Iron-based FTS catalysts

Cobalt- rather than iron-based FT catalysts have been examined, in order to minimize the competing water-gas shift reaction, which would result in a lowered carbon efficiency. Most cobalt FT catalysts have been prepared by coprecipitation of Co salts with various promoters onto a slurried oxide support to afford mixed phase systems (J ). Reduction to the active catalyst was controlled by addition of various promoters (e.g. MgO, Th02, AI2O3) (2). In part, these promoters are necessary to maintain good metal dispersion in the catalyst and resistance to sintering. Dispersion... 

Traditionally, iron-based catalysts have been used for FT synthesis when the syngas is coal derived, because of their activity in both FTS and WGS reactions. Complex mixtures of straight-chain paraffins, olefins, and oxygenate (in substantial proportions) compounds are known to be formed during iron-based FTS. Olefin selectivity of iron catalysts is typically greater than 50% of the hydrocarbon products at low carbon numbers, and more than 60% of the produced olefins are a-olefins.13 For iron-based catalysts, the olefin selectivity decreases asymptotically with increasing carbon number. 

Practically, natural gas following partial oxidation gives essentially the desired H2 CO ratio of 2 1 and water-gas shift inactive cobalt-based FT catalysts can be used to produce these hquid fuels. However, in the case of coal, water-gas shift is effected in situ using an iron-based catalyst Alternatively, the hydrogen fraction in the syngas feed can be maximized by subsequently applying the exothermic water-gas shift reaction (7.9) ... 

The overall goal of this study is to develop superior catalysts suitable for use in modem advanced slurry phase or membrane reactors. Successful investigations have produced an iron-based unsupported catalyst with high activity and extended catalyst life (improved attrition resistance). This catalyst was used by the authors to understand the phase transformations from iron oxide precursors after activation to iron carbide crystallites and to characterize the presence of characteristic amorphous carbon species that have been reported to envelop the spent Fe-FTS catalyst grains in form of surface layers. Previous studies suggest two different phases to be considered as the active phase including iron oxide and a mixture of x snd e -carbides (Table 1) and in some instances minor amounts of metallic iron, however the spatial distribution of the different... 

CO can be converted into either hydrocarbon products and water (via FTS) or C02 and Fl2 via the water-gas shift (WGS) reaction. The reversible WGS reaction accompanies FTS over the iron-based catalyst only at high temperature conditions. The individual rates of FTS (rFTS) and the WGS reaction (rWGS) can be calculated from experimental results as rWGS = r(,and rFTS = rco-rc02, where rCo2 is the rate of C02 formation and rco is the rate of CO conversion. 

A continuous cross-flow filtration process has been utilized to investigate the effectiveness in the separation of nano sized (3-5 nm) iron-based catalyst particles from simulated Fischer-Tropsch (FT) catalyst/wax slurry in a pilot-scale slurry bubble column reactor (SBCR). A prototype stainless steel cross-flow filtration module (nominal pore opening of 0.1 pm) was used. A series of cross-flow filtration experiments were initiated to study the effect of mono-olefins and aliphatic alcohol on the filtration flux and membrane performance. 1-hexadecene and 1-dodecanol were doped into activated iron catalyst slurry (with Polywax 500 and 655 as simulated FT wax) to evaluate the effect of their presence on filtration performance. The 1-hexadecene concentrations were varied from 5 to 25 wt% and 1-dodecanol concentrations were varied from 6 to 17 wt% to simulate a range of FT reactor slurries reported in literature. The addition of 1-dodecanol was found to decrease the permeation rate, while the addition of 1-hexadecene was found to have an insignificant or no effect on the permeation rate. 

Iron-based Fischer-Tropsch synthesis (FTS) catalysts are preferred for synthesis gas with a low H2/CO ratio (e.g., 0.7) because of their excellent activity for the water-gas shift reaction, lower cost, lower methane selectivity, high olefin... 

