PREAMBLE (NOT PART OF THE STANDARD)

END OF PREAMBLE (NOT PART OF THE STANDARD)

Indian Standard
DEMINERALIZATION PLANT—GUIDELINES

March 1992
iWater Sectional Committee, CHD 013

FOREWORD

This Indian Standard was adopted by the Bureau of Indian Standards, after the draft finalized by the Water Sectional Committee had been approved by the Chemical Division Council.
Demineralized water is required for a wide range of industries involving production of chemicals, pharmaceuticals, fertilizers, steel, power, etc. Besides its other uses, the major use of demineralized water is as boiler feed water in boilers, ranging from low pressure to supercritical pressure. With the advent of high pressure and super-critical pressure boilers, the quality of demineralized water has acquired greater importance. This makes it essential to develop and make available the required specification for the guidance of users to procure and instal efficient and economical system for production of demineralized water.
The specification for demineralization plant varies from one place to other depending upon the source of water available, ionic load of water, treated water quality desired, regenerant availability, etc. These factors are to be taken into account for proper selection of demineralization plant, and to develop their detailed specifications.
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1 SCOPE

1.1

This standard covers (a) the basic details of demineralization plant, (b) brief guidelines for framing the specification of demineralization plant, (c) brief details of various systems currently in use for production of demineralized water, and (d) the various considerations required for making the buyers specification complete in all respects.

1.2

Attempts have been made to expose the buyers to different systems of demineralization plant, so that it may be easier for them to compare and select the best possible system suiting their particular requirements.

2 REFERENCES

The Indian Standards listed below are necessary adjuncts to this standard:

IS No.	Title
252: 1973	Caustic soda, pure and technical (second revision)
265: 1987	Hydrochloric acid (third revision)

3 FACTORS FOR DRAWING UP SPECIFICATION

3.1

The following factors are to be kept in view before drawing up the specification for demineralization plant:

1. The source of water (river water, well water, etc) available for treatment;
2. Quality of treated water;
3. End-use of demineralized water;
4. The availability of regenerants in the vicinity of the proposed plant;
5. Disposal of regeneration wastes; and
6. The availability of utilities, such as steam, instrument air, etc.

3.2 Quality of Feed Water

3.2.1

The quality of water to be treated plays an important role in drawing up the specifications. The system has to be designed to process raw water available from different sources, such as rivers, tube wells, rivulets, wells, ponds, lakes, etc.

3.2.2

The first step is to make a detailed analysis of raw water for various parameters including organics, colour, suspended solids, iron, manganese besides other dissolved solids. The analysis of raw water shall be carried out throughout the year to determine its profile variations with the change of seasons. Records of analysis of at least two years shall be made available before fixing up the treated water quality. Sufficient margin in various constituents of water may be kept in order to take care of variations in the coming years based on yearly seasonal variations in water analysis. The tube-well water composition does not vary much with the season, so in that case, it becomes easier to fix up the design parameters of water analysis; but in surface water, fluctuations are quite high, so it becomes difficult to arrive at the designed analysis. However, a proper assessment has to be made for fixing up designed water analysis.

3.2.3

The next is to properly pretreat the raw water to obtain water suitable for feeding into demineralization plant as the ion exchange resin used in demineralization plant are susceptible to various constituents commonly present in water including iron, manganese, colour, suspended solids, residual chlorine, etc. The feed water for demineralization plant shall be free from chlorine, organics, iron, manganese, suspended solids within 2 to 3 mg/l. It shall also be free from oil and grease to ensure long life of the ion exchange resins. All these considerations have to be kept in view in the design of a demineralization plant.

3.3 Regenerant Chemicals

The availability of regenerant chemicals in the neighbourhood of demineralization plant also plays a decisive role for fixing the guidelines. It is economical to make use of chemicals available in nearby areas for regeneration of various ion exchange resins. This will also lead to substantial savings in storage capacity of chemicals in the plant due to their availability at a short notice. Regenerant chemicals like hydrochloric acid (IS 265: 1987) and pure caustic soda (IS 252: 1973) used shall conform to the relevant Indian Standards.

