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Reverse Osmosis Plant Process Equipment Industrial Water Treatment System
Project Introduction
Principle of reverse osmosis system
At a certain temperature, a semi-permeable membrane is used to separate the fresh water from the saline. The fresh water moves to the saline through the semi-permeable membrane. As the liquid level on the saline side of the right ventricle rises, a certain pressure is generated to prevent the fresh water from the left ventricle from moving to the saline side, and finally equilibrium is reached. The equilibrium pressure at this time is called the osmotic pressure of the solution, and this phenomenon is called osmosis. If an external pressure exceeding the osmotic pressure is applied to the saline side of the right ventricle, the water in the salt solution of the right ventricle will move to the fresh water of the left ventricle through the semi-permeable membrane, so that the fresh water can be separated from the salt water. This phenomenon is the opposite of the permeability phenomenon, called the reverse permeability phenomenon.
Thus, the basis of reverse osmosis desalination system is
(1) The selective permeability of semi-permeable membrane, that is, selectively let water through but not allow salt through;
(2) The external pressure of the saline chamber is greater than the osmotic pressure of the saline chamber and the fresh water chamber, which provides the driving force for water to move from the saline chamber to the fresh water chamber. Typical osmotic pressures for some solutions are shown in the table below.
The above semi-permeable membrane used to separate fresh water from salt water is called reverse osmosis membrane. Reverse osmosis membrane is mostly made of polymer materials. At present, the reverse osmosis membrane used in thermal power plants is mostly made of aromatic polyamide composite materials.
RO(Reverse Osmosis) reverse osmosis technology is a membrane separation and filtration technology powered by pressure difference. Its pore size is as small as nanometer (1 nanometer =10-9 meters). Under a certain pressure, H20 molecules can pass through RO membrane, Inorganic salts, heavy metal ions, organic matter, colloids, bacteria, viruses and other impurities in the source water cannot pass through the RO membrane, so that the pure water that can pass through and the concentrated water that cannot pass through can be strictly distinguished.
In industrial applications, reverse osmosis plants use specialized equipment to facilitate the reverse osmosis process. Industrial reverse osmosis systems are designed to treat large volumes of water and are used in various industries including agriculture, pharmaceuticals, and manufacturing. The equipment used in these systems is specifically designed to ensure that the reverse osmosis process is efficient and effective in producing fresh water from salt water sources.
The reverse osmosis process is an important technology for seawater desalination, which can provide fresh water to areas where water is scarce or where traditional water sources are polluted. As reverse osmosis equipment and technology advance, the process remains a key solution to water shortages and quality issues around the world.
The Main characteristics of reverse osmosis membrane:
Directionality and separation characteristics of membrane separation
Practical reverse osmosis membrane is asymmetric membrane, there are surface layer and support layer, it has obvious direction and selectivity. The so-called directivity is to put the membrane surface in high pressure brine for desalting, the pressure increases the membrane water permeability, desalting rate also increases; When the supporting layer of the membrane is placed in high pressure brine, the desalination rate is almost 0 with the increase of pressure, but the water permeability is greatly increased. Due to this directionality, it cannot be used in reverse when applied.
The separation characteristics of reverse osmosis for ions and organic matter in water are not the same, which can be summarized as follows
(1) Organic matter is easier to separate than inorganic matter
(2) Electrolytes are easier to separate than non-electrolytes. Electrolytes with high charges are easier to separate, and their removal rates are generally in the following order. Fe3+> Ca2+> Na+ PO43-> S042-> C | - for the electrolyte, the bigger the molecule, the easier to remove.
(3) The removal rate of inorganic ions is related to the hydrate and the radius of hydrated ions in the ion hydration state. The larger the radius of the hydrated ion is, the easier it is to be removed. The order of removal rate is as follows:
Mg2+, Ca2+> Li+ > Na+ > K+; F-> C|-> Br-> NO3-
(4) Separation rules of polar organic matter:
Aldehyde > Alcohol > Amine > Acid, tertiary amine > Secondary amine > Primary amine, citric acid > Tartaric acid > Malic acid > Lactic acid > Acetic acid
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(5) Pair isomers: tert- > Different (iso-)> Zhong (sec-)> Original (pri-)
(6) The sodium salt separation performance of organic matter is good, while the phenol and phenol row organisms show negative separation. When aqueous solutions of polar or non-polar, dissociated or non-dissociated organic solutes are separated by membrane, the interaction forces between solute, solvent and membrane determine the selective permeability of membrane. These effects include electrostatic force, hydrogen bond binding force, hydrophobicity and electron transfer.
