Separation of Fischer−Tropsch Wax from Catalyst Using Near-Critical Fluid Extraction: Analysis of Process Feasibility

Abstract

The technical feasibility of a near-critical fluid extraction (NCE) process for the recovery of heavy normal paraffins from a Fischer−Tropsch slurry reactor has been analyzed. Process simulations were carried out using the ASPEN PLUS program, considering 100 individual compounds from C1 to C100. Four light solvents were evaluated: n-pentane, n-hexane, n-heptane, and n-octane. These four compounds are major products of the F−T reaction. Most of the analysis was concentrated in two regions: (1) high solvent/product ratios (ca. 20/1), such that product could be recovered by temperature-retrograde condensation; and, (2) low solvent/product ratios (ca. 3/1). The latter region appeared to require higher extraction temperatures and higher slurry flowrates, but had several attractive features such as lower vapor flowrates, lower solvent makeup rates, and lower energy requirements. The concentration of solvent in the product from the NCE process was never low enough for the process to be self-sufficient in solvent, when the flowsheet contained only one product recovery unit. Self-sufficiency was obtained with multiple recovery units. The NCE process appears to be feasible, and has many attractive features. However, the existing thermodynamic database is not sufficient to support an optimum process design.