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Products from Syngas—Methanol (Catalyst)	Printer Friendly

Methanol (CH3OH) can be used as a transportation fuel, but currently, its principal use is as a chemical intermediate in the production of a number of other chemicals (e.g.,formaldehyde, dimethyl ether (DME), methyl tert-butyl ether (MTBE), acetic acid, olefins, methyl amines, and methyl halides). Today, world methanol production is almost exclusively (90%) produced from syngas derived from the steam reforming of natural gas.   Syngas is a gaseous compound consisting mostly of carbon monoxide (CO) and hydrogen (H2) and produced from the gasification of feedstocks at temperatures in excess of 1100°F under conditions where the amount of oxygen (from air, pure oxygen, or steam) is less than what is needed for complete combustion. Syngas can be produced from any hydrocarbon feedstock, including natural gas, petroleum products, coal, and biomass.   Methanol synthesis. Methanol synthesis is a well-developed commercial process with high activity and very high selectivity. Modern methanol plants produce 1 kg of MeOH/liter of catalyst/hr with >99.5% selectivity for methanol. The syngas produced from reforming natural gas is fed to a reactor containing a catalyst which produces methanol and water vapor. The crude methanol contains up to 18% water, ethanol, and other chemicals (e.g., higher alcohols, ketones, and ethers). Purification is by distillation in one unit which removes volatile compounds and a second unit which removes the water and higher alcohols. Unreacted syngas is recirculated back to the methanol reactor resulting in an overall conversion efficiency of over 99%. A generic methanol synthesis process flow diagram is shown in figure 1.   Catalytic production of methanol from syngas is a high-temperature, high-pressure exothermic (heat generating) reaction. The reaction conditions prefer that there be just enough hydrogen needed for methanol synthesis. The presence of carbon dioxide (CO2) in the syngas makes the reaction about 100 times faster than when absent.        Methanol catalysts. Catalysts play a pivotal role in syngas conversion reactions. The basic concept of a catalytic reaction is that reactants adsorb onto the catalyst surface, rearrange, and combine into products that then desorb from the catalyst surface. One of the fundamental functional differences between syngas synthesis catalysts is whether or not the adsorbed CO molecule dissociates on the catalyst surface. For methanol synthesis, the CO bond remains intact.   The first methanol catalysts were zinc-chromium catalysts and operated at high temperatures and pressure. They demonstrated high activity and selectivity for methanol synthesis and were resistant to sulfur poisoning, but as the ability to remove syngas impurities improved, interest shifted to other catalysts. Today, all commercial production of methanol from syngas uses low temperature and pressure processes based on Cu catalysts. Copper catalysts are extremely sensitive to sulfur poisoning and sulfur concentrations need to be kept below 1 part per million (ppm), and preferably below 0.1 ppm. The addition of ZnO to the Cu catalyst limits sulfur poisoning. Sulfur contamination is generally not a problem in the commercial production of methanol because the syngas used is produced from natural gas using catalysts that are very intolerant of sulfur and sulfur compounds are generally removed from the feedstock prior to gasification. Other catalyst poisons that may be contained in syngas include metal carbonyl compounds (particularly from iron and nickel), alkali metals, arsenic, phosphorus, chlorine.   Methanol synthesis catalysts are commercially produced by a number of manufacturers (e.g., IFP, ICI, BASF, Shell, Sud Chemie, Dupont, United Catalysts, and Haldor Topsoe MK-121). Commercial methanol catalysts have lifetimes of 3-5 years under normal operating conditions.   Methanol converters. The two major challenges to the design of methanol converters (reactors) is the need to remove the large amount of heat produced during methanol synthesis, and achieving high conversion rates. Methanol synthesis reactions are typically run at lower temperatures to limit competeing side reactions and to extend catalyst lifetime. However, catalyst activities decrease at lower temperatures. The maximum conversion efficiency of syngas to methanol per single pass of syngas over the catalyst is about 25%. Removing the methanol as it is produced can improve the per pass conversion process efficiencies.   Numerous methanol converter designs have been commercialized and can be roughly categorized into two types-adiabatic and isothermal converters. Adiabatic converters often contain multiple catalyst beds separated by gas cooling devices and involve either direct heat exchange or injection of cooled, fresh or recycled syngas. The ICI Low pressure Quench Converter is the most widely used adiabatic methanol converter. Other adiabatic reactors in use include the ARC converter (an improved version of the ICI converter); the Halliburton converter; the Haldor-Topsoe Collect, Mix, Distribute (CMD) adiabatic converter; and the Toyo Engineering Corporation multistage radial flow converter (MRF-Z™). Isothermal converters are designed to continuously remove heat and operate like a heat exchanger.One of the more widely used commercial isothermal converters is the Lurgi Methanol Converter. Other isothermal converters in use include the Tube Cooled Converter; the Linde isothermal reactor (the Variobar converter); and the Mitsubishi MGC/MHI Superconverter. Other converter designs under development include three phase systems (similar to the slurry reactors used for Fischer Tropsch synthesis) and reactors better able to convert low stoichiometric ratio (CO rich) syngas to methanol which enhances the potential to produce methanol from coal or biomass feedstocks.  Commercial production. Global annual methanol production capacity was 12.8 billion gallons in 2002. About 20% of world production occured in North America (2001), and U.S. consumption was 28% of world supply. Methanex and SABIC are the world’s largest producers accounting for 17 and 6.5% of global capacity, respectively. There are 18 methanol production plants in the U.S.  U.S. spot market prices for methanol ranged from $0.20 to $1.50/gallon between January 1993 and October 2001, with prices generally in the $0.30/gallon ($5/GJ, LHV) to $0.70/gallon ($12/GJ LHV) range. Price fluctuations result from inbalances of methanol supply and demand (largely as a result of the availability of imports), and the price of natural gas.    Most of the capital cost for methanol production from natural gas is for reforming and conditioning the gas. For the ICI process, the allocation of capital costs are 2% for desulfurization, 32% for reforming/gas cooling, 14% for steam production, 24% for compression, 22% for methanol synthesis, and 6% for distillation. Slurry converters are expected to reduce capital costs. Production of methanol from biomass is estimated to increase the cost by $0.30/gallon to $0.55/gallon (depending on analysis assumptions) compared with natural gas.