Basics of designing demineralizer plant using ion exchange resin.

1. Ion exchange plant design
Some basic principles
You will not find here a complete plant design manual. Only a few general
recommendations to ensure that an ion exchange system is designed economically
and to achieve good performance, and a simple, but detailed example. Basic column
types are shown in another page.
Reputable water treatment companies have their own technologies and design
methods. We will cover in this page some of the basic parameters to consider when
designing an ion exchange plant.
These parameters are:
Feed water analysis
Production flow rate
Cycle length
Required quality of the treated water
Regeneration technology
Dimensions of the vessels
Selection of resin types
This page is mainly focused on demineralisation systems, but most of the principles
and recommendations apply to other ion exchange processes: softening,
dealkalisation, nitrate removal etc.
Analysis of the feed water
All ion exchange systems are designed for a given feed water. Some variations of the
feed water analysis are acceptable, and should be taken into account, but an ion
exchange system cannot be designed efficiently for vastly different water types. For
instance, a demineralisation system designed for the treatment of deep well water is
completely different from a system designed to treat reverse osmosis permeate.
The first thing to do is thus obtain a
reliable water analysis. Details are
shown in another page.
When the water analysis is not
constant, e.g. due to seasonal
variations, do not take an "average
composition" as the basis of your
design. Instead, use the "most probable"
case, design with this water, and check
as a second step what will happen with
the "minimum" and "maximum"' waters.
All water analyses must be perfectly
balanced, as shown in the example on
the right.
The water analysis will determine what
resin combination is required, and if a
degasifier should be considered.
Production flow rate
It is important to know whether the system will operate at constant or variable flow
rate. Some system designs require a minimum flow rate (e.g. AmberpackTM).
Obviously, the system should be able to operate at both limits.
In general, it is not advisable to operate intermittently, i.e. to stop production in the
middle of the run and re-start it. Treated water quality may be affected after a stop
Update
8 Sep 2015
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2. not followed by regeneration.
Cycle length
A short cycle length is desirable in most cases. The practical limit is that the
production run should be at least as long as the regeneration process. As most ion
exchange systems are regenerated automatically, the duration of the production run
does not have to be "at least one day" as was the rule at the time (many decades ago)
when the morning shift would regenerate manually every day at 7 o'clock. Efficient
systems have been designed with running times as short as 3 hours.
The limits of the running time are also related to resin kinetics. When reading ion
exchange resin product data sheets, you will typically see that the specific flow rate
in water treatment should be between 5 and 50 bed volumes per hour (m3/h per m3
of resin). At lower flow rates, hydraulic distribution in the resin bed may be poor, and
at higher flow rates, kinetic effects may affect the speed of exchange, resulting in
both cases in deterioration of the treated water quality.
So in practice the running time must be selected as a function of the following
parameters:
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. AmberpackTM) and during regeneration (e.g. UpcoreTM).
With low salinity waters, e.g. when the feed water is good RO permeate, the running
time can be several days. Mixed bed polishers after a primary demineralisation will
run for several weeks before regeneration is required.
See the description of a full cycle.
Treated water quality
In ion exchange the quality of the treated water does not depend much on the feed
water analysis. Factors affecting the treated water quality are essentially related to
the regeneration process.
To a minor extend, temperature may affect the residual silica leakage in the treated
water: at temperatures higher than about 50 °C, silica is hardly removed by strongly
basic anion exchange resins (SBA).
Other than that, you can expect the treated water quality of a regeneration system
regenerated in reverse flow to be:
Conductivity: ~ 1 µS/cm
Silica: 10 to 25 µg/L
For polishing MB units, conductivity is generally around 0.1 µS/cm, and silica less than
10 µg/L. Well designed and operated mixed bed polishers can achieve a conductivity
close to that of pure water (0.055 µS/cm) and silica in the single µg/L range, or
below.
Regeneration technology
Details of the regeneration are given in a separate page. Another page shows the
corresponding column designs.
