What is a desiccant and how is it used in maritime shipping?

A desiccant is a hygroscopic substance that absorbs moisture from the surrounding environment and plays a crucial role in protecting goods during international maritime shipping. Without effective moisture management, millions of tons of goods annually would suffer irreversible damage from condensation, mold, corrosion, and decay. This comprehensive guide will introduce you to what a desiccant is, how it works, what types exist, and how to use it correctly in maritime shipping practice.

What is a desiccant and why is it important?

Definition and basic principle

A desiccant (also called a drying agent or moisture absorber) is a hygroscopic substance capable of attracting and absorbing moisture from the surrounding air. The term comes from the Latin word “desiccare,” which means “to dry.” These are materials with a porous structure or chemical affinity for water that reduce relative humidity in an enclosed space to a minimum level.

In the context of maritime shipping, a desiccant works on a simple physical principle: when humid air in a container comes into contact with desiccant material, water molecules are attracted and retained in the pores or on the surface of the material. This process is called adsorption (physical adhesion) or absorption (penetration into the structure). The result is a reduction in absolute humidity in the container, which prevents condensation on the walls and ceiling of the container — a phenomenon known as “container rain” or “container sweat.”

The importance of desiccants in maritime shipping cannot be overstated. During long sea voyages lasting weeks or months, the temperature and humidity in the container change dramatically. When the container moves from a warm port to cooler zones or vice versa, the air inside cools, and if it contains high absolute humidity, it condenses. Without desiccants, water would accumulate on the surface of goods, causing metal corrosion, mold growth on textiles, cardboard breakdown, and other damage. Experts estimate that desiccants protect goods worth billions of dollars annually.

Historical development of desiccants

The use of desiccants has a surprisingly long history. Even in ancient times, people were aware of the moisture problem in storing goods and used natural materials such as ash, salt, or fired clay to absorb moisture. However, when international trade increased in the 19th century and maritime shipping became the dominant mode of transport, the problem of condensation in enclosed spaces became critical.

The breakthrough came in the early 20th century when chemists developed synthetic silica gel — an industrial drying agent with exceptional absorption capacity. Silica gel was originally developed during World War I for military applications and was later adapted for commercial maritime shipping. In the 1950s and 1960s, when containerization of cargo expanded (standardized steel containers), desiccants became an essential part of logistics processes.

In the 1970s and 1980s, the use of bentonite (natural clay desiccant) and calcium chloride expanded, offering lower prices and better performance in some applications. Today, there is an entire industry dedicated to the manufacture, testing, and distribution of desiccants, with standards and regulations (such as ISO 12103) regulating their quality and effectiveness.

How does a desiccant work and what is the absorption mechanism?

Process of adsorption and absorption

Understanding the mechanism of how desiccants work is key to their proper use. There are two main processes by which a desiccant removes moisture:

Physical adsorption is a process in which water molecules adhere to the surface of a material without chemical bonding. Silica gel and bentonite work primarily through physical adsorption. Their porous structure creates an enormous surface area — one gram of silica gel has a surface area larger than 800 square meters! When humid air comes into contact with this surface, water molecules are attracted by Van der Waals forces and settle in the pores. This process is reversible — when the desiccant is heated, the moisture evaporates and the material can be reused.

Chemical absorption is a process in which the desiccant chemically reacts with moisture. Calcium chloride is a classic example. When it absorbs moisture, it creates hydrated compounds (e.g., CaCl₂·6H₂O), which is a chemical bond. This method is very effective and can absorb up to 200% of its own weight in moisture, but regeneration requires higher temperatures and the process is energy-intensive.

The process works best in an enclosed space, such as a shipping container. Once the desiccant is placed inside the container, it begins to absorb moisture from the surrounding air. The higher the relative humidity and temperature, the faster the desiccant works. Optimal conditions are at relative humidity of 50–90% and temperature of 15–35 °C.

Capacity and efficiency

The absorption capacity of desiccants is measured as the percentage of moisture they can absorb relative to their own weight. Typical values:

• Silica gel: 30–40% of its weight (sometimes up to 50% for high-quality products)
• Bentonite: 20–30% of its weight
• Calcium chloride: 150–200% of its weight

These figures are important for calculating the required amount of desiccants. For example, a 20-foot shipping container (TEU) contains approximately 33 cubic meters. If the container has wet cargo and the expected relative humidity reaches 90%, a significant amount of desiccants will be needed — usually 5–15 kilograms, depending on the type of goods and length of shipping.

