YOUR PROBLEMS & OUR SOLUTIONS Index
Demineralization plant
is employed for removal of minerals or dissolved salts from the water.
Salts on dissolving dissociate into electrically charged particles
called ions: for example common salt will be split into sodium ion (a
positively charged ion or cation) and chloride (a negatively charged
ion or an anion). If such a solution is brought into contact with a
suitable ion exchange material (called resin), some ions from the
solution are taken up by the resin and an equivalent number are transferred
from the resin to the solution. Ion exchange is thus a reversible
interchange of ions between a liquid and a solid.
A simple Demineralization
Plant consists of two beds of chemically treated resin beads
operating in series. The first column- cation exchanger- converts
the dissolved solids in the raw water to the equivalent acids; these acids are
removed as the water passes through the second column- anion exchanger. The
final product from this process consists essentially of pure
water. When exhausted, the cation exchange resin is regenerated with acid and
the anion exchange resin with alkali.
In essence the DM plant comprises
of resin vessels with charge of strong cation and anion resin;
control-panel encompassing a conductivity measurement and alarms, etc; acid
and caustic injection facility from bulk, semi-bulk or carboy
containers.
The high-purity water from a
demineralization
plant is typically used as feed
water for high pressure boilers in many industries; as wash water in
computer chip manufacture and other micro-electronics manufacturing processes,
as pharmaceutical process water, and any process where high-purity water
is a requirement. DM water is used as process water in
the manufacture of chemicals and fertilizers, food products such as soft
drinks, automobiles for rinsing of parts,
textiles, etc.
Two-bed INDION DM plants are made in all sizes, from
small portable units for laboratories to large multi-stream installations for
Thermal power stations, refineries, petrochemical and steel plants.
The
type of resins employed
and selected depends on numerous factors: Treated water quality
required- If silica removal is not required, anion exchange resin
used in two- bed DM plants is usually INDION 850 weak base
anion resin. If silica level of 1.0 ppm can be tolerated, then INDION
N-IP strong base Type -2 resin is offered. When water free from silica is
required, the anion exchanger is charged with INDION FF-IP strong base
Type -1 anion resin.
Input water
quality
Presence of organic foulants- In cases where water has high level
of organic foulants such as humic and fulvic acids occurring in natural
surface waters, Macroporous resins such as INDION 810- Type 1strong base
resin are better suited for the application than INDION FF-IP
Flow through plant required
Considerations of minimization of operating costs in terms of regenerant chemical consumption: In order to reduce regenerant chemical consumption in large plants, INDION 850 resin (which is very efficient for removal of strong acids such as HCl and H2SO4 with minimal requirement of alkali for regeneration) is used in combination with INDION FF-IP strong base resin which is best suited for removal of weak acids such as carbon dioxide and silica from water
The regeneration is usually carried out in three steps. Firstly, the ion exchange column is backwashed with an upflow of water. The pressure vessel has about 50% free space above the resin bed (known as free board). This free space allows removal of any entrained solids, and re-classification of the resin bed by backwashing. Backwashing also relieves bed compaction. Secondly, a predetermined amount of acid or alkali is injected into the column in a downward direction (same direction as the service flow or co-current) to displace sodium/calcium/magnesium in the cation exchanger and chlorides/sulphates/alkalinity in the anion exchanger taken up during the service cycle. Lastly, the column is rinsed to remove excess regenerant. The entire operation takes about 3 hours for a two-bed DM plant.
With counter-flow regeneration, the regenerant acid or caustic passes in the direction opposite to the flow of water during the service cycle. With counter-flow regeneration, the fresh regenerant enters at the bottom of the resin bed and passes in an upward direction (opposite to the downflow direction during service cycle- or counter-current). Hence, bottom layer of the resin bed is always in highly regenerated condition. This means lower leakage or slip of ions during the service cycle producing better quality of treated water than the co-current method.
The mixed bed is a single
column of INDION 225 cation exchanger and INDION FF-IP anion
exchanger mixed together. Water passing through the column comes into
contact with these materials and is subjected to almost infinite number of
demineralizing stages. Thus demineralized water of extreme purity is
produced.