3.4 Disposal of Regeneration Wastes

The disposal of regeneration wastes plays an important role. The toxicity, acidity and alkalinity of the waste water have to be within the specified limits. These are strictly monitored vis-a-vis pollution control and environmental protection measures. Normally, the pH of effluents of demineralization plant varies depending upon the regeneration of cation or anion. In case of cation regeneration, the waste acid comes to drain, whereas, in case of anion, the
1waste alkali is drained. On mixing of both acidic and alkaline wastes the effluent gets neutralized to a certain extent. However, it is essential to ensure full neutralization to about pH 7.5 before the disposal off as plant effluent.

3.5 Utility and Cost

3.5.1

Proper assessment of the availability of utilities is imperative before putting up the plant. Depending upon the availability, the complete scope of demineralization plant can be developed. In some cases heating of regenerant is required, for which arrangement for steam is to be made; otherwise electrical heating is to be resorted to. Besides this, compressed air may be required for operating various instruments/mechanical equipment. If the existing infrastructure does not include arrangements for supply of compressed air, the same are to be provided in plants specification.

3.5.2

The type of operation, namely, automatic, semi-automatic or manual has got a major bearing on the cost of the plant. In some cases only manual operation is preferred; whereas in other cases semi-auto or auto operation is being considered. In case semi-auto operation is desired, proper care has to be taken in developing the specification because this greatly depends upon many process sequences.

3.5.3

The process sequence adopted for the plant, requires special attention as it makes the plant operation more economical.

4 BASIC PARAMETERS

4.1

Considerable importance is to be given for determination of the basic parameters so as to get a plant suiting the requirements of the client.

4.1.1

Buyer specification consists of (a) design, (b) engineering, (c) procurement, (d) transportation, (e) storage, (f) erection, and (g) commissioning of all the work including mechanical, electrical, instrumentation besides civil work.

NOTE—Sometimes civil work is excluded from the scope, and included in the scope of main civil contractor, who executes civil work for the entire plant including demineralization plant. It becomes advantageous to adopt this, as the complete responsibility lies with a single civil contractor, following the same norms for the complete factory. It has got certain disadvantages as well because increased co-ordination is needed between demineralization plant supplier and civil contractor to complete the civil work in time so that the erection work of the demineralization plant is started as scheduled. Hence, it is preferred to have one source responsible for demineralization plant supply in all respects including civil works.

4.1.2

The specification shall clearly give the minimum, normal and maximum flow rates desired for the system. The flow and number of streams required largely depend on the requirement of demineralized water in the end use in other plants which has to be assessed prior to framing the specifications. The number of streams are also to be clearly identified as it makes a big cost impact in the plant. Sometimes, it is preferred to have 100 percent spare stream, whereas in other cases, no spare stream is desired as it is being compensated by creating a large capacity for storing treated water. The single stream demineralization plant is designed with higher capacity to get extra water for storage to take care of any extreme urgency. However, it is definitely preferable to go in for a minimum of two streams. One can go for any number of streams, but then the cost of the plant would increase with increase in number of the streams. Therefore, an optimum balance has to be struck for fixing the number of streams for a given end use.

4.1.3

The header system shall also be clearly marked in the plant specification. Sometimes, it is desirable to have a single header system, whereas in other cases, separate headers for each stream are favoured. In some cases, a mixed approach is being adopted having both single header, and separate headers for some of the process fluid streams. Single header system is having certain flexibility, as any unit of this stream can be easily connected to other unit or the other stream. So single header system is commonly preferred. Some clients do prefer individual streams but the cost implications require to be looked into seriously.

4.2 Storage Capacity

The storage capacities for the feed water tank, degassed water tank, demineralized water tank and acid and alkali tank are also to be predetermined, and clearly defined in the specification of demineralization plant.

