What Is Vacuum Filtration?

Vacuum filtration is based on the principle that the vaporization temperature of oil and water is different (the vaporization temperature of water is lower than the vaporization temperature of oil) at the same pressure (vacuum degree), and a vacuum oil filter is made according to this principle.

Vacuum filtration is the most widely used filtration method in theory and practice. Like other filter cake filtration methods, vacuum filtration technology also uses porous filter media to support the filter cake. The difference is that the driving force used in vacuum filtration is small, so it is suitable for some special applications. ^[1]

Vacuum filtration is due to the action of a vacuum machine, and a negative pressure is drawn in the filter drum of the filter to form a pressure difference between the two sides of the filter medium layer (filter cloth). With the rotation of the filter, the mother liquor in the lye is pumped away, and the heavy alkali is adsorbed on the filter cloth and finally scraped

Vacuum filtration is to achieve a certain degree of negative pressure (vacuum) on the side of the filter medium to discharge the filtrate to achieve solid-liquid separation, so its driving force is small, generally 0.04 to 0.06 MPa, and in some cases, it can reach 0.08 MPa, due to the lower pressure drop on both sides of the filter cake, the filtration speed is slower, and the moisture content of the fine material filter cake is higher, which is the main disadvantage of the vacuum filter; but its advantage is that it can be used in relatively simple Continuous operation under mechanical conditions, and in most cases can obtain a relatively satisfactory working index. Therefore, compared with other types of filters (such as filter presses), vacuum filters have long been favored by users. Unless the filter press has made major breakthroughs in continuous operation, mechanical properties, manufacturing and operating costs, and can meet the comprehensive requirements of the application sector, the important role of vacuum filtration in the field of solid-liquid separation will be difficult to shake.

The working cycle of the vacuum filter can generally be divided into the following phases: cake forming phase; dehydration phase; washing phase, compaction phase; drying phase; cake unloading phase. Among them, the washing, compacting, and drying stages are determined by actual needs, and most of the vacuum filters are used for cake formation, dehydration, and unloading (the unloading stage is not included in the filtration cycle of the horizontal belt vacuum filter). Have the basic working process. In the filtering cycle, the share of time occupied by each operation varies with the filter.

The cake forming stage is a filtration process in the strict sense (filtration in its own sense refers to the process of continuously separating the filtrate from the solid material through the medium). In this process, the solid material is vacuumed (lower feed) or vacuum. Combined with gravity (upper feed) and adsorbed on the surface of the filter medium, gradually forming a filter cake of a certain thickness; as the filter cake gradually thickens, the corresponding filtration resistance gradually increases, so the filtration speed at this stage (I.e., the volume of filtrate passed by the filter medium per unit area per unit time) gradually decreased. However, the main task at this stage is to form a filter cake of a certain thickness to achieve the desired processing capacity. After the cake is formed, it leaves the feed tank and enters the dehydration stage. During this stage, the relative saturation of the filter cake (ie, the ratio of the volume of filter cake moisture to the total volume of filter cake pores) is to be reduced from 100% at the beginning to about 10% to 20%. Under vacuum, most of the pores occupied by water are replaced by air.

The excluded water is basically gravity water and pore water, because vacuum suction is not enough to exclude surface water and capillary water. The hydrodynamic characteristics of the filter cake during the dehydration stage are the displacement of two fluids in a porous medium. For the gas used as a replacement medium, the effective permeability gradually increases, and the flow resistance gradually decreases, but for the liquid to be displaced, the situation is exactly the opposite.

Use air to expel the liquid in the pores. On the one hand, it is possible to reduce the water content of the filter cake, but on the other hand, it is very easy to cause the decrease of the vacuum degree, which is not conducive to the further removal of water. This is because the gas easily penetrates the filter cake and causes the cake to crack. Therefore, how to prevent cracking of the filter cake in the dehydration stage is a very important issue in the actual filtration process. After the filter cake is dehydrated, if there is no other treatment, it will enter the unloading stage. The tasks at this stage; one is to remove the filter cake; the other is to restore the permeability of the filter medium so that it enters the next filtration cycle. Therefore, how to remove the filter cake with the lowest remaining moisture and maximize the recovery of the filtration medium's permeability (that is, avoid clogging) are the two major factors to consider when choosing the cake removal method.

