Liquid Fuels Synthesis

Two, commercially proven technologies for converting the synthesis gas to liquid transportation fuels are considered. One of the technologies, Fischer-Tropsch synthesis, was developed in Germany in the 1920s, and several commercial plants were constructed to produce transportation fuels during World War II. The technology was commercialized by Sasol in South Africa in the 1950s and expanded in the early 1980s to produce about 150,000 bpd of transportation fuels from coal. Early versions of the technology used iron-based catalysts, which produce a broad range of products from methane to high molecular-weight waxes, but also produce a range of oxygenated materials. Figure 3.2 schematically shows the process layout of a Fischer-Tropsch process.

The primary Fischer-Tropsch synthesis reaction is illustrated by:

The hydrocarbon distribution follows an Anderson-Schultz-Flory distribution given by:

• Wn is weight fraction of hydrocarbon with n carbons

• a is chain growth probability

On a molecular basis, methane is most prevalent product. Higher a increases the molecular weight of the product. a increases with decreasing temperature and is also affected by catalyst type.

Because of the broad molecular weight range of products, their normal-paraffinic nature, and the presence of oxygenates, Fischer-Tropsch product requires a large

Purge /

O2/Steam Fuel Gas

Purge /

O2/Steam Fuel Gas

( for internal use )

Fig. 3.2 Schematic of Fischer-Tropsch process producing liquid transportation fuels from coal [1]

( for internal use )

Fig. 3.2 Schematic of Fischer-Tropsch process producing liquid transportation fuels from coal [1]

Table 3.1 Typical product distributions in wt% for iron Fischer-Tropsch catalysts operating at low temperature (LTFT) and at high temperature (HTFT)

Product_LTFT_HTFT

CH4 4 7

C2C4 olefins 4 24

C2-C4 paraffins 4 6

Gasoline 18 36

Middle distillate 19 12

Heavy oils and waxes 48 9

Water soluble oxygenates 3 6

amount of refining to produce finished transportation fuels. Recently developed catalysts, based on cobalt, are more selective to hydrocarbon products but still produce a broad molecular-weight range of normal-paraffinic compounds that require upgrading. The cobalt-based product distribution requires somewhat less refining to produce final transportation fuels. Cobalt catalysts are very sensitive to poisoning by sulfur and other impurities. Synthesis temperature markedly affects product distribution as indicated in Table 3.1. A number of reactor types have been applied to Fischer-Tropsch synthesis. Each has advantages and disadvantages. The best reactor type and catalyst for commercial-scale coal-to-fuels is yet to be clearly demonstrated. The two primary contenders appear to be ebulated, slurry-bed (fixed fluid-bed) or fixed-bed reactors.

FT technology continues to be improved. The large Sasol plants in South Africa originally used circulating fluid-bed Synthol reactors, but these have been replaced with fixed fluid-bed reactors (SAS reactors). These form the basis for the large (36 ft diameter) slurry reactors, which produce ~17,000 bpd of fuels each, installed at the Oryx gas-to-liquids plant in Qatar. Shell has developed improved fixed-bed Fischer-Tropsch reactor technology which forms the basis of their Shell Middle Distillate Synthesis (SMDS) process. They built a 12,000 bpd plant in Bintulu, Malaysia, which has been operating since 1980. Others have developed or are developing their own version of FT technology.

Another technology for production of liquid transportation fuels from coal or biomass involves the production of methanol from the synthesis gas, followed by its conversion to gasoline by methanol-to-gasoline (MTG) technology developed by Mobil Oil. The main reactions include:

• Syngas of methanol

• Methanol dehydration to dimethyl ether

2CH3OH ^ CH3OCH3 + H2O

• MTG Reactions over a shape selective zeolite catalyst

where (n-CHy-) represents gasoline, which is a complex hydrocarbon mixture.

Figure 3.3 illustrates the main processing steps in coal to gasoline using MTG. Methanol synthesis is large-scale commercial technology that can be supplied by several technology licensors and is used to produce methanol from coal today. It is well-developed and highly selective to methanol [2]. Single-train methane-based methanol plants up to 5,500 tonnes of methanol per day have been built.

Natural Gas Coal BioMass

Natural Gas Coal BioMass

Commercial technology with multiple technology suppliers

Primarily Gasoline

Fig. 3.3 Gasoline production from coal, biomass, or natural gas using methanol to gasoline (MTG) technology. Feeds are natural gas, coal, or biomass, water and oxygen

Primarily Gasoline

Fig. 3.3 Gasoline production from coal, biomass, or natural gas using methanol to gasoline (MTG) technology. Feeds are natural gas, coal, or biomass, water and oxygen

Table 3.2 Typical gross (wt.) hydrocarbon product distribution from methanol conversion to gasoline by MTG [5]. Given in tons of product per 1,000 tonnes of methanol feed

Products

Production (1,000 tonnes MeOH)

Gasoline (88% of HC)

Fuel gas

387 7

MTG, developed by Mobil Oil in the 1970s, is based on shape-selective zeolite catalysts that produce almost-exclusively hydrocarbon molecules in the gasoline range [3, 4]. The principle product is high-octane gasoline. Methanol synthesis is highly selective, and methanol is almost the only product. Methanol is then converted to gasoline in another highly selective process. Table 3.2 gives a typical gross (wt.) product distribution for the hydrocarbon fraction in the conversion of methanol by MTG [5]. The gasoline fraction has an average research octane number of 92.2 and an average motor octane number of 82.6 and meets all other specifications for U.S. commercial gasoline [5]. A 14,500 bpd gasoline plant was started-up in New Zealand in 1985 and operated for about 10 years using natural gas as feed to produce methanol, which was then converted to gasoline [5]. The MTG portion of the plant was shut down when it became economically attractive to sell methanol rather than gasoline because of decreasing crude price. A second-generation, 100,000 tonnes/ year MTG plant was constructed in Shanxi Province, China and started up in June 2009 [6]. Plans to increase the scale to one million tonnes/year have been announced [6]. Two MTG projects have been under consideration for the U.S. although lower crude prices have slowed progress on them. A variant of MTG involves conversion of methanol to olefins and the conversion of these olefins to gasoline and diesel. It is referred to as methanol to olefins, gasoline, and diesel.

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