A turbo-expander, also referred to as an expansion turbine, is a centrifugal or axial flow turbine in which a high pressure gas expands to produce useful work, generally to drive equipment or machinery. The device often provides an attractive option for recovering energy when the pressure of a gas stream needs reducing — and so finds use in a wide variety of plants. Because the work comes from the expanding high pressure gas, the expansion is approximated by an isentropic (nearly constant entropy) process; the reduced pressure exhaust gas from the turbo-expander is at a lower temperature than that of the inlet gas.
Turbo-expanders handle a wide range of services, from those involving cold gases (say, -270°C) to hot gases (above 350°C). Typically, use of a turbo-expander makes sense only for relatively large gas flows. As a very rough indication, units currently in operation range in power ratings from about 25 kW to around 25 MW. However, some special applications have used turbo-expanders with ratings lower than 25 kW.
A turbo-expander operates according to the thermodynamic and fluid-dynamic laws of physics. Operational flexibility is a key requirement because the device often must handle a wide range of flowrates and pressures. Therefore, it usually has not just one rated operating point but also several alternative ones. When designed and manufactured properly, a turbo-expander can yield very high efficiencies at its rated point and reasonable efficiencies at alternative operating points.
In a simple, single-stage turbo-expander, the high pressure gas flows through variable inlet nozzles (or inlet guide vanes) and then through the wheel, exhausting at a lower pressure and substantially colder temperature. In many applications, the outlet gas goes to a downstream process; therefore, turbo-expander nozzles are used to control the gas flowrate and conditions to maintain the operating conditions (flowrate, pressure, etc.) required downstream. The operation of some turbo-expanders can pose some operational risks. For instance, due to fast temperature reduction of the gas, partial liquefaction or condensation of the expanded gas isn’t uncommon.
Manufacturers of turbo-expanders rely heavily on standardization; most components are pre-designed. Parts normally needing to be customized for each specific application are the wheels, shafts, nozzle assembly, gear systems, auxiliaries and controls. Each standard size/model is linked directly to a casing and, therefore, the overall dimension of the machinery. Every standard model or frame can accommodate a specific range of turbo-expander wheel diameters. Model sizes also are directly distinguished by the design pressure and flowrate. The pressure sets the flange ratings. As a very rough indication, wheel diameter can reach up to around 2 m. Fabricated casings are common but ones made by casting or other methods are used sometimes. A fabricated casing provides flexibility to design and manufacture for a broad range of applications, ratings and nozzle loads. The design temperatures typically set the materials of construction for the components. Hot gas turbo-expanders differ completely from cold gas ones.
The inlet nozzle system, often known as adjustable inlet guide vanes (IGVs), is the primary control tool of a turbo-expander; therefore, its mechanism, configuration and details demand great care. The key requirements are precise flow regulation and reliability. These are needed to accurately control the speed of the turbo-expander to avoid speed fluctuations, particularly at part-load and low loads. Some modern multilink mechanisms use sophisticated techniques to adjust the IGVs for precision flow control and minimal actuating forces.
Nozzle segments must contend with severe working conditions due to high gas velocities and other effects. Special anti-friction and anti-wear coatings usually are needed on the nozzle segments to minimize losses during the first isenthalpic expansion. The presence of solid particles and impurities can pose a great risk for turbo-expanders. For this reason, nozzles typically receive some sort of treatment, such as tungsten-carbide or special coatings, surface induction hardening, etc., to minimize erosion problems and other damaging effects. The appropriate choice depends on the particular application.
Hot Gas Applications
Lots of plants have streams of high pressure hot process or waste gases that require cooling before further processing or disposal. Not infrequently, sites opt for turbo-expanders to recover useful work from these streams.
Some turbo-expanders have been designed and built specifically for hot gas services but often steam turbines have been adapted and used. Basically, these latter units feature the same working principle as conventional steam turbines except for the working fluid. However, because the operating, thermodynamic and fluid-dynamic behavior of a hot gas in each service differs from that of steam, successful application requires many checks and verifications to avoid potential problems or issues. Hot gas turbo-expanders can use low alloy carbon steels to some extent. Extreme temperatures and pressures, though, call for suitable alloys, many of which are employed for steam turbines.
