High Performance Electro-deposition Reactor For Water Softening

The recirculated cooling water has been broadly used in various applications including power generation,petrochemical industry,steel manufacturing and heating ventilation.In order to prevent scale formation,treatment of this type of water is necessary.Current available recirculated cooling water treatment processes including inhibitor addition,magnetization,chemical precipitation,reverse osmosis,electrodialysis,ion exchange and electro-deposition.Among all the recirculated cooling water treatment processes,electro-deposition has become the hot research topic in recent years due to its environmental friendliness,fast reaction rate,wide applicability and simultaneous production of chlorine for disinfection.However,currently available electro-deposition reactor has many problems,such as laborious detachment,low softening efficiency and high energy consumption.This thesis systematically developed electro-deposition reactor with high softening and detachment performance.Major research works include four sections,that is,cathode selection,scale detachment methods investigation,single stage electro-deposition reactor treatment performance,multistage electro-deposition reactor treatment performance.The softening performance,energy consumption and detachment performance of six different cathodes,which including sheet normal stainless steel,reticular stainless steel,sheet mirror stainless steel,sheet frosted stainless steel,sheet nickel and sheet titanium,were investigated.The experimental results showed that the mirror stainless steel had best performance,and therefore was selected as the proper cathode for use in the electro-deposition reactor.The deposition rate and detachment efficiency of mirror stainless steel could reach 34 g/h/m² and 87.5%,which was 1-1.3 and 1.2-2.2 fold higher than those obtained on the other cathodes;while the energy consumption consumed was only 17.4 k Wh/kg,which was 66%-98%of those of other cathodes.Two originally detachment methods,that is air-scoured washing and current pulsation were proposed.Air-scoured and current pulsation,which detached scale using shear force produced by air bubbles and H₂ produced by enhanced water electrolysis,not only had satisfactory detachment,but also had long electrodes service life,and accordingly could be used as alternative methods for conventional ones.The detachment efficiency could reach 84.8%and 92.1%,respectively,for air-scoured washing and current pulsation,which were 1.2-1.3 fold higher than that of conventional mechanical scraping;while the electrodes service life could reach 15 y,for both methods,which were 13 times longer than that of the conventional polarity reversal.Based on the results of cathode selection study and detachment method study,single stage electro-deposition reactor was improved.The hydraulic characterization experimental results showed that the single stage reactor exhibited completely mixed flow behavior.In single stage reactor,the mean retention time was and 7.45 min;the dispersion value was 0.110;variance was 14.6;MDI was 4.33;VE was 23%;dead volume ratio was 11.30%,without the current supplied.Due to the optimization of the cathode and the detachment method,the inter electrode spacing of the single-stage reactor had been reduced remarkably,and the effective cathode area has been increased significantly.Therefore,the softening performance has been enhanced remarkably,whereas the energy consumption has been reduced significantly.The deposition rate was as high as 40.47 g/h/m²,which was 2-10 fold higher than of that of the traditional reactor;while the energy consumption and residence time are only 14.8 k Wh/kg and0.6 min,respectively,which were 10%-90%and 10%-29%of that of the traditional reactor.Long term operational experiment indicated that the single stage electro-deposition reactor could run steadily,without any performance deterioration.In order to overcome the structure problems occurred in single stage electro-deposition reactor,the multistage electro-deposition reactor was successfully proposed.The hydraulic characterization experimental results showed that the multistage connection not only can reduce back mixing,short circuit,dead volume and axial dispersion significantly,leading to plug flow behavior accordingly,but also can increase the volumetric efficiency remarkably.In multistage reactor,the mean retention time was 8.20 min;the dispersion value was 0.032;variance was 4.5;MDI was 1.90;VE was 53%;dead volume ratio was 2.32%,without the current supplied.This allowed the multistage reactor to have better performance than the single stage reactor in water softening.Under the same conditions,the softening efficiency,current efficiency and energy consumption of the multistage reactor could be 1.8-3.1 times,1.8-2.4 times and21%-46%of those of the single stage reactor.Compared to the uniform electrolysis,the tapered electrolysis not only can strength the softening performance of the front-end multistage reactor,but also can inhibit the mass transfer occurred in the back-end of the reactor,and accordingly leading to better technical superiority.Long term operational experiment indicated that the multistage electro-deposition reactor could run steadily,without any performance deterioration.