Joule - Thomson effect

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

The advantages of this indirect p, V, T method over direct methods are (0 that the extent of the gas imperfection is measured directly, and not as a small difference between two large quantities, and (m) that the results are uncomplicated by adsorption effects. 

The Joule-Thomson effect is a measure of the deviation of the behavior of a real gas from what is defined to be ideal-gas behavior. In this experiment a simple technique for measuring this effect will be applied to a few common gases. 

An ideal gas may be defined as one for which the following two conditions apply at all temperatures for a fixed quantity of the gas (1) Boyle s law is obeyed i.e.. 

It is apparent that the enthalpy Hof an ideal gas is also a function of temperature alone  

Accordingly, we can write for a definite quantity of an ideal gas at all temperatures 

Since the process is adiabatic, the change in internal energy is 

Isenthalpic Nature. As the Joule-Thomson experiment is carried out adiabati-cally, we can write 

However, it does not follow from this fact alone that AH also is zero, because the process involves a change in pressure. Nevertheless, it can be shown that the process is an isenthalpic one that is, AH is zero. 

The work performed by the gas is that accomplished in the right chamber. 

As 2 = 0, it follows from the first law of thermodynamics that U2 - Ui = W = PiVi - P2V2 

we have proved that the Joule-Thomson experiment is isenthalpic as well as adiabatic. 

A gas passes a membrane from the high pressure side (subscript 1) to the low pressure side (subscript 2). This process is assumed to occur adiabatically. i.e. the whole system has been isolated and no heat transfer occurs (q = 0). The internal energy change of this process AD is equal to 

This implies that this process occurs isenthalpic. The temperature change in this process is expressed by the differential equation (3T/3P)h, which is called the Joule-Thonq son coefficient (Ijt. If the enthalpy of a gas H is considered to be dependent on T and P then 

Depending on the relative magnitude of the two terms between brackets the gas is either cooled or wanned upon pressurizing. Some values of fijT of various gases is given in table VI.13. - 

It can be seen clearly that temperature decrease in gas separation depends on the type of gas. Hydrogen will give a small temperamre difference only but carbon dioxide may give a tremendous temperature decrease at high applied pressure. It is clear that in in the latter case the separation performance is affected as well and that the Joule-Thomson effect should be taken into account when carbon dioxide is removed at a high pressure. 

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Joule-Thomson effect, Joule-Kelvin effect... 

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. 

Many gases can be liquefied by making use of the Joule-Thomson effect, cooling... 

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. 

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