(359b) Assessment of Pore Diffusion Limitations for the near Critical and Supercritical Fischer-Tropsch Synthesis

The objective of the present work is to provide an assessment of the diffusional limitations of catalyst particles of a few millimeter diameters in a fixed bed reactor operating under near- critical and super critical Fischer-Tropsch synthesis conditions. To conduct such assessment a conventional theory of diffusion-reaction in porous pellets has been employed. The kinetic model used for this purpose has been developed by Mogalicherla and Elbashir (2011) using a Langmuir-Hinshelwood-Hougen-Watson (LHHW)  kinetic type for a cobalt catalyst.¹ In the aforementioned kinetic model the concentration of reactant gases had been represented by fugacity rather than partial pressure to better account for the non-ideality of such reaction  mixture. The binary diffusional coefficients of all reactants and products in supercritical solvent were estimated from correlation proposed by Hong He (1998) that accounts for the molecular weights and critical properties of all compounds in addition to the  reactor operation conditions and reaction mixture density.² The following conditions have been considered in developing this model: temperature ranged from 220 - 250°C, pressure ranged from 35-80 bar, and H₂/CO feed ratios ranged from of 1/1-3/1, with supercritical solvent hexanes/syngas molar ratio ranged from of 1/1-4/1. The catalyst effectiveness factor and concentration profiles of reactants and products along the pellet radius were estimated for 1 mm size catalyst particles. A single chain growth probability (α-value) model was assumed in all the simulations (i.e. standard ASF product distribution). The other catalyst properties such as porosity, pore volume, surface area, and tortuosity factor were adopted from the properties of commercial Co/Al₂O₃ pellets.

Considering the aforementioned operating conditions, our findings with regards to the reactants concentration profile inside the catalyst pores showed a rapid drop of CO concentration along the catalyst radius, specifically within the initial layers of the catalyst particle. On the other hand, the higher diffusivity of hydrogen creates intense hydrogen rich environment inside catalyst particles, which can lead to the termination of the chain growth process as well as to unwanted reactions such as methanation. The trends of catalyst effectiveness factor with operating parameters indicate that the catalyst particle effectiveness factor is proportional to the fugacity of CO in the bulk phase instead of its effective diffusivity. Our findings also showed that the isothermal catalyst effectiveness factor is only sensitive to the operating temperature specifically at low pressures, which is the region of the critical pressure of the supercritical solvent.

References

1. Mogalicherla, A.K. and Elbashir, N.O. Development of kinetic model for supercritical Fischer-Tropsch synthesis. Energy & Fuel 2011, 25(3), 878-889.
2. Hong He, C. Infinite-dilution diffusion coefficients in supercritical and high-temperature liquid solvents. Fluid Phase Equilibria 1998, 147, 309-317