Energy efficient hybrid gas separation process with ionic liquids

Hybrid gas separation processes, making good use of the advantages in different technology, have attracted much attention to be more energy intensive. Due to non-volatility, good stability and tailor-made properties, ionic liquids (ILs) have received much attention as novel potential solvents for specific gas absorption.

Thus, for a gas mixture separation, the hybrid scheme where IL is used for specific gas absorption and other technologies applied for the rest gases separation is proposed in this thesis. However, the problem is selecting the optimal IL for a specific gas separation among the enormous number of potential options.

Existence of a standard procedure/framework for ILs screening together with IL-based separation process development will assist scientist and companies to reach more easily and in a better way remove to their targets. In this work, before establishing methodology to develop the IL-based gas separation, experimental information on gas solubility in different ILs and pure IL thermodynamic properties are necessary to be collected.

Thus, a comprehensive Henry’s law constants of 11 gases in various ILs and pure IL properties database are first established. Additional experimental gas solubility data are implemented in an already established gas solubility database. Based on all available experimental data, following three thermodynamic models on Henry’s law constant and activity coefficient of gas in various ILs are developed. (1) Group contribution based Henry’s law constant model of gas in three imidazolium ILs with different number of carbon atom of alkyl chain. (2) Corrected COSMO-RS model for the infinite dilution activity coefficient of three gases in ILs. (3) UNIFAC-IL activity coefficient model for four gas-IL systems.

A relatively good agreement between the model predicted and the experimental solubility data is observed. Moreover, the developed UNIFAC-IL model can be used to predict the solubility of gases in new ILs that are not included in parameter fitting due to its group contribution basis. For this reason, the model represents a very useful tool for a task-specific ILs design.

With the database and models available, a three-stage methodology is proposed for ILs screening and process development. The application of the three-stage methodology is highlighted through a conceptual example of a model shale gas separation. Two ILs providing good separation performance for CO2 removal are identified, one ([thtdp][phos]) is screened based on Henry’s constant database, the other ([MMPY][eFAP]) is generated through computer-aided IL design. [MMPY][eFAP] shows a three times higher selectivity of CO2/CH4 than [thtdp][phos].

Then the hybrid shale gas separation scheme where IL used for CO2 removal and distillation for other light hydrocarbon gas separation is designed. Thermodynamic modelsincluding pure IL properties and phase equilibrium model for gas-IL systems are established.The shale gas separation processes with two ILs, together with a traditional MDEA based CO2 removal process, are simulated and analyzed through both energy and economic consumption.Compared with traditional separation technology, the IL-based separation could achieve to be energy-saving and cost-efficient.