Separation of NH₃/CO₂ from melamine tail gas with ionic liquid: Process evaluation and thermodynamic properties modelling

Melamine, as a typical chemical intermediate, is widely applied in the plastics and coatings industry. However, its synthesis process releases large amounts of NH3 and CO2 gas mixtures. A tail gas treatment is therefore a necessary process followed by its production steps. The traditional separation technology for tail gas treatment is water scrubbing for NH3 absorption, but this option suffers from high energy consumption and lots of NH3containing waste water.

In this work, a novel ionic liquid (IL)-based solvent, is proposed for the separation of NH3/CO2 mixtures from melamine tail gas. As models for prediction of solubility of NH3 in ionic liquids are not available, first the UNIFAC-IL model is adopted and extended to the NH3-IL systems based on available measured data together with other Group Contribution-based pure component property models.

Then, a systematic computer-aided IL design for NH3/CO2 separation is performed by formulating and solving a mixed-integer nonlinear programming problem. Considering NH3 solubility, NH3/CO2 selectivity and feasibility for industrial application as selection criteria, a hydroxyl-functionalized imidazolium-based IL is found to be optimal.

Based on the established thermodynamic models, both the IL-based process and a traditional water scrubbing process for NH3/CO2 separation are simulated and evaluated in terms of energy consumption, economic analysis and net CO2 emission under the same NH3 recovery. The simulation results show that compared with the traditional processes, the IL-based process represents a 45%, 35% and 19% reduction in overall equivalent energy, separation cost and net CO2 emission, respectively.