As with two-bed demineralizers, mixed bed units are regenerated
with acid and alkali: but the ion exchange resins must be separated before
this can be done. Bed separation is accomplished by backwashing: this carries
the lighter INDION FF-IP resin to the top of the bed and the heavier INDION
225 sinks to the bottom. Two completely separated layers are thus formed, into
which the acid and alkali solutions and rinse water are introduced through
specially designed distributors. After regeneration, the two resins are mixed
with compressed air.
Normally mixed bed unit treats water from the two-bed
DM plant that is already of high purity and their ionic load is low. They can
consequently be operated at high flow rates, and are of relatively smaller
size.
Electrical conductivity is
used to express the purity of demineralized water. Depending on the
application pH and/or reactive silica in DM water may also
be specified as parameters to measure the purity of DM water.
The
quality of the water depends on the type of scheme used:
Cation-Anion-Polishing
Mixed Bed
For standard plants our guarantees are as follows:
1)
Conductivity 0.1 micromhos /cm-1.0 micromhos/cm. at 25°C (We guarantee
conductivity of 0.1 micromhos/cm in very large projects only)
2)
Sodium 0.01 mg/l - pH: 7 +/- 0.2
3) Reactive silica 0.02
mg/l -0.05 ppm
Cation-Anion
(Counter-Current Regeneration)
For standard plants our guarantees are
as follows:
1) Conductivity 0.5 to 1.0 µS/cm at 25°C- 30 micromhos/cm
(We guarantee conductivity less than 10 micromhos/cm in large projects
only)
2) Sodium 0.05 to 0.1 mg/l - pH: 7.5 - 9.0
3) Reactive
silica 0.025 mg/l - less than 0.5 ppm (with FF-IP we can
guarantee less than say 0.3 ppm)
Cation-Anion (Co-Current
Regeneration)
With typical co-current regeneration, the outlet quality
will depend on the regenerant applied, resin employed and raw water
quality
1) Conductivity 5 to 30 µS/cm at 25°C- conductivity can
be upto 2 to 5 % of conductivity of raw water
2) Sodium 0.5 to 3
mg/l
3) Silica 0.1 to 0.3 mg/l - less than 1.0 ppm
For the sizing of a demineralization plant, a good in-depth water analysis is normally required which gives the breakdown of total anions and total cations and any potential organic foulants. The final water quality specification, as well as flow rate and water used per day is required.
The alkalinity or bicarbonates and carbonates present in raw water appear as carbonic acid or dissolved carbon dioxide at the outlet of cation exchanger. Weak base anion resin such as INDION 850 does not remove weak acids such as carbon dioxide or silica. The demineralized water is therefore passed through a degassing tower for removal of carbon dioxide or CO2. The tower, made of rubber-lined steel is filled with packing rings through which the demineralized water percolates. Low pressure air introduced at the bottom of the tower scrubs out CO2, and the degassed water collects in a sump beneath the tower.
All INDION DM plants are provided with conductivity indicators that have two basic elements: a conductivity cell with electrodes of special design between which demineralized water flows and a sensitive milliammeter for measuring the current passing between the electrodes. This current is proportional to conductivity of the water.
