Regeneration methods
for ion exchange units

Introduction

• Co-flow regeneration, where the fluids are flowing from the top to the bottom of the column both during the service run as well as during regeneration.
• Reverse flow regeneration, where the fluids are flowing alternatively upwards and downwards during service and regeneration.

Co-flow regeneration (CFR)

Reverse flow regeneration (RFR)

1. Upflow loading and downflow regeneration, as in the floating bed and Amberpack^TM processes.
2. Downflow loading and upflow regeneration, as in the UFD^TM and Upcore^TM processes.

1. The treated water has a much higher purity than with co-flow, due to a very low leakage.
2. Less regenerant is required, as the contaminating ions don't have to be pushed through the whole bed, and the leakage is almost independent of the regenerant dosage.

Treated water quality

No backwash with RFR

Regeneration steps

1. Backwash resin bed (co-flow regeneration only) to remove suspended solids and decompact the bed.
2. Inject regenerant diluted in appropriate water quality. The injection is at a low flow rate, so that the contact time is 20 to 40 minutes.
3. Displace the regenerant with dilution water at the same flow rate.
4. Rinse the bed at service flow rate with feed water until the desired treated water quality is obtained.

Mixed bed regeneration

1. Backwash resin bed to separate the cation from the anion resin.
2. Let the resins settle.
3. Optionally: drain the water down to the resin bed surface.
4. Inject caustic soda diluted in demineralised water.
5. Displace the caustic with dilution water.
6. Inject acid diluted in demineralised water.
7. Displace the acid with dilution water.
8. Drain the water down to the resin bed surface.
9. Mix the resins with clean compressed air or nitrogen.
10. Refill the unit slowly with water.
11. Do the final rinse with feed water at service flow rate until the desired treated water quality is obtained.

Regeneration efficiency

1. Hydrochloric acid is more efficient than sulphuric acid to regenerate a strongly acidic cation exchange resin (SAC) initially in the Na⁺ form.
   With 50 g HCl per litre of resin, a conversion of 60 % to the H⁺ form is achieved.
   With 50 g H₂SO₄, a conversion of only 40 % is achieved.
2. Even expressed as equivalents, hydrochloric acid is more efficient: 36.5 g HCl (1 eq) will convert the resin to 45 %, whereas 49 g H₂SO₄ (1 eq) convert only 39 %.
3. To obtain total conversion, i.e. 100 % in the H⁺ form, we need about 6.5 eq HCl (240 g/L) but 8 eq H₂SO₄ (400 g/L).
4. This is due to the fact that the second acidity of sulphuric acid is considerably weaker than the first acidity.
5. Regeneration of a strongly basic anion exchange resin (SBA) initially in the Cl^— form with caustic soda is more difficult:
   With 50 g NaOH per litre, only 37 % of the resin are converted; with 40 g (1 eq) only 32 %.
   As much as 37.5 eq NaOH (1500 g) are required to convert the SBA resin to about 100 % in OH^— form.
6. The reason why SBA resins of type 1 are more difficult to regenerate than SAC resins is the selectivity coefficient:
   K_(Cl/OH) = 22 whilst K_(Na/H) = 1.7.

Regeneration ratio

Definition:

• The regeneration ratio — or regenerant ratio — is calculated as the total amount of regenerant (in equivalents) divided by the total ionic load (also in equivalents) during one cycle.
• It is is also equal to the number of eq/L regenerant per eq/L of resin operating capacity.
• A (theoretical) regenerant ratio of 1.00 (i.e. 100 %) would correspond to the stoichiometric quantity.
• All resins need a certain excess of regenerant above the stoichiometric quantity.

