Ion exchange is an effective,versatile means of conditioning boiler feedwater. The term “ion
exchange” describes the process:
as water flows through a bed of ion exchange material, undesirable ions are removed and
replaced with less objectionable ones.
For example, in softening processes,
calcium and magnesium ions (hardness) are exchanged for sodium ions.
In dealkalization, the ions contributing to alkalinity (carbonate, bicarbonate, etc.) are removed and
replaced with chloride ions.
Other dealkalization processes
utilizing weak acid cation resin or strong acid cation resin in a split stream process, exchange cations with hydrogen.
This forms carbonic acid which can be removed in a decarbonator tower.
Demineralization is simply replacing all cations with hydrogen ions (H+) and all anions with hydroxide ions (OH–).
Ion exchange materials are like storage batteries; they must be recharged (regenerated) periodically
to restore their exchange capacity. With proper design and operation, ion exchange processes are capable of removing selected ions almost completely (in some cases to a fraction of a part per million).
Ion exchange processes
Table 1 — Types of ion exchange materials
Capacity Exchangers kgr/ft3
CATION
Inorganic (zeolites)
Natural (greensand) 3–5
Synthetic 12–16
Organic
Sulfonated coal — 5–7
(carbonaceous)
Synthetic – (phenolic types) 6–18
(styrene base) 20–30
ANION
Inorganic Not widely
Metallic oxides used
Organic
Synthetic resins 10–22
Cation exchangers
There has been constant improvement in ion exchange materials since the first use of natural and
synthetic inorganic products. Sulfonated coal, styrene-base resins, phenolic resins and acrylic
resins are some that have been developed. Exchange capacities were greatly increased with the
development of the styrene-base exchangers. These resins are manufactured in spherical, stress
and strain-free form to resist physical degradation. They are stable at temperatures as high as
300°F and are applicable over a wide pH range. More dense ION EXCHANGE MATERIALS
Ion exchange processes are versatile — specific types of ions can be removed from water depending on the choice of exchange material and regenerant used.
Development
Slightly more than 100 years ago,two English agricultural chemists, H. S. Thompson and J. Thomas Way, noted that certain soils had a greater ability than others to absorb ammonia from fertilizers.
They found that complex silicates in the soil performed an ion exchange function. They were able to prepare materials of this type in the laboratory from solutions of sodium aluminate and sodium silicate. In 1906,
Robert Gans used materials of this type for softening water. The early materials used were slow in
regenerating and lacked physical stability. These first synthetic,inorganic exchangers were called
zeolites. Today, zeolites are almost totally replaced by synthetic ion exchange materials. The basic types of ion exchangers in use for water conditioning are
Anion exchangers
Anion exchange materials are classified as either weak base or strong base depending on the
type of exchange group. Weak base resins act as acid adsorbers,efficiently removing strong acids
such as sulfuric and hydrochloric.
However, they will not remove carbon dioxide or silica. They are used in systems where strong
acids predominate, where silica reduction is not required, and where carbon dioxide is removed
in degasifiers. Preceding strong base units in demineralizing processes, weak base resins give
Typical Minerals Types of Minerals in Influent Exchanger Converted to
Cation
(A) Ca(HCO3)2 ® Na+ ® NaHCO3
CaSO4 ® Exchanger ® Na2SO4
Cation
(B) Ca(HCO3)2 ® H+ ® H2CO3
CaSO4 ® Exchanger ® H2SO4
Cation
(C) Ca(HCO3)2 ® H+ ® H2CO3
Exchanger
(Weak Acid)
Anion
(D) Na2SO4 ® Cl– ® NaCl
NaHCO3 ® Exchanger ® NaCl
Anion
(E) H2CO3 ® OH– ® H2O
H2SO4 ® Exchanger ® H2O
Conventional softening — process (A)
Dealkalization by split stream softening — blending effluents from (A) and (B)
Dealkalization by anion exchange — process (D) proceded by (A)
Dealkalization by weak acid cation exchanger followed by conventional softening
process (C) followed by (A)
Demineralizing — combination of (B) and (E)
Table 2 — Types of ion exchange processes
more economical removal of sulfates and chlorides.
