Gas-to-liquids production

The gas-to-liquids (GTL) process converts natural gas to liquid products such as transportation fuels (diesel, gasoline), or other products such as dimethyl ether or methanol. It has been in use for decades (e.g., the Shell Middle-Distillate Process has been online in Malaysia since 1993) [12,19] and has recently received a sudden increase in attention because of game-changing discoveries of shale gas reserves and corresponding technical advances in shale gas recovery [20]. 

The Sasol processes are of interest now that the conversion of synthesis gas-to-liquid products is being developed by a number of companies. The gas-to-liquid (GTL) process is of particular interest in the production of sulfur-free distillates. 

Products from coking processes vary considerably with feed type and process conditions. These products are hydrocarbon gases, cracked naphtha, middle distillates, and coke. The gas and liquid products are characterized by a high percentage of unsaturation. Hydrotreatment is usually required to saturate olefinic compounds and to desulfurize products from coking units. 

Conversion from coal to natural gas. Sasol 1 was designed as a coal-to-liquids facility. A natural gas pipeline was constructed and commissioned in 2004. This allowed the Sasol 1 facility to be converted to a gas-to-liquids plant. Although it implied that the associated coal tar refinery would become redundant, the decision was made by Sasol to keep the coal-to-chemicals units at Sasol 1 in operation by supplying coal pyrolysis products from its larger CTL facility in Secunda. 

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... 

Remote and relatively small gas fields cannot justify the high investment cost associated with liquefied natural gas (LNG) production or a gas pipeline system. Conversion of the natural gas from such gas fields to liquids by a gas-to-liquids facility allows these gas fields to be exploited. 

Gas-to-liquids (GTL) is the chemical conversion of natural gas into petroleum products. Gas-to-liquid plants use Fischer-Tropsch technology, which first converts natural gas into a synthesis gas, which is then fed into the Fischer-Tropsch reactor in the presence of a catalyst, producing a paraffin wax that is hydro-cracked to products (see also Chapter 7). Distillate is the primary product, ranging from 50% to 70% of the total yield. 

Estimates of capital costs of GTL plants display a wide range while the EIA (2006) indicates capital costs at US 25 000 45 000 per barrel of daily capacity, depending on production scale and site selection, the IEA (2006) reports capital costs of GTL plants currently completed or under construction with US 84 000 per barrel. By comparison, the costs of a conventional refinery are around 15 000 per barrel per day. Gas-to-liquid is assumed profitable when crude oil prices exceed 25 per barrel and natural gas prices are in the range of 0.5-1.0/GJ (EIA, 2006). The economics of GTL are extremely sensitive to the cost of natural gas. 

Second-generation biofuel technologies make use of a much wider range of biomass feedstock (e.g., forest residues, biomass waste, wood, woodchips, grasses and short rotation crops, etc.) for the production of ethanol biofuels based on the fermentation of lignocellulosic material, while other routes include thermo-chemical processes such as biomass gasification followed by a transformation from gas to liquid (e.g., synthesis) to obtain synthetic fuels similar to diesel. The conversion processes for these routes have been available for decades, but none of them have yet reached a high scale commercial level. 

Gas-to-liquids plants are generally located close to natural gas fields, as the transport costs for liquid fuels are less than those for gaseous fuels. The production of GTL is considered to be an alternative to liquefied natural gas (LNG), specifically when focusing on the end-product vehicle fuel and not the long distance transport of energy. In 1993, a first large-scale GTL plant was erected by Shell in Bintulu, Sarawak in Malaysia, based on Fischer-Tropsch synthesis. The plant s total thermal process efficiency is about 63% (Shell, 1995) (see Table 7.11) a second plant is under construction in Qatar, with production expected to begin in 2010. 

Palm oil has been cracked at atmospheric pressure and a reaction temperature of 723 K to produce biofuel in a fixed-bed microreactor. The reaction was carried out over microporous HZSM-5 zeolite, mesoporous MCM-41, and composite micromesoporous zeolite as catalysts. The products obtained were gas, organic liquid product, water, and coke. The organic liquid product was composed of hydrocarbons corresponding to gasoline, kerosene, and diesel boiling point range. The maximiun conversion of palm oil, 99 wt.%, and gasoUne yield of 48 wt.% was... 

In trickle beds, the gas-to-liquid, kigaGL, and liquid-to-particle, kfaLS, coefficients are used to represent the effect of the external mass transfer resistances. The interfacial areas aGl and <2ls refer to the effective mass transfer surface per unit volume of empty reactor. Due to the fact that the coefficients kig and klL cannot be easily estimated independently from the corresponding interfacial areas aGL and aLS respectively, by simple experimental techniques, correlations are normally reported for the products kigaGL and k,a]S (Smith, 1981). 

The fact that dissolved ozone is produced brings a very important advantage to subsequent ozone applications mass transfer from the gas to liquid phase is not required. Efficient mixing (e. g. with static mixers) of the ozone-rich pure water stream with the (waste-)water stream to be treated, though, is required. During this in-situ ozone production, the liquid ozone concentration (cL) can easily reach the solubility level (cr ), depending on the pressure (P) and temperature (T) in the cell. Oversaturation of the feed-water will immediately occur, when the pressure drops. Due to this potential degassing, vent ozone gas destruction is also required for this system. 

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