Compressed air is an important utility in refining and chemical
industries and is widely used to drive pneumatic equipment and machinery. Air
is also utilized as an oxygen/nitrogen source in manufacturing processes.
The drying of compressed air upstream of process equipment is crucial to
avoid moisture condensation in downstream piping and to satisfy moisture specifications
if air is used as the process gas. Among the available technologies, desiccant drying
has been recognized as one of the most cost-efficient and versatile options, allowing
water dewpoints down to –70°C (–94°F) degrees.
The desiccants that are conventionally offered into this service include
activated alumina, molecular sieves and silica gels. Although these materials
have been well established in industry over the years, misconceptions often
remain about some of the benefits they offer in compressed air plants. This is
especially pronounced for silica gels, as there are a variety of grades offered
in a market with substantial variability in properties and performance
characteristics in compressed air applications. This article briefly reviews the
relevant benefits and challenges associated with using the most common
desiccants and highlights the advantages of proprietary premium adsorbentsa,b
for high energy efficiency air dehydration, comparing them to the most common silica
gels desiccants offered on the market.
ADSORBENTS
Activated alumina. This is one of
the most established adsorbents used in the compressed air industry. It offers a
large surface area and a high-porosity matrix with good affinity toward polar
compounds, especially water. The highly hydrophilic nature of the alumina
surface makes it an ideal choice for adsorbing moisture. Dewpoints down to –50°C (–58°F) are accessible with
activated alumina, and regeneration temperatures are normally within the 200°C–250°C (392°F–482°F) range. Major
advantages of activated alumina include fairly high “in service” robustness and
high stability towards alkaline components (e.g., ammonia, amines or alkali
hydroxides). Combined with wide availability and a comparable low cost,
activated alumina are an ideal adsorbent for heatless dryers and a good
conventional choice for heat-regenerated dryers.
Molecular sieves. These sieves are
crystalline aluminosilicates with frameworks stabilized by monovalent or
multivalent cations from the alkali or alkaline earth group, as well as water
in its as-synthesized form. The crystal water is removed by thermal treatment
without damaging the crystalline structure to create the conditions for reversible
water adsorption/desorption. The microporous and highly ordered structure of
molecular sieves ensures very high surface areas in such materials, often in a
700 m2/g–900
m2/g range. Molecular sieves are normally used for applications
where very low dewpoints down to –100°C
(–148°F) are required or the compressed
air must be dried at higher temperatures. The steep shape of the isotherm allows
it to reach low moisture content in the product gas. However, the downside of
such high affinity to moisture is the need to heat up molecular sieve beds to
high temperatures to drive the water off. Regeneration temperatures in the 240°C–290°C (464°F–554°F) range are often used
for efficient reactivation.
Silica gel. This amorphous
and highly porous form of silicon dioxide (SiO2) exhibits high
surface areas and favorable 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 more than 50 yr. Despite
its long commercial history, users are often challenged when selecting the
proper silica gel desiccant for dehydration of compressed air. This is mostly
due to the abundance of various silica gel grades and types on the market as
well as a poor understanding of the differences in properties and performance
between these materials—this
often results in an unfavorable decision towards a lower cost but less-efficient
product.
Proprietary adsorbentsa,b. Unlike standard
market silica gels, the author’s company’s adsorbentsa,b are
alumino-silicate gels produced using a unique proprietary manufacturing
process. The patented line of highly efficient adsorbents are optimized for the
most energy-efficient drying.
Some of the key factors that are normally considered when selecting an
adsorbent include equilibrium and dynamic water capacities, regeneration
temperature (which is related to the heat of moisture adsorption on the desiccant),
and durability and hydrothermal stability of the adsorbent in service. The
company’s adsorbentsa,b exceed these performance parameters. High
dynamic water uptake combined with comparatively low moisture desorption
temperatures and outstanding hydrothermal resistance are the major benefits
making these products some of the most energy-efficient in industry.
Desiccant selection for the compressed air plant is of crucial
importance. Not only is it important that the proper adsorbent warrants steady
and reliable operation of the dryer, but it saves the user in costs. As
adsorbent replacement terms are often advised by the dryer manufacturer, the
change-outs are conducted in accordance with these guidelines. The desiccant
selection and service periods are normally not challenged by users and the
associated costs are assumed. Part of this is due to adsorbent being viewed as a
commodity and the poor differentiation between products offered on the market—this is especially
relevant to silica gels, which are available in various grades, shapes,
compositions and colors. A fundamental misconception is to equate the author’s
company’s adsorbents to conventional silica gels or alumina-promoted silica
gels.
The data provided below compares the properties and performance of the proprietary
adsorbentsa,b to some of the most common silica gels offered on the
market and underlines the importance and necessity of comparing adsorbents to
select the proper product.
The company’s adsorbents’ performance is compared with conventional and
alumina promoted silica gels that are widely offered. To make this comparison
meaningful, the desiccants were divided in two types: regular grade, which are
compared with the first proprietary adsorbenta (TABLE 1); and water
stable grades, which are compared with the second adsorbentb (TABLE 2).
Regular grade silica gels are normally characterized by high surface
area and pore volume. Products A and B in TABLE 1 exhibit high surface area pore volumes
that, at first glance, point to similarities with the company’s adsorbenta.
