Synthesis into liquid fuels

The Fischer-Tropsch process converts synthesis gas into hydrocarbon products. It was extensively used by Germany in the Second World War and developed in South Africa during the Apartheid years. It is now subject to extensive research and development for the conversion of coal into liquid fuels as an alternative to crude oil. The general process flow-sheet is shown in Figure 11.4. 

One of the most commercially desirable appHcations for OTMs is the spontaneous partial oxidation of natural gas into synthesis gas. This latter feedstock is of major importance for its subsequent conversion to methanol and to hydrogen. Additionally, synthesis gas can be converted, via Fischer-Tropsch chemistry, into liquid fuels (gas to liquids, GTL) particularly under circumstances where natural gas is located remotely from the place of consumption. Because the resulting hquid fuels possess an intrinsically higher energy density than natural gas, they would be less expensive to transport. One prime example is the Trans-Alaska Pipeline System, which in the future could be used to transport synthetic liquid fuels derived from Alaska s vast reserves of natural gas in the Prudhoe Bay area to the Port of Valdez [36]. This is one of many remote natural gas locations around the world which could take advantage of OTM technology. 

Dry reforming of methane is gaining great interest owing to the fact that this process efficiently converts two greenhouse gases (CH4 and CO2) into synthesis gas (CO -I- H2), which can be further processed into liquid fuels and chemicals by Fischer-Tropsch and similar processes. 

Another application of the hydrocracking process could become important during the next decade, when the transformation of natural gas into liquid fuels is considered to be economically feasible. In a route going via the manufacture of synthesis gas and the production of heavy paraffins by Fischer-Tropsch synthesis, a hydrocracking operation could be the final step in order to produce a high quality diesel engine fuel. 

Coal is currently economically useful only in plants that are equipped for large-scale handling of solids, and it is used only indirectly as a raw material for chemical synthesis. Accordingly, there has been considerable research on processes for converting coal into gaseous or liquid fuels and chemicals. Only gasification has advanced to commercial status. 

The South African government initiated the Mossgas project in the mid-1980s to investigate the conversion of gas and associated natural gas liquids into transportation fuel. This eventually led to the construction of the Mossgas gas-to-liquids plant (presently known as PetroSA) in Mossel Bay, South Africa. It was designed as a 33,000 barrels per day oil equivalent facility, with two thirds of the production being derived from Fischer-Tropsch synthesis and the remainder from associated gas liquids. This facility reached full commercial production in 1993 and was aimed at the production of transportation fuel only.50... 

Acidic chloroaluminate ionic liquids are excellent media for polymer cracking reactions. With the huge quantities of polymers that need to be disposed of each year the ability to break them down into useful compounds for new synthesis or to use as liquid fuels is extremely important. While certain polymers such as poly(methyl methacrylate) are easily cracked into their constituent monomers that can be reused, the majority of polymers are extremely difficult to crack into useful organic compounds. However, merely dissolving polyethylene in acidic chloroaluminate ionic liquids containing a proton source results in the formation of a mixture of alkenes and cyclic alkenes [48], The key compounds produced are shown in Figure 10.10. 

The first step toward making liquid fuels from coal involves the manufacture of synthesis gas (CO and H ) from coal. In 1925, Franz Fischer and Hans Tropsch developed a catalyst that converted CO and at 1 atm and 250 to 300°C into liquid hydrocarbons. By 1941, Fischer-Tropsch plants produced 740 000 tons of petroleum products per year in Germany (Dry, 1999). Fischer-Tropsch technology is based on a complex series of reactions that use to reduce CO to CH groups linked to form long-chain hydrocarbons (Schulz, 1999) ... 

It was discovered in the late nineteenth century that coal can be incompletely burned to yield a gas consisting primarily of CO and H2, and many people were undoubtedly asphyxiated and kUled by explosions before these processes were harnessed successfully. We wfil see later that the use of a CO + H2 mixture (now called synthesis gas) for the production of chemicals has had an important role in chemical synthesis (it was very important for explosives and synthetic fuels in both World Wars), and it is now one of the most promising routes to convert natural gas and coal into liquid diesel fuel and methanol. We will describe these processes in more detail in later chapters. 

The FTS converts synthesis gas into mostly liquid hydrocarbons [12-15]. Depending on the origin of the synthesis gas, the overall process from carbon feedstock to liquid product is called gas-to-liquids (GTL), coal to liquids (CTL), or biomass to liquids (BTL). The product spectrum, however, is broader than liquid hydrocarbons alone and can include methane and alkanes, C H2 +2 (with n from 1 — 100), alkenes or olefins (C H2 n > 2), and to a lesser extent, oxygenated products such as alcohols. Hence the FTS offers the opportunity to convert gas, coal, or biomass-derived syngas into transportation fuels, such as gasoline, jet fuel, and diesel oil, and chemicals, such as olefins, naphtha, and waxes. The reactions need a catalyst, which in commercial applications is either based on cobalt or iron. 

The conversion of natural gas into liquid transportation fuel is a well-proven technology today. There are two basic technologies available the Fischer-Tropsch synthesis and the methanol route developed by Mobil Research and Development Corporation, USA. 

It was estimated that this process route was approximately twice as costly as converting natural gas to liquid fuels via synthesis-gas-based routes such as Fischer-Tropsch. However, some novel developments in gas separation technology and high temperature heat recovery could bring the process back into contention. 

Since the late 1940s Royal Dutch/Shell companies have been carrying out research and development work on hydrocarbon synthesis for the conversion of various raw materials, such as coal and natural gas, into liquid transportation fuels. When crude from the Middle East became increasingly available, some reduction in this work occurred, but interest revived in the early 1970s. 

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