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Sasol  (Page 2/4)

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  1. Coal to liquid fuels Sasol is involved in mining coal and converting it into synthetic fuels, using the Fischer-Tropsch technology. [link] is a simplified diagram of the process that is involved.
    The gasification of coal to produce liquid fuels
    Coal gasification is also known as the Sasol/Lurgi gasification process, and involves converting low grade coal to a synthesis gas. Low grade coal has a low percentage carbon, and contains other impurities. The coal is put under extremely high pressure and temperature in the presence of steam and oxygen. The gas that is produced has a high concentration of hydrogen ( H 2 ) and carbon monoxide (CO). That is why it is called a 'synthesis gas', because it is a mixture of more than one gas. In the Sasol Advanced Synthol (SAS) reactors , the gas undergoes a high temperature Fischer-Tropsch conversion. Hydrogen and carbon monoxide react under high pressure and temperature and in the presence of an iron catalyst, to produce a range of hydrocarbon products. Below is the generalised equation for the process. Don't worry too much about the numbers that you see in front of the reactants and products. It is enough just to see that the reaction of hydrogen and carbon monoxide (the two gases in the synthesis gas ) produces a hydrocarbon and water. ( 2 n + 1 ) H 2 + nCO → C n H 2 n + 2 + nH 2 O A range of hydrocarbons are produced, including petrol, diesel, jet fuel, propane, butane, ethylene, polypropylene, alcohols and acetic acids.
    Different types of fuels It is important to understand the difference between types of fuels and the terminology that is used for them. The table below summarises some of the fuels that will be mentioned in this chapter.
    Compound Description
    Petroleum (crude oil) A naturally occurring liquid that forms in the earth's lithosphere (see Grade 11 notes). It is a mixture of hydrocarbons, mostly alkanes, ranging from C 5 H 12 to C 18 H 38 .
    Natural gas Natural gas has the same origin as petroleum, but is made up of shorter hydrocarbon chains.
    Paraffin wax This is made up of longer hydrocarbon chains, making it a solid compound.
    Petrol (gasoline) A liquid fuel that is derived from petroleum, but which contains extra additives to increase the octane rating of the fuel. Petrol is used as a fuel in combustion engines.
    Diesel Diesel is also derived from petroleum, but is used in diesel engines.
    Liquid Petroleum Gas (LPG) LPG is a mixture of hydrocarbon gases, and is used as a fuel in heating appliances and vehicles. Some LPG mixtures contain mostly propane, while others are mostly butane. LPG is manufactured when crude oil is refined, or is extracted from natural gas supplies in the ground.
    Paraffin This is a technical name for the alkanes, but refers specifically to the linear alkanes. Isoparaffin refers to non-linear (branched) alkanes.
    Jet fuel A type of aviation fuel designed for use in jet engined aircraft. It is an oil-based fuel and contains additives such as antioxidants, corrosion inhibitors and icing inhibitors.
    You will notice in the diagram that Sasol doesn't only produce liquid fuels, but also a variety of other chemical products. Sometimes it is the synthetic fuels themselves that are used as feedstocks to produce these chemical products. This is done through processes such as hydrocracking and steamcracking . Cracking is when heavy hydrocarbons are converted to simpler light hydrocarbons (e.g. LPG and petrol) through the breaking of C-C bonds. A heavy hydrocarbon is one that has a high number of hydrogen and carbon atoms (more solid), and a light hydrocarbon has fewer hydrogen and carbon atoms and is either a liquid or a gas.
    Hydrocracking
    Hydrocracking is a cracking process that is assisted by the presence of an elevated partial pressure of hydrogen gas. It produces chemical products such as ethane, LPG, isoparaffins, jet fuel and diesel.
    Steam cracking
    Steam cracking occurs under very high temperatures. During the process, a liquid or gaseous hydrocarbon is diluted with steam and then briefly heated in a furnace at a temperature of about 850 c i r c C . Steam cracking is used to convert ethane to ethylene . Ethylene is a chemical that is needed to make plastics. Steam cracking is also used to make propylene, which is an important fuel gas.
  2. Production of crude oil Sasol obtains crude oil off the coast of Gabon (a country in West Africa) and refines this at the Natref refinery ( [link] ). Sasol also sells liquid fuels through a number of service stations.
    Crude oil is refined at Sasol's Natref refinery to produce liquid fuels
  3. Liquid fuels from natural gas Sasol produces natural gas in Mozambique and is expanding its 'gas to fuel' technology. The gas undergoes a complex process to produce linear-chained hydrocarbons such as waxes and paraffins ( [link] ).
    Conversion of natural gas to liquid fuels
    In the autothermal reactor , methane from natural gas reacts with steam and oxygen over an iron-based catalyst to produce a synthesis gas . This is a similar process to that involved in coal gasification. The oxygen is produced through the fractional distillation of air .
    Fractional distillation
    Fractional distillation is the separation of a mixture into its component parts, or fractions. Since air is made up of a number of gases (with the major component being nitrogen), fractional distillation can be used to separate it into these different parts.
    The syngas is then passes through a Sasol Slurry Phase Distillate (SSPD) process. In this process, the gas is reacted at far lower temperatures than in the SAS reactors. Apart from hard wax and candle wax, high quality diesel can also be produced in this process. Residual gas from the SSPD process is sold as pipeline gas while some of the lighter hydrocarbons are treated to produce kerosene and paraffin. Ammonia is also produced, which can be used to make fertilisers.
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Source:  OpenStax, Siyavula textbooks: grade 12 physical science. OpenStax CNX. Aug 03, 2011 Download for free at http://cnx.org/content/col11244/1.2
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