(106d) A Short-Cut Method for Synthesis of Solvent-Based Separations

In the process industry, solvent-based separations, e.g., extraction, extractive distillation, and azeotropic distillation, are widely applied to separate nonideal mixtures by adding an external component to the system. The optimization of solvent-based separations includes two sequential activities, solvent selection and process design/control. First, the solvent is selected based on the physical properties of the solvent, such as solubility power, boiling point, selectivity, etc. [1]. Then, the process variables such as solvent flowrate, column stages, and feed inlet stages are determined by sensitivity analysis or simulation-based optimization for the selected solvent. For example, Luyben [2] designed the optimum acetone/chloroform azeotropic distillation system by varying the solvent flowrate and reflux ratio with a fixed number of stages. Similarly, You et al. [3] designed an acetone/methanol extractive distillation process by minimizing the process energy consumption. However, these process design methods are based on knowing the identity of the solvent(s), which are usually selected based solely on their physical properties. Kossack et al. [4] pointed out that solvent screening based on physical properties such as selectivity alone may result in unfavorable entrainer choice. It is necessary to take separation process properties such as energy consumption, number of stages, etc., into account during solvent selection. A short-cut evaluation model which can quickly evaluate both the solvents’ physical/mixture and process properties is important in designing an optimal separation-based process. In other words, with the assistance of the short-cut evaluation model, different solvent-based separation alternatives can be quickly generated and evaluated.

This work focuses on developing a hybrid separation synthesis framework for the separation of nonideal mixtures. First, solvents/entrainers that can break the azeotropic barrier are designed by a computer-aided molecular design method or identified via literature search. In this step, the solvent/entrainer physical and mixture properties are applied as constraints. Then, the solvents’ process properties, like energy consumption and the number of stages, are calculated using the developed short-cut evaluation model. In the short-cut evaluation model, for extractive distillation systems, the extractive column is divided into three sections (rectifying, extractive, stripping), and the Underwood and Fenske equations are applied in each section separately. The analysis of each section separately enables consideration of variations in the relative volatility across the whole column, thereby providing accurate estimates of the minimum reflux ratio for the column and the number of stages in each section. The minimum reflux ratio is used to estimate the minimum reboiler duty through enthalpy balance. The short-cut evaluation model employs a scoring approach, which considers physical properties, mixture properties, and process parameters to select the optimal solvent-based separation process. Finally, the top-ranked alternatives are verified using rigorous models.

The developed hybrid separation synthesis framework is applied to different case studies, such as dimethyl carbonate and isobutanol production. For all the examples, the framework was able to generate multiple process alternatives that are quickly evaluated to identify the optimal separation process.

References

[1] Shen, W., Dong, L., Wei, S. A., Li, J., Benyounes, H., You, X., & Gerbaud, V. (2015). Systematic design of an extractive distillation for maximum‐boiling azeotropes with heavy entrainers. AIChE Journal, 61(11), 3898-3910.

[2] Luyben, W. L. (2008). Control of the maximum-boiling acetone/chloroform azeotropic distillation system. Industrial & engineering chemistry research, 47(16), 6140-6149.

[3] You, X., Rodriguez-Donis, I., & Gerbaud, V. (2015). Improved design and efficiency of the extractive distillation process for acetone–methanol with water. Industrial & Engineering Chemistry Research, 54(1), 491-501.

[4] Kossack, S., Kraemer, K., Gani, R., & Marquardt, W. (2008). A systematic synthesis framework for extractive distillation processes. Chemical Engineering Research and Design, 86(7), 781-792.