Turboexpander [TE] and Joule Thomson [JT] expansion

In one line, TE is idealized as an adiabatic reversible process while JT is approximated as an irreversible adiabatic process. In JT skin friction between the flowing gas and the non-moving surface of the valve, material generates irreversible heat transfer to the valve surface. This heat goes to excite the molecules of the valve material causing the excitation of electrons that cannot be reversed. In the case of TE, its design eliminates friction losses. This makes TE an internally reversible adiabatic isentropic [ no entropy] process while the irreversibility in the JT due to frictional loss of heat makes JT just an adiabatic process with an overall much less efficient cooling process than TE.?The adiabatic process is used to approximate both TE and JT. Because the process of gas expansion is so rapid, it is assumed that the heat is contained within the system. The irreversibility of JT distinguishes it from an internally reversible TE.

Turboexpander [TE]

Turbo-expander is a centrifugal or axial-flow turbine, through which 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.

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Turbo-expanders handle a wide range of services, from those involving cold gases (say, -270°C) to hot gases (above 350°C). Typically, the use of a turbo-expander makes sense only for relatively large gas flows.

Typical applications of Turboexpander

Turboexpander for power generation

As a very rough indication, units currently in operation range in power ratings from about 25 kW to around 25 MW. The work produced by the gas expanding through the turboexpander is available at room temperature. This work may be absorbed as heat into a cooling loop. The heat is then recovered for use elsewhere. In some large cryo-plants, the turboexpanders may be on the same shaft as compressors elsewhere in the cycle. The work produced by the gas expanding through the turboexpanders in this case then helps drive the compressors.

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The use of turboexpanders in cryogenic plants has been the result of several technological advances. The development of sophisticated computational fluid dynamic models coupled with computer-aided design and manufacturing has allowed the design and construction of efficient turboexpanders operating at cryogenic temperatures. The development of reliable rotating bearing systems that function at high speeds and cryogenic temperatures has also been necessary for the successful use of turboexpanders.

Hot Gas Applications

Lots of plants have streams of high-pressure hot processes 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.

Cold Gas Services

Many turboexpanders find use in low-temperature, refrigeration, and cryogenic services. Such turbo-expanders primarily serve to efficiently reduce the temperature in a high-pressure gas stream. The expansion causes the gas to cool dramatically while providing mechanical energy to rotate equipment to do useful work. 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.

Joule Thomson expansion: JTE describes the temperature change of a real gas or liquid (as differentiated from an ideal gas) when it is forced through a valve or porous plug while keeping it insulated so that no heat is exchanged with the environment. This procedure is called a throttling process or Joule–Thomson process. ?JTE is considered to be an isenthalpic process. When the stream of a real gas is forced through a valve or porous plug while keeping it insulated [image above] so that no heat and work are exchanged with the environment, there is a change in the temperature of the stream of gas. An isenthalpic process works at constant U + W [ U is internal energy and W is work] with no change in enthalpy. At room temperature, all gases except hydrogen, helium, and neon cool upon expansion. These three gases experience the same effect but only at lower temperatures.

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The simple answer is that Turboexpander does not produce entropy, whereas JT does. TE is an adiabatic process that is internally reversible. As a result, TE has more energy to expand and perform mechanical work. TE produces more cooling as a result.

Fundamentals

Gases are a phase of matter where all the atoms or molecules are moving independently of each other all the time. All these molecules move at different speeds but if we take the average speed of all the molecules; we get the temperature of the gas. KE [average] = 1/2Kb T [ Kb is Boltzmann's constant and T is temperature]. If a particular gas has a large number of fast molecules, the gas is hot. On the other hand, if a particular gas has mostly slow molecules, then the gas is cold.?

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The average speed of a gas is also called the kinetic energy of the gas. When you combine the average speed for each molecule with all the molecules in the gas you get heat. Heat is the average speed of a single gas molecule multiplied by all the molecules in the gas. Heat can be seen as the total amount of energy, H [enthalpy]. dH/dT = Cp, [Cp is the heat capacity of the gas] of all the molecules in a certain gas.

At the fundamental level, every gas has some kinetic energy, according to kinetic theory, "the average kinetic energy of gas molecules depends upon the absolute temperature. When a gas expands, the gas works to overcome the intermolecular forces of attraction (it implies that the gas is spending its energy). This results in a decrease in the internal energy of the system. Since internal energy is a function of temperature, the expansion of gas decreases the temperature of the gas.

Key points of distinction between TE and JT:

[1] Both JT and TE [turboexpander] are adiabatic processes;

[2] JT is an irreversible process due to huge frictional irreversible energy losses as heat converts to work when the gas is throttled;

[3] TE is also an adiabatic process;

[4] TE does not generate entropy; Which means TE is an internally reversible process;

[5] Because there are no entropy losses in TE, there is more energy available for gas to work and expand, resulting in more cooling.

?[6] With the same base value, the output pressure in TE is much smaller because it does more work and produces more cooling.

[7] JT is more economically viable at a low gas flow rate. A normal throttling pressure-reducing valve does not have to withstand the stresses and temperature differences imposed on a cryogenic valve.

[8] TE offers fewer economic advantages below 10 MMScfd and lose efficiency below 5 MMScfd for ethane recoveries of 10 to 30% JT expansion may be sufficient.

[9] JT is insensitive to variable flow rates while the turboexpander loses efficiency when operating off the design rates. Isentropic expansion in turboexpander makes it much more effective than JT. JT is less costly than turboexpander. Turboexpander is very sensitive to feed gas quality while JT is flexible regarding feed gas quality.?

Cooling curve of JTE vs turboexpander

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Summary

A turboexpander 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 using 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.