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Saturday 3rd October 2015
In previous articles we've highlighted the importance of understanding that not all desiccants are the same. They each exhibit different properties depending on factors such as humidity and temperature. We've also talked about how different suppliers produce different quality desiccant products and the dangers of using poor quality desiccants.
How Do Silica Gel and Molecular Sieves Compare with Varying Relative Humidity?
This graph below shows how silica gel and molecular sieve perform over the range of relative humity from zero to 100% humidity. The Y axis shows the capacity of the adsorbent to lock moisture at 25 degrees C measured as the mass of moisture locked as a percentage of the mass of the desiccant. For example at 10% humidity 100g of molecular sieve is locking away 15g of moisture (or 15% of it's own mass).
So Which is the Better Desiccant, Silica Gel or Molecular Sieve, for Adsorbing Moisture?
It really depends on what the desired relative humidity (RH) is of your application. The key word here is "desired". Many people make the mistake of considering only the RH of the packing environment, the start of the process before the desiccant has done its job. They don't consider the desired RH of the sealed and packed environment.
Consider for example Company X who pack products at 40% RH (25 Deg C). They look at the graph above and mistakingly draw the conclusion that at 40% RH (see point 1. on the graph) both silica gel and molecular sieves perform the same and so either desiccant would do.
But what if once the product is sealed within its packaging it needs to maintain a very dry environment, say 10% RH? At the moment the packaging is sealed, the amount of moisture is fixed (assuming no leaks). Both desiccants will start to adsorb this moisture within the packaging environment thus reducing the RH. But as they adsorb moisture they loose capacity to adsorb more. Furthermore their capacity depends on the RH they are in so as the RH changes their performance changes. You can see that the capacity of a desiccant to adsorb moisture at the target RH becomes a very important factor to consider.
Let us now look at the two desiccant options shown, and see how they would perform for this theoretical product that requires a low RH of 10% within its packaging:
Molecular Sieve Desiccant:
At 10% RH molecular sieve desiccant can adsorb 15% of it's own mass in moisture (see point 2. on the graph).
Silica Gel Desiccant:
At 10% RH silica gel only has the capacity to adsorb about 5% of its own mass in moisture. So you would need 3 times as much silica gel to adsorb the same amount of moisture as molecular sieve!
The Relative Humidity You Want After the Desiccants Have Removed Moisture is Important
What we want to achieve is a stable relative humidity within the packaging or equipment. Knowing what the desired RH is provides a starting point for choosing your desiccant.
Just looking at another example: If we knew that the desiccant was going to have to operate in the 70% RH range (see point 3. on graph) then silica gel has almost 50% more capacity than molecular sieve desiccant. So although the packaging environment in this example was 40% where both desiccants behave the same, you can see that the target RH really does determine which disiccant is better! If you were aiming for 10% RH then molecular sieve is 300% better. If you were aiming for 70% RH then silica gel is 50% better than molecular sieve.
To keep things simple, we have only shown two disiccants (silica gel and molecular sieves) but we also provide other types of desiccants, which our in-house experts are happy to discuss, when you contact us.
What are Other Factors Involved in Choosing the Best Desiccant for Your Application?
We have only considered one factor here which is Relative Humidity (RH), but these are just some of the other factors we will take into consideration when advising you of the best desiccant for your application:
- Temperature range,
- moisture fluctuations,
- pressure changes,
- mechanical forces,
- other contaminants present,
- structural integrity of desiccant,
- desiccant reactivity to materials present.
Our desiccant experts are available for a free desiccant product consultation to help you make sure you've got the right desiccants and adsorbents in your application, whether that's automotive, air conditioning, pharmaceutical, biomedical, product packaging or engineering. In fact any application, even custom projects, where controlling moisture is an issue.
Posted by John Logan at 15:32