10 Minute Read
By Martin Scott
BASF are the world’s largest chemical manufacturer and produces market-leading desiccants for the compressed air industry.
In this article, we take a closer look at desiccant technology and BASF's range of products for the compressed air industry.
A desiccant is a hygroscopic material with a high surface area due to an intricate microporous structure. The desiccant can form physical bonds with different molecules and hold them within the structure. The more polar the molecule, the stronger the substrate is bound onto the desiccant. Water being of a high polarity is what makes a desiccant ideal for the drying of air.
Desiccants are used within industry in drying units to remove moisture from compressed air for many different processes. Desiccant containing compressed air dryers are the secret behind many manufacturing processes ranging from the food and feed industry through to automotive manufacturers, electronics and many more.
The type of desiccant that should be used in a dryer is very much based on the requirements of the application and the air quality standard required.
The International Standards Organisation (ISO) established quality standards for compressed air to guide the design and selection of various air compressor and dryer components. ISO 8573 air quality standard and ISO 12500 compressed air filter standard make the basis for air treatment product selection much easier. The air quality standards are specified into 9 different classes with class 0 being the driest and highest quality of compressed air widely used in the medical and food sectors for example. These air quality class ratings can be seen in Figure 1.
Figure 1: Classification of air quality according to ISO 8573-1:2010
A compressed air dryer can be designed and operated to achieve a particular level of Pressure Dew Point (PDP). The PDP is the indicator in which the dryness of the air can be described as the temperature at which water vapour condenses at the current working pressure. As far as desiccants are concerned, different types may be recommended depending on the PDP required by the process.
The two most common types of desiccant dryers are pressure swing adsorption (PSA) and heat-regenerative adsorption (HRA) dryers. HRA dryers use heat to desorb the moisture from the desiccant therefore regenerating the desiccant for use in a new cycle whereas PSA dryers use a change in pressure to desorb the desiccant which requires the use of a purged dried compressed air to regenerate the desiccant. This can mean up to 25% of dried compressed air may be needed from the currently active tower to regenerate the desiccant in the tower not in use.
There are two main types of HRA dryer systems which may be described as purged and purgeless. A standard adsorption dryer is desorbed with externally heated fans, similar to the more economical purgeless system. However, in contrast to the modern purgeless systems, a partial flow of compressed air (purge) is required for cooling. The pressure dew points of a standard heat regenerative dryer are -25 to -40°C and -70°C.
A more economic system, known as a purgeless unit does not use any purged air to help with the regeneration of the desiccant. A purgeless unit configuration can be seen below in Figure 2.
Figure 2: Compressed air dryer with purgeless regeneration
HRA dryers are low-energy units that are desorbed and cooled with drawn-in ambient air (blower air). An external electric heater, steam or other medium can be used for heating. These units are lower-energy dryers and depending on the dew point required depends on the type of desiccant recommended for the units. The pressure dew point of an economic system is typically -25 to -60°C.
A third dryer type, a Heat-of-Compression (HOC) unit is a heat-regenerated adsorption dryer that uses hot gas from an oil-free compressor for full or split stream desorption. The pressure dew point achievable with a Heat-of-Compression dryer is typically -15 to -40°C.
Figure 3: Compressed air dryer with Heat-of-Compression (HOC) regeneration
BASF offers a portfolio of desiccants for the drying of compressed air. These include their F-200 activated alumina, Sorbead® AIR silica gels, and BASF Molecular Sieves. These desiccants reach dynamic adsorption capacities above 20% by weight and can reach pressure dew points to < -70 °C.
The type of desiccant that should be used inside a unit is based on the type of unit that you have and the dew point that you wish to reach.
The efficiency of a compressed air unit is strongly influenced by the adsorption capacity, regeneration characteristics and the practical working lifetime of the adsorbent. The lower the desorption temperature and the longer the lifetime of the adsorbent, the higher the efficiency of the dryer system and the plant utilising it.
HRA dryer units may take advantage of higher efficient desiccant and as a result, Sorbead® Air (previously known as KC-Trockenperlen®) should be the first choice for HRA units due to it being an economical and more environmentally friendly solution due to it’s lower desorption temperature when compared to activated alumina.
Sorbead® Air has a high adsorption capacity and an energetically favourable regeneration temperature of 120 - 150°C which may lead to lower running costs based on the energy consumed to regenerate every cycle. With potential PDPs down to -60°C and a lifetime of 10 years Sorbead® Air should be the desiccant of choice for most HRA dryer units in the market.
Sorbead Air® R and Sorbead Air® WS are available through Lawrence Industries for the UK market. Sorbead Air® WS is the only water-resistant high-capacity, porous silica on the market and can be fully submerged in water without breaking down which is why it is commonly used as a “guard” layer used at around 15% with Sorbead Air® R.
Some of the following unique properties are the reason behind the efficiency of Sorbead® Air range:
Figure 4 and Figure 5 below shows an example of the energy costs and savings that may be achieved with a dryer filled with Sorbead® Air compared to a dryer with standard desiccant.
Figure 4: Sorbead® Air energy-saving benefits based on average energy consumption and energy savings at a compressed air unit, externally heat regenerated 66 m³/min, 7 bar, 35°C, -40°C PDP.
Figure 5: Lifetime of Sorbead® Air versus standard systems. Sorbead® Air achieves 3x the lifespan of conventional systems.
BASF Molecular Sieves (also known as zeolites) are used when particularly deep pressure dew points to -100°C are required and also in cases where the compressed air to be dried has a low relative humidity or is already pre-dried. Molecular sieves are often used when Class 0 and Class 1 air standards are required due to the low PDP achievable which means they can also be used in conjunction with BASF F-200 in a mixed bed drying unit to achieve a lower PDP than alumina can manage alone.
Molecular sieves also can be used as a suitable material for the selective separation of gas mixtures due to the range of controlled pore sizes available.
BASF F-200 activated alumina is rapidly becoming the industry standard for the drying of compressed air. BASF’s material is known for its uniform bead size and for its high abrasion resistance leading to very low levels of dust formation from attrition. Desorption at temperatures of 180-200°C, similar to that of molecular sieve is the exact reason why they can be used in conjunction as a mixed bed where lower PDP’s are needed. F-200 is available in a range of different bead sizes to suit different drying units. Sizes range from a small bead of 7x14 Tyler Mesh (2.0 mm) through to the largest bead sizes which is 1/4” (6.4 mm).
Desiccant selection depends on the application, the required pressure dew point, the type of dryer and the energy savings you wish to achieve over a long-term period. For heat-regenerated adsorbent dryers which are economical systems then Sorbead® Air is the best choice. For other dryers where desorption temperature is not a factor or perhaps a lower PDP is needed, BASF offer Molecular Sieves and F-200 activated alumina.
The selection guide in Table 1 outlines the usage scenarios using BASF’s desiccants.
Table 1: BASF adsorbents selection table for compressed air dryers
For further advice and recommendations please contact us and we will be more than happy to discuss your requirements.
Martin completed his Chemistry degree with a focus on Medicinal Chemistry at the University of Warwick in 1999. Since then he has held several Sales and Management positions in the chemical industry. He has now been with us since 2014 and looks after accounts in the Coatings and Agrochemical industries.
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