At present, the main reverse osmosis membrane materials are cellulose acetate, aromatic polyamides and polypiperazine amides. Cellulose acetate reverse osmosis membrane for the asymmetric membrane, although in the alkali resistance, bacterial resistance, water production is not as good as the polyamide membrane, but because of its excellent chlorine resistance, resistance to contamination is still in use today. Aromatic polyamide can be divided into linear aromatic polyamide and crosslinked aromatic polyamide, the former for the asymmetric membrane, the latter for the composite membrane. These membranes can be used in the manufacture of ultrapure water and seawater desalination, which require high solute removal performance, because of their high cross-linking density and high hydrophilicity, as well as their excellent desalination rate, water yield, oxidation resistance, organic matter removal rate and silica removal rate, and so on. Polypiperazine amide can be divided into linear polypiperazine amide membrane and crosslinked polypiperazine amide membrane, the latter has products on the market. The membrane is characterized by high water yield, chlorine resistance, hydrogen peroxide resistance, and can be used in water purification and foodstuffs that require high desalination performance.
According to the operating pressure reverse osmosis membrane can be divided into three categories: high-pressure reverse osmosis membrane, low-pressure reverse osmosis membrane and ultra-low pressure reverse osmosis membrane. High-pressure reverse osmosis membrane for seawater desalination, there are five main types: cellulose triacetate hollow fiber membrane, straight-chain all-aromatic polyamide hollow fiber, crosslinked all-aromatic polyamide rolled composite membrane, aryl-alkyl polyether urea rolled composite membrane and cross-linked polyether composite membranes. The original brackish water desalination reverse osmosis operating pressure is as high as 2.8 ~ 4.2MPa, while the use of low-pressure reverse osmosis membrane can be in the 1.4 ~ 2.0MPa low operating pressure to remove salt, energy consumption is greatly reduced. In addition, low-pressure reverse osmosis membrane can also be used for the production of high-purity water in the electronics and pharmaceutical industries, food industry wastewater treatment, beverage water production, etc. The use of low-pressure reverse osmosis membrane can improve the selective separation of certain organic and inorganic solutes while reducing the cost of equipment, operating costs, and improving production capacity. Ultra-low pressure reverse osmosis membrane is also known as sparse reverse osmosis membrane or nanofiltration membrane.
Due to the different membrane processes, the performance of different separation membranes made from the same membrane material will be very different, so the reasonable and advanced membrane process and the optimal process parameters are an important guarantee for the preparation of separation membranes with excellent performance.
Physical or chemical methods, or a combination of physical and chemical methods, can be prepared with good separation performance of polymer separation membrane. Commonly used membrane preparation methods include phase conversion method (salivation, spinning) and composite method.
1, the phase transition method
Phase transition of the various methods of membrane production in the second chapter has been partially introduced. Phase conversion membrane can be roughly divided into the following six stages:
(1) the polymer and additives dissolved in a solvent, the preparation of membrane liquid;
(2) the membrane liquid through the salivation method into a flat plate and tubular membrane, or through the spinning method can be made into a hollow fiber-type membranes;
(3) so that part of the membrane solvent evaporation;
(4) will be impregnated with a membrane in a non-solvent liquid of polymer, the membrane of the membrane, the membrane, the membrane, the membrane, the membrane, the membrane, the membrane, the membrane, the membrane, the membrane, the membrane, the membrane, the membrane, the membrane and the solvent. polymers in a non-solvent liquid (most commonly water), and the liquid phase of the membrane is solidified and molded in water;
(5) heat treatment of the solidified and molded membrane. Non-acetate cellulose membranes, such as aromatic polyamide membranes, generally do not require heat treatment;
(6) pre-pressurization of the membrane.
The polymer concentration in the membrane making solution is generally about 10% to 40%, when the solution concentration is too low, the strength of the membrane is low, and the practical performance is poor; solution concentration is high, the polymer dissolution effect is poor, the uniformity of the membrane produced is poor, and the performance is not guaranteed. The solvent used should be able to dissolve the polymer, miscible with water, and no chemical reaction with other components. If the membrane is made at room temperature, the solvent is preferably a low-boiling polar solvent, the content of 60% to 90%. Additives should be able to miscible with the components of the film-making liquid, but also soluble in water, preferably high boiling point polar substances, the general content of 0% to 30%.
