Three-Phase Distillation: Nonequilibrium Modeling And Parameter Estimation

From a conventional point of view, the presence of more than one liquid phase within a distillation column should be generally avoided due to the uncertainties caused by multi-liquid-phase in operation and control. However, the appearance of a second liquid sometimes is inevitable in chemical industry, especially in some high-value added processes like azeotropic distillation, extractive distillation and reactive distillation, where the complex vapor-liquid-liquid three-phase distillation may appear.In the past, the research on three-phase distillation has been generally ignored because of the complexity of the three-phase hydrodynamics. These years, with the aid of advanced computation ability and experiment facilities, it is time to explore the essence of three-phase distillation, and to utilize its unique characteristics to enhance efficiency, optimize operation, save energy and reduce consumption.This thesis focuses on the aspects of modeling and simulation of three-phase (packed) distillation. Modeling concepts of equilibrium (EQ) and nonequilibrium (NEQ) are employed. Meanwhile, systematic experimental investigations on packed distillation have been conducted at TU Berlin. Due to the nonequilibrium nature of three-phase hydrodynamics, the EQ concept using stage efficiency or HETP is found not suitable for modeling three-phase distillation. The NEQ concept should be primarily considered. However, the three-phase NEQ modeling was severely restricted due to the lack of methods to predict the interphase transfer parameters, which is the main research topic of this dissertation.The contributions and innovations of this thesis are as follows:1. To describe the diversity of vapor-liquid-liquid flow, three improved NEQ models (i.e. M₁, M₂, M₃) were proposed based on literature results and experimental observations. Each model can be applied to certain three-phase flow on a tray and a packing. One unique feature of the models is the using of embedded phase stability algorithm in the NEQ simulation, which is of great significance to the mixed two/three-phase distillation largely existing in real distillation practice.2. NEQ model based parameter estimations using multiple experimental data sets have been performed. A hierarchical estimation strategy has been proposed for parameter classification and problem decomposition. As a result, some literature correlations have been extended, and can be used in the transfer parameters prediction for three-phase distillation. Model validation has shown that the prediction ability of three-phase NEQ model can be significantly improved by using the extended correlations.3. Principle three-phase mass transfer model has been developed on the basis of a thorough analysis of the three-phase flow behaviors on packings. The model enables, for the first time, rigorous computation of all the transfer parameters. The flow pattern of the second liquid was found to play a critical role on the efficiency of three-phase distillation. Therefore, its unknown flow pattern has been identified based on comprehensive experimental data sets. The identified flow behavior can give a reasonable explanation to the discrepancy existing in the separation efficiency of three-phase packed distillation.4. In the scope of distillation operation and control, dynamic EQ and NEQ models for three-phase distillation have been developed. (EQ and NEQ) model validation and comparison with experimental data have been carried out. Moreover, dynamic parameter estimation based on the NEQ model has been conducted, the identified dynamic flow pattern of the second liquid phase corresponds well with previous studies.Finally, conclusions and future research prospects are given.