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What is reverse osmosis membrane technology? What is its principle

2022-09-02

What is reverse osmosis membrane technology

reverse osmosis membrane

Reverse osmosis membrane is an artificial semipermeable membrane with certain characteristics made by simulating biological semipermeable membrane, and it is the core component of reverse osmosis technology. The principle of reverse osmosis technology is that under the action of higher than the osmotic pressure of the solution, these substances and water are separated according to the fact that other substances cannot pass through the semi-permeable membrane. The pore size of the reverse osmosis membrane is very small, so it can effectively remove dissolved salts, colloids, microorganisms, organic matter, etc. in water. The system has the advantages of good water quality, low energy consumption, no pollution, simple process and easy operation.

 

 

The reverse osmosis membrane is the core element to realize reverse osmosis, and it is an artificial semipermeable membrane with certain characteristics made by simulating biological semipermeable membrane. Generally made of polymer materials. Such as cellulose acetate film, aromatic polyhydrazide film, aromatic polyamide film. The diameter of the surface pores 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 permeation speed of water is not good. The chemical structure of some polymer materials has more hydrophilic groups, so the permeation speed of water is relatively fast. Therefore, a satisfactory reverse osmosis membrane should have an appropriate amount of permeation or salt rejection.

 

The reverse osmosis membrane should have the following characteristics:

 

(1) It should have high-efficiency desalination rate at high flow rate;

 

(2) It has high mechanical strength and service life;

 

(3) Can function under lower operating pressure;

 

(4) Resistant to chemical or biochemical effects;

 

(5) It is less affected by factors such as pH value and temperature;

 

(6) The source of film-making raw materials is easy, the processing is simple, and the cost is low.

 

There are two types of reverse osmosis membranes: asymmetric membranes and homogeneous membranes. The currently used membrane materials are mainly cellulose acetate and aromatic polyamides. Its components are hollow fiber, coil, plate and frame and tubular. It can be used for chemical unit operations such as separation, concentration and purification, and is mainly used in pure water preparation and water treatment industries.

 

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. Apply pressure to the feed liquid on one side of the membrane. When the pressure exceeds its osmotic pressure, the solvent will reverse osmosis against the direction of natural osmosis. Thereby, the permeated solvent, ie permeate, is obtained on the low pressure side of the membrane; the concentrated solution, ie the concentrated solution, is obtained on the high pressure side. 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.

 

During reverse osmosis, the permeation rate of the solvent, that is, the 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 a 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 in 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, so as to obtain purified water. It can also be used for pre-concentration of macromolecular organic matter solutions. Due to the simple process of reverse osmosis and low energy consumption, it has developed rapidly in the past 20 years. It has been widely used in seawater and brackish water (see brine) desalination, boiler water softening and wastewater treatment, and combined with ion exchange to produce high-purity water. and separation and concentration of biological agents.

 

Reverse osmosis membrane filtration accuracy

 

The reverse osmosis membrane can intercept substances larger than 0.0001 micron, and is the most delicate membrane separation product. It can effectively intercept all dissolved salts and organic substances with a molecular weight greater than 100, while allowing water molecules to pass through.