Methanation poisons

CHLOROPICRIN, LIQUID Trtdoronitro-methane, Nitrochloroform, Picfume, Nitrotrichloro-methane Poisonous, Class B, I 4 (1 3 ... 

METHYL BROMIDE Bromomethane, Embafume, M-B-C Fumigant, Monobromo-methane Poisonous Liquid or Solid, Class B 3 1 0 ... 

This reaction is an undesirable side reaction in the manufacture of hydrogen but utilised as a means of removing traces of carbon monoxide left at the end of the second stage reaction. The gases are passed over a nickel catalyst at 450 K when traces of carbon monoxide form methane. (Methane does not poison the catalyst in the Haber process -carbon monoxide Joes.)... 

Final Purification. Oxygen containing compounds (CO, CO2, H2O) poison the ammonia synthesis catalyst and must be effectively removed or converted to inert species before entering the synthesis loop. Additionally, the presence of carbon dioxide in the synthesis gas can lead to the formation of ammonium carbamate, which can cause fouHng and stress-corrosion cracking in the compressor. Most plants use methanation to convert carbon oxides to methane. Cryogenic processes that are suitable for purification of synthesis gas have also been developed. 

Most commercial methanator catalysts contain nickel, supported on alumina, kaolin, or calcium aluminate cement. Sulfur and arsenic are poisons to the catalyst, which can also be fouled by carry-over of solvent from the CO2 removal system. 

The methanation reaction is currently used to remove the last traces (<1%) of carbon monoxide and carbon dioxide from hydrogen to prevent poisoning of catalysts employed for subsequent hydrogenation reactions. Processes for conversion of synthesis gas containing large quantities of carbon monoxide (up to 25%) into synthetic natural gas have been investigated to serve plants based on coal-suppHed synthesis gas. 

In Lurgi coal gasification, an example of extremely important treating is in the sulfur removal step ahead of methanation where the catalyst is poisoned by even small traces of any sulfur compound. The sulfur removal step is a relatively high capital and operating cost item. 

The influence of Zn-deposition on Cu(lll) surfaces on methanol synthesis by hydrogenation of CO2 shows that Zn creates sites stabilizing the formate intermediate and thus promotes the hydrogenation process [2.44]. Further publications deal with methane oxidation by various layered rock-salt-type oxides [2.45], poisoning of vana-dia in VOx/Ti02 by K2O, leading to lower reduction capability of the vanadia, because of the formation of [2.46], and interaction of SO2 with Cu, CU2O, and CuO to show the temperature-dependence of SO2 absorption or sulfide formation [2.47]. 

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 ... 

The second step after secondary reforming is removing carbon monoxide, which poisons the catalyst used for ammonia synthesis. This is done in three further steps, shift conversion, carbon dioxide removal, and methanation of the remaining CO and CO2. 

This paper surveys the field of methanation from fundamentals through commercial application. Thermodynamic data are used to predict the effects of temperature, pressure, number of equilibrium reaction stages, and feed composition on methane yield. Mechanisms and proposed kinetic equations are reviewed. These equations cannot prove any one mechanism however, they give insight on relative catalyst activity and rate-controlling steps. Derivation of kinetic equations from the temperature profile in an adiabatic flow system is illustrated. Various catalysts and their preparation are discussed. Nickel seems best nickel catalysts apparently have active sites with AF 3 kcal which accounts for observed poisoning by sulfur and steam. Carbon laydown is thermodynamically possible in a methanator, but it can be avoided kinetically by proper catalyst selection. Proposed commercial methanation systems are reviewed. 

Nickel. As a methanation catalyst, nickel is presently preeminent. It is relatively cheap, it is very active, and it is the most selective to methane of all the metals. Its main drawback is that it is easily poisoned by sulfur, a fault common to all the known active methanation catalysts. The nickel content of commercial nickel catalysts is 25-77 wt %. Nickel is dispersed on a high-surface-area, refractory support such as alumina or kieselguhr. Some supports inhibit the formation of carbon by Reaction 4. Chromia-supported nickel has been studied by Czechoslovakian and Russian investigators. 

It is highly active but easily poisoned by sulfur and not particularly selective to methane. Oddly enough, carbon monoxide appears to inhibit the rate of methane formation. 

