New ideas for circulating water treatment, published in Soda Ash Industry Issue 3, 2016

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New ideas for circulating cooling water treatment New ideas for circulating cooling water treatment in soda ash production Lianyungang Alkali Industry Co., Ltd. Shi Jinfu 222042 <Abstract> This article briefly analyzes the current situation and existing problems of circulating cooling water treatment in the soda ash production process, and introduces the circulating electrochemical treatment technology and its application. It provides new ideas for the soda ash plant to improve the level of circulating water treatment, extend the life of the heat exchanger equipment of the soda ash unit, and reduce the consumption of soda ash production. <Keywords> Electrochemical treatment and sterilization of circulating cooling water 1 Brief description: A large amount of circulating cooling water is used in the soda ash production process. The existing treatment method is usually chemical treatment technology, generally three types of antiscalants, dispersants, and bactericides. This technology has certain limitations. There are scaling and slime phenomena in carbonization, ammonia coolers, ammonia brine heat exchangers, and compressors in the final cooler. Key equipment must be pickled every year, which not only increases operating costs but also shortens equipment life. The main issues are as follows: (1) The concentration ratio is limited and there is sewage discharge. According to water quality control requirements and to meet production needs, the concentration multiple of circulating water is limited, which is determined by the concentration of scaling ions and chloride ions in the circulating water. (2) Microbial growth and slime. The drug resistance of bacteria and algae determines the frequent need for shock dosing, causing fluctuations in water quality and excessive scaling index. The large amount of sludge deposited in the circulating pool and thin heat exchanger tubes is the deposit of killed bacteria and algae. (3) There is still scaling in the system, especially in high-temperature heat exchangers. This is due to the principle of dosing. On the one hand, scaling ions represented by bicarbonate are only dispersed in the water, causing instability in the heat exchange area, such as scaling on the filler, which requires frequent replacement of the filler. On the other hand, the relationship between the dosage and the concentration ratio is determined under certain setting conditions. In a very complex circulating water system, there are many heat exchangers with different working conditions. Sometimes there are some high-temperature heat exchange equipment. Theoretically, the control standard requirements for the water quality of heat exchangers under different process conditions are also different. In a large system, it is impossible to have the best of both worlds. It is impossible to take care of all heat exchangers, and only some heat exchangers can run in better working conditions. Even in the simplest power plant cooling system, the working conditions of the heat exchanger will also change due to load and seasonal changes. Even in the most critical part of the carbonization device for soda ash production, the heat exchange temperature is not high, and obvious scaling was found during maintenance. (4) In order to ensure production heat exchange needs, the heat exchanger needs to be pickled every year, which will adversely affect the life of the equipment. (5) Fouling affects heat transfer efficiency and reduces energy utilization efficiency. In the power plant condenser, even if rubber balls are used for cleaning, scaling will inevitably occur. Regular pickling or mechanical cleaning is still required. Even half-side cleaning will have a huge impact on power generation. 2 Working principle and advantages of electrochemical equipment Electrochemical equipment uses the electrochemical properties of water and minerals in the water to adjust the balance of minerals in the water through electrochemistry to achieve the purpose of removing scaling substances, anti-corrosion and preventing microorganisms. It is a clean circulating cooling water treatment technology. 2.1 The operating current maintained in the cathode electrochemical reaction chamber of electrochemical equipment is approximately 50-90 DC amps. A high concentration of hydroxyl radicals is formed near the cathode (the wall of the reaction chamber). This elevated pH environment (pH as high as 13) allows minerals that are prone to scaling to pre-scale and precipitate from the water. In fact, the chemical environment created by the local high hydroxide concentration near the cathode is similar to the cold lime softening environment created by lime treatment. 