4.2.1 Feed Water Tank

The feed water tank capacity largely depends upon the availability of feed water, chances of failure of feeding system, fluctuations in the pressure, and flow of feed water. But in most of the cases, this tank acts as a buffer tank, and is not provided with more than 2 to 3 hours capacity which is just sufficient to provide suction to the feed water pump, so as to maintain constant pressure to the demineralization plant systems.

4.2.2 Degassed Water Tank

The capacity of degassed water tank largely depends upon the frequency of the regeneration of ion exchange resins, waste water used in the regeneration and extra capacity desired for emergency in the plant. All these factors shall be kept in view while evolving the minimum capacity of degassed water tank so that it covers all the contingencies in the plant. Normally it
2is sized at half an hour pumping capacity of degassed water pump.

4.2.3 Demineralized Water Tank

The capacity of the demineralized water tank greatly varies from one client to the other. The capacity is normally fixed on the basis of exigencies occurring in the plant, the variations in use of demineralized water in down stream plants, use of demineralized water for regeneration, etc. Normally, in power plants clients prefer to have the storage capacity for 16 to 24 hours, but in other plants it is being kept for 4 to 8 hours. However, there is no strict rule for it as this entirely depends upon the client’s requirements.

4.2.4 Acid and Alkali Tank

The storage capacities of acid and alkali tanks required for regenerations are also to be clearly indicated in the specification. These capacities are dependent upon the consideration of the time it takes to procure the chemicals at plant site. Where it may take 10-15 days for obtaining the chemicals, it becomes advisable to go in for at least one month’s storage at the plant site. In normal cases, where a tanker of acid or alkali is expected within 4 to 6 days time, storage capacity of a minimum of 15 days may be desirable. Wherever the regenerant chemicals are available in the plant, a limited storage of 3 to 4 days only may be considered.

5 FEED WATER

5.1

Before developing a system and fixing specifications for demineralization plant, the source of feed water and its availability has got to be established. Sometimes, water is available in the form of filtered water after proper chlorination, coagulation, flocculation, clarification and filtration. The filtered water is being directly fed to the feed water tank followed by ion exchangers for production of demineralized water. However, it becomes essential to establish water quality, which shall be free from colour, organics, etc. In case of any colour, organics or free chlorine, the water has to be treated with active carbon to take care of minor quantities of contaminants coming in feed water. In case of unfiltered water, the active carbon filters are to be preceded by filters for which pressure sand filters are normally used; sometimes dual media filter having sand and anthracite are also used.

5.2

For dechlorination, sometimes sodium sulphite is used, which is dosed in feed water before filtration. Sometimes the system is having only one dosing pot, where solid sodium sulphite is added alongwith water to make solution. The solution thus prepared is dosed at a desired rate under pressure before filtration. In other situation, a solution preparation tank with agitator is provided. The solution is dosed to feed water by means of sulphite dosing pump.

6 DEMINERALIZATION SYSTEM

6.1

There are different kinds of systems for treatment of water in order to get demineralized water. Nowadays, as the cost of regenerant chemical is high, it is advisable to select an economical system to reduce recurring cost on chemicals. Systems in operation are described in brief to guide the buyers in selecting a viable and stable demineralization water plant.

6.1.1

Cation exchanger unit having strong acidic-cation exchange resin followed by anion exchanger unit having strong basic anion exchange resin without any degasser system in between. This system is prepared for water having less alkalinity of 100 ppm and capable of giving demineralized water suitable for low pressure and to a certain limit for medium pressure boilers. Besides, this system can come handy also for industries using demineralized water for processing. In this system, both cocurrent and countercurrent techniques can be adopted depending upon the quality of feed water, but countercurrent technique is more economical (see Fig. 1).

