Second law analysis of non-equilibrium fluid streams

Abstract

A methode for obtaining second law analysis, of heat and mass transfer unit operations between two fluid streams, is presented. For single phase heat exchangers, the heat exchanger temperature characteristic T h =T h (T c ) is introduced where T h and T c stands for the stream temperature of the hot and cold stream along the heat exchanging surface. If the two streams involve two-phase fluids, additional information is needed to define the vapour content and the existing deviations from the equilibrium. This can be given by the vapour/liquid ratio and the relevant fluid temperature characteristic T v =T v (T l ) connecting the vapour and the liquid temperature of the corresponding phases of each stream. A non-equilibrium vapour-liquid mixture can be represented by its corresponding equilibrium state and a non-equilibrium-equilibrium deviation (NED) measured by the corresponding entropy difference. It is shown, how the two streams of the heat exchanger can be combined to form an equivalent single stream representing the behaviour of the heat exchanger. The same approach is extended further to include the mass transfer operations between two-phase binary mixture streams. In this case in addition to the temperature characteristic mentioned above, the mass fraction characteristic for the liquid-vapour coexisting phases are needed or, instead of that, the temperature characteristics of liquid and vapour as a function of the local equilibrium temperature. It is shown how the equivalent single stream can be obtained and the special cases of evaporator, absorber, rectificator, condenser and dephlegmator are considered in brief. Combination of such equivalent single streams, compose the description of complex thermodynamic cycles as for instance is the cycle of non-equilibrium absorption refrigeration.