Gas-phase Fischer-Tropsch Synthesis Research Articles

Effect of catalyst shape and multicomponent diffusion flux models on intraparticle transport-kinetic interactions in the gas-phase Fischer-Tropsch synthesis

Intraparticle Fischer-Tropsch (FT) transport-kinetics interactions for various Fe-based catalyst particle shapes (sphere, solid cylinder, hollow cylinder, 4-hole cylinder, modified 4-hole cylinder, and 7-hole cylinder) under typical gas-phase FT process conditions (T = 493–533 K, P = 25–30 bar, and H2/CO = 2) are investigated using different multicomponent diffusion flux models. A Fe-based microkinetic gas-phase FT model is used to describe the intrinsic reaction rates for all reactants and products. The Soave-Redlich-Kwong (SRK) EOS is used to describe the vapor-liquid equilibrium associated with the FT product distribution. The effect of process conditions and catalyst shape on the effectiveness factor, particle volume-averaged key component concentrations, liquid-to-vapor (L/V) ratios and diesel range concentration profiles, along with methane-based diesel selectivities, are presented. It is found that the solid cylinder has the highest volume-average diesel concentration (ca. 4.4 mol/m3 at T = 493 K and P = 25 bar) when compared to the other shapes. The hollow cylinder and 7-hole cylinder shapes have the lowest intraparticle liquid-to-vapor (L/V) ratio (ca. 0.004 at T = 493 K and P = 25 bar) and the highest effectiveness factor (η) of 0.35. Temperature-based diffusivity correlations do not consider the change in effective diffusivities of the species in the FT reaction network. The simulations show that the inclusion of Knudsen diffusion through the Wilke-Bosanquet and Dusty-Gas Models resulted in an intraparticle FT product distribution that closely approximate literature results.

Product Analysis of Supercritical Fischer-Tropsch Synthesis: Utilizing a Unique On-Line and Off-Line Gas Chromatographs Setup in a Bench-Scale Reactor Unit

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].

Fischer-Tropsch Synthesis over Skeletal Co@HZSM-5 Core-Shell Catalysts

We used skeletal Co as the core to prepare a skeletal Co@HZSM-5 core-shell catalyst by growing an HZSM-5 membrane on skeletal Co via hydrothermal synthesis. The physicochemical properties of the catalyst were determined using elemental analysis, N2 physisorption, X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and NH3 desorption. In gas-phase Fischer-Tropsch synthesis (FTS), the skeletal Co@HZSM-5 core-shell catalyst was more efficient than a physically mixed skeletal Co-HZSM-5 catalyst in cracking long-chain hydrocarbons, giving higher selectivity for C5-C11 gasoline products. The thickness of the zeolite shell on the skeletal Co@HZSM-5 core-shell catalyst was easily tuned by adjusting the hydrothermal time. At a suitable zeolite shell thickness, the long-chain hydrocarbons were cracked completely, with high FTS activity, leading to high selectivity for the gasoline fraction. Increasing the reaction temperature resulted in higher

Middle distillates production via Fischer–Tropsch synthesis with integrated upgrading under supercritical conditions

A vertically aligned fixed‐bed reactor system with a cascade of three sequential catalyst beds has been used to incorporate Fischer–Tropsch synthesis (FTS) in the first bed, oligomerization (O) in the second bed, and hydrocracking/isomerization (HC or C) in the third bed (FTOC). Compared to gas phase FTS (GP‐FT) alone, gas phase FTS with the subsequent upgrading beds (GP‐FTOC) is demonstrated to result in a reduction in the olefin selectivity, a reduction in the C26+ selectivity, and a marked enhancement in the production of branched paraffins and aromatics. Utilization of supercritical hexane as the reaction medium in supercritical FTS (SC‐FT) and supercritical phase FTOC (SC‐FTOC) resulted in a significant reduction in both CH4 selectivity and CO2 selectivity. Interestingly, significant amounts of aldehydes and cyclo‐paraffins were collected in SC‐FT and in SC‐FTOC, respectively, while not being observed in traditional gas phase operation (both GP‐FT and GP‐FTOC). © 2014 American Institute of Chemical Engineers AIChE J, 60: 2573–2583, 2014

Enhancement in the intraparticle diffusion in the supercritical phase Fischer–Tropsch synthesis

The objective of this study is to provide a systematic assessment of the intraparticle mass transfer diffusional limitations in a Fischer–Tropsch synthesis (FTS) tubular fixed bed reactor under both conventional gas-phase media and supercritical phase media. Our results showed that the measured catalyst effectiveness factors are proportional to both the fugacity of CO in the bulk phase as well as to its effective diffusivity. The diffusional flux of CO in catalyst pores in the gas phase FTS was found to be higher than that of the supercritical phase FTS, but only at the entrance of the reactor bed. As the conversion increases, the reaction products completely fill the catalyst pores in the gas phase FTS, which results in drastic decrease in the CO concentration and diffusional flux in catalytic pores. Therefore, the catalyst effectiveness factor at the reactor exit drops more than 65% to that of the reactor entrance value. Nevertheless, in supercritical phase FTS, the variations in diffusional flux with conversion are not as intense, therefore the catalyst effectiveness factor along reactor length vary within 20% of the reactor entrance value.

Supercritical Activity Restoration for Fischer Tropsch Synthesis

Previous research has demonstrated that supercritical phase Fischer Tropsch Synthesis (SC-FTS) using hexanes as the solvent medium offers superior activity maintenance to traditional fixed-bed (gas phase) Fischer Tropsch Synthesis (GP-FTS) (Elbashir et al., 2005). This has prompted an investigation into whether supercritical media can reactivate a catalyst used in GP-FTS. To that end, a study was conducted in which a series of GP-FTS experiments of approximately 2 days each were performed, separated by in situ periods of catalyst reactivation using supercritical hexanes as well as SC-FTS.It was found that the supercritical hexanes media and SC-FTS reactivation periods were effective in partially restoring lost catalyst activity and selectivity. GP-FTS was demonstrated to have detrimental effects on subsequent SC-FTS operation while SC-FTS operation showed beneficial effects on subsequent GP-FTS operation. Specifically, SC-FTS reactivations reduced the methane selectivity in subsequent GP-FTS periods. The CO2 selectivity in SC-FTS is consistently lower than that in GP-FTS, while the olefin selectivity into the diesel range is higher in SC-FTS. While the propagation probability was initially higher in SC-FTS than GP-FTS, the propagation probability values converged as the experiment progressed.

Rapidly Quenched Skeletal Fe-Based Catalysts for Fischer−Tropsch Synthesis

A novel, rapidly quenched skeletal Fe catalyst (RQ Fe) has been prepared by alkali leaching of the Fe 50 Al 50 alloy solidified by the rapid quenching technique and tested in gas phase Fischer—Tropsch synthesis (FTS). Characterizations demonstrate that the RQ Fe catalyst has larger specific surface area, smaller crystallite size, and higher population of the Fe(111) surface than the conventional Raney Fe catalyst prepared from the naturally solidified Fe 50 Al 50 alloy. As compared to Raney Fe, which has FTS activity equivalent to the precipitated Fe catalyst while higher than the fused Fe catalyst, RQ Fe is 25% more active. Promotion of the RQ Fe catalyst with Mn or K further improves the FTS activity, selectivities to alkenes and higher alkanes, as well as the catalytic stability, showing that the rapid quenching technique is promising in the preparation of skeletal Fe-based catalysts with improved FTS performance.