The principle, structure and effect of reverse osmosis membrane

Reverse osmosis is a new membrane separation technology developed in the 1960s. It relies on a reverse osmosis membrane to separate the solvent and solute in the solution under pressure. To understand the principle of reverse osmosis desalination, we must first understand the concept of “osmosis”. Osmosis is a physical phenomenon. When two waters with different concentrations of salts are separated by a semi-permeable membrane, it will be found that the water on the side with less salt content will penetrate through the membrane to the higher salt content. The water contained in it does not penetrate, so that the salt concentration on both sides is gradually melted until it is equal. However, it takes a long time to complete this process, also known as natural infiltration. However, if you try to add a pressure on the water side with high salt content, the result can also stop the above penetration, and the pressure at this time is called the osmotic pressure. If the pressure is increased, the water can permeate in the opposite direction, leaving the salt behind. Therefore, the principle of reverse osmosis desalination is to apply a pressure greater than the natural osmotic pressure in salty water (such as raw water), so that the osmosis proceeds in the opposite direction, and the water molecules in the raw water are pressed to the other side of the membrane , into clean water, so as to achieve the purpose of removing salt in the water, which is the principle of reverse osmosis desalination.

At present, reverse osmosis membranes are divided into two categories, cellulose membranes and non-cellulose membranes, according to the chemical composition of their membrane materials. According to the physical structure of membrane materials, it can be roughly divided into asymmetric membranes and composite membranes.

The most widely used cellulose film is cellulose acetate film (referred to as CA film). The total thickness of the membrane is about 100 μm, the thickness of the entire skin layer is about 0.25 μm, and the skin layer is full of micropores with a pore size of about 5-10 angstroms, so it can filter out extremely fine particles, while the pore size in the porous support layer It is very large, about several thousand angstroms, so the membrane with this asymmetric structure is also called asymmetric membrane. In the reverse osmosis operation, the cellulose acetate membrane can achieve the expected desalination effect only when the skin layer is in contact with the high-pressure raw water, and must not be inverted.

Non-cellulose membranes are mainly aromatic polyamides. Others include polynecamide membranes, benzodiazepine membranes, polysulfone amide membranes, polytetrafluoroethylene graft membranes, polyethyleneimine membranes, etc.

The polyamide composite membrane developed in recent years is made of a polyester non-woven fabric as the support layer. Because the polyester non-woven fabric is very irregular and too loose, it is not suitable for the bottom layer of the salt barrier layer, so the microporous engineering Plastic polysulfone is cast on the nonwoven surface. The pores on the surface of the polymaple layer are controlled at about 150 angstroms. The barrier layer is a highly cross-linked aromatic polyamide with a thickness of about 2000 angstroms. Aromatic polyamides with high cross-linking degree are polymerized from trimesoyl chloride and phenylenediamine. Because this kind of film is composed of three layers of different materials, it is called a composite film.

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Further reading:

RO membrane package

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