Cooling turboexpander

The need to obtain greater recoveries of the C9, C3, and C4S in natural gas has resulted in the expanded use of low-temperature processing of these streams. The majority of the natural gas processing at low temperatures to recover light hydrocarbons is now accomphshed using the turboexpander cycle. Feed gas is normally available from 1 to 10 MPa. The gas is first dehydrated to a dew point of 200 K and lower. After dehydration the feed is cooled with cold residue gas. Liquid produced at this point is separated before entering the expander and sent to the condensate stabilizer. The gas from the separator is... 

Gas can be condensed by (a) mechanically refrigerating it, (b) compressing and expanding it, using turboexpanders, or, (c) pressure effects such as by Joule-Thomson cooling and overcoming the vapor pressure. The liquefaction of methane can involve all three of these effects. These effects can be separately evaluated to show the effectiveness of each in producing liquid. 

As stated earlier, turboexpanders are normally used in cryogenic processes to produce isentropic expansion to cool down the process gas. Two common applications are natural gas processing plants and chemical plants. In natural gas processing plants, turboexpanders are installed to liquify heavier hydrocarbon components and produce lean natural gas with specified dew point limits to meet required standards. 

At the central platform, water and hydrocarbon liquids are first removed in knockout drums. Then saturated natural gas, free of any liquid droplets, enters the twin expanders. The gas is cooled below its dewpoint, allowing heavy hydrocarbon components and water vapor to condense in the discharge stream. Turboexpanders were chosen for two main reasons They are more compact than competing methods of controlling the dewpoint and their operating costs are typically lower than those of many alternatives. 

The gas is routed through heat exchangers where it is cooled by the residue gas, and condensed liquids are recovered in a cold separator at appro.ximately -90°F. These liquids are injected into the de-methanizer at a level where the temperature is approximately -90°F. The gas is (hen expanded (its pressure is decreased from inlet pressure to 22.3 psig) through an expansion valve or a turboexpander. The turboexpander Lises the energy removed from the gas due to the pressure drop to drive a compressor, which helps recompress the gas to sales pressure. The cold gas f-)50°F) then enters the de-methanizer column at a pressure and temperature condition where most of the ethanes-plus Lire in the liquid state. 

We will then describe the Collins helium liquefier and coolers which use turboexpanders (Linde He liquefier). At the end of the chapter, we will describe cooling cycles which use regenerators and other cycles used in cryocoolers (Philips-Stirling, Gifford-McMahon (GM), Klimenko cycles and pulse tube refrigerators (PTRs)). 

Any work developed by the turboexpander is at the expense of the enthalpy of the process stream, and the latter is correspondingly cooled. A low inlet temperature means a correspondingly lower outlet temperature, and the lower the temperature range, the more effective the expansion process becomes. 

Joule-Thomson Valves The principal function of a J-T valve is to obtain isenthalpic cooling of the gas flowing through the valve. These valves generally are needle-type valves modified for cryogenic operation. They are an important component in most refrigeration systems, particularly in the last stage of the liquefaction process. Joule-Thomson valves also offer an attractive alternative to turboexpanders for small-scale gas-recovery applications. 

The air feed is first compressed and heat of compression is removed from the stream by intercooling, aftercooling and direct water quench. The elevated pressure airstream is purified to remove water and other impurities and it is then expanded to a lower pressure to generate the reduced temperature necessary for liquefaction. Expansion takes place either across a valve (Joule-Thompson expansion) or through a turboexpander producing useful work. The compression, cooling and subsequent expansion of the air feed stream constitutes the refrigeration cycle. 

The nitrogen feed stream is split and part enters a second plate-fin exchanger to be cooled further against the nitrogen recycle streams. The other part is sent to a turboexpander to be cooled by expansion in the... 

The employment of turboexpander (TDA) for gas cooling allows us to decrease pressure by 4 MPa and to cool the gas by more than 20 °C. It should be noted that the same pressure drop on a throttle would cool the gas by only 12 °C. 

The main cooling field devices are throttles, heat exchangers, turboexpanders, air cooling apparatuses, and artificial cold mechanisms. 

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