Today, methanol is almost exclusively produced from syn-gas, which is a mixture of hydrogen, carbon monoxide and CO2 over a heterogeneous catalyst according to the following equations:
The two reactions in Eqs. (1) and (2) are exothermic, with heats of reaction equal to -21.7 kcal mol-1 and -9.8 kcal mol-1, respectively. They both result in a decrease of volume as the reaction proceeds. According to Le Chatelier's principle, the conversion to methanol is therefore favored by increasing pressure and decreasing temperature. Equation (3) describes the endothermic reverse water gas shift reaction (RWGSR) which also occurs during methanol synthesis, producing carbon monoxide which can then further react with hydrogen to produce methanol. In fact, Eq. (2) is simply the sum of the reactions in Eqs. (1) and (3). Each of these reactions is reversible, and thus limited by thermodynamic equilibria depending on the reaction conditions, mostly temperature, pressure and composition of the syn-gas.
Synthesis gas for methanol production can be obtained by reforming or partial oxidation of any carbonaceous material such as coal, coke, natural gas, petroleum, heavy oils and asphalt. Economic considerations dictate the choice of raw materials. The long-term availability of raw materials, energy consumption and environmental aspects will however also play increasingly important roles.
The composition of syn-gas is generally characterized by the stoichiometric number S. Ideally, S should be equal to or slightly above 2. Values above 2 indicate an excess of hydrogen, whereas values below 2 mean a hydrogen deficiency relative to the ideal stoichiometry for methanol formation.
Synthesis gas from coal has less than the optimum hydrogen to carbon content. Treatment of the gas before methanol synthesis or addition of hydrogen is therefore needed to avoid the formation of undesired byproducts. Reforming of feeds with a higher H/C ratio such as propane, butane, or naphthas leads to S values in the vicinity of 2, which is ideal for conversion to methanol. Steam reforming of methane on the other hand, yields syn-gas with a stoichiometric number of 2.8 to 3.0. In this case, the addition of CO2 can lower S close to 2. Excess hydrogen can also be used in an adjacent plant to produce ammonia.
The production of methanol from syn-gas on an industrial scale using high pressures (250-350 atm) and temperatures (300-400 °C) was first introduced by BASF during the 1920s. From then until the end of World War II, most methanol was produced from coal-derived syn-gas and off-gases from industrial facilities such as coke ovens and steel factories. The use of such feedstocks containing high levels of impurities, was made possible by the design of a catalyst system consisting of zinc oxide and chromium oxide, which was highly stable to sulfur and chlorine compounds. After World War II, the feedstock for methanol synthesis shifted rapidly to natural gas, which became widely available at very low costs, particularly in the United States. Whereas 71% of the methanol in the United States was still derived from coal in 1946, by 1948 almost 77% was obtained from natural gas. Natural gas is still the preferred feedstock for methanol production because it offers, besides a high hydrogen content, the lowest energy consumption, capital investment and operating costs. Furthermore, natural gas contains fewer impurities such as sulfur and halogenated compounds or metals, which can poison the needed catalysts. When present, these impurities (mostly sulfur in the form of H2S, COS or mercaptans) can, however, also be removed relatively easily. Lower levels of impurities in the syn-gas allowed the use of more active catalysts, operating under milder conditions. This led, during the 1960s, to the development by ICI (now Synetix) of a process using a copper-zinc-based catalyst allowing the conversion of syn-gas to methanol at pressures of 50-100 atm and temperatures of 200-300 °C. This low-pressure route is the basis for most current processes for methanol production. The formation of byproducts (dimethyl ether, higher alcohols, methane, etc.) associated with the old high-pressure technology was also drastically reduced or even eliminated . Production using the high-pressure process is no longer economical, and the last plant based on this technology closed in the 1980s. Worldwide, the production capacity for methanol is dominated by processes from only a few companies, with Synetix accounting for some 60% of the installed capacity and Lurgi for 27% (Fig. 12.3).
All present processes use copper-based catalysts which are extremely active and selective, and are almost exclusively used in gas-phase processes. They mostly dif-
fer only in the type of reactor design and catalyst arrangement (in fixed beds, tubes, suspension, etc.). Control of the temperature is very important, and overheating of the catalyst should be avoided as it will rapidly decrease its activity and shorten its lifetime . In one pass over the catalyst only a part of the syn-gas is converted to methanol. After separation of methanol and water by condensation at the reactor's outlet, the remaining syn-gas is recycled to the reactor. Recently, liquid-phase processes for methanol production have also been introduced. Air Products in particular, developed the Liquid Phase Methanol Process (LPMEOH) in which a powdered catalyst is suspended in an inert oil, providing an efficient means to remove the heat of reaction from the catalyst and control the temperature (Fig. 12.4). The syn-gas is simply bubbled into the liquid. This process promotes also a higher syn-gas to methanol conversion, so that a single pass through the reactor is generally sufficient .
Generally, modern methanol plants have selectivities to methanol higher than 99%, with energy efficiencies above 70% (Fig. 12.5). Crude methanol leaving the reactor contains, however, water and small amounts of other impurities, the distribution and composition of which will depend on the gas feed, reaction conditions, and type and lifetime of the catalyst. Impurities that may be present in methanol include dissolved gases (methane, CO, CO2), dimethyl ether, methyl formate, acetone, higher alcohols (ethanol, propanol, butanol) and long-chain hydrocarbons. Commercially, methanol is available in three grades of purity: fuel grade, "A" grade, generally used as a solvent, and "AA" or chemical grade. Chemical grade has the highest purity with a methanol content exceeding 99.85% and is
Figure 12.4 Synthesis of methanol in the
Figure 12.4 Synthesis of methanol in the
LPMEOH™ slurry bubble reactor developed by Air Products (Source: DOE).
the standard generally observed by the industry for methanol production. Depending on the amount of impurities and purity desired, methanol will have therefore to be purified by distillation systems using one or more distillation columns.
Asides from the methanol synthesis step, the most crucial part of present methanol plants is the syn-gas generation and purification system, which will depend on the nature and purity of the feedstock used. Although natural gas is generally the preferred feedstock due to the simplicity of obtaining an adequate syngas with low levels of impurities, other routes are also used under given circumstances. Regions rich in coal or heavy oils, but having limited natural gas sources, in view of the high natural gas prices, can turn to these resources to produce methanol despite higher cost for the needed syn-gas purification system.
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