(7) Generally, solutes have little influence on the physical properties or transfer properties of the membrane. Only phenol or some low molecular weight organic compounds will make cellulose acetate expand in aqueous solution. The existence of these components will generally make the water flux of the membrane decrease, sometimes a lot.
(8) The removal effect of nitrate, perchlorate, cyanide and thiocyanate is not as good as chloride, and the removal effect of ammonium salt is not as good as sodium salt.
(9) Most of the components with relative molecular mass greater than 150, whether electrolyte or non-electrolyte, can be well removed
In addition, the reverse osmosis membrane for aromatic hydrocarbons, cycloalkanes, alkanes and sodium chloride separation order is different.
(2) High Pressure pump
In the operation of reverse osmosis membrane, water needs to be sent to the specified pressure by high pressure pump to complete the desalting process. At present, the high pressure pump used in thermal power plant has centrifugal, plunger and screw and other forms, among which, the multi-stage centrifugal pump is the most widely used. This can reach more than 90% and save energy consumption. This kind of pump is characterized by high efficiency.
(3) Reverse osmosis ontology
The reverse osmosis body is a combined water treatment unit that combines and connects the reverse osmosis membrane components with pipes in a certain arrangement. A single reverse osmosis membrane is called a membrane element. A sensing number of reverse osmosis membrane components are connected in series according to certain technical requirements and assembled with a single reverse osmosis membrane shell to form a membrane component.
1. Membrane element
Reverse osmosis membrane element A basic unit made of reverse osmosis membrane and support material with industrial use function. At present, coil membrane elements are mainly used in thermal power plants.
At present, various membrane manufacturers produce a variety of membrane components for different industry users. Membrane elements applied in thermal power plants can be roughly divided into: high pressure seawater desalination reverse osmosis membrane elements; Low pressure and ultra-low pressure brackish water desalting reverse membrane elements; Anti-fouling membrane element.
The basic requirements for membrane elements are:
A. Film packing density as high as possible.
B. Not easy to concentration polarization
C. Strong anti-pollution ability
D. It is convenient to clean and replace the membrane
E. The price is cheap
2.Membrane shell
The pressure vessel used to load the reverse osmosis membrane element in the reverse osmosis body device is called membrane shell, also known as "pressure vessel" manufacturing unit is Haide energy, each pressure vessel is about 7 meters long.
The shell of the film shell is generally made of epoxy glass fiber reinforced plastic cloth, and the outer brush is epoxy paint. There are also some manufacturers of products for stainless steel film shell. Because of the strong corrosion resistance of FRP, most thermal power plants choose FRP film shell. The material of the pressure vessel is FRP.
The affecting factors of reverse osmosis water treatment system performance:
For specific system conditions, water flux and desalting rate are the characteristics of reverse osmosis membrane, and there are many factors affecting the water flux and desalting rate of reverse osmosis body, mainly including pressure, temperature, recovery rate, influent salinity and pH value
(1) Pressure effect
The inlet pressure of reverse osmosis membrane directly affects the membrane flux and desalting rate of reverse osmosis membrane. The increase of membrane flux has a linear relationship with the inlet pressure of reverse osmosis. The desalination rate has a linear relationship with the influent pressure, but when the pressure reaches a certain value, the change curve of desalination rate tends to be flat and the desalination rate no longer increases.
