Desiccants

A desiccant is a dehumidifying agent which attracts moisture from the atmosphere. It adsorbs and holds particles of water to itself.

The three most common types of desiccants are clay, silica gel, and molecular sieve.

Clay desiccants are non-hazardous, moisture adsorbent substances created by the controlled drying of a calcium (bentonite) aluminosilicate clay. This naturally occurring montmorillonite clay has a special affinity for moisture and is chemically inert and non-corrosive. The layered structure of clay desiccant attracts and adsorbs moisture onto its surface and between its layers.

Desi Pak® is the trade name of the packaged clay (bags and packets) sold by AGM. Desi Pak will protect the contents of a properly sealed container during shipping and storage (i.e., the contents of a sealed container will be in the same condition as when they were packaged for shipment or storage).

Clay is the least expensive of all desiccants and highly effective within normal temperature and relative humidity ranges. It is used in most industrial applications requiring humidity control. Clay desiccant can adsorb up to 28% of its weight in water vapor at 77°F (25°C) and 80% relative humidity. At full moisture vapor capacity, Desi Pak remains dry and free-flowing. There is no apparent change in size, shape, or texture of the desiccant.

Clay desiccant works satisfactorily below 120°F (49°C). Above 120°F (49°C), there is a possibility that the clay will give up moisture rather than pull it in (silica gel or molecular sieve are suitable for such temperatures). Accordingly, anticipated storage and transportation conditions should be considered when deciding whether or not to use clay.

Indicating silica gel is a bead or granule that has been washed with a concentration of cobalt chloride (a heavy metal salt). The cobalt chloride is a deep blue color when dry and turns from blue to purple to pink as it becomes saturated with moisture.

AGM also carries orange indicating silica gel. This type is a bright orange color when dry and turns green as it becomes saturated with moisture.

The most common form sold by AGM is bulk silica gel, primarily used in air drying applications.

Another use of indicating silica gel is with a moisture-sensitive product that will be inspected regularly, as the indicating silica gel gives a quick visual indication of how well the product is doing. Due to the presence of cobalt chloride, indicating silica gel desiccant should never be used in contact with products for consumption, such as food or pharmaceuticals.

Desi View® is the trade name of a bagged indicating silica gel mixture sold by AGM. The Desi View mixture consists of either 90% clay or 90% silica gel; the remaining 10% is always blue indicating silica gel. Desi View bags are only 10% indicating silica gel for two reasons: 1) a 10% indicating silica gel content is enough of a color change indicator to be easily viewed through the bag’s polyethylene side; 2) indicating silica gel is more expensive than non-indicating silica gel.

Indicating silica gel is also available in special packet sizes for use in electronics, pharmaceutical, or food packaging.

In desiccant terminology, a “unit” is a quantity of desiccant defined in the U.S. Military’s specification (MIL-D-3464E) for packaged desiccant products. Under this specification, a unit is the quantity of desiccant which will adsorb 3.00 grams of water vapor at 20% relative humidity or 6.00 grams of water vapor at 40% relative humidity at 77°F (25°C).

It may also be helpful to think of a unit as equating to approximately one ounce of material. More specifically, a one unit bag of Desi Pak® (clay) contains approximately 33 grams per bag, a one unit bag of Sorb-It® (silica gel) contains 26 grams per bag, and one unit bag of Tri-Sorb® (molecular sieve) contains 32 grams per bag.

You need a humidity indicator card if you are concerned as to whether or not the desiccant in your package is still active. Desiccant becomes inactive when it has adsorbed as much moisture as it can for the conditions it is being exposed to. This can occur under the following conditions:

if your package is not properly sealed;
if you don’t use enough desiccant in your package to draw down the relative humidity;
if the barrier material (i.e., plastic bag) you use to package your product has a high moisture vapor transmission rate (which means that a significant amount of humid air is entering your package through the barrier material itself).

You need a humidity indicator card if you are concerned as to whether or not the desiccant in your package is still active. Desiccant becomes inactive when it has adsorbed as much moisture as it can for the conditions it is being exposed to. This can occur under the following conditions:

1) If your package is not properly sealed;
2) If you don’t use enough desiccant in your package to draw down the relative humidity;
3) If the barrier material (i.e., plastic bag) you use to package your product has a high moisture vapor transmission rate (which means that a significant amount of humid air is entering your package through the barrier material itself).

In order to achieve optimal protection, desiccants should be used within a closed/sealed moisture barrier or a rigid/sealed container. Humidity Indicator Cards are generally placed inside the container to show the humidity level and to indicate when the desiccant needs to be replaced.

Standard sizes for unit bags are as follows: 1/6, 1/3, 1/2, 1, 2, 4, 8, 16, and 80 units. A unit is roughly one ounce of desiccant material.

Standard sizes for packets are as follows: 1/4, 1/2, 1, 2, 3, 5, and 10 grams.

