Application of Cryogenic Turboexpanders

The practical applications for cryogenic turboexpanders are best illustrated by industry case history reports that follow. 

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. 

The removal of heat at a low temperature and its rejeetion at a higher temperature requires work. This is stated simply by the Seeond Law of thermodynamies, one form of whieh is 

A net quantity of 99 Btu of work is required to transfer 1 Btu of heat from 50°R to 500°R. Substituting in the Seeond Law equation gives this same result  

The heat quantities in the eountereunent heat exehanger streams are balaneed by a small amount of propane refrigeration at -15°F. 

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The primary function of cryogenic expansion equipment is the reduction of the temperature of the gas being expanded to provide needed refrigeration. The expansion of a fluid to produce refrigeration may be carried out in two distinct ways (l)in a Joule-Thomson valve where no work is produced, and (2) in an expander where mechanical work is produced. Although both Joule-Thomson valves and expanders are discussed, turboexpanders are presently preferred for most dry expansion applications. 

Jotile-Thomson Valves The principal function of a J-T valve is to obtain isenthalpic coohng 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 liquefac tion process. Joule-Thomson valves also offer an attractive alternative to turboexpanders for small-scale gas-recovery applications. 

In each case, the primary objective of turboexpanders is to conserve energy. Contemporary turboexpanders do this either by recovering energy from cold gas (cryogenic type) or from hot gases at temperatures of over 1,000 degrees. Current commercial models exist in the power range of 75 kW to 25-i- MW, so many applications are possible. 

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