Process design, exergy analysis, and multi-objective optimization of a coupled triethylene glycol dehydration and gradient condensation process for sustainable DMF recovery from synthetic leather exhaust gas with inherent safety considerations

Abstract

To address the bottlenecks of high energy consumption and secondary pollution in the conventional water absorption‑distillation process for N,N-dimethylformamide (DMF) recovery from synthetic leather exhaust gas, a novel physical separation process is proposed that integrates triethylene glycol dehydration, multi-stage gradient condensation and cold energy recovery. The process deeply dehydrates the exhaust gas using triethylene glycol (TEG), lowering the dew point of the dry gas to below −40 °C and fundamentally eliminating the risk of icing during cryogenic condensation. A gradient cooling strategy of pre-cooling at 35 °C, second-stage condensation at −5 °C and third‑stage condensation at −35 °C is adopted, and the cold energy carried by the non-condensable gas from the third stage is cascaded back to the second-stage condenser. Using R290 as the refrigerant, the three-stage condensation process achieves an average energy saving of 50.1% compared with a single-stage process. A stripping tower installed downstream of the TEG regenerator with a stripping gas flow rate of 500 kg·h⁻¹ increases the lean TEG purity to 99.98 wt%, satisfying the stringent dew point requirement for ice prevention at −35 °C, i.e., a water content below 308 ppm. Multi-objective optimization shows that without end-of-pipe treatment, the Pareto optimal solution is a DMF recovery rate of 93% with a total annual cost of 469 k$. When the cost of activated carbon adsorption is considered, the total annual cost versus DMF recovery curve becomes U-shaped with a global optimum near 99% recovery.