• Amberjet 1000
• regenerated with 55 g HCl per litre
• operating capacity : 1.20 eq/L
• 55 g/L HCl = 55/36.5 = 1.507 eq/L
• Regenerant ratio = 1.507/1.20 = 1.26 = 126 %

• WAC resins require just above the stoichiometric quantity. A safe number is 105 to 110 %.
• WBA resins require 115 to 140 %, because most of them they have some strongly basic functional groups.
• When regenerated with ammonia or sodium carbonate, WBA resins require a regenerant ratio of 150 to 200 %. These regenerants can be used for WBA only, not for SBA resins.
• SAC and SBA resins require a larger excess than their weak counterparts.
• Co-flow regenerated SAC and SBA resins require more than those regenerated in reverse flow.
• SAC resins regenerated in reverse flow with hydrochloric acid need an absolute minimum of 110 % regeneration, but a safer value is 120 %. If the water contains high hardness or low alkalinity, the minimum value must be increased.
• SAC resins regenerated with sulphuric acid require a larger excess than those regenerated with HCl. At least 40 % more.
• For SBA resins, there is no easy way to estimate a minimum, as it depends on the type of SBA resin (styrenic type 1 vs type 2 or acrylic resins).
• Important note: when calculating the regenerant ratio for SBA resins, one must take 2 equivalents of NaOH for each equivalent of CO₂ or SiO₂.
• WAC/SAC couples can be regenerated with a global ratio of about 105 %.
• WBA/SBA couples can be regenerated with a global ratio of 110 to 120 %. More is required if the silica level is high in the feed water.
• The regenerant ratio for silica should be at least 800 %. This should be calculated separately as the quantity of NaOH (in eq) divided by the load of silica (in eq) during one cycle. One equivalent of silica is taken as 60 g as SiO₂.

Thoroughfare regeneration

1. The feed water must pass first through the weak, then only through the strong resin.
2. The regenerant must pass first through the strong, then through the weak resin.

Separate columns in service

Separate columns in regeneration

1. The weak resin has a high capacity and good regeneration efficiency, but does not remove all ions. Therefore it must be placed first, and the strong resin will be used to remove whatever the weak resin has not removed, albeit with a lower efficiency.
2. The strong resin requires a high excess of regenerant. The weak resin requires almost no excess. Therefore the regenerant passes through the strong resin first, and the weak resin is regenerated with the excess regenerant coming out of the strong resin.

Amberpack in service

Amberpack in regeneration

Regenerant types and concentrations

Types of regenerant

• Sodium chloride (NaCl) is normally used to regenerate SAC resins in the softening process, and SBA resins used for nitrate removal.
• For softening, potassium chloride (KCl) can also be used when the presence of sodium in the treated solution is undesirable.
• In some hot condensate softening processes, ammonium chloride (NH₄Cl) can be used.
• For nitrate removal, the SBA resin can be regenerated with other compounds providing chloride ions, such as hydrochloric acid (HCl).
• For decationisation — the first step of a demineralisation process — SAC resins must be regenerated with a strong acid. The most common acids are hydrochloric and sulphuric acids.
  • Hydrochloric acid (HCl) is very efficient and does not cause precipitations in the resin bed.
  • Sulphuric acid (H₂SO₄) is sometimes cheaper and easier to store and to handle in general, but less efficient than hydrochloric acid: the operating capacity of the SAC resin is lower. Additionally, its concentration must be carefully adjusted to prevent calcium sulphate precipitation (see below). Once a CaSO₄ precipitate is formed, it is very difficult to remove from the resin bed.
  • Nitric acid (HNO₃) can also be used in principle, but is not recommended as it can cause exothermic reactions; explosions have been observed in some cases, so that the use of nitric acid is considered dangerous.
• For dealkalisation, the WAC resin is best regenerated with hydrochloric acid (HCl). When using sulphuric acid, the concentration must be kept under 0.8 % to avoid calcium sulphate precipitation. Other, weaker acids can also regenerate WAC resins, such as acetic acid (CH₃COOH) or citric acid, a molecule containing three —COOH groups: (CH₂COOH-C(OH)COOH-CH₂COOH = C₆H₈O₇). Have a look at the 3-dimensional formula.
• SBA resins are always regenerated with caustic soda (NaOH) in the demineralisation process. Caustic potash (potassium hydroxide KOH) is in principle also applicable, but usually more expensive.
• WBA resins are usually also regenerated with caustic soda, but other regenerants— weaker alkalis — can also be used, such as:
  • Ammonia (NH₃)
  • Sodium carbonate (soda ash, Na₂CO₃)
  • A lime (calcium hydroxide, Ca(OH)₂) suspension

