Selecting a desiccant for a dehumidifier: Comparing different desiccant types

Author: Mycond Technical Department

The correct choice of adsorbent (desiccant) is a key factor in the effectiveness of dehumidification systems. In industrial and commercial air drying, different types of adsorption materials show significant differences in performance, energy consumption, and operational characteristics. This article provides an engineering analysis of five main desiccant types and proposes a methodology for selecting the optimal material for specific applications.

The physical basis of adsorption dehumidification

Adsorption dehumidification is based on the ability of porous materials to capture water vapour molecules from the air. This process occurs through two primary mechanisms: physical adsorption (retention of moisture on pore surfaces due to Van der Waals forces) and chemisorption (formation of chemical bonds between water molecules and the adsorbent material).

A key characteristic of an adsorbent is its adsorption isotherm—the relationship between the amount of adsorbed moisture and the relative humidity of the air at a constant temperature. The shape of this curve determines the effectiveness of the material under different operating conditions.

It is important to distinguish between static (equilibrium) and dynamic capacity. Dynamic capacity is the actual amount of moisture a material can capture under operating conditions, taking into account contact time, flow rate, and regeneration cycles.

Five main desiccant types are used in industrial and domestic systems: silica gel, natural zeolites, synthetic molecular sieves, activated alumina, and composite and hybrid materials.

Silica gel for air drying

Silica gel is an amorphous silicon dioxide (SiO₂) with a well-developed pore system. Its structure contains macropores (over 50 nm), mesopores (2–50 nm), and micropores (under 2 nm), which determine the adsorption rate and overall capacity of the material.

The adsorption isotherm of silica gel has a characteristic S-shaped form with maximum capacity in the 40–70% relative humidity range. At lower humidity levels, the capacity of silica gel decreases sharply, limiting its effectiveness for deep drying.

The operating process temperature range for silica gel is from -10°C to +50°C. Regeneration temperature is typically 100–150°C depending on saturation level and available thermal power.

Under optimal regeneration conditions, standard silica gel provides a dew point of -40°C to -50°C. This characteristic depends on adsorbent bed thickness, flow rate, and the duration of the adsorption–regeneration cycle.

Silica gel is widely used in industrial ventilation, warehouses, and domestic dehumidifiers where ultra-deep drying is not required, but moderate cost and low regeneration energy are important.

Natural zeolites for adsorption drying

Natural zeolites are aluminosilicates with a crystalline structure and a microporous system formed by a framework of silicon and aluminium tetrahedra. The pore size of natural zeolites varies from 0.3 to 1 nm depending on the mineral type (clinoptilolite, mordenite, chabazite).

The water vapour adsorption characteristics of natural zeolites feature a steeper isotherm compared to silica gel. This is due to the higher affinity of polar water molecules for the cations in the zeolite structure.

Regeneration temperatures for natural zeolites are typically 150–200°C, higher than for silica gel due to stronger adsorption bonds. With sufficient regeneration, natural zeolites provide dew points of -50°C to -60°C.

Natural zeolites are used where deeper drying is required than silica gel can provide, but without the need to achieve cryogenic dew points. Their advantage is lower cost compared with synthetic molecular sieves thanks to raw material availability and simpler production technology.

Synthetic molecular sieves

Synthetic molecular sieves are artificially synthesised zeolites with precisely controlled pore size and chemical composition. The main molecular sieve types include:

• Type 3A with an effective pore diameter of 3 Å for adsorbing only water
• Type 4A with 4 Å pores for adsorbing water and small molecules
• Type 5A with 5 Å pores for a broader range of substances
• Type 13X with 10 Å pores for a wide spectrum of molecules

High affinity for water due to a high concentration of cations and pore uniformity enables adsorption even at very low relative humidity. With a properly designed regeneration cycle, molecular sieves can achieve dew points down to -70°C.

However, effective regeneration of molecular sieves requires high temperatures—typically 180–250°C depending on sieve type and the required drying depth. This is due to the strong adsorption bonds between water molecules and the sieve structure.

Molecular sieves are used in compressed air treatment systems for instrumentation and control (I&C), in cryogenic air separation units, and in pharmaceutical and food industries where extremely low dew points are required. High effectiveness is accompanied by significant cycle energy consumption and higher material cost.

Activated alumina and composite desiccants

Activated alumina is a porous material with amphoteric properties, capable of adsorbing both acidic and alkaline impurities in addition to water vapour. Its structure is characterised predominantly by mesopores with some micropores, providing intermediate characteristics between silica gel and zeolites.

Depending on regeneration conditions, activated alumina provides dew points in the range of -50°C to -65°C. Regeneration temperatures are typically 150–200°C.

A key advantage of alumina is its increased chemical resistance to acidic gases (hydrogen sulphide, carbon dioxide) and organic impurities. This makes it suitable for drying process gases containing contaminants.

Composite and hybrid desiccants are created by combining the properties of base materials. Examples include:

• Silica gel impregnated with lithium chloride, providing increased dynamic capacity at low regeneration temperatures (60–80°C)
• Mixed layers of different adsorbents in a single rotor or cassette to optimise the process
• Metal-organic frameworks (MOFs) with record specific surface areas up to 7000 m²/g
• Polymer adsorbents with tunable porosity

Composite materials can deliver increased capacity at reduced regeneration temperatures or improved selectivity for water in the presence of other components. However, most new materials remain at the laboratory research stage or in limited industrial deployment due to high synthesis costs and insufficiently studied long-term stability.

