Fundamental of filtration

Fundamental of filtration

1. filter

Filtration is the operation of the solid and liquid separation by allowing the liquid in the suspension to pass through the pores of the porous medium under the action of an external force, and the solid particles in the suspension are trapped on the medium.

Description
1). wherein the porous medium is referred to as a filter medium; the treated suspension is referred to as a filter slurry; the solid particles retained by the filter medium in the filter slurry are referred to as filter cakes or filter cakes; the liquid passing through the filter medium is referred to as a filtrate.
2). driving force of the liquid through the filter medium can have gravity, pressure (or pressure difference) and centrifugal force;
3). The purpose of the filtration operation may be to obtain a clean liquid product, or to obtain a solid product.
4). Washing effect: recover the residual filtrate in the filter cake or remove the soluble salt in the filter cake.

2. Filter media

The filter medium acts to support the filter cake and allows the filtrate to pass through. The basic requirement is that it has sufficient mechanical strength and as little flow resistance as possible, and at the same time, it should have corresponding corrosion resistance and heat resistance.

Commonly used filter media in the industry:
1). Fabric medium: Also known as filter cloth, it is a fabric woven from natural fibers and synthetic fibers such as cotton, wool, silk, hemp, and a web woven from glass or metal wire. The minimum diameter at which such media can retain particles is 5-6 micron. Fabric media is the most widely used in industry.
2). Stacking medium: It is made up of various solid particles (sand, wood carbon, asbestos, diatomaceous earth) or non-woven fabrics, and is mostly used in deep bed filtration.
3). Porous solid medium: a solid material with many fine pores, such as a porous ceramic, a porous plastic, a tube or plate made of porous metal, and capable of intercepting fine particles.
4). Porous membrane: various organic polymer membranes and inorganic material membranes for membrane filtration. Widely used are two types of organic polymer membranes, cellulose acetate and aromatic amide. Can be used to trap 1 micron the following tiny particles.

3. Depth filtration and filter cake filtration

1). Filter cake filtration: The size of the particles in the suspension is mostly larger than the pores of the medium. The suspension is placed on one side of the filter medium during filtration. At the beginning of the filtration operation, some of the small particles enter the media channels and may pass through the channels without being trapped, leaving the filtrate still turbid. As the process progresses, the particles gradually build up on the media, forming a layer of particles called a filter cake. After the filter cake is formed, it becomes the medium that acts as the primary entrapment for the subsequent particles. Therefore, the ever-increasing filter cake is a truly effective filter medium, and the liquid that passes through the filter cake becomes a clear liquid.
2). Deep filtration: At this time, the particle size is much smaller than the size of the medium channel, and the particles easily enter the medium channel. However, since the tunnel is curved and elongated, the particles adhere to the wall of the tunnel under the action of surface force and electrostatic force as the fluid flows through the meandering tunnel. Therefore, in the depth filtration, the filter cake is not formed on the medium, and the solid particles are deposited inside the filter medium. This filtration is suitable for the treatment of suspensions with very low solids content.

4. Filter cake compressibility and filter aid

The compressibility of the filter cake refers to the phenomenon that the void ratio of the filter cake is significantly reduced after being pressed, which causes the filtration resistance to increase significantly when the filtration pressure is increased, and the greater the filtration pressure, the more serious this situation will be.
In addition, the particles contained in the suspension are very fine. When the filter enters the pores of the medium at the beginning of filtration, the pores are blocked. Even if it is not so severe, the filter cake formed by these fine particles is liquid. The permeability is also very poor, that is, the resistance is large, making filtration difficult.
In order to solve the above two problems, filter aids are often used in industrial filtration.