(55b) Modeling the Start-up Phase of Fischer Tropsch Synthesis in a Fixed Bed Reactor: Effect of Pore Filling and Heat Transfer Through the Catalyst Bed

Fischer Tropsch Synthesis is a highly exothermic process. During the startup, when the catalyst support pores are empty; there is a strong possibility of catalyst sintering due to rapid heat generation and poor heat transfer. As the reaction proceeds, these pores begin to fill with product hydrocarbons. As a result, the overall heat transfer improves, first, due to diffusion limitations imposed by the liquid in the pores and second, due to the change in the heat capacity of the bed as the pores are filled.

The objective of this paper is to explore the combined effects of pore filling and heat transfer in the bed on the startup behavior of FTS fixed bed reactor with a view towards suggesting startup procedures that avoid sintering and localized temperature runaways. Such a task has earlier been undertaken by Satterfield et al. [1] however; there were some limitations due to the nature of assumptions taken.

Finite difference approximation is used to model the intra-pellet reaction and heat transport. However, finite element analysis was used to model the inter-pellet heat and mass transport. Vapor-liquid equilibrium was assumed to be attained at each pore, thus virtual pressure and fugacity coefficients were used to develop a quantitative correlation. The goal was to estimate the time required to fill the catalyst pores. This time represents the unsteady transient phase which requires fine temperature adjustments to avoid reaction run away.

The resulting kinetic model, coupled material and energy balance are used to assess the optimum reactor startup procedures. Another goal is to identify the impact of different types of inert packing used to control heat transfer and conversion in the fixed bed reactor with a view towards avoiding the hot spots and steep temperature gradients. The combined effect of heat transfer and kinetics present a challenging problem for both the bed design and for safe startup sequence of the highly exothermic FTS reactions in jacket cooled fixed bed reactors.

[1]          G.A. Huff, C.N. Satterfield, Industrial & Engineering Chemistry Process Design and Development. 24 (1985) 986-995.