Process Design and Control of Extractive Distillation
This chapter presents case studies to improve process insight and develop a methodology for the process design and control of extractive distillation systems. This material attempts to bridge the gap between process design and control by introducing dynamic methods into the process design.
A computerized process design procedure will be developed. Graphical residue maps and column operating curves of the extractive distillation column improve process insight into the critical design variables. Dynamic simulation and multivariable control system analysis will lead to robust and very practical single-input-single-output control scheme solutions.
System 1-methyl acetate, methanol, and water.
System 2-methyl acetate, methanol, and ethylene glycol.
System 3-ethanol, water, and ethylene glycol.
The case studies will be referenced as Systems 1, 2, and 3 throughout this chapter. These systems illustrate significant variation but enough similarity so a generic understanding of the process is obtained.
Air Products and Chemicals, Inc. practices the methyl acetate, methanol, and water system. The models and methods developed in this chapter have been verified against operating data from this real plant. I will point out the significant findings from a critical evaluation of these cases in this chapter. A more detailed and complete description of these cases studies is contained in Grassi (1991).
Extractive distillation affects the liquid phase activity of the components so the mixture may be efficiently separated into pure products. This is done by adding a third, heavy, component termed the solvent. The solvent has an affinity for one component, causing it to boil intermediate in the ternary mixture. The overhead product from the extractive distillation tower contains the lightest component. The intermediate component leaves with the solvent in the bottoms stream of the extractive distillation tower.
The extractive tower bottoms stream feeds a solvent recovery distillation tower. The solvent recovery tower overhead product contains the intermediate component. The solvent recycles from the bottom of the solvent recovery tower to the extractive distillation tower.
Figure 18-1 presents a simplified flow sheet of the extractive distillation process. This process requires very nonideal vapor-liquid equilibrium and tight process integration within the double column system. 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 is not obvious.
KeywordsDistillation Tower Solvent Flow Extractive Distillation Reflux Ratio Relative Volatility
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