These are two general classes of strong base anion exchangers,
Types I and II, denoting differences in chemical nature. Both remove silica and carbon dioxide
as well as other anions.
Type I is more effective in removing silica,and is used when the combined
silica and carbon dioxide content of the water contacting the exchanger is more than 25% of
the total anions. When there is contamination of the water with organic matter, a more porous
form of Type I resin is recommended.
The Type II anion
material is used in treating waters where the combined carbon dioxide and silica content is less
than 25% of the total anions. This is often the case when carbon dioxide is taken out in a degasifier
ahead of the anion exchanger unit.
2
Figure 1 — Typical ion exchange unit
Equipment operation Ion exchange material is housed in specially constructed tanks (Figure 1) where it forms a bed,
usually 30–60 inches deep. In some cases, it is supported by another bed of graded gravel or anthracite filter media. In other cases, some special methods of support, without gravel or anthracite, are used. During normal operation, water enters the top of the tank through a pipe, which distributes it over the surface of
the exchanger bed. The treated water is drawn off by collector piping at the bottom.
Several newer designs for ion exchange are now coming into common use. These are becoming
popular because of their advantages of higher operating efficiencies and lower leakage rates. In
countercurrent regeneration procedures, the regenerant flow is opposite in direction to the service
water flow. Therefore, the resin located in the bed where the finished water leaves the vessel, is
the most highly regenerated. This results in lower leakage rates and slightly higher operating capacities
at equal regenerant dosages to cocurrent operated vessels.
Physical Characteristics of
Resin
Anion and cation resins can be obtained in several different physical forms. They can be
obtained with different ions located on the exchange site.
This has importance in applications such as mixed beds where minimal leakage rates are
required even from a newly installed bed.
Particle size can also be specified.
Uniform particle sized (UPS)
resins are now available where all beads fit into a very close particle size range. For practical
purposes, all of the beads are the same size. Beds of UPS resins have some unique operating
characteristics which offer advantages
when they are used in mixed bed, layered bed, and packed bed applications. Macroporous resins are highly porous which give them advantages when used in processes that have high fouling potential.
ION EXCHANGE PROCESSES
Ion exchange processes fall into several categories: softening (including removal of iron and
manganese), dealkalization, and demineralization. Examples of these processes are listed in
Table 2.
3
Figure 2 — Cocurrent vs. countercurrent regeneration
Cocurrent vs countercurrent: With downflow operation, ion distribution at end of operating cycle
is the same for cocurrent and countercurrent operation. But at end of regenerating cycle, ion
distribution is revised when cocurrent is compared with countercurrent regeneration. Key to
optimum results is quality of rinse water used with countercurrent regeneration.
R– H+ R– H+
R–2 Ca+2
R–2 Mg+2
R– Na+
R–2 Ca+2
R–2 Mg+2
R– Na+
R– H+
R– H+
R–2 Ca+2
R–2 Mg+2
R– Na+
R–2 Mg+2
R–2 Ca+2
R– Na+
4
However, countercurrent vessels are more sensitive to operating problems. The bed must be immobilized during the regeneration process and the influent water must be very low in suspended solids.
Figure 2 illustrates what happens in the two regeneration modes at exhaustion and after regeneration.
Packed bed systems essentially fill the vessel with resin. The systems are countercurrent in design and offer the low leakage rate advantage. However, packed beds also offer the advantage of reduced waste generation. Since there is no space for proper backwash, packed bed systems usually are built with external backwash tanks which allow the resin to be backwashed after it is sluiced out of the operating
vessel. All countercurrent ion exchange systems require feedwater that is very low in suspended solids.
Regeneration of the exchange material involves three steps: backwash, introduction of the regeneration chemicals, and rinse.
Figure 3 shows valve arrangements on a salt regenerated unit.
Backwash is simply a reversal of the normal flow to wash out any suspended matter in the bed and
to “fluff” the bed, to break up packed areas. This is done just before the unit is regenerated.
During regeneration, chemicals are introduced at the top surface of the bed and removed through
the bottom outlet. The rinse washes out the last traces of regenerant chemical.
Figure 3 — Valve arrangements for regeneration with salt
Ion exchange units are usually installed in duplicate to permit continuous service during regeneration. Some typical equipment arrangements are
shown in Figure 4.
Regeneration procedures When regeneration is ineffective, the bed is usually fouled with suspended matter. This emphasizes the importance of proper backwash procedures. During backwash, the cation exchanger bed should expand at least 50%, while the anion exchanger bed should expand at least 75%.
How much the bed expands depends on the backwash water temperature, backwash rate and density of the ion exchanger.
Figure 5 shows the expansion characteristics of typical cation and anion resins.
The capacity of ion exchange material varies according to the amount and concentration of regenerating chemical used and the time the chemical contacts the exchanger. Table 3 shows this variation in capacity with regenerant dosages.
Selecting the optimum dosage level depends mainly on the quality of finished water required, considering both economic and operating factors.
5
KEY:
WAC – Weak acid cation WBA – Weak base anion SAC – Strong acid cation SBA – Strong base anion
6
Figure 4 — Typical equipment arrangements
CONCLUSIONS
The use of ion exchange processes affords numerous efficient and effective means of conditioning
feedwater. The proper selection of the specific ion exchange process depends on water quality needs, operating convenience, and economic considerations. For effective results, the system must be
carefully selected, designed, operated and maintained. Because m
the decision is complex,
an experienced ion exchange
engineer should be consulted to
Regenerant Operating assist in selection and design.
Exchanger Dosage Capacity
Type Regenerant (lb/ft3) (kgr/ft3)
Cation (high capacity) Salt 6 – 8 20 – 24
10 – 15 25 – 30
Cation (high capacity) Sulfuric acid 4 – 6 10 – 12
8 – 10 14 – 16
Anion (weak base) Ammonia 1.5 – 2 20 – 22
Caustic 3 – 4 20 – 22
Soda ash 3 – 5 12 – 15
Anion (strong base) Caustic 3.5 – 5 10 – 12
7
Figure 5 — Expansion characteristics of exchange beds
Table 3 — Effect of regenerant levels on exchange capacity
exchange” describes the process:
as water flows through a bed of ion exchange material, undesirable ions are removed and
replaced with less objectionable ones.
For example, in softening processes,
calcium and magnesium ions (hardness) are exchanged for sodium ions.
In dealkalization, the ions contributing to alkalinity (carbonate, bicarbonate, etc.) are removed and
replaced with chloride ions.
Other dealkalization processes
utilizing weak acid cation resin or strong acid cation resin in a split stream process, exchange cations with hydrogen.
This forms carbonic acid which can be removed in a decarbonator tower.
Demineralization is simply replacing all cations with hydrogen ions (H+) and all anions with hydroxide ions (OH–).
Ion exchange materials are like storage batteries; they must be recharged (regenerated) periodically
to restore their exchange capacity. With proper design and operation, ion exchange processes are capable of removing selected ions almost completely (in some cases to a fraction of a part per million).
Ion exchange processes
Table 1 — Types of ion exchange materials
Capacity Exchangers kgr/ft3
CATION
Inorganic (zeolites)
Natural (greensand) 3–5
Synthetic 12–16
Organic
Sulfonated coal — 5–7
(carbonaceous)
Synthetic – (phenolic types) 6–18
(styrene base) 20–30
ANION
Inorganic Not widely
Metallic oxides used
Organic
Synthetic resins 10–22
Cation exchangers
There has been constant improvement in ion exchange materials since the first use of natural and
synthetic inorganic products. Sulfonated coal, styrene-base resins, phenolic resins and acrylic
resins are some that have been developed. Exchange capacities were greatly increased with the
development of the styrene-base exchangers. These resins are manufactured in spherical, stress
and strain-free form to resist physical degradation. They are stable at temperatures as high as
300°F and are applicable over a wide pH range. More dense ION EXCHANGE MATERIALS
Ion exchange processes are versatile — specific types of ions can be removed from water depending on the choice of exchange material and regenerant used.
Development
Slightly more than 100 years ago,two English agricultural chemists, H. S. Thompson and J. Thomas Way, noted that certain soils had a greater ability than others to absorb ammonia from fertilizers.
