demineralization

DEMINERALIZATION OF WATERFOR HIGH PRESSURE BOILERS

Dilip KumarNTPC Ltd.

DEMINERALIZATION TECHNIQUES

DISTILATION

ELECTRODIALYSIS

REVERSE OSMOSIS

ION EXCHANGE

DISTILLATION

Distillation is one of the oldest methods of water treatment and is still in use today though not commonly as a home treatment method. It can effectively remove many contaminants from drinking water, including bacteria, inorganic and many organic compounds.

ELECTRODIALYSIS

Electro dialysis is a membrane process in which ions are transported through ion permeable membranes from one solution to another under the influence of an electrical potential gradient.

REVERSE OSMOSIS

Osmosis occurs when two solutions of different concentrations are separated from one another by a membrane which is permeable to solvent but impermeable to solute. Solvents flows from dilute to the concentrated solution, until, at equilibrium, the chemical potential of the solvent is equal on both sides of the membrane.

REVERSE OSMOSIS CONTI...

A pressure at which just prevent the solvent flow is called Osmotic pressure. If the pressure greater than the osmotic pressure is applied to the concentrated solution, the solvent can be forced through the membrane leaving the dissolved substances behind. This method of purifying water istermed reverse osmosis.


REVERSE OSMOSIS CONTI...A typical reverse osmosis plant consists of the following items:

Pre-treatment including acid dosing for pH control and dosing of scale control additives.

High pressure pumps which may be high speed centrifugal, multi-stage centrifugal or reciprocating type.

The reverse osmosis membranes. The membranes or permeators are usually connected in series/ parallel stages is used as the feed to the latter stages. This increases the plant conversion.

A pressure regulating valve, this is used to maintain the necessary reject flow and control the inlet membrane pressure.

The post treatment system, this is usually includes a degasser to remove carbon dioxide formed when acid is used for pH control.

A TYPICAL REVERSE OSMOSIS PLANT

RAW WATER

LOW PRESSURE PUMPS PARTICULATE

FILTERSHIGH PRESSURE

PUMPS

RO plant

Stage-1 7 modules

Stage-24 modules

Stage-3 2 modules

CONCETRATE TO WASTE

PRODUCT WATER

DEGASSING TOWER

PRODUCT WATER PUMPS

STORAGE


RAW WATER



PUMPS

RO plant

Stage-1 7 modules

Stage-24 modules

Stage-3 2 modules

CONCETRATE TO WASTE

PRODUCT WATER

DEGASSING TOWER

PRODUCT WATER PUMPS

STORAGE


RAW WATER



PUMPS

RO plant

Stage-1 7 modules

Stage-24 modules

Stage-3 2 modules

CONCETRATE TO WASTE

PRODUCT WATER

DEGASSING TOWER

PRODUCT WATER PUMPS

STORAGE


Water analyses from the reverse osmosis plant at Hartlepool power station

Analyses Pre-treated water

Product water

Reject water

Conductivity µS/cm

1560 145 6050

Total hardness mg/kg CaCO3

560 30 2700

Sodium mg/kg Na

100 15 600

Sulphate mg/kg SO4

455 15 2300

Chloride mg/kg Cl

180 23 800

DEMINERALIZATION BY ION- EXCHANGE PROCESS

Ion exchange is the reversible interchange of ions between a solid (ion exchange material) and a liquid in which there is no permanent change in the structure of the solid. Ion exchange is used in water treatment and also provides a method of separation for many processes involving other liquids. It has special utility in chemical synthesis, medical research, food processing, mining, agriculture, and a variety of other areas. The utility of ion exchange rests with the ability to use and reuse the ion exchange material.

DEMINERALISATION SREAM

ACTIVATED CARBON FILTER (ACF)Sl.No Characteristics Unit NTPC Specification

3.2.1

3.2.2

3.2.3

3.2.4

3.2.5

3.2.6

3.2.7

3.2.8

Total surface, Min

Particle density, wetted in water

Mean particle diameter

(i) In case of needle / cylindrical type

(ii) In case of granular type

Adsorption capacity in terms of iodine number, Min

Abrasion Number (by ASTM method), Min.