6.1.2

Cation exchanger unit having strong acidic-cation exchange resin followed by degasser system having degasser tower, and degassed water tank followed by strong basic anion exchanger. This system gives demineralized water for low pressure, and to a certain limit for medium pressure boilers as well. Here also, both cocurrent and countercurrent regeneration techniques can be used depending upon the quality of feed water but countercurrent technique is more economical. This system is suitable for water having moderate alkalinity of about 250 mg/l alkalinity. Here also strong basic anion exchange resin of Type 1 or Type 2 is used depending upon the requirement of silica leakage (see Fig. 2).

6.1.3

Cation exchanger unit having strong acidic-cation exchange resin, followed by anion exchanger unit having strong basic anion exchange resin, followed by mixed bed exchanger unit, having a mixture of strong acidic-cation exchange resin, and strong basic anion exchange resin Type 1. This system gives improved quality demineralized water, sometimes called polished water because of the use of mixed bed exchanger unit which is also named as polishing unit because of its basic role to polish (refine) the demineralized water. Here also, cocurrent or countercurrent regeneration techniques can be adopted for both cation and anion exchanger, but for mixed bed exchanger it is always preferable to adopt cocurrent regeneration. This system gives demineralized water of high purity, which is required for use in medium pressure, high pressure boilers and other chemical processing industries where purity of water is of major concern. The system is suitable for water having alkalinity less than 100 ppm. Here also in anion exchanger, strong basic anion exchange resin of Type 1 or Type 2 can be used depending upon silica leakage (see Fig. 3).
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FIG. 1 FLOW DIAGRAM SHOWING THE ARRANGEMENT OF A DEMINERALIZATION PLANT

FIG. 2 FLOW DIAGRAM SHOWING THE ARRANGEMENT OF A DEMINERALIZATION PLANT WITH A DEGASSER TOWER

FIG. 3 FLOW DIAGRAM SHOWING THE ARRANGEMENT OF A DEMINERALIZED PLANT WITH A MIXED BED UNIT

6.1.4

Cation exchanger unit having strong acidic-cation exchange resin followed by degasser system having degasser tower and degassed water tank followed by anion exchanger unit having strong basic anion exchange resin followed by mixed bed exchanger unit having a mixture of strong acidic-cation exchange resin and strong basic anion exchange resin of Type 1. Here also both kinds of regeneration techniques as in 5.1.3 be used depending upon the quality of demineralized water desired except for mixed bed unit. The system yields demineralized water of high purity which is useful for medium and high pressure boilers. This system is suitable for water having moderate alkalinity of 250 ppm. Use of Type 1 or Type 2 strong basic anion exchange resin in system depends greatly upon leakage of silica from the system can (see Fig. 4).
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FIG. 4 FLOW DIAGRAM SHOWING THE ARRANGEMENT OF A DEMORALIZATION PLANT WITH A DEGASSER AND A MIXED BED UNIT

6.1.5

The cation exchanger unit in the systems described above may further be split into a system consisting of weak acid cation exchanger unit having weak acidic-cation exchange resin, followed by strong acid cation exchanger unit having strong acidic-cation exchange resin. This system is more useful for water having high alkalinity of more than 300 ppm and high hardness of more than 300 ppm. The modified systems are commonly adopted to conserve the regenerant chemicals. Here, the regeneration is adopted in thoroughfare manner involving passing of regenerant from one unit to the other unit in series. Normally, the regeneration is being done from strong acid cation resin to weak acid cation resin by adopting cocurrent thoroughfare technique, that is, using both the regeneration in cocurrent manner in series or using countercurrent thoroughfare regenerations technique involving countercurrent regeneration of strong acid cation exchanger in series with cocurrent regeneration of weak acid cation resin (see Fig. 5).

6.1.6

The anion exchanger unit in the above systems can also contain two separate anion exchangers involving weak base anion exchanger followed by strong base anion exchanger. This system is also used to conserve the regenerant chemicals and to make the plant more economical by adopting either coccurent thoroughfare technique, involving regeneration of both weak base and strong base anion unit in cocurrent manner in series or countercurrent thoroughfare technique with countercurrent regeneration of strong base anion exchange resin with cocurrent regeneration of weak base anion exchange resin in series is adopted. The system is used when the water is having a high amount of chlorides and sulphates (see Fig. 6).

