Industrial silica gel for air and gas drying

What silica gel is

Silica gel is a synthetic, amorphous form of silica, commonly written as SiO₂·nH₂O. It is produced as beads or irregular granules with a porous internal structure. The pores give the gel a large internal surface, and this surface is where water vapour is held.

Industrial silica gel is not one product. Grades differ in pore size, pore volume, resistance to liquid water and composition. Alumino-silicate gels also contain aluminium oxide. These differences affect where each grade is used.

How silica gel adsorbs water

Water molecules are polar and are attracted to the surface of the gel. At first they are bound directly on the pore walls. As the load increases, water vapour also condenses in the narrow pores; this is called capillary condensation. Both processes are physical and reversible.

Adsorption releases heat. Regeneration releases the adsorbed water and removes it from the bed. In heat-regenerated systems, hot gas carries the released water away, and the bed is then cooled before reuse. Follow the instructions for the selected grade and equipment; see the FAQ below for regeneration temperatures.

Pore size and humidity: pore size affects how much water vapour a gel adsorbs at a given relative humidity. The amount also depends on temperature and the selected grade. The Types and data tab explains the differences and compares two example grades.

Beads and granules: silica gel is supplied as spherical beads or irregular granules in different sizes. The form and size are chosen for the equipment and the flow through the bed. Selected beaded grades are designed for applications where dust and attrition are critical.

Moisture indicator: indicating grades contain an indicator that changes colour as the gel adsorbs moisture. The colour change gives a visual indication of moisture uptake. The loading at which the colour changes depends on the indicator, so it does not mean the same thing for every product.

How to choose a silica gel type

Four properties help distinguish industrial silica gels: pore structure, resistance to liquid water, composition and the presence of a moisture indicator. These properties can overlap. Our orange indicating gel, for example, is a narrow-pore type A gel.

They are a starting point for selection. The grade, particle size and bed arrangement must match the gas or liquid and the inlet conditions. They must also suit the required performance and the equipment’s regeneration method.

Narrow-pore silica gel (type A) has narrow pores and a large internal surface. Type A generally adsorbs more water vapour than type B at low relative humidity. The chart below compares two example grades at 25 °C.

Mesoporous silica gel (type B) has medium-sized pores. It generally adsorbs less water vapour than type A at low relative humidity and more at high relative humidity. Compare the data for the selected grades under the same conditions.

Wide-pore silica gel has wide pores and a high pore volume. It can take up large amounts of water vapour at high relative humidity. Selected grades are used as moisture-protection layers in drying processes, but this does not mean they are resistant to liquid water.

Water-resistant silica gel (FNG type) is a gel with medium to wide pores in hard beads that resists contact with liquid water. It is used as an inlet guard layer to protect the main adsorbent bed.

Alumino-silicate gels, type R and type WS, are hard beads of silicon dioxide that also contain aluminium oxide. Type R is used as the main drying bed and is not resistant to liquid water. Type WS is water-resistant and protects type R or other adsorbents; suitable WS grades can also form a standalone bed.

Resistance to liquid water and suitability for hot, humid regeneration gas are two different properties. A gel that tolerates contact with water is not automatically suitable for heat-of-compression dryers, so check the data sheet of the selected grade.

Indicating silica gel contains a moisture indicator. Our orange gel turns from orange to dark green, and its indicator contains no cobalt compounds. The yellow gel turns light blue.

Silica gel types at a glance

For these two example grades, type A adsorbs about twice as much water vapour as type B at 20% and 50% RH. At 90% RH, the values are 35% by weight for type A and 60% for type B. These are typical data-sheet values at 25 °C from one manufacturer. Lines connect the listed values as a guide to the eye; they are not measured isotherm curves or dryer performance data.

Data for selected grades

Orange type A silica gel, 2-5 mm: typical equilibrium water adsorption is 10% by weight at 20% RH, 25% at 50% RH and 35% at 90% RH, all at 25 °C. Its average pore diameter is about 2-3 nm.

Wide-pore granules, 5-8 mm: the TDS gives a minimum water adsorption of 80% at 100% RH and an average pore diameter of 8-11 nm. The adsorption test temperature is not stated in this TDS, so this value is not directly comparable with the values at 25 °C.

Original BASF Sorbead® Air R and Air WS: their data sheets give typical equilibrium water capacities of 42% by weight at 80% RH and 25 °C for both products. At 10% RH and 25 °C, the values are 6.5% for Air R and 4% for Air WS. These figures describe the named BASF products. Use the TDS of the selected equivalent grade for comparison.

