Approximate procedures for simulation and synthesis of nonideal separation systems

• Main Author: Matias, Teresa do Rosario Senos
• Published: University of Edinburgh 1997
• Subjects: 502.85
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.657436

Simulation and synthesis of nonideal separation systems is computationally intensive. The main reasons for this are the time used in the calculation of physical properties, which cannot be assumed constant throughout the calculation, and the elaborate methods required for the full rigorous simulation and design of distillation units. The present work looks at two different ways of reducing computing time in steady state simulation and in synthesis of nonideal separation systems: • Use of approximate models for physical property calculation. • Use of 'shortcut' procedures, which are thermodynamically rigorous, in simulation and synthesis of nonideal distillation. Approximate models are derived for the liquid activity coefficient and for relative volatilities within a simplified flash unit. Liquid activity coefficient models include a Margules-like equation generalised for a multicomponent mixture and other equations of the form of rational functions. They are tested with several nonideal ternary mixtures and it is shown how their behaviour changes across the ternary composition diagram. The development of simplified flash units with approximate physical properties is done in a dual level flowsheeting environment. One level is used to solve the material balance assuming given fixed relative volatilities. The other level approximates the physical property values based on rigorous bubble point data obtained from a rigorous physical property package, using an 'ideal' correction to calculate the vapour liquid equilibrium conditions. It is shown how the two levels can be used in different arrangements, by converging them simultaneously or one within the other. The performance of the dual level flowsheeting arrangements is tested using the Cavett problem structure for several mixtures and compared against the conventional method where the flash is performed directly by the rigorous physical property package. Finally a rigorous shortcut procedure has been developed for designing nonideal distillation processes. The procedure is based on a nonideal variation of Fenske equation with rigorous physical properties using an iterative method. The procedure is implemented in a package for automated synthesis incorporating heat integration. An example case is studied and the results obtained in the synthesis are compared will a full rigorous simulation of the same process. It is shown for the first time how a rigorous shortcut procedure can be used in synthesis to produce results that consider heat integration in the initial stages of design, within a reasonable amount of time.