(2) Temperature effect
The desalting rate decreases with the increase of the inlet temperature of reverse osmosis. However, the water yield flux increases almost linearly. The main reason is that when the temperature increases, the viscosity of water molecules decreases and the diffusion ability is strong, so the water flux increases. With the increase of temperature, the rate of salt passing through the reverse osmosis membrane will be accelerated, so the desalination rate will be reduced. Raw water temperature is an important reference index for reverse osmosis system design. For example, when a power plant is undergoing technical transformation of reverse osmosis engineering, the water temperature of raw water in the design is calculated according to 25℃, and the calculated inlet pressure is 1.6MPa. However, the water temperature in the actual operation of the system is only 8℃, and the inlet pressure must be increased to 2.0MPa to ensure the design flow of fresh water. As a result, the energy consumption of the system operation increases, the life of the internal seal ring of the membrane component of the reverse osmosis device is shortened, and the maintenance amount of the equipment is increased.
(3) Salt content effect
The concentration of salt in water is an important index affecting the membrane osmotic pressure, and the membrane osmotic pressure increases with the increase of salt content. Under the condition that the inlet pressure of reverse osmosis remains unchanged, the salt content of the inlet water increases. Because the increase of osmotic pressure offsets part of the inlet force, the flux decreases and the desalination rate also decreases.
(4) The influence of recovery rate
The increase in the recovery rate of the reverse osmosis system will lead to a higher salt content of the inlet water of the membrane element along the flow direction, resulting in an increase in osmotic pressure. This will offset the driving effect of the inlet water pressure of reverse osmosis, thus reducing the water yield flux. The increase of the salt content in the inlet water of the membrane element leads to the increase of the salt content in the fresh water, thus reducing the desalination rate. In the system design, the maximum recovery rate of reverse osmosis system does not depend on the limitation of osmotic pressure, but often depends on the composition and content of salt in the raw water, because with the improvement of recovery rate, micro-soluble salts such as calcium carbonate, calcium sulfate and silicon will scale in the concentration process.
(5) The influence of pH value
The pH range applicable to different types of membrane elements varies greatly. For example, the water flux and desalination rate of acetate membrane tend to be stable in the range of pH value 4-8, and are greatly affected in the range of pH value below 4 or higher than 8. At present, the vast majority of membrane materials used in industrial water treatment are composite materials, which adapt to a wide pH value range (the pH value can be controlled in the range of 3~10 in continuous operation, and the membrane flux and desalination rate in this range are relatively stable.
Reverse osmosis membrane pre-treatment method:
Reverse osmosis membrane filtration is different from filter bed filter filtration, filter bed is full filtration, that is, raw water all through the filter layer. Reverse osmosis membrane filtration is a cross-flow filtration method, that is, part of the water in the raw water passes through the membrane in the vertical direction with the membrane. At this time, salts and various pollutants are intercepted by the membrane, and carried out by the remaining part of the raw water flowing parallel to the membrane surface, but the pollutants cannot be completely taken out. As time goes by, the residual pollutants will make the membrane element pollution more serious. And the higher the raw water pollutants and recovery rate, the faster the membrane pollution.
1. Scale control
When the insoluble salts in the raw water are continuously concentrated in the membrane element and exceed their solubility limit, they will precipitate on the surface of the reverse osmosis membrane, which is called "scaling". When the water source is determined, as the recovery rate of the reverse osmosis system increases, the risk of scaling increases. At present, it is customary to increase recycling rates due to water shortages or environmental impacts of wastewater discharge. In this case, thoughtful scaling control measures are particularly important. In reverse osmosis system, the common refractory salts are CaCO3, CaSO4 and Si02, and other compounds that can produce scale are CaF2, BaS04, SrS04 and Ca3(PO4)2. The common method of scale inhibition is to add scale inhibitor. The scale inhibitors used in my workshop are Nalco PC191 and Europe and America NP200.
2.Control OF colloidal and solid particle contamination
Colloid and particle fouling can seriously affect the performance of reverse osmosis membrane elements, such as a significant reduction in fresh water output, sometimes also reduce desalination rate, the initial symptom of colloid and particle fouling is the increase in the pressure difference between the inlet and outlet of reverse osmosis membrane components.
The most common way to judge the water colloid and particles in reverse osmosis membrane elements is to measure the SDI value of water, sometimes called F value (pollution index), which is one of the important indicators to monitor the operation of reverse osmosis pretreatment system.