The efficiency of desiccants also changes with temperature. At lower temperatures (near the dew point), the desiccant becomes less effective because the air contains less moisture. At very high temperatures (above 40 °C), silica gel may partially release already absorbed moisture. Therefore, desiccants are most effective in the moderate temperature range typical for maritime shipping.

What are the main types of desiccants?

Silica gel

Silica gel is a synthetic material created from silicon dioxide (SiO₂). It is made by dehydrating sodium silicate and creates transparent to milky white beads or granules with a microscopic porous structure. It is the most commonly used desiccant in industry and maritime shipping.

Advantages of silica gel:

• High absorption capacity (30–40% of its weight)
• Safe — not toxic or corrosive
• Easily regenerable by heating (can be reused)
• Available and relatively inexpensive
• Effective over a wide temperature range
• Available with color indicators (blue → pink when saturated)

Disadvantages of silica gel:

• Slower absorption than calcium chloride
• At very high humidity (above 95%), it becomes less effective
• Requires careful storage — must not be exposed to high humidity before use

Silica gel is an ideal choice for shipping electronics, textiles, pharmaceuticals, and other sensitive products where safety and reusable use are required.

Bentonite and clay desiccants

Bentonite is a natural mineral, primarily montmorillonite, which is mined from deposits around the world. It is a clay material that is processed into granules or powder and used as a desiccant. Bentonite is a more environmentally friendly choice than synthetic desiccants because it comes from natural sources.

Advantages of bentonite:

• Lower price than silica gel
• Natural origin — more environmentally friendly
• Good absorption capacity (20–30% of its weight)
• Safe and non-toxic
• Suitable for large-volume shipping

Disadvantages of bentonite:

• Slower absorption than silica gel
• Lower capacity than silica gel
• Harder regeneration — requires high temperatures
• Can break down and create dust
• Poorer performance at lower temperatures

Bentonite is often used in combination with other desiccants to optimize costs and performance. It is popular in maritime shipping for transporting wood, paper, and other cheaper commodities where high safety is not required.

Calcium chloride

Calcium chloride (CaCl₂) is an inorganic salt that functions as a desiccant through chemical absorption. It is available in granulated or powder form and is known for its exceptional absorption capacity.

Advantages of calcium chloride:

• Highest absorption capacity (150–200% of its weight)
• Very fast absorption — reacts quickly to increased humidity
• Effective at very high humidity (up to 95–100%)
• Effective over a wide temperature range
• Ideal for long sea voyages

Disadvantages of calcium chloride:

• Higher price than bentonite
• Chemical absorption — harder regeneration
• Can be slightly corrosive if moisture condenses
• Requires special packaging (often in non-woven fabric)
• After saturation, it becomes liquid — must be contained in bags

Calcium chloride is most commonly used for shipping goods sensitive to moisture on very long routes, such as shipments from Asia to Europe or America.

How is desiccant used in maritime shipping?

Placement in the container

Proper placement of desiccants in the container is critical to their effectiveness. The desiccant must be positioned to have the greatest contact with the humid air in the container.

Optimal placement:

• Top of the container: Place some desiccants on top of the cargo or on the container ceiling. Moisture accumulates at the top, where temperature drops fastest.
• Sides of the container: Place desiccants along the inner walls of the container to allow air circulation around them.
• Between cargo layers: If the cargo is stored in several layers, place desiccants between individual layers.
• Near openings: If possible, place desiccants near the container doors, where temperature changes fastest.

Incorrect placement:

• Placing desiccants on the bottom of the container — moisture accumulates at the top
• Wrapping desiccants in tight packaging — limits contact with air
• Placing too close to furniture or goods — may result in direct contact and contamination

When loading the container, it is important to ensure that the desiccants are accessible for inspection and that their bags are not damaged during shipping.

Calculation of required quantity

Calculating the correct amount of desiccants is not an exact science, but there are proven practices. The amount depends on several factors:

1. Container size and volume: A 20-foot container (TEU) has a volume of approximately 33 m³, a 40-foot container (FEU) has a volume of approximately 67 m³.
2. Type and moisture of cargo: Some goods (e.g., wood, paper, textiles) naturally contain moisture and release it into the air. Such goods require more desiccants.
3. Length of shipping: Long routes (e.g., from Asia to Europe — 3–4 weeks) require more desiccants than short routes (e.g., within Europe — several days).
4. Climate conditions: Shipping in summer months or through tropical areas requires more desiccants.
5. Type of desiccant: Calcium chloride is more effective, so less is needed; bentonite requires more.