Comparing desiccant effectiveness

Below is a comparative table of the main parameters of different desiccant types. All values are indicative and depend on specific operating conditions, equipment design, and the regeneration regime.

Algorithm for selecting a desiccant for a project:

1. Determine the required dew point:
   • Dew point above -40°C: consider silica gel
   • Dew point from -40°C to -55°C: natural zeolites or activated alumina
   • Dew point below -55°C: molecular sieves
2. Analyse available regeneration temperature:
   • Up to 120°C: silica gel or composite materials with low-temperature regeneration
   • 150-200°C: all options except molecular sieves
   • Above 200°C: all desiccant types, including molecular sieves
3. Assess the presence of contaminants:
   • With acidic gases, organic vapours, or particulate contamination present: give preference to activated alumina
   • For clean air: this factor does not constrain selection
4. Calculate regeneration cycle energy consumption for each option
5. Compare economic metrics (initial cost, service life, operating costs)

Example selection logic: For an air dehumidification system in pharmaceutical production with a required dew point of -65°C and 6 bar steam (160°C) available:

• Silica gel will not deliver the required dew point — excluded
• Natural zeolites can theoretically reach -60°C, but with insufficient safety margin — risky
• Activated alumina can deliver -65°C at a regeneration temperature of 180–200°C, but only 160°C steam is available — ineffective
• 4A molecular sieves provide the required dew point but require 200–220°C for regeneration, which is above what is available
• Solution: implement a two-stage system with pre-drying on zeolites to -55°C and final drying on molecular sieves to -65°C with electric regeneration of the second stage

Common mistakes and proper adsorbent selection

Typical engineering mistakes:

1. Selecting silica gel for systems requiring a dew point below -50°C due to insufficient understanding of the adsorption isotherm limits
2. Confusing natural zeolites with synthetic molecular sieves, leading to incorrect expectations of achievable dew point
3. Underestimating the regeneration energy demand of molecular sieves (4A requires a minimum regeneration temperature of 200°C)
4. Ignoring chemical incompatibility of adsorbents with impurities present in the air or gas
5. Overestimating expected adsorbent service life under harsh operating conditions

Misconceptions:

• Higher initial desiccant characteristics always mean better system operating performance
• Composite desiccants universally outperform traditional materials

Application limitations:

• At air temperatures below -10°C, adsorption rate decreases for all desiccant types
• At relative humidity above 90% and temperature over 30°C, silica gel may reach its capacity limit
• For facilities with airflows over 50,000 m³/h, it is advisable to consider hybrid schemes
• When liquid water is present, moisture separators must be installed upstream of the adsorber

Conclusions and recommendations

The key principle in desiccant selection is balancing the required drying depth, regeneration energy consumption, and total life-cycle costs:

• Silica gel – optimal for most industrial and commercial applications with a dew point of -30…-50°C due to the lowest cost and regeneration energy consumption
• Natural zeolites – for dew points of -50…-60°C where higher effectiveness than silica gel is required
• Synthetic molecular sieves – indispensable for dew points below -60°C, despite significant operating costs
• Activated alumina – for drying gases with impurities where chemical resistance is important
• Composite materials – for specific applications with special requirements

For design engineers, it is critical to carry out a comprehensive analysis, including calculation of the cycle energy balance, assessment of available heat sources for regeneration, and forecasting operating costs for a period of at least five years.

FAQ: Answers to frequently asked questions

Why is silica gel unsuitable for achieving a dew point of -60°C even with deep regeneration?

The adsorption isotherm of silica gel shows that at relative humidity below 5% (which corresponds to a dew point of -50°C at 20°C), dynamic capacity drops below 2% by mass, while effective system operation requires a capacity of at least 5–8%. Even at a regeneration temperature of 180°C, silica gel cannot adsorb enough moisture at such low water vapour partial pressures.

How do you determine the required regeneration temperature for a specific desiccant type?

The regeneration temperature is determined from the material’s desorption isotherm. For silica gel, when drying air to a dew point of -40°C, regeneration at 120°C is sufficient. For molecular sieves at a dew point of -65°C, a regeneration temperature of at least 200°C is required.

Can one desiccant type be used for all applications?

No, this is technically inefficient and economically impractical. For domestic dehumidifiers (dew point -20…-30°C), silica gel is optimal. For industrial systems (dew point -40…-55°C), silica gel or natural zeolites are suitable. For cryogenic plants (dew point below -60°C), molecular sieves are necessary.

What affects the actual service life of an adsorption material?

Service life is defined as the number of adsorption–regeneration cycles until capacity falls below 80% of the initial value. It is affected by: thermal stresses during regeneration, the presence of liquid water or condensate, chemical impurities, and mechanical vibration and shock. Typical service life is 50–80 thousand cycles for silica gel and 80–120 thousand cycles for molecular sieves, but under adverse operating conditions it may drop to 10–20 thousand cycles.

Selecting the optimal adsorbent for an air dehumidifier requires careful analysis of project requirements and material characteristics. The right decision ensures not only effective drying but also economically justified system operation throughout its life cycle.