More about Adsorption, Desiccants
and Sorbents
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What is adsorption?
By definition adsorption refers
to the concentration of a fluid component (gas or
liquid phase) onto the surface of a solid. Activated
aluminas refer to that class of aluminum oxides that
are very hydrophilic in nature and possess high
degrees of bulk porosity. As a result of chemical
structural properties activated aluminas exhibit
high adsorption capacity for water and well as other
polar substances. Once saturated with the adsorbed
material the alumina are easily regenerated by
thermal desorption of the adsorbed species. Their
traditional use lay as bulk desiccation agents in
the dehydration of gases and organic solvents.
Adsorbate binding energies on alumina are greater
than that of silica, making activated aluminas more
attractive when low to moderate water levels are
specified in process effluent. Activated aluminas
are low in cost, costing less than ¼ of the price of
molecular sieves, and are the best adsorbent in
terms of resistance of acid, performing far better
than silica gels, alumino silicates, or molecular
sieves. Spherical activated alumina has high surface
area, adsorptive capacity and abrasion resistance.
It is regenerated for reuse by purging or evacuating
at elevated temperatures.
What is the adsorption capacity?
Adsorption capacity is defined
as the accumulation of the solute molecules at the
surface of a solid. This capacity is directly
proportional to the area of the surface exposed and
is dependent on the solute partial pressure and the
temperature. An increase in temperature reduces the
adsorption capacity of activated alumina as the
adsorption of water on alumina is exothermic.
Adsorption capacity depends on
the surface site reactivity and is measured by the
volume of adsorbed water per unit of surface area.
Water in the air actually sticks to the alumina
itself in between the tiny passages as the air
passes through them. The water molecules become
trapped so that the air is dried out as it passes
through the filter. This process is reversible, and
when the alumina desiccant is heated to between
350-600 F (177-316 C) it releases all of the water
stored inside it. The process of heating the
activated alumina is called regenerating the
desiccant.
When a gas is compressed the
partial pressure of the water present increases. At
a constant temperature the adsorptive capacity for
water increases with increasing water partial
pressure (and relative humidity).
Desiccants are manufactured to
meet standards such as “Standard Methods of Testing
Sorbent Performance of Absorbents” published by ASTM
International, as well as other US government
standards.
Water removal from ambient air
is the important first step in the production of
nitrogen enriched gas from nitrogen containing gas
mixtures such as air. This need for water removal is
essential for all gas separating processes including
cryogenics, membrane permeation and adsorption
techniques.
What is activated alumina?
Activated alumina is
manufactured from aluminum hydroxide by
dehydroxylating it in a way that produces a highly
porous material. This material yields a surface area
of between 120-250 square meters per gram of
material. Activated aluminas do not soften or
disintegrate when immersed in liquids. A high
internal surface area through the presence of pores
or micropores is necessary to create adsorption
sites.
What are desiccants?
Desiccants and sorbents are
specialized moisture absorbing minerals or
chemicals. Alumina, silica gel, calcium sulfate,
barium oxide, lithium chloride, perchlorates (such
as lithium, barium or magnesium perchlorate) and
molecular sieves such as zeolite are commonly used
as desiccants. These materials are used as
desiccants because they possess both high water
adsorption capacities as well as favorable water
adsorption isotherm shapes. (discussed below) The
water adsorption capacity of these materials varies
from 20 to 50 wt %.
Desiccants attract moisture from gases and liquids.
The desiccant material becomes saturated as moisture
is adsorbed onto its surface. The best adsorbents
will therefore have the greatest surface area
available for adsorption. Superior adsorbents are
designed to maximize the available surface area for
moisture removal. DrysphereTM by Dynamic Adsorbents,
Inc. is designed to increase surface area by
containing multiple channels and pores which
dramatically increase the available surface area for
physical and chemical interaction.
How are desiccants and sorbents different?
Desiccants and sorbents are
related products, but differ. Sorbents recover
liquids through absorption, adsorption and
chemically react with or otherwise remove water.
Absorbents retain liquid through the molecular
structure, causing more than 50% swelling.
Adsorbents are coated by a liquid on the surface
(including pores) without swelling more than 50%.
Adsorption is a phenomenon which involves the
fixation of a material present in a fluid on a
solid. A selective mass transfer occurs between
these two phases. Since adsorption is a surface
phenomenon the best adsorbents have large surface
areas per unit mass and high attractive forces for
the compounds they are adsorbing.
Desiccants and sorbents differ in how performance is
measured. One unit of desiccant will adsorb 3 grams
of water vapor at 20% relative humidity, or 6 grams
of water vapor at 40% relative humidity at 77 C. To
achieve very dry air, a dehumidifier desiccant can
be rotated through process air and heated to remove
moisture. Desiccants can be reactivated using heat
as the method to remove captured moisture, thus
allowing the desiccant to once again become fully
activated. Superior desiccant materials such as
alumina break down slowly and therefore can be used
and regenerated multiple times.
How does one determine how much moisture
can be
removed?
The water content of a gas is
defined as the weight or volume of water vapor per
unit weight or volume of gas. This is expressed as
parts per million weight (ppmw) or parts per million
volume (ppmv). For any given temperature water
content can also be expressed by its relative
humidity, which is defined as the ratio of its
partial pressure to its saturation pressure.
The water adsorption isotherm
relates the equilibrium amount of water adsorbed
onto a solid and the water content in a fluid at any
constant temperature and pressure. The amount of
water trapped on a solid at a given relative
humidity and temperature depends on its chemical
affinity for the solid and the number of available
sites for interaction. The capacity of a desiccant
for water is expressed as the mass of water adsorbed
per mass of desiccant.
Water adsorption is a combined
result of three phenomena:
-
Chemisorption
-
Physisorption – hydrogen
bonding with some surface forces (Van der Waals’
forces) due to the formation of multiple layers
by hydrogen bonding in the pores of the
desiccant.
-
Capillary Condensation –
growth of multilayers of water (Kelvin’s law)
where localized condensation takes places at
temperatures above that of the bulk fluids dew
point.
As the relative humidity begins
to increase the first step in the process of
adsorbing a liquid or gas onto a solid is
chemisorption with a monomolecular layer forming on
the surface of the adsorbent. As the relative
humidity increases, the less active sites adsorb
water more gradually, with the monolayer completely
bound when the relative humidity reaches
approximately 10%. At this point multilayers of
water vapor form, reflective of the fact that
physisorption is taking place. When the water vapor
pressure (relative humidity) reaches 40% the pores
begin to fill in by capillary condensation. When the
relative humidity of the inlet gas attains 100% and
the bed is in equilibrium there is no further
adsorption through these three mechanisms of
adsorption and complete saturation occurs.
Alumina and other materials used as desiccants have
water adsorption isotherms that are concave to the
pressure axis, particularly at low pressure, which
helps in forming short, sharp mass transfer
zones. (See below) These materials are polar, and as
such they selectively adsorb polar molecules like
water and alcohol, even though they adsorb all
liquid and gases to some extent. For example, water
as a polar compound is more strongly adsorbed than
hydrocarbons.
How much desiccant is required for
any given
job?
The amount of desiccant required
depends on several factors including the amount of
water present, the capacity of the selected
desiccant to take up water, and the presentation of
the desiccant relative to the components containing
water. Stream conditions such as pressure,
concentration and molecular weight of the molecules,
temperature and site competing molecules affect the
efficiency of adsorption.
THINK ALUMINA -- THINK DYNAMIC
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