Regarding reverse osmosis, I hope everyone will participate

This post was last edited by cdpulin on 2012-3-1 at 13:38. Reverse osmosis, also known as inverse osmosis, is a membrane separation process that uses a pressure difference as the driving force to separate the solvent from a solution. Pressure is applied to the liquid on one side of the membrane; when this pressure exceeds its osmotic pressure, the solvent will undergo reverse osmosis, moving in the opposite direction to its natural tendency to permeate. Thus, the permeated solvent, that is, the permeate, is obtained on the low-pressure side of the membrane ； The concentrated solution is obtained on the high-pressure side, that is, the concentrate. When seawater is treated using reverse osmosis, fresh water is obtained on the low-pressure side of the membrane, while brine is obtained on the high-pressure side. 　　During reverse osmosis, the permeation rate of the solvent, that is, the flow energy N, is given by: N = Kh(Δp – Δπ). Here, Kh is the hydraulic permeability coefficient, which increases slightly as the temperature rises ； Δp is the static pressure difference across the membrane ； Δπ is the osmotic pressure difference between the solutions on both sides of the membrane. The osmotic pressure π of a dilute solution is given by: π = iCRT, where i is the number of ions produced by the ionization of solute molecules ； C is the molar concentration of the solute ； R is the molar gas constant ； T is the absolute temperature. 　　Reverse osmosis typically uses asymmetric membranes and composite membranes. The equipment used for reverse osmosis is mainly hollow fiber or spiral-wound membrane separation devices. 　　Reverse osmosis membranes can retain various inorganic ions, colloidal substances, and macromolecular solutes in water, thereby producing purified water. It can also be used for the pre-concentration of solutions of macromolecular organic compounds. Due to its simple process and low energy consumption, reverse osmosis has seen rapid development over the past 20 years. It is now widely used in the desalination of seawater and brackish water (see brine), the softening of water for boilers, and wastewater treatment. Combined with ion exchange, it is used to produce high-purity water. Its scope of application is expanding; it is now being used in the concentration of dairy products and fruit juices, as well as in the separation and concentration of biochemicals and biological agents. 　　 II. Basic Principles A dilute solution (such as fresh water) and a concentrated solution (such as seawater or saltwater), both of the same volume, are placed on opposite sides of a container, with a semipermeable membrane separating them. The solvent in the dilute solution will naturally pass through the semipermeable membrane and flow toward the concentrated solution side. As a result, the liquid level on the concentrated solution side will be higher than that on the dilute solution side, creating a pressure difference. This pressure difference is known as osmotic pressure. The magnitude of osmotic pressure depends on the type of concentrated solution, its concentration, and temperature; it is unrelated to the properties of the semipermeable membrane. If a pressure greater than the osmotic pressure is applied to the side of the concentrated solution, the solvent in the concentrated solution will flow toward the dilute solution; this flow of solvent occurs in the opposite direction to the original osmotic flow, and this process is known as reverse osmosis. 1. Dissolution-diffusion model Lonsdale and others proposed a dissolution-diffusion model to explain the reverse osmosis phenomenon. He considered the active surface layer of reverse osmosis to be a dense, pore-free membrane, and assumed that both solutes and solvents could dissolve in the homogeneous, non-porous surface layer of the membrane, diffusing through it driven by the chemical potential resulting from concentration or pressure. Differences in solubility and the diffusivity of solutes and solvents in the membrane phase affect the amount of energy required for them to pass through the membrane. The specific process is divided into the following steps: First, the solute and solvent are adsorbed and dissolved on the outer surface of the feed side of the membrane ； In the second step, there is no interaction between the solute and the solvent; driven by their respective chemical potential differences, they pass through the active layer of the reverse osmosis membrane via molecular diffusion ； In the third step, the solute and solvent desorb on the surface of the permeate side of the membrane. 　　In the process of solute and solvent permeation through the membrane, it is generally assumed that the first and third steps occur rapidly; at this point, the permeation rate depends on the second step, namely the diffusion of solutes and solvents through the membrane driven by a chemical potential difference. Due to the selectivity of the membrane, gas mixtures or liquid mixtures can be separated. The permeability of a substance depends not only on the diffusion coefficient but also on its solubility in the membrane. 2. Preferential adsorption—capillary flow theory: When different types of substances are dissolved in a liquid, its surface tension changes in distinct ways. For example, the presence of organic substances such as alcohols, acids, aldehydes, and fats in water can reduce its surface tension. However, the addition of certain inorganic salts can cause the surface tension to increase slightly, as the distribution of these solutes is uneven – the concentration of solutes in the surface layer of the solution is different from that in the interior of the solution. This is what is known as surface adsorption in solutions. When an aqueous solution comes into contact with a polymer porous membrane, if the chemical properties of the membrane cause it to exhibit negative adsorption for solutes and positive adsorption preferentially for water, a layer of pure water of a certain thickness, adsorbed by the membrane, will form at the interface between the membrane and the solution. Under external pressure, it passes through the capillaries on the membrane surface, thereby enabling the acquisition of pure water. 