Joule-Thomson valve

Expanders 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 (1) in an expander where mechanical work is produced, and (2) in a Joule-Thomson valve where no work is produced. 

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. 

The turboexpander lowers the temperature of the product to -100°F, causing it to liquify. Now at 350 psig pressure, the liquid from this process enters the demethanizer tower where it mingles with the previously introduced stream of liquid. The turboexpanders provide a 92% recovery rate while the former system, a backup Joule-Thomson valve, was able to provide only a 60% recovery rate. The volume of gas entering the turboexpanders can vary up to 10% yet, the different flowrates do not significantly affect the efficiency of these units, which are rated at 2,400 hp at 16,000 rpm. 

The expansion valve or Joule-Thomson valve, as it is often called, is an important component in any liquefaction... 

Figure 5.6 A schematic of the PRICO (LNG) process. 1) compressor, 2) partial condenser, 3) accumulator, 4) refrigerated heat exchanger, 5) Joule-Thomson valve, 6) refreigerant suction drum, 7) centrifugal pump, 8) partial condenser. Source. [77].

The main disadvantage in membrane technology is methane co-permeation. Another approach that has been developed is to allow the gas to expand adiabatically in a nozzle where the temperature drops from an inlet value of 30°C to -50°C, causing condensation of water and heavier hydrocarbons. Separation of water droplets is achieved by a high-G swirl initiated by a wing (see Fig. 6.17). Twister isentropic efficiency is about 90%, and it is more efficient than a Joule Thomson valve and a Turbo-expander. 

Most practical refrigeration and liquefaction systems obtain a reduction in temperature with the aid of an expansion valve (Joule-Thomson valve), an expansion engine, or a combination of the two devices. In the case of the expansion valve, the flow within the valve is irreversible as well as non-isenthalpic. However, the inlet and outlet conditions have the same enthalpy. The change in temperature of a fluid obtained with an isenthalpic change in pressure is represented by the Joule-Thomson (IT) coefficient, defined as... 

Joule-Thomson valves, besides being widely used in refrigeration processes, now are offering an attractive alternative to the turboexpander for small-scale light hydrocarbon (LPG) recovery applications, provided that ethane recovery is limited to below 30%. In a Joule-Thomson plant, the expander would be eliminated, but to obtain the same level of recovery as in a turboexpander plant, the compressor would have to be scaled up. Another advantage attributed to a Joule-Thomson plant besides overall simplicity of operation is the ability to operate under more widely varying inlet gas flow rates. 

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