GTL technology enables conversion of natural gas to refined oil products as diesel, LPG and naphtha and base oils in three major process steps.
Synthesis gas production
Syngas is generated through a reaction of preheated natural gas with oxygen and steam to produce carbon monoxide (CO) and hydrogen (H2) together with some carbon dioxide (CO2).
The Fischer-Tropsch conversion
The F-T reactor converts synthesis gas to oil and wax (long chained hydrocarbon molecules) in a catalytic reaction in a 3-phase slurry bubble column reactor using cobalt based catalyst.
Product upgrading
The Upgrading Unit converts the wax into diesel and naphtha through hydrocracking and hydro isomerisation followed by fractionation.
FT Reactor / the GTL conversion reactor
The F-T reactor is a three-phase, slurry bubble column reactor in which syngas is fed into a suspension of cobalt-based catalytic particles. The syngas enters the slurry reactor and bubbles up through the slurry consisting of finely divided cobalt catalyst and molten wax (a product of the chemical reaction). The high heat of reaction is removed by steam generation in a heat exchanger inside the reactor. The wax is separated from the reactor slurry using a proprietary filtration system located inside the reactor.
Due to the excellent heat transfer properties of the 3-phase system, the reactor can be operated at essentially isothermal conditions, which is ideal for maximising the yield of the desired C5+ products. Furthermore, the use of small catalyst particles allows use of a high-activity catalyst and boosts selectivity by avoiding particle diffusion limitations.
The capacity of such a reaction system is two to three times greater than conventional (fixed-bed) reactors.
FT Catalyst / the GTL conversion catalyst
The Fischer-Tropsch (FT) catalyst can be considered the heart of the GTL technology, determining the fraction obtained of the primary paraffinic wax product. Operated in the slurry bubble column, it has to fulfil a number of requirements including an appropriate catalytic activity, a moderate deactivation with time, suitable dispersion in the reactor medium, and low degradation by chemical or mechanical attrition as well as being adapted to the slurry separation technology at hand. The GTL.F1 2nd generation FT-catalyst fulfils all these requirements.
The catalyst is based on cobalt to achieve the highest possible product yield starting with natural gas. Cobalt is distributed on the highly porous surface of a catalyst support so that a maximum of metal atoms can be exposed for the reaction to take place. Cost-effective commercial scale manufacturing technology has been developed in a joint development program with the catalyst manufacturer Johnson Matthey, and the performance demonstrated at semi-commercial scale.
The catalyst formulation and preparation involve wet impregnation of a cobalt/rhenium solution on porous alumina or modified alumina particles (the alumina support), followed by drying and calcination – i.e. the application of strong heat to decompose the cobalt and rhenium salts. This treatment results in a nominal composition of 12-30 weight per cent cobalt and 0.2 to 1.5 weight per cent of rhenium. Additional promoters can be added. Several key performance factors have to be balanced to obtain the best result; for example, the selection of the right type of alumina porosity versus the physical properties of the catalyst.
A particularly important feature of the 2nd generation catalyst is its unique resistance to attrition and abrasion that allows operation in the harsh and turbulent environment of a slurry reactor. This property is achieved through a proprietary formulation that uses a catalyst support containing a divalent transition metal that forms a spinel compound with the alumina.
Product Upgrade / “Upgrading to commercial products”
The Upgrading Unit consists basically of a hydrocracking reactor and a fractionation column. In the reactor the FT-waxes are converted to petrochemical naphtha, auto diesel and base oils through hydro isomerisation and hydrocracking reactions.
The key to successful processing in the Upgrading Unit is to find an optimal balance between the FT-feed properties, the catalysts, the operating conditions and the product properties.
The catalysts are bifunctional and contain Pt, Ni and/or Mo as the hydrogenation agent.
