Steady state simulation of chemical processes
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AbstractThe steady state simulation of single and multistage separation processes is examined in this investigation. In particular, a new formulation of the stability criterion for multiphase reacting/ non-reacting systems is presented. This formulation permits the simultaneous computation of stability and flash calculations for single stage separation processes. Furthermore, the formulation is extended for the simulation of multistage separation processes involving multiple phases. These formulations are subsequently utilized to develop algorithms for single and multistage multiphase separation processes. For single stage processes, algorithms for isothermal-isobaric and isenthalpic-isobaric multiphase flash are presented. Algorithms for multistage multiphase separation processes developed in this work permit the simulation of three-phase and two-phase distillation columns, absorbers and reboiled absorbers. The algorithms are evaluated using a number of test cases. Applications of the algorithms are illustrated. The new development of the stability criterion has led to the formulation of a set of coupled nonlinear algebraic equations which describe both the stability and the equilibrium calculations of reacting and non-reacting systems. Two algorithms for the simultaneous solution of stability and multiphase isothermal-isobaric flash calculations are presented. These algorithms differ in the manner in which the mole fraction summation and the stability equations are solved. The first algorithm solves these equations by using an active set solution strategy while the second one utilizes the Newton-Raphson method. The second algorithm is extended to handle the isenthalpic flash computations in non-reacting systems. The effectiveness and efficiency of the proposed algorithms are illustrated by solving several typical multiphase problems. Furthermore, the active set algorithm is utilized to study a number of typical phase equilibrium problems encountered in gas processing and petrochemical industries, in enhanced oil recovery schemes and in systems containing gas hydrates. The formulation of simultaneous stability and equilibrium calculations for single stage separation processes is extended to permit the formulation for multistage multiphase separation processes. New independent variables which represent the total moles of a component leaving a stage are introduced. An algorithm based on the Newton-Raphson method is proposed for the solution of the coupled nonlinear equations. The algorithm is utilized to simulate three and two phase distillation columns, absorbers and reboiled absorbers. The performance of the two phase multistage algorithm is compared with some of the existing algorithms. It is found to be efficient and effective. The three phase distillation algorithm is easily able to handle the appearance or disappearance of a phase during the computations. An acceleration technique, based on the Dominant Eigenvalue Method of Orbach and Crowe (1971), is evaluated for computational efficiency for single stage flash calculation. It is found to be efficient. This investigation provides a unified approach for the steady state simulation of multiphase single and multistage separation processes.
Bibliography: p. 179-190.