Joule-Thompson valve

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 Linde cycle is a simple cryogenic process based on Joule-Thompson effect. It is composed of different steps the gas is first compressed, then preliminarily cooled in a heat exchanger using liquid nitrogen, finally it passes through a lamination throttle valve to exploit the benefits of Joule-Thomson expansion. Some liquid is produced, and the vapour is separated from the liquid phase and returns back to the compressor through the heat exchanger. A simplified scheme of the overall process is reported in Fig. 2.9. 

A metering valve PRV for pressure reduction after the vessel. It must be heated to compensate for Joule-Thompson effects during expansion. 

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. 

A schematic of the system and the corresponding temperature-entropy diagram are shown in Figure 3. A schematic of the system is shown on the left side of the diagram. The equipment in this system are a gas compressor, compressor aftercooler, plate-fin heat exchanger, Joule-Thompson expansion valve and liquid-vapor separator. 

The expander cycle and its corresponding temperature-entropy diagram are shown in Figure 4. Notice that the expander cycle is similar to the simplified Joule-Thompson cycle, except that the Joule-Thompson expansion valve has been replaced by an expansion turbine. The performance of this cycle differs in several ways. The expansion of the gas is no longer isenthalpic, but with the expansion turbine, it is isentropic that is, there is a change in... 

Isothermal compression and heat rejection take place between points 1 and 2. A portion of the compressed gas is diverted from the plate-fin heat exchanger and expanded through an expansion turbine to lower the temperature and produce mechanical work. The expanded and cooled gas (point 11) reenters the heat exchanger between point 9 and point 10. The balance of the feed gas is further cooled and expanded through a Joule-Thompson expansion valve from point 5 to point 6. Liquid is formed in the Joule-Thompson expansion. 

The part of the feed nitrogen that passes through the second plate-fin exchanger is expanded through a Joule-Thompson expansion valve and the two phase mixture enters a medium pressure vapor-liquid separator. The vapor from this vessel is mixed with the discharge from the turboexpander... 

Here, F, Zf and h are, respectively, the molar flow rate, mole fraction of component of i and total enthalpy, all in cell k their subscripts, ret and perm, refer to retentate and permeate streams. Equations (10.4) and (10.5) are mass balances and mass-transfer equations for each of the components present in the membrane feed. The cross-flow model [Equations (10.3)-(10.7)] was implemented in ACM v8.4 and validated against the experimental data in Pan (1986) and the predicted values of Davis (2002). The Joule-Thompson effect was validated by simulating adiabatic throttling of permeate gas through a valve in Aspen Hysys. Both these validations are described in detail in Appendix lOA. 

Many relief valves leak hydrocarbons to the flare. If the vessel with a leaking relief valve is operating at a substantial pressure (about 30 or 40 psig), then the line downstream of the relief valve will be noticeably (perhaps 10°F) colder than the relief valve connection itself, due to a Joule-Thompson expansion. 

If the vessel with the leaking relief valve is liquid full with light hydrocarbons (ethane, propane, or butane), then a leaking relief valve will be quite obvious. The line downstream of the faulty pressure relief valve will be covered with ice. The ice is atmospheric moisture freezing on the line to the flare. This is not due to a Joule-Thompson expansion, but due to the auto-refrigeration of the volatile hydrocarbon liquid as it flashes to a vapor in the flare header piping. 

On expansion the gas cools rapidly (Joule-Thompson effect) and tends to block the back pressure restrictor valve (particularly when the simple manual valves are used) with solid carbon dioxide and precipitated solutes. 

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