Isothermal Joule-Thomson

It is also possible to measure (dH/dP)T, the isothermal Joule-Thomson coefficient directly, which is a quantity more diiectly related to deviations from ideality [14]. 

There are two variations of the basic set-up of the Joule-Thomson experiment which both yield practical information. In the isothermal Joule-Thomson experiment the temperature is held constant with a downstream heater, and the resultant heat input for the pressure decrease permits an experimental evaluation of (8H/8P)T, the isothermal Joule-Thomson coefficient. In the other variation there is no throttling device used, and the pressure is held constant. For the steady-state flow of gas the temperature change is measured for measurable inputs of heat. This experiment, of course, yields (8H/8T)P, or CP. Thus, the variations of this constant-flow experiment can yield all three of the important terms in Equation (7.46). 

If this coefficient is constant in an isothermal Joule-Thomson experiment, then the heat which must be supplied to maintain constant temperature is AH in the following relationship... 

Flow calorimetric measurements of the isothermal Joule-Thomson coefficient of a vapour also provide information on gas non-ideality which is fiee from adsorption errors. Basically, all that is required is a fixed-throttle flow calorimeter, free of heat leaks, fitted with an electric heater as shown in Figure 9 so that isothermal measurements can be made [77-alb/wor]. 

Fig. 9. Schematic diagram of an isothermal Joule-Thomson apparatus...

Calorimetric Measurements.— The relation between the equation of state and the isothermal Joule-Thomson coefficient ft = (dHjdp)j. can be obtained from equation (16b). In the low pressure limit it is simply... 

A flow calorimeter for measurement of the isothermal Joule-Thomson coefficient of vapours has been described by Francis, McGlashan, and Wormald and an apparatus said to be useful for studies on mixtures has been described by Dawe and Snowdon, but very few measurements of ft for mixtures have been... 

Use eqn 17.45 to compute the limiting isothermal Joule-Thomson coefficient dB... 

Such measurements are usually made with no throttle (pi = p, heat capacity), or with no power (P = 0 Joule-Thomson), or with the power adjusted so as to restore the temperature of the gas after it has passed through the throttle (71 = T isothermal Joule-Thomson). The equations for these special cases can be readily derived from equation (32). 

Joule-Thomson Effect, Francis, McGlashan, and Wormald have measured the isothermal Joule-Thomson coefficient... 

TABLE 46. Comparison of Calculation Results with Experimental Data [4.4] for the Isothermal Joule-Thomson Effect for Freon-23... 

Raskazov D. C., Krukov L. A. Experimental study of isothermal Joule-Thomson Effect of Freon-23.—Thermophysical Properties of Substances and Materials. Gosstandart SSSR, GSSSD, 1975, 8, p. 84—99. 

A characteristic of the non-ideal gas is that it has a finite Joule-Thomson effect. This relates to the amount of heat which must be added during an expansion of a gas from a pressure Pi to a pressure P2 in order to maintain isothermal conditions. Imagine a gas flowing from a cylinder, fitted with a piston at a pressure Pi to a second cylinder at a pressure Pi (Figure 2.2). 

For an ideal gas, under isothermal conditions, AU = 0 and /V 2 = Pp - Thus q = 0 and the ideal gas is said to have a zero Joule-Thomson effect. A non-ideal gas has a Joule-Thomson effect which may be either positive or negative. 

The reader interested in the liquefaction technologies can see, for example, ref. [14,15], We will only remind that in most cases, the gas cooling is obtained by the Joule-Thomson process an isothermal compression of the gas is followed by an expansion. This procedure leads to a cooling only if the starting temperatures are lower than the inversion temperature 7] = 6.75 TCI (for a Van der Waals gas), where TCI is the critical temperature. 

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. 

Heat capacity, molar Heat capacity at constant pressure Heat capacity at constant volume Helmholtz energy Internal energy Isothermal compressibility Joule-Thomson coefficient Pressure, osmotic Pressure coefficient Specific heat capacity Surface tension Temperature Celsius... 

To reduce the work of compression in this cycle a two-stage or dualpressure process may be used whereby 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-Thomson cooling is roughly proportional to... 

Dielectric and pressure virial coefficients of NzO have been measured at 6.5, 30.1, and 75.1 °C. The dipole moment, polarizability, and molecular quadrupole moment were determined to be 0.18 D, 3.03 x 1CT24 cm3, and 3.4 xlO 26 e.s.u. cm2, respectively.91 A lower limit of —0.15 0.1 eV has been calculated for the molecular electron affinity of N20, using molecular beam studies.92 The enthalpy-pressure behaviour for N20 along eleven isotherms in the vapour phase has been determined from measurements of the Joule-Thomson effect.91... 

Temperature, pressure, volume, amount, energy, enthalpy, heat capacity, expansion coefficient, isothermal compressibility, and Joule-Thomson coefficient. 

The Linde>Hampson process uses a thermodynamic process. Isothermal compression and subsequent cooling along an isobar is done in a heat exchanger. Joule-Thomson expansion connected with an irreversible change in entropy is used as the refrigeration procedure. Despite its simplicity and reliability, this method is now less attractive compared with new ones where cooling is primarily carried out in reversible processes (expander) and where less energy is required. 

THERMAL PROPERTIES OF PROPANE. HEAT CAPACITY, JOULE-THOMSON COEFFICIENT, ISOTHERMAL THROTTLING COEFFICIENT, AND LATENT HEAT OF VAPORIZATION. FROM PROCEEDINGS OF THE 4TH SYMPOSIUM ON THERMOPHYSICAL PROPERTIES, UNIV. MARYLAND COLLEGE PARK, MD. 

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