(265e) Supercritical Adiabatic Reactor for Fischer Tropsch Synthesis

The Fischer Tropsch reaction is highly exothermic, with an adiabatic temperature rise on the order of 1800^oC.  Managing the released heat is a major factor in Low Temperature Fischer Tropsch (LTFT) reactor design.  The two conventional LTFT reactor designs are fixed bed (with a high gas velocity in a large number of narrow tubes) and slurry bed (with a powder catalyst dispersed in a liquid wax media). 

Since 1989, when Kaoru Fjumoto’s research group first published the use of a supercritical fluid as the reaction media for fixed bed LTFT, a number of research groups have compared Supercritical LTFT (SC-FTS) with traditional fixed-bed LTFT (GP-FTS), consistently finding that SC-FTS gives lower methane and CO₂ selectivities and enhanced olefin or aldehyde selectivities in the gasoline and heavier fractions.  These benefits motivate the possible industrial utilization of SC-FTS.  However, the presence of the solvent and the higher reaction pressure lead to cost penalties.  In order for SC-FTS to be industrially viable, the costs the “upgrade” must be less than the benefits.

One of the less noted benefits SC-FTS is the decreased adiabatic temperature rise.  Simulation based on using hexane as the media at a ratio of 3.5 moles of media per mole of syngas drops the adiabatic temperature rise to 60^oC.  Consequently, it is feasible to carry out SC-FTS in a series of modules each with an adiabatic reactor and a heat exchanger.

While upgrading Sasol’s ARGE reactor for SC-FTS would approximately double its cost, preliminary economic analysis suggests that this system should have a cost comparable to the ARGE reactor, lowering the capital cost of a SC-FTS plant by approximately $1000 per BPD.  This reactor system also allows for far easier catalyst replacement and greater reactor control flexibility than the traditional ARGE reactor.

This work does not address the operating cost penalty of SC-FTS or the balance of plant capital cost penalty, though a thorough analysis of these costs is underway.