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Discussion on filter characteristics and selection principle

Sep 23, 2019

Filtration is an indispensable treatment in the purification process of fluids, mainly used to remove particulate matter or other suspended matter in the fluid. The principle of the filter is to use a porous medium to remove contaminants from the fluid (liquid or gas) to achieve the desired level of cleanliness. Filters are often considered to be a simple mesh or screen, and filtration or separation is performed on a surface. This is the way of the past. Most filter walls now have a certain thickness, which means that the filter has a depth that acts as a “bending channel” to remove contaminants. The filter is a device for removing a small amount of solid particles in the liquid. When the fluid enters the filter cartridge with a certain size filter, the impurities are blocked, and the clean filtrate is discharged from the filter outlet. When the cleaning is required, the screw is charged. The bottom screw plug, drain the fluid, remove the flange cover, take out the filter cartridge, and reload it after processing. It is well known that the core of the filter is a filter membrane, which is a film that is formed on a microporous support layer (support) and filled with finer pores. There are many materials for making filters, which are classified into organic films (such as polysulfone hollow fiber membranes) and inorganic membranes (such as ceramic membranes). The membrane filter has high filtration precision, stable particle size control, and easy recovery performance. Therefore, it is convenient to use maintenance**.

1 Filtration mechanism and influencing factors

1.1 Filtration mechanism

There are two main types of fluid filtration mechanisms. One is based on the size of the particles to separate, such as interception, sieving and surface capture; the other is adsorption, that is, the particles adhere to the filter under chemical/charge action?

1.2 Factors affecting filtering

1.2.1 Characteristics of fluids

It is related to the characteristics of the fluid. For example, the viscosity of the fluid and the chemical/ionic composition, the greater the viscosity of the fluid, the slower the flow rate under the same pressure conditions, the more contact between the fluid and the membrane, and the better the filtration effect; for example, the mixing of the fluid and the membrane/ The contact time also has a large effect on the filtration effect, and the longer the mixing/contact time, the better the filtration effect. In addition, it should be noted that the characteristics of the fluid only affect the adsorption retention effect of the membrane on the fluid without affecting the exclusion of the particle size.

1.2.2 Operating conditions

It is related to actual operating conditions such as the flow rate of the particles and the filtration pressure. To achieve good filtration results, generally choose a lower flow rate, the lower the flow rate, the better the retention effect. Practice has shown that the structural movement of the membrane is detrimental to filtration. Once the structure of the membrane changes during the filtration process, the particles and fibers can be precipitated from the depth filter, affecting the filtration effect. However, the velocity/pressure differential has only a significant effect on the adsorption cut-off and has a relatively small effect on size exclusion.

1.2.3 particle type

The type of particles is also highly dependent on the filtration effect. The particles are classified into two types: variable particles and non-deformable particles. Under a certain pressure, the deformable particles will enter the filter membrane and cause more filter mesh plugging, which will affect the filtration effect, such as gel filtration. However, when the immutable particles are filtered, a layer-like object is formed on the filter.

1.2.4 Filter membrane type

Depending on the type of filter membrane, the pore size and structure of the different membranes are different, some membranes are rigid and some membranes are mobile. The nominal pore size of the pre-filtration membrane does not have a uniform national standard. Different manufacturers have their own definitions and methods, so it is necessary to pay attention to the selection and replacement of the business. The same is the 0.22μm pre-filtration membrane, which is filtered by different manufacturers. The effect will vary greatly. The common pore size of the sterilization filter is defined by regulations, and each merchant performs the same standard, which is relatively simple when selecting and replacing.

1.2.5 filter material

In relation to the material to be filtered, the filter material is classified into two types: hydrophilic (water infiltable) and hydrophobic (water insoluble) according to the relationship with water. Hydrophilic filters are mainly used in water or water/organic solution mixing filtration and sterilization filtration, such as cellulosic materials (regenerated cellulose, mixed cellulose ester), PVPP polycarbonate, PVDF modified polydisperse Ethylene; hydrophobic filters are intercepted or "guided" into the membrane by water, mainly used in solvent, acid, alkali and chemical filtration, tank / equipment respirator, process gas, fermentation intake / exhaust filtration, Such as PTFE polytetraethylene, PVDF polyvinylidene, polypropylene, polysulfone, polycarbonate and the like.

2 filter characteristics and structure

Filters are generally classified into three types: depth filter (primary filter), surface filter (intermediate filter), and membrane filter (depth filter). The fibers of the deep filter are detached and cannot give an exact pore diameter. The thickness is generally 3 to 20 mm. It usually has adsorption and has a large soil-containing capacity. The fibers of the surface filter are generally thermally bonded or coated. Coated, can give the rated pore size, relatively thin (<1 mm), less adsorption capacity; membrane filter is characterized by hard texture, not easy to break, with tortuous channels and very high internal surface area, there is a certain The open cell rate, which can be used for integrity testing, is often used for deep filtration, such as sterile filters.https://www.inocofiltration.com/