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An Introduction to
Zeolite Molecular Sieves
Dry Purify Separate Dry Purify Separate Dry Purify Sep
Hundreds of systems for the
drying and purification of
liquids and gases rely on the
high adsorption efficiency of zeolite
molecular sieves. These unique
adsorbents are a result of synthetically
produced crystalline metal alumino-
silicates that have been activated for
adsorption by removing their water of
hydration. Since little or no change
in structure occurs during this
dehydration, highly porous adsorbents
are formed that have a strong affinity
for water and specific molecules.
Unlike other adsorbents, zeolite
molecular sieves have precisely
uniform pore sizes and molecular
dimensions. This translates into a
sieve-like selectivity where molecules
of varying size and polarity may be
readily adsorbed, slowly adsorbed or
completely excluded. This selectivity,
combined with a high capacity over
a wide range of operating conditions,
gives each zeolite molecular sieve a
high level of adsorption efficacy.
Why they are usedUse of zeolite molecular sieves to dry,
purify and separate liquids and gases
prevents unwanted side reactions,
helps meet product specifications,
and avoids costly complications from
equipment corrosion and freeze-up.
Other beneficial performance
characteristics include:
• Dehydration to water content less
than 0.1ppm
• High capacity for water above
200°F (93°C)
What are zeolite molecular sieves?
Used successfully in hundreds of commercial systems
for drying and purifying liquids and gases, zeolite molecular
sieves are the most universally applicable adsorbents in the
process industries.
• Purification and dehydration
in one operation
• Dehydration without adsorbing
valuable product or altering the
composition
• High product recovery
• Numerous purification and
dehydration cycles are possible due
to the reversible adsorption process
• High cyclic capacity with sufficient
thermal or pressure swing purging
Table of Contents
Page 2 What are zeolite molecular sieves?
Page 4 Zeolite research and synthetic production
Page 6 Crystal structure and molecular sieve types
Page 8 Adsorption based on molecular size,polarity and degree of unsaturation
Page 12 Zeolite molecular sieves and adsorption efficiency
Page 13 Zeolite molecular sieves and co-adsorption
Page 15 Regeneration cycles
Page 17 Applications
Page 18 Put UOP’s experience and technology to work for you
MOLSIVTM
Adsorbents
Specific, uniform pore size is the key toadsorbent efficiencyand selectivity…Based on size and charge distributionin a molecule, zeolite molecular sievescan adsorb individual moleculesreadily, slowly or not at all.
TM
Naturally occurring crystalline
zeolites, a subset of molecular sieves,
were first noted two centuries ago.
Their ability to release water when
heated and readsorb upon cooling
was known at that time, but their
capacity to selectively adsorb
molecules other than water was not
recorded until the 1920s. In the early
1930s, X-ray diffraction studies
revealed the zeolites as crystalline
materials with precisely arrayed
cavities and pores within each crystal.
Since zeolites found in nature have a
high degree of chemical and physical
variability, these products were not
viable for commercial separation
processes. In the early 1950s,
a division of Union Carbide
Corporation, that is now part of UOP,
was searching for an adsorbent to
separate atmospheric gases and to be
used in other industrial applications.
As a result of this research, structures
of silicon and aluminum oxides with
uniform pore sizes and precise
molecular dimensions were
synthesized and patented. The
synthetic zeolites’ sieve-like selectivity
offered the consistent performance
necessary for commercial use. By
1953, more than 30 pure zeolite
species had been prepared. Their
crystal structures and adsorption
properties had been characterized,
and researchers had learned how to
regenerate them for repeated use in
commercial applications.
had for commercial use, they began to
delve into adsorption technology and
design processes that could rely on
these new materials.
As a consequence of their research,
zeolite molecular sieves were
substituted into existing dryer and
simple adsorber systems with amazing
results. The use of zeolite molecular
sieves improved the drying and
purifying of various gas and liquid
process streams with minimal changes
in technology. For more advanced
uses, however, additional process
engineering knowledge was required.
