Author: Mycond Technical Department
Adsorption air dehumidification is a technological process of removing moisture from air or gas streams using solid porous materials (desiccants). Unlike condensation methods, adsorption dehumidifiers can achieve extremely low dew points, which makes them indispensable in many industrial processes. The correct choice of adsorption material critically affects the energy efficiency, reliability, and operating costs of the entire system.
Physical fundamentals of adsorption dehumidification
The operating principle of adsorption dehumidifiers is based on two main mechanisms: physical adsorption and chemisorption. In physical adsorption, water vapor molecules are held on the desiccant surface by van der Waals forces, whereas chemisorption involves the formation of chemical bonds between water molecules and the active sites of the adsorbent.
A key concept in selecting a desiccant is the adsorption isotherm—the graphical dependence of the amount of adsorbed moisture on relative humidity at a constant temperature. The shape of this curve determines the material’s effectiveness across different humidity ranges.
It is important to distinguish between static and dynamic capacity. Static capacity is the maximum amount of moisture that can be adsorbed under equilibrium conditions. Dynamic capacity is the actual amount of moisture absorbed under operating conditions with limited contact time.
Let’s consider five main types of desiccants used in modern adsorption dehumidifiers:
- Silica gel
- Natural zeolites
- Synthetic molecular sieves
- Activated alumina
- Composite and hybrid materials
Silica gel: a universal solution for standard dehumidification tasks
Silica gel is an amorphous silicon dioxide (SiO₂) with a developed pore system. Structurally, it includes macropores (over 50 nm), mesopores (2–50 nm), and micropores (less than 2 nm), providing a high specific surface area of 600–800 m²/g.
The adsorption isotherm of silica gel has an S-shaped curve with maximum capacity in the 40–70% relative humidity range. This feature makes it effective for drying air of moderate humidity, but less productive at very low relative humidity.
Silica gel operates over a wide temperature range from -10°C to +50°C. Typical regeneration temperature is 100–150°C depending on the degree of saturation and available thermal power.
Under optimal regeneration conditions, standard silica gel allows achieving a dew point in the range of -40°C to -50°C. The actual achievable dew point depends on bed thickness, airflow rate, and cycle duration.
Main application areas for silica gel:
- Industrial ventilation
- Warehouse dehumidification
- Household adsorption dehumidifiers
- Systems where low regeneration energy consumption is critical
Natural zeolites: enhanced performance at moderate cost
Natural zeolites are aluminosilicate minerals with a crystalline structure and a micropore system formed by a framework of silicon and aluminum tetrahedra. The main types used in dehumidifiers include clinoptilolite, mordenite, and chabazite.
The pore size of natural zeolites varies from 0.3 to 1 nanometer depending on the mineral type. This feature provides greater selectivity for water molecules compared to silica gel.
The adsorption isotherm of zeolites is steeper due to the higher affinity of polar water molecules for the cations in the material’s structure. This enables efficient moisture uptake even at low relative humidity.
The regeneration temperature of natural zeolites is typically 150–200°C, which is higher than for silica gel. This is due to stronger adsorption bonds.
With sufficient regeneration, natural zeolites provide a dew point in the range of -50°C to -60°C, making them effective for applications where silica gel does not provide the required depth of drying.
An advantage of natural zeolites is their lower cost compared to synthetic molecular sieves due to the availability of natural raw materials and simpler production technology.
Synthetic molecular sieves: maximum drying depth
Synthetic molecular sieves are artificially created zeolites with precisely controlled pore size and chemical composition. Their production enables a uniform structure with predictable adsorption properties.
The main types of molecular sieves include:
- Type 3A – effective pore diameter of 3 angstroms for adsorbing only water
- Type 4A – 4 angstrom pores for adsorbing water and small molecules
- Type 5A – 5 angstrom pores for a broader range of substances
- Type 13X – 10 angstrom pores for a wide range of molecules
Due to the high concentration of cations and uniform pores, molecular sieves exhibit exceptional affinity for water. They effectively adsorb moisture even at very low relative humidity, which is unattainable for other desiccants.
With a properly designed regeneration cycle, molecular sieves deliver dew points down to -70°C, making them indispensable for cryogenic processes and high-precision manufacturing.
However, the high performance comes at a cost: molecular sieves require high regeneration temperatures (180–250°C) and are characterized by increased cycle energy consumption.
Typical applications include compressed air treatment systems for instrumentation and control (I&C), cryogenic air separation units, pharmaceutical manufacturing, and the food industry.
Activated alumina: resistance to contaminants
Activated alumina (Al₂O₃) is a porous material with amphoteric properties, capable of adsorbing both acidic and alkaline impurities, in addition to water vapor.
The structure of activated alumina is characterized predominantly by mesopores with a fraction of micropores, providing intermediate adsorption characteristics between silica gel and zeolites.
Depending on regeneration conditions, alumina provides a dew point in the range of -50°C to -65°C, making it suitable for many industrial applications.
A key advantage of activated alumina is its increased chemical resistance to the presence of acidic gases (hydrogen sulfide, carbon dioxide) and organic impurities. This property makes it ideal for drying process gases containing contaminants.
Typical regeneration temperature is 150–200°C, higher than for silica gel but lower than for most molecular sieves.
Main application areas include natural gas treatment systems, air separation processes, and chemical production where not only drying depth but also adsorbent resistance to contaminants is important.
