First, the principle of filtration

(1) Basic concept filtration is the operation of separating a suspension by using a porous medium. During operation, under the action of external force, the liquid in the suspension flows out through the pores of the medium, and the solid particles are trapped, thereby achieving separation. The suspension to be filtered is generally referred to as a filter; the porous medium employed is referred to as a filter medium; the liquid passing through the pores of the medium is referred to as a filtrate; the retained solid is referred to as a filter cake or filter residue.

The driving force of the filtering operation is the pressure difference between the upstream and downstream sides of the filter medium. The methods for generating the pressure difference are as follows:

1 using the indenter of the filter itself;
2 pressurizing the surface of the filter;
3 vacuuming on the downstream side of the filter medium;
4Using inertial centrifugal force.

Filtering operations can be divided into two categories based on the principle of action:

1 The number of pores of the sieve filtration filter medium is smaller than the diameter of the solid particles, and the solid particles are removed from the suspension by sieve analysis.

The fine particles flowed out at the initial stage of the filtration, and the filtrate was turbid. As the cake layer is formed and thickened, the filtrate gradually becomes clear. As the sieve holes are gradually blocked, the filtration speed is reduced. During the filtration process, when the filter cake layer is formed, the sieve layer is produced by the cake layer, and the filter medium loses the sieve separation effect, and only serves to support the cake layer, and the cake layer is filtered.

2 adsorption filtration (deep bed filtration): the mesh size of the filter medium is larger than the diameter of the solid particles, and the solid particles are adsorbed by the surface of the medium after entering the pores of the filter medium.

In actual production, the sedimentation and adsorption simultaneously act, and the adsorption filter medium intercepts larger particles; the sieve layer of the filter medium can adsorb smaller particles.

Second, the filter medium and filter aid

1. Filter medium? The main function of the filter medium is to support the filter cake. It must have a porous structure, sufficient mechanical strength and as little flow resistance as possible, and corrosion resistance.

Commonly used filter media are of the following types:

1 fabric medium, such as industrial filter cloth, wire mesh, etc.;

2 granular medium, such as perlite powder, cellulose, diatomaceous earth, etc.;

3 solid cardboard, such as bleached wood board, synthetic fiber board, etc.;

4 filter membrane, composed of cellulose and other polymers. The granular medium is used as a filter aid for pre-coating on the surface of the fabric medium for coarse filtration; solid cardboard medium is mostly used for semi-fine filtration and fine filtration; and the filtration membrane medium is used for fine filtration and ultra-fine filtration.

2. The filter aid pre-applies some hard granules to the surface of the filter medium or to the filter slurry to form a relatively hard and loose filter cake, so that the filtrate can pass smoothly. The granule is called filter aid. . Requirements for filter aids: The filter aid is a hard, loose structure of powdery or fibrous solids. The shape aid should be well suspended in the feed. The particles are suitable in size and contain no soluble. The substance in the filtrate.

Commonly used filter aids are diatomaceous earth, cellulose, and the like.

The purpose of using a filter aid is to prevent clogging of the filter media pores or to reduce the filtration resistance of the filter cake.

The filter aids and methods are pre-coated and pre-mixed.

The filter aids used in the filtration operations in wine production are mainly diatomaceous earth and cellulose. Diatomaceous earth is a fossil of aquatic plant diatoms, which is composed of the remains of paleontological sedimentary diatoms. Its main component is SiO2 (85-90%), which is grayish white powder, the particle size is generally 30-40μm, the minimum is 2μm, the maximum is 300-400μm, and the specific gravity is 0.31-0.7.

Its main features are:

1 The main component is SiO2, which has good chemical stability. It is insoluble in any strong acid other than hydrofluoric acid and soluble in caustic. For different concentrations and different temperatures of the filter can be applied;

2 When used as a filter aid, a higher filtration speed and a desired clarity are obtained. Diatomaceous earth has numerous small and complex pores with a porosity of 80-90%, which provides a large space for trapping solid particles. In addition, the skeleton is hard, and the formed filter cake has substantially incompressible characteristics, and can still maintain a porous state under the condition of pressure change, and the filtration resistance is small, and a high filtration speed and an ideal clarity can be obtained;

Third, the filtering speed

1. Filtration speed and filtration rate The volume of the filtrate passing through the unit filtration area per unit time is called the filtration speed, ie: ?u=V/Atm3/m2 where: A——filtration area, m2;?t—filtration time, s;?V——the volume of the filtrate flowing out at time t, and the volume of filtrate obtained per unit time in m3 is called the filtration rate, indicating the production capacity of the filtration equipment. ? Filtration rate? Q = V / t = uA? Its unit m3 / h and L / min.

2. Factors affecting the filtration speed

1 The viscosity viscosity of the filter is larger, the greater the filtration resistance, the smaller the filtration speed;

2 filter cake thickness The greater the thickness of the filter cake, the greater the resistance, the smaller the filtration speed. When the thickness reaches a certain level, the filtration is terminated;

3 filter cake nature of the incompressible filter cake particles are hard, the flow path through which the filtrate passes does not become smaller due to the increase of pressure, the resistance remains basically unchanged, or the resistance increases with the continuous increase of filtration time, the filtration speed is basically with the pressure The increase is proportionally increased; the small particles will block the pores under the action of large pressure, so the initial pressure of the filtration should not be too large to avoid premature clogging of the flow passage.

Fourth, the way to filter operations

Constant pressure filtration

The filtration operation is carried out under a constant pressure difference, called constant pressure filtration. Constant pressure filtration is the most commonly used filtration method. During the filtration process, as the cake layer is continuously thickened, the resistance increases greatly, and the filtration speed will gradually decrease due to the constant pressure difference.

2. Constant speed filtration

The filtration operation is carried out at a constant speed and is called constant velocity filtration. As the filtration time continues, the thickness of the cake layer increases, the resistance increases, and the filtration pressure difference increases linearly with the continuation of time. Filtration equipment has limited pressure resistance and pressure cannot be too high. For compressible filter cakes, an excessive increase in pressure does not increase the speed, which in turn causes the filtration to terminate.

3. Constant pressure and constant pressure filtration

This filtering method starts with constant speed filtration at a lower speed to prevent the pressure from rising too early to form a flow channel blockage, and then uses constant pressure filtration when the pressure rises to a given value. The metering pump is used to maintain a constant speed at the initial stage of filtration. As the time continues, the pump outlet pressure rises. If the pressure rises to a set value that enables the bypass valve to automatically open, the branch opens and some of the slurry returns to the pump inlet. The filter entering the filter press is reduced, and the inlet pressure of the filter press is maintained constant, thereby starting the constant pressure filtration.