To address this problem, Union
Extensive QC testing insures superior
product quality and consistency
Zeolite research and synthetic production
Sodium Aluminate
Sodium Silicate
Makeup Tank
Crystallization Tank
Steam
Crystal Slurry
Wash Water
Filter
Zeolite Crystals
Weigh Hopper
Clay Binder
Ion Exchange Tank
Salt
Steam
Ion-Substituted Zeolite
4
Zeolite research spawns commercial adsorption technology
Once engineers recognized
the incredible potential
zeolite molecular sieves
Mixer Particle Forming
Dryer
Screen
Kiln
Activated Molecular
Sieve Product
Filter
Wash Water
Manufacturing process
for the production of activated
zeolite molecular sieves.
Carbide formed a large, process engineering group to develop new and
comprehensive adsorption technology and design guidelines. Starting with
fundamental adsorbent data, the researchers studied adsorption equilibria,
adsorption kinetics, deactivation phenomena, cyclic life and scale-up factors.
After much research, the group discovered how to economically manufacture
the zeolites in commercially useful forms without adversely affecting their
adsorption properties.
In November of 1954, Union Carbide announced the availability of the first
limited commercial quantities. The pure zeolites were then used within the
chemical, manufacturing and petroleum refining industries to solve difficult gas
purification and dehydration problems. Today, by altering existing crystalline
structures for improved functionality, UOP continues to manufacture many
types of zeolites for a myriad of industries.
How zeolite products are manufacturedSodium silicate, alumina trihydrate and sodium hydroxide are batch-weighed
into mix tanks and stirred until homogenous. The mixture forms a gel that is
pumped into a crystallization tank where it is monitored under closely
controlled conditions.
Filter, wash and exchangeAfter crystallization is complete, a rotary filter separates and washes the
zeolite crystal slurry. For cationic exchange to take place (calcium, potassium
or other cations substituted for sodium in the crystal), the filter cake is
transferred to a heated tank where it will be mixed with a solution of the
appropriate metal salt. The exchanged forms will then be washed and
filtered in the same manner as the original crystal slurry.
5
Forming final productOnce separated and washed, the filter
cake is conveyed to hoppers. To form
commercial 1/16-in and 1/8-in (about
1.6-mm and 3.2-mm) pellets (extrudates) or
beads (spheres),crystals from the filter are
mixed with specially formulated clay
binders. The crystals are then fed through
forming equipment to produce pellets
or beads. The various product forms are
then dried, screened and fired in a rotary
kiln to drive out the water and activate the
zeolite molecular sieves. The adsorbents are
then immediately packaged to prevent any
moisture pick up.
Many tests are used to determine product
quality from crystallization to final firing.
Examples include x-ray diffraction,McBain-
Bakr adsorption, loss on ignition,crush
strength,density and particle size. Quality
control techniques including Statistical
Process Control and adherence to ISO 9000
standards ensure that crystallization and
other manufacturing processes achieve
exact specifications.
The basic formula for zeolite molecular sieves is M2/nO • Al2O3 • xSiO2 • yH2O
where M is a cation of n valence. The fundamental building block of the
molecular sieve crystal structure is a tetrahedra with four oxygen anions
surrounding a smaller silicon or aluminum cation. Sodium ions or other cations make up
the positive charge deficit in the alumina tetrahedra,and each of the four oxygen anions is
shared, in turn,with another silica or alumina tetrahedron to extend the crystal lattice in
three dimensions. In all molecular sieve types, the sodium ion can be exchanged to form
other functional products.
The crystal structure of zeolite molecular sieves is honeycombed with relatively large
cavities. Each cavity is connected through apertures or pores. The water of hydration is
contained within these cavities. Before product is used, the water of hydration is
removed by heating.
Crystal structure andmolecular sieve types
Illustrations of the rigid, three-dimensional
framework of SiO4 and Al04 tetrahedra
The crystallization of molecular sieve Type A from a hydrous
gel as seen through the electron microscope. Photo 1 shows
development of crystallization after two hours at 100º C.
Photo 2 shows completely crystallized A.
Structural model of a zeolite.
SkeletalTetrahedron
PackedSpheres
SolidTetrahedron
6
1 2
7
In general, the elasticity and kinetic
energy of incoming molecules allows
for easy passage of molecules of up
to 0.5 angstroms larger than the
free diameter of the aperture. In
addition, the size and position of the
exchangeable cations may affect the
free aperture size in any type of
molecular sieve. The zeolite molecular
sieves that are most commonly used
include Types A and X. Unit cell
formulas and structural details for
each type are outlined below.