Standard sizes for canisters is as follows: 40 grams.

Custom sizes are available to fit your packaging needs, though such sizes typically carry a significant minimum order quantity.

The largest users of desiccants are the industrial, pharmaceutical, and the electronic components packaging industries.

Desiccants are generally packaged in bags, packets, or canisters. Packets are sometimes referred to as “sachets” or “pouches.”

Desiccants in bags are commonly referred to as industrial desiccants. They are primarily used by industry because their medium to large size enables them to adsorb moisture in heavy duty applications. However, these bags are commonly used in non-industrial applications as well.

Bagged desiccants are available in:

• Tyvek® (heat sealed)
• Kraft paper (heat sealed)
• Heavy, non-woven rayon/polyester blend (string sewn)

The term “bagged” desiccants commonly refers to any desiccant which is packaged by the unit.

Desiccants in packets or canisters are commonly referred to as pharmaceutical desiccants. They are primarily used by the pharmaceutical industry because of their small size. However, other industries, such as electronics, optics, and food packaging, utilize these packets and canisters as well.

Packet desiccants are available in:

• Tyvek (heat sealed)
• GDTII (heat sealed)

It depends. The amount of desiccant required is usually dependent on the following factors:

• Volume of air space to be desiccated
• Nature of the material to be protected
• Moisture surrounding the package
• Type of desiccant to be used
• Length of time for protection
• Atmospheric conditions (temperature and relative humidity) when/where the product is sealed and during subsequent shipment and storage
When AGM receives a call from someone looking for desiccant, our Desiccant Specialists will ask a number of questions to determine responses to the above information so they can calculate which type and size of desiccant will work best for the particular application.

You may use our Desiccant Calculator to determine the amount of desiccant for your application.

For further information on how to determine the amount of desiccant required for your application, please contact one of our Desiccant Specialists at 520-412-7511 or email us.

Custom desiccants provide moisture control when standard bagged or canister desiccant is inadequate to your project’s needs due to size, shape, or moisture adsorption capability. Custom desiccants are commonly formed, pressed, or bagged desiccants that are shaped to fit your project without requiring you to perform a redesign. These desiccants are intended to properly fit into the cavities and open spaces of a specific product or its housing. An example of a custom desiccant is AGM's H20Lock Pressed Desiccant.

AGM manufactures two main types of custom desiccants: H20Lock Pressed Desiccant and Custom Desiccant Bags.

H2OLock Pressed Desiccant is a is a composite desiccant designed to provide an optimum balance between strength, adsorption rate, moisture capacity, and thermal resistance. H2OLock is comprised of 4A molecular sieve and heat resistant binders. The desiccant’s composition provides H2OLock with:
• A serviceable maximum temperature of 150C (302F)
• Compressive strength of 6,000 psi
• High-performance moisture adsorptive capabilities

Due to its durability, H2OLock can operate as a structural component and moisture adsorber in one, frequently replacing plastic and metal parts, insulators, shock pads, and other pieces.

AGM can produce Custom Desiccant Bags in non-standard bag sizes using a benchtop heat sealer, up to 11.3" x 11.3" (interior dimensions). Bags may also be handsewn. The bag material is coated Tyvek® and may be filled with molecular sieve or silica gel. Bentonite clay may also be used; however, we advise you to contact AGM's Engineering Department for bentonite clay availability.

AGM also offers custom desiccant bags in 10, 15, and 20 lbs weights for larger volumes. A variety of sizes, bag materials, and configurations are available upon request."

Most of our customers use white, granular silica gel with a 1-3mm grain size. Beaded silica gel is also available, although it is not as widely used as the granular form for this application. White (non-indicating) silica gel is normally specified because the indicating chemical in blue (indicating) silica gel slightly reduces its adsorption capacity.

Particle size affects pressure drop through a bed of desiccant. Typically, the larger the particles, the lower the pressure drop at a given flow rate through the bed. The beaded form of silica gel will generally have a smaller pressure drop across the bed for a given particle size than granular silica gel.

The lower the moisture in the gas stream, the higher the adsorption of siloxane by the silica gel. This is because the desiccant prefers the H2O molecule due to its polar nature.

We do not know of a correlation between pressure and siloxane adsorption, and our experience has shown that pressure has minimal effect on H2O absorption. However, the LFG combustion system that we supply silica gel for in Southern California is run at 80 psig. This system pressure is believed to not be related to siloxane adsorption, but a result of other system requirements. Another user has had good results using a system pressure of 5-10 psig.

Downstream gas monitoring is required to watch for an increase in siloxane levels. When the silioxane levels begin to trend upward, the bed needs to be replaced. There is no change in the physical appearance of the desiccant when it is saturated.

Regeneration is possible. However, due to the large amount of desiccant used in a normal LFG system, it is usually not economically feasible.