Concentrations

• NaCl (softening and nitrate removal): 10 %
• HCl (decationisation, de-alkalisation and demineralisation): 5 %
• NaOH (demineralisation): 4 %
• H₂SO₄: for SAC resins, the acid concentration must be carefully selected between 0.7 and 6 % as a function of the proportion of calcium in the feed water (which is the same in the SAC resin). For WAC resins, the concentration is usually 0.7 %. Too high a concentration may cause calcium sulphate precipitation.
  For SAC resins, stepwise concentrations are often used: after a first step at a low concentration, a second step is carried out at a higher concentration once a great part of the calcium on the resin has been eluted. In rare cases, three steps are used. The steps at higher concentrations reduce the quantity of dilution water and increase the sulphuric acid efficiency.

Special applications

Sweetening-off and -on

1. Backwash with the raw solution (optional)
2. Sweetening off: displacement of the solution with water
3. Regenerant injection
4. Displacement of the regenerant with water
5. Fast rinse with water
6. Sweetening on with the solution to be treated

Merry-go-round

Ion exchange columns

Introduction

Co-flow regenerated column

• The water enters from the top of the column. So as not to disturb the surface of the resin bed, the incoming water stream is stopped by a simple jet breaker.
• The column has a large freeboard, usually about the same height as the resin bed, so that the resin can be backwashed inside the column to remove suspended solids accumulated on the bed surface.
• A manhole (shown on the left side) is necessary to inspect and possibly repair the column inside.
• Two sight glasses are also shown, one at the top, one at the level of the resin bed surface.
• An air vent is also necessary at the top, to empty the column by draining the water out for inspection or a resin change.
• One of the most important features of the vessel is the collector at the bottom: nowadays, one of the most popular types of collector is a plate with densely distributed nozzles .
• The reinforced plate disk of steel onto which the nozzles are screwed.
• It is supported by poles or L-shaped beams (here two poles are visible).
• A regenerant distributor is sometimes — but not always — mounted in the middle of the vessel to ensure a uniform distribution of the regenerant. In absence of such a collector, the regenerant is introduced from the top of the column, which results in some dilution of the chemicals.

• Columns with freeboard
  • Co-flow regenerated vessels
  • Reverse flow regeneration with air hold-down
  • Reverse flow regeneration with water hold-down
  • Stratabed columns
  • Split-flow columns
• Packed bed columns
  • Amberpack^TM and floating beds
  • Multi-compartment Amberpack
  • Upcore^TM (Amberpack Reverse)
  • Stratapack^TM
• Some other technologies
  • Econex^TM (Italba)
  • ReCoFlow^TM (Ecotec)
  • ISEP^TM (Calgon Carbon)
• Polishers
  • Mixed bed units
  • Triobed^TM
  • Amberpack Sandwich^TM
  • Multistep^TM (Bayer)
  • Tripol^TM (Vivendi/Permutit)

Columns with freeboard

In production

In regeneration

In production

In regeneration

Stratified beds

These are hold-down units containing a pair of cation or anion resins: the weakly functional resin has a smaller particle size, and its density is lower than that of the strongly functional resin allows the two components to be kept separated. Some mixing of both layers at the interface is inevitable, though, and periodical re-separation is required. The stratified bed technology saves a column and brings the benefit of a good regeneration efficiency. As the weak resin always has a density smaller than the strong base, stratified bed must always be regenerated in reverse flow. For co-flow regeneration, two separate columns are necessary.
Stratified beds are also called layered beds. Stratabed^TM is a trademark of Dow.

Split-flow units

In split-flow vessels, regeneration is carried out simultaneously from the top and from the bottom of the bed. The regenerant collector is located in the upper third of the resin bed. An additional regenerant distributor is required above the resin bed. The idea is to allow the upper part of the bed to be backwashed to remove accumulated debris without disturbing the lower layers of the bed that are responsible for the good treated water quality. There is no inert or inactive resin, and the system does not consume extra water, but is more complicated and the regeneration flows are sometimes difficult to adjust.

