Research On The Cooling Crystallization Process For Hyperpure AdBlue Preparation

AdBlue is a hyperpure urea aqueous solution with mass fraction being 32.5% and can be applied in treatment technology of heavy duty diesel engine exhaust—SCR?selective catalytic reduction?for NO_x abatement by reducing NO_x to pollution-free N₂ under the effects of catalyst.Due to the catalyst deactivation caused by impurities of urea,it's vital to prepare high quality hyperpure urea product.Research involving the crystallization thermodynamics,cooling crystallization two-dimensional kinetics and crystallization process successfully revealed the growth mechanism and controlled the aspect ratio.First,the solubility of urea in the water at different temperature was measured by the equilibrium method,and the solubility equation of urea was fitted via the solid-liquid equilibrium experiential equation.The viscosity of saturated urea aqueous solution at different temperature was measured by Ubbelohde viscometer.The metastable zone width of urea aqueous solution under different cooling rates and stirring speed was measured by laser method.The mathematical model for metastable zone width of urea aqueous solution based on the measured data was also calibrated and can be used for optimizing the operational conditions.Based on the thermodynamics data,the variations of urea crystal morphology and size,nucleation numbers and supersaturation against crystallization process under different cooling rates were measured by batch transient sampling method.Various particle size analysis methods were then introduced and compared during the investigation of two-dimensional growth mechanisms.Considering the fact that fewer works study the nucleation and multi-dimensional growth together,contribution factor was defined for analyzing the effects on two-dimensional growth and nucleation.Several modified operation curves based on metastable zone width measurement and two-dimensional kinetic evaluation were outlined and compared aiming to acquire urea crystals with improved crystal size distribution?C.V.=31.4?and aspect ratio?4.97?.Compared with the classical cooling profile,kinetics validation of optimized profile was performed.Under the same washing conditions,the purity of crystal products with different aspect ration caused by different crystallization process was measured.The urea crystals which came up to the national standards,had been produced through optimized profile.The total ions contentwere 1.2ppm,below the national standards?1.5ppm?,.Moreover,the quantitative evaluation system involving CSD?average size distribution,size distribution on the characteristic dimensions,etc.?and A.R.established in the work can provide the comprehensive analysis of the experimental results obtained from the various particle size analysis technology and other process analysis instruments.