Poisoning iron catalysts

Remaining trace quantities of CO (which would poison the iron catalyst during ammonia synthesis) are converted back to CH4 by passing the damp gas from the scmbbers over a Ni methanation catalyst at 325° CO -t- 3H2, CRt -t- H2O. This reaction is the reverse of that occurring in the primary steam reformer. The synthesis gas now emerging has the approximate composition H2 74.3%, N2 24.7%, CH4 0.8%, Ar 0.3%, CO 1 -2ppm. It is compressed in three stages from 25 atm to 200 atm and then passed over a promoted iron catalyst at 380-450°C ... 

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... 

How can this enigma be answered Put away a sample of pure harmaline, with its spectral identification, onto the shelf for 50 or 100 years, and then re-analyze it Who knows, but what might be needed for this conversion is heat, or a bit of iron catalyst, or some unknown species of South American mold. Acid is certainly known to promote this oxidation. It would be very much worth while to answer this question because some, perhaps much, of the results of human pharmacological studies that involve harmaline as a metabolic poison, may be influenced by the independent action of harmine as a harmaline contaminant. 

Catalyst Poisons. Synthesis gas prepared by the partial combustion of sweet natural gas can be charged to the reactors directly without purification. However, synthesis gas containing more than 0.1 grain of sulfur per 100 cubic feet must be purified before use over fluidized iron catalysts. Other catalyst poisons are known, such as chlorine (14), but they are not likely to be encountered in the natural gas to gasoline process. 

The character of the chemisorption of nitrogen can be also judged from the results of studies of ammonia synthesis kinetics at the reversible poisoning of the catalyst with water vapor (102,103). If a gas mixture contains water vapor, an adsorption-chemical equilibrium of adsorbed oxygen, hydrogen gas, and water vapor sets in on the iron catalyst. 

Both type-4 and type-B adsorption on iron have a poisoning effect on the H2 and D2 exchange on iron catalysts at very low temperatures... 

One strong point of SIMS is its ability to detect elements that are present in trace amounts, and as such the technique is highly suited to the detection of poisons on a catalyst caused by contaminants in the reactor feed. Chlorine, for example, poisons the iron catalyst used in ammonia synthesis because it suppresses the dissociation of nitrogen molecules. Plog et al. [18] used SIMS to show that chlorine impurities may coordinate to potassium promoters, as evidenced by a KCI2- signal, or to iron, visible by an FeCh- peak. The SIMS intensity ratio... 

The iron catalyst used in the synthesis of ammonia (Haber s process) is poisoned by H2S. 

The catalyst may combine chemically with the impurity The poisoning of iron catalyst by H2S comes in this class. 

Long Life. This is determined essentially by resistance to a) thermal degradation and b) irreversible poisoning. In some plants, conventional iron catalysts have achieved service lives of up to 14 years. 

Poisoning of transition metals by sulfur is a serious problem encountered in many industrial processes [1-4]. In the Fisher-Tropsch synthesis of hydrocarbons from CO/H2 mixtures, the presence of a few ppm volume ratio of a sulfur containing gas can have a drastic effect on the life time of an iron catalyst [1]. On the other hand, a partial and well controlled treatment of a metal by sulfur can result in desirable effects, particularly with regard to the manipulation of catalyst selectivity for certain reactions [5]. 

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. 

Hydrogen sulfide causes a permanent poisoning of iron catalysts. Methane does not poison ammonia catalysts under normal synthesis conditions. Equilibrium data (Browning, De Witt, and Emmett, 77 Browning and Emmett, 78) should be mentioned in this connection. 

Fig. 22. Poisoning of reduced fused iron catalyst by H2S 7 = 535 K, H2/CO = 1, P = 2.16 MPa. Sulfur concentration in feed (rag S/m3) (O) 6.9, ( ) 23.0, ( ) 69.0. Reprinted with permission from Ref. 199. Copyright 1963, 1964 American Chemical Society.

Poisoning of iron catalysts during ammonia synthesis by sulfur compounds has received relatively little attention (154, 240-244). Nevertheless, the previous work provides information on the poisoning mechanism and interesting examples of how oxide promoters may influence the sulfur poisoning behavior of a catalytic metal. 

The palladised cathode gave the greatest percentage of PCP removal due to its superior capability to produce and store hydrogen. The poor performance at the iron cathode was mainly a result of its instability under the HDH condition. Both instability and poisoning of catalyst by intermediates were responsible for the poorer performance of the Ni cathode. 

Deviations from the Schulz-Flory distribution arc possible if secondary reactions such as cracking on acidic supports or insertion of product olefins into the growing chain occur [42]. It has been reported recently that the Schulz Flory constant a has a tendency to increase from C3 to C, [45]. This may be the reason why the values found are usually higher for methane and lower for Cj and C) j.)., as would be expected for an ideal Schulz-Flory distribution [40]. Investigations by Madon et at. on partly sulfur-poisoned iron/copper catalysts revealed a dual product distribution. This was explained by the assump tion of > 2 types of active sites for hydrocarbon chain formation, each with a slightly different value of the chain growth probability [46]. 

Besides sulfur, cliloride and bromide ions also deactivate iron catalysts. Other catalyst poisons are Pb at 0.5 wi Sn at O.l wt % and hi concentrations of Bi (10wi%) [15]. 

Electronic promoters, for example, the alkali oxides, enhance the specific activity ofiron-alnmina catalysts. However, they rednce the inner snrface or lower the thermal stability and the resistance to oxygen-containing catalyst poisons. Promoter oxides that are rednced to the metal during the activation process, and form an alloy with the iron, are a special group in which cobalt is an example that is in industrial use. Oxygen-containing compounds such as H2O, CO, CO2, and O2 only temporarily poison the iron catalysts in low concentrations. Sulfur, phosphorus, arsenic, and chlorine compounds poison the catalyst permanently. 

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