Desiccants are compounds or agents, such as Montmorillonite Clay or Silica Gel, used in facilitating low-humidity environments by adsorbing moisture content from the air. Typically used in transport, storage, or maintenance of materials and products, desiccants are used to keep everything from gym shoes to military munitions dry.
There are four sources of water contamination in a closed container or package that desiccants are designed to combat:
- Water vapor in the air inside the package;
- Moisture contained in the materials inside the package;
- Moisture in or on the walls of the package; and
- Moisture entry into the package due to permeation or leakage.
Each issue is best mitigated with a different type or combination of desiccant. An effective desiccant will absorb the water vapor in the air, lowering the relative humidity to the point where water cannot condense (the dew point).
First, Know the Product:
Is the cargo highly sensitive to humidity and moisture, or is it fairly tolerant? In other words, how dry does the product need to remain?
This information will help establish acceptable humidity and moisture levels.
Second, Know the Environment:
Monitor the temperatures and weather ranges to which the product’s storage container will be exposed. The weather
has a great affect upon humidity levels inside the container.
This information will help predict fluctuations inside the container
Third, Know the Packaging:
Packaging can be plastic bags, cardboard boxes, Tupperware, even large Conex storage containers – the list is endless. However, it is the characteristics of the container (the material’s permeability, seal type, corrosion resistance, container dimensions, etc.) that is important.
This information will help determine how much of a particular desiccant is needed and in what form.
As you may have surmised, precisely determining all of the above listed information for storing anything can be difficult. For that reason, the table below shows the adsorptive tendencies of five common desiccants, including effectiveness at elevated temperatures and extreme water vapor concentrations. To that end, the table may be used as a very general reference. For those with more specific moisture goals, contact AGM for a consultation.
|Metric||Molecular Sieve||Silica Gel||Montmorillonite Clay||Calcium Oxide||Calcium Sulfate|
|Adsorption Capacity in Low H2O Concentrated Environments||High||Low||Average||High||Above Average|
|Adsorption Rate||High||Above Average||Above Average||Low||Above Average|
|Water Capacity at 77° F & 40% RH||High||High||Average||High||Average|
|Adsorption Capacity at High Temperatures||High||Low||Low||Average||Low|
|Desiccant||Common Uses and Points of Interest|
|Molecular Sieve is often used to dry gags streams via high-pressure desiccators and is ideal for applications in extreme temperature settings (e.g. -40℃ to 230℃).|
Pro: Molecular Sieve will not as readily desorb moisture back into packaging when temperatures rise as will Silica Gel or Clay.
Con: Molecular Sieve is comparatively expensive.
|Silica Gel is perhaps the most commonly used desiccant, and is often used in desiccant breathers to adsorb moisture from the air inside of sealed containers. In addition, packaged silica gel is often found in consumer products, such as shoes and electronics.|
Pro: Silica Gel is predominantly non-corrosive and non-toxic in nature. In fact, some grades have received U.S. government approval for use in food and drug packaging, but not all.
Con: Silica Gel, though it outperforms clay, does not function well in extreme heat and environments with a low H20 concentration.
|Packaged Montmorillonite Clay is most often used in storage applications, though it is occasionally used in the food and pharmaceutical industries, as well.|
Pro: Clay will successfully regenerate at very low temperatures without substantial deterioration. It is inexpensive and highly effective within normal temperature and relative humidity ranges.
Con: Clay may readily desorb moisture back into the container as temperatures rise.
|Calcium Oxide is most commonly used in the packaging of dehydrated food.|
Pro: Calcium Oxide will adsorb a much greater amount of water vapor at a very low relative humidity compared to other desiccants. It is most effective where a low critical relative humidity is necessary, and where there is a high concentration of water vapor present.
Con: Calcium Oxide adsorbs moisture from the air very slowly and swells as it takes on more and more moisture.
|Calcium Sulfate is used for a wide range of applications, including drying air and industrial gases, organic liquids and solids, as well as refrigerants, predominately in sealed storage spaces.|
Pro: Calcium Sulfate is a general-purpose desiccant intended mainly for laboratory use. It is chemically stable, non-disintegrating, non-toxic, non-corrosive, and does not release its absorbed water when exposed to higher ambient temperatures.
Con: Calcium Sulfate has a low adsorption capacity, generally adsorbing only up to 10% of its weight in water.
ANY ADDITIONAL INFORMATION?
For those wishing to dive deeper into the above desiccant, Figures 1 and 2 illustrate the adsorption rates (how quickly water vapor is adsorbed) and the adsorption capacities (how much water vapor is adsorbed to reach equilibrium at various relative humidity readings) of the above five common desiccants.
It should be noted that some desiccant have a specialized function. For example, activated alumina (a very porous material) is extremely effective for drying compressed gases; activated carbon has been used extensively for many years as an adsorbent of odors and toxic gasses and has long been used in military gas masks. Others, ranging from metal salts to phosphorus compounds, have specific strengths that would be impossible to address individually. As such, it is recommended that anyone purchasing desiccant consult a supplier first.
DO NOT EAT DESICCANTS!
Some desiccants are toxic. If you accidentally consume desiccant, seek immediate medical attention or call poison control!
Selecting the right desiccant can be tricky, that’s why AGM has an entire division dedicated to desiccants and their use. In addition, AGM offers a moisture analysis service which takes into account the wide range of variables necessary to prescribe desiccant combinations that maintain targeted humidity-level goals for any storage duration.