The beauty of Silica Gel

MERYT Technical Bulletin - December 2020

Among the many adsorbents that exist, there is one that stands out especially for its beauty. This is the silica gel. An adsorbent not only used in the industry, but that we find very often in our daily life, for example in small bags protecting electronic devices, footwear and various objects from moisture.

A few days ago I was very lucky to share the supervision of an industrial loading of silica gel in compressed air dryers at a refinery and I was very pleased with the comment of one of the cargo managers: "How beautiful it is, they look like diamonds." And that's right, the little translucent gel silica balls shining in the sunlight looked like precious stones.

Therefore, we thought it was a good idea for our December MERYT technical bulletin to write about this adsorbent that is both so familiar and so unknown to many of us.

What is silica gel?

Silica gel, as directed by its name, is not a gel, but is a solid adsorbent, which is usually produced in balls of different sizes, or can also be produced in powder.

Its chemical composition is silicon oxide (SiO2), the most abundant product on our planet. However, it is not an adsorbent that is obtained naturally (such as a clay) but requires a manufacturing process to confer on it the desired properties of maximum moisture adsorption capacity. The term gel is given because at one stage of this manufacture it goes through a state in which the solution is a gel.

Its structure, as shown below, is based on links by siloxane bridges (Si-O-Si) resulting in a material with amorphous characteristics, resistant and very porous, which allows adsorption of both inorganic and organic compounds. Its surface is covered by hydroxyl groups (in the form of Silanol Si-OH groups) that are very important to promote adsorption.

How is the silica gel produced?

The silica gel is produced from an aqueous solution of sodium silicate, which is acidified with sulfuric acid, to produce a gelatinous precipitate (hence the gel name). This precipitate is washed with water and then carefully dehydrated, thus obtaining the silica gel (SiO2.nH2O).

Characteristics of the silica gel

It is an extremely porous structure, characterized by the uniformity of the arrangement of pores and their sizes. The surface area of the pores of the silica gel varies according to the manufacturing method, reaching values up to 800-850 m2/g. This large surface area formed by the interconnected pores retains water by adsorption and capillary condensation, allowing the silica gel to adsorb up to 40%of its weight in water, that is, for each ton of silica gel we will be able to adsorber up to 400 kilos of water, depending on the conditions.

With regard to pores, depending on the type of silica gel, their size can vary from 5? to 300?, while the volume of them can be between 0.3 to 1.3 cm3/g.

This adsorbent is non-toxic, non-flammable and not chemically reactive, and this is the most common form of silica gel, in the form of beautiful translucent spheres:

There are also two other forms of silica gel that are made with color indicator to alert us visually when the silica gel has become saturated with moisture.

One is the blue one, which many of us remember from the desiccators present in all laboratories. This blue color is due to the addition during the manufacturing process of the compound cobalt chloride, which is blue in the absence of moisture and turns pink when the silica gel is saturated with moisture. It tends to avoid this type of adsorbent due to the toxicity of cobalt, because it requires its disposal as hazardous waste. 

The other form of silica gel with indicator is orange. This color is due to the violet compound of methyl, which turns from moisture-free orange to dark green when saturated with moisture.

Below are the main characteristics of the type A gel silica that we supply in MERYT:

Applications of silica gel

The gel silica has multiple applications, although its best known application is that of adsorption of moisture in air. However, it can also be used to absorb water from other gases such as carbon dioxide, oxygen, nitrogen, hydrogen, carbon monoxide, noble gases and acid gases. It can also be used to dry LPG-type liquids, solvents, chlorinated hydrocarbons and to pretreat oils.

One of the most well-known applications is the drying of compressed air that is required in industrial plants. In many of these plants, there are dryers formed by two gel silica beds in which one bed is in adsorption mode and the other bed is in regeneration mode. Operating cycles can be several hours and air quality 1, 2 and 3 and air dew points below -20C (and up to -70C) can be obtained according to ISO 8573-1. The temperature at which the silica gel is regenerated (150-160C) is lower than that of activated alumina (180-200C), another classic adsorbent for air drying, so that it allows significant energy savings and CO2 emissions, compared to alumina.

The gel silica plays a key role in electrical power transformers, where it dries the air that is in contact with the oil. This oil should be as dry as possible to avoid negative effects on the equipment and also to extend its service life. 

Another very important use of silica gel is in maritime containers, in which, on its internal walls, bags full of gel silica are hung to remove moisture from the container that can infiltrate along the crossing, and in this way protect the goods transported in it. 

Silica gel is also used as a conservation agent for book collections in museums and libraries, as very old specimens must be preserved from moisture to prevent deterioration.

With regard to its use to purify oils, it is used both in biodiesel production plants, for removing contaminants from the oil, and for the purification of oils for food use, thanks to its null toxicity.

Many more applications of silica gel are found in industries such as electronics, pharmaceuticals, paint industries ,steel, paper, glass and also in pets???, where it is widely used as an alternative to cat sand.

As you can see, this nice adsorbent is very useful in multiple fields of our lives. We hope that this newsletter has been interesting to you and, as always, we will be very happy to give you additional information about this or other adsorbent and/or catalyst. And, of course, remember that we can provide any of them.

Happy Christmas holidays and all the best for the New Year 2021!!