Filtering Principle Of Metal Filter Element

This blog mainly talks about the filtering principle of the metal filter element. The main filter elements involved are stainless steel pleated filter element, stainless steel sintered filter element, stainless steel Johnson screen(wedge wire screen) and so on.

Fundamental Principles of Filtration - Folding Filter, Sintered Filter, Johnson Screen

Filtration is a process in which suspension is separated from the liquid through a permeable medium capable of intercepting solid particles.

(1) Characteristics of the filtration process

From the point of view of physics, filtration operation belongs to the flow process, that is, the flow process of multiphase fluid through porous media. This process has the following two distinct characteristics.

The flow of fluid through porous media (including filter media and cake, mostly stainless steel pleated filter element) belongs to very slow motion (i.e., retention motion).

There are two kinds of factors affecting the flow, one is macro-hydrodynamic factors, such as the characteristics of filter media, cake structure, pressure difference, the viscosity of filtrate, and the other is micro-physical and chemical factors, such as electrodynamic phenomena, capillary phenomena, flocculation phenomena and so on.

The larger the particle size of solid, the more dominant the macro-factors are. On the contrary, microphysical and chemical factors dominate.

The solid particles suspended in the fluid are continuously deposited on the pore or surface of the filter medium, and the filter cake deposited on the surface of the medium is continuously compressed. Therefore, with the filtration process, the flow resistance increases. The pressure difference inside and outside the stainless steel pleated filter element becomes larger.

(2) Driving Force of Filtration Process

In order to filter and obtain the liquid flow through the filter medium, it is necessary to maintain a certain pressure difference on both sides of the filter medium to overcome the resistance of the filtration process. There are four types of impetus infiltration operation:

Gravity

Vacuum Degree

Pressure

Centrifugal Force

Correspondingly, filtration operations are called gravity filtration, vacuum filtration, pressure filtration and centrifugal filtration.

Gravity filtration refers to the operation of suspension as a driving force of the process by virtue of its own column height. Because the pressure difference provided by slurry column is generally low, about 4.9 *104Pa, it is less used.

Vacuum filtration is accomplished by varying degrees of vacuum on both sides of the filter medium. Usually, the air pressure is on the side contacting the filter slurry, and the back side of the filter surface is connected with the vacuum source. The commonly used vacuum is (5.33-8.00)*104Pa.

Pressure for pressure filtration is provided by a compressor or pump. If the compressor is used to supply pressure, the commonly used filtration pressure difference is (4.9-29.4)*104Pa; when supplied by a pump, it usually does not exceed 4.9*104Pa.

The pressure difference of centrifugal filtration is provided by a centrifuge with a filter medium. The commonly used strength is 1.5 x 106 Pa.

(3) Classification of filtration process

There are two main forms of filtration operation in the industry: cake filtration and deep filtration.

1. Filter Cake Filtration

(1) Characteristics of cake filtration

After the slurry passes through the filter medium, the solid particles are intercepted by the filter medium, and a filter cake with a thickness of 6 mm or more is formed on the surface of the medium. Most industrial processes belong to cake filtration.

Cake filtration is essentially a surface filtration mechanism, which can be accomplished in two steps: first, clean filter media acts, solid particles, and liquids are 90. Angular flow to the filter medium, through the filtering effect of the filter medium, the solid particles whose particle size is equal to or slightly larger than the pore of the medium are intercepted, deposited on the surface of the medium, and many narrow channels are formed to intercept the smaller particles in the liquid flow, resulting in more and more particles deposited on the surface of the medium, forming filter cake.

Subsequently, the formed cake acts as a filter medium in the filtration process of the suspension added continuously.

Cake filtration is mainly suitable for suspensions with high solid particle concentration (volume fraction greater than 1%). This is because in the process of cake filtration, for the dilute suspension, the solid material with smaller particle size may pass through the pore of the filter medium or deposit in the pore, causing a blockage.

In order to effectively prevent this phenomenon, it is often necessary to take certain measures. Usually, filter aids are used to form a pre-coated layer on the surface of the filter media (i.e. the initial cake layer) to solve this problem.

It should be pointed out here that although Johnson screen filter element is surface filtration, it can not form a filter cake. Johnson screen has the advantage of easy blockage and cleaning. Johnson screen is the best choice if it is filtered under special circumstances.

Another disadvantage of cake filtration is that in the process of filtration, the resistance of filtrate flow will gradually increase so that when filtering a certain amount of slurry, it must increase the filtering time or improve the filtering impetus.

In order to overcome this shortcoming, many new methods of cake filtration have been developed to restrict the formation or thickening of the cake layer. Typically, hydraulic or mechanical methods are used to prevent the formation of cake layer or maintain a thin layer of cake, such as "delayed cake filtration" proposed by Tiller and Cheng. Mechanical removal methods such as brush, liquid jet or scraper are used to disturb and return solid particles to the suspension so that the suspension is continuously enriched and solid-liquid separation is achieved.

At this moment, the direction of particle movement is parallel to the surface of the filter medium, while the liquid moves towards the filter medium at a certain angle. Other methods to restrict the thickening or formation of filter cakes depend on their industrial use. Generally, there are the following:

1. The filter cake is removed by gravity, centrifugal force or electrophoretic force tangent or disconnected with the filter medium.

2. Removal of filter cake by intermittent reverse flow.  

3. Vibration prevents cake deposition

4. Cross-flow filtration is used to make the slurry and the filter medium move tangentially so that the filter cake can be cut continuously.

