How to judge molecular sieves saturation
Time:2025-07-28
There are several drying towers in a certain factory that often emit an acidic odor. After dismantling and inspection, it was found that the molecular sieve had turned black, and it was then realized that the molecular sieve had failed.
So how to determine if the molecular sieve is out of use? We can simply judge with our eyes:
*Color: First of all, the brand new molecular sieve is usually off white or off yellow. If the molecular sieve turns black, it needs to be stopped and replaced.
*Particle state: Molecular sieves are adsorbents that do not break when exposed to water, and good molecular sieves are hard and do not lose powder. If it shatters at a pinch, it means it has already powdered and needs to be replaced immediately.
*Odor: Molecular sieve is a non-toxic adsorbent with little odor. If there is an acidic or burnt smell, it needs to be shut down for inspection.
*Bed state: The normal state of molecular sieve loading is loose and uniform. If clumping or other phenomena are found, the tower needs to be dismantled and cleaned.
If it cannot be seen with our eyes, then it is necessary to check the operating data:
|
Testing index |
Normal range |
Failure critical value |
Reason |
|
Adsorption time |
Design value±10% |
Shorten over 30% |
Adsorption capacity decreased |
|
Regeneration temp. |
200-300℃ |
>350℃ regeneration |
Pore diameter blockage |
|
Product purity |
Over 99.5% |
3 times <98% |
Selective failure |
|
Pressure variation |
Within 10kPa |
Increase 50% suddenly |
Bed pulverization and clumping |
If it is still difficult to judge, the following methods can be used for simple testing:
Method 1: Horizontal shaking method - testing water absorption capacity
1. Pour 30g of molecular sieve and 100ml of water into a mineral water bottle
2. Shake vigorously for 30 seconds and then let it rest
3. Observation phenomenon: Normally, the bottle wall heats up and the molecular sieve quickly sinks to the bottom; If the bottle wall is not hot and the suspended particles are>50%, it indicates that it has failed.
Method 2: Weighing method - detecting water absorption rate
1. Weigh 50g of molecular sieve (record weight W₁)
2. Put it in a bowl and soak it in water for 10 minutes
3. Remove and use a tissue to absorb surface water
4. Weigh W₂
If (W2-W1)/W1>20%, the molecular sieve can continue to be used; If it is less than 15%, it indicates that the molecular sieve has failed.
Let's list several different scenarios of molecular sieve failure characteristics:
|
Application scenario |
Failure performance |
Solution |
|
Air dryer |
Compressed air dew point > -40℃ |
Replace 3A molecular sieve |
|
Oxygen generator |
Oxygen concentration <90% |
Replace 13X molecular sieve or LiLSX type |
|
Refrigerant drying |
Compressor abnormal |
Replace molecular sieve 5A |
|
Alcohol dehydration |
Alcohol content >0.5% |
Replace 3A molecular sieve |
Air adsorption dryers are familiar in the chemical industry, and their core components are adsorbents , such as activated alumina, molecular sieves, silica gel. They adsorb water and gas through a porous structure, and their performance affects the drying effect directly. As the adsorption regeneration cycle progresses, the adsorption capacity gradually decreases and needs to be replaced regularly to ensure stable system operation.
The replacement cycle is usually 3-5 years, but it needs to be adjusted based on the following factors:
1. Frequency of use: High frequency use will accelerate the saturation of the adsorbent, and the cycle needs to be shortened; Long term high load operation will accelerate the decline of adsorbent performance;
2. Environmental humidity: The higher the humidity of the inlet air, the greater the adsorption load and the corresponding decrease in lifespan; High temperature, high dust and other harsh environments may require early replacement.
3. Operating parameters: Abnormal pressure or increased dew point temperature may indicate adsorbent failure.
Meanwhile, during work, it is important to pay attention to the following matters:
1. Monitoring indicators: Regularly monitor the dew point temperature at the outlet, and if it continues to exceed the standard, check the status of the adsorbent.
2. Replacement operation: Thoroughly clean the adsorption tower during replacement to avoid mixing of new and old adsorbents; The filling should be evenly compacted to prevent air flow short circuits.
3. Selection matching: The new adsorbent should be consistent with the prototype number to ensure that the porosity and mechanical strength meet the requirements.
4. Regeneration inspection: After replacement, verify the regeneration effect and confirm that the heating temperature and cooling time are normal.
So how to choose the right adsorbent for an adsorption dryer?
1) Molecular sieve: It is a crystal structure with uniform pore size and strong adsorption ability for water molecules, especially suitable for precision industrial scenarios with dew point below -60 ℃. However, due to its low strength, it is easy to be pulverization under high-intensity compressed air impact, so it can only be used in small quantities in traditional twin tower adsorption dryers.
2) Activated alumina: It is a high-strength adsorbent and the most widely used adsorbent in adsorption dryers currently. It can withstand the high-pressure impact of compressed air without powdering. Excellent performance in moderate humidity environments, suitable for places with dew point below -40 ℃, with a regeneration temperature 30-50 ℃ lower than molecular sieves.
3) Silica gel adsorbent: It’s getting less and less in compressed air drying. It is only suitable for drying at room temperature and is used in pressure dew point environments of -20 ℃.
In terms of drying performance, molecular sieves are much stronger than activated alumina, but the replacement cost is higher. Therefore, before choosing which adsorbent, it is necessary to understand the user's requirements or expectations for dew point, so as to choose the best solution that can meet the user's requirements and reduce the cost of use.
