Investigation On The Crystallization Mechanism And Chemical Scale Inhibition Of Calcium Sulfate

As one of the main unresolved problems in heat transfer,scaling has been existing widely in water desalination process,cooling water system,alkali evaporator system,geothermal system,power plant boiler system and dairy process.The industrial scaling process is a relatively complex process,and subjected to factors such as crystallization process kinetics and equilibrium thermodynamics.Therefore,it has become an urgent problem to be solved at this stage that combining theoretical models to explore the effects of different influencing factors on the crystallization process kinetics and equilibrium thermodynamics.There are many problems that resulted from the scaling to the actual industrial production,which mainly include reducing the heat transfer coefficient and endangering the safety of the equipment.In order to solve these problems,the physical,technological,and chemical methods were adopted to inhibit scale in the industry.However,the current physical method of descaling technology is immature and will cause damage to the heat exchanger and pipe wall.The technical requirements and the initial investment of the process method are relatively large.Due to the small investment and good anti-scaling effect,chemical method is considered to be the best anti-scaling method at this stage.Among chemical agents,copolymer scale inhibitors have become one of important direction of current research due to their excellent scale inhibition performance and stability.In view of the above problems,the crystallization kinetics of calcium sulfate and the influence mechanism of operating conditions such as different temperatures and stirring speeds on calcium sulfate crystallization are systematically studied through the combination of isothermal crystallization experiment and chemical potential gradient model modeling analysis in this paper.In addition,a novel polymer scale inhibitor with a good inhibitory effect on calcium sulfate scale was designed and synthesized in this paper.The main research contents and results of this work are as follows:1.The effect of different factors on the crystallization kinetics of calcium sulfate was explored.The crystallization kinetics curves of calcium bisulfite at different temperatures,stirring speeds,pH values,and polymer concentrations were measured by isothermal crystallization method.It was found that the crystallization rate of calcium sulfate dihydrate increases with the increase of crystallization temperature.Compared with the temperature,the increase of stirring rate and pH value of solution has little influence on crystallization rate.As the concentration of polyacrylic acid increases,the crystallization rate of calcium sulfate dihydrate showed a downward trend.In addition,polyacrylic acid will be combined with calcium ions in the solution,thus the solubilization effect of calcium sulfate dihydrate will be achieved.2.Combining chemical potential gradient model based on the principle of non-equilibrium thermodynamics and extended Debye-Hückel model,the crystallization process was modeled.The results show that,compared with the other two models,the crystallization kinetics data calculated and predicted by the adhesive-type growth mechanism was in the best agreement with the experimental data.The effects of different factors on the crystallization rate constant k_t(kinetics)and driving forceΔμ(Thermodynamics driving force)of calcium sulfate dihydrate crystallization were further analyzed.The results indicated that temperature has a great influence on the crystallization rate constant k_t and the thermodynamic driving forceΔμ,and temperature was the key factor influencing the crystallization rate of calcium sulfate dihydrate.At the same time,the pH nearly has no significant effect on them,while the stirring speed and polymer concentration primarily affected constant k_t and thermodynamics driving forceΔμ,respectively.In addition,studies had shown that the crystal growth mechanism of calcium sulfate dihydrate belongs to adhesive-type growth mechanism.By constructing the linear function relationship between k_tand temperature,pH and polyacrylic acid concentration,the growth kinetics of calcium sulfate dihydrate crystal can be successfully predicted.3.A novel ternary polymer scale inhibitor containing carboxyl,sulfonic and amide groups was designed and synthesized.Through single factor experiment,the polymerization process was optimized,and the optimal synthesis conditions were determined as follows:the mass ratio of polymerization monomer was 1:1:1,the reaction temperature was 90℃,the APS content was 10%of the total mass of polymerization monomer,the IPA content was 4%of the total mass of polymerization monomer,and the polymerization time was 3 h.The polymer scale inhibitors were characterized by FTIR,~1H NMR,TGA,and GPC.4.The static scale inhibition method was used to evaluate the inhibition efficiency of the synthesized polymer scale inhibitor on calcium sulfate scale.The results show that the polymer scale inhibitor had great stability,heat resistance and salt resistance.When the polymer concentration was 16 mg/L,the inhibition efficiency of the synthesized scale inhibitor was close to 100%.The inhibition efficiency of PIA-SAS-AM is higher than that of polyacrylic acid at the same polymer concentration.The XRD analysis of the products after static evaluation showed that all the products were calcium sulfate dihydrate.The SEM analysis of the product after static evaluation showed that the crystal morphology of calcium sulfate dihydrate without polymer was regular flake and rod-shaped crystal,and with the increase of polymer concentration,defects began to appear on the crystal surface.In addition,the scale inhibition mechanism of PIA-SAS-AM was analyzed.The results showed that the ternary polymer PIA-SAS-AM had the functions of chelation,aggregation and dispersion,and lattice distortion.