Partially extracted Raney cobalt is very active, but it is easily poisoned by sulfur and tends to lay down carbon more readily than Raney nickel (21). Cobalt is less active than nickel and much less selective to methane... 

Catalyst Poisons. Hausberger, Atwood, and Knight (33) reported that nickel catalysts are extremely sensitive to sulfides and chlorides. If all materials which adversely affect the performance of a catalyst were classified as poisons, then carbon laydown and, under extreme conditions, water vapor would be included as nickel methanation catalyst poisons. 

It was shown in laboratory studies that methanation activity increases with increasing nickel content of the catalyst but decreases with increasing catalyst particle size. Increasing the steam-to-gas ratio of the feed gas results in increased carbon monoxide shift conversion but does not affect the rate of methanation. Trace impurities in the process gas such as H2S and HCl poison the catalyst. The poisoning mechanism differs because the sulfur remains on the catalyst while the chloride does not. Hydrocarbons at low concentrations do not affect methanation activity significantly, and they reform into methane at higher levels, hydrocarbons inhibit methanation and can result in carbon deposition. A pore diffusion kinetic system was adopted which correlates the laboratory data and defines the rate of reaction. 

Benzene. Benzene is ordinarily scrubbed out by a Rectisol system before the methanators. However, if a different H2S removal system were used, benzene could pass through the system and then hydrogenate, plug up the catalyst pores, or reform. Benzene was therefore included in this poison study. 

For the C02, the literature kinetics gave more reasonable correlation than the simple kinetics though the difference is not great. However, Ref. 15 (16) involves methanation of > 50% C02 in H2 under conditions where Equation 3 would break down, and Ref. 17 (18) involves only the initial hydrogenation (less than the first 1 or 2% ) of the C02 present. Furthermore, there is a possibility that the reverse shift would produce enough CO to poison the C02 methanation in these experiments which would make it difficult to obtain agreement between various runs. 

A good methanation catalyst is one which is physically strong, is reducible at 300°C (570°F) and has high activity. In order to provide a long life, it must retain these properties in use. Lives of 3-5 years are commonly obtained from charges of Imperial Chemical Industries, Ltd. (ICI) catalyst 11-3, depending on the temperature of operation and the presence of poisons in the synthesis gas, factors which are discussed below. These properties can be obtained by careful attention to the formulation and manufacture of the catalyst. 

The poisons most likely to be encountered in an ammonia plant are those originating in the C02-removal system which precedes the metha-nator. Carry-over of a small amount of liquid into the methanator, which is almost inevitable, is not normally serious. Plant malfunction, however, can sometimes result in large quantities of C02-removal liquor being pumped over the catalyst, and this can be very deleterious. Table I lists the effects of common C02-removal liquors on methanation catalyst activity. 

These tests demonstrated that the Lurgi Rectisol process provides an extremely pure synthesis gas which can be charged directly to the metha-nation plant without problems of sulfur poisoning of the nickel catalyst. However, in order to cope with a sudden sulfur breakthrough from Rectisol as a result of maloperation, a commercial methanation plant should be operated with a ZnO emergency catchpot on line. 

These tests were performed to establish the limits in flexibility and operability of a methanation scheme. The two demonstration plants have been operated in order to determine the optimum design parameters as well as the possible variation range which can be tolerated without an effect on catalyst life and SNG specification. Using a recycle methanation system, the requirements for the synthesis gas concerning H2/CO ratio, C02 content, and higher hydrocarbon content are not fixed to a small range only the content of poisons should be kept to a minimum. The catalyst has proved thermostability and resistance to high steam content with a resultant expected life of more than 16,000 hrs. 

The space velocity was varied from 2539 to 9130 scf/hr ft3 catalyst. Carbon monoxide and ethane were at equilibrium conversion at all space velocities however, some carbon dioxide breakthrough was noticed at the higher space velocities. A bed of activated carbon and zinc oxide at 149 °C reduced the sulfur content of the feed gas from about 2 ppm to less than 0.1 ppm in order to avoid catalyst deactivation by sulfur poisoning. Subsequent tests have indicated that the catalyst is equally effective for feed gases containing up to 1 mole % benzene and 0.5 ppm sulfur (5). These are the maximum concentrations of impurities that can be present in methanation section feed gases. 

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