Packed bed units

Floating bed units

Amberpack in service and regeneration

Called "Schwebebett" by Bayer AG in 1963, this design has upflow loaded, downflow regenerated columns in which the bed was initially partially fluidised. The floating bed technology has been a big success since the mid 1970's has been adopted, with some variations, by most of the European OEMs. Today, these units are fully packed with practically no fluidised resin.
Bayer's Schwebebett (called WS system in some countries) uses an inert floating material at the top of the resin chamber. It cannot be backwashed.

Amberpack^TM

Amberpack with double chamber

The Amberpack system of Rohm and Haas is a variation of the floating bed and enables resin transfer to a backwashing column. The ion exchange vessels are similar to Bayer's Schwebebett units, the differences being the absence of inert floating material and the presence of at least two transfer ports per resin chamber, through which resins can be taken out for backwashing. The resins are exhausted upflow and regenerated downflow. In multicompartment Amberpack columns, two (sometimes three) resins are separated by a plate fitted with double nozzles. This is the ideal system for WBA/SBA and for WAC/SAC combinations: it gives the best efficiency and treated water quality. Each compartent has two transfer ports for external backwashing.
The WAC/SAC combination with sulphuric acid regeneration requires a special version with secondary dilution.

A full Amberpack demineralisation line followed by a mixed bed polisher

Upcore^TM

Upcore column in service and regeneration

UpCoRe stands for "Upflow Countercurrent Regeneration". Dow licensed this technology from the Dutch engineering company Esmil in the late 1970s. The units are exhausted downflow and regenerated upflow. A special inert polymer called Dowex Upcore IF62 fills the upper part of the columns.
Dow claim that their system is "self-cleaning", and that suspended solids accumulated during the exhaustion run escape during the first stage of regeneration, but this is only partially true: the system is not capable of eliminating large amounts of suspended matter, unlike Amberpack with its dedicated backwashing tower.
Besides, upflow regeneration is more difficult than downflow, particularly with hydrochloric acid, because of the high velocity required to compact the bed and the resulting short contact time. This system consumes a little more water, as an additional step is required to compact the bed against the top nozzle plate before regenerant injection.
Upcore is also available as Amberpack Reverse, the main difference being the presence of a backwash column as a safety feature.
Upcore is useful when the plant works intermittently or when large flow rate variations are expected.

Stratapack^TM

A Stratapack vessel

Stratapack columns, which are Amberpack Reverse units in the Stratabed fashion, and offer the advantages of both. Inert resin is necessary in view of the relatively fine particle size of the weak base or weak acid resin. The column has three transfer ports. The system is also available as Upcore layered bed.
Stratapack cation units are not recommended with sulphuric acid regeneration.
Because a little mixing at the interface cannot be prevented, Stratapack is not quite as efficient as a double-compartment Amberpack. Additionally, it consumes a little more water, like Amberpack Reverse. To minimise the effect of resin mixing, a higher dosage of regenerant is often used. The common characteristic of all Amberpack systems is the presence of a backwash column, which is an essential safety device to ensure smooth operation of the water treatment plant.

Some other technologies

The Econex system

Econex^TM
This system was developed in the 1970s by Italba-Ionics and Davy-Bamag. The columns have a freeboard, but in normal operation it is filled with inert material so the columns do not need an air or water hold-down during regeneration. When the resin has to be backwashed, the inert is extracted to a holding tank and backwashing is performed in the ion exchange column.
The columns are simple, but large and a transfer system is needed. Some units have double chamber anion columns to house a WBA above and a SBA below.

A ReCoFlo unit

ReCoFlo^TM
This technology was developed by the Canadian Ecotec Corporation. The columns are very shallow, with a bed depth of typically only about 15 cm (6 inches). Cycles are very short, the service run being only about 20 minutes. Special, fine resins are used. This technology was mainly applied in the surface treatment industry.

