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Alcohols – From Waste

Alcohols can be made from organic materials by fermentation, and there is the potential for the production of alcohols from organic waste. Historically, the production of methanol, ethanol, and higher molecular alcohols from syngas 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 ration of 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: (1) modified high pressure methanol synthesis catalysts, such as alkali-doped ZnO/Cr₂O₃, (2) modified low pressure methanol catalysts, such as alkali-doped Cu/ZnO and Cu/ZnO/Al₂O₃, (3) modified Fischer-Tropsch catalysts, such as alkali-doped CuO/CoO/Al2O3, and (4) 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 (http://www.fuelfrontiers.com/technology.htm) 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.