Filtration can be divided into constant pressure filtration and constant speed filtration. Constant pressure filtration refers to filtration under a constant pressure difference. As the filtration progresses, the thickness of the filter cake gradually increases, the resistance increases and the filtration rate continues to decrease. Constant speed filtration refers to the filtration process that maintains the filtration rate unchanged. In order to maintain the filtration rate constant, the pressure difference must be continuously increased to overcome the rising resistance due to the thickening of the filter cake. Since the pressure difference is constantly changing, constant-speed filtration is difficult to control, so constant-pressure filtration is generally used in production. Sometimes, in order to avoid clogging and damage of the filter cloth due to high pressure difference in the initial stage of filtration, the operation method of first constant speed and then constant pressure can also be adopted.
Filtration rate refers to the volume of filtrate that can be obtained per unit time and unit filtration area. The greater the filtration driving force, the faster the filtration rate, the greater the filtration resistance, and the slower the filtration rate. In differential pressure filtration, the driving force is the differential pressure, and the resistance is related to many factors such as the structure and thickness of the filter cake and the nature of the filtrate. The driving force and resistance directly affect the filtration rate. The following mainly analyzes the influence of various factors on the filtration rate from three aspects.
1. The nature of the filter medium
The influence of filter media is mainly manifested in the resistance and efficiency of filtration. Since the concentration and particle size in different filtrates are different, if the pores of the selected filter medium are much larger than the particle size, the required filtering effect will not be achieved. If the pores of the selected filter medium are much smaller than the size of the particles, the filter medium will be blocked quickly, greatly shortening the service life. For example, to filter paint with high concentration, we generally choose String Wound Cartridge instead of Pleated Filter or Melt Blown Cartridge. The reason is that Pleated Filter and Melt Blown Cartridge have small pores, which are suitable for filtering liquids with small particles and low concentration liquid. If filter the liquid with too high concentration, it will be blocked quickly, shortening the service life.Therefore, we have to select a suitable filter medium for filtration treatment according to the concentration in the filtrate and the size of the particles.
2. Filtered pressure difference
Generally, for incompressible filter cakes, increasing the driving force can increase the filtering rate. But for compressible filter cake, pressure can not effectively increase the filtration rate. In the filtration operation, it can be divided into pressure filtration, vacuum filtration, gravity filtration and centrifugal filtration according to the different driving force. The filtration rate can be improved by increasing pressure difference appropriately. By the way, HONGTEK' s High Flow Filter, Pleated Filter, Melt Blown Cartridge, String Wound Cartridge, Carbon Filter, Filter Bag, Ultrafiltration Membrane Module are adopted pressure filtration, while Membrane Bioreactor (MBR) Module are adopted vacuum filtration.
3. The nature of the suspension
The viscosity of the suspension has a greater impact on the filtration rate, the smaller the viscosity, the faster the filtration rate. The two factors that affect the viscosity the most are temperature and concentration. The higher the temperature in the liquid, the lower the viscosity, otherwise the higher the viscosity. In some areas when the liquid is filtered with low temperature, sometime the liquid needs to be heated to an appropriate temperature to ensure that the filtration rate meets the requirements. The greater the concentration in the liquid, the greater the viscosity, otherwise the lower the viscosity. If the liquid concentration is too high, we can properly dilute the liquid without affecting the properties of the liquid to make the filtration rate reach the expected standard.
All in all, the filtration rate is affected by the nature of the suspension, the pressure difference of the filtration and the nature of the filter medium. Therefore, we should solve these three problems, ensure the accuracy and efficiency of filtration operation and separation in the filtration process, improve the production capacity of filtration equipment, reduce power consumption and the cost of filtration.