However, closer inspection shows that the high values of these commonly
recognized parameters are insufficient to derive performance features. Product A
is a conventional pure SiO2-based adsorbent and was shown to exhibit
inferior mechanical properties compared to the adsorbenta (i.e., on
average, a higher attrition rate and more pronounced aging under thermal swing
cycling). It is important to highlight that lower mechanical strength and
enhanced fines generation are among the common features exhibited by pure
silica gel desiccants. Another meaningful disadvantage of SiO2 adsorbents
is comparatively low hydrothermal stability. As desiccant is subject to multiple
thermal regeneration cycles in commercial service, hydrothermal aging plays a
role. While equilibrium moisture uptake on “fresh” Product A was measured as reasonably
high, a pronounced decline in moisture pickup was shown when the sample was
exposed to prolonged thermal regeneration cycles. The precise effect is highly
dependent on the specific operating conditions of the machine.
Alumina is often added to the formulation to enhance the hydrothermal
stability of silica gel adsorbents and modify some of the adsorbent properties.
Product B contains about 20 wt% aluminum oxide. While silica gel desiccants promoted
with a certain amount of alumina could feature enhanced stability towards
liquid water, the exact response is highly dependent on the manufacturing
process and the amount of alumina added in a composition. An important aspect
to consider when selecting SiO2-Al2O3
desiccants with high Al2O3 content is the altered shape
of the water adsorption isotherm, as compared to pure silica gels. As alumina
content increases, water adsorption isotherm deviates from the isotherm typically
observed for pure SiO2 and shows features more typical for alumina.
Without elaborating on the fundamental scientific nature of this
phenomena, the direct consequence (relevant to operations of an industrial
dryer) is reduced water dynamic capacity of the desiccant at humidities and
pressures normally used for commercial compressed air dryers. While the heat of
adsorption—and,
therefore, moisture uptake properties—are normally not substantially affected at alumina levels of 1
wt%–5 wt%, higher content
often results in lower moisture pick-up compared to pure SiO2
adsorbents. Product B with an Al2O3 content of 20 wt% reports
an equilibrium uptake much lower than normally observed for the proprietary
adsorbentsa,b. As a result, more adsorbent would be required (i.e.,
a larger vessel size) to achieve similar performance.
Another consideration when comparing SiO2 adsorbents promoted
with alumina is water desorption temperature. Desiccants promoted with high amounts
of alumina often exhibit higher heat of moisture adsorption and require somewhat
higher regeneration temperatures. Since an increase in moisture heat of adsorption
is almost directly proportional to alumina content, higher alumina content in
silica gels often translates into an increase in nominal regeneration temperature.
Temperatures of ~140°C are sufficient to
regenerate the adsorbenta while Product B requires ~160°C for reactivation. Although
some manufacturers do claim regeneration temperatures as low as 160°C degrees for high
alumina-containing SiO2-based desiccants, users and dryer original
equipment manufacturers (OEMs) must always execute caution when using or
designing dryers with these adsorbents.
The properties of some of the available water stable desiccants are
shown in TABLE 2
compared to the proprietary adsorbentb. This type of adsorbent is
resistant to liquid water and often used as protective layers on top of regular
grade desiccants. Product D is 99.8 wt% SiO2 pure silica gel with a surface
area of 240 m2/g. The comparably low surface area coincides with the
reduced equilibrium moisture uptake, which is almost 30% lower than what the adsorbentb
offers. As a result, Product D would have a limited contribution to the overall
desiccant bed moisture removal performance; therefore, a larger vessel size would
be needed for newly designed dryers that use this adsorbent. Another related
disadvantage is lower achievable dewpoint temperatures for a fixed volume of
adsorbent if compared to the author’s company’s adsorbentb, even for
configurations where water stable grade is used as the layer protecting a regular
grade adsorbent.
Product E is alumina promoted silica gel with a reported 16 wt% Al2O3
content with a surface area of about 450 m2/g. It features a somewhat
higher equilibrium moisture uptake as compared to Product D, but is inferior to
the adsorbentb, which offers about 20% higher loading. An important
aspect to consider when selecting a high alumina promoted silica gels-based
desiccant as a protective layer is long-term hydrothermal stability. Activated
alumina is known to undergo partial rehydration in the presence of liquid
moisture at elevated temperatures upon extensive thermal swing cycling, conditions
that are typical for the front section of the adsorbent in heat-reactivated
dryers. The adsorbentb is designed to deliver outstanding
performance at most severe hydrothermal conditions due to its proprietary
manufacturing process, with a compositional alumina of ~3 wt%.
Takeways. Selecting the
most-effective adsorbent for a compressed air dryer is important and could be
very challenging if a user or OEM are not well educated on properties of the common
desiccant grades offered on the market. This difficulty is especially
pronounced for silica gels with an abundant product selection across
applications in dryer plants. The adsorbentsa,b discussed here are high-performance
desiccants optimized for air drying with very clear value propositions and
competitive offerings. HP
NOTES
a Sorbead®
Air R
b Sorbead®
Air WS
ARTEM
VITYUK is the Global Market Manager in BASF’s Adsorbents Solutions
group, where he is responsible for compressed air and intermediates portfolios and
leads efforts to develop and commercialize new products for sustainable
processes. Dr. Vityuk earned a PhD in chemical engineering (heterogenous
catalysis) from The University of South Carolina, U.S. The author can be
reached at artem.vityuk@basf.com.