Study On The Characteristics Of The Spray Structure And The Unsteady Evaporation And Crystallization Of The Droplets For The Desulfurization Wastewater

The Zero Discharge Techonoly for the desulfurization wastewater becomes a hot issue because of the high efficiency of dealing with the wastewater and the low investment.In the real application of this technology,the complicated conditions of the flue gas affect the spray,the evaporation and crystallization processes.The negative pressure in the flue changes the spray structure.The particles in the high temperature flue could change the homogeneous nucleation process to the hetergeneous nucleation process of the wastewater droplet.Also,the concentration and water activity changes with water vaporization process because the wastewater contain different kinds of solutes.This leads to the change of the evaporation and crystallization rates.So,in this study,the spray structure and the evaporation and crystallization processes of the droplet were investigated for the desulfurization wastewater.An unsteady evaporation model for the multiphase droplet was built considering the effects of the flue gas and different kinds of solutes on the spray,the droplet vaporization and crystallization processes.This study is meaningful and important for the promotion of the Zero Discharge Techonoly.The spray,droplet vaporization and the droplet crystallization in the zero discharged technology for the desulfurization wastewater was investigated experimentally and theoretically.First,the spray characteristics were investigated experimentally under different operating conditions in the flue.Then,the effects of different solutes and different solid particles on the evaporation and crystallization processes were investigated by the measuring the mass loss and the heat flux.At last,a new unsteady mathematic model was built for the wastewater droplet vaporization considering the effect of the water activity and density change.The model was also vaildated by measuring the suspended droplet evaporation rate at different temperatures and different solut concentrations.First,in this study,the effects of the gas temperature,gas pressure,solutes concentrations,solution temperature and spray pressure on the spray penetration,spray angle and spray distribution,were investigated.The results showed that,with the increase of the spray pressure（from 2 to 5 MPa）and the decrease of the gas pressure（from 0.1 to 0.04 MPa）,the initial spray speed could be improved and the penetration for the wastewater spray was also increased.The increase of the gas temperature（from90°C to 170°C）could decrease the density of the gas and the decrease of the concentration of the solute in the wastewater could decrease the density of the wastewater solution.These could improve the penetration of the spray slightly.The angle decreased 4°,when the gas pressure increased from 0.1 MPa to 0.27 MPa.By the mutiple effects of density,viscosity and surface tension,the spray angle increased initially and decreased then,when the solutes concentration was increasing.The structure of the spray could be divided to the sheet structure and the droplet sturcture.In the sheet stucture,the distance between the droplets was short and the size of the droplet was big.In the droplet structure,the distance between the droplets was long and the size of the droplet was small.No clear division between the sheet structure and droplet structure was found and the size of the droplet was big,when the spray pressure was low（from 2 to 3 MPa）.The whole spray sturcture could not keep hollow cone.No clear sheet structure was found when the gas pressure was low（from 0.04 to 0.1 MPa）.The droplet size was small and the distance between the droplets was long.The whole spray structure kept hollow cone.Second,the evaporation and crystallization processes were investigated experimentally by measuring the mass loss and the heat flux of the desulfurization wastewater droplet.The effects of the volume of the droplet,different kinds of solid particles,different anions(SO₄^2-and Cl^-)and different cations(Ca²⁺,Mg²⁺,and Na⁺)on thermal physics,supersolubilities and nucleation of the droplet were investigated.These could affect the evaporation rate,crystallization rate and the structure of the drying crystals.The effect of operating parameters（temperatures,volume of the droplet and adsorption density）on the Pelect number was studied.The relationship between the operating parameters and the homogeneous and heterogeneous nucleation was investigated.The crystallization rate and the structure of the drying crystals could be analyzed by the Pe number.With the increase of the mass of the flying ash on the droplet,the crystallization rate was increased.The partieles（CaCO₃ and Fe₃O₄）could improve the crystallization rate.However,the particles of Al₂O₃ could not increase the crystallization rate and might inhibit the crystallization process.With the increasing concentration of SO₄^2-and decreasing concentration of Cl^-,the evaporation rate of the droplet was increased because of the increase of the saturated vapor pressure around the droplet.Also,it decreased the crystallization rate of the droplet due to the increase of the supersolutbility of the solutes.With the increasing concentration of Mg²⁺and decreasing concentration of Na⁺,the evaporation rate of the droplet changed little because of the similar saturated vapor pressures.Also,it could increase the crystallization rate of the droplet because of the decrease of the supersolubility of the solutes.Finally,an unsteady mathematic model considering the change of the thermal physics for the mutilphase electrolytes was built.The evaporation rate of the suspended droplet was obtained to validate the model.The temperatures of the droplet with the gas temperature from 90°C to 170°C were measured.At these droplet temperatures,the evaporation rate of the wastewater droplet was investigated for the solute concentrations of 0,1 and 5 times.Comparing the calculating results of the model with the experimental results,the model could predict the evaporation rate of the droploet accurately with the error less than 5%.Comparing with the results of the uniform concentration model,the results of diffusion model showed that the water activity was lower because the surface concentration was higher.The lower water activity slightly decreased the vapor pressure and the evapration rate of the droplet.Comparing with the effect of water activity,the increase of the density dominated the decline of the droplet evaporation rate.