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Classification and Application Areas of Membrane Technology I. Membrane Separation Technology Membrane separation is a new separation technology that emerged at the beginning of the 20th century and saw rapid development after the 1960s. Due to its capabilities in separation, concentration, purification, and refinement, as well as its efficiency, energy savings, environmental friendliness, molecular-level filtration, and simple and easy-to-control filtration process, membrane separation technology is now widely used in fields such as food, pharmaceuticals, biology, environmental protection, chemicals, metallurgy, energy, petroleum, water treatment, electronics, and bionics. It has generated significant economic and social benefits, and has become one of the most important methods in separation science today. The membrane can be in a solid phase, liquid phase, or even a gas phase. There are a wide variety of membranes made from various natural or synthetic materials, exhibiting diverse properties in terms of physical, chemical, and biological characteristics. Most people would think that membranes are very far removed from our lives. In fact, that’s not the case; membrane separation technology is very closely related to our daily lives. Membrane separation technology is used in various things we come into contact with on a regular basis, such as water, fruit juices, milk, health products, traditional Chinese medicines, tea-based foods, beverages, and seasonings. With the rapid development of the national economy, the application areas of membrane separation technology will not only become increasingly widespread, but it will also be recognized and accepted by more and more people. According to preliminary estimates, global sales of membranes and membrane components in 2001 reached nearly 8 billion US dollars, while the market for complete systems and membrane engineering amounted to hundreds of billions of US dollars. These figures are increasing at a rate of 10% to 20% per year, indicating the broad prospects for this emerging industry. II. History, Current Status, and Prospects of Membrane Separation Technology (1) History of Development Membranes are widely present in nature, especially within living organisms, yet the historical process of human understanding, utilization, simulation, and ultimately artificial synthesis of them has been long and tortuous. The development of membrane science and technology in our country began in 1958 with research on ion exchange membranes. The 1960s marked the beginning of its formative stage. Research on reverse osmosis began in 1965, and the national campaign for seawater desalination that started in 1967 **promoted the development of membrane technology in our country. It entered the development phase in the 1970s. During this period, various membrane and device technologies such as microfiltration, electrodialysis, reverse osmosis, and ultrafiltration were successively developed, and the 1980s saw their entry into the stage of widespread application. The 1980s was another period of gas separation and the development of other new membranes. (II) Current Situation: With the development of membrane science and technology in China, corresponding academic and technical organizations have also been established one after another. Their establishment plays a crucial role in standardizing the membrane industry and promoting its development. Over the past half century, membrane separation has evolved from a laboratory technique to a large-scale industrial application, becoming an efficient and energy-saving new separation technology. Since 1925, a new membrane process has been put into industrial use almost every decade. Due to the superior properties of membrane separation technology itself, membrane processes have now received widespread attention around the world. In an era characterized by energy shortages, resource scarcity, and deteriorating environmental conditions, the industry and scientific community regard membrane processes as a highly important new technology for the transformation of industrial technologies in the 21st century. Some experts have said that whoever masters membrane technology holds the key to the future of the chemical industry. Since the 1980s, membrane technology in our country has entered the application phase, which was also a period of development for new membrane processes. During this period, membrane technology was developed and applied on a large scale in fields such as food processing, seawater desalination, pure water and ultrapure water production, pharmaceuticals, biology, and environmental protection. Moreover, during this period, **membrane-related topics were included in both key scientific and technological research projects and the Natural Science Foundation. Currently, this highly promising emerging industry is challenging the market with great enthusiasm, bringing significant economic, social, and environmental benefits to numerous enterprises. (III) Outlook At present, membrane separation technology has made great progress, but it is still in an early stage of development, and there is still much work to be done. In the 21st century, membrane science and technology will further improve and refine existing membrane processes, continuously explore and develop new processes and materials, and expand the existing areas of application, thereby enabling membrane technology to play an even greater role. One of the outlooks: We must strive to integrate emerging membrane separation technologies with traditional process technologies, and continuously move the research achievements in membrane technology from the laboratory stage to industrial application. Vision 2: We must commit to researching new membrane materials and developing new polymeric membrane materials. Vision 3: We must commit to researching and developing new film-forming processes in order to further advance the technologies and methods for creating ultra-thin, highly uniform, defect-free asymmetric film coatings. Vision 4: We must strive to advance the development of inorganic membranes. Inorganic membranes are receiving increasing attention from both the academic community