Charging system of boiler water

1. Ammonia: adjusting pH value

2. Hydrazine: deaeration

The two agents are pumped to deaerator through dosing to adjust pH value of feed water and remove oxygen in water.

3. Phosphate (trisodium phosphate) is added directly to the boiler drum. It also regulates pH to prevent scaling.

The general function of all agents is to minimize the scale and corrosion of the inner pipe system of boiler.

Raw water tank

Raw water tank addition (sodium hypochlorite, NAOC):

The chemical name of bleach is hypochlorite (sodium hypochlorite, NaOCl), which is a strong oxidant and a cheap and easy to obtain sterilization agent. Its bactericidal effect is that sodium hypochlorite is decomposed into hypochlorite, which is unstable, and it is decomposed into new ecological oxygen and chlorine in water solution, which makes bacteria die due to strong oxidation, which is effective for killing bacteria and phages.

Pretreatment

Flocculant added before pretreatment: flocculant mainly includes inorganic flocculant, organic polymer flocculant and microbial flocculant, which are mainly used for various sewage treatment. Specifically, organic polymer flocculant has a wide range of uses in the treatment of refinery wastewater, other industrial wastewater, high suspended matter wastewater and solid-liquid separation. Especially, the acrylamide series of organic polymer flocculants have shown their superiority in water treatment because of their high molecular weight and strong bridging ability. The flocculant has a wide range of flocculation, high flocculation activity, and extensive conditions, most of which are not affected by ionic strength, pH value and temperature, so it can be widely used in wastewater and industrial wastewater treatment.

Adding flocculant is to make water and impurities separated quickly and thoroughly.

Ultrafiltration backwash

Ultrafiltration backwash and dosing: sodium hydroxide, hydrochloric acid, sodium hypochlorite

Sodium hydroxide and sodium hypochlorite do not react, and HCl + NaClO = NaCl + HClO occur between hydrochloric acid and sodium hypochlorite. If HClO is unstable under acidic conditions, it is easy to generate Cl2 toxic. Hydrochloric acid reacts with sodium hydroxide, so it cannot be added together. Then sodium hypochlorite is mainly oxidation and sterilization. Microorganism is organic substance. Alkali washing mainly eliminates organic matter, and alkali softens The effect of loosening sediment is better. The alkali washing should also be beneficial to remove the inorganic salt deposition after pickling. Acid washing or alkali washing first mainly depends on the types of membrane pollutants

reverse osmosis

Reverse osmosis dosing: add scale inhibitor at the inlet of high-pressure pump, and nahs03 reductant before RO water inflow: (sodium bisulfite, a common reductant)

In order to ensure the long-term stable operation of the reverse osmosis device and prevent the membrane surface from being damaged by the pollution of microbial oxidant and suspended impurities, the redox and dosing treatment of oxidant should be carried out before RO water inflow to eliminate the influence of oxidant on the membrane; In order to prevent fouling on the surface of the membrane and reduce the desalination performance of the membrane, a scale inhibitor dosing system is set at the inlet of each high-pressure pump to improve the water flux of the membrane

The alkali addition in the first stage reverse osmosis is less. Alkali solution was injected into the water of reverse osmosis to improve pH. The general use of alkaline agent is only sodium hydroxide (NaOH), easy to purchase, and easy to dissolve in water. Generally, industrial sodium hydroxide without other additives can meet the needs. Commercial sodium hydroxide has 100% of the flake base, and 20% and 50% of the liquid alkali. When adding alkali to increase pH, it is necessary to pay attention to that the increase of pH will increase LSI and decrease the solubility of calcium carbonate, iron and manganese. The most common alkali adding application is the secondary RO system.

In the secondary RO system, the primary RO water supply the secondary ro as raw water. The second stage RO process the first stage RO water production by polishing, and the water quality of the secondary RO water can reach 4 megaohms.

There are four reasons for adding alkali in the secondary RO water inlet:

a. Above ph8.2, carbon dioxide is transformed into carbonate ion, which can be removed by reverse osmosis. Carbon dioxide is a gas, which will enter RO water freely with the liquid, which will cause improper load for the polishing treatment of ion exchange bed downstream.

b. Some TOC components are easier to remove at high pH.

c. The solubility and removal rate of silica are higher at high pH (especially higher than 9).

d. The removal rate of boron is also higher at high pH (especially higher than 9). There is a special case for alkali addition applications, which is usually called the hero process, which regulates the pH of the water to 9 or 10. The first-class reverse osmosis is used to deal with the brackish water, which will have pollution problems (such as hardness, alkalinity, iron, manganese, etc.) at high pH. The weak acid cation resin system and degassing device are usually used to remove these pollutants.

The free chlorine in RO and NF water should be reduced to below 0.05ppm to meet the requirements of polyamide composite membrane. There are two methods for chlorine removal, granular activated carbon adsorption and the use of reducing agents such as sodium sulfite. In small system (50-100gpm), activated carbon filter is generally used, and the investment cost is reasonable. It is recommended to use the high-quality activated carbon which has been treated by acid washing to remove hardness and metal ions. The content of fine powder should be very low, otherwise the membrane will be polluted. The newly installed carbon filter material must be fully washed until the toner is completely removed, usually for several hours or even days. We can't rely on five μ M security filter to protect the RO membrane from carbon powder pollution. The advantage of carbon filter is that organic matter which will cause membrane pollution can be removed, and the treatment of all water inflow is more reliable than adding chemicals. But its disadvantage is that carbon will become a feed for microorganisms. In carbon filter, the bacteria are produced, which results in the biofiltration of reverse osmosis membrane. picture

Sodium bisulfite

Sodium bisulfite (SBS) is a typical reducing agent used in large ro plant. The purity of commercial sodium bisulfite is 97.5-99%, and the dry storage period is 6 months. SBS solution is unstable in air and will react with oxygen. Therefore, it is recommended that the service life of 2% solution is 3-7 days and that of less than 10% solution is 7-14 days. Theoretically, 1.47ppm SBS (or 0.70ppm sodium metabisulfite) can reduce 1.0ppm of chlorine. Considering the safety coefficient of industrial brackish water system, the SBS addition is set to be 1.8-3.0ppm per 1.0ppm chlorine. The injection port of SBS should be upstream of the membrane element, and the distance should be set to ensure that there is 29 seconds reaction time in the membrane element. It is recommended to use a suitable on-line agitator (static mixer).

If cationic coagulant or filter aid is used in pretreatment, special attention should be paid to when using anionic scale inhibitor. It will produce a viscous pollutant, which will increase the operating pressure, and it is very difficult to clean the pollutants.

Sodium hexametaphosphate

Sodium hexametaphosphate (SHMP) is a common scale inhibitor used in reverse osmosis in the early stage, but with the appearance of special scale inhibitor, the amount of it has been greatly reduced. There are some restrictions on the use of SHMP. The solution should be prepared every 2-3 days, because it will be hydrolyzed when exposed to air. After hydrolysis, it will not only reduce the scale inhibition effect, but also cause the possibility of scaling calcium phosphate. The scale of calcium carbonate can be reduced by using SHMP, and LSI can reach + 1.0.

Scale inhibitor

Scale inhibitor hinders the growth of RO water and salt crystallization in concentrated water, so it can allow the insoluble salt to exceed the saturated solubility in the concentrated water. The use of scale inhibitor can replace acid addition, or can be used in combination with acid addition. There are many factors that affect the formation of mineral scale. The decrease in temperature reduces the solubility of scaling minerals (except calcium carbonate, which, in contrast to most substances, decreases with the increase of temperature), and the increase of TDS increases the solubility of insoluble salts (because the high ion strength interferes with the formation of seed).