RH means relative humidity. Equilibrium capacity is not the working capacity of a dryer bed and does not determine the bed quantity on its own.

Storage: store silica gel in tightly closed packaging in a dry room to prevent moisture uptake before use. Reseal opened packaging carefully.

Documents: TDS and SDS are available on request for the selected grade. The SDS gives handling information.

Where industrial silica gel is used

Compressed air drying is the main application of the gels on this page. They also have other roles: drying the air that enters equipment, protecting adsorbent beds and treating gases and liquids. The table gives a starting point for selection; the sections below explain the role of each grade.

Compressed air drying

The desiccant for a compressed air dryer must match its regeneration method and the required pressure dew point. In heat-regenerated dryers, alumino-silicate gels are used as the main bed, usually type R with a layer of water-resistant type WS on the inlet side. In heat-of-compression dryers, the gel must be suitable for hot, humid regeneration gas. The table compares the main dryer types.

The ranges summarise manufacturer guidance for suitable dryer designs; they are not guaranteed properties of every gel. The pressure dew point is specified at the operating pressure. Selection also depends on regeneration temperature, cooling method and cycle time.

Source: our article on desiccants for compressed air dryers, based on BASF and Atlas Copco information.

Technical and process gases

Suitable silica gel grades are used for technical gas drying. The Sorbead® Air R data sheet lists nitrogen, oxygen, carbon dioxide and liquefied gases. For an equivalent grade, check its data sheet against the requirements of the gas and process.

Natural gas: dedicated grades such as Sorbead® H remove water and heavy hydrocarbons in the same adsorption unit, so they control both the water and the hydrocarbon dew point. Narrow-pore beaded gels such as SYLOBEAD® SG B 127 are also used for gas drying and hydrocarbon dew point control. Where a very low water content is required, for example before cryogenic processing, molecular sieve beds are used downstream of the silica gel or instead of it.

Air separation plants: EIGA guidance describes silica gel as an adsorbent used in cryogenic adsorbers for carbon dioxide and hydrocarbon removal. This application requires a grade selected for the specific process. Separately, BASF lists air separation plants among the applications of Sorbead® WS involving regeneration with humid gas.

Protection of adsorbent beds

Liquid water carried into an adsorber can severely damage the first adsorbent layers. A water-resistant layer on the inlet side takes up this water and protects the main bed. The water-resistant FNG type protects standard silica gel, molecular sieve and other adsorbents; suitable type WS grades protect type R, molecular sieve, activated alumina, activated carbon and catalysts.

In natural gas processing, selected buffer grades also capture entrained liquids, such as heavy hydrocarbons and glycol, before they reach the molecular sieve bed.

Silica gel for transformer breathers and other breathers

Transformers with a conservator, gearboxes, hydraulic units, storage tanks, drums and IBCs draw in ambient air when the temperature or the liquid level falls. A breather filled with silica gel dries this air before it reaches the oil or the stored product. Suitable indicating silica gel is commonly used because its colour change gives a visual indication of moisture uptake. Replace or regenerate the gel according to the product and equipment instructions.

Catalysts and liquids

Selected mesoporous and wide-pore grades are used as catalysts and catalyst carriers in chemical and petrochemical processes. Other uses include petroleum refining and the dewatering and purification of liquids. Select the grade for the specific process and liquid.

Other uses

Depending on the grade, silica gel is also used for moisture control in enclosed spaces without forced air flow, as packing in chromatography columns and in adsorption cooling systems.

BASF Sorbead® and water-resistant gels

Originals and equivalents: we supply the original BASF Sorbead® products. We also supply equivalent grades for datasheet comparison. Compare the selected product with the original data sheet and the dryer specification before ordering. Products we supply include:

• Sorbead® Air R and Sorbead® Air WS for compressed air dryers, and Sorbead® WS as a water-resistant layer that protects other adsorbents and catalysts: originals, or equivalent type R and type WS grades for datasheet comparison,
• Sorbead® H for hydrocarbon dew point control in natural gas,
• Sorbead® Orange CHAMELEON®, an indicating gel used in transformer breathers. It changes from orange to colourless as it adsorbs moisture, before reaching its full adsorption capacity,
• KC-Trockenperlen®, the German trade name of the same BASF product family,
• equivalents of Grace SYLOBEAD® SG B 127, a narrow-pore beaded silica gel for gas drying and hydrocarbon dew point control.

Sorbead® and KC-Trockenperlen® are trademarks of BASF. CHAMELEON® is a registered trademark of its owner. SYLOBEAD® is a trademark of its owner. Product names are used only to identify the products.