Development of a full process simulation package for tail gas cleaning from melamine production with ionic liquids

Melamine production is known to produce a tail gas with a significant amount of NH₃ and CO₂. The most common separation methods applied to melamine tail gas are water scrubbing and urea co-production technology. With good stability, non-volatility and tailored properties, ionic liquids (ILs) are regarded as vital potential solvents for gas separation. Therefore, two new process technologies, one is the ionic liquid-based process (IL-0), and the other is the enhanced ionic liquid process (IL-En) were employed and evaluated for energy and cost efficiency. The IL-En employs stripping on the treatment of melamine tail gas. The protic ionic liquid named 1-butyl imidazolium bis (trifluoromethylsulfonyl) imide ([Bim][NTf2]) was selected for the evaluation of the melamine tail gas cleaning process. Thermodynamic data were fitted to the NRTL equations. Three full process flowsheets were simulated in Aspen Plus V11 ^TM. A basic and an enhanced ionic liquid process (IL-0 and IL-En), a conventional water scrubbing (WS) technology as a comparison, process sensitivity analysis and energy/economic evaluation were carried out. The results showed that the total separation cost of the IL-En can be reduced by 61% compared to that of the WS process. Moreover, the IL-based flowsheet is simpler than WS and avoids wastewater discharge.

In order to give deep analysis of the proposed technology, a detailed multi-objective optimization (MOO) for the NH₃/CO₂ separation of the tail gas by IL in technical-energy-economic evaluation. According to the optimization results, specific cases with different objective functions were chosen, and the relationship between key parameters was analyzed. The results showed that the ionic liquid-based technology could realize a better performance regarding the technical-energy-economic than the base case. Deeper analyses with the influence of operation parameters with evaluation index were carried out, thus providing support to the industrial application of the new separation technology.

On the basis of the process steady-state design and multi-objective optimization work, the dynamic control behavior and evaluation of balanced IL-based NH₃/CO₂ separation were carried out. In order to analyze the dynamic behavior of this new technology, the two step test were considered: the tail gas flowrate and composition fluctuation were introduced to the process. With the procedure of definition of control problem, conversion from steady-state to dynamic mode and successful inventory control, the simulation results showed that NH₃ concentration and NH₃ recovery have a positive response with more tail gas and NH3 in the system. With the analysis of the parameter behavior on the different tail gas fluctuations, basic dynamic control could be built to guarantee stabilization with different fluctuations. Based on the detailed investigation about this process’s dynamic behavior, this new technology is fairly easy and simple to operate. Finally, the full-scale simulation package for the IL- based NH₃/CO₂ separation process will be established and provide strong support to this novel and new technology industrial application.