Published November 2010
Fischer-Tropsch (F-T) synthesis, the well-known technology for conversion of primary fuels (natural gas, coal, oil-refinery residues and biomass) into secondary fuels (naphtha, gasoline, kerosene, jet fuel, diesel and fuel oil) is also a good potential source of many important oxygenated chemicals (aldehydes, ketones, alcohols, acids, etc.). In addition to chemicals, the process generates a large amount of water. For example, a high-temperature, iron-catalyzed F-T process producing 15,000 b/d of fuels can generate about 240–250 thousand pounds of water per hour from the F-T reaction alone. This water may contain up to 6.0 wt % oxygenates, about 80–85% of which is comprised of alcohols, aldehydes and ketones. Detailed composition information is provided in this Review. If a Synthol advanced reactor system is used, the amount of the three major oxygenates (ethanol, propanol and acetone) produced in the F-T reaction may be about 4,320 lb/hr, 1,185 lb/hr, and 3,315 lb/hr, respectively. On a yearly basis, the production rate of the aforementioned oxygenates is equivalent to 34.1 million lb, 9.3 million lb, and 26.2 million lb, respectively. The market price of the three chemicals in the U.S. is about 25¢/lb, 55¢/lb and 37¢/lb, respectively. The cumulative market value of the three oxygenates is, therefore, equivalent to $23.3 million.
Apart from recovery of valuable oxygenated products, in certain areas where carbonaceous feedstocks are found, water may be in short supply and, thus, a relatively costly commodity. Also in some regions, environmental regulations prevent the disposal of the polluted water derived from the F-T process into the natural waterways. In such situations, purification and recovery of the process wastewater become necessary.
The cost of purifying water and recovering oxygenated chemicals therefrom could be prohibitive where energy costs are high. But, in some instances, it could be inevitable.
This Review presents a detailed account of the oxygenated products contained in F-T reaction waters, and the various water purification schemes that have been researched and proposed. Some of them are already in industrial use. Cost data for the extraction of oxygenates are beyond the scope of this Review. Analysis of those costs will be provided in a future Review.