The overall objective of present contribution is to model and numeric analyze an industrial relevant extractive distillation column equipped with Intalox Metal Tower Packing (IMTP) for purification of n-butane from 1-butene,cis-2-butene and trans-2-butene mixture (C4) using N-Methylpyrrolidone (NMP) as entrainer.
In order to highlight inconsistencies in the modeling of mass and heat transfer in nonideal multicomponent extractive distillation column, both equilibrium stage models (EQ) and nonequilibrium model models (NEQ) incorporating with Maxwell-Stefan diffusion equations were successfully developed and implemented in the framework of Aspen Custom Modeler. To calculate thermodynamic properties of the C4 mixture and to estimate the activity coefficients the thermodynamic model UNIQAC was used in the present work. The limitations of the Fick's law for describing diffusion and advantages of Maxwell-Stefan diffusion theory to model transport process in nonideal multicomponent system were discussed respectively. Maxwell-Stefan formulation takes proper account of thermodynamic nonideality, and hence was proved to be the most general and convenient approach for describing transfer processes in nonideal multicomponent mixtures. To explore and understand the reasons behind the different trajectories followed by EQ and NEQ models explicit investigations were made on component Murphee efficiency, effective interfacial area, heat mass transfer coefficient and interfacial heat mass transfer rate. It was found that in the majority part of column the diffusion process in nonideal multicomponent system performs odd behavior like reverse diffusion, diffusion barrier and osmotic diffusion. Moreover, the component Murphree efficiency demonstrates different and unbounded values throughout the column. As a result, the real plant performances a reduced capacity. In addition, the nonequilibrium model requires the equipment design parameters, like column diameter, tray or packing type and design, to be available, the simulation results of nonequilibrium model can therefore be used to diagnose operating and design problems, identify equipment design parameters that can be altered to improve column performance. From a pragmatic view point of industrial application the results of present work has implication on industrial design and application associated with extractive distillation of nonideal multicomponent mixture.
The overall presentation falls into five chapters. Chapter I deals mainly with the statement of problem and background of this work, it also includes an introductory treatment of distillation process and review of literature in corresponding field. Chapter II introduces the fundamental theory incorporating with Maxwell-Stefan diffusion relation; it is, in many ways, the cornerstone for model development and well understanding of fluid behavior in column. Procedures for estimating diffusion coefficient in nonideal multicomponent mixtures and describing interphase interaction are suggested. Part III, the core portion of this work, covers applications of Maxwell-Stefan theory on modeling of nonideal multicomponent extractive distillation process; constitutive relations for simultaneous heat and mass transfer in conjunction with irreversible thermodynamic were developed and implemented; various correlations for estimating effective interface area, heat and mass transfer coefficients were discussed and integrated in the implementation. Chapter IV presents the results of numeric analysis. The limitation of the effective diffusivity approach is highlighted, and the likely pitfalls in misapplying it are warned. Chapter V winded up the contribution with synoptic conclusions and tentative perspect. Keywords:Maxwell-Stefan diffusion theory; extractive distillation column; nonideal multicomponent mixture; equilibrium nonequilibrium models; numerical analysis; aspen custom modeler.