EFECTO JOULE THOMSON, Guías, Proyectos, Investigaciones de Termodinámica Aplicada

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EFECTO JOULE-THOMSON

EFECTO JOULE-THOMSON 11.1 JOULE-THOMSON EXPANSION In this chapter, we shall study the behavior of open systems by means of phase transitions. In the best-known first-order phase transitions, namely, the melt- ing of ice and the vaporization of water, the regions of temperature and pressure are easily accessible without special apparatus. Some of the most interesting materials, however, such as nitrogen, hydrogen, and helium, whose phase transitions are well understood, exist only at low temperatures. It is important, therefore, to learn how these low temperatures are achieved and maintained. The first step is to liquefy nitrogen, which is produced by means of the Joule-Thomson expansion or, as it is also called, a throttling process, as discussed in Sec. 10.2. that is, the Joule-Thomson coefficient is the slope at a point on the isenthalpic expansion curve. The locus of all points at which the Joule-Thomson coeffi- cient is zero (the locus of the maxima of the isenthalpic curves) is known as the inversion curve and is shown for nitrogen in Fig. 11-2, as a heavy closed curve. The region inside the inversion curve, where y is positive, is called the region of cooling, that is, the final temperature of the gas is less than the initial tem- perature; whereas outside the inversion curve, where ¡e is negative, it is called the region of heating, that is, the final temperature is more than the initial temperature. For example, expansion represented by movement from point (a) on Fig. 11-2 to either (5) or (c) raises the temperature of the gas, whereas movement from points (c) or (4) to (e) lowers the temperature of the gas. Since the Joule-Thomson coefficient involves T, P, and h, we seek a rela- tion among the differentials of T, P, and h. In general, the difference in molar enthalpy between two neighboring equilibrium states is dh= T ds +vdP, and, according to the second T ds equation, Úv Tds=cpdT — r(5r) nde: Substituting for T ds, we get