BASF Desiccants and their role in industrial air drying
BASF is the world’s largest chemical manufacturer and produces the highest quality desiccants for a wide range of drying applications across many industry sectors. Their market-leading desiccants range is not only high performing but also stable and reliable over extended time periods. This article discusses the different types of desiccants in the range, their uses in various applications and their wider influence on the chemical industry.
In this technical article
What is a desiccant?
A desiccant is a hygroscopic material which is used to maintain a dry environment by the process of adsorption, an exothermic surface phenomenon where molecules bind to the surface of the adsorbent. This is used in liquid or gas drying and plays a vital role in dehydration, purification and separation.
Why are desiccants used?
During industrial processes when the ambient air is compressed, water, in liquid form, is produced by condensation. This water needs to be removed to reduce the risk of corrosion and operational disturbances in the system as these could lead to additional plant maintenance. Compressed air is widely used across many industry sectors such as the Automotive, Brewing, Paper, Textile, Pharmaceutical and Petrochemical industries to name a few, so the need to remove water is widespread.
Figure 1: Desiccants are commonly found in shoes and bags in little packets like the above to prevent any moisture damage.
Desiccants in everyday life
Desiccants are not only used in industrial applications, but also in our day to day life. They are commonly used in the manufacture of insulated windows where a spacer is filled around the perimeter of the panes of glass to prevent the formation of moisture and condensation. Desiccants are also used in air conditioning systems as well as protecting consumer goods such as clothes, shoes and electronics which would otherwise be susceptible to damage caused by mould or damp due to an excess of water vapour.
What is a dew point?
The dew point is the temperature air must be cooled to, for water to turn from water vapour into liquid water. This is dependent on temperature, air pressure and humidity as they all affect the equilibrium of water between these two states. The dew point is often given as a requirement when deciding on a desiccant depending on how dry the air needs to be for the application.
What are the different types of desiccants and what do each of them do?
Activated Alumina as a desiccant
These are hard, spherical beads made from aluminium hydroxide and are highly porous; an excellent adsorbent for drying a wide variety of liquids and gases. They also have a very high surface area (in the region of 800 m2 / g) and are a uniform size which helps minimise fluids moving across one side of the vessel and bypassing the desiccant bed, a phenomenon known as channelling. This makes them highly effective in the drying process and typically molecules with the highest polarity are preferentially adsorbed. The efficiency of adsorption is affected by many factors, namely temperature, pressure and the molecular weight of the molecules in the gas stream.
Advantages and disadvantages of Activated Alumina
The major advantages of activated alumina are high robustness in the presence of liquid moisture and good stability against alkaline components such as ammonia and amines. They are widely available and very cost-effective, making them a good choice for heat-regenerated dryers. BASF’s F-200 range is available in a wide range of sizes from 1/16’’ to 1/4".
However, some disadvantages include sensitivity to compressor oil vapours which can block the pores on its surface and significantly reduce its water adsorption capabilities. Furthermore, rehydration can also have an adverse effect; this is when a portion of aluminium oxide is converted to aluminium hydroxide in the presence of moisture at high temperatures. Unlike aluminium oxide, aluminium hydrates do not exhibit the high surface area and porosity required for efficient water adsorption.
Figure 2: Activated Alumina beads (above) look harder than the silica gel beads but this offers excellent robustness in the presence of moisture.
Molecular Sieves as desiccants
Molecular sieves belong to the zeolite class of materials such as hydrated alkali metal or alkaline earth aluminosilicates. They are typically synthetic crystalline alumino-silicate with a regular micropore structure. Commercially referred to as 3A and 4A, the A refers to Angstroms, Å, meaning the name is indicative of the size of the molecular pore. Molecular sieves allow some molecules to pass through whilst inhibiting others (hence behaving as a sieve). The two most common types used in commercial dehydration applications are zeolite A and zeolite X. Zeolite X varies from A as it has a lower Si/Al atomic ratio, the higher the ratio, the higher the thermal stability. They are commonly used for drying organic liquids, air, liquid gases and noble gases.