Composite and hybrid desiccants: innovative solutions for specific tasks
Composite and hybrid desiccants are created by combining the properties of base materials to achieve specific adsorption characteristics.
Popular examples include silica gel impregnated with lithium chloride, which provides increased dynamic capacity at low regeneration temperatures (60–80°C). This makes such materials ideal for systems with limited access to high-temperature heat sources.
Another approach is the use of mixed layers of different adsorbents in a single rotor or cassette to optimize the drying process.
Among new classes of materials under development, noteworthy are:
- Metal-organic frameworks (MOFs) with a record specific surface area up to 7000 m²/g and controlled hydrophilicity
- Polymeric adsorbents with adjustable porosity
- Salt-based composites for thermochemical heat storage systems
However, most innovative materials currently have limited industrial adoption due to the high cost of synthesis and insufficiently studied long-term stability. For mass applications, traditional silica gel and zeolites still dominate due to their optimal balance of performance and cost.
Comparative analysis and desiccant selection methodology
Below is a comparative table of the main characteristics of different desiccant types to help engineers with initial material selection:
| Desiccant type | Dynamic capacity, % by weight | Achievable dew point, °C | Typical regeneration temperature, °C | Relative cycle energy consumption | Chemical resistance to impurities | Typical lifetime, thousand cycles | Relative material cost |
|---|---|---|---|---|---|---|---|
| Silica gel | 8-12 | -40 to -50 | 100-150 | 1.0 | Moderate | 50-80 | 1.0 |
| Natural zeolites | 10-15 | -50 to -60 | 150-200 | 1.3-1.5 | High | 60-100 | 1.2-1.8 |
| Molecular sieves | 15-22 | -60 to -70 | 180-250 | 1.8-2.5 | Low to moderate | 80-120 | 2.5-4.0 |
| Activated Al₂O₃ | 8-15 | -50 to -65 | 150-200 | 1.4-1.8 | Very high | 70-100 | 1.8-2.5 |
| Composite desiccants | 10-18 | -40 to -60 | 60-180 | 0.8-1.5 | Depends on composition | 40-90 | 1.5-5.0 |
Note: The values provided are approximate and depend on specific operating conditions, equipment design, and regeneration mode.
The algorithm for selecting a desiccant for a specific project includes the following steps:
- Determine the required dew point of the dried air:
- Dew point above -40°C → silica gel (most economical option)
- Dew point from -40°C to -55°C → natural zeolites or activated alumina
- Dew point below -55°C → molecular sieves
- Analyze the available heating medium temperature for regeneration:
- Temperature limited to 120°C → silica gel or composite materials
- Available temperature 150–200°C → all options except 3A and 4A molecular sieves
- Available temperature above 200°C → all desiccant types
- Assess the presence of impurities in the air or gas:
- Acidic gases present, organic vapors, particulate contamination → activated alumina preferred
- Clean air → selection is not limited
- Calculate the specific energy consumption of the regeneration cycle for each option
- Compare economic indicators, considering the initial adsorbent cost, its lifetime, and operating expenses
Common engineering mistakes when selecting a desiccant
In the practice of designing adsorption dehumidifiers, engineers often make typical mistakes that can lead to unsatisfactory system performance:
- Choosing silica gel for systems requiring a dew point below -50°C. The adsorption isotherm of silica gel shows a sharp drop in capacity at low partial pressures of water vapor, making it ineffective for deep drying.
- Confusing natural zeolites with synthetic molecular sieves. Despite similar names, these materials have different characteristics. Natural zeolites do not provide dew points below -60°C.
- Underestimating the regeneration energy demand of molecular sieves. For 4A molecular sieves, a regeneration temperature of at least 200°C is required. Insufficient temperature leads to gradual adsorbent degradation due to residual moisture accumulation.
- Ignoring chemical incompatibility of adsorbents with impurities. Silica gel deteriorates upon contact with liquid water, molecular sieves degrade under acidic gases, activated alumina can chemisorb hydrogen sulfide.
- Overestimating the expected adsorbent lifetime. A typical lifetime of 50–100 thousand cycles is achievable only when recommended operating conditions are observed.
Conclusions: balancing technical and economic factors
Selecting the optimal desiccant for an adsorption drying system is always a balance between the required drying depth, regeneration energy consumption, and overall life-cycle costs.
Silica gel remains the optimal choice for most industrial and commercial applications with dew points in the range of -30°C to -50°C due to the lowest material cost, minimal regeneration energy consumption, and sufficient chemical resistance.
Natural zeolites occupy an intermediate niche for dew points of -50°C to -60°C, where higher performance than silica gel is needed without the extreme requirements of cryogenic applications.
Synthetic molecular sieves remain indispensable for achieving dew points below -60°C in cryogenic units, instrument air preparation systems, and pharmaceutical production, despite significant operating costs.
Activated alumina is used in specific conditions for drying gases with impurities, where chemical resistance is more important than maximum capacity.
For design engineers, it is critically important to conduct a comprehensive analysis, including calculating the energy balance of the cycle, assessing available heat sources for regeneration, and forecasting operating costs for a period of at least five years.
The final decision should be based on a techno-economic comparison of options, taking into account the specifics of the particular facility, rather than dogmatically following one type of material for all applications.
