(124a) Production of Middle Distillate Range Liquid Fuels From Syngas Using Fischer-Tropsch Synthesis and Associated Upgrading Technology Under Supercritical Phase Conditions and Multiple Reactor Configurations

Fischer-Tropsch Synthesis (FTS) is a nearly century old method for producing low sulfur liquid fuels from a carbon monoxide and hydrogen gas feedstock. Dating back to Germany between WWI and WWII, FTS has been used industrially in South Africa for over 50 years. FTS is the highly exothermic surface-catalyzed hydrogenation and polymerization of carbon monoxide. Traditionally, the reaction is conducted on either an iron or cobalt based catalyst in either a gaseous or slurry phase. Under these reaction conditions, FTS has a few significant problems. As a polymerization reaction which has generally been shown to follow Anderson-Schulz-Flory (ASF) reaction kinetics, FTS inherently produces a broad range of products including gaseous light hydrocarbons, fuel range liquids, heavy waxes, CO₂ and water. Additionally, FTS products consist primarily of n-paraffins and 1-olefins with very few branched compounds, aromatics, or oxygenates. This leads to low octane numbers and poor cold flow properties, which are unfavorable for both gasoline and diesel engines. Traditional FTS also has a high selectivity towards CH₄ and CO₂ whichare unwanted side products whose production leads to lower carbon efficiency. 

This study sought to enhance FTS productivity of fuel-range (C₅-C₂₂) hydrocarbons by modifying the traditional mode of operation in two ways. First, the reactor configuration was altered in order to shift the product distribution towards the target hydrocarbon range and also to produce more branched and aromatic compounds. Initially, two additional reaction beds were added sequentially below the FTS reaction bed. The three bed system consisted of a traditional FTS reactor loaded with Fe-based catalyst, an oligomerization bed loaded with amorphous silica alumina (ASA), and a hydrocracking/isomerization bed utilizing 1 wt. % Pd loaded onto ASA. The additional reactors served to oligomerize light olefins and crack heavy waxes down towards the target product range. The product distribution of this system was then compared to that of a single-bed FTS reactor as well as to that of a single bed reactor loaded with a mixture of Pd-ASA cracking catalyst and Fe based FT catalyst.

As a second modification to the traditional FTS process, the reactions were carried out in a supercritical fluid (hexanes) medium. Supercritical fluids (SCFs) have liquid-like densities and heat transfer properties, have high diffusivities, are miscible with gases, and can be excellent solvents for both liquids and waxy products. Gas phase FTS reactors suffer from poor heat management, wax buildup on catalyst active sites, and poor economies of scale. In contrast, slurry phase FTS reactors have better heat management, but suffer from interphase diffusion resistance, low conversion, catalyst abrasion, media maintenance complications, and difficult product separation. The unique properties of SCFs help to offset the problems of both gas and slurry phase systems by allowing gaseous reactants and liquid products to freely diffuse to and from surface reaction sites on the catalyst as well as by aiding in the removal of excess heat of reaction.

Relative to traditional gas phase FTS, the three bed system showed a significant shift of products towards middle distillates, an increase in aromatics and branched paraffin production, and a reduction in olefins. The further addition of SCF markedly reduced CH₄ and CO₂ selectivity, increased heavy product yields, and helped maintain the catalyst activity.