Thermodynamic Properties of Fluids

The phase rule (Sec. 2.7) tells us that specification of a certain number of intensive properties of a system also fixes the values of all other intensive properties. However, the phase rule provides no information about how values for these other properties may be calculated. 

Numerical values for thermodynamic properties are essential to the calculation of heat and work quantities for industrial processes. Consider, for example, the work requirement of a compressor designed to operate adiabatically and to raise tire pressure of a gas from P[ to P2. This work is given by Eq. (2.33), wherein the small kinetic- and potential-energy changes of the gas are neglected  

the shaft work is simply AH, the difference between initial and final values of the entlialpy. 

Our initial purpose in tliis chapter is to develop from tlie first and second laws the fundamental property relations which imderlie the mathematical structure of tliennodynamics. From tliese, we derive equations which allow calculation of enthalpy and entropy values from f V T and heat-capacity data. We tlien discuss tlie diagrams and tables by which property values are presented for convenient use. Finally, we develop generalized correlations which provide estimates of property values in tlie absence of complete experimental information. 

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If M represents a molar thermodynamic property of fluid solution, then by definition. a homogeneous... 

This equation is seldom used, because the tables of the thermodynamic properties of fluids (steam tables) allow the values of the fluid/gas vapor to be accurately obtained. 

Examination of the thermodynamic properties of fluid tables shows how the viscosity varies with temperature. In order to obtain a general impression of this, consider the data in the thermal properties of fluid tables and the various values at different temperatures. 

It is necessary to be able to calculate the energy and momentum of a fluid at various positions in a flow system. It will be seen that energy occurs in a number of forms and that some of these are influenced by the motion of the fluid. In the first part of this chapter the thermodynamic properties of fluids will be discussed. It will then be seen how the thermodynamic relations are modified if the fluid is in motion. In later chapters, the effects of frictional forces will be considered, and the principal methods of measuring flow will be described. 

Haar L., Gallager J. G., and Kell G. S. (1979). Thermodynamic properties of fluid water. In Contributions to the 9th Int. Conf. on Properties of Steam, Munich, Western Germany. 

Equations of state have a much wider application. In this chapter we first present a general treatment of the calculation of thermodynamic properties of fluids and fluid mixtures from equations of state. Then the use of an equation of state for VLE calculations is described. For this, the fugacity of each species in both liquid and vapor phases must be determined. These calculations are illustrated with the Redlich/Kwong equation. Provided that the equation of state is suitable, such calculations can extend to high pressures. 

Hie thermodynamic properties of fluids consisting of light molecules sometimes departs markedly from those of heavier molecules. These departures, the so-called quantum effects, result from two different phenomena, the exchange effect and the diffraction effect... 

Confinement in porous materials is known to modify the thermodynamical properties of fluids. Capillary condensation is an example where a dense phase appears before saturating pressure is reached. Such phenomenon is not yet well understood in disordered mesoporous materials presenting highly interconnected pores. 

Preliminary to such a search we examine several thermodynamic properties of fluids at or close to criticality, that clearly show why and how fluctuations dominate under such conditions, (i) Consider first the isothermal compressibility, kj = —(dV/dP)T/V. At the critical point the isotherm dP/dV)r has zero slope thus, Ki grows indefinitely as T —> Tc. (ii) Using Eq. (1.3.13) and the definition for K one finds that (dV/dT)p = -(dV/dP)TidPldT)v = KiV dP/dT)y, wherein (dP/dT)v does not vanish. Therefore, the coefficient of thermal expansion, = i /V) dV/BT)p also grows without limit as the critical point is approached, (iii) According to the Clausius-Clapeyron equation in the form AH = T(Vg — Vi)(dP/dT), the heat of vaporization of the fluid near the critical point becomes very small, since Vg — Vi 0, whereas dP/dT remains finite. 

Pitzer, K.S. (1955a). The Volumetric and Thermodynamic Properties of Fluids. I. Theoretical Basis and Virial Coefficients. J.Am.Chem.Soc., 77, 3427-3433. 

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