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. 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; the liquefaction of gases; and other low-temperature processes.

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 a) performing work to compress hydrogen, and, b) 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. The other is that some of the gas loses so much energy that is stops being a oexpander gas and 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 liquified. It usually takes at least two steps of expansion/work to liquify all of the Methane in the stream. During this two stage expansion/work process the gas pressure may drop from 500 psig to 25 psig and the temperature may drop from -150 degrees F to -260 degrees F.

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 Joules-Thompson (J-T)valve. This valve is named after two scientists that discovered that if you drop the pressure of a gas through a small orifice, which is the opening in the control valve, without adding or subtracting heat (adiabatically) that the gas will undergo a tremendous drop in temperature. Even though the gas does drop in pressure and become colder as a result, none of the gas is liquified because none of the gas is performing work such as compressing a gas in the turboexpander. So you can see that the turbexpander is critical in the separation of Hydrogen from Methane because just dropping the gas pressure is insifficient to cause the gas to liquify.

See also

• Air cycle machine
• Compressed air energy storage
• Turbopump