Ash content, Max

Mean particle length

(iii) In case of needle / cylindrical type

(iv) In case of granular type

Bulk Density, min

m2/g

g/cc

mm

mm

mg/g

% by mass

mm

mesh

Kg/m3

850

1.3 – 1.4

0.6 – 0.8

1.5 – 2.0

1000

95

7.0

2. – 2.44 – 16

400

ACTIVATED CARBON

ACTIVATED CARBON FILTER (ACF)

ACTIVATED CARBON

Acts on principle of adsorption which is a surface active phenomenon .

It removes residual turbidity (<2 NTU) of water to its 1/10 level.

It removes organic molecules to control color and odor.

It removes free residual chlorine present in filtered water(0.5 ppm Nil)

PREPARATION OF RESINS

TYPES OF RESIN

(R)

SAC: Strong Acid CationWAC: Weak Acid CationSBA: Strong Base Anion WBA: Weak Base Anion

R-SO3H

Sulphonic Acid

(SAC)

R-CH2CHCH3

|

COOH

Carboxylic Acid

(WAC)

CH3

|

R-CH2-NH+ OH

|

CH3

Tertiary Ammonium

(WBA) CH3

|

R-CH2-N-CH3 OH

|

CH3

Quarternary Ammonium

(SBA)

VESSEL DESIGN

WEAK ACID CATION (WAC)

Weak acid cation exchange resins derive their exchange activity from a carboxylic group (-COOH). When operated in the hydrogen form, WAC resins remove cations that are associated with alkalinity, producing carbonic acid as shown:

R-CH2CHCH3

|

COOH

WEAK ACID CATION (WAC) CONT….

These reactions are also reversible and permit the return of the exhausted WAC resin to the regenerated form. WAC resins are not able to remove all of the cations in most water supplies. Their primary asset is their high regeneration efficiency in comparison with SAC resins. This high efficiency reduces the amount of acid required to regenerate the resin, thereby reducing the waste acid and minimizing disposal problems.

WEAK ACID CATION (WAC) CONT….

Weak acid cation resins are used primarily for softening and dealkalization of high-hardness, high-alkalinity waters, frequently in conjunction with SAC sodium cycle polishing systems. In full demineralization systems, the use of WAC and SAC resins in combination provides the economy of the more efficient WAC resin along with the full exchange capabilities of the SAC resin.

CATION EXCHANGE MECHANISM

START OF RUN DURING THE RUN END OF RUN

CaMgNa Ca

NaMg

Ca

Mg

Na

Un-exchanged ResinNa leakage

STRONG ACID CATION (SAC)

SAC resins can neutralize strong bases and convert neutral salts into their corresponding acids.SAC resins derive their functionality from sulfonic acid groups (HSO3¯). When used in demineralization, SAC resins remove nearly all raw water cations, replacing them with hydrogen ions, as shown below:

Chemical structural formula of sulfonic strong acid cation resin (Amberlite IR-120)

(XL): cross link(PC): polymer chain(ES): exchange site(EI): exchangeable ion

STRONG ACID CATION (SAC) CONTI...

Strong acid cation exchangers function well at all pH ranges. These resins have found a wide range of applications. For example, they are used in the sodium cycle (sodium as the mobile ion) for softening and in the hydrogen cycle for decationization.


A measure of the total concentration of the strong acids in the cation effluent is the free mineral acidity (FMA). In a typical service run, the FMA content is stable most of the time. If cation exchange were 100% efficient, the FMA from the exchanger would be equal to the theoretical mineral acidity (TMA) of the water. The FMA is usually slightly lower than the TMA because a small amount of sodium leaks through the cation exchanger. The amount of sodium leakage depends on the regenerant level, the flow rate, and the proportion of sodium to the other cations in the raw water. In general, sodium leakage increases as the ratio of sodium to total cations increases.