Except for very small ion exchange units (and for de-alkalisation with a WAC resin
only), plants should always be designed using reverse flow regeneration. Packed bed
columns are particularly useful, as they offer a compact and economical design, and
very good treated water quality. They are normally sized for relatively short cycles.
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3. One should however pay attention to the following points:
AmberpackTM and other floating bed columns
Those have upflow loading and downflow regeneration. The service flow rate must be
high enough to keep the bed compacted. For SAC resins, that have the highest
specific gravity, the linear flow rate must be comprised between 25 and about
70 m/h (at about 20 °C). Other resins have a lower specific gravity and are
compacted at a lower flow rate, the minimum being about 16 m/h.
UpcoreTM and similar units
With downflow loading and upflow regeneration, the regeneration flow rate must be
high enough to keep the bed compacted. This is achieved using the following tricks:
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.
Short contact times and lower regenerant concentration may however affect the
efficiency of regeneration.
Vessel sizing
For a given resin volume, it is generally cheaper to
make a tall and narrow column rather than a wide
and short unit: in the illustration, both columns
contain the same resin volume. Column B is
cheaper, because the major cost components of the
column are the dished ends and nozzle plates.
There is no limit in height, except that the pressure
drop at maximum flow rate should not exceed 100
to 150 kPa (1 to 1.5 bar) at maximum flow rate with
clean resins.
When selecting the vessel diameter, the limits of
the preceding section (regeneration technology) should also be considered.
Resin choice
You will have to refer to the resin manufacturer. However, a few general
recommendations can be made:
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. StratabedTM or
StratapackTM)
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
You can make an approximate calculation by hand even without using a computer
program or the engineering data of the resin manufacturer. The results may be only
20 % precise, but will give you an idea. In any case, it is a good exercise for
understanding the basic principles presented above.
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4. This calculation can be done for softening units and for simple demineralisation trains
comprising a single (strongly acidic) cation exchange resin column an optional
degasifier and a single (strongly basic) anion exchange resin column.
Here is the procedure for a simple demineralisation plant:
1. Examine water analysis (details above)
2. Calculate cation concentration Cc [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 Ca [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] = Cc · Q
Anion load [eq] = Ca · Q
8. Consider the approximate operating capaciy of the resins as follows:
SAC: capc = 1.0 eq/L with HCl regeneration or
SAC: capc = 0.8 eq/L with H2SO4 regeneration
SBA: capa = 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: Vc = Cc · Q / capc [L]
SBA: Va = Ca · Q / capa [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 Vc and Va are too
high, increase the running time t. If they are too low, reduce the running time
t.
This calculation is obviously only approximate, as we have taken in point 8 an
estimated operating capacity for both resins, not taking into account several
parameters that, in reality, affect this capacity: regeneration level, exact water
composition, temperature, selected endpoint, etc.
Furthermore, the additional ionic load caused by the quantity of ancillary water
required to dilute regenerants and rinse resins has not been taken into account.
Depending on the feed water salinity, this extra water can increase the ionic load by
2 to 10 %.
Besides, the calculation of WAC/SAC or WBA/SBA resin couples cannot be done by
hand, as it requires iterations for the optimisation of the "overrun".
A precise calculation can be done with a dedicated software, such as IXCalcTM for the
resins produced by Dow.
Example
Using the 10 point procedure
described above.
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. Cc = 4.8 meq/L
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5. 3. HCO3 = 2.9 meq/L — a degasifier is recommended
Residual CO2 after degasifier = 0.25 meq/L
4. Ca = 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: capc = 1.0 eq/L
Anion regeneration with NaOH: capa = 0.5 eq/L
9. Resin volumes
SAC: Vc = 3456 / 1.0 = 3456 L
SBA: Va = 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: Vc = 2304 / 1.0 = 2304 L
SBA volume: Va = 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.
© François de Dardel
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