Membrane basics

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Advantages of membrane separation 1. Good selectivity: Applying the molecular cutting theory, it can realize the separation and classification of substances at the molecular level, and has excellent performance that cannot be replaced by common filter materials. 2. Normal temperature operation, no phase change: The membrane separation process can be carried out at room temperature without phase change. It is especially suitable for the purification of heat-sensitive products. It maintains the original quality of the product and has low energy consumption. Its cost is about 1/3-1/8 of evaporation concentration or freeze concentration. ； 3. Wide application fields: It can be used to replace traditional filtration, and can also be used for material separation, purification, concentration, desalination, resource recycling, etc., involving chemical industry, petroleum, food, textile, printing and dyeing, metallurgy, environmental protection, biopharmaceuticals and other fields. ； 4. No chemical changes: The membrane separation process is a physical process, without adding additives and chemical reagents, with little product loss and no pollution to the product. ； 5. Strong matching: The scale of the membrane separation system can be designed according to needs, and the membrane software can be developed according to actual conditions. The processing scale can be large or small, and can be carried out continuously or intermittently. The process is simple, easy to operate, and easy to automate. ； Membrane element configuration 1, plate membrane element 2, tubular membrane element 3, rolled membrane element 4, hollow fiber capillary element membrane separation process and principle membrane materials: Filter material with selective separation function. membrane separation technology: Chemical unit technology that utilizes the selective separation characteristics of membranes to achieve concentration, classification, purification, etc. ； It is a method of separating, classifying, purifying and enriching substances by using selectively permeable materials as the separation medium, using external energy as the driving force, and relying on the difference in the mass transfer rate of each component in the multi-component fluid within the membrane. Principle of membrane separation and filtration: Membrane is a selective separation barrier between two or more concentration phases. It adopts cross-flow filtration separation method and uses the selective separation function of membrane material to separate and purify the components. ； The cross-flow method can effectively reduce membrane fouling and prevent concentration polarization, and the system operates continuously. Schematic diagram of membrane separation principle. Traditional dead-end filtration and cross-flow filtration. Membrane types classified by molecular weight cutoff 1. Microfiltration (MF): The basic principle of intercepting substances with a diameter of more than 0.1um is the sieve separation process. The materials of microfiltration membranes are divided into two categories: organic and inorganic. Organic polymers include cellulose acetate, polypropylene, polycarbonate, polysulfone, polyamide, etc. Inorganic membrane materials include ceramics and metals. In view of the separation characteristics of microporous filter membranes, the application scope of microporous filter membranes is mainly to intercept particles, bacteria and other pollutants from the gas phase and liquid phase to achieve the purpose of purification, separation and concentration. ； ※ Substances with interception size above 0.1UM ； ※ Usually used as sterilization and clarification filtration of materials ； It is also often used as prefiltration for ultrafiltration, nanofiltration, and reverse osmosis ※ Large flux and low operating cost ； 2. Ultrafiltration (UF): It is a membrane separation technology that can purify, separate, and concentrate solutions. The ultrafiltration process can usually be understood as a screening process related to the membrane pore size. ※Soluble substances with a molecular weight cutoff between 1,000 and 500,000 ※ Used for the removal and concentration of proteins, tannins, viruses, bacteria, and enzymes ； ※Decolorization and grading of biochemical, pharmaceutical, natural products, peptides, sugar and other products ； 3. Nanofiltration (NF): Substances with a molecular weight cutoff of more than 150 and a diameter of about 1 nm are a membrane separation technology between ultrafiltration and reverse osmosis. Features ※ Retention pore diameter is about 1nm ； ※ Used to remove monovalent inorganic salts and water from solutions ； ※Allowing monovalent salt to pass through reduces the osmotic pressure, so the operating pressure is low and energy consumption is saved. ； 4. Reverse osmosis (RO): Water and some small molecular substances can pass through. It is mostly used in pure water preparation, seawater desalination and other fields. It is also used for the concentration of small molecules such as amino acids. ※Except for water molecules that can pass through the membrane, other molecules and ions are intercepted by the membrane ※Classification chart of membrane interception substances with different molecular weight cutoffs most commonly used in the preparation of pure water http://www.starmem.com.cn/images/ti_bottom.gif Membrane Fouling and Cleaning Membrane Fouling and Cleaning Definition: The adsorption and deposition of solids or solutes on the membrane surface or in the membrane pores causes the membrane pore size to become smaller or blocked, resulting in temporary irreversible changes in membrane flux and deterioration of separation performance. At this time, the operation must be stopped, the membrane system must be cleaned, and the membrane module separation performance must be restored. Classification: Depending on the pollution points, it can be divided into internal and external pollution. ◇ Internal pollution: When the solute is concentrated, it crystallizes or precipitates in the membrane pores, causing blockage, resulting in a decrease in the effective porosity of the membrane ◇ External pollution: Adsorption and sedimentation pollution sources of certain solid components on the membrane surface: ◇ organic pollution: Proteins, fats, polypeptides, polysaccharides, dyes and other macromolecules ◇ Inorganic pollution: Calcium and barium carbonates, sulfates, and silicate scales ◇ Microbial contamination: Bacteria adhere to the surface of the membrane to form bacterial colonies, and the secretions of these bacterial colonies are conducive to the adhesion of other organic matter to form bacterial films. Cleaning method: Factors affecting cleaning of physical cleaning, chemical cleaning, and biological cleaning ◇ Time ◇ Temperature ◇ Drugs: Type of cleaning agent, appropriate concentration, pH value and ionic strength. ◇ Transmembrane pressure difference ◇Hydraulic parameters (flow speed, flow mode) ◇Membrane properties (membrane material, membrane morphology, membrane surface properties) ◇Properties of feed liquid: