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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