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Gas to liquid catalysis

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Gas to liquid catalysis (GTL) is a family of reactions, in chemical engineering, which produce more complex organic compounds from simple hydrocarbons available from petroleum. It is not a trivial process, with plants potentially costing in the hundreds of millions and with uncertainty on the best reactions, but it is still cheaper than some conventional petroleum industries. [1] For countries without extensive chemical industries, it offers a relatively simple method to convert trapped natural gas to more easily shipped compounds.

Design

In a given plant, the major design steps are:

  • Definition of the basic technology components
    • Catalyst screening
    • Selection of the reactor system
    • Preliminary modeling.
    • Process integration
  • Pilot plant: validation of the catalysts and reactors; further modeling
  • Demonstration plant: continued validation on a larger scale

Economics

The economic argument in the Middle East, for countries with reserves of natural gas but little local market for it and no pipeline infrastructure to ship it to larger economies, are that "GTL can convert it into a liquid form that is easier to export. This is the same reason such countries crack ethane to make ethylene and convert that into polyethylene, ethylene glycol, and other petrochemicals. It is also why they convert methane into methanol and liquefied natural gas (LNG)."[2]

Cobalt salts, especially cobalt(II) nitrate, are the most common catalysts, but by no means the only ones. When South Africa was under economic embargoes due to its apartheid policy, its Sasol company pioneered GTL using iron catalysts to in high-temperature processes with significant sulfur in the feedstock. Iron, however, produces less desirable aromatic hydrocarbons, oxides, and other by-products less desirable than aliphatic hydrocarbons.

Regardless of the catalyst, most plants begin with a reformer or a gasifier to convert natural gas into carbon monoxide and hydrogen, much as has been used in methanol and ammonia production. These feedstocks then go to a Fischer-Tropsch reactor, which produces paraffin. Paraffins, in turn, is hydrocracked to make diesel fuel and also naphtha, lubricating oil base stocks, and gases.

References

  1. R. L. Espinoza, D. S. Jack, H. A. Wright, J. D. Allison and S. R. Landis, Steps in the Development of the GTL Process, ConocoPhillips
  2. Alexander H. Tullo (21 July 2003), "Catalyzing GTL: Gas-to-liquids fuels are becoming a reality and looming as a robust market for catalysts", Chemical and Engineering News