Poisoning Fischer-Tropsch catalysts

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... 

It is well known that all sulfur compounds rapidly deactivate iron, cobalt and nickel Fischer-Tropsch catalysts. However, due to the efficiency of modem gas purification processes such as (he Lurgi Kectisol process, the sulfur level in synthesis gas can be reduced below 0.03 tng/mj. Tiiis level is tolerable and a constant synthesis gas conversion can be achieved [15], Iron catalysis which have been poisoned by sulfur are not readily reactivated. Only very thorouglt reoxidation by which all traces of sulfur are burnt away efficienily. followed by reduction, is effective [15,21J. 

Within the Fischer-Tropsch research ECN Biomass concentrates on the definition of the gas cleaning with respect to the typical B R in urities, like NHj, HCl, HCN, H]S, COS, tars (heavy organic molecules), soot, and alkali metals, Traces (< ppm) of these compounds can already be a poison for the Fischer-Tropsch catalysts. For the implementation of B R and Fischer-Tropsch ECN its strategy is on the demonstration of integrated systems to reduce the time necessary to realise a first full-scale installation for conversion of biomass and residue, gas cleaning, and Fischer-Tropsch synthesis. To achieve this ECN focuses on two lines of development ... 

Inhibition of chemi-desorption reactions is probably mainly caused by the CO, which is known to be strongly adsorbed on the Fischer Tropsch catalyst metals and which is also known as a poison of hydrogenation reactions or a firmly bound tt-1 igand in coordination chemistry, which reacts via insertion. Thus in this paper a kinetic concept of Fischer Tropsch surface polymerization is developed, whereas the nature of surface species is only regarded in general. 

Fischer-Tropsch catalysts are sensitive to poisoning by sulfur-containing compounds. The sensitivity of the catalyst to sulfnr is greater for cobalt-based catalysts than for their iron connterparts. 

Ruthenium is a known active catalyst for the hydrogenation of carbon monoxide to hydrocarbons (the Fischer-Tropsch synthesis). It was shown that on rathenized electrodes, methane can form in the electroreduction of carbon dioxide as weU. At temperatures of 45 to 80°C in acidihed solutions of Na2S04 (pH 3 to 4), faradaic yields for methane formation up to 40% were reported. On a molybdenium electrode in a similar solution, a yield of 50% for methanol formation was observed, but the yield dropped sharply during electrolysis, due to progressive poisoning of the electrode. 

If the gasifier product stream is intended for downstream use as the feedstock for further upgrading such as methanation, methanol or Fischer Tropsch synthesis, very thorough desulphuri-sation is essential since the catalysts in these upgrading processes are highly sensitive to sulphur poisoning. The methanation catalysts normally cannot tolerate more than 0.05 ppm of sulphur in the feedstock. In addition to H2S sulphur values in the gasifier product it may contain COS, CS2, mercaptans and thiophenes. These are normally removed by activated carbon or zinc oxide filters ahead of the sensitive synthesis catalyst beds. 

The purification step in the route 1 approach removes all of the H2S and COS in the raw product gas from the gasifier in addition to the carbon dioxide. Sulfur acts as a catalyst poison to Fischer-Tropsch, methanation and methanol catalyst systems, so pure sulfur-free gases must be used in these synthesis reactions. 

The deactivation of a Fischer-Tropsch precipitated iron catalyst has been investigated by means of a novel reactor study. After use of the catalyst in a single or dual pilot plant reactor, sections of the catalyst were transferred to microreactors for further activity studies. Microreactor activity studies revealed maximum activity for catalyst fractions removed from the region situated 20 - 30% from the top of the pilot plant reactor. Catalyst characterization by means of elemental analyses, XRD, surface area and pore size measurements revealed that (1 deactivation of the catalyst in the top 25% of the catalyst bed was mainly due to sulphur poisoning (2) deactivation of the catalyst in the middle and lower portions of the catalyst bed was due to catalyst sintering and conversion of the iron to Fe304, Both these latter phenomena were due to the action of water produced in the Fischer-Tropsch reaction. 

Sulfur poisoning is a key problem in hydrocarbon synthesis from coal-derived synthesis gas. The most important hydrocarbon synthesis reactions include methanation, Fischer-Tropsch synthesis, and methanol synthesis, which occur typically on nickel, iron, or cobalt, and ZnO-Cu catalysts, respectively. Madon and Shaw (2) reviewed much of the early work dealing with effects of sulfur in Fischer-Tropsch synthesis. Only the most important conclusions of their review will be summarized here. 

Although metals or even promoted metals have very low sulfur tolerances in synthesis reactions, other materials, such as metal oxides, nitrides, borides, and sulfides, may have greater tolerance to sulfur poisoning because of their potential ability to resist sulfidation (18). The extremely low steady-state activities of Co, Ni, and Ru metals in a sulfur-contaminated stream actually correspond to the activities of the sulfided metal surfaces. However, if more active sulfides could be found, their activity/selectivity properties would be presumably quite stable in a reducing, H2S-containing environment. This is, in fact, the basis for the recent development of sulfur active synthesis catalysts (211-215), which are reported to maintain stable activity/ selectivity properties in methanation and Fischer-Tropsch synthesis at H2S levels of 1% or greater. Happel and Hnatow (214), for example, reported in a recent patent that rare-earth and actinide-metal-promoted molybdenum oxide catalysts are reasonably active for methanation in the presence of 1-3% H2S. None of these patents, however, have reported intrinsic activities... 

Carbon forms play important roles as intermediates, catalyst additives and deactivating species in Fischer-Tropsch synthesis on iron catalysts. Deactivation may be due to poisoning or fouling of the surface by atomic carbidic carbon, graphitic carbon, inactive carbides or vermicular forms of carbon, all of which derive from carbidic carbon atoms formed during CO dissociation (ref. 5). While this part of the study did not focus on the carbon species responsible for deactivation, some important observations can be made to this end. 

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. 

Tjeed gases, CO and H2, for the Fischer-Tropsch synthesis obtained from coal gasification contain sulfur compounds that have been acknowledged as catalyst poisons. Since the early work of Fischer and Tropsch (I), the need for scrupulous removal of sulfur compounds from reactant gas streams has been stressed. Hence to date, little work has... 

Some of the products of Fischer-Tropsch synthesis are troublesome to the process, particularly through catalyst poisoning. These include waxes, salts, and organometallic componnds, which form deposits on the catalyst surface, thus poisoning it. 

Stenger HG, Satterfield CN. Effects of sulfur poisoning of a reduced fused magnetite catalyst in the Fischer-Tropsch synthesis. Ind. Eng. Chem. Proc. Des. Dev. 198534 415-420. 

Bambal, A.S., et al., 2014. Poisoning of a silica-supported cobalt catalyst due to presence of suUiir impurities in syngas during Fischer-Tropsch synthesis effects of chelating agent. Industrial Engineering Chemistry Research 53 (14), 5846—5857. 

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