Type A
Na12 [(AlO2)12 (SiO2)12] • 27H2O
Note: Na+ (sodium) can be replaced
by other cations.
Type A contains roughly spherical
cavities that are approximately 11
angstroms in diameter and about 925
cubic angstroms in volume. They
account for nearly half of the total
crystalline volume that is available
for adsorption.
• The Type A molecular sieve
has a framework composed of
truncated octahedra joined in
a cubic array. The result is
a central truncated
cube-octahedron with an
internal cavity 11 angstroms
in diameter (alpha cage).
• Each central cavity, or alpha
cage, is entered through six
circular apertures formed by
a nearly regular ring of eight
oxygen atoms with a free
diameter of 4.2 angstroms.
• The cavities are arranged in a
continuous three-dimensional
pattern forming a system of
unduloid-like channels with a
maximum diameter of 11
angstroms and a minimum
of 4.2 angstroms.
• The truncated octahedra enclose
a second set of smaller cavities
6.6 angstroms in internal
diameter (beta cages). The
smaller cavities are connected to
the larger cavities via a distorted
ring of six oxygen atoms of
2.2 angstroms free diameter.
Type 3A
Type 3A crystals are produced when
some of the sodium ions are replaced
by potassium ions. Since potassium
ions are larger than sodium ions, the
pore size is effectively reduced to
about 3.2 angstroms.
(1) Truncated octahedron. (2) Face of cubic
array of truncated octahedra.
1
2
4.2 A°
11.4 A°
6.6 A°
2.6 A°
4 A°
Above: Two adjacent unit cells of
Type 4A — light circles represent
oxygen ions and dark circles
represent sodium cations.
Type 4A
Type 4A sodium-bearing crystals have
a free aperture size of 3.5 angstroms
in diameter. At typical operating
temperatures, molecules with an
effective diameter of up to four
angstroms may be passed through
this aperture.
Commercially useful zeolite species
Type† Nominal Pore Common Bulk Density Heat of Adsorption Equilibrium MoleculesDiameter Form lb/cu-ft (max) btu/lb H2O H2O Capacity* Adsorbed**
Angstroms (gm/cc) (kcal/kg H2O) wt-%
3A 3 Powder 35 (0.56) 1800 26 Molecules with an effective1/16-inch Pellets 40(0.64) (1,000) 21 diameter <3 angstroms1/8-inch Pellets 40 (0.64) 21 including H2O and NH3
8 x 12 Beads 44 (0.71) 214 x 8 Beads 44 (0.71) 21
4A 4 Powder 32 (0.51) 1800 27 Molecules with an effective1/16-inch Pellets 44 (0.71) (1,000) 22 diameter <4 angstroms1/8-inch Pellets 44 (0.71) 22 including ethanol, H2S, CO2, SO2,
8 x 12 Beads 44 (0.71) 22 C2H4, C2H6 and C3H64 x 8 Beads 44 (0.71) 22
14 x 30 Mesh 44 (0.71) 22
5A 5 Powder 32 (0.51) 1800 26 Molecules with an effective1/16-inch Pellets 44 (0.71) (1,000) 21.5 diameter <5 angstroms including1/8-inch Pellets 44 (0.71) 21.5 n-C4H9OH, n-C4H10,
C3H8 to C22H46, R-12
13X 8 Powder 27(0.43) 1800 30 Molecules with an effective 1/16-inch Pellets 40 (0.64) (1,000) 26 diameter <8 angstroms1/8-inch Pellets 40 (0.64) 26 including C6H6, C7H8
8 x 12 Beads 40 (0.64) 264 x 8 Beads 40 (0.64) 26
Zeolite molecular sieve characteristics and applications
Type 5A
When some of the sodium ions in
Type 4A are replaced with calcium
ions,Type 5A is produced. It features
the largest pore opening of the A
types, with a free aperture size of
4.2 angstroms.
Type X
Na86 [(AlO2)86 (SiO2)106] • 264H2O
Note: Na+ (sodium) can be replaced
by other cations.
Although Type X is based on the
same building blocks as Type A, the
beta cages are linked tetrahedrally
instead of in a cubic arrangement.