Practical rules:

• For a 20-foot container with standard cargo: 5–10 kg of desiccants
• For a 40-foot container with standard cargo: 10–15 kg of desiccants
• For wet cargo (wood, paper): +50% to the above values
• For very long routes (more than 30 days): +50% to the above values

Example: Shipping paper from Indonesia to Belgium (40 days) in a 40-foot container would require approximately 15 kg + 50% (wet cargo) + 50% (long route) = approximately 23–25 kg of desiccants.

Protection of cargo from moisture

The primary function of desiccants is to prevent condensation in the container — a phenomenon known as “container rain.” This phenomenon is one of the most common reasons for cargo damage during maritime shipping.

How container rain occurs:

When a container is loaded in a warm and humid environment (e.g., in a tropical port), the air in the container is heated and saturated with moisture. Over the following days and weeks, as the container moves to cooler zones or as the temperature drops at night, the air cools. Cold air cannot hold as much moisture as warm air, so the moisture condenses on the coldest surfaces — on the walls and ceiling of the steel container. This condensed water drips down onto the cargo like rain.

How desiccants prevent this:

The desiccant absorbs moisture from the air before the air cools. By reducing the absolute humidity in the container, the amount of water that can condense is reduced. With proper application of desiccants, the relative humidity in the container can be maintained below 50%, effectively eliminating condensation.

Examples of damage that desiccants prevent:

• Corrosion: Moisture causes corrosion of metal parts, machinery, and tools
• Mold growth: Mold develops on textiles, paper, and wood
• Cardboard breakdown: Wet cardboard loses strength and deforms
• Electronics breakdown: Moisture causes circuit corrosion and component failure
• Color and odor change: In food and cosmetics, quality deteriorates

How is desiccant regenerated and reused?

Regeneration of silica gel

One of the greatest advantages of silica gel is its regenerability. When silica gel becomes saturated with moisture (usually changes color from blue to pink if it has an indicator), it can be easily regenerated and reused.

Method 1: Regeneration in a home oven

1. Remove bags of silica gel from the container
2. Open the bags and pour the silica gel into a shallow container (e.g., baking sheet)
3. Preheat the oven to 120–150 °C
4. Place the container with silica gel in the oven for 2–4 hours
5. The silica gel will gradually change from pink back to blue
6. Remove and let cool
7. Return to bags and reuse

Method 2: Regeneration in a microwave

1. Pour the silica gel into a microwave-safe container
2. Heat at 50% power for 5–10 minutes, stirring every minute
3. Check the color — if it returns to blue, you’re done
4. Caution: The microwave will get very hot, use gloves

Method 3: Regeneration in the sun

1. Spread the silica gel in a bright place in direct sunlight
2. Leave in the sun for 6–8 hours
3. Less effective, but safer than heating methods

Number of regenerations:

Silica gel can be regenerated dozens to hundreds of times without loss of capacity. In industry, silica gel regenerated more than 100 times is often used. Over time, however, silica gel can degrade, especially if exposed to extreme temperatures or mechanical damage.

Regeneration of bentonite and calcium chloride

Regeneration of bentonite and calcium chloride is much more difficult and energy-expensive.

Bentonite:

• Requires heating to 150–200 °C for 4–8 hours
• Regeneration is less effective — capacity gradually decreases
• In practice, bentonite is often discarded after saturation rather than regenerated
• Economically, regeneration often does not pay for small volumes

Calcium chloride:

• Requires heating to 200–250 °C
• Regeneration is very energy-intensive
• Chemical structure may change — regenerated calcium chloride may not have the same capacity
• In practice, calcium chloride is usually replaced with new material after saturation

For these reasons, silica gel is the most commonly used desiccant in maritime shipping — its low regeneration costs and high reusability make it the most economical choice on a long-term basis.

Saturation indicators

Modern desiccants, especially silica gel, are often equipped with color indicators that signal when the desiccant is saturated with moisture.

Color indicators:

• Blue silica gel: Fresh, dry silica gel; ready to use
• Pink/purple silica gel: Saturated; requires regeneration
• Clear silica gel: Without indicator; requires weight check or special equipment

How to check saturation without an indicator:

• Weigh the desiccant — if the weight has increased by more than 30–40%, it is saturated
• Visually inspect — saturated desiccant may appear wetter or darker
• Feel — saturated desiccant is heavier and may smell damp

It is recommended to check desiccants halfway through and at the end of shipping to ensure they are still working effectively.

What are common mistakes when using desiccants?

Insufficient quantity

The most common mistake is underestimating the required amount of desiccants. Many shippers try to save costs by using less desiccant, which often leads to failure.