3. Hydrogen bond theory: In cellulose acetate, due to the effects of hydrogen bonds and van der Waals forces, the membrane consists of crystalline regions and amorphous regions. Regions where macromolecules are firmly bonded together and arranged in parallel are the crystalline regions, whereas regions where the macromolecules are completely disordered are the amorphous regions; water and solutes cannot enter the crystalline regions. Near the cellulose acetate molecules, water forms hydrogen bonds with the oxygen atoms on the carbonyl groups of cellulose acetate, thereby creating what is known as bound water. When cellulose acetate absorbs the first layer of water molecules, it causes a significant decrease in the entropy of those water molecules, resulting in a structure similar to ice. In the large pore spaces of the amorphous region, the proportion of bound water is very low; water with a normal structure exists at the center of the pores. Ions or molecules that cannot form hydrogen bonds with the cellulose acetate membrane enter the bound water and migrate in an orderly manner, passing through the membrane by continuously changing the locations where hydrogen bonds are formed with cellulose acetate. 　　Under pressure, hydrogen bonds are formed between the water molecules in the solution and the oxygen atoms of the carbonyl groups, which are the activation sites of cellulose acetate. The hydrogen bonds previously formed by the water molecules are broken, causing the water molecules to dissociate and move to the next activation site where new hydrogen bonds are formed. Through this series of bond formations and breakages, the water molecules leave the dense active layer on the surface of the membrane and enter its porous layer. Since the porous layer contains a large amount of capillary water, water molecules can flow out of the membrane easily. III. Reverse Osmosis Mechanism Models A unified “dry-closed, wet-open” reverse osmosis mechanism model; there are several classic models. 1. Preferential adsorption capillary pore model: A weakness is that no pores were detected under dry-state membrane microscopy. Wet membrane specimens are not suitable as samples for electron microscopy. 　　2. Dissolution diffusion model: Pores are not considered. 　　3. Dry-closed, wet-open model: Proposed by Deng Yu and others in the 1980s and 1990s, it is the most appropriate modern reverse osmosis mechanism model that can explain Models 1 and 2. The \"dry-closed, wet-open\" reverse osmosis model unifies the two most classic mechanisms for reverse osmosis: the pore model and the dissolution-diffusion model. That is, when the membrane is dry, its pores contract and become compact, the pores close, and under an electron microscope, no dry membrane prepared in a dry state for examination can be seen ； 　　When the membrane is wet, the membrane material swells; the pores of the membrane are swollen by the solvent, causing the pores to open. The combination is the “dry-close, wet-open” desalination model. IV. Grand Vision: Seeking New Technologies for Seawater Desalination 　　 Non-pressurized osmotic adsorption method (1990s) 　　The non-pressurized adsorption osmosis seawater desalination method, also known as “forward osmosis,” involves allowing water to pass through a porous membrane and move forward into a highly absorbent adsorbent or a solution/solid with a salt concentration even higher than that of seawater. No external pressure is required; however, the special salts in the solution can evaporate easily, and little heat is needed (what is the ratio of heating energy to the energy required for reverse osmosis under pressure?) ). It is divided into solid salt and liquid salt categories. Solid-state salt hydrolysis adsorption requires less energy. 　　Seawater desalination technology: Non-pressure adsorption osmosis seawater desalination method: Invented by Deng Yu in the 1990s, and included in the American Chemical Abstracts. 　 　The other two methods also feature innovations and improvements in the film structure. Carbon nanotube film: A type of membrane with pores made from carbon nanotubes; another type is the protein membrane of living cells. 　　The pores in the membrane are formed by proteins that facilitate the passage of water molecules through the cell membranes of living cells. V. Application Scope 　 Preparation of space water, pure water, distilled water, etc ； Alcohol production and water for dilution ； Preliminary preparation of water for use in industries such as pharmaceuticals and electronics ； Concentration, separation, purification, and water preparation in chemical processing processes ； Desalinated soft water for boiler make-up water ； Seawater and brackish water desalination ； Water and wastewater treatment for industries such as papermaking, electroplating, and printing and dyeing. 　　Membrane separation technology, represented by polymer separation membranes, as a new and efficient unit operation for fluid separation, has achieved remarkable rapid development over the past 30 years and is now widely used in various fields of the national economy. VI. Current Status of Reverse Osmosis – Membrane Applications Among various membrane separation technologies, reverse osmosis is the one that has seen the most successful application, fastest development, and widest adoption in China in recent years. It is estimated that since 1995, the usage of reverse osmosis membranes has been increasing by an average of 20% per year ； According to conservative estimates, in 1999 the market supply of industrial reverse osmosis membrane elements was 6,000 units of 8-inch membranes and 26,000 units of 4-inch membranes. The markets in 2000 and 2001 were stronger, with the consumption of membranes increasing significantly from one year to the next. It is estimated that the use of reverse osmosis technology has generated an annual output value of over 1 billion RMB for the water treatment industry. 　　