Advantages and disadvantages of Molecular Sieves
Molecular sieves are used for applications where very low dew points are required, typically down to - 100 °C and are hence extremely efficient desiccants.
An important difference between molecular sieves and activated alumina/silica gels is their ability to maintain water uptake at high temperatures and low water partial pressures. The downside of such a high affinity to moisture is that a high regeneration temperature is needed to drive the water off; This is typically in the region of 230–290 °C. A major disadvantage of molecular sieves for gas dehydration is a high sensitivity towards impurities in the gas streams, such as heavy hydrocarbons, or very acidic or basic compounds (SOx, NOx or NaOH). If molecular sieves suffer structural disintegration due to these contaminates, it can result in dusting which can cause an increase in pressure drop. This in turn will cause additional stress inside the system. They are also unable to tolerate liquid water as it can cause them to break down, so beds need to be protected from this.
Silica gels as desiccants
Silica gel is an amorphous and highly porous form of silicon dioxide (SiO2) exhibiting high surface areas and favourable water adsorption properties. Silica gel is commercially available as granular and spherical bead material of various size ranges and has been widely used in the compressed air industry for over 60 years. Their typical surface area is 700 – 750 m2 / g and they can achieve a dew point of -40 °C.
Some silica gels are colour indicating, changing for example from orange to colourless (such as in BASF’s Sorbead® Orange Chameleon) to indicate saturation. Equipment such as gearboxes and pumps need to take in air or ‘breathe’ during operation, and when they do they typically introduce moisture which acts as a contaminant. Breathers containing silica gel adsorb this moisture, thereby protecting and prolonging the life of the equipment. The colour change of the desiccant indicates when these breathers need to be changed.
Advantages and disadvantages of silica gels
Some of the advantages are that they have very stable thermal and chemical characteristics, are non-toxic, odourless and available in a wide range of bead sizes to suit any application. They can also be regenerated over and over again and hence have a very long shelf life. However, they can be very sensitive to water droplets and hence they require a protective layer often called a ‘guard layer’ to keep them away from direct exposure to liquid water which would cause cracks, reducing the mechanical strength of the beads. An example of this is BASF’s Sorbead® Air WS which stands for ‘water stable’.
Figure 3: Silica gel is one of the most common recognisable types of desiccants. It's ability to be readily regenerated gives it an extremely long shelf life.
Regeneration of Desiccants
Regeneration can either be achieved by an increase in temperature, known as ‘thermal swing adsorption’ or TSA or through a reduction in pressure known as ‘pressure swing adsorption’ or PSA. The purpose of regeneration is to get the desiccant to relinquish its water. Both methods have their advantages and disadvantages.
TSA is the most effective at removing adsorbed contaminants such as Carbon Dioxide and water, and is often used in cryogenic oxygen plants and natural gas dehydration. This method though can be very expensive. Regeneration temperatures can vary greatly, from 170-230 °C for Activated Alumina to 200 °C for Molecular Sieves.
PSA occurs when the pressure is reduced across the sieve bed resulting in the adsorbent being removed. Sometimes a vacuum can also be used to reduce the pressure even further. Although PSA is not as effective as TSA in desiccant regeneration, it uses less heat and hence could be more cost-effective. PSA dryers are also not as energy efficient as a significant amount of compressed air is lost.
Summary
Desiccants are widely used throughout various industrial sectors to remove water and other contaminants from gas streams. This serves to increase the lifespan of plant equipment by reducing the risk of corrosion by removing moisture in the form of a condensate, as well as producing a higher quality end product. There are three main types of desiccants all which have their advantages and disadvantages depending on the system.
To get more information about the desiccants or with help as to which would be best for your application, get in touch at 01827 314151 to speak to our technical sales manager for the process chemicals industry.
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