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3-200nm3 / h Oxygen Plant Spare Parts Skid Mounted Cryogenic Turbo Expander
A turboexpander, also referred to as a turbo-expander or an expansion turbine, is a centrifugal or axial-flow turbine, through which a high-pressure gas is expanded to produce work that is often used to drive a compressor or generator.
Because work is extracted from the expanding high-pressure gas, the expansion is approximated by an isentropic process (i.e., a constant-entropy process), and the low-pressure exhaust gas from the turbine is at a very low temperature, −150 °C or less, depending upon the operating pressure and gas properties. Partial liquefaction of the expanded gas is not uncommon.
Turboexpanders are very widely used as sources of refrigeration in industrial processes such as the extraction of ethane and natural-gas liquids (NGLs) from natural gas,the liquefaction of gases (such as oxygen, nitrogen, helium, argon and krypton) and other low-temperature processes.
Raw natural gas consists primarily of methane (CH4), the shortest and lightest hydrocarbonmolecule, as well as various amounts of heavier hydrocarbon gases such as ethane (C2H6), propane(C3H8), normal butane (n-C4H10), isobutane (i-C4H10), pentanes and even higher-molecular-masshydrocarbons. The raw gas also contains various amounts of acid gases such as carbon dioxide(CO2), hydrogen sulfide (H2S) and mercaptans such as methanethiol (CH3SH) and ethanethiol(C2H5SH).
When processed into finished by-products (see Natural-gas processing), these heavier hydrocarbons are collectively referred to as NGL (natural-gas liquids). The extraction of the NGL often involves a turboexpander and a low-temperature distillation column (called a demethanizer) as shown in the figure. The inlet gas to the demethanizer is first cooled to about −51 °C in a heat exchanger (referred to as a cold box), which partially condenses the inlet gas. The resultant gas–liquid mixture is then separated into a gas stream and a liquid stream.
The liquid stream from the gas–liquid separator flows through a valve and undergoes a throttling expansion from an absolute pressure of 62 bar to 21 bar (6.2 to 2.1 MPa), which is an isenthalpicprocess (i.e., a constant-enthalpy process) that results in lowering the temperature of the stream from about −51 °C to about −81 °C as the stream enters the demethanizer.
The gas stream from the gas–liquid separator enters the turboexpander, where it undergoes an isentropic expansion from an absolute pressure of 62 bar to 21 bar (6.2 to 2.1 MPa) that lowers the gas stream temperature from about −51 °C to about −91 °C as it enters the demethanizer to serve as distillation reflux.
Liquid from the top tray of the demethanizer (at about −90 °C) is routed through the cold box, where it is warmed to about 0 °C as it cools the inlet gas, and is then returned to the lower section of the demethanizer. Another liquid stream from the lower section of the demethanizer (at about 2 °C) is routed through the cold box and returned to the demethanizer at about 12 °C. In effect, the inlet gas provides the heat required to "reboil" the bottom of the demethanizer, and the turboexpander removes the heat required to provide reflux in the top of the demethanizer.
The overhead gas product from the demethanizer at about −90 °C is processed natural gas that is of suitable quality for distribution to end-use consumers by pipeline. It is routed through the cold box, where it is warmed as it cools the inlet gas. It is then compressed in the gas compressor driven by the turboexpander and further compressed in a second-stage gas compressor driven by an electric motor before entering the distribution pipeline.
The bottom product from the demethanizer is also warmed in the cold box, as it cools the inlet gas, before it leaves the system as NGL.
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