25 TREATMENT AND CONDITIONING

OF INDUSTRIAL WATER

1. BOILER WATER

1.1. TREATMENT OF MAKE-UP WATER 1.1.1. Carbonate removal and softening facilities

Boiler make-up water must be treated in an ion exchanger to reduce hardness as much as possible. This is the absolute minimum in terms of treatment.

At very low pressure levels, simple softening techniques (ration exchangers regenerated with sodium chloride) are still sometimes used, whilst demineralization is preferred at high pressure levels. At intermediary pressure levels, carbonate removal, silica removal (where necessary) and softening are combined in a variety of different methods. The three most commonly used techniques are described below: - carbonate removal using cold lime process followed by optional silica removal using iron chloride and aluminate and a softening treatment (see Pages 146 and 149); - carbonate removal using hot lime

process (between 95 and 110C) followed by a softening treatment (this method is not widely used nowadays); - carbonate removal through a carboxylic ration exchanger (see Page 236) followed by a softening treatment and including the physical removal of CO2.

In order to facilitate the user's choice, table 96 shows the results that may be expected of each method.

Whichever process is adopted, the oxygen must subsequently be removed from the water through physical (deaeration) or chemical means. The water must then be conditioned.

Figures 898 and 899 show two possible installations.

1.1.2. Total demineralization systems

When the above processes do not allow the production of water of satisfactory quality, total demineralization must be performed.

The technique adopted will depend on the composition of the make-up water, the pressure and the type of boiler. Possible methods are listed in table 97.

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Chap. 25: Treatment and conditioning of industrial water Table 96. Results of carbonate removal and softening treatments.

P alk. M alk. TDS pH SiO2 r. deg. Fr. deg. Fr. deg. in mg.l-1

Treatment: cold lime 0.5 2 to 4 TDS - M alk. 8.5 unchanged + FeCl3 + softening to 2 + 3 to 6 to 10 Treatment above + idem idem idem 8.5 2 to 5 sodium aluminate to 10 Treatment: hot lime 1 TDS - M alk. 8.5 to + hot magnesium ox- to 2 to 2.5 + 2 to 2.5 10 1 to 2 ide + softening 1.5 6 to 7 1 to 3 TDS - M alk. with- without + 1 to 3 out Carboxylic cation ex- pH cor- changer + softening correction rection + intermediate CO2 0 removal 2 to 5 TDS - M alk 7.5 with + 2 to 5 to 8.5 pH with pH with unchanged correction correction cor- rection

TDS: salinity of the raw water (expressed in Fr. deg.) M alk.: M alk. of the raw water (expressed in Fr. deg.) If the requisite SiO2 ratio is not attained at the minimum values of M alk., the levels of silica and alkalinity in the

M alk. water of the boiler must be readjusted.

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1. Boiler water

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Chap. 25: Treatment and conditioning of industrial water

Table 97. Performance of total demineralization systems.

Resistivity in thousands of ohms.cm

Silica

in mg.l-1 1. Primary demineralization cocurrent regeneration 10 to 1000 100 to 500 2. Primary demineralization countercurrent regeneration (UFD, etc.) 500 to 2000 20 to 100 3. Primary demineralization continuous system 500 to 2000 20 to 100 4. Demineralization through reverse osmosis 10 to 300 100 to 2000 Primary demineralization (1, 2, 3) completed by a polishing cation exchanger 500 to 3000 20 to 500 Primary demineralization (1, 2, 3, 4) completed by a mixed bed or polishing treatment line 5000 to 20 000 5 to 30

These values are given for reference only. Values may differ for highly mineralized polluted raw water or water that has undergone insufficient preliminary treatment.

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1. Boiler water

Figure 900 shows an installation with a

single cation exchanger and a combination of anion exchangers of varying base properties.

The type of installation used for HP boilers could include: - preliminary treatment: clarification or removal of carbonates; - a primary system of ion exchangers comprising a strong acid cation exchanger, a strong base anion exchanger, and possibly a (carboxylic) weak acid cation exchanger and a weak base anion exchanger;

- a secondary system comprising either a strong acid cation exchanger and a strong base anion exchanger, or a mixed bed or, just a single polishing cation exchanger; - a conditioning system.

Figure 902 shows a total demineralization installation in a nuclear power station. Using carbonate-free water as a basis, the system comprises a complete cation-anion system with countercurrent regeneration and a polishing mixed bed.

The innovative treatment plant at the Mannheim power station in Germany (flow rate 210 m3.h-1) offers another example of how advanced technology can be used in water treatment (Figure 903). 1.1.3. Deaeration facilities

The oxygen contained in make-up water is removed by deaeration. This can be performed using a physical medium such as vacuum deaerators or deaerating heaters (Chapter 16) or a chemical medium such as oxygen scavengers (polishing treatment) or catalytic resins.

Carbon dioxide is always removed using a physical medium.

Deaerating heaters are by far the most commonly used medium. Operating temperatures vary between 105C and 140C and tank capacity is calculated for between 15 and 60 minutes of storage at the nominal flow rate.

Condensates containing no oxygen are sent directly to the storage tank whilst polluted condensates (O2) must be deaerated with the make-up water.

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Chap. 25: Treatment and conditioning of industrial water

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1. Boiler water

1.2. CONDITIONING BOILER WATER

The treatment and conditioning of water must satisfy three main objectives:

- continuous heat exchange; - corrosion protection; -production of high-quality steam. The water treatment facilities purify and

deaerate make-up water or feedwater.

During the subsequent conditioning process, which is an essential complement to the water treatment program, specific doses of conditioning products are added to the water by means of metering systems.

The most commonly used products include: - phosphates - dispersants, polyphosphates - dispersants.

Reacting with the alkalinity of boiler water, these products neutralize the hard

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Chap. 25: Treatment and conditioning of industrial water

ness of the water by forming tricalcium phosphate, an insoluble compound that can be dispersed and blown down on a continuous basis or periodically through the bottom of the boiler; - natural and synthetic dispersants: . natural polymers: lignosulphonates, tannins; . synthetic polymers: polyacrylates, malefic acrylate copolymer, malefic styrene copolymer, polystyrene sulphonates, etc. All of the above increase the dispersive properties of the conditioning products; - sequestering agents: such as organic phosphates, which act as inhibitors and implement a threshold effect;

- oxygen scavengers: sodium sulphite, tannins, hydrazine, hydroquinone / pyrogallol-based derivatives, hydroxylamine derivatives, ascorbic acid derivatives, etc.

These scavengers, catalyzed or not, reduce the oxides and dissolved oxygen. Most also passivate metal surfaces. The choice of product and the dose required will depend on whether a deaerating heater is used; - anti-foaming or anti-priming agents: mixtures of surface-active agents that modify the surface tension of a liquid, remove foam and prevent the carry-over, of fine water particles in the steam.

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