| Defects | Causes | Remedies |
| Decrease in capacity between two successive regenerations |
a. Increase in ionic load b. Flow recorder defective c. Insufficient chemicals used d. Resin dirty e. Plant being used intermittently f. Channelling in bed g. Resin fouled h. Resin deteriorated i. Resin quantity insufficient in unit
|
Check by analysis Check Check Give prolonged backwash Avoid this Check and ensure uniform distribution /collection If cation, give HCl wash; if anion, resin give alkaline brine treatment Check and replace charge Check and top up
|
| Treated quality not upto the standard |
a. Cation exhausted b. Anion exhausted c. Mixed bed exhausted d. MB resin not in uniform mixed state
f. Na slip from cation high
i. Unit not sufficiently rinsed j. Excessive/low flow rate
m. Resin deteriorated |
Check Check Check Repeat air mix and rinse Check Check raw water analysis; change in Na/TA and SiO2/TA ratio; use more chemicals Check raw water analysis; change in Na/TA and SiO2/TA ratio; use more chemicals Check Rinse to satisfactory quality Adjust to between unit min/max flow rate Check and ensure uniform collection/distribution Check resin and give alkaline brine/ HCL treatment Check resin and replace |
| Mixed bed quality not good |
a. Resin not separated during backwash properly
c. Final rinse not proper d. Some valves may be leaking and contaminating the treated water
|
Give extended backwash after exhausting the bed Repeat Repeat Check and examine
|
| High residual CO2 from degasser |
a. It can be due to choked suction filter of degasser air blower b. Improper air flow to the degasser
d. Air seal not fitted/broken resulting in short circulating of air
|
Check and clean filter
Check and operate blower Check and replace fitting
|
| Unit rinse takes long time |
a. Flow rate too high b. Unit exhausted c. Backwash valve passing d. Anion resin organically fouled e. MB air mix not satisfactory f. Acid/alkali pockets formed in unit
|
Increase flow rate Regenerate unit Check and rectify Give alkaline brine treatment Carry out air mix once again Faulty design check and rectify. Temporarily backwash (followed by air scour if MB) and rinse again
|
| Flow rate too less |
a. Choked valve and suction strainer of pump b. Cavitation in the pump c. Low inlet pressure d. Distribution or collecting system choked e. Resin trap at outlet choked f. Control valve shut due to low off-take
|
Check Check Check-pump Check Check and clean Increase off-take
|
| Pressure drop across the bed increasing day by day |
a. Defective valves b. Packed resin bed and resin fines present
c. Collecting system choked d. Pressure gauge defective
|
Check Give extended backwash with open manhole and scrap off fines from top surface of the resin Check, repeat backwash Check and rectify/ replace
|
| Flooding in degasser |
a. Very high air flow rate
|
Reduce air flow rate by adjusting damper Open and check
|
| Resin being lost |
a. Excessive backwash pressure
c. Inlet strainer damaged
|
Check inlet pressure and reduce if necessary Examine same for breakage Check and replace
|
| Ejector not working |
a. Low power water pressure b. Air lock in the unit c. Choked or defective valves d. Ejector nozzle may be choked e. Too much back pressure from the unit
|
Check Backwash & open air release Examine and rectify Check Check for chokage of collecting system; passage of inlet/outlet valves Check and rectify
|
| Incorrect reading rota-meters |
a. Chocked orifice lines/orifice b. Dirty glass and float
|
Check and clean check and clean |
| Improper reading from flow recorder integrator |
a. Choked impulse lines/orifice b. DP transmitter requires recalibration c. Leakage in signal tube between transmitter and panel d. Low air pressure for DP transmitter or recorder
|
Check and clean Recalibrate Check Check
|
| Level electrodes system for measuring and dilution tank not functioning properly |
a.
Improper contact between electrodes and control b.
Shorting of the two electrodes due to moisture or c. Improper working of the level controllers
|
Check contact and rectify Check and dry the contacts of moisture and dirt Check
|
| Leakage from acid injection/unloading/transfer pumps |
a. Improper adjustment of the mechanical seal b. Low strength of sulphuric and presence of ferrous sulphate
|
Check and adjust Check concentration and take appropriate action
|
| Corrosion in concentrated acid tanks and lines |
a. Low concentration of sulphuric acid
b. Lining of HCl tank/pipe line damaged
|
Check silica gel breather in acid storage tank and replace silica gel charge if exhausted Rectify
|
| Improper opening and closing of pneumatically operated valves |
a. Defective solenoid valves b. Leakage in airline from solenoid valve to the respective control valve. c. Improper contact of micro switch giving false indication to panel d. Fused mimic lamp giving false indication to the panel
|
Check Check Check Check
|
| Improper operation of a certain regeneration cycle | a. Defective relays in the control circuit | Check and replace relays |
| Solid state programme not functioning properly |
a. The controller can be kept in "hold" due to the reasons explained under operation b. Improper operation of the controls for the controller c. Defect in the inside of the controller
|
Remove conditions which cause "hold" of controller Press test switch & check the complete cycle Check the instruments thoroughly from inside. Meanwhile, operation may be continued by using bypass toggle switches
|