Temperature Excursions of Fischer-Tropsch Synthesis Occurring in a Fixed Bed Investigated Using a 2-D (r,z) Reactor Model

Friday, November 12, 2010: 10:36 AM
251 C Room (Salt Palace Convention Center)
Donald Cahela, Min Sheng, Tunde Dokun and Bruce Tatarchuk, Department of Chemical Engineering, Auburn University, Auburn, AL

The highly exothermic nature of the Fischer-Tropsch synthesis reaction results in a temperature excursion at the tube centerline in a tubular fixed bed reactor. Pseudo-homogeneous models assume a uniform velocity across the radius of the tubular reactor. This approach ignores the vacuuming effect caused by the reduction in moles for the FTS reaction. The accentuation of the “hot spot” in a fixed bed tubular reactor for FTS is studied using a model that accounts for the radial velocity.

Using microfibrous entrapped catalyst (MFEC) made from copper fibers has potential to improve selectivity for Fischer-Tropsch synthesis by minimizing temperature excursions, resulting in improved selectivity causing a reduction of the size and weight of a processing system for producing JP-5 type products. Selectivity for the Fischer-Tropsch Synthesis (FTS) reaction is sensitive to temperature, the chain propagation probability dropping by about 0.1 for every 25ºC increase in temperature. For the highly exothermic FTS reaction, a 50ºC temperature increase from wall to centerline has been indicated in two inch diameter fixed bed reactors for FTS. Minimizing the temperature excursion at the centerline of a tubular reactor for the FTS reaction is important to produce the desired product such as diesel fuel or JP-5. Microfibrous entrapped catalysts manufactured using copper fibers demonstrate effective bed thermal conductivities of about 7-10 W/m-K while the thermal conductivity of packed beds is about 0.2 W/m-K, similar to that of the gas phase. This significant increase in the effective thermal conductivity of the MFEC catalyst bed allows a much larger diameter reactor to be used for FTS to produce JP-5 while minimizing the plant footprint and weight.

A 2-D (r,z) model of a tubular reactor packed with extruded catalyst pellets or with an MFEC catalyst is solved to estimate the temperature excursion encountered in the FTS reaction. The accentuation of the hot spot by the increased reaction rate due to the hot spot at the centerline of the reactor and inward radial flow due to the reduction in the number of moles by the FTS reaction are accounted for in this model. This model uses segregated energy balances on the gas, catalyst and fiber phases. Using this approach avoids estimation of an overall effective thermal conductivity. The heat of reaction is generated inside the catalyst particles and the catalyst temperature will be assumed to be uniform inside the catalyst particles. A material balance on CO is solved for the conversion. Decrease in the number of total moles due to the FTS Rxn will create a vacuuming effect that will further accentuate production of undesired low molecular weight materials in the center of the tubes where the temperature excursion is the greatest. This effect is accounted for by incorporating a semi-implicit pressure linked equation (SIMPLE) using a modification of the continuity equation and momentum balances (extended Brinkman-Forchheimer equation) to estimate the axial and radial velocities. A simulation using Euler implicit integration to determine the steady-state is implemented using three-point central finite difference formulas. The complex rate expression is reduced to a first order reaction form by lumping the volumetric reaction rate with the film mass transfer coefficient. A mass balance on CO is written as a 2-D (r,z) cell model. The axial Peclet number is very large, for the relevant cases simulated, indicating that very little axial dispersion of the gaseous components occurs. Comparisons of the performance of a packed bed of extruded pellets with MFEC catalyst are made.


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See more of this Session: CO Hydrogenation
See more of this Group/Topical: Catalysis and Reaction Engineering Division