Continuing this 4-part article series on natural gas conversion, this next piece looks at the technology involved in the conversion process. You can read part 2 on the history of conversion here.
Technology and Gas Conversion
The F-T process uses a specialized catalyst to facilitate the conversion of CO into heavier molecules, mostly alkanes. An alkane is a class or family of chemicals that all share the general formula CnH2n+2. Don’t let the symbols throw you. The formula just says that for every (n) number of carbon atoms there are (2n+2) hydrogen atoms, for example C3H8 or C6H14. The family includes many useful compounds whose names are in general use by the public, such as methane, butane, propane, and octane, to name a few. As the number of carbon atoms in each molecule increases, the higher the compound’s boiling point becomes. Methane with one carbon is a gas, even at the North Pole in January. Propane with three carbons is a gas at room temperature but can be compressed to form a liquid for use in gas grills, and home heating as well as other uses. Octane, with eight carbons, is a liquid with about the same boiling point as gasoline. Paraffin wax is an alkane with 20 to 30 carbon atoms and is a solid at room temperature but is still flammable if given enough encouragement. The F-T process produces a mixture of alkanes from methane (one carbon) to paraffin wax (30 or more carbon atoms).
The percentage of each alkane produced is controlled by the selection of catalyst and the operating conditions in the F-T catalytic converter. The most commonly used catalysts include those made from transition metals like cobalt, iron and ruthenium. For natural gas F-T processes, cobalt is the most commonly used. Unfortunately, it is impossible to have the process produce only the alkane desired. Mixtures of different alkanes are inevitable. So, the F-T unit is often followed by a catalytic cracker where the heavier alkanes can be thermally “cracked” to reform lighter, often more desirable liquid fuels. The process produces very pure liquid fuels, such as ultra-low sulfur synthetic diesel and high purity jet fuel.
Technology has surged forward since WWII and modern materials science has created new catalysts that are significantly more efficient at converting CO into alkanes and allowing simpler upstream gasification steps. The newer catalysts also help narrow the range of alkanes produced, making refining the final products easier and more cost effective. It is now possible to produce ultra-low sulfur diesel at prices less than the current retail price of petroleum sourced ultra-low sulfur diesel. Modern designs also include secondary processes to capture and make use of what were once waste materials, such as carbon dioxide, extra hydrogen and organic chemicals that were formed as byproducts.