Study On The Synthesis And Performance Of Special Inhibitor In Lt-med Seawater Desalination

Scale was one of the main problems that encountered in operation of thermal seawater desalination plants. In general, the scale in heat transfer surfaces was prevented by adding scale inhibitor in operation of large LT-MED seawater desalination plants domestic and foreign. At present, the most scale inhibitors used in thermal seawater desalination were phosphates and polymers. Phosphates were mainly used in low temperature distillation plants under 90℃which was not used widely. Polymers especially polycarboxylic were used widely in thermal seawater desalination because of its good anti-scale efficiency and thermal stability.The terpolymer inhibitor PAMA was synthesized in water solvent by using acrylic acid, maleic anhydride and 2-acrylamido-2-methyl-1-propanesulfonic acid as monomers and potassium persulfate as initiator. Result of orthogonal test indicated that the best synthesis conditions were n(AA):n(AMPS)= 2:1, 15% potassium persulfate accounting for monomers'quality, 3% isopropyl alcohol accounting for monomers'quality, 4h for reaction time, n(MA):n(AMPS)=6:1 and 90℃for temperature. IR spectra showed that there were special carboxylates, sulfonic group and carbon-nitrogen absorption peak of the polymer synthesized. The molecular weight of polymer synthesized in the best conditions was 4511 which measured by viscosimetry. Result of thermal analysis showed that the polymer began to decompose at 372.5℃.The scale inhibiting performance to calcium carbonate of the polymer inhibitor PAMA was studied by national standard GB/T16632-1996. Effects of PAMA amount, Ca2+ concentration,alkalinity and temperature to scale inhibiting percentage were relatively large. The scale inhibiting percentage increased with increased in PAMA amount and decreased with increased in Ca2+ concentration, alkalinity and temperature. Effect of constant temperature time was relatively small. Results of seawater concentration method showed that the scale inhibiting percentage was increased with increased in PAMA amount. Effect of temperature and constant temperature time to scale inhibiting percentage was larger than the effect of concentration times. The scale inhibiting percentage decreased with increased in temperature, constant temperature time and concentration times.In iron oxide dispersion test, the transmissivity decreased with increased in amount which was opposite to the dispersion performance and increased with increased in temperature. The transmissivity firstly decreased and then increased with increased in Fe2+ amount.It was good at the scale inhibiting performance in two methods of polymer scale inhibitor PAMA which was a better inhibitor. The national standard method which had shortness needed to be improved. As an inhibitor of individual component, PAMA had better scale inhibiting performance, but it also had a measurable distance comparing with commercial products. It was necessary to do some study on combination, improved his scale inhibiting performance and made commercial product.The combinated inhibitor was prepared by the synthesized scale inhibitor PAMA and foreign scale inhibitor BelgardEV2035, BelgardEV2050 and its performance and economical evaluation were investigated. The combination between PAMA and BelgardEV2035 or BelgardEV2050 had synergistic effect. The contrast analysis of combination result from molecular weight showed that molecular weight was one of the important effects to scale inhibiting performance and there was synergistic effect of two different molecular weight scale inhibitors. The contrast analysis of combination result from functional groups showed that the import of phosphono improved the scale inhibition performance to calcium carbonate and there were synergistic effect in carboxy, phosphono and sulfonic. But the import of phosphono would bring environmental protection problem. It was an important direction of synthesizing scale inhibitor of decreasing the percentage of phosphono in scale inhibitor molecular or searching for no phosphorus group which substituted the phosphono. The economical evaluation of combinated inhibitor showed that the combination increased the scale inhibiting percentage and decreased the cost of singly using inhibitor of BelgardEV2035 and BelgardEV2050. Scale samples of blank and adding inhibitor were prepared and investigated by scanning electron microscope. Scale inhibiting mechanism was initially exploited. Result of scanning electron microscope indicated that lattice distortion was the scale inhibiting mechanism of PAMA and its combinations to calcium carbonate scale. The mechanism was explained by the geometry match theory, surface energy theory and molecular weight, functional groups. The combination between PAMA and BelgardEV2050 not only made lattice distort but also converted the type of the crystal. This scale inhibiting mechanism needed to further exploit.The terpolymer inhibitor of PAMA synthesized by free-radical polymerization had better scale inhibition performance to calcium carbonate and iron oxide dispersion performance and its combinations with other scale inhibitors indicated good synergistic and economical effect. It was a novel, high performance and non-phosphorus inhibitor. The study provided the experiment basis for the application in LT-MED seawater desalination. On the basis of further consummation of synthesis condition, evaluation method and combination technology, the PAMA would be used in LT-MED seawater desalination.