Multiphase flows, with phase change

Flashing flow and condensing flow are two examples of multiphase flow with phase change. Flashing flow occurs when pressure drops below the bubble point pressure of a flowing liquid. A frequently... 

Discussions in Chapter 2 may be referred to for explanations of the various symbols. It is straightforward to apply such conservation equations to single-phase flows. In the case of multiphase flows also, in principle, it is possible to use these equations with appropriate boundary conditions at the interface between different phases. In such cases, however, density, viscosity and all the other relevant properties will have to change abruptly at the location of the interface. These methods, which describe and track the time-dependent behavior of the interface itself, are called front tracking methods. Numerical solution of such a set of equations is extremely difficult and enormously computation intensive. The main difficulty arises from the interaction between the moving interface and the Eulerian grid employed to solve the flow field (more discussion about numerical solutions is given in Chapters 6 and 7). 

A perennial problem in multiphase reactors is scale-up, that is, how to achieve the desired results in a large-scale reactor based on the observations made in the laboratory unit, which remains elusive due to complexities associated with transport-kinetic coupling [14]. The success of scale-up of trickle-bed reactors is based on the ability to understand and quantify the transport-kinetic interactions at the particle scale level (or single eddy scale), the interphase transport at the particle and reactor scales, and the flow pattern of each phase and phase contacting pattern and their changes with the changes in reactor scale and operating conditions [1]. 

For non-ideal multicomponent mixtures the multiphase flow calculation can be combined with a more rigorous thermodynamic equilibrium calculation to determine the mixture properties at the interface as discussed by [60, 70, 98]. However, describing the chemical reactor performance under industrial operation conditions the heat balance is normally dominated by the heat of reaction term, the transport terms and the external heating/cooling boundary conditions, hence for chemical processes in which the phase change rates are relatively small the latent heat term is often neglected. 

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