECC bed for same operating condition is only , which proves tremendous improvement upon conventional packed bed design. This work is a first stage towards tackling challenges related to reactor scale up and runaway hotspot formation in a fixed bed FT reaction, and is a part of broader project involving numerous experimental and simulation studies.

References:

Sheng, M., et al., High conductivity catalyst structures for applications in exothermic reactions. Applied Catalysis A: General, 2012. 445: p. 143-152.

Tatarchuk, B., et al., Microfibrous media and packing method for optimizing and controlling highly exothermic and highly endothermic reactions/processes. 2013, US Patent: 8420023 B2