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    2021-10-21 17:42:22
    What is a Ceramic Filter?

    Abstract: Ceramic filter is a widely used filter that uses rigid microporous material as a filter element. It is composed of ceramic, corundum, silicon carbide, sand chip, and so on. The ceramic filter has the characteristics of good heat resistance, uniform gas distribution, large rigidity, and no exudate in liquid. It is mainly used in fluidized bed combustion, calcination, power generation through organic waste gasification, building materials, and other fields.


    Catalog:

    I Working Principle of Ceramic Filter
    II Main Features of Ceramic Filter
    III Structure and Material of Ceramic Filter
    IV Application of Ceramic Particulate Filter


    I Working Principle of Ceramic Filter 

    The ceramic filter is composed of a porous body with several main flow paths of fluid needed to be purified penetrating from one end face to the other end face and filter membrane installed on the inner wall surface of the main flow path. (1)The fluid needed to be purified flows into the main flow path through an opening on the end surface, then it passes through the filter membrane and the inside of the porous body to be purified and is taken out as purified fluid from the outer peripheral surface of the porous body. (2) The fluid needed to be purified flow into it through the outer peripheral surface of the porous body, then it passes through the inside of the porous body and the filter membrane to be purified, and is taken out as purified fluid from the opening on the end surface of the main flow path.

    II Main Features of Ceramic Filter

    1. Except for hydrofluoric acid and concentrated alkali, it has excellent corrosion resistance to all corrosive liquids;

    2. Good heat resistance, no thermal deformation, softening, or oxidation. It can still be used at higher temperatures;

    3. The gas is evenly distributed, and the pore size of 0.1~600mm can be formed as required;

    4. The rigidity is large, and it does not cause shape change and pore deformation under the action of fluid pressure;

    5. In the case of precision filtration (as a special case), in order to prevent from polluting the liquid, trace amounts of exudate are not allowed. When ceramic filters are used, there is usually no exudate, and it is also qualified according to the Food Sanitation Law. Consequently, it can be adopted in the poultry industry.

    6. The solid collected by the ceramic filter accumulates outside the filter layer or inside it, clogging the pores of the filter, and thus the filtering capacity will decrease. This clogging state can take several forms, such as solids collected on the surface; solids entering the inner layer of the filter; solids accumulated on the surface forming a rough surface (like a filter layer). In these cases, although filtering capacity is gradually decreasing, the degree of decrease is different.

    7.Although ceramic filters can be used in high-temperature areas that other porous bodies cannot reach, it is best used in high-temperature areas below 1000°C when filtering combustion exhaust gas, molten metal, and blowing gas into the nozzles of high-temperature furnaces.

    Figure 1: high-temperature strength of  ceramic filter

     

    8. For clean fluids, the amount of fluid penetration through the ceramic filter layer is proportional to the area and pressure difference, and inversely proportional to the plate thickness and viscosity coefficient. The filter (filter) layer is regarded as a collection of capillaries, and the fluid flow through the capillaries can be expressed by the following formula:

    v=(π·d4·ΔP)/(128η·l)=(π·d4·ΔP)/(128α·η·L)

    In addition, the relationship between the air binding rate ε and the number of capillaries per unit area N and the diameter of the capillary d can be expressed by the following formula:

    ε=(π/4)d2·N

    Therefore, the penetration amount V can be expressed by the following formula:

    V=N·v=(d2·ε·ΔP)/(32α·η·L)

    In the formula, 

    V——permeation volume per unit time per unit area (cm3/cm2·sec);ΔP——pressure difference (dyne/cm2);

    η——Viscosity coefficient (dyne·sec/cm2);

    L——The thickness of the filter (filter) layer (cm);

    d——diameter of the capillary tube (cm);

    l——length of the capillary tube (cm);

    α——curvature (1~3).

    In other words, the amount of fluid penetration through the ceramic filter layer is proportional to the porosity of the filter and the square of the diameter of the capillary. Increasing the amount of binder, expanding the range of particle diameter distribution, etc. will reduce the porosity ε and the amount of fluid penetration. Therefore, the amount of binder and particle diameter distribution should be used as the manufacturing conditions of the filter. 

    III Structure and Material of Ceramic Filter

    The research of ceramic filtration mainly focuses on two aspects: one is the material of the ceramic filter, including its manufacturing process and performance; the other is the structure of the ceramic filter, including its shape, placement position, filtering mechanism, and filtering effect.

