Transport Properties: Theory

Abstract

In the preceding chapters, we have considered in detail prediction of the thermodynamic properties of fluids as this is of prime importance in the design of chemical process systems. In many areas of design, quantitative knowledge of the transport properties of the process fluids is also required and it is to the theory and practice of predicting such properties that we now turn.Unlike thermodynamic quantities, which refer fundamentally to properties of a system at equilibrium, transport properties determine the rate at which processes such as heat and mass transfer occur in a system that is not at equilibrium. Practical process situations often involve a system at or near to a steady state but, neverthe less, far from true equilibrium. In particular, there may be large spatial gradients in temperature, concentration or fluid velocity which give rise to non-linear transfer processes. In contrast, the science of fluid transport properties is concerned almost exclusively with somewhat idealised systems displaced only slightly from equili brium. The well known linear rate laws of Fourier, Fick and Newton arise in this context and each is associated with a transport coefficient that relates flux to gradient. The connection between these situations is that empirical models which are often, out of necessity, adopted in the description of practical transfer processes involve so-called transfer coefficients which depend parametrically on the formal transport properties. Here we shall be concerned only with the latter.Although for engineering purposes temperature and pressure are usually specified quantities, in the case of the transport properties both theory and expe rience show that temperature and density (or molar volume) are the fundamental state variables while, in essence, pressure is of no direct importance. Indeed, a transport property X, where X may be the viscosity rj, the thermal conductivity X, the diffusion coefficients D H and Dy, or the thermal diffusivity a, is written most conveniently as the sum of three contributions: X{p m T) = X 0 (T) + AX(p n ,r> + AX c (p",r).(9.1) Thermophysical Properties of Fluids Downloaded from www.worldscientific.com by UNIVERSITY OF BIRMINGHAM LIBRARY -INFORMATION SERVICES on 03/22/15. For personal use only.