In order to improve the quality of the membrane, in the process of membrane production should pay attention to the following aspects:
(1) purification and ripening Due to the water absorption of polar polymers and polar solvents, attention should be paid to constant their water content, if necessary, polymers and solvents need to be purified before the preparation of membrane fluid; polymers - solvents - additives are completely solved and ripened, and the surface of the uniform membrane fluid tends to be molecularly Dispersed thermodynamically unstable system, this system will sooner or later split phase, the membrane should be carried out in a homogeneous situation; mechanical impurities in the membrane fluid can be used under the action of inert gas 200 ~ 240 mesh filter to remove by pressure filtration; residual in the membrane fluid gas can be removed by decompression; containing acetone and other low-boiling solvents, can be used to remove the static method; in order to prevent the solvent from evaporating and some components of the In order to prevent the volatilization of solvents and self-polymerization of certain components, the film-making liquid should be stored under sealed and lightproof conditions.
(2) the requirements of the environment film, to ensure that the environment is clean and clean salivary substrate, for this reason, the flow of the glass plate needs to be cleaned with a 1:1 solution of anhydrous alcohol and ether, so that the effective removal of oil and grease; film-making liquid flow, to prevent the entrainment of the gas; the flow of the film and the solvent evaporation should pay attention to the control of the environment, temperature, humidity, and other conditions of the constancy of the flow of the film and the solvent evaporation to avoid turbulence, the turbulent flow of the gas. The turbulence of the airflow is often one of the causes of membrane defects (( pinholes and bright spots.
(3) other requirements Membrane in solidification molding, in order to make the solvent and additives from the membrane completely leached, depending on the different forms of the membrane, need to be kept from a few hours to dozens of days; membrane evaporation contact with the air on the side of the membrane is the membrane of the surface of the surface active layer, or the surface layer of dense, the layer of dense play a role in the separation; heat treatment of the membrane so that the membrane pore size contraction, which led to the increase in the rate of separation and the decrease in flux, thus pay attention to control the time of heat treatment, so the membrane is not a good choice. Therefore, attention should be paid to control the time and temperature of heat treatment; membrane pre-pressure treatment before use, in order to stabilize the membrane performance.
2, composite method
The reverse osmosis membrane produced by the phase transition method, the solute to play a role in the separation of only a very thin surface dense layer, the thickness of which is about 1/100 of the membrane thickness. the membrane permeability and the thickness of the surface dense layer is inversely proportional to the thickness of the surface layer can be reduced by the surface layer of dense layer to increase the speed of the membrane through the surface dense layer thickness, but the study showed that, in order to produce a thickness of less than 0.1 (m) of the surface dense layer is extremely difficult to make. However, studies have shown that it is extremely difficult to produce a surface layer with a thickness of less than 0.1(m).
Under pressure, membrane compaction decreases the transmission rate of the membrane. Membrane compaction occurs mainly in the transition layer between the surface dense layer and the porous support layer below, which increases the membrane's permeation resistance. Although some studies have pointed out that the decrease in transmission rate is related to the structural change of the surface dense layer, as long as the operating pressure does not exceed the yield point of the polymer in the surface dense layer, the main reason for the decrease in transmission rate still lies in the densification of the transition layer. Therefore, from the point of view of reducing the thickness of the surface dense layer and solving the densification of the transition layer, there is a limit to improve the membrane performance by simply relying on the improvement of the phase conversion method.
Using other processes to prepare dense ultra-thin desalination layer and porous support layer, and then the two parts of the composite, which can not only reduce the thickness of the surface dense layer, but also cancel the transition layer is easy to cause the pressure density, but also choose the tough material to prepare the porous support layer, and choose the high desalination of the material to prepare the ultra-thin desalination layer, which makes the membrane at the same time has a higher solute separation rate and solvent transmission rate, which is the basic production of membrane. Speed, which is the basic conception of making composite membrane.
1. Characteristics of composite membrane
(1) Different materials can be used to produce the ultra-thin desalination layer and the porous support layer, so that their functions can be optimized to optimize the performance of the composite membrane.