SDI(silt density index) is the change of water filtration speed per unit time to indicate the pollution of water quality. The amount of colloid and particulate matter in water will affect the SDI size. SDI value can be determined by SDI instrument.
3. Control of membrane microbial contamination
Microorganisms in raw water mainly include bacteria, algae, fungi, viruses and other higher organisms. In the process of reverse osmosis, microorganisms and dissolved nutrients in water will be continuously concentrated and enriched in the membrane element, which becomes the ideal environment and process for the formation of biofilm. The biological contamination of reverse osmosis membrane components will seriously affect the performance of reverse osmosis system. The pressure difference between the inlet and outlet of reverse osmosis components increases rapidly, resulting in the decrease of water yield of membrane components. Sometimes, biological contamination will occur on the water production side, resulting in the contamination of product water. For example, in the maintenance of reverse osmosis devices in some thermal power plants, green moss is found on the membrane elements and fresh water pipes, which is a typical microbial pollution.
Once the membrane element is contaminated by microorganisms and produces biofilm, the cleaning of membrane element is very difficult. In addition, biofilms that are not completely removed will cause rapid growth of microorganisms again. Therefore, the control of microorganisms is also one of the most important tasks of pretreatment, especially for reverse osmosis pretreatment systems using seawater, surface water and wastewater as water sources.
The main methods to prevent membrane microorganisms are: chlorine, microfiltration or ultrafiltration treatment, ozone oxidation, ultraviolet sterilization, adding sodium bisulfite. The commonly used methods in thermal power plant water treatment system are chlorination sterilization and ultrafiltration water treatment technology before reverse osmosis.
As a sterilizing agent, chlorine is able to rapidly inactivate many pathogenic microorganisms. The efficiency of chlorine depends on the concentration of chlorine, the pH of the water, and the contact time. In engineering applications, the residual chlorine in water is generally controlled at more than 0.5~1.0mg, and the reaction time is controlled at 20~30min. The dosage of chlorine needs to be determined by debugging, because organic matter in water will also consume chlorine. Chlorine is used for sterilization, and the best practical pH value is 4~6.
The use of chlorination in seawater systems is different from that in brackish water. Usually there is about 65mg of bromine in seawater. When seawater is chemically treated with hydrogen, it will first react with hypochlorous acid to form hypobromous acid, so that its bactericidal effect is hypowet acid rather than hypochlorous acid, and hypobromous acid will not decompose at a higher pH value. Therefore, the effect of chlorination is better than in brackish water.
Because the membrane element of composite material has certain requirements on the residual chlorine in the water, it is necessary to carry out dechlorination reduction treatment after chlorine sterilization.
4. Control of organic pollution
The adsorption of organic matter on the membrane surface will cause the decrease of membrane flux, and in severe cases, it will cause irreversible loss of membrane flux and affect the practical life of the membrane.
For surface water, most of the water is natural products, through coagulation clarification, DC coagulation filtration and activated carbon filtration combined treatment process, can greatly reduce the organic matter in the water, to meet the requirements of reverse osmosis water.
5. Concentration polarization control
In the process of reverse osmosis, there is sometimes a high concentration gradient between the concentrated water on the membrane surface and the influent water, which is called concentration polarization. When this phenomenon occurs, a layer of relatively high concentration and relatively stable so-called "critical layer" will be formed on the surface of the membrane, which hinders the effective implementation of the reverse osmosis process. This is because the concentration polarization will increase the solution permeable pressure on the membrane surface, and the driving force of the reverse osmosis process will be reduced, resulting in the reduction of water yield and desalination rate. When the concentration polarization is serious, some slightly dissolved salts will precipitate and scale on the membrane surface. In order to avoid concentration polarization, the effective method is to make the flow of concentrated water always maintain a turbulent state, that is, by increasing the inlet flow rate to increase the flow rate of concentrated water, so that the concentration of micro-dissolved salt on the membrane surface is reduced to the lowest value; In addition, after the reverse osmosis water treatment device is shut down, the concentrated water on the side of the replaced concentrated water should be washed in time.
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