Cold Gas Services
Many turbo-expanders find use in low temperature, refrigeration and cryogenic services. Such turbo-expanders primarily serve to efficiently reduce temperature in a high pressure gas stream. Expansion causes the gas to cool dramatically while providing mechanical energy to rotate equipment to do useful works. Some configurations couple the turbo-expander to a compressor, with the generated work used for the compression of the gas in the process. Sometimes, the turbo-expander and compressor are packaged in a single unit on a single shaft.
A turbo-expander can generate low temperature gas far more efficiently than options such as a “Joule-Thomson” (JT) valve or others in many refrigeration, cryogenic and low temperature gas services. Given a certain pressure reduction, the almost isentropic expansion in a turbo-expander allows for a lower temperature of the expanded gas than an isenthalpic expansion by means of a throttling valve or other devices. Indeed, the application of a cold gas turbo-expander instead of low efficiency, traditional methods (such as a JT valve) can significantly improve the cooling capacity, performance, efficiency and operational costs of such a processing plant. The lower temperature considerably increases the overall cold gas or refrigeration cycle efficiency. In addition, the turbo-expander generates useful work.
The casing material for turbo-expanders for cold gas and cryogenic applications typically is a stainless steel; special alloys sometimes are chosen. Many such turbo-expanders use active magnetic bearings (AMBs) because traditional oil bearings usually won’t suffice. Modern canned-type magnetic bearings are popular. These high performance bearings suit aggressive, sour or difficult gases typically not tolerated by traditional magnetic bearings and electrical devices. They encapsulate traditional electrical components of the AMB within a metal can made of an advanced material (such as a high nickel alloy) that prevents any contact with the gas.
Challenges From Non-Ideality
In many applications, turbo-expanders operate, at least partly, in the dense gas thermodynamic region where the ideal gas law poorly approximates true thermodynamic behavior. Therefore, assuming an ideal gas can lead to inaccurate predictions of the flow structure and performance parameters of these turbo-expanders. Unfortunately, for some gases, accurate thermodynamic and fluid-dynamic behaviors may not be readily available for the intended operating range. Yet, improving dense gas turbo-expander performance through fluid dynamics depends upon properly taking into account the non-ideal thermodynamic behaviors of the operating fluid. This demands a good understanding of how the fluid dynamics deviate from ideal gas behavior as well as of the capabilities and limitations of the available thermodynamic models for each specific fluid and application. Expansions in the dense gas region often involve subcritical and supercritical inflow conditions, representing cases of moderate to high thermodynamic non-ideality. Some turbo-expanders require practical simulations of real gas flow through more complex three-dimensional geometries.
A Cold Gas Turbo-Expander
Let’s look at a case study for the application of a cold gas turbo-expander in a gas recovery process. The inlet gas is a mixture of different gases destined for a cryogenic separation/recovery process. In that process, the inlet gas first is cooled to about -50°C in a heat exchanger (cold box); this partially condenses the gas. The resultant gas/liquid mixture then is separated into a liquid stream and a gas stream. The liquid stream from the separator flows through a valve and undergoes a throttling expansion from about 63 Barg to around 22 Barg, which is an isenthalpic (constant enthalpy) process that results in lowering the temperature of the stream from about -50°C to about -80°C as it enters the separation/recovery tower. The gas stream from the separator goes into the turbo-expander where it undergoes an isentropic expansion from around 63 Barg to about 22 Barg that lowers its temperature from about -50°C to about -90°C as it enters the recovery tower to serve as distillation reflux.
Liquid (at about -90 °C) from the top tray of the recovery tower goes through the cold box where it is warmed to about 0°C as it cools the inlet gas; it then returns to the lower section of the recovery tower. Another liquid stream (at about 2°C) from the lower section of the recovery tower goes through the cold box and returns to the recovery tower at about 12°C. In effect, the inlet gas provides the heat required to re-boil the bottom of the recovery tower and the turbo-expander cools the gas flow (removes the heat as useful work) to provide reflux in the top of the tower.
The overhead gas (at about -90°C) from the recovery tower is a pure gas suitable for the downstream process. This gas passes through the cold box where it is warmed as it cools the inlet gas. It then is compressed in a gas compressor driven by the turbo-expander and further compressed in a second-stage gas compressor powered by an electric motor. The bottom product from the tower also is warmed in the cold box, as it cools the inlet gas, before leaving the system as a marketable byproduct.
AMIN ALMASI is a mechanical consultant based in Sydney, Australia. Email him at [email protected].