Joule valve

If high wellhead pressures are available over long periods, cooling can be achieved by expanding gas through a valve, a process known as Joule Thomson (JT) throttling. The valve is normally used in combination with a liquid gas separator and a heat exchanger, and inhibition measures must be taken to avoid hydrate formation. The whole process is often termed low temperature separation (LTS). 

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. 

For a compressible fluid that undergoes exansion through a valve or an orifice, the Joule-Thompson coefficient is defined as ... 

Figure 9-3 shows a typical cryogenic plant where the gas is cooled to -100°F to -150°F by expansion through a turbine or Joule-Thompson (J-T) valve. In this example liquids are separated from the iniei gas at 100 F and 1,000 psig. It is then dehydrated to less than I ppm water vapor to assure that hydrates will not form at the low temperatures encountered in the plant. Typically, a mole sieve dehydrator is used. 

The flow through the sampling valve is a true Joule-Thompson expansion. 

Figure 5.9 The Joule-Thompson cycle (Linde cycle). The gas is first compressed and then cooled in a heat exchanger, before it passes through a throttle valve where it undergoes an isenthalpic Joule-Thomson expansion, producing some liquid. The cooled gas is separated from the liquid and returned to the compressor via the heat exchanger.

The Joule-Thomson coefficient is the slope of the isenthalpic lines in the P-T projection. In the region where iJt<0, expansion through the valve (a decrease in pressure) results in an increase in temperature, whereas in the region where pJt >0, expansion results in a reduction in temperature. The latter area is recommendable for applying the PGSS process. 

Temperature changes as pressure is reduced when a flowing stream of gas passes through a throttle, i.e., a valve, choke, or perforations in casing. This is called the Joule-Thomson effect. The change in temperature is directly related to the attraction of die molecules for each other. 

Gas expanders are used to recover energy from high pressure process gas streams in a plant when the lower pressure is adequate for further processing. Power calculations are made in the same way as those for compressors. Usually several hundred horsepower must be involved for economic justification of an expander. In smaller plants, pressures are simply let down with throttling valves (Joule-Thomson) without attempt at recovery of energy. 

Figures 4.7 through 4.9 are provided for hydrate limits to isenthalpic Joule-Thomson expansions, such as that which occurs when a gas with entrained free water droplets flows through a valve. A similar set of charts could in principle be determined for hydrate limits to isentropic (AS = 0) expansions such as would occur when a gas flows through a perfect turboexpander of a modern gas processing plant. To date, however, no such charts have been generated.

When water-wet gas expands rapidly through a valve, orifice or other restriction, hydrates form due to rapid gas cooling caused by adiabatic (Joule-Thomson) expansion. Hydrate formation with rapid expansion from a wet line commonly occurs in fuel gas or instrument gas lines. Hydrate formation with high pressure drops can occur in well testing, start-up, and gas lift operations, even when the initial temperature is high, if the pressure drop is very large. 

Pressure reduction is accompanied at the hydrate interface by a temperature decrease to the equilibrium temperature. Normally the pipeline cannot be depressured sufficiently rapidly for Joule-Thomson (isenthalpic) cooling to lower the temperature this would occur through a restriction such as a valve. If the pressure is reduced slowly, a vertical isothermal depressurization (AT = 0) results. Usually an intermediate pressure reduction rate causes the hydrate interfacial temperature to be significantly less than the surroundings, causing heat influx from the surroundings to melt hydrates from the pipe boundary inward. 

The expansion of an ideal gas in the Joule experiment will be used as a simple example. Consider a quantity of an ideal gas confined in a flask at a given temperature and pressure. This flask is connected through a valve to another flask, which is evacuated. The two flasks are surrounded by an adiabatic envelope and, because the walls of the flasks are rigid, the system is isolated. We now allow the gas to expand irreversibly into the evacuated flask. For an ideal gas the temperature remains the same. Thus, the expansion is isothermal as well as adiabatic. We can return the system to its original state by carrying out an isothermal reversible compression. Here we use a work reservoir to compress the gas and a heat reservoir to remove heat from the gas. As we have seen before, a quantity of heat equal to the work done on the gas must be transferred from the gas to the heat reservoir. In so doing, the value of the entropy function of the heat reservoir is increased. Consequently, the value of the entropy function of the gas increased during the adiabatic irreversible expansion of gas. 

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

If, in the heat pumps, the energy of compression is not recovered but is wasted in letdown valves (as the pressure of the working fluid is reduced to the low pressure of the evaporator (Joule-Thomson cycle), the liquefaction efficiency will be low (35-60%). This range of efficiencies is a function of the liquefier size and refrigerant used. If the letdown valves are replaced by turbo expanders (Brayton cycle), which recover some of the compression energy during pressure letdown, and if helium or neon refrigerants are used, the efficiency can theoretically reach 80-90%. 

Insulated Joule-Thomson cell similar to that of Fig. 2 (suitable stainless steel frits can be obtained from chromatographic parts suppliers, e.g., Upchurch Scientific part C-414) metal or nylon tees, crosses, and reducers (available from Swagelok and other manufacturers) -in. Teflon rod type T insulated copper-Constantan thermocouples with 0.010-in.-diameter wires voltmeter with 0.1-jU.V resolution (e.g., Keithley 196), null voltmeter (e.g., Hewlett Packard 419A or Keithley 155), or sensitive potentiometer (e.g., Keithley K-3). Cylinders of CO2, N2, and He with regulators and control valves 50 ft of l-in. copper coil, -in. and 1-in. polyethylene tubing 0- to 10-bar Bourdon gauge 25°C water bath. 

At the wellhead there is a choke valve, and the fluid pressure drops to the actual injection pressure. There is a Joule-Thomson temperature drop associated with this pressure drop, but because the acid gas is in the liquid phase, the temperature drop is quite small. 

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