(68f) Modeling Diffusional Limitations In Fischer Tropsch Synthesis Catalyst at near Critical and Supercritical Solvent Conditions

Aswani K Mogalicherla, Elfatih E.Elmalik, Nimir.O.Elbashir*

Chemical Engineering Program, Texas A&M University at Qatar, PO Box 23874, Doha Qatar

*Corresponding author: nelbashir@tamu.edu

Abstract: Conducting Fischer-Tropsch synthesis (FTS) reaction in near critical and supercritical fluid (SCF) media has been demonstrated to have certain advantages over the traditional routes because of the unique characteristics of the supercritical phase. It combines the desirable properties of gas-like diffusion along with liquid-like heat transfer and solubility to overcome many of the limitations of the current industrial FTS reactors.¹ In the process of tuning transport properties, there is a possibility that the reaction moves from a kinetic control regime to diffusion control regime. Therefore, analyzing only the variations in transport properties (diffusivity, density and viscosity) with operating conditions is not a sufficient task for optimal reactor design. The relative change in the diffusion rate with respect to the rate of reaction has to be determined for identifying the controlling regime.² Very few studies have been reported on diffusional limitations in FTS catalysts at supercritical conditions.³ Those studies were not accounting the details of reaction kinetics, product distribution and density dependency of diffusivities. In most of the cases first order reaction kinetics were assumed, which is not a valid assumption. Even though a large number of kinetic models for FTS have been developed, these models were not used in estimating diffusional effects in FTS catalysts.

The objective of the present work is the assessment of diffusional limitations in fixed bed tubular reactors which typically contain catalyst particles of a few millimeter diameters. For this a conventional theory of diffusion-reaction in porous pellets is employed. To account the details of reaction kinetics, LHHW model developed by Mogalicherla et al. (2010) from cobalt catalyst at near critical and supercritical fluid (n-hexane) conditions was used.⁴ The binary diffusional coefficients of all reactants and products in supercritical solvent were estimated from correlation proposed by Hong He (1998) which accounts the molecular weights and critical properties of all compounds, reactor operation conditions and reaction mixture density.⁵ The following conditions have been considered in developing this model: temperature range from 220 - 260°C, pressure range from 35-80 bar, and H₂/CO feed ratios of 1-2, with supercritical solvent hexanes/syngas molar ratio of 3:1. The catalyst effectiveness factor and concentration profiles of reactants and products along the pellet radius were estimated in three different size catalyst particles (0.5mm,1 mm and 2 mm) and at three CO conversion levels (30, 60, 80%). A single chain growth probability (α-value) model was assumed in all the simulations. The variation of α and product distribution with conversion and operating conditions was taken into account from the experimental data of Elbashir and Roberts (2005).⁶ The other catalyst properties such as porosity, pore volume, surface area, and tortuosity factor were adopted from the properties of commercial Co/Al₂O₃ pellets. The effectiveness factor for LHHW type kinetics was estimated using an approximation method proposed by Hayes et al.( 2007).⁷ The optimal operating conditions (temperature pressure, density, one pass conversion and catalyst particle size) for SCF-FTS operated in a fixed bed reactor were identified.

References

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