Turboexpander

A turboexpander, also referred to as a turbo expander, expansion turbine or simply expander, is a centrifugal or axial flow turbine through which a high pressure gas is expanded to produce work that is typically used to drive a compressor. Because work is extracted from the expanding high pressure gas, the expansion is isentropic and the low pressure exhaust gas from the turbine is at a very low temperature, often as low as 200 K (-100 °F) or less.^[1] Turbo expanders are very widely used as sources of refrigeration in industrial processes such as the extraction of ethane as well as natural gas liquids (NGLs) from natural gas^[2]; the liquefaction of gases;^[3][4][5][6] and other low-temperature processes.

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Operation

The heart of a turboexpander is a composite metal shaft that has a compressor wheel attached to one end of the shaft and an expander wheel attached to the other end of the shaft. Each wheel is contained completely separate from the other.

A sealing system along the common shaft is used to prevent the process fluids present at each wheel from coming in contact from leakage along the shaft. The sealing systems are usually both mechanical and hydraulic in nature although some systems use a gas (seal gas) purge. The advantage of using a gas seal system is that sealing fluids do not leak into and accumulate in downstream equipment.

In an ethylene production plant, a high pressure, low temperature mixture of hydrogen and methane gas from the demethanizer overhead stream is passed through the expander side of a turboexpander. As the gas passes through the expander side, work is performed because at the other end of the expander-compressor shaft is a compressor wheel that is compressing purified product hydrogen gas for distribution to customers. Because the expanding demethanizer overhead gas is performing work to compress hydrogen, and is experiencing a large pressure drop across the expander wheel, two important things happen. One is that the expanded gas drops in temperature (loses energy) anywhere from 50 to 75 degrees F (28 to 42 K). The other is that some of the gas loses so much energy that it changes to a liquid. If one is trying to separate hydrogen from methane, it is is vital that a portion of the expanded gas is liquefied. It usually takes at least two steps of expansion/work to liquefy all of the methane in the stream. During this two-stage expansion/work process, the gas pressure may drop from 515 psia to 40 psia (35 atm to 2.7 atm) and the temperature may drop from -150 degrees F to -260 degrees F (172 K to 111 K).

Once the hydrogen and methane are separated, both streams are usually sent to a coldbox (a large aluminum block heat exchanger) to use the extremely cold temperatures to finish cryogenically treating the main cracked gas stream (see ethylene production).

When a turboexpander is not in operation it is still necessary to cool the demethanizer overhead stream by dropping it from a higher pressure to a lower pressure. While the turboexpander is out-of-service, the demethanizer overhead stream is allowed to bypass the turboexpander via a control valve that is also known as a Joule-Thomson (J-T) valve. This valve is named after two scientists who discovered that, if a gas undergoes a drop in pressure as it flows through an orifice such as the opening in a valve (adiabatically, meaning without adding or subtracting heat), the gas temperature will drop. Even though the gas does drop in pressure and become colder in flowing through the J-T valve, none of the gas is liquefied because the gas is not performing work such as compressing a gas in the turboexpander. Thus, the turbexpander is critical in the separation of hydrogen from methane because just dropping the gas pressure through a J-T valve is insufficient to cause the gas to liquefy.

See also

• Air cycle machine
• Compressed air energy storage
• Turbopump

References