The largest application area for reverse osmosis membrane technology in China remains in the supply water for large-scale boilers and various industrial pure water systems; the drinking water market comes next in terms of scale. Applications in industries such as electronics, semiconductors, pharmaceuticals, healthcare, food, beverages, alcohol, chemicals, and environmental protection also account for a significant portion of its use. 　　Recent Advances in Reverse Osmosis Membranes Ultra-low pressure membranes have seen an increasing use since 1999, thanks to advantages such as reduced electricity consumption and lower pressure requirements for related mechanical components, which in turn leads to lower material costs. This is particularly evident in small systems that use 4-inch membranes; there is also a rising trend in the use of ultra-low pressure membranes in larger systems. The maximum water production capacity of systems currently using such membranes is 650 tons per hour. 　　Low-pollution membranes: Membrane fouling is the biggest hazard in reverse osmosis applications. Several low-pollution membranes with strong anti-pollution properties, long service life, low cleaning frequency, and easy cleaning have already been developed. 　　Positively charged reverse osmosis membranes: The materials used in low-pressure and ultra-low-pressure composite membranes, which are widely used today, are aromatic polyamides, and their surfaces carry a negative charge. Membrane manufacturers have now developed low-pressure composite membranes with positively charged surfaces; such membranes are currently used primarily in systems for producing high-purity water with high resistivity. The positive charge membrane ES10C produced by Nippon Denko Corporation has enabled the production of high-purity water with a resistivity of 10–15 megohms in tertiary reverse osmosis systems used in the semiconductor industry ； The resistivity of the produced water from the three-stage reverse osmosis systems at the three production plants of Korea Modern Electronics Company, which have a total production capacity of 800 tons per hour, is 8–9 megohms ； The 170 tons per hour tertiary reverse osmosis system at a semiconductor factory in Shanghai also meets the aforementioned standards. Furthermore, in two-stage reverse osmosis systems with a capacity of 5–20 tons per hour at several domestic pharmaceutical factories, a resistivity of 1.7–3 megohms for the reverse osmosis product water has also been achieved. 　　High-temperature resistant, food-grade, and sanitary-grade reverse osmosis membranes: The operating temperature for reverse osmosis membranes used in ordinary water treatment is 0–45 degrees Celsius. However, in situations where high temperatures of up to 90 degrees Celsius are required for sterilization, high-temperature resistant and chemical-resistant reverse osmosis membranes can be used. In addition, food-grade or sanitary-grade reverse osmosis membranes with various special membrane element structures have also begun to be used in China. VII. Current Status and Recent Advances in the Application of Reverse Osmosis Seawater Desalination Membranes Abroad, there are reverse osmosis seawater desalination systems with a daily production capacity of 100,000 tons; the current large-scale roll-type membrane seawater desalination systems have a single-unit capacity of 6,000 tons per day. The reverse osmosis seawater desalination units that have been built or are under construction in China have a daily production capacity of 350–1000 tons. Abroad, the water utilization rate for single-stage reverse osmosis seawater desalination can reach up to 45%, while in China it is currently around 35%. Additionally, the reverse osmosis desalination membranes used on fishing boats in China are mostly small membrane elements with a diameter of 2.5 inches. At present, there are no more than 10 companies in China that produce desalination units on a large scale. The “Yahai Shui” desalination plant built in Hebei, with a daily production capacity of 18,000 tons, is the largest reverse osmosis system in the country to use desalination membranes. In the future, the application of seawater desalination membranes in China will enter a new era, and in the near future, the country will also build seawater desalination facilities with a daily production capacity of tens of thousands of tons. In addition, commercial production of reverse osmosis membrane elements for seawater desalination has already begun in China. Section 8: Current Challenges in Reverse Osmosis The main difficulty at present is the development of reverse osmosis membranes that are inexpensive, stable, and remain intact under long-term pressure. Starting from the early 2000s, China acquired the technology for producing reverse osmosis membranes independently. With strong support from **, this project was included in the special plan for the industrialization of high-tech technologies established by the **Planning Commission. It was developed successfully by Hangzhou Beidouxing Membrane Products Co., Ltd., a subsidiary of the Hangzhou Water Treatment Research and Development Center under the **Marine Bureau. Currently, 95% of the reverse osmosis membrane market is occupied by imported membranes, with domestic membranes accounting for only about 5% of the market share; China still has a long way to go in terms of reverse osmosis technology.

The above covers the most basic knowledge of reverse osmosis, intended to serve as a starting point for further discussion.

Reverse osmosis is just a broad concept that encompasses many aspects. Take the operating environment for example: brackish water membranes, seawater membranes. As for operating pressure, there are low-pressure membranes, ultra-low-pressure membranes, and extremely low-pressure membranes. In short, it cannot be explained in just a few words; let’s discuss it gradually