Realizing the above three basic stages on a vacuum filter is a relatively simple matter. This is because the pressure used in vacuum filtration is low, and the conversion between stages does not require high mechanical requirements. However, in the actual filtration cycle, the phases are not distinct, but there are certain transition phases. Therefore, in the design of the filter, the dehydration zone should be enlarged as much as possible while the other zones should be reduced. For example, if the feed concentration is sufficient to ensure the formation of a certain thickness of filter cake, the time taken for the cake formation phase can be appropriately reduced and the dehydration time can be increased. As for the size of the transition zone, it mainly depends on the mechanical switching performance of the distribution head. Improvement can also increase dehydration time. ^[1]

The vacuum filter is a device that performs normal pressure on the upstream of the filter medium and vacuum on the downstream side. The filtration pressure is formed by the pressure difference between the upstream and downstream sides to perform solid and liquid separation. The vacuum degree commonly used in vacuum filters is 0.05 ~ 0.08MPa, but there are also more than 0.09MPa. There are two types of vacuum filters: batch and continuous. Intermittent and continuous vacuum filters have their own characteristics, but continuous vacuum filters are more widely used. Common continuous vacuum filters include drum type and rotary disc type.

The advantages of the vacuum filter are as follows: low labor intensity; can directly observe the filtering situation, find problems in time, and facilitate inspection; low maintenance costs; continuous vacuum filter has high working efficiency.

The disadvantages of the vacuum filter are as follows: can not filter the low boiling point filtrate material: can not filter the difficult to filter material that will form a compressible filter cake; the vacuum system needs frequent maintenance; continuous vacuum filter if the filter process If the concentration of solid particles and particle size distribution in the slurry vary greatly, the filtering effect is poor.

Vacuum filtration drum vacuum filter

The rotary vacuum filter is shown in the right figure (a). Its body is convertible

Rotary Vacuum Filter

A moving horizontal cylinder (shown in section (b) on the right) is called a rotating drum, with a diameter of 0.3 to 4.5 m and a length of 3 to 6 m. The outer surface of the cylinder is composed of a perforated plate or a special drainage member, and the top is covered with a filter cloth. The inside of the cylinder is divided into a number of fan-shaped cells. Each cell has a straw that communicates with the channels in the hollow shaft, and the channels in the hollow shaft lead axially to the rotating disk located at the shaft end and rotating with the shaft. The rotating disc is closely matched with the fixed disc to form a special rotary valve, called a distribution head, as shown in the right figure (c). The fixed plate of the distribution head is divided into several arc-shaped gaps, which are respectively communicated with the pressure reducing pipe, the washing liquid storage tank and the compressed air pipeline.

When the drum rotates, vacuum and pressurization are obtained in the sector by the function of the dispensing head, so that the operations such as filtering and washing can be controlled in sequence.

The advantages of rotary vacuum filter: continuous production, high degree of mechanization; can be made into various types using different materials according to the nature of the material and liquid, process requirements to meet different filtration requirements (usually, for suspensions The materials with medium particle size and medium viscosity are not suitable for rotary vacuum filter); filter thickness and washing effect can be controlled by adjusting the rotating speed of the drum; filter cloth loss is smaller than other types of filter.

The shortcomings of rotary vacuum filter: The filtering driving force is small, it just uses vacuum as the driving force. Due to the loss of pipeline resistance, the maximum does not exceed 80kPa, generally 26.7 ~ 66.7kPa, so it is not easy to drain, and the final humidity of the filter cake is generally above 20%; The equipment is complicated to manufacture and the main equipment and auxiliary vacuum equipment are expensive. The electric energy consumed for vacuuming is high, and the larger the filtering area, the more difficult it is to manufacture. The largest filtration area currently produced in China is about 50

, Generally 5 40

.

Vacuum filter turntable vacuum filter

As shown in the figure on the right, a schematic diagram of a rotary vacuum filter is shown. It consists of a set mounted on a horizontal shaft

Turntable vacuum filter

It is composed of a filter disc (or turntable) that rotates with the axis. The structure and operating principle of the rotary vacuum filter and its rotary disk are similar to those of the rotary vacuum filter. Each sector of the disc has its own outlet pipe leading to the central axis, and when several discs are connected together, the exit of the sector of a turntable and the corresponding exits of other discs of the same phase angle form a continuous channel. Similar to rotary vacuum filters, these continuous channels are also connected to shaft-end rotary valves (dispensing heads). The operation of rotary vacuum filters is very similar to that of rotary vacuum filters. Each turntable is equivalent to a rotating drum, and the operation cycle is also controlled by a rotary valve. Each turntable has its own filter cake discharge device, but the discharge is more difficult.

Turntable vacuum filter has a very large filtration area, which can be as large as 85

, Its unit filtration area occupies less land, filter cloth is easy to replace, consume less, and energy consumption is also low. The disadvantage is that the filter cake is poorly washed, and the wash water and the suspension are easily mixed in the filter tank. ^[3]