The Type X crystal has a larger,
elliptical-shaped internal cavity of 13
angstroms in diameter with a pore
diameter of approximately 8
angstroms for the sodium form.
Numerous zeolite species that differ in chemical composition, crystal
structure and adsorption properties are known. By selecting the
appropriate adsorbent — one that allows entry of those molecules small
enough to pass into the pore system — and by choosing the proper operating
conditions, zeolite molecular sieves can be adapted to suit specific applications.
While the external surface area of the molecular sieve crystal is available for
adsorption of molecules of all sizes, the internal area is available only to those
Adsorption based on molecular size,polarity and degree of unsaturation
High silica molecular sievesLike Types A and X, high silica zeolites selectively adsorb molecules based
on their size. However, they differ from Types A and X in that they have
a significantly higher proportion of SiO2 to AlO2 in their molecular
structure. With the reduced amount of AlO2 and the corresponding
reduction in cation density, the high silica zeolites are hydrophobic and
organophilic adsorbents. The high silica zeolites are also stable at low
pH ranges and high temperatures up to 1,292ºF (700ºC).
†Chart depicts basic molecular sieve types only. In all applications, these basic forms are customized for specific use.*Lbs H2O/100 lbs activated adsorbent at 17.5 torr H2O at 25ºC. **Each type adsorbs listed molecules plus those of preceding type.
Molecules ApplicationsExcluded
Molecules with an effective • Preferred adsorbent for commercial dehydration of unsaturateddiameter >3 angstroms (ethane) hydrocarbon streams (cracked gas, propylene, butylene
and acetylene)• Dries polar liquids such as methanol and ethanol• Static desiccant in household refrigeration systems
Molecules with an effective • Adsorbent for static dehydration in a closed gas or liquid systemdiameter >4 angstroms (propane) • Used in the packaging of drugs, electronic components and
perishable chemicals• Water scavenger in paint and plastic systems• Used commercially in drying saturated hydrocarbon streams
Molecules with an effective • Separates normal paraffins from branched-chain and cyclicdiameter >5 angstroms hydrocarbons through a selective adsorption process
(iso compounds and all 4-carbon rings) • Pressure swing purification of hydrogen
Molecules with an effective diameter >8 • Used commercially for general gas drying, air plant feed purificationangstroms (C4F9)3N (simultaneous removal of H2O and CO2), and liquid
hydrocarbon and natural gas sweetening (H2S and mercaptans removal)
Exposed cations within the
crystal structure act as sites of strong
localized positive charge. These sites
electrostatically attract the negative
end of polar molecules.
The role of cationsThe strong adsorptive forces in
zeolite molecular sieves are primarily
due to the cations that are exposed
within the crystal lattice. They act as
sites of strong localized positive
charges that electrostatically
attract the negative end
of polar molecules. The greater the
polarity of the molecule, the more
strongly it will be attracted
and adsorbed.
Polar molecules are generally those
that are asymmetrical and contain O,
S, Cl or N atoms. Carbon monoxide,
for example, will be adsorbed in
preference to argon.
In fact, under the influence of
localized, strong positive charges on
the cations, polarity may be induced
in the molecules. The polarized
molecules are then adsorbed strongly
due to the electrostatic attraction of
the cations. In hydrocarbons, the more
unsaturated the molecule, the more
polarizable it is and the more strongly
it is adsorbed. As an example, zeolite
molecular sieves will effectively
remove acetylene from olefins and
ethylene or propylene from saturated
hydrocarbons.
Adsorption, desorption and hysteresisSince zeolite molecular sieves rely on
strong physical forces rather than
chemisorption to retain adsorbates,
their adsorption is characterized by a
Langmuir-type isotherm (the amount
of a given compound adsorbed
increases rapidly to a saturation
value as its pressure or concentration
increases in the external bulk phase).
Any further increase in pressure at
constant temperature causes no
further increase in the amount
adsorbed. With zeolite molecular
sieves, this equilibrium saturation
value typically corresponds to a
complete filling of the internal void
volume with the adsorbate. When
adsorbed molecules are desorbed
via heat or by displacement with
another material, the crystal’s
chemical state remains unchanged.
molecules small enough to enter the
pores. The external area is about one
percent of the total surface area.
Materials that are too large to be
adsorbed internally will typically be
adsorbed externally to the extent of
0.2 to 1 weight percent.