and industrial applications due to certain advantages that other polymer membranes do not possess. These advantages include resistance to acids and bases, resistance to organic solvents, good chemical stability, high mechanical strength, strong resistance to microbial contamination, tolerance to high temperatures, a narrow pore size distribution, and high separation efficiency. In future development, researching new materials and new processes for inorganic membranes is an inevitable trend. Prospect 5: Whether in academic research or industrial applications, the study of techniques such as microfiltration, ultrafiltration, nanofiltration, reverse osmosis, electrodialysis, gas separation, and pervaporation will remain of top priority. III. Characteristics of membrane separation technology: The membrane separation process is an efficient and environmentally friendly separation method. It represents a high-tech field that combines elements from multiple disciplines, and it exhibits a variety of properties in terms of physical, chemical, and biological aspects, offering numerous advantages. Compared with traditional separation techniques such as distillation, adsorption, absorption, extraction, and cryogenic separation, membrane separation technology has the following characteristics. ※ Efficient separation process ※ Low energy consumption ※ Operating temperature close to room temperature ※ Good quality stability ※ Continuous operation ※ High flexibility ※ Purely physical process ※ Environmentally friendly ※ …… IV. Basic characteristics of membrane separation processes Membrane separation technology is widely favored by users for its significant energy-saving effects, simple equipment, easy operation, and ease of control. By selecting the appropriate membrane separation process, various traditional separation and filtration methods such as drum vacuum filtration, plate and frame filter pressing, ion exchange, centrifugal separation, solvent extraction, electrostatic dust removal, bag filtration, adsorption/regeneration, flocculation/copolymerization, decantation/sedimentation, evaporation, and crystallization can be replaced. V. Common Membrane Separation Processes (I) Microfiltration: Given the separation properties of micro-porous filter membranes, their main application is to retain particles, bacteria, and other contaminants from gas and liquid phases, thereby achieving purification, separation, and concentration. The specific fields involved mainly include: the pharmaceutical industry, the food industry (gelatin, wine, liquor, fruit juice, milk, etc.), high-purity water, municipal wastewater, industrial wastewater, drinking water, biotechnology, biological fermentation, etc. (II) Ultrafiltration: Early industrial applications of ultrafiltration were in wastewater and sewage treatment. Over the past thirty years, with the development of ultrafiltration technology, it is now applied in a wide range of fields including food processing, the beverage industry, the pharmaceutical industry, biologic preparations, traditional Chinese medicine preparations, clinical medicine, treatment of printing and dyeing wastewater, treatment of wastewater from the food industry, resource recovery, and environmental engineering. (III) Nanofiltration: The main application areas of nanofiltration include the food industry, advanced processing of plants, the beverage industry, advanced processing of agricultural products, biomedicine, biological fermentation, fine chemicals, and the environmental protection industry… (IV) Reverse osmosis: Due to its advanced, efficient, and energy-saving characteristics, reverse osmosis separation technology is widely used in various sectors of the national economy. It is primarily employed for water treatment and the concentration of heat-sensitive substances. Its main application areas include the food industry, the dairy industry, the beverage industry, advanced processing of plants (agricultural products), biomedicine, biological fermentation, as well as the production of drinking water, pure water, ultra-pure water, desalination of seawater and brackish water, use in the power, electronics, and semiconductor industries, as well as in pharmaceutical processes, for the preparation of formulations, injectable solutions, sterile and pyrogen-free pure water. It is also used in the food and beverage industry, chemical and other industrial processes, as boiler water, washing water, and cooling water. (V) Other common membrane separation processes: In addition to these four common membrane separation processes, there are also dialysis, controlled release, membrane sensors, and membrane-based gas separation. VI. Inorganic Membranes Section (I): Overview and Characteristics of Inorganic Membranes. The development of inorganic membranes began in the 1940s, and to date it has gone through three stages. Due to the excellent properties of inorganic membranes and the progress in inorganic materials science, the application fields of these membranes are expanding steadily. Membrane-catalyzed reaction processes, which combine inorganic membranes with catalytic reaction mechanisms, are considered one of the three major future development directions in the field of catalysis. Therefore, the application of inorganic membranes has become a hot topic in research and development within the field of membrane technology. Research on inorganic membranes in China began in the late 1980s; with the support of the National Natural Science Foundation and various government departments, China has been able to produce inorganic microfiltration membranes, ultrafiltration membranes, and metal palladium membranes in laboratory settings. Entering the 1990s, the **Ministry of Science and Technology organized scientific and technical efforts to develop the industrialization techniques for inorganic ceramic microfiltration membranes, thereby accelerating the industrialization process of such membranes. The 7th International Conference on Inorganic Membranes was held in China in 2002, marking that the research and industrial application of inorganic membranes in our country had reached an internationally leading level. Inorganic membranes are made from inorganic materials and are