A Unified Modeling Framework for the Design of Complex Separation Processes

A unified modeling approach that combines rigorous rate-based balance equations with the model-order reduction properties of orthogonal collocation on finite elements (OCFE) approximation techniques is employed for the design and optimization of complex staged separation processes. The process model involves the rigorous description of mass and heat transfer phenomena, phase equilibrium, and chemical reactions in both gas and liquid phases in a sufficient number of selected collocation points which are less than the number of stages in the column. In this way a significant degree of detail is maintained that is absolutely necessary for the accurate representation of complex reactive absorption and distillation processes but in a more compact form than the equivalent full-order (tray-bytray) model representation. The unified modeling framework has been proved particularly efficient in the optimal design of single or multiple columns, the optimization of operating conditions and the dynamic simulation of complex staged separation units mainly due to the elimination of integer variables associated with the column stages and the accurate representation of the process. Results from the implementation of the compact model formulation to the design optimization and the dynamic simulation of the reactive absorption of nitrogen oxides (NO x) from a gas stream by a weak HNO3 aqueous solution as used in an industrial nitric acid production process are reported. Optimal column configurations with multiple side feed and draw streams are obtained.