GAS-TO-LIQUIDS TURBOMACHINERY

Gas to liquids (GTL) is a refinery process to convert natural gas, flare gas or other gaseous hydrocarbons into longer-chain hydrocarbons, such as gasoline or diesel fuel. Methane-rich gases — natural gas or associated gases — are converted into liquid synthetic fuels in different processes, such as FT (Fischer–Tropsch) and others. In this column, we discuss turbomachinery for GTL with a focus on the emerging market of offshore GTL.

The FT process starts with partial oxidation of methane to carbon dioxide, carbon monoxide, hydrogen and water (steam). The ratio of carbon monoxide to hydrogen is adjusted using the water-gas shift reaction, while excess carbon dioxide is removed by a dedicated process. Removing the water yields synthesis gas (syngas), which is chemically reacted over a catalyst to produce liquid hydrocarbons and other byproducts. Oxygen is usually provided from a cryogenic air separation unit.

An alternative methanol-to-gasoline (MTG) process converts natural gas to syngas, and then methanol. The methanol is usually polymerized over a catalyst to form alkanes. A third GTL process builds on the MTG technology by converting natural gas-derived syngas directly into drop-in gasoline and jet fuel via a thermo-chemical, single-loop process. This is usually known as the syngas to gasoline plus (STG+) process. STG+ generally follows four principal steps in one continuous process loop. It often consists of reactors in which syngas is converted to synthetic fuels. Most often, four fixed bed reactors in series are used. Alternative equipment can also be employed.

GTL turbomachinery

The GTL process begins with the air compression system. It feeds the syngas unit directly, or through an air separation unit (ASU) if production of oxygen is required as part of the process. A compact, lightweight and efficient air compressor package is important for any FTGTL unit. An axial air compressor is one option for a large FT-GTL unit. Another solution is a hybrid compressor, which contains initial axial stages and final centrifugal stages. This combination benefits both technologies.

Turbocompressors are required for various services in a FT-GTL, such as recycle applications, syngas compression units and others throughout the entire GTL process. They are usually compact barrel-type centrifugal compressors, like those used in methanol and ammonia synthesis trains. Since various GTL process steps are exothermic, steam turbines could be a good option for drivers. Steam is usually generated as part of the process. The large volume of low-caloric tail gas in air-based processes can also be used to generate steam using a compact steam generation unit (compact boiler or similar).

Steam turbines use high enthalpy steam, which is at high pressure and temperature in an expansion process. Steam enthalpy is converted into mechanical energy as it passes through a turbine stage. Each stage consists of nozzles and rotor blades. In the nozzles, the steam is accelerated and transformed into kinetic energy with a reduction in potential energy. The flow is directed onto the rotor blades, which convert kinetic energy to mechanical energy.

Steam turbines for offshore applications are usually compact and relatively light. They often use impulse blading for high reliability and efficiency and require low maintenance. Steam turbines are proven in offshore and marine applications. Those driving compressors or other machinery used in GTL usually require large outputs. The flow at low pressure sections might be greater than typical offshore applications. To meet these requirements, advanced blade cascades should be used for variable speeds, high loads, and low pressures. Steam turbines require less space and are lighter than variable-speed drives (VSD), such as variable-speed gas turbines or VSD-driven electric motors.