What are the four reasons for the decrease in membrane flux to be investigated first? Concentration polarization, gel layer, plugging and compaction

What are the four reasons for the decrease in membrane flux to be investigated first? Direct answer: Divided into four categories based on the order of occurrence and reversibility - concentration polarization, where intercepted components accumulate near the membrane surface to form a high concentration boundary layer, resulting in a decrease in flux accompanied by a synchronous increase in transmembrane pressure difference; The gel layer is polluted, and the polarization is so serious that the solute forms a layer on the membrane surface, which can be recovered only after chemical cleaning after shutdown; Blockage of membrane pores, particles or precipitates getting stuck in the pores, and unsatisfactory cleaning and recovery; Membrane compaction can cause deformation of membrane pores due to excessive operating pressure, leading to an irreversible decrease in flux. The first three types can all be restored, while the fourth type can only be prevented - distinguish before taking action, don't rush to replace the film.

Concentration polarization: first look at transmembrane pressure difference

Concentration polarization is the most common and lightest type of flux decline. The judgment method is very clear: while the flux decreases, the transmembrane pressure difference synchronously increases, which is caused by polarization. Its essence is that the solute concentration near the membrane surface is much higher than that of the main feed liquid, and the mass transfer driving force is weakened. The disposal method is to increase the membrane surface flow velocity - increase the cross flow velocity to wash away the sediment. In terms of operational design, optimized flow channels and uniform flow velocity distribution can make polarization come later or slower.

Gel layer: boundary of chemical cleaning

Further polarization, solutes cluster on the membrane surface to form gel like substances, which cannot be washed away by increasing the flow rate, and chemical cleaning must be stopped. Its recognition features are: a large decrease in flux, long recovery time, and a significant rebound after cleaning but quickly falling back down. The gel layer should be treated from both ends: the disposal end should be chemically cleaned according to the procedures to restore the membrane surface; Control the concentration factor at the prevention end to prevent the concentration of the feed solution from entering high-risk areas. Matching the cleaning agent and cleaning cycle according to the type of pollution is the core discipline of this process.

Membrane pore blockage and membrane compaction

Membrane pore blockage is one layer deeper than surface pollution - particles or precipitates get stuck in the pores, and cleaning solution needs to drill into the pores to flush out the blockage. The characteristics are a decrease in flux, an increase in pressure difference, and poor cleaning recovery. Disposal relies on pre-processing: block large particles in advance and choose aperture to prevent narrow roads from carrying large vehicles. Membrane compaction is the most unique: this type of damage is irreversible, and excessive operating pressure or long-term high-pressure operation causes the membrane material to become increasingly dense. Its characteristics include irreversible decrease in flux, normal pressure difference, and minimal cleaning effect. The answer to compaction is to control the operating pressure. Low pressure is more efficient than high-pressure hard extrusion.

Comparison and disposal of four types of reasons

Troubleshooting steps

1. Record the current values of flux, transmembrane pressure difference, and operating pressure;
2. Flux decrease and pressure difference increase should be treated as concentration polarization first;
3. After increasing the cross flow velocity and recovering, return to polarization and switch to optimizing operating parameters;
4. If the recovery is not obvious, arrange chemical cleaning to verify the gel layer;
5. If the recovery is still poor after cleaning, check the pre-treatment and pore size matching;
6. If the pressure difference is normal but the flux continues to irreversibly decrease, replace it according to the compaction evaluation.

Replacing the film is an option after all four reasons have been ruled out. Most flux issues do not require membrane replacement - by focusing on operating parameters, pretreatment, and cleaning procedures, the flux curve will provide positive feedback.

What is the problem with low flux from the beginning?

First, check whether the operating conditions are consistent with the design values - changes in the concentration, temperature, and pressure of the feed solution will change the flux benchmark. After excluding operating conditions, investigate according to the four types of reasons.

How often should chemical cleaning be done?

Set according to trend rather than calendar: Arrange when the flux decreases by more than 20% or the transmembrane pressure difference deviates significantly from the baseline, and for those that require monthly cleaning, check the pre-treatment and concentration factors.

Can membrane compaction prevent it?

Can. The core is pressure management - operate according to the designed pressure, without relying on high-pressure hard extrusion flux; For devices that operate under long-term high pressure, include the pressure curve in the monthly review.

This article by LuoyangWohong PetrochemicalEquipment Co., Ltd. arranges that the company provides design, manufacturing and operation and maintenance services for condensation recovery, membrane separation recovery and complete solvent recovery devices for chemical and pharmaceutical enterprises. If you need process evaluation or scheme configuration, please contact and exchange.