Example: A shipper places only 5 kg of silica gel in a 40-foot container instead of the recommended 10–15 kg. During a 30-day voyage, the silica gel becomes saturated and stops absorbing moisture. The result is condensation and cargo damage worth thousands of euros.

Solution: Always calculate the required amount based on container size, type of cargo, and length of shipping. It is better to have more desiccants than to have damaged cargo.

Incorrect placement

Placing desiccants on the bottom of the container or wrapping them in tight packaging limits their effectiveness. The desiccant must have contact with the air in the container.

Example: A shipper places desiccants in a closed cardboard box on the bottom of the container. The air in the container cannot circulate around the desiccants, so moisture does not accumulate where the desiccants are. The result is condensation on the upper parts of the cargo.

Solution: Place desiccants on top of the cargo, along the walls of the container, and between cargo layers. Ensure good contact with air.

Ignoring regeneration

Many shippers discard saturated desiccants instead of regenerating them. This is a waste of resources and increases costs.

Example: A shipper discards 10 kg of saturated silica gel after each shipment. If instead the shipper regenerated the silica gel, they would save costs on new material and also reduce environmental impact.

Solution: Implement a desiccant regeneration program. Silica gel can be regenerated dozens to hundreds of times, significantly reducing long-term costs.

Frequently asked questions about desiccants

What is a desiccant and how does it work?

A desiccant is a hygroscopic substance that absorbs moisture from the surrounding air. It works through physical adsorption (in silica gel and bentonite) or chemical absorption (in calcium chloride). The material has a porous structure or chemical affinity for water that attracts water molecules and retains them in pores or in a chemical bond.

How is desiccant used in maritime shipping?

In maritime shipping, desiccant is placed in shipping containers before loading cargo. It is usually used in the form of bags, which are placed on top of the cargo, along the walls of the container, and between cargo layers. The desiccant absorbs moisture from the air in the container, preventing condensation and cargo damage.

What are the types of desiccants?

The main types of desiccants are: silica gel (synthetic, high capacity, easily regenerable), bentonite (natural, lower price, harder regeneration), and calcium chloride (highest capacity, fastest absorption, hard regeneration). Each type has its advantages and disadvantages and is suitable for different applications.

What is the difference between silica gel and bentonite?

Silica gel is a synthetic material with higher absorption capacity (30–40%), easier regeneration, and higher price. Bentonite is a natural clay material with lower capacity (20–30%), harder regeneration, and lower price. Silica gel is suitable for sensitive goods and long routes, bentonite for cheaper commodities.

How is desiccant regenerated?

Silica gel is regenerated by heating to 120–150 °C in a home oven, microwave, or in the sun. Bentonite and calcium chloride require higher temperatures (150–250 °C) and are energy-intensive. Silica gel can be regenerated dozens to hundreds of times without loss of capacity.

How much desiccant is needed per container?

For a 20-foot container with standard cargo, 5–10 kg of desiccants is recommended. For a 40-foot container, 10–15 kg is recommended. For wet cargo or very long routes, the amount is increased by 50%. The exact amount depends on the type of cargo, length of shipping, and climate conditions.

What damage does moisture in a container cause?

Moisture in a container causes condensation (“container rain”), which leads to: metal corrosion, mold growth on textiles and paper, cardboard breakdown, electronics failure, color and odor change in food and cosmetics. Desiccants prevent these damages by absorbing moisture.

Is desiccant safe for cargo?

Yes, silica gel and bentonite are completely safe for cargo. They are not toxic, not poisonous, and do not emit any chemical vapors. Calcium chloride is also safe, but if moisture condenses, it can be slightly corrosive. All desiccants are used in industry without health risks.

Can desiccant be reused?

Yes, silica gel can be regenerated and reused dozens to hundreds of times. Bentonite and calcium chloride can also be regenerated, but it is more energy-intensive and capacity decreases over time. Regeneration of desiccants is economically advantageous and environmentally responsible.

How long does desiccant regeneration take?

Regeneration of silica gel in a home oven takes 2–4 hours. In a microwave, it takes 5–10 minutes. In the sun, it takes 6–8 hours. Regeneration of bentonite and calcium chloride takes 4–8 hours in an oven with higher temperature.

What are future trends in desiccant technologies?

The future of desiccants is heading towards: higher capacity and efficiency, lower costs through material innovations, more environmentally friendly solutions (natural materials), smart desiccants with digital humidity indicators, and automated regeneration systems. Research is also exploring new materials, such as MOF (Metal-Organic Frameworks), which have the potential to surpass current desiccants in capacity and selectivity.

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