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Alcohols – Production

Alcohols can be produced from a variety of feedstocks. For example, ethanol is produced from organic feedstocks, such as biomass, by a fermentation process. However, recent efforts have focused on the production of ethanol from waste materials using a gasification process. The overall process consists of a thermochemical conversion of synthesis gas which is then converted to higher molecular weight alcohols by a catalytic process, after which high purity ethanol is separated by distillation. The process is similar to the methanol and gas-to-liquid process. The key differentiating factors are the catalysts and their operating parameters.

Alcohols can be made directly from organic feedstocks (such as sugars and sugar derivatives) by fermentation or indirectly by the production of synthesis gas, and there is the potential for the production of alcohols from organic waste.

Historically, the production of methanol, ethanol, and higher molecular alcohols from synthesis gas has been known since the beginning of the 20^th century. There are several processes that can be used to make mixed alcohols from synthesis gas including iso-synthesis, variants of Fischer-Tropsch synthesis, oxo-synthesis involving the hydroformylation of olefins, and homologation of methanol and lower molecular weight alcohols to make higher alcohols. In the context of the Fischer-Tropsch process, depending on the process and its operating conditions, the most abundant products are usually methanol and carbon dioxide, but methanol can be recycled to produce higher molecular weight alcohols.

With the development of various gas-to-liquid processes, it was recognized that higher alcohols were by-products of these processes when catalysts or conditions were not optimized. Modified Fischer-Tropsch (or methanol synthesis) catalysts can be promoted with alkali metals to shift the products toward higher alcohols. Synthesis of higher molecular weight alcohols is optimal at higher temperatures and lower space velocities compared to methanol synthesis and with a hydrogen/carbon monoxide ratio of approximately 1 rather than 2 or greater.

In the process, the feedstock enters the process and is converted to synthesis gas with the desired carbon monoxide/ hydrogen ratio, which is then reacted, in the presence of a catalyst, into methanol (CH₃OH), ethanol (CH₃CH₂OH), and higher molecular weight alcohols.

Thus,

Stoichiometry suggests that the carbon monoxide/hydrogen ratio is optimum at 2, but the simultaneous presence of water-gas shift leads to an optimum ratio closer to 1.

As in other synthesis gas conversion processes, the synthesis of higher molecular weight alcohols generates significant heat and an important aspect is choice of the proper reactor to maintain even temperature control which then maintains catalyst activity and selectivity. In fact, the synthesis of higher molecular weight alcohols is carried out in reactors similar to those used in methanol and Fischer-Tropsch synthesis. These include shell and tube reactors with shell-side cooling, trickle-bed, and slurry bed reactors.

Catalysts for the synthesis of higher molecular weight alcohols generally fall mainly into four groups: (i) modified high pressure methanol synthesis catalysts, such as alkali-doped ZnO/Cr₂O₃, (ii) modified low pressure methanol catalysts, such as alkali-doped Cu/ZnO and Cu/ZnO/Al₂O₃, (iii) modified Fischer-Tropsch catalysts, such as alkali-doped CuO/CoO/Al2O3, and (iv) alkali-doped sulfides, such as mainly molybdenum sulfide (MoS₂).

The catalytic synthesis process makes several different alcohols depending, in part, on residence time in the reactor and the nature of the catalyst. The alcohols can be separated by distillation and dried to remove water.

A further aspect of the waste-to-alcohols concept is the use of a plasma field in which temperatures are reputed (but not yet proved) to reach 30,000°C (54,000°F). The feedstock can be materials such as waste coal, used tires, wood wastes, raw sewage, municipal solid wastes, biomass, discarded roofing shingles, coal waste (culm), and discarded corn stalks. The plasma field breaks down the feedstock into their core elements in a clean and efficient manner.

Most efforts for alcohol production have centered on bacteria, enzyme, and acid/solvent hydrolysis fermentation– based ethanol production. However, the action of bacteria and enzymes is often very dependent on the feedstock. In addition, these processes generate a small quantity of ethanol over a period of days and the dilute aqueous ethanol product must be distilled to recover the ethanol. In the gasification-synthesis process, various carbonaceous feedstocks can be used, with appropriate modifications in the synthesis gas production step.

A new system for converting trash into ethanol and methanol could help reduce the amount of waste piling up in landfills while displacing a large fraction of the fossil fuels used to power vehicles in the United States. The technology developed does not incinerate refuse, so it does not produce the pollutants that have historically plagued efforts to convert waste into energy. Instead, the technology vaporizes organic materials to produce synthesis gas that can be used to synthesize a wide variety of fuels and chemicals. In addition to processing municipal waste, the technology can be used to create ethanol out of agricultural biomass waste, providing a potentially less expensive way to make ethanol than current corn-based plants.

The new system makes synthesis gas in two stages. In the first stage, waste is heated in a 1,200°C (2,190°F) chamber into which a controlled amount of oxygen is added to partially oxidize carbon and free hydrogen. Not all of the organic material is converted, and some forms char which is then gasified when researchers pass it through arcs of plasma. The remaining inorganic materials, including toxic substances, are oxidized and incorporated into a pool of molten glass which hardens into a material that can be used for building roads or discarded as a safe material in landfills. The second stage is a catalyst-based process for converting synthesis gas into equal ethanol and methanol.

Ethanol from cellulosic biomass materials (such as agricultural residues, trees, and grasses) is made by first using pretreatment and hydrolysis processes to extract sugars, followed by fermentation of the sugars. Although producing ethanol from cellulosic biomass is currently more costly than producing ethanol from starch crops, several countries (including the United States) have launched biofuel initiatives with the objective of the economic production of ethanol from bio-sources. In the process, the carbohydrate from the biomass is converted to a sugar derivative; this is converted to ethanol in a fermentation process that is similar to the process for brewing beer; typical feedstocks for the process are sugar beets and from molasses.

Ethanol can also be produced by synthesis from the chemical compound ethylene, which is derived from crude oil or natural gas, or by the fermentation of carbohydrates.

Methanol (methyl alcohol, wood alcohol, CH₃OH) is mainly manufactured from natural gas, but biomass can also be gasified to methanol. Methanol can be made with any renewable resource containing carbon such as seaweed, waste wood, and garbage. Methanol is stored and handled like gasoline because it is produced as a liquid. Methanol is currently made from natural gas, but it can also be made from a wide range of renewable biomass sources, such as wood or waste paper.

Methanol can be used as one possible replacement for conventional motor fuels and, like ethyl alcohol, has a high octane rating; hence an Otto engine is preferable. If an ignition booster is used, methanol can be used in a diesel engine.

Methanol also offers important emissions benefits compared with gasoline. It can reduce hydrocarbon emissions by 30 to 40% with M85 and up to 80% with M100 fuels. Methanol costs less than gasoline, but has lower energy content. Taking this into account, costs for methanol in a conventional vehicle are slightly higher than those for gasoline.

However, methanol is not miscible with hydrocarbon derivatives and separation ensues readily in the presence of small quantities of water, particularly with reduction in temperature. Anhydrous ethanol, on the other hand, is completely miscible in all proportions with gasoline, although separation may be effected by water addition or by cooling. If water is already present, the water tolerance is higher for ethanol than for methanol, and can be improved by the addition of higher alcohols, such as butanol. Also, benzene or Acetone can be used.