A simplified ISEP system

ISEP^TM
ISEP is a simulated moving bed, operating in a quasi continuous, stepwise fashion. It was developed by the US company Advanced Separation Technologies (AST), now a subsidiary of Calgon Carbon. The columns typically 30 of them) are arranged in a carrousel (merry-go-round). The feed and elution solutions are connected to a stationary upper distributor fitted with typically 20 ports, and the raffinate as well as the extract are connected to a lower stationary connector fitted with the same number of ports. The columns themselves are on a rotating frame. The carrousel rotates continuously at a speed of 0.1 to 1.5 revolutions per hour, and the ports are thus successively connected to all columns. A simplified schema is shown here with only 8 columns and 6 inlet and outlet ports. Simulated moving beds can be used for chromatography, for purification of fermentation broth, sugar syrup deashing, colour removal from various solutions, separation of metals and other applications. The major problem with this system is leaks between the heads of the rotating columns and the fixed distributor.

Higgins loop (click)

Polishing units

1. Units with mixed resins
2. Units with separate resins, in one or several vessels

Mixed bed units

A mixed bed in service

Strongly acidic cation resin is mixed with strongly basic anion resin. The quality of treated water is excellent, typically with a conductivity of less than 0.1 µS/cm and a residual silica of less than 10 or even down to 1 µg/L when properly designed and operated. The resins must however be separated for regeneration. This is a delicate and lengthy operation.
Regeneration involves the following steps:

• Backwash for separation
• Settling
• Acid injection from the bottom, extraction through the middle collector
• Acid displacement
• Caustic injection from the ad hoc distributor, extraction through the middle collector
• Caustic displacement
• Air mixing
• Final fast rinse

See details in the regeneration page.

Spherical units

A spherical vessel

Some power stations have condensate polishers operating under high pressure: 4 to 5 MPa (40 to 50 bar, 600 to 700 psi). In this case the shell of the vessel must be very thick. For this reason spherical vessels are built, because a sphere has a better resistance to pressure than a cylinder, and one can save in the thickness of the vessel shell. Those have disadvantages compared to cylindrical columns, as the flow through the resin bed is less uniform. See the condensate polishing page.

Triobed^TM units

A Triobed after separation

Triobed is a trademark of Rohm and Haas. The concept was developed by Duolite in the 1970's, and had immediate success. The idea was to mix a third, inert component to the active SAC and SBA resins. Density and particle size of the three components are precisely adjusted so that the inert forms a separating layer between cation and anion resins after backwash. Click on the adjacent picture for a better understanding of the principle. Triobed doesn't have only advantages:

• The inert resin has sometimes problems: it may float if there are traces of oil in the water or condensate, or attract air bubbles at the time of backwashing.
• The inert "dilutes" the active resins, and uses space: the total capacity of the bed is reduced.
• The SAC resin is very coarse, which is detrimental to its kinetics, and requires a high backwash flow rate for separation.

For these reasons, conventional two-component mixed beds are now preferred. Amberjet and other uniform resins resins give a very sharp separation, and if absolutely no cross-contamination is acceptable, other techniques are available with external regeneration.
For existing Triobed units, only very specific resin combinations are allowed.

Amberpack^TM Sandwich^TM

Amberpack Sandwich unit

A different type of polisher, with separate cation and anion resins, not a mixed bed. Amberpack Sandwich polishing units are double chamber Amberpack columns with a strong acid and a strong base resin, separated by a nozzle plate. Twice the efficiency of a mixed bed, and half the size.
The small freeboard (see picture) is usually be filled with a floating inert material. A collector is located just below this separation plate. The advantages of Amberpack Sandwich compared to mixed bed units are:

• As the column is almost full, it contains twice the resin amount, and offers thus a double capacity for the same vessel size.
• As resins are never mixed, all problems of cross-contamination found in MBs are avoided.
• Regeneration is in counterflow mode, thus a much smaller quantity of regenerants is required.
• Sandwich gives the same treated water quality as a conventional MB.

It is sometimes preferred to have the anion column in the bottom compartment: in this way, traces of Na from the anion resin are eliminated by the cation resin. This arrangement is possible only when the feed does not contain any hardness.
Sandwich units can also be used, like "working MBs", for the treatment of low salinity water, such as RO permeate.

Construction material

In the laboratory, glass columns are used for resin testing or quality control.	Small units are usually made of fiberglass reinforced plastic. See also SDI.
Industrial units are made of steel with an internal hard rubber coating.	In the food industry and some other industries, ion exchange columns are often made of stainless steel.