Typical effluent profile for strong acid cation exchanger.


The exchange reaction is reversible. When its capacity is exhausted, the resin can be regenerated with an excess of mineral acid.

Thoroughfare Counter-flow Regeneration

EXHAUSTED CATION RESIN REGENERATION

Thoroughfare Counter-flow Regeneration

The regeneration efficiency of WAC is very high compared to the strong acid resin. Therefore it is possible to utilize the regenerant acid strength from the strong acid unit to regenerate the weak acid unit.

DEGASIFIER DESIGN

In water demineralization, a degasifier, or degasser, is often used to remove dissolved carbon dioxide after cation exchange. The most common degassers are of the so-called forced draft or atmospheric type.

THEORY OF DEGASIFICATION

The solubility of CO2 in pure water is high: about 1.5 g/L or more than 30 meq/L at 25°C and atmospheric pressure. When you stir the water and divide it into small droplets in an atmospheric degasifier and blow air through the "rain", the gas tends to move into the air because the partial pressure of CO2 in air is much below the equilibrium pressure. The residual CO2 after an atmospheric degasifier is 0.20 to 0.25 meq/L (typically 10 mg/L as CO2. Therefore such degassers are used when the bicarbonate concentration plus free carbon dioxide in the feed water to separate column demineralization systems is at least 0.6 to 0.8 meq/L.

DEGASIFIER DESIGN

After cation exchange, the bicarbonate and carbonate (if any) ions are converted to carbonic acid, or carbon dioxide. CO2 is soluble in water, but it tends to escape into the air, much as it does in a glass of Cold drink when you stir it. Using a degasser to remove CO2 reduces the ionic load on the strong base anion resin, and the consumption of caustic soda is thus lower.

DEGASIFIER

To be effective, the degasifier must be placed after the cation exchange column. Before cation exchange, the water is containing bicarbonate. After it, the cations in water (Ca++, Mg++ and Na+ principally) are converted to H+ ions, which combine with the HCO3

— bicarbonate anions to produce carbonic acid.

WEAK BASE ANION EXCHANGER

Weak base resin functionality originates in primary (R-NH2), secondary (R-NHR'), or tertiary (R-NR'2) amine groups. WBA resins readily re-move sulfuric, nitric, and hydrochloric acids, as represented by the following reaction:

STRONG BASE ANION EXCHANGER

SBA resins derive their functionality from quaternary ammonium functional groups. When in the hydroxide form, SBA resins remove all commonly encountered anions as shown below:

As with the cation resins, these reactions are reversible, allowing for the regeneration of the resin with a strong alkali, such as caustic soda, to return the resin to the hydroxide form.


Demineralization using strong anion resins removes silica as well as other dissolved solids. Effluent silica and conductivity are important parameters to monitor during a demineralizer service run.

Conductivity/silica profile for strong base anion exchanger


When silica breakthrough occurs at the end of a service run, the treated water silica level increases sharply. Often, the conductivity of the water decreases momentarily, then rises rapidly. This temporary drop in conductivity is easily explained. During the normal service run, most of the effluent conductivity is attributed to the small level of sodium hydroxide produced in the anion exchanger. When silica breakthrough occurs, the hydroxide is no longer available, and the sodium from the cation exchanger is converted to sodium silicate, which is much less conductive than sodium hydroxide. As anion resin exhaustion progresses, the more conductive mineral ions break through, causing a subsequent increase in conductivity.

EXHAUSTED ANION RESIN REGENERATION

Strong base anion exchangers are regenerated with a 5% sodium hydroxide solution. As with cation regeneration, the relatively high concentration of hydroxide drives the regeneration reaction. To improve the removal of silica from the resin bed, the regenerant caustic is usually heated to 120°F or to the temperature specified by the resin manufacturer. Silica removal is also enhanced by a resin bed preheat step before the introduction of warm caustic.


