Types of reverse osmosis membranes and their structural and morphological classification

Classification according to the structure and morphology of the membrane itself

  • Homogeneous membrane is a membrane of the same material and uniform thickness. In order to increase the strength to withstand pressure, the thickness of the film is thicker, and the entire film thickness acts as a shielding layer, so the water permeability is poor.
  • The asymmetric membrane is made of the same material and is made into a dense skin layer and a porous support layer. The epidermis layer is very thin and plays the role of salt separation, with a thickness of about 0.1 to 0.2 μm. Because of the smaller resistance, the water flux of the membrane is higher than that of the homogeneous membrane.
  • The composite membrane is a composite of several layers of membranes made of different materials. The surface layer is a dense shielding skin (which acts to prevent and separate salt), with a thickness of about 0.2 μm. The skin is applied on a porous layer with high strength, with a thickness of about 40 μm, and the bottom layer is a non-woven fabric support layer with a thickness of about 120 μm. , acts to support the entire membrane.

Classification by film processing shape

  • Flat membrane The flat membrane is used as the intermediate raw material, which can be processed into plate type, tube type or roll type reverse osmosis membrane.
  • Hollow fiber membrane is a very thin and asymmetric hollow fiber membrane made by melt spinning through hollow fiber spinning, heat treatment and other processes.

Classification by film material

  • Cellulose acetate film is generally made of cellulose triacetate by lipidation, and then hydrolyzed twice to form a mixture of one, two, and triacetate cellulose.
  • Aromatic polyamide membranes are generally highly cross-linked aromatic polyamides as a dense desalination layer of the membrane skin.

Performance comparison of aromatic polyamide ultra-thin composite membrane and cellulose acetate membrane

  1. The chemical stability of the composite membrane is good, and the cellulose acetate membrane will inevitably be hydrolyzed.
  2. The composite membrane has good biological stability and is not easily attacked by microorganisms, while the cellulose acetate membrane is vulnerable to microorganisms.
  3. The transmission property of the composite membrane is good.
  4. The composite membrane will not be compressed during operation, so the water production will change little with the use time, while the acetate membrane will be compressed during operation, so the water production will continue to decrease.
  5. The desalination rate of the composite membrane changed little with time, while the desalination rate of the cellulose acetate membrane decreased continuously due to the inevitable hydrolysis.
  6. Due to the large Kw of the composite membrane, its working pressure is low, and the electricity consumption of the reverse osmosis feed pump is almost reduced by half compared with the cellulose acetate membrane.
  7. The lifespan of cellulose acetate membrane is generally only three years, while the composite membrane can be used for five years.
  8. The disadvantage of composite membrane is that it has poor chlorine resistance and is more expensive.

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