They found that complex silicates in the soil performed an ion exchange function. They were able to prepare materials of this type in the laboratory from solutions of sodium aluminate and sodium silicate. In 1906,
Robert Gans used materials of this type for softening water. The early materials used were slow in
regenerating and lacked physical stability. These first synthetic,inorganic exchangers were called
zeolites. Today, zeolites are almost totally replaced by synthetic ion exchange materials. The basic types of ion exchangers in use for water conditioning are
Anion exchangers
Anion exchange materials are classified as either weak base or strong base depending on the
type of exchange group. Weak base resins act as acid adsorbers,efficiently removing strong acids
such as sulfuric and hydrochloric.
However, they will not remove carbon dioxide or silica. They are used in systems where strong
acids predominate, where silica reduction is not required, and where carbon dioxide is removed
in degasifiers. Preceding strong base units in demineralizing processes, weak base resins give
Typical Minerals Types of Minerals in Influent Exchanger Converted to
Cation
(A) Ca(HCO3)2 ® Na+ ® NaHCO3
CaSO4 ® Exchanger ® Na2SO4
Cation
(B) Ca(HCO3)2 ® H+ ® H2CO3
CaSO4 ® Exchanger ® H2SO4
Cation
(C) Ca(HCO3)2 ® H+ ® H2CO3
Exchanger
(Weak Acid)
Anion
(D) Na2SO4 ® Cl– ® NaCl
NaHCO3 ® Exchanger ® NaCl
Anion
(E) H2CO3 ® OH– ® H2O
H2SO4 ® Exchanger ® H2O
Conventional softening — process (A)
Dealkalization by split stream softening — blending effluents from (A) and (B)
Dealkalization by anion exchange — process (D) proceded by (A)
Dealkalization by weak acid cation exchanger followed by conventional softening
process (C) followed by (A)
Demineralizing — combination of (B) and (E)
Table 2 — Types of ion exchange processes
more economical removal of sulfates and chlorides.
These are two general classes of strong base anion exchangers,
Types I and II, denoting differences in chemical nature. Both remove silica and carbon dioxide
as well as other anions.
Type I is more effective in removing silica,and is used when the combined
silica and carbon dioxide content of the water contacting the exchanger is more than 25% of
the total anions. When there is contamination of the water with organic matter, a more porous
form of Type I resin is recommended.
The Type II anion
material is used in treating waters where the combined carbon dioxide and silica content is less
than 25% of the total anions. This is often the case when carbon dioxide is taken out in a degasifier
ahead of the anion exchanger unit.
2
Figure 1 — Typical ion exchange unit
Equipment operation Ion exchange material is housed in specially constructed tanks (Figure 1) where it forms a bed,
usually 30–60 inches deep. In some cases, it is supported by another bed of graded gravel or anthracite filter media. In other cases, some special methods of support, without gravel or anthracite, are used. During normal operation, water enters the top of the tank through a pipe, which distributes it over the surface of
the exchanger bed. The treated water is drawn off by collector piping at the bottom.
Several newer designs for ion exchange are now coming into common use. These are becoming
popular because of their advantages of higher operating efficiencies and lower leakage rates. In
countercurrent regeneration procedures, the regenerant flow is opposite in direction to the service
water flow. Therefore, the resin located in the bed where the finished water leaves the vessel, is
the most highly regenerated. This results in lower leakage rates and slightly higher operating capacities
at equal regenerant dosages to cocurrent operated vessels.
Physical Characteristics of
Resin
Anion and cation resins can be obtained in several different physical forms. They can be
obtained with different ions located on the exchange site.
This has importance in applications such as mixed beds where minimal leakage rates are
required even from a newly installed bed.
Particle size can also be specified.
Uniform particle sized (UPS)
resins are now available where all beads fit into a very close particle size range. For practical
purposes, all of the beads are the same size. Beds of UPS resins have some unique operating
characteristics which offer advantages
when they are used in mixed bed, layered bed, and packed bed applications. Macroporous resins are highly porous which give them advantages when used in processes that have high fouling potential.