Zeolites will preferentially adsorb
molecules based on polarity and
degree of unsaturation in organic
molecules, in addition to selectivity
based on size and configuration. In a
mixture of molecules small enough to
enter the pores, the molecules with
lower volatility, increased polarity, and
a greater degree of unsaturation will be
more tightly held within the crystal.
General flow chart for liquid drying.
In
Heater
PurgeGas
In
Out
CoolingGas
In
Out
LiquidStream
Adso
rptio
n
Des
orpt
ion
Cooler Condenser
Out
10
With zeolite molecular sieve
powders, no hysteresis occurs
during desorption. Adsorption and
desorption are completely reversible
with their respective isothermal
curves coinciding completely.
However, with zeolite molecular sieve
pellets or beads, further adsorption
may occur at pressures near the
saturation vapor pressure. This can
occur as a result of condensation in
the pellet or bead voids external to
the zeolite crystals. In addition,
hysteresis may take place during
desorption of the adsorbate in the
macro-pore region of the binder.
A brief review of adsorption principles and systemsThe rate at which molecules are
adsorbed into formed zeolite
molecular sieves depends on the
following four variables:
1. The rate at which molecules
being adsorbed can diffuse to
activated crystals within the
pellet or bead
2. The relative size of molecules
and molecular sieve pores
3. The strength of adsorptive
forces between molecular
sieves and adsorbate
4. Adsorption temperature
Fundamental adsorption systemsDepending on the type of operation,
zeolite molecular sieves may be used
in one of three basic types of
adsorption systems:
• Multiple-bed adsorption
• Single-bed adsorption
• Static adsorption
Multiple-bed adsorptionMultiple bed adsorption is ideal for
most commercial, large-scale fluid
purification operations. Conventional
fixed-bed, heat-regenerated adsorption
systems are commonly used. A typical
dual-bed installation places one bed
on-stream
Adsorption on zeolite molecular sieves
produces a Langmuir-type isotherm.
25
20
15
10
5
010 20 30 40 50
MolecularSieveType A
SilicaGel
ActivatedAlumina
Water Vapor Adsorption at 25° C(Equilibrium Data)
Cap
acity
wt-%
Relative Humidity Percent
to purify the fluid while the other
bed is being heated, purged and
cooled. When the process design
requires less than six hours for the
adsorption step, additional beds can
be added to permit continuous
processing of the feed.
Single-bed adsorptionSingle-bed adsorption can be used
when interrupted product flow can
be tolerated. When the adsorption
capacity of the bed is reached, it can
be regenerated for further use either
in place or at another location.
Alternatively, it can be discarded
if economically feasible.
Static adsorptionWhen manufactured into various
physical forms, zeolite molecular
sieves can be used as static desiccants
in closed gas or liquid systems.
Multiple bed adsorption for
H20 and C02 removal from natural gas
before methane liquification.
11
25
20
15
10
5
0100(38)
200(93)
300(149)
400(204)
500(260)
Water Vapor Adsorption Isobarsat 10mm Hg Partial Pressure
(Equilibrium Data)
Wat
er A
dsor
bed
wt-%
Temperature °F (°C)
0(-18)
Zeolite MolecularSieves
ActivatedAlumina
Silica Gel
Drying power of silica gel, zeolite molecular sieves and activated alumina
under various operating temperatures.
12
Zeolite molecular sieves and adsorption efficiency
Zeolite molecular sieves are
employed in numerous
installations and operations
due to their exceptional adsorption
efficiency. The following details
typical conditions where they
are effectively used.
When very dry streams are requiredIn industry, drying by adsorption is
favored due to its ability to produce
a much drier liquid or gas than
other commercial methods. When
extremely dry streams are required,
zeolite molecular sieves are selected
because they can reduce water
concentrations to less than 0.1 ppm.
In addition, they are effective over a
wide range of operating conditions.
When operating at high temperaturesZeolite molecular sieves are also a
good choice when drying streams at
high temperatures. In fact, they are
the only adsorbents that remain
effective under very hot conditions.
For example, at 200ºF (93ºC) and
above, zeolite molecular sieves have
more than 13 weight-percent capacity
while other adsorbents have none.
