(532d) A Step Towards Scaling Up the Design of a Non-Conventional Fischer Tropsch Reactor: Validation of Simulation Studies On Heat and Mass Transfer Behavior Inside the Reactor Bed

The Fischer Tropsch synthesis is the focal process in which the synthesis gas (a CO and H₂ mixture) is converted to ultra-clean liquid based fuels and value-added chemicals. Many factors impede the current commercial FTS reactor technologies which can mainly be attributed to transport and thermal limitations due to the complex nature of the reaction. Our efforts have been mainly focusing on scaling up reactor technology that facilitate running the reaction in the supercritical fluid (SCF) media to leverage several advantages over the conventional technologies [1]. We started with a series of macro and micro scale investigations aiming at better understanding the phase behavior of the non-ideal reaction mixture while simultaneously identifying its influence on the reaction kinetics [2] and the in-situ diffusion behavior [3] of the reactants and products inside catalyst pores. On the macro scale investigations we conducted series of process integration and optimization studies coupled with the development of sophisticated dynamic control systems [4]. Furthermore, we identified the most applicable solvent(s) for the SCF-FTS using different but interrelated approaches that include phase behavior, techno-economic and safety analysis for the proposed process [5]. These investigations supported our efforts on building a bench-scale reactor in order to verify several simulation studies, which is the focus of this communication.

Our modeling efforts to scale up this reactor initially focused on establishing sufficient tools and parameters to differentiate between operating the fixed-bed reactor on either conventional gas phase or in SCF-FTS media. We utilized different diffusion rates into the catalyst particle throughout the catalyst bed as well as different assumptions on the amount of wax and liquids present in capillary pores and the bulk phase [2, 3]. We utilized different diffusion rates into the catalyst particle throughout the catalyst bed as well as different assumptions on the amount of wax and liquids present in capillary pores and the bulk phase.

The reactor bed behavior is normally combined with a severe pressure/temperature gradient depending on the reaction media. In essence, the catalyst bed has been modeled as various sections whereby different regimes exist and local conditions apply. By generating diffusion profiles for each regime we were able to determine a catalyst effectiveness profile over the catalyst bed for the specified rate of reaction expression. Our efforts continued by simultaneously calculating the effect of heat and mass diffusion in and out of the particles through different media such as wax and supercritical hexane. The uniqueness of our model is that it is capable of simultaneously evaluating the catalyst effectiveness profiles that relates in and out catalyst pores while at the same time measures the temperature profiles inside catalyst pores The results presented in Figure 1a show an example of the diffusion of CO from the catalyst boundary inwards while the composition of the fluid inside the catalyst pore is changing as a result of CO consumption. In one case we have considered the catalyst pores as filled with supercritical hexane (equivalent to SCF-FTS), and in the second case we have considered the pores to be filled with wax (equivalent to gas-phase FTS). These reaction mixtures have different effective diffusivities, resulting in different levels of penetration of the reactant CO into the catalyst capillary pores. For this reason, the level of H₂ remains near constant and high throughout the catalyst particle. Figure 1b shows the temperature distribution changes when modeling for SCF-hexane as the dominating reaction mixture whereby the obtained profile shows almost isothermal behavior. This is in an excellent agreement with our previous experimental investigations of the lab scale reaction behavior as reported for the SCF-FTS [4].  Nevertheless, scaling up the SCF-FTS reactor technology requires experimental validation of the capabilities of these simulations models to predict the reaction behavior over a wide range of operating conditions and catalyst bed sizes, which will be the focus of the current study utilizing our sophisticated bench scale reactor bed.

Figure 1a, and b: (a) Left: intraparticle CO diffusion into the particle. (b) Right: intraparticle temperature distribution as heat is generated by the product synthesis inside the catalyst particle.                                                          

Related Literature:

1. Blank J., Bani Naser L., Hussain, R., Elbashir, N. O. (2013) “Modeling of Heat and Mass Transfer Limitations in Fischer Tropsch’s Fixed Bed Reactor: Comparisons between Supercritical Phase and Gas Phase” 10^th Natural Gas Conversion Symposium (NGCS) Extended Abstract, Synfuels Session, Conference Proceedings, January 2013, Doha, Qatar.
2. Mogalicherla A., Elbashir N. O. (2011) "Development of a Kinetic Model for Supercritical Fluids Fischer Tropsch Synthesis" Energy & Fuels; 25(3); 878-889.
3. Mogalicherla A., Elmalik E; Elbashir N. O.  (2012) "Enhancement in the Intraparticle Diffusion in the Supercritical Phase Fischer-Tropsch Synthesis" Chemical Engineering and Processing: Process Intensification, 62; 59-68.
4. Elbashir N. O., Bukur D. B., Durham E., Roberts C. B. (2010) “Advancement in Fischer-Tropsch Synthesis via Utilization of Supercritical Fluids as Reaction Media” AIChE Journal; 56 (4)  997-1015
5. Elmalik E., Tora, E., El-Halwagi M. M., Elbashir N. O. (2011) "Solvent Selection for Commercial Supercritical Fischer-Tropsch Synthesis Process" Fuel Processing Technology, 92; 1525-1530.