Filter characteristics and selection

Filtration is an indispensable treatment method in the purification process of fluids, and is mainly used to remove particles 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), so that the fluid reaches the required cleanliness level. A filter is often thought of as a simple mesh or sieve, and filtration or separation is performed on a surface. This is the past method, and now most filters have a certain thickness of the filter wall, that is to say, the filter equipment has a depth, in the form of a "curved channel" played an auxiliary role in the removal of contaminants. The filter is a device that removes a small amount of solid particles in the liquid. When the fluid enters the filter cartridge with a certain specification filter screen, its impurities are blocked, and the clean filtrate is discharged from the filter outlet. When cleaning is needed, the pipe is turned Bottom screw plug, drain the fluid, remove the flange cover, take out the filter cartridge, and reinstall it after treatment. As we all know, the core element of the filter is the filter membrane, which is a membrane filled with smaller pores prepared on a microporous supporting layer (support). There are many materials for making filter membranes, which are divided into organic membranes (such as polysulfone hollow fiber membranes) and inorganic membranes (such as ceramic membranes). Membrane filters have high filtration accuracy, relatively stable particle size control, and backwashing is easy to restore performance. Therefore, it is extremely convenient to use and maintain.

    1 Filtration mechanism and influencing factors

    1.1 Filtration mechanism

    There are two main fluid filtration mechanisms. One is separation based on the size of the particles, such as interception, sieving, and surface capture; the other is adsorption, that is, the particles adhere to the filter under chemical/charge action. This requires each pharmaceutical factory to choose different filter membranes according to their actual needs.

    1.2 Factors affecting filtration

    1.2.1 Characteristics of fluid

    It is related to the characteristics of the fluid. For example, fluid viscosity and chemical/ionic composition, the greater the fluid viscosity, the slower the flow rate under the same pressure conditions, the more contact between the fluid and the membrane, the better the filtering effect; another example is the mixing of fluid and membrane/ The contact time also has a greater impact on the filtering effect. The longer the mixing/contact time, the better the filtering effect. In addition, it should be noted that the characteristics of the fluid only affect the adsorption and retention effect of the membrane on the fluid and not the elimination of particle size.

    1.2.2 Operating conditions

    It is related to actual operating conditions, such as particle flow rate and filtration pressure. To achieve a good filtration effect, generally choose a lower flow rate, the lower the flow rate, the better the retention effect. Practice has proved that the structure movement of the membrane is unfavorable to filtration. Once the structure of the membrane changes during the filtration process, particles and fibers can be precipitated from the deep filter, which will affect the filtration effect. However, the velocity/pressure difference only has an important effect on the adsorption retention, and has a relatively small effect on the size exclusion.

    1.2.3 Particle type

    The type of particles is also closely related to the filtering effect. There are two types of particles: deformable particles and non-deformable particles. Under a certain pressure, deformable particles will enter the filter membrane and cause more filter meshes to be blocked, thereby affecting the filtration effect, such as the filtration of gels. However, when immutable particles are filtered, a cake-like object is formed on the filter membrane.

    1.2.4 Filter membrane type

    Depending on the type of filtration membrane, the pore size and structure of different filtration membranes are different. Some membrane structures are rigid, and some membrane structures are movable. There is no unified national standard for the rated pore size of the pre-filtration membrane. Different manufacturers have their own definitions and methods. Therefore, you need to pay great attention to the selection and replacement of vendors. The same is the 0.22μm pre-filtration membrane. Choose filters from different manufacturers. The effect will be very different. The public pore size of sterilization filtration is defined by laws and regulations, and all businesses implement the same standard, and it is relatively simple to select and replace.

    1.2.5 Filter material

    It is related to the filter material. According to the relationship with water, the filter material is divided into two types: hydrophilic (water can be wetted) and hydrophobic (water can not be wetted). Hydrophilic filters are mainly used in water or water/organic solution mixing filtration and sterilization filtration, such as cellulose materials (regenerated cellulose, mixed cellulose ester), PVPP polycarbonate, PVDF modified polyvinylidene fluoride ; Hydrophobic filter is through the water is intercepted or "guided" into the filter membrane, mainly used in solvent, acid, alkali and chemical filtration, tank/equipment respirator, process gas, fermentation intake/exhaust filtration, such as PTFE polytetrafluoroethylene, PVDF polyvinylidene fluoride, polypropylene, polysulfone, polycarbonate, etc.

    2 Filter characteristics and structure

    Filters are generally divided into depth filters (primary filtration), surface filters (intermediate filtration) and membrane filters (depth filtration). The fibers of the depth filter sometimes fall off, and an exact pore size cannot be given. The thickness is generally 3-20mm, which usually has adsorption effect and has a large dirt bearing capacity; the fibers of the surface filter are generally thermally bonded or membranes Coated, can give rated pore size, relatively thin (<1 mm), and small adsorption capacity; the main characteristics of membrane filter are hard texture, not easy to break, tortuous channels and very high internal surface area, and have a certain The open porosity can be used for integrity testing, and is often used in deep filtration, such as sterile filters.