The isobars plotted below illustrate zeolite molecular sieve performance
over a spectrum of operating temperatures. The solid lines assume the
use of completely regenerated adsorbents. The capacity is lowered by any
residual water left on the adsorbent, a factor of particular importance in
high temperature drying operations. As an example, the dotted line isobars
show the effect of two percent residual water at the start of adsorption
on silica gel, zeolite molecular sieves and activated alumina. In some
applications, this residual water can completely consume the adsorption
capacity of silica and alumina type adsorbents. For this reason, it is best
to use silica and alumina type adsorbents for the bulk separation of water.
They are very effective for this purpose and offer the additional benefit
of extending the life of zeolite molecular sieves. After bulk separation
processes have taken place, zeolite molecular sieves can then be used
to achieve very low dew point levels.
Due to the ability of zeolite molecular
sieves to produce a drier liquid or gas,
industry operations typically favor
drying by adsorption over other
commercial methods.
Co-adsorption and pore sizeCo-adsorption can be avoided through
proper selection of zeolite molecular
sieve type. The zeolite molecular sieve
should have a critical pore diameter small
enough to prevent all stream components
except water from being admitted to the
active inner surfaces of the adsorption
cavities. In this way,co-adsorption of
molecules other than water (including
polar and unsaturated components), is
eliminated. By eliminating co-adsorptions
the molecular sieve will provide
maximum capacity for water and
reduce outlet water concentrations
to less than 10 ppm.
Co-adsorption and affinity for waterZeolite molecular sieves feature an
extremely high adsorptive attraction
for water. This affinity is so strong
that water will normally displace any
other material that is already
adsorbed. To further enhance this
selectivity for water, the temperature
of the adsorbent bed can be raised.
Although the rate of adsorption will
be somewhat reduced if the water
has to displace another material
before it can be adsorbed, zeolite
molecular sieves still offer better
performance when compared
to other adsorbents.
13
Zeolite molecular sieves’strong attractionfor water prevents co-adsorption problems in chemicalprocess streams.
In some drying applications,components other than water may be adsorbed.
In many chemical process streams, this altering of stream composition,or
co-adsorption,can cause serious problems. When product composition is
critical, zeolite molecular sieves can be used to solve these co-adsorption difficulties.
of 5 to 12, and a few are stable in
solutions having a pH as low as 3.
They are stable in most organic
streams, however in vapor phase
processes, gases that will hydrolize to
form strong acids will readily react
with the adsorbents.
When purifying acidic streams
The chemical stability of
zeolite molecular sieves
allows them to dry, purify
and separate numerous types of
materials including inorganic
gases, hydrocarbons, halogenated
hydrocarbons, alcohols, esters, ethers,
amines, amides, ketones and others.
Zeolite molecular sieves are alkaline
in nature with a pH range in water
slurry of 9 to 11. Most types are
stable in solutions within a pH range
Zeolite molecular sieves and co-adsorption
One-step drying and purifyingIn addition to water, impurities in a process stream can be
removed via proper operating conditions and appropriate zeolite
molecular sieve selection. Since zeolite molecular sieves adsorb
water more strongly than other material, the adsorbed water
concentrates at the inlet end of the bed. Here, it displaces other
impurities that have been previously adsorbed. These desorbed
impurities are then re-adsorbed farther down the column. The
desorbed impurities will begin to appear in the effluent stream
as displacement continues. This displacement can be allowed to
continue until little adsorbate, other than water, is left on the bed.
However, it is possible to design and operate a zeolite molecular
sieve adsorption system so that impurities are retained on
the adsorbent rather than re-entering the purified stream.
To accomplish this, sufficient bed must be provided to contain
the impurities in addition to the water. See the figure below
for an example of a co-adsorption system.