Nozzles

A nozzle plate with stiffening beam

Backwash column

A backwash tower. Left: principle, right: photograph of the bottom of the column.

Ion exchange plant design

Some basic principles

• Feed water analysis
• Production flow rate
• Cycle length
• Required quality of the treated water
• Regeneration technology
• Dimensions of the vessels
• Selection of resin types

Analysis of the feed water

Production flow rate

Cycle length

• Specific flow rate between 5 and 50 bed volumes per hour (BV/h).
• Mixed bed units should be designed to operate at a minimum of 12 to 15 BV/h.
• Make the system as small as possible for economical reasons (lower investment in hardware and resins).
• For packed bed systems, ensure that bed compaction is good both in the production phase (e.g. Amberpack^TM) and during regeneration (e.g. Upcore^TM).

Treated water quality

• Conductivity: ~ 1 µS/cm
• Silica: 10 to 25 µg/L

Regeneration technology

• An initial short compaction step is performed at about 30 m/h before regenerant injection
• Regenerant concentration may have to be reduced so that the acid solution can be injected at 7 or more m/h in the SAC unit, and the caustic solution at more than 5 m/h in the anion exchange unit.
• Contact time of the regenerant solution may have to be reduced.

Vessel sizing

Resin choice

• Macroporous resins are normally not required for demineralisation or softening
• An exception: all styrenic WBA resins are macroporous
• Special particle sizes are required depending on the design technology:
  • uniform or semi-uniform resins are necessary for packed beds
  • special grades are required for stratified beds (e.g. Stratabed^TM or Stratapack^TM)
  • special grades are also required for mixed bed polishers
• When the feed water contains high organics, acrylic anion resins are a good choice

How to calculate by hand, approximately

1. Examine water analysis (details above)
2. Calculate cation concentration C_c [meq/L]
3. Decide about the use of a degasifier:
   If the bicarbonate content is greater than 0.6 to 1.0 meq/L a degasifier may be justified
4. Calculate the anion concentration C_a [meq/L]: it contains
   Cl^—, SO₄⁼, NO₃^—, SiO₂, HCO₃^—or residual CO₂ after degasser if any
5. Decide about a reasonable running time t in hours between regenerations
6. Using the flow rate f in m³/h calculate the throughput Q [m³]:
   Q = f · t [m³]
7. Calculate the ionic load per cycle in eq (concentration in meq/L times throughput in m³):
   • Cation load [eq] = C_c· Q
   • Anion load [eq] = C_a· Q
8. Consider the approximate operating capaciy of the resins as follows:
   • SAC: cap_c = 1.0 eq/L with HCl regeneration or
     SAC: cap_c = 0.8 eq/L with H₂SO₄ regeneration
   • SBA: cap_a = 0.5 eq/L
9. The resin volume V required (in litres) is equal to the ionic load [eq] divided by the operating capacity [eq/L]:
   • SAC: V_c = C_c· Q / cap_c [L]
   • SBA: V_a = C_a· Q / cap_a [L]
10. At the end of this calculation, we must make sure that the specific flow rate of both resin columns is compatible with the general recommendations of the resin producer. The specific flow rate in h^—1 (often expressed in bed volumes per hour BV/h) is equal to the flow rate in m³/h divided by the resin volume in m³. The usual range is 5 to 50 h^—1. For a compact plant with minimum investment cost, use a specific flow rate around 30 to 35 h^—1.
    If the specific flow rates calculated from the resin volumes V_c and V_a are too high, increase the running time t. If they are too low, reduce the running time t.