These measures have been used in a large number of practices and show that they can effectively improve the filtering effect.

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This product is called solid liquid separator or manure separator. Its core separator -  drum screen, is manufactured by our second workshop (wedge wire screen, wedge wire filter production workshop), and then assembled in the third workshop (filter and solid-liquid separator production assembly workshop).

(2) Structure and characteristics of filter cake

It can be seen from the foregoing that in the process of cake filtration, the structural characteristics of cake have a decisive influence on the filtration.

The filter cake can be divided into two categories: incompressible filter cake and compressible filter cake. The incompressible filter cake means that the characteristic parameters of the filter cake are not affected by the compressive force of the solid particles, while the compressible filter cake is affected by the compressive force of the solid particles.

When the cake is incompressible, the fluid resistance produced by a certain volume of cake is neither significantly affected by the compressive force of solid particles nor by the deposition rate of solid particles, which is relatively beneficial to filtration. In practice, the incompressible filter cake is an ideal state, because the filter cake composed of solid particles which are not easy to deform may also show some compressibility due to the rearrangement of particles when they are compressed.

For compressible filter cake, when the pressure difference or flow rate at both ends of the cake increases, it will promote the formation of more compact filter cake, so it has greater resistance. Therefore, the material with high compressibility can only slightly increase the pressure, so that the filtration rate can be limited increased. If it exceeds a certain critical pressure, the filtration rate will decrease, and the filtration effect will be affected.

(3) Filtration steps of filter cake

A complete cake filtration operation consists of four steps: filtration, washing, dehumidification, and unloading. The following focuses on the process of cake washing and dehydration.

Filter cake washing  

When the filter cake is finished, a part of the filtrate is usually kept between the cake particles by the action of surface tension. This part of the filtrate generally needs to be replaced by another liquid (also called washing liquid, which can completely dissolve with the filtrate, usually water) through the cake layer without impurities or near impurities. This operation is called cake washing. Cheng.

The purpose of washing filter cake in industrial production is to recover valuable filtrate, which is a product or to remove impurities in filter cake. At this time, the product is not allowed to be polluted by filtrate.

There are two ways to wash filter cake, one is to wash the cake directly with washing liquid on the filter, the other is to remove the cake from the filter and put it in the storage tank to mix and stir with washing liquid, and then remove the washing liquid by filtering method.

The following three methods are usually used for cake washing.

A. Pure replacement washing. Substitution washing includes washing liquid directly on the surface of filter cake and subsequently infiltrating into the pore of filter cake for replacement and combination, thus removing the dissolved substances.

B. Countercurrent washing. It is a method of keeping filter cake and detergent in a reverse flow system. It is usually used on the occasion of utilizing valuable detergent effectively.

C. The filter cake is made into slurry by washing liquid and then filtered twice. In some cases, such as the resistance of liquid flow filter cake is too large to be washed by replacement, or the time required for washing by replacement is too long to reach the expected level of solute removal, or because the cake is cracked, replacement washing cannot be used.

Dewetting

After washing, the filter cake usually has a high moisture content, and generally needs to be dehumidified (or dehydrated). The desaturation force is applied to the filter cake to remove the washing residue remaining in the pore of the filter cake, which is called cake dehydration.

The basic methods of dehydration are as follows.

A. Vacuum suction or air (other gases or superheated steam may be considered when air is not available) is used on the side of the cake to blow the cake, and the liquid in the cake is carried away by the gas.

B. Pressing method. Mechanical, hydraulic, mechanical and hydraulic combination and rheological hydraulic force are used as driving forces to squeeze out the liquid in the filter cake.

C. Use capillary dehydration.

Any kind of filter must be able to filter, wash, dehumidify and unload these four operations well. Their suspensions can be carried out either intermittently or continuously, and selected according to production needs.

2. Deep Filtration

In the metal filter element, there is a stainless steel filter element product - sintered filter element, which is often seen in the industry.

There are two kinds of the sintered filter element, stainless steel wire mesh sintered filter element, and metal powder sintered filter element. Among them, metal powder sintered filter element is a typical deep filter product.

Deep filtration is a method of filtering by using the inner surface of the filter medium (i.e. pore). As shown in the figure, solid particles are trapped in the inner pore of the filter medium instead of forming a filter cake on the surface of the medium.

Deep filtration media generally use sand grains of O.4-2.5mm or other porous media. The direction of the slurry flow is usually downward. Intermittent operation is mostly used in deep filtration.

Deep filtration is often used when slurry concentration is very thin and solid particle size is very fine. Such as drinking water purification and the removal of very fine solid particles from synthetic fiber spinning belong to this category. When deep filtration is used, the slurry concentration is generally less than 0.1% (volume fraction). In addition, in deep filtration, the particle size of solid particles is smaller than the pore diameter of filter media.

The disadvantage of the deep filtration process is the blockage of filter media. Therefore, in order to maintain the required flow rate, it is necessary to continuously increase energy to overcome the increasing resistance. When the required energy increases to the maximum available value, measures should be taken to clean the filter media. Therefore, deep filtration includes two processes: filtration and cleaning.

When the filtering environment is bad, the wedge wire screen filter (Johnson screen filter) is the best choice.

For stainless steel filters, including Johnson screen filters, pleated filters, sintered filters and so on, each has its own advantages, we should choose according to the specific use.