(22c) Systematic Approach for Selecting Hydrocarbon Solvents to Enhance Fischer Tropsch's Catalyst Bed in Situ Behavior

The current focus on the commercial Fischer-Tropsch (FT) reactor technology is on two types of reactors; the first one is the fixed bed reactor, which is part of Shell’s Pearl Project in Qatar, the world largest gas-to-liquid (GTL) plant. The second one is the slurry reactor, which is part of Sasol’s Qatar GTL plant the OryxGTL plant. Despite the fact that the FT reactor is not as expensive as the reformer or the gasification units of the GTL plant, it still represents the heart of the technology, whereby the processes before and after mainly depend on the operation requirements and performance of the FT reactor. For increasing the total conversion, special consideration must go into improving mass transfer to and from the catalyst particles and controlling the thermal stability along the reactor bed. These two aspects often go hand in hand and have been investigated since the invention of this chemistry by Fischer and Tropsch. Application of a solvent helps with heat transfer as well as it improves the in situ extraction of the heavy hydrocarbons from catalyst pores. Utilization of hydrocarbon solvents as reaction media for FT has been considered as part of the operation in some commercialized GTL processes to assist in catalyst regeneration cycles. However, current research and development efforts now also consider these solvents as suitable reaction media to facilitate properties in between gas phase and liquid phase by employing them as (near) supercritical fluids[1]. Supercritical hydrocarbon solvents were found to provide more tunable product distribution and can selectivity maximize specific fuels yields without the need for conventional high temperature FT (HTFT) or low temperature FT (LTFT) operation conditions, which require different reactors and/or catalysts.[2] In this study we will describe our systematic approach to select appropriate hydrocarbon solvent(s) for this process drawing on different investigations, such as: (1) experimental and modeling studies to understand the influence of these solvents on the phase behavior of the reaction media [3]; (2) safety assessments of these solvents as part of the GTL process and methods to select the best solvent accordingly [4]; (3) solvent recoverability based on optimized separation cycle to minimize energy use [5]; and finally (4) techno economic evaluation of the whole GTL process in the presence and absence of solvents. The optimization of the global process using ASPEN Plus for parts (3) and (4) was found to be more complex, and therefore the global energy integration has been performed to target the minimum energy cost. This novel procedure provided us with number of applicable hydrocarbon solvents for FT that have the potential to enhance the overall performance of the FT fixed-bed reactor without considerable modifications in the existing reactor structure whilst the utilizing typical separation units of GTL plant albeit at a bigger size.

References:

1. 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.
2. Elbashir N. O. ; Roberts C. B. (2005) "Enhanced Incorporations of  -olefins in the Fischer-Tropsch Synthesis Chain-Growth Process Over an Alumina Supported Cobalt Catalyst in Near-Critical and Supercritical Hexane Medium" Industrial & Engineering Chemistry Research. 44, 505-521.
3. 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.
4. Hamada N. A., El-Halwagi M. M., Elbashir N.O., Mannan S. M. (2012) “Safety Assessment of Potential Supercritical Solvents for Fischer-Tropsch Synthesis.” Journal of Loss Prevention in the Process Industries. Accepted.
5. Bao B., Elmalik E, E., Elbashir N. O. , El-Halwagi M. M., “Elevated Pressure Supercritical Fischer-Tropsch Synthesis: Product Separation and Energy Integration” Proceedings of the 10^th  International Symposium on Supercritical Fluids. San Francisco, California May 2012.