Example

1. Water analysis [meq/L]
   

   Cations			Anions
   Ca	3.2		Cl	1.1
   Mg	0.7		SO4	0.6
   Na	0.9		NO3	0.2
   			HCO3	2.9
   Σ Cations	4.8		Σ Anions	4.8
   			SiO2	0.4

2. C_c = 4.8 meq/L
3. HCO₃ = 2.9 meq/L — a degasifier is recommended
   Residual CO₂ after degasifier = 0.25 meq/L
4. C_a = 1.1 + 0.6 + 0.2 + 0.25 = 2.15 meq/L
5. Select runnning time t = 12 h
6. Flow rate 60 m³/h
   Throughput 60 · 12 = 720 m³
7. Ionic load
   • Cation load [eq] = 4.8 · 720 = 3456 eq
   • Anion load [eq] = 2.15 · 720 = 1548 eq
8. Operating capacity
   • Cation regeneration with HCl: cap_c = 1.0 eq/L
   • Anion regeneration with NaOH: cap_a = 0.5 eq/L
9. Resin volumes
   • SAC: V_c = 3456 / 1.0 = 3456 L
   • SBA: V_a = 1548 / 0.5 = 3096 L
10. Specific flow rate
    • SAC: 60 / 3.456 = 16.9 h^—1
    • SBA: 60 / 3.096 = 19.4 h^—1
    The specific flow rate values are OK, but one could reduce the running time to say 8 hours to make the plant smaller. The new data would then be:
    • Throughput 60 · 8 = 480 m³
    • Cation load [eq] = 4.8 · 480 = 2304
    • Anion load [eq] = 2.15 · 480 = 1032 eq
    • SAC volume: V_c = 2304 / 1.0 = 2304 L
    • SBA volume: V_a = 1032 / 0.5 = 2064 L
    • SAC flow rate: 60 / 2.304 = 26.0 h^—1
    • SBA flow rate: 60/2.064 = 29.1 h^—1
    In most cases, the demineralisation system will comprise at least 2 trains operating alternatively, with automatic regeneration, so the shorter running time is an advantage. We have assumed in our example (see picture above) that the colums are regenerated in Amberpack mode, which will guarantee a very good treated water quality, with typically less than 1 µS/cm conductivity and less than 10 µg/L residual silica.
     
    Amberpack, Upcore, Stratabed, and Stratapack are trademarks of the Dow Chemical Company.
    

Laboratory trials and setup

Introduction

Resin volume

Basic setup

1. A tank containing the solution to be treated.
2. A small tap or a rubber tube with a screw clamp to control the flow rate.
3. A column fitted with a rubber stopper.
4. The ion exchange resin to be tested.
5. The bottom of the column must be fitted with fritted glass. Another option is a second rubber stopper covered with a nylon cloth and a small layer of glass beads.
6. Another small tap.
7. The bottom tank collects the treated solution.

The picture shows a quantity of solution above the resin bed. It is most important never to let the column run dry, otherwise the hydraulic distribution may be disturbed and the resin may be damaged. Regeneration of the resin can be carried out in the same equipment. Regeneration steps include:

• Backwash (optional)
• Regenerant injection
• Regenerant displacement
• Rinse

  Recommended operating conditions

Resin volume	50 to 250 ml
Column diameter	20 to 30 mm
Resin bed depth	150 to 800 mm
Loading flow rate	2 to 40 BV/h
Regenerant flow rate	2 to 5 BV/h
Regenerant contact time	20 to 60 minutes
Regenerant displacement	2 to 4 BV of water
Final rinse	2 to 10 BV at service flow rate

The flow rate is expressed in bedvolumes per hour (BV/h), i.e. in litres of solution per litre of resin per hour. In general, the highest the concentration of contaminants to remove, the lowest the flow rate to use. When filling the column with resin, a short backwash with deionised water is recommended to ensure proper packing and classification of the bed.

More than one cycle may be required

To determine the operating capacity and the leakage, it is often necessary to perform several cycles of exhaustion and regeneration. The first run made with a new, totally regenerated resin, produces generally a higher capacity and a lower leakage. After two or three cycle, an equilibrium is obtained, and the subsequent cycles should be more or less identical.
 