Thoroughfare Counter-flow RegenerationThe regeneration efficiency of WBA is very high compared to the strong base resin. Therefore it is possible to utilize the regenerant alkali strength from the strong base unit to regenerate the weak base unit.


Demineralizers with weak and strong base anion units can experience silica fouling because of the use of waste caustic from the strong base anion vessel to regenerate the weak base anion resin during thoroughfare regeneration. To avoid this, most of the impurities from the strong base anion resin are dumped to the drain before the thoroughfare begins (generally, the first third of the regenerant). To be confident that the right amount is dumped, an elution study can be performed.

RESIN STABILITY AND FACTORS

Oxidation

Exposing an ion exchange resin to a highly oxidative environment can shorten resin life by attacking the polymer crosslinks, which weakens the bead structure, or by chemically attacking the functional groups. One of the most common oxidants encountered in water treatment is free chlorine (Cl2). Hydrogen peroxide (H2O2), nitric acid (HNO3), chromic acid (H2CrO4), and HCl can also cause resin deterioration.Dissolved oxygen by itself does not usually cause any significant decline in performance, unless heavy metals and/or elevated temperatures are also present to accelerate degradation, particularly with anion exchange resins.


Oxidation When a strong base anion resin experiences chemical attack, the polymer chain usually remains intact, but the quaternary ammonium strong functional group (trimethylamine for type 1 anion resins) splits off. Alternately, the strong base functional groups are converted to weak base tertiary amine groups, and the resin becomes bifunctional, meaning it has both strong base and weak base capacity. The decline in strong base (salt splitting) capacity may not be noted until more than 25% of the capacity has been converted.


Irreversible sorption or the precipitation of a foulant within resin particles can cause deterioration of resin performance. The fouling of anion exchange resins due to the irreversible sorption of high molecular weight organic acids is a well-known problem.

Although fouling rarely occurs with cation exchange resins, difficulties due to the presence of cationic polyelectrolytes in an influent have been known to occur. Precipitation of inorganic materials, e.g. CaSO4, can sometimes cause operating difficulties with cation exchange resins.

FAULING


Silica fouling:

Silica (SiO2) exists in water as a weak acid. In the ionic form, silica can be removed by strong base anion exchange resins operated in the hydroxide cycle. Silica can exist as a single unit, (reactive silica) and as a polymer (colloidal silica). Colloidal silica exhibits virtually no charged ionic character and cannot be removed by the ionic process of ion exchange. Ion exchange resins do provide some colloidal silica reduction through the filtration mechanism, but they are not very efficient at this process.

Silica is a problem for high-pressure boilers, causing precipitation on the blades, which reduces efficiency. Both types of silica, colloidal and reactive, can cause this problem.

MIXED BED EXCHANGERS

A mixed bed exchanger has both cation and anion resin mixed together in a single vessel. As water flows through the resin bed, the ion exchange process is repeated many times, "polishing" the water to a very high purity. Due to increasing boiler operating pressures and the manufacture of products requiring contaminant-free water, there is a growing need for higher water quality than cation-anion demineralizer can produce.

MIXED BED EXCHANGER REGENERATION

During regeneration, the resin is separated into distinct cation and anion fractions as shown in Figures

1. SERVICE

2. BACKWASH

3. SIMULTANEOUS REGENERATION

4. DRAIN DOWN

5. MIXING WITH AIR

6. FINAL RINSE

MIXED BED EXCHANGER REGENERATION

The resin is separated by backwashing, with the lighter anion resin settling on top of the cation resin. Regenerant acid is introduced through the bottom distributor, and caustic is introduced through distributors above the resin bed. The regenerant streams meet at the boundary between the cation and anion resin and discharge through a collector located at the resin interface. Following regenerant introduction and displacement rinse, air and water are used to mix the resins. Then the resins are rinsed, and the unit is ready for service.

demineralization

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