solid-state membranes. Based on the science of inorganic materials, they possess advantages that polymer separation membranes cannot match: ※ a narrow pore size distribution, high separation efficiency, and stable filtration performance. ※ It has good chemical stability and is resistant to acids, bases, and organic solvents. ※ Heat-resistant, it can be regenerated by steam backflushing and disinfected at high temperatures. ※ It has strong resistance to microbial contamination, making it suitable for use in the biomedical field. ※ It has high mechanical strength, can be backwashed under high pressure, and possesses strong regeneration capacity. ※ No leachables are generated, there is no secondary pollution, and it has no negative impact on the separated materials. ※ The separation process is simple, requires low energy consumption, and is easy to operate. ※ The membrane has a long service life. ※ ……………… (II) Classification of inorganic membranes 1. In terms of their surface structure, inorganic separation membranes can be divided into two main categories: dense membranes and porous membranes. To meet the needs of industrial production, porous ceramic membranes are currently used in a mature and widespread manner. 2. Based on the material used for their fabrication, inorganic membranes can be classified into ceramic membranes, metal membranes, alloy membranes, polymer-metal complex membranes, molecular sieve composite membranes, zeolite membranes, glass membranes, and so on. 3. Based on their structural characteristics, inorganic membranes can be divided into non-supported membranes (primarily used in laboratories and research work) and supported membranes (mainly used in industrial production). (III) Inorganic membrane structures: The inorganic porous separation membrane elements that have been highly successful in industrial applications consist mainly of a three-layer structure: a porous support layer, a transition layer, and an active separation layer. (IV) Inorganic membrane elements and membrane modules 1. Inorganic membrane elements From a microscopic perspective, inorganic membrane elements have two types of structures: symmetric and asymmetric. Due to advantages such as stable treatment performance, high mechanical strength, and high permeation flux, asymmetric membrane elements are currently the main form used in industrial applications. In terms of geometric shape, commercial inorganic membranes come in various forms: multi-channel, tubular, flat-plate, honeycomb, etc. Given that industrial production requires a large filtration area, and multi-channel membrane elements utilize cross-flow filtration, which is different from other membrane filtration methods, coupled with their advantages such as easy installation, high mechanical strength, and suitability for large-scale industrial applications, multi-channel inorganic membranes have become the main products used in industrial settings. 2. Inorganic membrane modules: To ensure the proper operation of inorganic membrane elements, it is necessary to use these elements in conjunction with membrane housings; hence, membrane modules were developed. Typically, membrane modules are named according to the number of membrane elements installed, which is very simple and straightforward. The specific applications of inorganic membranes are in separation and reaction processes in the form of membrane modules. Inorganic membrane modules are composed of 1, 3, 7, 19, 37, or more membrane elements, depending on the actual production or experimental requirements of the user. An inorganic membrane system usually consists of many membrane modules. (5) Filtration method of inorganic membrane separation system: The inorganic membrane separation system includes membrane modules, raw material delivery systems, pressure and flow measurement and control systems, etc. The main method used is cross-flow filtration; unlike end-filtering, cross-flow filtration involves two types of fluids: the permeate and the circulating fluid. (VI) Application areas of inorganic membranes Membrane separation technology is widely favored by users due to its significant energy-saving effects, simple operation and maintenance, and easy control. By selecting the appropriate membrane separation process, various traditional separation and filtration methods such as vacuum filtration, plate and frame filter pressing, and ion exchange can be replaced. The applications of inorganic membranes mainly involve three areas: liquid phase separation and purification, gas separation and purification, and membrane reactors. The industrial application of inorganic membranes is primarily focused on the field of liquid separation. Their use in liquid separation involves microfiltration and ultrafiltration, with ceramic membranes being the most commonly used type, accounting for 80% of the market share. Food and beverages; deep processing of plants (herbs); fruit juices, vegetable juices; dairy industry; deep processing of agricultural products; sugars; food additives, natural pigments, flavorings; alcoholic beverages, etc.; biochemistry and pharmaceuticals; biological fermentation liquids; proteins, enzymes; traditional Chinese medicines, health product oral liquids; animal plasma, serum; pharmaceuticals and intermediates; chemical industry; synthetic dyes and intermediates; fine chemicals; chemical raw materials and intermediates; synthetic and microbial pesticides; recycling of catalyst particles; recovery of organic chemical raw materials; purification of chemical acids and alkalis; environmental engineering; preparation of process water for industries such as the beverage and food industries; treatment of wastewater from food production, biological fermentation, dye manufacturing, etc.; treatment of wastewater from pulp and paper manufacturing, textile industries, and degreasing processes; treatment of oily wastewater; air filtration. In the field of gas separation, its applications mainly include the purification of gases (air) and the separation of gas components; however, at present, only the separation of uranium isotopes has been successfully implemented, while other gas purification and separation processes are still in the research and development stage