The oil embargos of the 1970’s spurred interest in the use of syngas to produce alcohols (methanol, ethanol, butanol, etc.) for blending with gasoline. Currently, methanol is commercially produced from syngas, but higher alcohols (ethanol, butanol, etc.) are not.
In its simplest form, syngas is composed of carbon monoxide (CO) and hydrogen (H2). Syngas is 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) are less that what is needed for complete combustion. In principle, syngas can be produced from any hydrocarbon feedstock, including natural gas, naphtha, residual oil, petroleum coke, coal, and biomass.
Higher alcohols, or mixtures of higher alcohols with methanol, have better fuel properties than pure methanol. Compared to methanol, higher alcohols have higher octane ratings (greater resistance to uncontrolled ignition in internal combustion engines), are less volatile, display a lower tendency toward phase separtation in the presence of water, and are more compatible with certain engine components. Using mixed alcohols (i.e., a mixture of methanol and higher alcohols) avoids the problems associated with the use of methanol alone. Additionally, mixed alcohols have lower vapor pressure, better solubility with hydrocarbon components, improved water tolerance, and higher overall heating value compared to methanol. And, when used as a diesel substitute at levels of 20-30% weight, the calorific value, lubrication properties, and ignition properties are improved compared to pure methanol. Mixed alcohols are also more compatible with the existing fuel infrastructure relative to methanol.
Historically, several processes have been developed to make mixed alcohols from syngas, however, commercial production has been hampered by poor selectivity and low product yields (typically around 10 percent of the syngas is converted to alcohol with methanol the most prevelant component). Currently there are no commercial plants that produce mixed alcohols in the C2 to C6 range, largely due to the lack of appropriate catalysts.
Higher alcohol synthesis reactions. The mechanism for higher alcohol synthesis involves numerous reactions each with multiple pathways leading to a variety of products. The production of higher alcohols first involves the synthesis of methanol followed by the stepwise addition of carbon at the end of the methanol molecule to sequentially produce ethanol, propanol, butanol, etc. Branched higher alcohols (e.g., isobutanol, methyl ester) can also be produced.
The types of reactions that occur are affected by the operating conditions and the types of catalysts used. Higher alcohol production is favored at high pressures, higher temperatures, and a syngas H2/CO ratio close to 1. Thermodynamic constraints limit the theoretical yields of higher alcohols, and the heat generated during the chemical reactions must be removed to maintain control of process temperatures. Compared to methanol, production of higher alcohols generates more heat. Water and carbon dioxide are produced as by-products of higher alcohol production and secondary reactions can result in the production of other products (e.g., aldehydes, ketones, methane).
Catalysts play a pivotal role in syngas conversion reactions. The basic concept of a catalytic reaction is that chemical compounds adsorb onto the catalyst surface, rearrange, and combine into products that then desorb from the catalyst surface. A number of different types of catalysts can be used to produce higher alcohols.
Modified high pressure methanol synthesis catalysts operate at temperatures ranging from 300-425°C (~600-825°F) and at pressures ranging from 12.5 to 30 MPa (million pascals). They are used to produce branched primary alcohols and are composed of ZnO/Cr2O3 with an alkali added. Modified low pressure methanol synthesis catalysts are used at temperatures ranging from 275-310°C (~550-615°F) and at pressures ranging from 5 to 10 MPa. They consist of Cu/ZnO or Cu/ZnO/Al2O3 with alkali added and used to produce primary alcohols. Many of the early processes used these catalysts and methanol is the most abundant (~ 80%) alcohol produced. Modified Fischer-Tropsch catalysts, CuO/CoO/Al2O3 operate at temperatures between 260-340°C (~525-670°F) and pressures of 6 to 20 MPa and catalyze the production of linear primary alcohols. At optimal conditions, syngas is converted to a liquid product containing 30-50% higher alcohols. The lack of long-term stability and low activity of these catalysts hinders their commercial application. Modified sulfide catalysts (mainly MoS2) are used at temperatures between 260-350°C (~525-690°F) and pressures between 3 and 17.5 MPa and produce of linear alcohols. Cesium (Cs) or potassium (K) is often added as a promoter. They selectively (75-90%) produce higher alcohols from syngas with a 10% CO conversion efficiency when the H2/CO ratio is one. Sulfide catalysts are extremely resistant to sulfur poisoning, and in fact, require 50-100 parts per million (ppm) sulfur in the syngas to maintain the catalyst. However, the presence of carbon dioxide in the syngas can shift the reaction toward the production of methanol rather than higher alcohols.
A major hurdle to the commercial production of higher alcohols from syngas is the need to improve the selectivity and productivity of catalysts.
Production of higher alcohols occurs in reactors similar to those used in methanol and Fischer-Tropsch processes, and like those processes, removal of the large amount of excess heat generated during reactions is essential to maintaining control of the process temperature, to maximizing yields, and to minimizing deactivation of the catalysts. New designs are also being examined.
Currently higher and mixed alcohols are not commercially produced from syngas, but a number of companies are conducting research including Dow, IFP, Snamprogetti, the Institute of Energy Process Engineering (Research Centre Julich), and the Western Research Institute in collaboration with Power Energy Fuels, Inc.
A number of studies have examined the cost of producing higher alcohols using natural gas as the feedstock. Generally, about 50 percent of the costs of producing alcohols from syngas are for the capital costs associated with syngas production, with 29% of the costs attributed to alcohol synthesis, 17% for CO2 removal, and 4% for alcohol fractionation.