(705c) Startup and Steady State Operational Issues in Fixed-Bed Fischer-Tropsch Synthesis Reactors: A Modeling Study

In this paper we will present startup and steady state operational issues that arise in fixed-bed FTS reactors. We specifically address the interplay between catalyst design and process operation. One of the major concerns is sintering due to temperature variations in the reactor bed that arise especially during startup [1] . During startup, the catalyst pores are empty and diffusional limitations are low. The pores are gradually filled to reach a steady state. Accumulation of oil and wax in catalyst pores and on the surface of catalyst alters the nature of heat and mass transport and hence temperature gradients. Therefore it is important to consider the fill effects during startup phase of the fixed bed reactor. This process is slow and spans over a period of few hours to few days [2]. Similarly, with respect to steady state operation, catalyst size and morphology is important from the standpoint of conversion and selectivity. Morphology of catalyst has a strong effect on  product distribution [3-5], while particle size affects diffusion and related parameters (e.g. effectiveness factor) [6].

During the pore filling stage, reactor bed is vulnerable to uncontrolled reaction (or runaway) due to fresh native catalyst and smaller heat and mass transfer associated with an empty pellet. Empty pellet presents no diffusion limitations and maximum sites are available which can generate localized hot spots due to radial and axial temperature gradients [2]. The thermal energy released from the Fischer Tropsch reaction is typically managed via controlling the catalyst loading and by adding diluents to the feed syn-gas [7]. Similarly specialized procedures have been developed to control the rate of heat release during the startup [2]. Prior literature on theoretical modeling of FTS startup is limited. Huff and Satterfield[8] present a model for pore filling during startup phase. Here we present a comprehensive study taking into account intra-pellet and intra-reactor dynamic variations over the course of catalyst pore filling. 

This study covers the entire duration of reactor operation from initial gas injection on empty pellets to steady state operation using liquid filled catalyst pellets in the reactor bed. The study was motivated by various thermal and hydro-dynamic problems that we encountered in our prior experimental studies using a fixed bed reactor to evaluate catalyst performance. The reactor startup model is dynamic in time; however some components will be solved under the assumptions of “Pseudo Steady State” adopted due to significantly low rate of liquid accumulation within the pores. Intra and inter-pellet models are coupled to create a two dimensional comprehensive reactor simulation model. Mears’ criterion [9] has been applied to justify the model development in both axial and radial direction thus making it a 2-dimensional heterogeneous dynamic model.

References:

[1]          J.R. Inga, in: U.S. Patent (Ed.), United States 2012.

[2]          K.B. Arcuri, in: European .Patent. Office (Ed.), 1987.

[3]          E. Iglesia, S.L. Soled, J.E. Baumgartner, S.C. Reyes, Journal of Catalysis. 153 (1995) 108-122.

[4]          S.A. Gardezi, L. Landrigan, B. Joseph, J.T. Wolan, Industrial & Engineering Chemistry Research. 51 (2012) 1703-1712.

[5]          B. Sun, G.B. Yu, J. Lin, K. Xu, Y. Pei, S.R. Yan, M.H. Qiao, K.N. Fan, X.X. Zhang, B.N. Zong, Catalysis Science & Technology. 2 (2012) 1625-1629.

[6]          Y.W. Li, Y.N. Wang, Y.Y. Xu, H.W. Xiang, B.J. Zhang, Industrial & Engineering Chemistry Research. 40 (2001) 4324-4335.

[7]          H.A.J. van Dijk, Eindhoven University of Technology Madrid, Spain, 2001, p. 174.

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

[9]          R. Philippe, M. Lacroix, L. Dreibine, C. Pham-Huu, D. Edouard, S. Savin, F. Luck, D. Schweich, Catalysis Today. 147 (2009) S305-S312.