Cooling Joule-Thompson

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. 

Figure 11-58. Theoretical comparison of Joule-Thompson cooling effect with nitrogen vs. the use of a mechanical expander. (Used by permission Gibbs, C. W., (Ed.). Compressed Air and Gas Data, 1969. Ingersoll-Rand Co.)...

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.

By dissolving the compressible media in a liquid, a so-called gas-saturated solution may be formed. By expansion of such a solution in an expansion unit (e.g., a nozzle) the compressed medium is evaporated and the solution is cooled. Owing to the cooling caused by evaporation and/or the Joule-Thompson effect the temperature of the two-phase flow after the expansion nozzle is lowered. At a certain point, the crystallization temperature of the substance to be solidified is reached, and solid particles are formed and cooled further. 

When any vapor expands, due to a pressure reduction (other than H2 and C02), it cools off. This is called a Joule-Thompson expansion. The reduction in temperature of the steam is called a reduction in sensible-heat content. The sensible heat of the steam is converted to latent heat of condensation. Does this mean that the latent heat of condensation of 10-psig steam is much higher than that of 450 psig steam Let s see ... 

Joule-Thompson cooling The decrease in temperature of a gas when its pressure is rapidly reduced. 

The temperature of the seabed is known to be 42°F, but the downstream end of the pipeline will be colder than this (38°F) due to Joule-Thompson cooling when it is vented. A porosity of 0.5 and a permeability of 0.01 mD are default values. An annulus spacing of 0.1 is required for pressure flow communication. 

Liquefaction of the purified air is accomplished using the Joule-Thompson effect, which is the cooling effect obtained from a compressed gas when it is allowed to expand. By using this expansion-cooling effect repetitively, and by employing the chilled expanded gas to prechill the compressed gas before expansion, air may be liquefied by employing compression pressures of only about 10 atm (about 150 psig, Eig. 11.1). It is not possible to accomplish... 

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. 

Where they have a positive slope, water cools on adiabatic expansion and warms if adiabatically compressed, and the two regions are separated by the Joule-Thompson inversion curve. Much the same information is contained in the enthalpy-pressure diagram (Figure 8.6), where it can be seen that constant enthalpy changes in pressure lead to increases in temperature in one region and decreases in another. The effect of dissolved NaCl on the Joule-Thompson coefficient has been calculated by Wood and Spera (1984), and the effect will be similar for other electrolytes. Because the addition of most electrolytes to water results in a decrease in V and in a, fijT is smaller, and the net effect is to move the inversion curve to higher temperatures, as shown in Figure 8.5. 

In a PGSS experiment, the SCF is first dissolved in the HC by increasing the pressure until a melt is obtained. The gas-saturated solution is then expanded through a nozzle where it is cooled simultaneously by evaporation and Joule-Thompson effects. In this way supersaturation conditions are reached, and solid particles are formed. Again, the product is completely free of organic solvents. The amount of SCF needed is low, between 0.2 and 0.6 kg per kilogram of HC. 

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. 

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 vapor is expanded from the high pressure at the feed side to the low pressure at the permeate side. This will cause a Joule-Thompson effect, which in general will lead to a slight temperature drop from the feed to the permeate side and cool the membrane. Although this effect will reduce the temperature by only one to three degrees centigrade for most of the mixtures treated in practical application, it will lead to condensation of a small part of the vaporous feed. 

---