Energy Efficient Hybrid Gas Separation with Ionic Liquids

Abstract

Hybrid gas separation processes, combining absorption and membranes together with distillation require less energy and have attracted much attention. With the property of non-volatility and good stability, ionic liquids (ILs) have been considered as new potential solvents for the absorption step. In this PhD study, a systematic screening model for ILs is established by considering the needed properties for gas absorption process design. Rigorous thermodynamic model of IL-absorbed gas systems is established for process design-analysis. Then a strategy for hybrid gas separation process synthesis where distillation or other gas separation processes and IL-based absorption are employed for energy efficient gas processing is developed.

Introduction

Gas separation processes have been one of the most important operations in the oil and gas related industries, where gases of interest are present in significant amounts to justify their separation for use as raw gas in chemical production utilization. Distillation based gas separation processes is very common in the oil and gas industry. They however, consume large amounts of energy. Therefore, an alternative energy-saving and “green” technology is to replace, where possible, the distillation step with gas absorption employing IL as a solvent in a hybrid scheme. However, the enormous number of potential ILs that can be synthesized makes it a challenging task to search for the best one for a specific gas separation. In order to solve this problem, a systematic screening method is established by considering important properties for separation. Then rigorous thermodynamic model of IL-based system could be established. The hybrid gas separation process combining traditional technology with IL-based technology together could be designed for energy efficiency and economy-saving.

In this project, a three-stage methodology is proposed for hybrid gas separation process design and evaluation. The first stage involves IL screening, where a systematic screening method together with a database tool is established to identify suitable ILs based on a collection of gas solubility data, Henry’s constant data as well as data estimated through reliable predictive models (for example, COSMOS-RS). The second stage is process design, where the important design issues (amount of solvent needed, operating temperatures and pressure, evaporation conditions, etc.) are determined. A hybrid gas separation scheme is designed to replace the conventional distillation process. Since the only energy requiring step in the hybrid process is the flash-evaporation step (and the low energy consuming pre-distillation step, if employed), potentially a large reduction of energy consumption is possible by switching from distillation to the hybrid-absorption scheme the selective gas separation tasks. For example, replace distillation by absorption to remove only the gases present in smaller amounts in the gas mixture, thereby letting the larger amounts free to go out as the exit (raffinate) gas. This small amount absorbed gas is then easily separated through evaporation or distillation, which only consumes a small fraction of the total energy of the conventional distillation based process. The third stage involves verification and sustainability analysis based on rigorous process simulation of the generated hybrid gas separation process strategy.

Conclusion

The predictive Henry’s constant models have been found to be in good agreement with experimental data. On the basis of these validated models, a method for selection of the potential IL-solvent for gas separation by absorption has been highlighted. An IL-based hybrid separation scheme for significantly reduced energy has been introduced and the potential benefits highlighted through a conceptual example. Further work is necessary to fine-tune the IL screening tool (database, model, search engine). More detailed analysis is necessary to account for all possible performance criteria to establish the hybrid gas separation scheme.

Supervisors

Prof. Georgios M. Kontogeorgis (Principal supervisor)
Assistant Prof. 
Xiaodong Liang

PhD Study started: September 2016 to be completed: August 2019

Contact

Xinyan Liu
PhD student
DTU Chemical Engineering