ION EXCHANGE PROCESSES
Ion exchange processes fall into several categories: softening (including removal of iron and
manganese), dealkalization, and demineralization. Examples of these processes are listed in
Table 2.
3
Figure 2 — Cocurrent vs. countercurrent regeneration
Cocurrent vs countercurrent: With downflow operation, ion distribution at end of operating cycle
is the same for cocurrent and countercurrent operation. But at end of regenerating cycle, ion
distribution is revised when cocurrent is compared with countercurrent regeneration. Key to
optimum results is quality of rinse water used with countercurrent regeneration.
R– H+ R– H+
R–2 Ca+2
R–2 Mg+2
R– Na+
R–2 Ca+2
R–2 Mg+2
R– Na+
R– H+
R– H+
R–2 Ca+2
R–2 Mg+2
R– Na+
R–2 Mg+2
R–2 Ca+2
R– Na+
4
However, countercurrent vessels are more sensitive to operating problems. The bed must be immobilized during the regeneration process and the influent water must be very low in suspended solids.
Figure 2 illustrates what happens in the two regeneration modes at exhaustion and after regeneration.
Packed bed systems essentially fill the vessel with resin. The systems are countercurrent in design and offer the low leakage rate advantage. However, packed beds also offer the advantage of reduced waste generation. Since there is no space for proper backwash, packed bed systems usually are built with external backwash tanks which allow the resin to be backwashed after it is sluiced out of the operating
vessel. All countercurrent ion exchange systems require feedwater that is very low in suspended solids.
Regeneration of the exchange material involves three steps: backwash, introduction of the regeneration chemicals, and rinse.
Figure 3 shows valve arrangements on a salt regenerated unit.
Backwash is simply a reversal of the normal flow to wash out any suspended matter in the bed and
to “fluff” the bed, to break up packed areas. This is done just before the unit is regenerated.
During regeneration, chemicals are introduced at the top surface of the bed and removed through
the bottom outlet. The rinse washes out the last traces of regenerant chemical.
Figure 3 — Valve arrangements for regeneration with salt
Ion exchange units are usually installed in duplicate to permit continuous service during regeneration. Some typical equipment arrangements are
shown in Figure 4.
Regeneration procedures When regeneration is ineffective, the bed is usually fouled with suspended matter. This emphasizes the importance of proper backwash procedures. During backwash, the cation exchanger bed should expand at least 50%, while the anion exchanger bed should expand at least 75%.
How much the bed expands depends on the backwash water temperature, backwash rate and density of the ion exchanger.
Figure 5 shows the expansion characteristics of typical cation and anion resins.
The capacity of ion exchange material varies according to the amount and concentration of regenerating chemical used and the time the chemical contacts the exchanger. Table 3 shows this variation in capacity with regenerant dosages.
Selecting the optimum dosage level depends mainly on the quality of finished water required, considering both economic and operating factors.
5
KEY:
WAC – Weak acid cation WBA – Weak base anion SAC – Strong acid cation SBA – Strong base anion
6
Figure 4 — Typical equipment arrangements
CONCLUSIONS
The use of ion exchange processes affords numerous efficient and effective means of conditioning
feedwater. The proper selection of the specific ion exchange process depends on water quality needs, operating convenience, and economic considerations. For effective results, the system must be
carefully selected, designed, operated and maintained. Because m
the decision is complex,
an experienced ion exchange
engineer should be consulted to
Regenerant Operating assist in selection and design.
Exchanger Dosage Capacity
Type Regenerant (lb/ft3) (kgr/ft3)
Cation (high capacity) Salt 6 – 8 20 – 24
10 – 15 25 – 30
Cation (high capacity) Sulfuric acid 4 – 6 10 – 12
8 – 10 14 – 16
Anion (weak base) Ammonia 1.5 – 2 20 – 22
Caustic 3 – 4 20 – 22
Soda ash 3 – 5 12 – 15
Anion (strong base) Caustic 3.5 – 5 10 – 12
7
Figure 5 — Expansion characteristics of exchange beds
Table 3 — Effect of regenerant levels on exchange capacity
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