The Process Behind GTL Conversion

Liquid hydrocarbon fuels can be produced from associated natural gas via a well-known catalytic chemical reaction called Fischer-Tropsch (FT) synthesis. The FT synthesis is a collection of chemical reactions that converts a mixture of carbon monoxide and hydrogen into liquid hydrocarbons. It was first developed by Franz Fischer and Hans Tropsch at the "Kaiser-Wilhelm-Institut für Kohleforschung" in Mülhei an der Ruhr (Germany) in the 1920s. During World War II, FT synthesis provided the needed liquid hydrocarbon fuels for the German war effort. Later, facing isolation during the apartheid era, South Africa turned to FT synthesis from coal gasification to supply significant quantities of its hydrocarbon fuel needs. Since then, many refinements and adjustments to the technology have been made, including catalyst development and reactor design. The process, a key component of gas to liquids technology, produces a synthetic lubrication oil and synthetic fuel, from natural gas, yet it can also be produced from coal or biomass. The FT process has received intermittent attention as a source of low-sulfur diesel fuel and to address the supply or cost of petroleum-derived hydrocarbons. It is also a process that may allow a substantial decrease in offshore associated gas flaring.

The FT process is a catalytic chemical reaction in which carbon monoxide (CO) and hydrogen (H2) in the syngas are converted into hydrocarbons of various molecular weights according to the following equation:

(2n+1) H2 + n CO → Cn H(2n+2) + n H2O

Where n is an integer. Thus, for n=1, the reaction represents the formation of methane, which in most CTL (Coal to Liquid) or GTL applications is considered an undesirable byproduct. The FT process conditions are usually chosen to maximize the formation of higher molecular weight hydrocarbon liquid fuels, which are higher value products. There are other side reactions taking place in the process, among which the water-gas-shift reaction: CO + H2O → H2 + CO2 is predominant. Depending on the catalyst, temperature, and type of process employed, hydrocarbons ranging from methane to higher molecular paraffins and olefins can be obtained. Small amounts of low molecular weight oxygenates (e.g., alcohol and organic acids) are also formed. The FT synthesis reaction, in theory, is a condensation polymerization reaction of CO. FT synthesis is technically classified into two categories, the high-temperature (HTFT) and the low-temperature (LTFT) processes. The criterion for this classification is the operating temperature of the synthesis, which ranges between 310−340 °C for the HTFT process and 210−260 °C for the LTFT process. An FT facility can be divided into roughly three sections, synthesis gas (syngas) generation, FT synthesis, and refining of the synthetic crude (syncrude).

The FT technology has experienced dramatic improvements in catalyst technology and an aspect vital to fitting the FT reactor and ancillary reactors and equipment aboard FPSOs, has been the decrease in the reactor size and cost. Most of the worldwide capital investment in GTL is being made to build module plants that can fit on FPSOs. There are 2 processes being used; one is the FT process and the other from ExxonMobil, which converts natural gas to gasoline via methanol. General Electric is building a GTL module plant and a partnership between Petrobras and CompactGTL has a working test plant in Brazil, which is scheduled to be installed on SBM FPSOs for pre-salt production, thus beginning to eliminate flaring in Brazil.

With the long distances (around 300km) from the pre-salt plays to the coast of Brazil, the possibility of unloading oil offshore is attractive in comparison to using pipelines for oil transportation. Since the GTL product can be mixed with crude, it greatly simplifies the downstream process and may allow new fields to produce in less time. The Brazil plant demonstrates the world’s first fully integrated small scale GTL facility, at 200,000scf/d capacity, incorporating:

• Gas pre-treatment

• Pre-reforming

• Reforming

• Waste heat recovery

• Process steam generation

• Syngas compression

• Fischer-Tropsch synthesis

• FT cooling water system

• Tail gas recycling

Analysts forecast that this technology will soon be present at the GoM, WA and North Sea and hopefully we will see a drastic reduction in offshore gas flaring worldwide along the next decade.

Sources: U.S. Department of Energy, Petrobras, SBM, CompactGTL, Wikipedia, Biofuels Academy and NSF

 

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