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Gas to liquids (GTL) is a process for converting natural gas into synthetic oil, which can then be further processed into fuels and other hydrocarbon-based products. In the simplest of terms, the GTL process tears natural gas molecules apart and reassembles them into longer chain molecules, like those that comprise crude oil.
However, with this particular conversion process, the result is an extremely pure, synthetic crude oil that is virtually free of contaminants such as sulfur, aromatics, and metals. This synthetic crude can then be refined into quality products such as diesel fuel, naphtha, wax, and other liquid petroleum or specialty products.
The lubricant base stock created in the gas-to-oil conversion is called an ISO paraffin. ISO paraffins appear to be a viable alternative for use as a base oil for lubricants that are currently blended from API Group III and IV base stocks.
The dominant demand for Group III and IV base stocks is from the automotive manufacturers. Lubricant performance parameters are being stressed by the increasing demands for reduced emissions and increased energy efficiency placed upon internal combustion engine designers. High-quality base stocks are integral to the formulation of lubricants that meet the new demands.
ISO paraffins seem to provide the type of performance required for internal combustion engine operating conditions, including good viscosity properties (viscometrics), oxidation resistance, and low-temperature cold-cranking conditions. Development of these stocks into useful long-term lubricants could be employed as an alternative to crude oil-derived products.
Industrial machinery often requires lubricant performance in the same general temperature and film strength range as crankcase engine oils.
The GTL process is based on two primary steps:
The world’s proved and potential natural gas reserves are estimated to be more than 14,000 trillion cubic feet (tcf). Most of these reserves are considered stranded because they are too far from the consumers and difficult to transport. GTL has the potential to convert a significant percentage of this gas into several hundred billion barrels of liquid petroleum - enough to supply the world’s energy needs for the next 25 years.
GTL offers tremendous economic value to the countries and/or companies that control these reserves. GTL will permit the economic development of these remote natural gas discoveries that currently are deemed too far from market to be of economic value.
GTL will help eliminate the need for flaring natural gas associated with oil production. This will permit earlier development and production of oil fields shut in by the inability to dispose of the associated natural gas, and reduce the negative environmental impact of flaring.
GTL will yield synthetic hydrocarbons of the highest quality that can be used directly as fuels or blended with lower-quality crude oil-derived fuels to bring them up to compliance with increasingly stringent environmental and performance specifications.
Dr. H. Ernest Henderson1 referred to gas to liquids as the new horizon in lubricant base stocks at last year’s ILMA/ICIS-LOR Base Oils and Petroleum Additives Conference. In his presentation, Fischer-Tropsch Gas to Liquids: Performance Beyond Current Synthetics, he provided attendees with information on the history of the process, fluid properties, and performance evaluations.
German chemists Franz Fischer and Hans Tropsch invented the F-T process in 1923, and it was used in Germany to produce fuels during World War II. From 1948 to 1953, the F-T process was further developed in Brownsville, Texas, and is now used commercially by Sasol (South Africa), Mossgas (South Africa), and Shell. Several companies are currently considering or pursuing GTL developments, including BP/Amoco, Conoco, ExxonMobil, Rentech, and Syntroleum.
Iso-paraffins represent the ideal base stock chemical structure. These structures are readily produced during the F-T wax upgrading step and are consistent with those found in polyalphaolefins, or PAOs. The Viscosity Index (VI) of iso-paraffins is extremely high while providing excellent oxidation resistance and good pour points.
“GTL provides the best combination of kinematic viscosity VI, pour point, volatility and composition when compared with commercially available API Group III base stocks,” Henderson said. He further noted that in most cases, the properties of GTL fluids exceed the industry range for any base stock properties, as seen in Table 1.
“GTL fluids also provide an excellent match for PAOs,” Henderson said. “Their excellent VI, flash point, and volatility more than offset the marginal low-temperature property represented by pour point, which can be further improved, if necessary, through increased dewaxing severity.” Henderson also mentioned that key ILSAC GF-3 engine tests have been completed to demonstrate GTL performance.
“The Sequence IIIF test focuses on oil viscosity control, oil consumption, deposits and wear, while the Sequence VIB fuel economy test is a difficult procedure with major CAFE (Corporate Average Fuel Economy) implications,” he said.
“The Sequence IIIF test results included exceptional oil consumption for the GTL formulation as an SAE 0W-20 viscosity grade, excellent viscosity increase control, oil consumption comparable to API Group III/IV oils of higher SAE viscosity grades, and excellent deposit results. Wear performance was not representative of the oil/additive system based on discussions with industry experts and oil formulators.
“The Sequence VIB test showed the GTL oil easily met the ILSAC GF-3 fuel economy requirements and, in fact, exceeded the proposed ILSAC GF-4 limits,” Henderson said. “Although the proposed Sequence VIC test will require a longer oil conditioning period, this is not expected to be a difficulty with GTL synthetic fluids.
“Fischer-Tropsch GTL synthetic fluids are positioned for the future,” Henderson concluded. “They have an outstanding combination of viscometry, volatility, low-temperature, and compositional properties that exceed the American Petroleum Institute’s Group III ‘Best of Industry’ while matching PAO standards. GTL has demonstrated excellent performance in critical GF-3 engine tests while initial driveline testing is consistent with PAO. I’m very encouraged for the future of GTL.”