Abstract
The utilization of supercritical fluids (SCF) in the Fischer-Tropsch Synthesis (FTS) further complicates the hydrocarbon products identification and analysis process due to the dilution of hydrocarbon peaks by the predominant solvent peak. Therefore, in this project, a custom-made Gas Chromatography (GC) analysis system was designed and implemented to identify and quantify SCF-FTS products. The FTS products were identified using two different methods. The first was through retention time matching by injecting standard solutions, and the second was through the use of the GC/MS system. The quantification of CO and CH4 was achieved by using external standards, where the CO conversion was calculated by relating the peak area of CO to the peak area of an internal standard (argon) while the CH4 selectivity was calculated by relating the peak area of CH4 to that of CO. After setting and calibrating the GC system, two reaction conditions (gas phase: 240°C, 20 bar syngas with 2:1 H2:CO molar feed ratio and for the supercritical fluids FTS (SCF-FTS): 240°C, 65 bar with 20 bar syngas partial pressure and 2:1 H2:CO molar feed ratio) were used to compare the different FTS reaction media. The comparison between the gas phase FTS and the SCF-FTS showed the following: carbon monoxide conversion was improved by 14% in the SCF-FTS, while the hydrocarbon product profile SCF-FTS showed 78% reduction in light hydrocarbons (C1 - C4) products, 35% increase in middle distillates (C11 - C22) products compared to gas phase FTS. These improvements have resulted in higher chain growth probability for the SCF-FTS (α = 0.85) compared to the gas phase FTS (α = 0.76). These results are generally in agreement with previously reported enhancement in the SCF-FTS[1].
Highlights
Escalating concerns about the unstable oil prices and oil supply insecurity coupled with stricter environmental regulations have catalyzed the interest in the production of synthetic fuels from alternative resources such as natural gas, coal and biomass via the X-to-Liquid (XTL) process or other process
The main difference between the Gas Chromatography (GC) product distributions under the two reaction mediums is the dilution of the Fischer-Tropsch synthesis (FTS) products in the supercritical fluids (SCF)-FTS, as illustrated and compared in Figure 4 and Figure 5
Gas phase FTS samples were primarily used for the product identification and the product identification was validated for the supercritical fluids FTS (SCF-FTS) samples and found to be in good agreement
Summary
Escalating concerns about the unstable oil prices and oil supply insecurity coupled with stricter environmental regulations have catalyzed the interest in the production of synthetic fuels from alternative resources such as natural gas, coal and biomass via the X-to-Liquid (XTL) process or other process. Fixed bed and slurry bubble reactors are the two main reactor types used in industrial Low-Temperature-Fischer-Tropsch (LTFT) [9]. Slurry reactors were designed to overcome the temperature control issue facing fixed bed reactors, as they operate almost isothermally due to the enhanced heat capacity of the liquid medium (slurry). It makes sense that the idealistic FTS reactor should combine the advantages of commercial FTS reactors while simultaneously eliminates their disadvantages Such a combination was found possible to some extent through the application of supercritical fluids (SCF) as a FTS reaction media [11] [16]. SCF reaction media was reported to have unique characteristics offering single-phase operation and combined the desired properties of the gas phase (high diffusion) and the liquid phase (high solubility and high heat transfer) [1]. The liquid-like densities and heat transfer properties ensure efficient product desorption and heat removal while gas-like diffusivities ensure high reaction rates and high product removal rate [17]
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