Methanol, production catalyst poisoning

The advent of a large international trade in methanol as a chemical feedstock has prompted additional purchase specifications, depending on the end user. Chlorides, which would be potential contaminants from seawater during ocean transport, are common downstream catalyst poisons likely to be excluded. Limitations on iron and sulfur can similarly be expected. Some users are sensitive to specific by-products for a variety of reasons. Eor example, alkaline compounds neutralize MTBE catalysts, and ethanol causes objectionable propionic acid formation in the carbonylation of methanol to acetic acid. Very high purity methanol is available from reagent vendors for small-scale electronic and pharmaceutical appHcations. 

In addition to the direct use of ethanol as a fuel, its use as a source of H2 to be used with high efficiency in fuel cells has been thoroughly investigated. H2 production from ethanol has advantages compared vdth other H2 production techniques, including steam reforming of hydrocarbons and methanol. Unlike hydrocarbons, ethanol is easier to reform and is also free of sulfur, which is a well-known catalyst poison. Furthermore, unlike methanol, ethanol is completely renewable and has lower toxicity. 

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. 

Thus a variety of hydrocarbons, ranging from natural gas to coal, are used in methanol production. Regardless of the feedstock used to prepare the synthesis gas, it is necessary to remove sulfur so that the converter catalyst is not poisoned. Before natural gas or naphtha is reformed, the feedstock is desulfurized. In the partial oxidation and coal gasification processes, the feedstock is first oxidized and the resulting synthesis gas is desulfurized before entering the converter. 

The deactivation of methanol-synthesis catalyst was studied in laboratory and pilot-plant slurry reactors using a concentrated, poison-free, CO-rich feedstream. The extent of catalyst deactivation correlated with the loss of BET surface area. A model of catalyst deactivation as a function of temperature and time was developed from experimental data. The model suggested that continuous catalyst addition and withdrawal, rather than temperature programming, was the best way to maintain a constant rate of methanol production as the catalyst ages. Catalyst addition and withdrawal was demonstrated in the pilot plant. 

Only a few studies of the poisoning of copper/zinc oxide catalysts have been reported (refs. 4-6). Whether copper or zinc is most su.sceptible to attack by sulfur is still a question, Tlte literature findings on the sulfur tolerance of methanol synthesis catalyst are inconsistent with industrial experience. For example, observations from indusirinl production suggest that a... 

Then the product gas is fed to a low-temperature reactor where a Cu/Zn-Al2O3 particulate WGS catalyst works at about 200°C. Outlet CO concentration is decreased to <0.5%, while the remaining CO, which can poison downstream ammonia or methanol synthesis catalysts, is removed by pressure swing adsorption (PSA) unit. This method exploits the adsorption capacity of different molecular sieves or active carbon, which selectively permit the crossover of hydrogen but not of the other compounds present in the effluents. This technology has been... 

Although Maui gas is very low in sulphur, the incoming gas is desulphurised as a precaution against poisoning catalysts used in the process. Following desulphurisation, water, in the form of medium pressure steam, is added and the mixture passed through reformer reactor tubes which contain a nickel catalyst. The tubes are located inside the reformer furnace where the process temperature is raised to 900°C and the reaction to form synthesis gas occurs. The synthesis gas is cooled to 35°C, compressed to 100 bar, reheated and reacted at 250-300°C over a copper/zinc catalyst to form a water-methanol mixture with about 17 percent water. The methanol product is reduced in pressure and passed to the methanol-to-gasoline (MTG) plant. 

The principal catalyst poisons have been found to be oils, organic chlorine, or sulfur-containing compounds which usually enter with the air used for the oxidation.17 SOb The presence of acetone in the methanol had been found to be objectionable but with the substitution of the very pure synthetic alcohol for the product of wood distillation this difficulty has disappeared. Water vapor in appreciable quantities serves to decrease the temperature of the catalyst and hence, to slow down the reaction. Some patents have even claimed the addition of steam for the purpose of controlling the reaction rate. 

Production of formaldehyde by air-oxidation of methanol using a silver catalyst. The entering air is scrubbed with aqueous sodium hydroxide to remove any SO2 and COj which are catalyst poisons. 

Copper catalysts are very sensitive to poisonous compounds, especially when they are used in low-temperature processes, because adsorption of poison is thermodynamically favored. The significant poisons for copper catalysts in methanol production are sulfur and chlorine. Sulfur compounds - for example, H2S - form copper sulfides ... 

When the hydrocarbon steam reforming process conld provide poison free synthesis gas, the benefits of the more active copper catalysts were quickly reviewed. It was soon shown that the high activity of copper oxide-zinc oxide catalysts, compared with the zinc oxide-chrominm oxide types, particularly when alumina or chromia promoters were added, conld revolutionize methanol production. New, more efficient processes were nrgently needed in view of the increasing demand for methanol and the economies of high capacity units. 

A final difference between photocatalytic CO2 reduction and hydrogen evolution from water is the large number of possible reaction products that can be formed in the process. The products that have been detected in photocatalytic CO2 conversion include oxalic acid or oxalate, formate, methanol, methane, ethane, CO, and even elemental C. The problem arises from the fact that some of the possible products are gaseous, while others are liquids or even solids. If solids are deposited on the photocatalyst surface performing the reaction in the gas phase at low temperatures, deactivation of the catalyst should occur by blocking of the surface by carbon and liquid products that poison the photocatalyst. The large diversity of products that can be formed in CO2 reduction and their difference in physical states... 

Alkali AletalIodides. Potassium iodide [7681-11-0] KI, mol wt 166.02, mp 686°C, 76.45% I, forms colorless cubic crystals, which are soluble in water, ethanol, methanol, and acetone. KI is used in animal feeds, catalysts, photographic chemicals, for sanitation, and for radiation treatment of radiation poisoning resulting from nuclear accidents. Potassium iodide is prepared by reaction of potassium hydroxide and iodine, from HI and KHCO, or by electrolytic processes (107,108). The product is purified by crystallization from water (see also Feeds and feed additives Photography). 

Poisoning of platinum fuel cell catalysts by CO is undoubtedly one of the most severe problems in fuel cell anode catalysis. As shown in Fig. 6.1, CO is a strongly bonded intermediate in methanol (and ethanol) oxidation. It is also a side product in the reformation of hydrocarbons to hydrogen and carbon dioxide, and as such blocks platinum sites for hydrogen oxidation. Not surprisingly, CO electrooxidation is one of the most intensively smdied electrocatalytic reactions, and there is a continued search for CO-tolerant anode materials that are able to either bind CO weakly but still oxidize hydrogen, or that oxidize CO at significantly reduced overpotential. 

Sulfur is a potential problem even at low levels for synthesis gas systems using certain types of catalysts. The production of methanol from synthesis gas, for example, uses catalysts that are poisoned by sulfur. Some tar cracking catalysts are also sulfur sensitive. In those systems, thorough removal of sulfur will be required. Fuel cell systems are also sulfur sensitive. 

As shown by Re. 1—2, methanol oxidation to carbon dioxide is a six-electron reaction. This reaction, however, does not proceed by a simple single step. On the contrary, it is considered as a complex multi-step reaction involving several intermediates and by-product which may be different depending on the catalysts, media, temperature and other conditions. Although the details are yet to be found, it is widely agreed that some intermediates or by-products accumulate on the surface and poison the catalyst to decrease its activity. 

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