    The selection of filter material should be based on the type of inclusions to be removed by the filter, and the creep resistance and thermal vibration resistance of the material should also be considered. A large number of experiments show that the material, porosity and internal surface roughness of the ceramic filter affect the filtering effect.

    The structure of the ceramic filter is determined by its material. According to different materials, the ceramic filter can be classified into two categories: ceramic foam filter and particulate ceramic filter.

    (1) Ceramic foam filter

    The open-pore volume of ceramic foam filters is 75% to 90% and is usually classified according to the number of pores per inch (ppi). For example, the ceramic foam filter with 10 ppi has a pore size of 1778 μm; the ceramic foam filter with 30 ppi has a pore size of 711 μm. The thickness of the ceramic foam filter is generally 25 mm. In the pouring system, there are vertical installations and horizontal installations. The structure is designed according to the specific usage.

    The materials used for ceramic foam filters are NCL-Mullite, ZrO2, Zr-SiO4, Al2O3, and so on. Using Al2O3 foam ceramics, it is not fragile during operation, and has good resistance to thermal vibration and cracking. It has strong resistance to creep deformation under the flow of high-temperature molten metal at 1700℃. Because of the high porosity of the open pores (75% to 90%) and the thin pore walls, the ceramic foam filter does not need to be preheated before it comes into contact with the molten metal.

    (2) Ceramic Particulate Filter

    The structure of the ceramic particulate filter is a support plate with upper and lower pores, with particle filler in the middle, and a layer of active adsorption material is plated on the particle filler, as shown in the figure below. The thickness of the support plate is generally 12 mm, and the pore diameter is 4.5 to 13 mm. The ceramic particulate filter replaces MgO or Al2O3 as the filler and uses the active adsorbent according to the type of inclusions to be filtered. 

    Figure2: structure of  ceramic particulate filter

     

    CaO refractory is a kind of good filler for ceramic particulate filters. It can not only use the principle of physical adsorption, but also remove inclusions through chemical reactions. However, there are two reasons that limit its wide use. First, it needs extremely high sintering temperatures (above 1800°C) to obtain the necessary density and mechanical strength. Second, it is easy to hydrate under normal temperature and atmosphere.

    The advantages of CaO refractory are high refractoriness, high alkalinity, good filterability of molten steel, and extremely rich resources. The way to improve the hydration resistance of CaO refractory for liquid steel filtration is as following:

    ①vigorously increase the sintering degree of CaO refractory, larger crystal size, and use ultra-high calcined or fused lime; 

    ②form a protective film on the surface of CaO; 

    ③dip tar or organic resin film on the CaO burned product as an intermediate transition measure;

    ④add a small amount of chemical additives to CaO in order to reduce the sintering temperature of CaO.

    IV Application of Ceramic Particulate Filter

    At present, ceramic filters are more and more widely used in many industrial fields, such as catalytic precious metal recovery, fluidized bed combustion, calcination, power generation through organic waste gasification, building materials, chemical industry, and purification of flue gas with a high temperature in industrial processes such as various industrial kilns and furnaces. The application of high-temperature flue gas purification will also appear in smelting, material production, and glass manufacturing often carried out under conditions close to atmospheric pressure.

    The most prominent application of ceramic filters is the purification of soot in the field of coal-fired power generation. Because of the increasing demand for electric power worldwide, coal is the main source of solving power problems. Regarding the non-renewability of such fossil fuels, it is necessary to maximize the efficiency of power generation and reduce atmospheric pollution for all countries in the world, especially China, a large coal-burning country. Through circulating fluidized bed (CFBC) power generation and coal gasification (IGCC) power generation, and their combined power generation, improving power generation efficiency can be achieved. The power generation process of coal gasification is different from a traditional steam engine. After the coal is heated and vaporized, the coal gas needs to be purified before it enters the gas-fired generator (gas-fired engine). That is, any dust or other impurities must be removed. Most power plants limit the allowable dust concentration entering gas-fired turbines to less than 5 mg/m3.

    In theory, it is better to be less than 1 mg/m3. The working temperature of the dust removal system is often 350~1000℃, and the pressure is 1~2.5 SPa. Therefore, to achieve such a high purification effect under such high temperature and high pressure, ceramic filters must become the first choice.

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