A Joule-Thomson effect

For any gas, the sign of the Joule-Thomson effect depends on tanperature and pressure. The positive effect for each gas is observed only in the limited interval of temperatures and pressures. For each gas there are values of temperature and pressure at which the Joule-Thomson effect is equal to zero (no temperature changes occur at gas expansion in vacuum). These points (T, p,) are called points of inversion. At these points, the influence of forces of attraction is completely compensated for by the influence of repulsion forces consequently the gas temperature does not change. The set of inversion points forms an inversion curve in a p-T diagram. 

Upper and lower temperature inversion points for some gases (in K) 

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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. 

From a fundamental perspective the Joulc -Thotnson effect is important because it can be linked directly to the nature of intermolecular forces between gas molecules [205]. Consider, for example, a classic ideal gas as the simplest case in which molecules do not interact by definition. For this model it is simple to show that as a consequence of the absence of any intermolecular interactions a Joule-Thomson effect does not exist, that is, <5 = 0 [200, 201]. If, on the other hand, the ideal gas is treated quantum mechanically, it can be demonstrated [206] that a Joule-Thomson effect exists (S 0) despite the lack of intermolecular interactions. 

Does a Joule-Thomson effect exist in ideal quantum gases ... 

This observation is important because it also permits us to conclude that it will not be possible to construct a tangent through the origin at any point of the curve B2 T) no matter whether we consider a bulk or confined ideal quantum gas and irrespective of whether the quantum particles are Fermions or Bosons. In other words, for the ideal (bulk and confined) quantum gases, an inversion temperature Tj v does not exist because Eq. (5.155) does not have a solution. However, the reader should note that a Joule-Thomson effect does exist as pointed otit in Section 5.7.1, namely a dilute gas of Bosons is always cooled upon an iscnthalpie expansion B2 T) < 0), whereas a gas of Fermions is always heated during this process B2 (T) > 0). Tire extent to which this happens is modified in a nontrivial way by confinement according to the above discussion. 

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). 

FIGURE 4.31 Cooling bv the Joule-Thomson effect can be visualized as a slowing of the molecules as they climb away from each other against the force of attraction between them. 

Although this book is devoted to molecular fluorescence in condensed phases, it is worth mentioning the relevance of fluorescence spectroscopy in supersonic jets (Ito et al., 1988). A gas expanded through an orifice from a high-pressure region into a vacuum is cooled by the well-known Joule-Thomson effect. During expansion, collisions between the gas molecules lead to a dramatic decrease in their translational velocities. Translational temperatures of 1 K or less can be attained in this way. The supersonic jet technique is an alternative low-temperature approach to the solid-phase methods described in Section 3.5.2 all of them have a common aim of improving the spectral resolution. 

Joule-Thomson Coefficient. Knowing that a process is isenthalpic, we can formulate the Joule-Thomson effect quantitatively. 

In a relatively new process for production and fractionation of fine particles by the use of compressible media - the PGSS process (Particles from Gas-Saturated Solutions) - the compressible medium is solubilized in the substance which has to be micronized [58-61]. Then the gas-containing solution is rapidly expanded in an expansion unit (e.g., a nozzle) and the gas is evaporated. Owing to the Joule-Thomson effect and/or the evaporation and the volume-expansion of the gas, the solution cools down below the solidification temperature of the solute, and fine particles are formed. The solute is separated and fractionated from the gas stream by a cyclone and electro-filter. The PGSS process was tested in the pilot- and technical size on various classes of substances (polymers, resins, waxes, surface-active components, and pharmaceuticals). The powders produced show narrow particle-size distributions, and have improved properties compared to the conventional produced powders. 

Temperature changes as pressure is reduced when a flowing stream of gas passes through a throttle, i.e., a valve, choke, or perforations in casing. This is called the Joule-Thomson effect. The change in temperature is directly related to the attraction of die molecules for each other. 

JOULE-THOMSON EFFECT. In passing a gas at high pressure through u porous plug or small aperture, a difference r>f temperature between the compressed and released gas usually occurs. This phenomenon is called the. Imile-Thomson effect. The equation for this effect contains two partial derivatives and is... 

The temperatures on the envelope where pn = 0 are called inversion temperatures, Tt. At any given pressure, up to a maximum pressure, a given gas exhibits two inversion temperatures. The Joule-Thomson effect is important in refrigeration and in the liquefaction of gases. Modern refrigeration uses the larger effect of the evaporation of working fluids such as the chlorofluorocarbons. 

The temperature of the gas thus falls. The cooled air enters the chamber E from below and then goes up as shown. Thus, the gas cools the portion of the compressed air passing down the coil CE. This chilled gas then passes through a jet or throttle J and is further cooled by Joule-Thomson effect on account of expansion. This process goes on till the gas is converted into the liquid state. 

Joule-Thomson effect the change in temperature of a gas when it is compressed or allowed to expand through a small opening... 

In the process of liquefaction, one must also consider the inversion temperature (-361°F or -183°C or 90°K) of H2, because the behavior of this gas changes (inverses) at that temperature. Below the inversion temperature, when the pressure is reduced, the H2 temperature will drop. Above that temperature the opposite occurs a drop in pressure causes a rise in temperature. Therefore, in the process of liquefaction, H2 first has to be cooled below its inversion temperature—by such means as cooling with LN2—before the Joule-Thomson effect can be utilized. 

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