At this point, the system was tested with catalyst for activation and FTS, in the hopes that the seal leak rates would be impeded by the presence of small catalyst particles. The FTFE 20-B catalyst (L-3950) (Fe, 50.2% Cu, 4.2% K, 1.5% and Si, 2.4%) was utilized. This is part of the batch used for LaPorte FTS run II.20 The catalyst was activated at 543 K with CO at a space velocity (SV) of 9 sl/h/g catalyst for 48 h. A total of 1,100 g of catalyst was taken and 7.9 L of C30 oil was used as the start-up solvent. At the end of the activation period, an attempt was made for Fischer-Tropsch synthesis at 503 K, 175 psig, syngas SV = 9 sl/h/g catalyst, and H2/CO = 0.7. However, the catalyst was found to be completely inactive for Fischer-Tropsch synthesis. Potential reasons for catalyst poisoning under present experimental conditions were investigated. Sulfur and fluorine are known to poison iron-based Fischer-Tropsch catalysts.21,22 Since the stator of the pump is... 

The Fischer-Tropsch Synthesis (FTS) converts synthesis gas (a mixture of CO and H,) to hydrocarbons. Iron-based catalysts lose activity with time on stream (TOS). The rate of deactivation is dependent on the presence/absence of promoters such as potassium and/or binders such as silica [1.2]. Several possible causes of catalyst deactivation have been postulated [3] (i) Sintering, (ii) Carbon deposition, and, (iii) Phase transformations. With respect to phase transformations, there is considerable disagreement whether the active phase for the FTS is iron oxide or carbide [4,5]. In addition, certain reactor conditions, such as a high partial pressure of water, are known to cause a decline in activity [6]. 

Four precipitated iron-based catalysts were used. The first catalyst consisted of only iron. The other catalysts contained either added potassium, added silicon or both. The catalysts were designated in terms of the atomic ratios as lOOFe, 100Fe/3.6Si, 100Fe/0.71K and 100Fe/3.6Si/0.71K. The catalysts were prepared by continuous precipitation from iron (111) nitrate and concentrated ammonium hydroxide. For silica-containing catalysts, a colloidal suspension of tetraethyl ortho silicate was mixed with the iron nitrate solution prior to precipitation. Potassium was added to the catalysts in the form of potassium tertiary butoxide during the loading of the FTS reactor. 

Iron-based Fischer-Tropsch (FT) catalysts undergo a series of phase transformations during activation and use (1). Activation with carbon monoxide or syngas typically results in the conversion of Fe O to Fe O and ultimately to one or more iron carbides (2). During FT synthesis, iron carbides can be oxidized to Fe O if the or COj/CO ratios are high... 

Most of the more recently proposed GTL concepts are based on the use of the LTFT three phase fluid bed reactor. This development can also overcome the fixed bed reactor limitations on the temperature control of the very exothermic FT reaction. Additionally, a higher catalyst utilisation efficiency is possible and, as mentioned previously, larger reactor capacities can be attained, compared with tubular fixed bed units. The first commercial scale LTFT slurry reactor, a 2 500 bpd unit, was commercialised by Sasol in 1993 and is in continuous operation using an iron FT catalyst. 

Figure 1.6 Transformations of iron species on the catalyst during different steps in Fischer—Tropsch/water gas shift (FT/WGS) regimes in an FT reactor with iron-based catalysts (Pirola et al., 2009).

For fixed-bed reactors, upper limits to the temperatures should be used. For iron-based catalysts, for instance, carbon deposition occurs at higher temperatures (see section Carbon Deposition during the FT Synthesis ) and this would result in catalyst swelling and blockage of the reactor tubes. Both Co- and Fe-based catalysts operating at higher temperatures result in increases in the selectivity of the undesired methane and decreases in the desired heavier hydrocarbons. 

A study of the product selectivites of variously supported Co catalysts (kieselguhr, silica, alumina, bentonite, Y-zeolite, mordenite, and ZSM-5) was carried out by Bessel (37). AAdiereas the lower acidity supports such as silica and alumina produced mainly linear hydrocarbons, the acidic supports produced more branched products. At higher temperatures, the latter produced aromatics as well. The isomerization and aromatization are secondary, acid-promoted reactions of the FT olefins. This is then equivalent to a combination of the FT and the Mobil olefins to gasoline process. (With iron-based catalysts, this approach is unlikely to be successful because alkali promotion is essential and the alkali would neutralize the required acid sites on the zeolite support.) Calleja and coworkers (38) studied the FT performance of Co/HZSM-5 prepared by incipient wetness impregnation. Promotion with thorium, being basic, decreased the acidity of the zeolite and so less aromatics were formed and consequently more of the heavier hydrocarbons emerged from the reactor because of the depressed level of secondary reactions. 

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