(2) The ultra-thin desalination layer with high cross-linking degree and ionic groups can be made by different methods, and the thickness can be controlled to 0.01(0.1(m, which makes the membrane have a good separation rate of inorganic substances, especially organic substances, and a high water permeability, and at the same time, it has a good physical and chemical stability and compression denseness resistance.
(3) According to different application characteristics, different thicknesses of ultra-thin desalination layer can be made.
(4) Most of the composite membranes can be made into dry membranes, which is conducive to the transportation and preservation of the membrane.
At present, the production of composite membranes is usually the first production of porous support layer, and then directly on the porous support layer in a variety of ways to produce ultra-thin desalination layer. For the porous support layer, it is required to have appropriate size of pore density, pore size and pore size distribution, good compression densities and physical and chemical stability. Because polysulfone raw materials are cheap and easy to obtain, simple membrane production, good mechanical strength and compressive density, good chemical stability, non-toxic, resistant to microbial degradation, the membrane can be dried, and the rate of water permeability does not have much impact, so the vast majority of composite membranes in the industry are mainly used polysulfone porous support membrane as a support layer.
2. The main preparation methods of the ultra-thin desalination layer
The main preparation methods of the ultra-thin desalination layer include polymer coating, interfacial polymerization, in situ polymerization, plasma polymerization and so on. These have been described in Chapter II. In addition, the National Atomic Energy Research Institute in Oak Ridge, USA, has adopted a method for the preparation of composite membranes known as the dynamic molding method. In this method, colloidal particles or microparticles are attached and deposited on the surface of a porous support by means of pressurized closed-cycle flow to form a thin base film. Then the dilute solution of polymer polyelectrolyte, the same way to pressurized closed loop flow, they will be attached to the deposition of the bottom membrane, constituting a separation performance, double-layer structure of the composite membrane.
At present, less research on the mechanism of composite membrane formation to polyamine aqueous solution and chloryl chloride organic solution on the surface of polysulfone base film interface polymerization, for example, polysulfone porous membrane absorption of polyamine aqueous solution, chloryl chloride organic phase solution and then on the surface of polysulfone base film and the aqueous phase of the surface of the base film interfacial polymerization reaction to form an ultra-thin desalination layer. Due to the nature of the solute and the properties of the interface, the initial concentration at the interface of the two phases is high. When the two phases come into contact, the reaction starts rapidly, the concentration of the two monomers at the interface decreases rapidly, and a very thin polyamide film is formed at the interface. When the reaction time of the two monomers is too long, further reaction is controlled by the rate of diffusion through this film. It is generally recognized that the reaction of the acyl chloride with the polyamine is an irreversible nucleophilic reaction with a secondary reaction rate.
The reaction time of the chloryl chloride with amines during composite film preparation is generally short, in the range of a few seconds to a minute or so, because the ultrathin desalination layer of the composite is hopefully very thin, in the range of 50 to 300 nm. Too long a reaction time will thicken the ultrathin desalination layer and affect the transfer and selection properties of the composite film. During the composite process, the type of the two monomers, the initial concentration and ratio of the two monomers in the two phases, the type of the organic phase solvent, the temperature and time of the reaction, and the type and concentration of the acid receiver have a large impact on the good or bad film formation. In addition, although the interfacial reaction is not strict on the exact equivalence ratio of the two monomers, trying to make the two monomers react with the appropriate equivalence ratio will be conducive to the formation of high molecular weight composite membranes.
In addition, the polycondensation reaction is characterized by the generation of a larger number of intermediate products with different degrees of polymerization at the initial stage, with the extension of time, the degree of polymerization increases, so the film is formed at room temperature first, and then further reacted at higher temperatures, so as to make the ultra-thin desalination layer more structurally sound, thus facilitating the formation of high molecular weight composite membranes.
In short, reverse osmosis membrane process in each process, there are a series of factors affecting the performance of the membrane, the membrane should make better use of the changes in these factors, to coordinate their mutual constraints to make up for the intrinsic relationship, so as to prepare a better performance, quality and satisfaction of reverse osmosis separation membrane.