Effects of vapor -liquid mass transfer in reaction -separation systems
The effect of mass transfer on feasible products is analyzed for two industrially relevant classes of vapor-liquid reactive separation problems: reactive distillation in monomer production and melt polycondensation for engineering thermoplastics and synthetic fibers. Two different approaches are used, a bifurcation analysis for reactive distillation and the attainable region technique for melt condensation polymerization.
The Stefan-Maxwell model for mass transfer effects is combined with a flash cascade model for assessing feasible products of reactive distillation systems. The model fixed points are determined as a function of the Peclet number and other parameters, using a pseudo arc length continuation method. The approach is illustrated with two examples: a parameteric study of an idealized ternary system, and the esterification of acetic acid to produce isopropyl acetate. In the limit of little or no reaction, the fixed points are independent of the mass transfer resistance, and a model based on phase equilibrium can be used for feasibility. When the chemical reaction rates or extents of reaction are more significant, the fixed points do depend on the Peclet number, and mass transfer can influence the feasible products. However, best estimates of the mass transfer parameters and the processing conditions typical of reactive distillation indicate that these effects are small for the particular examples studied.
In the second part of the dissertation we consider step-growth melt polymerization systems using a different approach to mass transfer which is more common for high viscosity polymeric systems. A concentration-based formulation is applied to develop hybrid reactor-separator models for the two phase CSTR and PFR with simultaneous vapor removal. The evaporation of the volatile byproducts, which is typically limited by liquid-phase mass transfer, is characterized with a single dimensionless parameter (Thiele modulus). A reaction-separation vector is defined which satisfies the same geometric properties as the reaction vector, thus the candidate attainable region can be constructed following the existing procedure for reaction/mixing systems. The technique is demonstrated on the polycondensation step in the production of Nylon 6,6. The effect of temperature, pressure, and Thiele modulus on the candidate attainable region for number average molecular weight is analyzed.