Kluthe Magazine
What is the GtL Process?
Gas-to–Liquids is the term used for the conversion of natural gas into liquid hydrocarbons. In several process steps, GtL oil and fuels are produced that differ from those derived from crude oil due to their high purity. GtL fuels burn more cleanly, are biodegradable, and reduce environmental impact. Lubricants produced in this way are skin-friendly and nearly odorless. These advantages justify the complex production process.
Various Gas-to-Liquids Processes Available
In the GtL process, purified natural gas is first converted into synthesis gas, which is then further processed into long-chain hydrocarbons. These form the basis for the production of oil and fuels.
Treating Natural Gas
Natural gas is primarily composed of methane and contains a range of gaseous impurities (water vapor, simple hydrocarbons, carbon dioxide, hydrogen sulfide, nitrogen, noble gases). Before further processing, these substances must be removed. Water and hydrocarbons like ethane, butane, or propane are removed from the natural gas through drying methods. In most cases, absorption methods are used, where a hygroscopic liquid trickles countercurrent through the natural gas and binds the water along with the other substances. The degree of drying is described by the dew point. This is the temperature at which parts of the gas mixture just begin to condense. Carbon dioxide and hydrogen sulfide can be bound by washing the gas with special solvents. When larger amounts of hydrogen sulfide are present, sulfur is produced as a byproduct.
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The low boiling points of nitrogen and noble gases allow for separation at exceptionally low temperatures. Nitrogen only liquefies at -321 °F, and noble gases have similarly low boiling points. Methane liquefies at -260 °F. If the gas mixture is cooled to a temperature between -321 °F and -350 °F, the methane is mostly liquid, and the other gases escape from this liquid.
Producing Synthesis Gas
Synthesis gas is a mixture of carbon monoxide and hydrogen. Both components are highly reactive and can be combined into new chemical compounds using suitable methods. Producing these compounds and further processing them is the core of Gas-to-Liquids processes. Before these processes can begin, the relatively stable methane molecules must be split. Synthesis gas from methane is therefore also called split gas. A number of methods can be used to achieve this splitting,
In steam reforming, methane decomposes through the addition of hot steam at temperatures above 1650 °F. The process takes place in the presence of a catalyst.
Another method is partial oxidation, where pure oxygen is introduced at temperatures above 2200 °F. This leads to the partial combustion of methane, yielding carbon monoxide and hydrogen. Ultra-pure synthesis gas can also be produced using a plasma converter, which splits methane into carbon and hydrogen at extremely high temperatures. The resulting carbon reacts with carbon dioxide or steam to form carbon monoxide, thus producing pure synthesis gas.
Gas-to-Liquids: Converting Synthesis Gas into Liquids
Methanol Synthesis
Using copper, zinc oxide, and aluminum oxide compounds as catalysts, methanol can be produced from synthesis gas. This substance can be further processed into gasoline or diesel fuel. In reference to the term Gas-to-Liquids, the production of gasoline is also known as Syngas to Fuel.
Fischer-Tropsch Process
Developed in the 1920s by German chemists Franz Fischer and Hans Tropsch, this GtL process is based on the action of catalysts that temporarily form intermediate compounds during the chemical reaction and then release them again. This enables reactions that would otherwise not occur or would proceed only very slowly. Catalysts such as iron, nickel, or cobalt are used. During the reaction, temperatures range from 320 °F to 570 °F, and pressures reach up to 360 psi. The type of catalyst and the reaction conditions determine the composition of the resulting liquid mixture. A broad spectrum of hydrocarbons is created, with the main goal being the production of synthetic motor oils, lubricants, and fuels. In addition, long-chain wax-like substances can be produced. Byproducts often include oxygenated compounds, such as alcohols and acetone, which are used in the chemical industry as solvents or raw materials. In order to process and use the individual components, the liquid mixture must first be separated.
Processing the Hydrocarbon Mixture
Long-chain, wax-like hydrocarbons are converted into smaller molecules through a process known as cracking, which involves applying heat or using catalysts to break the bond. Fractional distillation uses the varying boiling points of the hydrocarbon components to separate them. This process takes place in tall columns. At the bottom (the sump), part of the liquid is vaporized. At the top (the head), part of the vapor is condensed and returned into the column. As the vapor rises through the column, it encounters the descending condensate.
The vapor increasingly absorbs the more volatile substances. The condensate becomes enriched with the less volatile components. By drawing from the column at different heights, it is possible to specifically extract individual components (fractions), such as GtL oil or fuels.
Advantages of Gas-to-Liquids Products
Unlike petroleum-based products, products obtained through Gas-to-Liquids processes contain no aromatic hydrocarbons and hardly any sulfur or nitrogen compounds. Most aromatic hydrocarbons are toxic, evaporate easily, and raise both the vapor pressure and flash point of fuels. Sulfur and nitrogen compounds can quickly cause corrosion to metal surfaces, equipment, and pipelines.
The high purity of GtL products is evident from their clear, bright appearance and barely perceptible odor.
Fuels burn with higher efficiency and release virtually no soot particles, sulfur oxides, or nitrogen oxides. As a result, local emissions and environmental impact are reduced. Lubricants from Gas-to-Liquids production are also characterized by good skin compatibility, which is particularly beneficial when using cooling lubricants that workers in metal processing regularly encounter.
