Influencing factors of ultrafiltration flux

1. Feed liquid flow rate

Although increasing the flow rate of feed liquid is beneficial to reduce concentration polarization and increase permeation flux, it needs to increase feed liquid pressure and increase energy consumption. Generally, the flow velocity in the turbulent flow system is controlled at 1-3m/s.

2. Operating pressure

The relationship between ultrafiltration membrane permeation flux and operating pressure depends on the properties of membrane and gel layer. The ultrafiltration process is a gelation model, and the membrane permeation flux has nothing to do with pressure, and the flux at this time becomes the critical permeation flux. The actual operating pressure should be near the limit flux, and the operating pressure at this time is about 0.5-0.6Mpa.

In the ultrafiltration process, only when the working pressure reaches a certain level, can the small molecules in the liquid material be separated through the membrane. If the working pressure is too low, the output of filtrate is small, which cannot meet the normal production. When the working pressure is too high, the thickness of the polarized layer will be increased to offset the speed-up effect of supercharging. At the same time, the deposited layer deposited on the membrane will be compacted, making it difficult to be washed, and the pores of the membrane will be blocked quickly, affecting the ultra-thickness. In addition, each ultrafiltration membrane has its pressure resistance range, and it should be used within this range.

3. Temperature

The operating temperature mainly depends on the chemical and physical properties of the materials being processed. Since high temperature can reduce the viscosity of the feed liquid, increase the mass transfer efficiency, and increase the permeation flux, it should be operated at the highest allowable temperature.
When the temperature rises, it can partially overcome the intermolecular force and reduce the viscosity. At the same time, it also affects the working performance of the membrane and increases the permeability. Excessive temperature will also affect the life of the ultrafiltration membrane.

4. Operation cycle

With the progress of the ultrafiltration process, a gel layer is gradually formed on the surface of the membrane, which reduces the permeation flux. When the flux reaches a certain minimum value, flushing is required, and this period of time becomes the operating cycle. The change in the operating cycle is related to the cleaning situation.

5. Feed concentration

As the ultrafiltration process proceeds. The concentration of the subject stream gradually increases. At this time, the viscosity becomes larger, which increases the thickness of the gel layer, thereby affecting the permeation flux. Therefore, the maximum allowable concentration should be set for the main liquid flow.
The concentration of the feed liquid directly affects the filtration rate. The flux of ultrafiltration has a linear relationship with the logarithm of the concentration. Generally speaking, as the concentration of the feed liquid increases, the viscosity of the feed liquid will increase, and the time for forming a polarized layer during ultrafiltration will be shortened, thereby reducing the speed and efficiency of ultrafiltration. Therefore, attention should be paid to controlling the concentration of the feed liquid during ultrafiltration.

6. Pretreatment of feed liquid

In order to improve the permeation flux of the membrane and ensure the normal and stable operation of the ultrafiltration membrane, the feed liquid should be pretreated as needed. The effect of pretreatment directly affects the pollution degree of the ultrafiltration membrane, the production capacity of the system and the service life of the ultrafiltration membrane. Pretreatment is generally carried out by high-speed centrifugation, microfiltration, pH adjustment, heat treatment, refrigeration or a combination of methods. The flocculant method developed in recent years can remove organic macromolecular unstable substances such as tannins, pigments, and pectin in the extract.

7. Membrane cleaning

The membrane must be flushed regularly to maintain a certain amount of permeation and extend the life of the membrane. Generally, under the specified feed liquid and pressure, within the allowable pH range, and the temperature does not exceed 60 ° C, the ultrafiltration membrane can be used for 12-18 months. If the membrane is not cleaned well, the life of the membrane will be shortened.

8. UF membrane pore size

The choice of ultrafiltration membrane pore size should be consistent with the size of the target components in the liquid medicine. If the pore size is too large, the separation effect will be poor, and the impurity content will be too high, which will affect the clarity and stability. If the pore size is too small, the permeability of active ingredients is low and the loss is large.

9. Elution amount

The amount of elution affects the content of target components in the filtrate. If the elution amount is too small, there will be more target components left in the concentrated solution and the loss will be greater; if the elution amount is too large, although the recovery rate will increase, post-treatment may be required or the original post-treatment process time will be reduced. To extend, attention should be paid to coordinating the relationship between them.

When higher yields are required, especially when studying sample objects in the microgram range, it is recommended that the following points be considered:

  1. According to the sample processing capacity, select the smallest available ultrafiltration container, repeatedly add samples in the small container, and repeat ultrafiltration.
  2. Within the appropriate range, select the ultrafiltration membrane with the lowest molecular weight cut-off.
  3. If possible, use a horizontal rotor instead of an angle rotor for centrifugation, which can reduce the surface area of the solution in contact with the centrifuge tube during centrifugation.
  4. Reduce the pressure or centrifugal force to about half of the maximum recommended value.
  5. Avoid over-concentration. The smaller the final volume, the more difficult it is to recover completely. If possible, after the first recovery, rinse the container with one or more drops of buffer, and then recover again.
  6. Pre-soak the ultrafiltration vessel overnight with 5% SDS, Tween 20, or Triton X dissolved in distilled water, then rinse thoroughly before use.

The main problem of ultrafiltration technology is ultrafiltration membrane fouling. As the performance of the ultrafiltration membrane increases with time during use, the permeation flow rate of the membrane will decrease rapidly, and the rejection rate of the solute will also increase significantly, which is caused by the inferior ratio of the membrane and the epiphytic fouling. The membrane is prone to fouling and clogging, and the service life of the membrane is relatively short. It is necessary to study the regeneration of the ultrafiltration membrane. On the one hand, the solution is to strengthen the pretreatment of the feed liquid, and on the other hand, it is to study the cleaning method of the ultrafiltration membrane.
In the application of ultrafiltration, it is necessary to clarify what is intercepted and what is removed. It is necessary to focus on the study of the interception rate of the target component, pay close attention to the technical standards and scope of application of various types of membranes, and objectively evaluate whether the treatment rate can meet the production requirements. Only Better solution volume and speed can better apply ultrafiltration technology.

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Further reading

Ultrafiltration water treatment modules

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