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Molecular Sieve versus Silica Gel

When people think of moisture protection and preventing water damage they often think Silica Gel, and with good reason, but there are alternatives and silica gel may not always be the best solution.

First we need to understand why desiccants are used.

A desiccant is an adsorbent material which removes moisture from the environment. Water is a chemically reactive molecule which can dissolve, react, oxidize and change other compounds. A classic example is the role water plays in rust. Or how dampness can lead to mould. So take for example the automotive industry, having water inside a closed system will lead to the system corroding as the water rusts metal parts. Another example is the pharmaceutical industry. Tablets in their packaging will react with moisture trapped in their packaging and this can lead to a shorter shelf life or even the drugs/supplements becoming chemically damaged. Imagine opening your effervescent vitamin C pastels and they are all soggy and stuck together!

So desiccants like Silica Gel are used to protect products, either on the shelf, prolonging their shelf life or within products, prolonging their functional life.

Comparing Silica Gel to Molecular Sieve Desiccants

Silica Gel:

  • Adsorbs up to 35% of its mass in water at high relative humidity
  • Releases moisture back into the environment at temperatures above 60 degrees Celcius and above about 150 Degrees Celcius it stops functioning as a desiccant.
  • Silica Gel can adsorb nitrogen.
  • Is better performing than molecular sieve at relative humidity above 40% when the temperature/pressure fluctuates.

Molecular Sieve:

  • Adsorbs up to 20% of its mass in water outperforming silica gel at lower relative humidities.
  • Releases moisture back into the environment at temperatures above 400 degrees Celcius.
  • Molecular Sieves have a very specific partical acceptance size of 3 angstroms which mean that only water molecules will adsorb, and hence will not adsorb other elements such as nitrogen.

Find out more about Silica Gel versus Molecular Sieve Desiccants here.

There are many other benefits and considerations when choosing the right desiccant. This article just covers some of the basics above and your choice of desiccant also varies depending on the operating parameters such as temperature and humidity. The quick and easy way to find out which is the right desiccant for your specific application is to contact our sales staff for a free no obligation consultation.

So let's look at two real world applications of desiccants, which we've already alluded towards:

Example: Pharmaceutical Application of Desiccants.

Vitamin C effervescent tablets are highly reactive to water. They are intended to effervesce and dissolve when the end user drops them in a glass of water. The last thing the manufacturer wants is any water in the packaging tube, but moisture is everywhere! So when the tubes are packed a small amount of silica gel is placed in the lid of the packaging. Once sealed this gel can take up to 30% of its own mass in water, which is more than enough to make the environment inside the tube dry. These plastic tubes are unlikely to be stored in hot temperatures so the temperature limits of Silica Gel are not an issue. Silica gel is also extremely clean to work with, an important consideration for medicines and tablets which will be ingested.

Example: Automotive Application of Desiccants.

The air conditioning system of a car is quite a challenging environment, it may get very hot. It is also a sealed environment so once it is built the inner chambers will never experience the outer environment. The challenge is that as it is being built, air and therefore moisture enter the system. This moisture if left, will cause corrosion and eventual failure of the automotive air conditioning units. The solution is to place a desiccant within the unit before it is sealed. Once sealed the desiccant will mop up the moisture ensuring the inner chambers are dry (low relative humidity) and free from moisture damage. Because of the high and fluctuating temperatures and the need to permanantly lock the moisture away, companies like GM, who we provide desiccants for prefer the properties of a molecular sieve desiccant.

You may be suprised at the many different ways desiccants are used in the automotive industry.

Automotive applications of desiccants

Posted by John Logan at 15:50

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