Modelling of a dynamic multiphase flash: the positive flashApplication to the calculation of ternary diagrams

R THERY, F LLOVELL, X MEYER, V GERBAUD, X JOULIA
2004 Computers and Chemical Engineering  
A general and polyvalent model for the dynamic simulation of a vapor, liquid, liquid-liquid, vapor-liquid or vapor-liquid-liquid stage is proposed. This model is based on the τ -method introduced as a minimization problem by Han & Rangaiah (1998) for steady-state simulation. They suggested modifying the mole fraction summation such that the same set of governing equations becomes valid for all phase regions. Thanks to judicious additional switch equations, the τ -formulation is extended to
more » ... ic simulation and the minimization problem is transformed into a set of differential algebraic equations (DAE). Validation of the model consists in testing its capacity to overcome phase number changes and to be able to solve several problems with the same set of equations: calculation of heterogeneous residue curves, azeotropic points and distillation boundaries in ternary diagrams. Topical Heading Process System Engineering CONCEPTS LEADING TO THE "POSITIVE FLASH" MODELLING The "negative flash" An approach called the "negative flash" in which the vapor -liquid phase split ratio is allowed to take a value outside the physical domain range was introduced (Neoschil and Chambrette, 1978; Withson and Michelsen, 1989). Later, this formulation was extended to liquid -liquid dynamic equilibrium (Heinichen, 1994) . The resolution of this model leads to a successful solution whatever the phase region (L or LL) as the set of equation is kept unchanged: If the phase split α takes a value between 0 and 1, two liquid phases exist. Otherwise, if α leaves the physical range, the set of equations remains the same but one of the liquid phases is non existent physically. Although this approach leads to
doi:10.1016/s0098-1354(04)00146-2 fatcat:kvbuxteewjhcnhoc6hfrtileoq