Sweet LPGProduct
Pad GasRegeneration
Gas In
Adsorption Regeneration
Fuel
Separator
Liquids
Cooler
Sour LPG Feed
Cooli
ng L
ine
Heater
Adsorption(Desulfurization
Step)
Regeneration(Heating
Step)
Ads
orpt
ion
(Des
ulfu
rizat
ion
Step
)
Rege
nera
tion
(Hea
ting
Step
)
20
15
10
5
0100 200 300 400 500
Carbon Dioxide Capacity at 25° C Molecular Sieve Type A
(Equilibrium Data)
Cap
acity
wt
-%
Carbon Dioxide Pressure, mm Hg0 600 700
0 2 4 6 8 100
5
10
15
20
15
10
5
050 100 150 200 250
Hydrogen Sulfide Capacity at 25° C Molecular Sieve Type A
(Equilibrium Data)
Cap
acity
wt
-%
Hydrogen Sulfide Pressure, mm Hg0 300 350
0 0.5 1.0 1.5 2.0 2.50
4
6
8
2
3.0
40
30
20
10
0100 200 300 400 500
Ammonia Capacity at 25° C Sulfur Dioxide Capacity at 25° C
Molecular Sieve Type A (Equilibrium Data)
Cap
acity
wt
-%
Pressure, mm Hg0 600 700
Sulfur Dioxide
Ammonia
These three graphs depict the equilibrium capacity of zeolite molecular sieves
for various gas impurities. Through co-adsorption, zeolite molecular sieves
will remove these materials in addition to water.
Typical co-adsorption system. Since zeolite molecular sieves have the
ability to adsorb hydrogen sulfide, mercaptans and water, the propane
feed is simultaneously purified (sweetened) and dried.
14
surface. Once the reactivation
temperature is reached, the bed is
flushed with a dry purge gas or
reduced in pressure. It is then
returned to adsorption conditions.
As a result, high loadings of water
and impurities on the adsorbent
can be obtained, following
a cooling step.
Pressure swing Pressure swing cycles, operating at
nearly isothermal conditions, use
either a lower pressure or a vacuum
to desorb the bed. Advantages of this
technique include fast cycling with
reduced adsorber dimensions and
adsorbent inventory, direct production
of a high purity product and the
ability to use gas compression as
the main source of energy.
Cyclic regeneration,
or desorption,
can be classified into
four types. Used separately or in
combination, the major adsorption-
desorption cycles are:
• Thermal swing
• Pressure swing
• Purge gas stripping
• Displacement
Thermal swing Thermal swing cycles reactivate the
sieve by elevating the temperature.
Typically, the operating temperature
is increased to 400 - 600ºF (204 –
316ºC). The bed is heated either by
direct heat transfer via hot fluid in
contact with the bed or by use of
indirect heat transfer through a
120 (+49)
100 (38)
200 (93)
300 (149)
400 (204)
500 (260)
Residual Loading After Regeneration Minimum Obtainable Dew Point
(Dynamic Data)
Dew
Poi
nt, °
F (°C
)
Bed Temperature, °F (°C)
0 (-18)
600 (316)
700 (371)
+80 (+27)
+40 (+4)
0 (-18)
-40 (-40)
-80 (-62)
-120 (-84)
-160 (-107)
-200 (-129)
4.0 WT–% 3.2 WT–% 2.3 WT–% 1.7 WT–% 1.0 WT–% 0 WT–%
This graph is used to find the minimum obtainable dew point as a function of
residual loading and effluent gas temperature during adsorption. Also shown
is residual loading after regeneration as a function of regeneration
temperature and purge gas dew point.
15
Regeneration cyclesPurge gas stripping This method uses non-adsorbing
purge gas. The purge gas desorbs the
bed by reducing the partial pressure
of the adsorbed component. The
higher the operating temperature and
the lower the operating pressure, the
more efficient the stripping. The use
of a condensable purge gas offers
the following advantages:
• Reduced power requirements
gained by using a liquid pump
instead of a blower
• An effluent stream that may
be condensed to separate the
desorbed material by simple
distillation
Displacement cycles Displacement cycles use an
adsorbable purge to displace the
previously adsorbed material. The
stronger the adsorption of the purge
media, the more completely the bed
is desorbed. In this case, lesser
amounts of purge can be used, but
it is consequently more difficult to
remove the adsorbed purge.
16
Air dryers with a desiccant-type in-line filtration system supplies
clean, dry air to truck air brake systems aiding in the prevention
of air line freezeups.
Zeolite molecular sieves are
used to purify industrial gases
and for the bulk separation of
oxygen from air.
Zeolite molecular sieves keep
dual pane windows free of
moisture and vapors.
The chart below provides a brief review of how and where zeolite molecular sieves are used in industry today.
