Reverse osmosis membrane is the core element to realize reverse osmosis. It is an artificial semi-permeable membrane with certain characteristics made by simulating biological semi-permeable membrane. Generally made of polymer materials. Such as cellulose acetate film, aromatic polyhydrazide film, aromatic polyamide film. The diameter of the surface micropores is generally between 0.5 and 10 nm, and the permeability is related to the chemical structure of the membrane itself. Some polymer materials have good repellency to salt, but the water penetration rate is not good. The chemical structure of some polymer materials has more hydrophilic groups, so the water penetration rate is relatively fast. Therefore, a satisfactory reverse osmosis membrane should have an appropriate amount of permeation or salt rejection rate.
Principle: reverse osmosis, also known as reverse osmosis, is a membrane separation operation that uses pressure difference as the driving force to separate the solvent from the solution. Pressure is applied to the material liquid on one side of the membrane. When the pressure exceeds its osmotic pressure, the solvent will reverse osmosis against the direction of natural permeation. In this way, the permeated solvent is obtained on the low pressure side of the membrane, that is, the permeate, and the concentrated solution, that is, the concentrated solution, is obtained on the high pressure side of the membrane. If reverse osmosis is used to treat seawater, fresh water is obtained on the low pressure side of the membrane and brine is obtained on the high pressure side. In reverse osmosis, the permeation rate of the solvent, that is, the liquid flow energy N is: N=Kh(Δp-Δπ) where Kh is the hydraulic permeability coefficient, which increases slightly with the increase of temperature; Δp is the static pressure difference on both sides of the membrane ; Δπ is the osmotic pressure difference of the solution on both sides of the membrane. The osmotic pressure π of the dilute solution is: π=iCRT where i is the number of ions generated 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 usually uses asymmetric membranes and composite membranes. The equipment used for reverse osmosis is mainly hollow fiber type or roll type membrane separation equipment. The reverse osmosis membrane can intercept various inorganic ions, colloidal substances and macromolecular solutes in the water to obtain clean water. It can also be used for pre-concentration of macromolecular organic solutions. Due to the simple reverse osmosis process and low energy consumption, it has been rapidly developed in the past 20 years. It has been widely used in seawater and brackish water (see brine) desalination, boiler water softening and waste water treatment, and combined with ion exchange to produce high-purity water. At present, its application range is expanding, and it has begun to be used for the concentration of dairy products and fruit juices. Separation and concentration of biochemical and biological agents.
The structure of reverse osmosis membrane has two types: asymmetric membrane and homogeneous membrane. The currently used membrane materials are mainly cellulose acetate and aromatic polyamides. Its components are hollow fiber type, roll type, plate and frame type and tube type. It can be used for separation, concentration, purification and other chemical unit operations, mainly used in pure water preparation and water treatment industries.