Iron FT catalyst

The phase transformations in the catalyst play an important role in determining the activity, attrition resistance, and deactivation of this catalyst. Activation of this precipitated catalyst transforms single crystals of hematite to smaller crystallites of carbide. While the transformation from hematite to magnetite is extremely rapid, the magnetite to carbide transition is much slower under the conditions of temperature and pressure employed in this study. As carbon deposits on the carbide particles, it serves to further prise the carbide particles apart. In a commercial slurry phase reactor the carbide particles break away leading to catalyst attrition. The implication of this work for the attrition resistance of iron FT catalysts is explored in detail elsewhere.18... 

The various factors that can contribute to deactivation of iron Fischer-Tropsch (FT) catalysts include transformation of the active phase into an inactive constituent, poisoning by carbonaceous species and heteroatoms, and loss of active phase surface area. Progress in elucidating the causes of deactivation is hampered by the inability to conclusively identify the active phase in iron FT catalysts. In recent work involving doubly promoted, unsupported iron catalysts, the sequence of phase transformations shown in Figure 1 that take the catalyst from its as-prepared hematite phase to iron carbide [1,2] was postulated. 

Previous work has indicated that attrition in iron FT catalysts is caused not just by a physical process, but that two chemical factors contribute to FT iron catalyst attrition. One is the catalyst transformation from magnetite to carbide which causes the crystals of magnetite to nucleate into ciystallites of the carbide phase [1,2]. The other cause of attrition is the deposition of carbon on the carbide surfaces which causes the carbide crystallites to further separate from one another [1,2]. Considering the discussion above, and the role carbide and the carbon covering the carbide surface plays in the activity of the catalyst, attrition resistance and activity may be at odds against one another. 

Figure 6. Relative amount of amorphous, carbidic, and graphitic carbon on unpromoted and potassium-promoted iron FT catalysts after 24 h on stream at a reaction temperature of 215°C.

Increasing and decreasing deactivation rates of iron FTS catalysts are obserx ed upon the addition of potassium and/or silicon respectively. There is a synergism in the maintenance of activity with the addition of both potassium and silicon leading to a low deactivation rate. 

Two iron FTS catalysts with an atomic ratio of K Fe=10 100 and Be Fe=1.44 100 were prepared and utilized in this study. Precipitated iron catalysts were prepared using Fe(N03)3 9H20 tetraethyl orthosilicate, Cu(N03)2 3H20, and K2CO3 or Be(N03)2 was used as the promoter precursor. Details of the preparation procedure was given elsewhere (5). In this study, the potassium promoted iron catalysts were pretreated with CO at 270°C and 1.2 MPa for 24 hours. The CO flowed through a catalyst slurry in 300 ml of Ethylflow oil... 

Potassium promoted iron FTS catalyst showed a low deactivation rate of 0.083% per day after passing an initial conditioning period of 300 hours. Higher temperature generated a shorter conditioning period. Beryllium promoted iron catalyst produced an even more stable activity than potassium. A deactivation rate as low as 0.0062% per day was obtained from beryllium promoted catalyst. For potassium promoted catalyst, FTS activity, CO2 selectivity, hydrocarbon productivity and water gas shift activity showed the same conditioning period of 300 hours of time on stream after passing a peak value at about 120 hours of reaction time. Methane selectivity, however, showed a monotonous decrease from an initial maxima to a stable level of 1.6%. 

Figure 1. Deactivation rate over potassium promoted iron FTS catalyst...

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. 

Table 1 Comparison of Cobalt and Iron FT Catalysts (Khodakov et al., 2007)...

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. 

Iron as catalyst for FT synthesis does not exhibit the dynamic characteristics observed with cobalt26 (Figure 9.14, left). 

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... 

The objective of the present study is to develop a cross-flow filtration module operated under low transmembrane pressure drop that can result in high permeate flux, and also to demonstrate the efficient use of such a module to continuously separate wax from ultrafine iron catalyst particles from simulated FTS catalyst/ wax slurry products from an SBCR pilot plant unit. An important goal of this research was to monitor and record cross-flow flux measurements over a longterm time-on-stream (TOS) period (500+ h). Two types (active and passive) of permeate flux maintenance procedures were developed and tested during this study. Depending on the efficiency of different flux maintenance or filter media cleaning procedures employed over the long-term test to stabilize the flux over time, the most efficient procedure can be selected for further development and cost optimization. The effect of mono-olefins and aliphatic alcohols on permeate flux and on the efficiency of the filter membrane for catalyst/wax separation was also studied. 

Therefore, when operating in the filter cake mode, the axial velocity should be maintained at a level such that an adequate shear force exists along the filter media to prevent excessive caking of the catalyst that could cause a blockage in the down-comer circuit. For the separation of ultrafine catalyst particles from FT catalyst/wax slurry, the filter medium can easily become plugged using the dynamic membrane mode filtration. Also, small iron carbide particles (less than 3 nm) near the filter wall are easily taken into the pores of the medium due to their low mass and high surface area. Therefore, pure inertial filtration near the filter media surface is practically ineffective. 

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. 

For FT catalysts operating below about 250 °C it is found that as the H2/CO ratio in the gas phase increases the average molecular weighr of the product decreases, as well as the amounts of olefins formed. This is in keeping with the mechanism depicted in Figure 4. For iron catalysts operating at temperatures above 300 °C the simple H /C0 ratio does not correlate well with selectivity. A more complex 0 5... 

After performing FT synthesis on an unreduced iron oxide catalyst, Kuivila et al.12 observed 22% carbide in the bulk by Mossbauer spectroscopy, but only —3% carbide on the surface by XPS, and therefore concluded that a sub-surface carbide phase had formed beneath a magnetite surface layer. Based in part on this result, they conclude that magnetite is the active phase for FT synthesis. Reymond et a/.10 also observed substantial amounts of carbide by XRD, but little or no carbide by XPS. The observation of a 2-4 nm thick carbon layer on the carbide phase, but not on the magnetite, allows a reinterpretation of the data in these two papers. Sputtering of the surface carbon layer permits the XPS to see the underlying carbide, and therefore it is not necessary that the carbide be present beneath an oxide layer. Thus, measurement of low carbide signals by XPS cannot be interpreted to mean that carbide is absent from the catalyst surface, and therefore not an important phase in FT... 

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