17
Application Role of zeolite molecular sieves
Air dryers • Dehydration of plastic pellets before they are molded• Dehydration for instrument air• Dehydration of room air with molecular sieve impregnated dessicant wheels
Oxygen concentrators for respiratory patients • Adsorption of nitrogen from compressed air using a pressure or vacuum swing system to obtain oxygen purity up to 95%
Air brakes • Dehydration of compressed air on brake systems of heavy- and medium-duty trucks, buses and trains
• Pressure swing dryers are used to reduce the dew point of air in the brake reservoir below ambient temperature to prevent freeze-up and corrosion
Insulated glass (dual-pane windows) • Removal of initial trapped moisture inside the dual-pane window and the moisture that will permeate during the life of the unit to prevent fogging
• Removal of vapors from organic sealing materials, paint and cleaning solvents introducedduring window manufacture
Polymer formulations • Dehydration of moisture-sensitive formulations — added to poly coatings, epoxiesand urethanes to control the curing process and coatings, adhesives, sealants, elastomers, metal-rich paints and vinyl foams to eliminate unwanted water reactions
Radioactive cleanup • Removal of radioactive nucleotides by ion exchange — cesium and strontium are exchanged preferentially into the zeolite molecular sieves to greatly reduce the volume of liquid waste
Refrigeration and air-conditioning (A/C) systems • Dehydration of automotive A/C, transport refrigeration, home refrigerators, freezers, residentialA/C, heat pumps and commercial refrigerants to prevent freeze-up and corrosion
• Dehydration to protect system materials from adverse chemical reactions
Deodorization • Removal of odor or taste from personal-care products and plastics with high silica (hydrophobic) zeolite molecular sieves. Odors are adsorbed, not masked
Package dehydration • Dehydration with zeolite molecular sieves when very low humidity conditions are required.Small desiccant packets or tablets protect products such as pharmaceuticals, medicaldiagnostic reagent kits, vitamins, food, candy, batteries, dry fuel propellants, machine parts,film and instruments
Air separation • Removal of water and carbon dioxide from air before liquefaction and cryogenic separation of nitrogen, oxygen and other atmospheric gases
• Separation of oxygen and nitrogen with pressure swing or vacuum swing adsorption systems
Natural gas • Dehydration before cryogenic recovery of hydrocarbon products and helium• Dehydration of high acid gas content (CO2 and H2S) natural gas and natural gas
condensate streams• Removal of sulfur compounds from ethane, propane and butane• Removal of water and CO2 before methane liquefaction• Removal of water and sulfur compounds to protect gas transmission pipelines• Dehydration of natural gas liquids• Desulfurization of feed streams for ammonia and other chemical plants• Removal of mercury, preventing damage to aluminum heat exchangers
Petroleum refining • Dehydration of alkylation feed, refinery gas streams prior to cryogenic separation, naphtha and diesel oil
• Purification of feedstocks to protect isomerization catalysts• Removal of water, HCl and H2S from reformer streams• Removal of oxygenates from etherification raffinate streams and alkylation feed• Removal of nitriles from etherification feed• Dehydration of ethanol• Dehydration and desulfurization of LPG streams• Separation of normal paraffins from branched chain and cyclic compounds• Purification by pressure swing adsorption for upgrading hydrocarbon streams
Petrochemicals • Dehydration and purification of NGL/ethane/propane feed • Dehydration of cracked gas, C2 and C3 splitter feed and hydrogen• Dehydration and purification of salt-dome-stored ethylene, propylene and various
other feedstocks• Removal of water, carbon dioxide, methyl alchohol and other oxygenates, hydrogen sulfide and
sulfur compounds, ammonia and mercury from ethylene, propylene, butylenes, amylenes and various solvents and co-monomers
Volatile organic compound removal • Removal of trace volatile organic compounds from air streams• Removal of volatile organic compounds from moisture-laden process streams
18
Put UOP's experience and technology to work for you
UOP's expertise and innovation extends from
research and development to manufacturing
and from application product selection to
technical services. To meet customer needs, UOP offers
the broadest portfolio of molecular sieve and activated
alumina products in the world. With sales,
technical support staff, and manufacturing facilities
located around the globe, UOP continues to lead the
industry through our commitment to our customers.
Whether you are looking to dry, purify or separate,
you'll find the adsorbent solution with UOP.
urify
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