Research On The Process Of Steamammonia Distillation In Coke Plant And Exploration Of Energy-saving Process

A large amount of residual ammonia wastewater is discharged from coke plantevery year. The wastewater can’t be treated by biochemistry directly, because of highconcentration of ammonia nitrogen. At present, ammonia distillation is widely used topretreat the wastewater before biochemical process. But there are some disadvantagesfor ammonia distillation, such as large energy consumption and high running cost. Inthis paper, the effect and steam consumption of ammonia distillation was mainlyanalyzed to explore the main cause of large energy consumption. In order to solve theproblem of large energy consumption, a new energy-saving process was put forward,which could provide reference and guidance for energy-saving technologicaltransformation of ammonia distillation process.Firstly, the simulated ammonia wastewater was treated by batch distillation. Theresults showed that total ammonia removal efficiency increased with increasing initialpH and evaporation. But as the removal of ammonia, the mass transfer rate descendedwhich resulted to removing unit mass of ammonia consumpted more energy.Continuous ammonia distillation process was further studied in a packed tower atatmospheric pressure. Main parameters affecting ammonia removal were studied.These parameters included feed temperature, inlet total ammonia concentration, initialpH and steam flow. The results showed that total ammonia removal efficiency wouldbe improved with the increase of feed temperature, pH and steam flow. While itwould be increased at first and then decreased with the increase of inlet total ammoniaconcentration, but the amount of removal ammonia increased. Based on the masstransfer theory, the fundamental factor affecting ammonia removal efficiency andsteam consumption was mass transfer rate. In addition, based on the mass transfertheory in packed tower, correlations between total ammonia concentration, freeammonia concentration and steam flow were established using the data obtained fromexperiment results. And the change curve of total ammonia concentration vs. steamconsumption was drawn. The curve displayed that when the required ammoniaconcentration in effluent was lower, steam consumption was larger and steamconsumption removing unit mass of ammonia increased more significantly. Therefore,to reduce the steam consumption at the stage of lower ammonia concentration is an effective way to save energy consumption in ammonia distillation process.In order to solve the problem of large energy consumption, a new process whichcombined ammonia distillation and air stripping was put forward. In this new process,ammonia distillation was substitute for air stripping to remove ammonia at the stageof lower ammonia concentration. And it mainly consisted of alkali addition, heattransfer, distillation, air stripping and absorption. Suitable operating parameters ofdistillation-air stripping were that: feed temperature was around90℃, pH value offeed was11.2and steam consumption was81.3kg/t (ammonia wastewater) in theammonia distillation process; feed temperature was60~70℃, pH of feed was10.4and gas-liquid ratio was2000in air stripping process. Distillation-air strippingprocess was used to treat simulated ammonia wastewater which total ammoniaconcentration was3500mg/L. Firstly, the wastewater was treated by distillation toreduce total ammonia concentration to about600mg/L, and then was further treatedby air stripping to reduce total ammonia concentration lower than200mg/L. Theresults showed that using distillation-air stripping process to treat ammoniawastewater, the steam consumption reduced48.7kg/t (ammonia wastewater), savingthe direct steam amounts37.5%, and the processing costs saved3.87yuan/t (ammoniawastewater). Therefore, the new energy-saving process would bring great benefits forcoke industry.