2H2O(l) + 2e- H2(gas)+ 2OH-(aq) CO2(aq)+ OH-(aq) HCO3-(aq) HCO3-(aq)+OH-(aq) CO32-(aq)+H2O(l) Ca2 +(aq) Calcium ions may form calcium hydroxide: Ca(OH)2(scale) Calcium carbonate: CaCO3(scale) 2.2 The anode electrochemical reaction current in the electrochemical equipment also converts part of the chlorine ions into chlorine gas, forming hypochlorous acid with a sustained bactericidal effect in the cooling water. Ozone, oxygen radicals, hydroxyl radicals and hydrogen peroxide are simultaneously produced. This family of products provides a biocidal effect that, combined with amperometric current and localized high and low (anodic) pH zones, maintains a de facto sterile environment within electrochemical equipment. Generate oxygen 4HO- O2(g) + 2H2O + 4e- Free chlorine Cl- – e- Cl0 Chlorine 2Cl-(aq) Cl2(g) + 2e- Ozone O2 + 2HO- – 2e- O3(g) + H2O Free radical OH0 OH- – e- OH0 Hydrogen peroxide 2H2O – 2 e- H2O2 + 2H+ Oxygen radical 2H2O – 2 e- O0 + 2 H+ 2.3 Advantages of electrochemical equipment 1. Safe and reliable. The equipment operates fully automatically to avoid contact * * chemicals. 2. One machine has multiple functions. It can solve the problems of scaling, microorganisms, biological slime and corrosion at the same time. It does not need to add chemicals, does not discharge any chemicals into the environment, and will not cause pollution to the environment. Possibility of system fouling due to removal of fouling factors * * The efficiency is reduced, the equipment operation cycle is extended, and the energy utilization rate is improved. Multiple sterilization conditions effectively control the growth of bacteria and algae. 3. Low operating cost. The maximum total power of a single equipment system is 3kW. 4. By properly installing electrochemical equipment and electrolytic water treatment equipment, completely destroy the slime at the bottom of the tower where microorganisms breed. 5. Dynamically adjust the LSI index of the circulating water system according to changes in conductivity, automatically adapt to changes in water quality, and ensure safe and stable operation of the circulating system. 6. The electrochemical system is controlled by a microcomputer and operates fully automatically. ； Maintenance is convenient and simple. 7. The electrochemical system equipment occupies a small area, is light in weight, and is easy and flexible to install. 8. Significantly reduce pollutant emissions. Except for the small amount of water that will be brought out by automatic cleaning, there is basically no need to discharge sewage, and this part of the water can be recycled into the system after being concentrated. 2.4 Schematic diagram of electrochemical equipment installation 3. Application of electrochemical treatment technology in circulating cooling water 3.1 Application examples The following is some data comparison of circulating cooling water chemical treatment and electrochemical treatment in a factory. Date 2015.6 PH Turbidity mg/L Total hardness (CaCO3) mg/L Calcium hardness (CaCO3) mg/L Total alkalinity (CaCO3) mg/L Iron ion (Fe2+) mg/L Chloride ion (Cl-meter) mg/L Conductivity us/cm Total phosphorus (PO43-mg/L) Normal use chemical data in June 201515 8.5 7.2 490 330 200 0.2 265 1620 1.5 16 8.6 6.6 470 244 225 0.1 260 1440 1.7 17 8.5 8.7 468 352 200 0.3 225 1500 2.1 18 8.6 14.4 500 320 210 0.4 225 1540 2.3 19 8.4 9.9 444 316 200 0.4 220 1500 1.3 20 8.6 10.5 474 318 190 0.5 230 1430 1.5 21 8.5 10.5 490 328 210 0.4 250 1520 1.4 22 8.5 6.2 450 320 165 0.3 290 1430 5.5 23 8.5 7.8 470 310 190 0.18 250 1450 1.5 24 8.6 13.4 484 344 200 0.54 235 1600 2 2016.1 month unmedicated data 8.8 on 20 12.2 664 396 190 0.72 475 2190 0.2 20 lower 8.7 11.4 664 404 162 0.5 500 2260 0 21 upper 8.8 12.3 652 388 160 0.58 500 2390 0.5 21 lower 8.8 12.6 656 396 165 0.52 520 2560 0.5 22 upper 8.8 13.4 672 398 145 0.6 475 1870 0.6 22 lower 8.9 14.1 678 400 145 0.84 515 2410 0.4 23 up 8.8 13.8 710 400 155 0.9 525 2170 0.5 23 down 8.8 11.9 694 424 145 0.62 560 2250 0.3 24 up 8.9 15.5 716 416 185 0.48 510 2160 0.4 8.8 12.1 702 406 165 0.42 500 2220 0.4 8.9 16.7 756 400 190 0.6 480 2190 0.6 8.9 20.1 648 396 175 0.8 460 2200 0.6 26 up 8.34 17 670 410 220 0.74 440 2220 0.4 26 down 8.8 15.3 680 394 150 1.2 250 2280 0.4 3.2 Daily operation and control of circulating cooling water system 3.2.1 Circulating water quality operation control indicators Circulating water analysis project analysis control indicators Normal operation analysis frequency requirements Remarks Turbidity ≤ 20NTU 1 time / day Forced pH value 7.8 ~ 8.8 1 time / day Forced conductivity (μs/cm) ≤9000 1 time / day Reference Cl- (mg/L) ≤1100 1 time/day forced calcium hardness (calculated as CaCO3) mg/L ≤700 1 time/day forced total alkalinity (calculated as CaCO3) mg/L ≤350 1 time/day forced total iron mg/L ≤1.0 1 time/week forced copper ion ug/L ≤100 1 time/week forced concentration multiple >5 1 time/day reference LSI 0.5-1.8 1 time/day mandatory 3.2.2 Water treatment operation effect standard After the circulating cooling water is electrochemically and chemically treated, its effect should reach * * "Design Code for Industrial Circulating Cooling Water Treatment" (GB50050-2007) requirements. 3.2.3 Dirt deposition controls water side dirt thermal resistance value