When it comes to desiccants, people have most commonly heard of silica gel packets. But there are a lot of different types of desiccants depending on the use-case scenario. For different needs, you’ll have to use a different type of desiccant.
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| Image Source :- Interteck |
In this article, we’ll explore the most common types of desiccants, learn about their composition, properties, and uses.
What is a desiccant?
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| Image Credit:- Interteck |
Desiccants are compounds or agents used in facilitating low humidity environments by absorbing moisture content from the air. They’re typically used in transport, storage, or maintenance of materials and products. They help keep everything from gym equipment to military munitions dry.
Why do you need desiccants?
Desiccants help tackle water contamination in close containers or packages. Water contamination can occur in a closed container or package due to the following 4 sources:
Presence of water vapor in the air inside the package,
Moisture in the materials inside the package,
Moisture inside or on the walls of the package, and
The entry of moisture into the package due to permeation or leakage
So, desiccants play an important role in the packaging, transport, and logistics of all kinds of goods. It is safe to say that they’re integral in ensuring that products don’t get contaminated during storage and transport.
Common types of Desiccants
1. Montmorillonite Clay
Montmorillonite clay is a naturally occurring adsorbent. It is prepared using controlled drying of magnesium aluminum silicate of the sub-bentonite type. This clay regenerates for repeated use at very low temperatures without substantial deterioration or swelling. But, it also causes the clay to readily release moisture back into the container as temperature increases.
Clay is a good desiccant that works well below 120°F. Above 120° F, the clay might start giving up moisture rather than pulling it in. This is why you must anticipate and consider storage and transportation conditions. Clay is normally the least expensive desiccant.
2. Silica Gel
One of the most commonly used desiccants, silica gel is an amorphous form of silica manufactured from sodium silicate and sulfuric acid. Its interconnected pores form a vast surface area that attracts and holds water by adsorption and capillary condensation. This enables silica gel packets to adsorb about 40% of its weight in water.
Silica gel is highly efficient at temperatures below 77°F (25°C), but it loses its adsorption capacity as temperature rises. In this aspect, silica gel is similar to clay. Silica gel has gained so much popularity because of its non-corrosive and non-toxic nature. Some grades of silica gel have even received the approval of the U.S. government for use in food and drug packaging.
Indicating silica gel is a specific type of silica gel that indicates when it is saturated by changing its color. Silica gel granules are washed with cobalt chloride to turn it into indicating silica gel. The cobalt chloride has a deep blue color when dry and turns pink as it becomes saturated with moisture.
3. Molecular Sieve
Molecular sieve is the best desiccant when thinking about technical performance characteristics. Its moisture absorption abilities are so pronounced that it can absorb trapped water molecules from a fully saturated silica gel granule.
Molecular sieves are synthetic porous crystalline aluminosilicates. They are engineered to have a very strong affinity for specifically sized molecules. The distinguishing feature of the molecular sieve structure is the uniformity of the pore size openings.
The pore size on the molecular sieve particles can be controlled as a part of the manufacturing process. The most commonly used pore size is 4 angstroms (4A). Other available sizes include 3 angstroms (3A), 5 angstroms (5A), and 10 angstroms (13X). With this, you can select a molecular sieve that can adsorb moisture but exclude most other molecules in the package.
4. Calcium Oxide
Calcium oxide is calcinated or re-calcinated lime. It has a moisture adsorbing capacity of not less than 28.5% by weight.
Calcium oxide, also known as quicklime, can adsorb a much greater amount of water vapor at a very low relative humidity than other materials. It is most effective in applications where a low critical relative humidity is necessary and where a high concentration of water vapor is present. Calcium oxide is mainly used in the packaging of dehydrated foods.
5. Calcium Sulfate
Calcium sulfate is an inexpensive desiccant packs available in suitable packaging forms. It is commercially known as Drierite.
Calcium sulfate is created by the controlled dehydration of gypsum. It acts as a general-purpose desiccant, mainly used in laboratories. It is chemically stable, non-corrosive, non-disintegrating, non-toxic, and does not release its adsorbed water when exposed to higher ambient temperatures.
Calcium Sulfate is a relatively cheap desiccant with a low absorptive capacity. It adsorbs only up to 10% of its weight in water. It also has regeneration characteristics that limit its useful life. Although available, it is not usually sold in package form.
More Absorbents
There are a lot more types of adsorbents available for specialized functions. A few of these other desiccant packs have a specialized function. For example, activated alumina is a porous desiccant that provides somewhat lower moisture capacity at low temperatures but slightly improved capacity at higher temperatures. It is effective for drying gases.
Activated carbon is another example that has been used extensively as an adsorbent of odors and toxic gases. Due to this property, it has long been used in military gas masks.
There are a lot more absorbents available on the market. They range from metal salts to phosphorus compounds. All of them have specific strengths that would be impossible to address individually.
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