Isenthalpic expansion

To reduce the work of compression in this cycle a two-stage or dualpressure process may be usedwhereby the pressure is reduced by two successive isenthalpic expansions. Since the isothermal work of compression is approximately proportional to the logarithm of the pressure ratio, and the Joule-Tnomson cooling is roughly proportional to... 

In a work-producing expansion, the temperature of the process fluid is always reduced hence, coohng does not depend on being below the inversion temperature prior to expansion. Additionally, the work-producing expansion results in a larger amount of coohng than in an isenthalpic expansion over the same pressure difference. 

It is not uncommon to utilize both the isentropic and isenthalpic expansions in a cycle. This is done to avoid the technical difficumes associated with the formation of liquid in the expander. The Claude or expansion engine cycle is an example of a combination of these meth-... 

A Mollier Diagram is useful for the expansion of a specific gas/vapor or multicomponent vapor fluid. See Figure 12-91 for comparison of (1) constant enthalpy (Joule-Thompson effect), isenthalpic, and (2) isentropic (constant entropy), which provides the colder temperature. Note that the expander indicated on the figure is somewhere between isenthalpic and isentropic or polytropic. See Figure 12-92. ... 

Thus, the Joule-Thomson expansion is an isenthalpic process. 

Figure 3.6 shows how pressure and temperature are related for a series of isenthalpic (Joule-Thomson) expansions. For example, if we start at the... 

Figure 5.15(b) shows that the final expansion stage occurs in a turbine, rather than in an isenthalpic Joule-Thomson orifice. It has a higher thermodynamic efficiency than that of the Joule-Thomson but is more complex and expensive. 

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. 

The pressure and temperature of the gas normally decreases upon expansion along an isenthalpic curve (AH = 0) until the intersection with the hydrate boundary of Figure 4.5 is encountered, which provided one point on a figure such as Figure 4.7. Multiple points were calculated to construct each figure. The charts enabled the user to estimate the limits to adiabatic expansion before... 

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.

Figure 8.14 Temperature changes as a result of depressurization (1) isenthalpic rapid expansion as through a valve, and (2) very slow depressurization, as in a large-volume pipeline. Note that for the rightmost case, a fluid system can be expanded into the hydrate region, as calculated by the methods in Section 4.2.1.1 and the programs of CSMGem on the CD accompanying this book.

Use the Plot tab on CSMGem to plot the isenthalpic expansion curve using MS Excel. Save this data under a different file name before continuing to plot-phase boundaries. The file will be overwritten if the name is not changed. 

Joule-Thomson expansion Expression representing an isenthalpic throttling process. 

FIGURE 5 (a) Schematic for combined isenthalpic and isentropic expansion refrigerator (b) temperature-entropy diagram for cycle. 

There is another mode of gaseous expansion called the Joule-Thomson expansion, in which the change in gas volume occurs at constant enthalpy AHW = 0 without any change in energy. The vector of the isenthalpic expansion then stands perpendicular to the abscissa on the ordinate and points in the negative direction (exergy consumption) as is shown in Fig. 11.10(b). 

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