Numerical Simulation And Optimization On Spray Drying Of Polycarboxylic Superplasticizer

With the continuous development of concrete industry in recent years,the demand for polycarboxylic superplasticizer,as the latest generation of concrete admixture,has been rising year by year.Polycarboxylic superplasticizer powder can greatly reduce the transportation and storage cost,facilitate storage,extend the shelf life,and it can be used in new products such as dry powder mortar,new coatings and sprayed concrete,which greatly broadens the engineering application field of polycarboxylic superplasticizer.Spray drying technology can realize the automatic and continuous production of polycarboxylate water reducer powder.It is of great engineering significance for the development of concrete industry to study the high-efficiency spray drying technology.The exhaust gas from hot air spray drying carries a large amount of waste heat,with low heat utilization rate,and the exhaust gas carries volatile components,which will cause environmental pollution.Using superheated steam instead of hot air as the spray drying medium,the whole spray drying process can form a closed cycle structure,the exhaust gas can recover the latent heat,and the whole process can achieve exhaust emission without VOC.In this paper,the flow field characteristics,droplet drying rate and reversal point temperature in the superheated steam spray drying tower of polycarboxylate superplasticizer were investigated based on with CFD numerical simulation method,and the inlet mode was optimized.Eulerian Lagrangian method was used to calculate the heat and mass transfer process between the drying medium and the droplet.And RNG k-ε model was used to describe the gas-phase turbulent process.REA model was used to calculate the drying kinetics of polycarboxylic superplasticizer,and a diffusion model was established to calculate the parameters of REA model.Hot air and superheated steam in 445 K were used as the drying medium respectively,the flow field characteristics and droplet drying rate in the drying tower under the two media are compared.It is found that the temperature distribution in the hot air and superheated steam drying tower is similar.With the decrease of the axial position of the tower,the average temperature of the section also gradually decreases,while the droplet drying rate first increases and then decreases,reaching the maximum at 4m.The temperature of each section in the superheated steam drying tower is higher than that of the hot air,and the drying rate of the superheated steam is lower than that of the hot air at this inlet temperature.By further comparing the droplet drying rate at each inlet temperature,it is determined that the reverse rotation point temperature of the superheated steam is 461 K when the water content of the polycarboxylic superplasticizer is 50% and the feed temperature is 20 ℃.The initial material moisture content is in the range of 40%-60%,and the temperature at the reversal point decreases with the increase of the initial material moisture content;The initial feed temperature is in the range of 10 ℃-30 ℃,and the reversal point temperature increases with the increase of the initial feed temperature.The air inlet mode of superheated steam spray drying was improved,combined with vertical inlet and swirl inlet,and the combined inlet drying tower was numerically simulated.The simulation results show that the swirl inlet can enhance the airflow disturbance in the drying tower and realize the flow pattern of vertical flow and circumferential flow.The swirl can further disperse the droplets and make the temperature distribution at the high section of the same tower more uniform.The droplet drying rate can be improved by increasing the swirl inlet velocity and vertical inlet velocity,but the moisture content of particles at the bottom of the tower is not significantly improved.Increasing the angle between the swirl inlet direction and the horizontal section can also improve the droplet drying rate to a small extent.