Details

Autor(en) / Beteiligte

• Grassi, Vincent G

Titel

Process design and control of extractive distillation

Ort / Verlag

ProQuest Dissertations & Theses

Erscheinungsjahr

1992

Link zum Volltext

• ProQuest

Quelle

ProQuest Dissertations & Theses A&I

Beschreibungen/Notizen

• Extractive distillation is an industrially important process to separate azeotropes or low relative volatility mixtures when conventional distillation becomes impractical. The process dynamics are highly nonlinear and multivariable with many interactions. Various methods of design and operation have been published, but industrial experience has shown that the process design and control of this process are not obvious. This research is a study to gain generic insight and develop a method for the process design and control of extractive distillation systems. The work bridges the gap between process design and control by introducing dynamic methods during the process design phase. Three industrially significant processes are studied. These are: (1) methyl acetate, methanol and water, (2) methyl acetate, methanol, and ethylene glycol and (3) ethanol, water, and ethylene glycol. These systems span a wide range of relative volatility between the components resulting in significantly different process designs. The methyl acetate, methanol and water system is practiced by Air Products and Chemicals, Inc. The models and methodology developed in this research are verified against operating data from this industrial facility. A computerized process design procedure is developed. Graphical residue maps and column operating curves of the extractive distillation column improve process insight into the critical design variables. First principles, dynamic simulation, multivariable dynamic analysis, and economic analysis lead to the conclusion that the optimum control structure is a simple robust single-input single-output scheme, termed E1-S1, where single temperatures in the extraction and solvent towers are controlled. The case studies serve to illustrate many important interrelations between the process design and control of extractive distillation systems. It is shown that additional stages in the extraction tower can significantly slow the process dynamics and result in a process design that is difficult to control. The economic cost of the solvent to feed recycle ratio is not significant over a wide range of solvent flow. This allows the process design engineer to design a system with more solvent than the true optimum to improve the controllability of the process. The solvent recycle must be of sufficiently high purity to ensure good control of the extraction tower overhead composition. Finally, the extraction tower must be designed so the process gain between overhead composition and reflux does not change sign over the control range. It is shown that the control range can be increased by increasing the solvent flow.