Process Development, Simualtion, And Industrial Implementation Of A New Coal-Gasification Wastewater Treatment Installation To Remove Phenols And Ammonia

Coal-gasification process produces a large quantity of gas-washing wastewater, which contains phenols, ammonia, and sour gas. Because of the toxicity of phenols, NH₃, and H₂S, the wastewater has to be treated and purified prior to discharge. Existing coal-gasification wastewater treatment plants usually consist of two sections: a chemical treatment process and a biological treatment process. However, the chemical treatment process can not remove phenols efficiently to meet the discharge standard by the following biological treatment.Based on the analysis of composition and thermodynamics properties of the wastewater, the complex multicomponent systems NH₃-CO₂-H₂S-NaOH-phenol-H₂O and H₂O-Diisopropyl Ether (DIPE)-Phenol-Hydroquinone were analyzed when simulating the wastewater stripping and solvent recovery process, respectively. And ELECNRTL and NRTL activity coefficient models are used to describe the wastewater stripping and solvent recovery process, respectively. Simulation results shows that the bottlenecks of the current wastewater treatment process are low phenols removal efficiency and plugging induced by ammonium bicarbonate crystals. This work proposes two methods to solve the problems. One is that ammonia and sour gas remove simultaneously in a wastewater stripper with side-draw, which reduce the pH value of the wastewater from 9 to below 7, and then remove phenols with solvent extraction in acidic condition. The other is that DIPE is replaced by MT, whose distribution coefficients to phenol and polyhydrics are greater than DIPE.The extraction process conditions of MT were determined, such as temperature, pH value, phase ratio, and extraction stages. The liquid-liquid equilibria of H₂O-MT-Phenol-Hydroquinone were studied. The experimental data were correlated with the NRTL activity coefficient model, and the corresponding binary interaction parameters were regressed. These parameters were used to simulate the new process.Process steady-state simulation was conducted to study the performance of the new process and perform detailed design, such as the number of stages, the top temperature of towers, and the top pressure of towers. And process dynamic simulation was conducted to design the control system of the new process.This new process has been successfully implemented in China Harbin Coal-chemicals Inc. for coal-gasification wastewater treatment with a capacity of 130t/hr. Satisfactory agreement was obtained between the simulation results and the industrial implementation of the new process. Compared with the old process, the new process recovers phenols more efficiently, and yields a satisfactory outflow for the later biological treatment of the wastewater.