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Glass columns in the Lanxess laboratory

Ion exchange capacity

Introduction

Definitions

Total capacity

Operating capacity

Zone of exchange

Ideal case

Start of the run

Middle of the run

End of the run

Case 1: the resin is totally regenerated at the beginning of the run (WAC & WBA)

Start of the run

Middle of the run

End of the run

Case 2: the resin is partially exhausted at the beginning of the run (SAC & SBA)

Start of the run

Middle of the run

End of the run

Case 2b: co-flow regenerated resins

Start of the run

Middle of the run

End of the run

Ion exchange kinetics

Low flow rate: the reaction zone is short

High flow rate: the reaction zone is long

Parameters affecting operating capacity

• Concentration and type of ions to be adsorbed
• Service flow rate
• Temperature
• Type, concentration and quantity of regenerant
• Type of regeneration process (co-flow, reverse flow...)
• Bed depth (reverse flow regeneration only)
• Particle size of the ion exchange resins

Measurement of the total capacity

1. For new resins, it gives information about the efficiency of the activation process: for instance, if every aromatic ring has been sulphonated in a strongly acidic resin, the theoretical maximum total dry weight capacity is about 5.5 eq/kg in H⁺ form.
2. For used resins, it gives information about a possible fouling: a fouled resin sample contains foreign matter, which increases the dry weight, and as a consequence the dry weight capacity (number of active groups per kg of dry matter) decreases, even if no functional group has been lost.

Operating capacity in practice

Experimental calculation of the operating capacity

Softening example

Demineralisation

Important remark

Water analysis details

Introduction

• Calcium and magnesium are the Total Hardness (TH).
• Bicarbonate, carbonate and hydroxide are the Total Alkalinity (m-Alk). Usually, natural water does not contain carbonate or hydroxide.
• Chloride, sulphate and nitrate are the Equivalent Mineral Acidity (EMA), also called Salts of Strong Acids (SSA) or, after cation exchange, Free Mineral Acidity (FMA).
• When hardness is greater than alkalinity (in meq/L) the bicarbonate hardness is called "temporary hardness" (= TH – Alk) and the remaining hardness is called "permanent hardness". The value of temporary hardness is then equal to that of alkalinity in meq/L.
• When hardness is smaller than alkalinity (in meq/L) there is no permanent hardness and temporary hardness is equal to total hardness.
• All natural waters are ionically balanced, i.e. the sum of cations in meq/L is equal to the sum of anions.

• barium (Ba⁺⁺) and strontium (Sr⁺⁺) are alkaline earth metals (see important note below) and belong thus to the hardness;
• for calculation with an ion exchange software, you would also add divalent iron (Fe⁺⁺), nickel (Ni⁺⁺) and copper (Cu⁺⁺) to the hardness group, by convenience;
• ammonium (NH₄⁺) and potassium (K⁺) are handled like sodium;
• lithium (Li⁺) also reacts like sodium (Na⁺);
• phosphate (PO₄^3–) belongs to the EMA;
• fluoride (F^–), bromide (Br^–) and iodide (I^–) are halogenides and behave like chloride.

• The solubility of barium sulphate is only 2 mg/L, thousand times lower than that of calcium sulphate.
• Ba and Sr are not well removed on WAC resins. These resins have a lower affinity for Ba and Sr than for Ca and Mg. See the table of selectivity values.
• Ba (and Ra) are very well removed on SAC resins. So well that regeneration may be difficult. Using H₂SO₄ to regenerate a SAC resin loaded — even partially — with barium may be close to impossible.

Units of concentration and capacity

Examples

A balanced analysis ?

• A small difference due to imprecision in the analytical procedures is acceptable as long as the difference between total cations and total anions is less than 3 %.
• At high pH (> 8.2), e.g. in the presence of ammonia or after lime decarbonation, there will be hydroxide or carbonate ions. Hydroxide ions are usually not reported separately. Carbonate ions are not always reported. In such a case, you would have more cations than anions.
• At low pH (say < 6.8), the water may contain either free mineral acidity (very rare for natural water) or free carbon dioxide, both producing H ions which are usually not reported separately.

An example of water analysis

m- and p-Alkalinity

• Hydrogencarbonate HCO₃^–, often called bicarbonate
• Carbonate CO₃⁼
• Hydroxide OH^–

• Phenolphthalein changing colour at pH 8.3 measures p-alkalinity
• Methylorange changing colour at pH 4.5 measures m-alkalinity

HCO₃^– + OH^– CO₃⁼ + H₂O

CO₂ + CO₃⁼ + H₂O 2 HCO₃^–

Free CO₂ and pH

H⁺ + HCO₃^– H₂CO₃ CO₂ + H₂O