6.1.7

In the systems given in 6.1.1 and 6.1.2, sometimes it becomes desirable to go in for weak base anion exchanger in place of strong base anion exchanger specially in cases where silica removal is not so critical from the feed water.

6.1.8

In the system given in 6.1.1 to 6.1.6 for strong base exchanger, sometimes Type 2 strong base anion exchange resin is used in place of Type 1 strong base anion exchange

FIG. 5 FLOW DIAGRAM SHOWING THE ARRANGEMENT OF A DEMINERALIZATION PLANT WITH A WEAK ACID CATION AND A MIXED BED UNIT
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FIG. 6 FLOW DIAGRAM SHOWING THE ARRANGEMENT OF A DEMINERALIZATION PLANT WITH A WEAK BASE ANION AND A MIXED BED UNIT
resin. This system is useful when silica leakage desired in the demineralized water is slightly higher.

6.1.9

The system sometimes necessitates to make use of two mixed bed exchanger unit in series, that is, one mixed bed unit followed by an other mixed bed unit in place of only one mixed bed unit. Such systems are normally employed to get highly pure demineralized water which is suitable for high pressure or super-critical pressure boilers or in cases where highly refined water is required.

6.1.10

The cation exchanger unit in the system given in 5.1.2 may also be selected in two separate cation exchanger units, each having strong acid cation exchange resin where the regeneration is made by countercurrent technique for second cation exchanger unit and the thoroughfare manner in series with first cation exchanger unit. This process becomes more advantageous than one single exchanger, as it takes care of any extra leakage coming from the first cation exchanger and thereby gives much better treated water than in single exchanger unit. This system becomes more useful with higher dissolved solids in feed water.

6.2

Any one of the above described systems can be selected for including in the specifications by client. However, analysis of water and economics of the process play the decisive role. As is evident from above, the regeneration technique plays an important role for achieving the desired quality of demineralized water. Depending upon the mode of regeneration, performance of exchangers varies. So it becomes important to fix the mode of regeneration in the specifications itself by the client.

6.3

The minimum depth of resin used in the above exchangers shall not be less than 91.5 cm (3 feet).

7 EQUIPMENT

7.1

The equipment details constitute an important criterion to be given in the specification. Basic parameters for each and every equipment are to be given in the specification.

7.1.1 Pressure Sand Filters

The pressure sand filters shall be either of vertical or horizontal type which is to be clearly mentioned in the requirement. Normally, vertical sand filters are preferred except in cases, where higher flow is required. The flow for the filters may clearly be established so as to cover the requirement of demineralized water, waste water for regeneration of exchangers, and filtered water for backwashing the filters. Back washing operation is adopted for cleaning the filter bed, and to make the bed loose, for reducing pressure drop while running the plant. The backwashing of filters is done by either filtered water alone or by air and filtered water together or independently. The desired mode of backwashing is to be clearly specified in the specification.
The storage tank shall be located either overground at a desired height to get the sufficient pressure for backwashing or on the ground level. In the latter case, extra pumps (1.5 kg/cm²) are required for backwashing the filters. Air requirement for backwashing should always be met by the rotary air blowers. Services air of 3 to 6 kg/cm² pressure shall never be used for air scouring as it will churn up the filter media. For provision of filtered water tank, its elevations, specification of filtered water pumps and air blowers shall clearly be stipulated to get the complete system. The guarantee of the filtered water coming out of filter shall be given in the specification based on which filter is designed. In general, turbidity is specified for filter design and water outlet of filter shall contain turbidity less than 2 NTU.
6The material of construction of filter is to be given clearly. The void space above the packed bed may be mentioned which is normally kept about 50 percent of the packed height.
The standard design specification are as follows:

1. Air blown—0.4 to 0.5 kg/cm² air discharge pressure 0.015 to 0.025 m³/m²sec of filter bed area (air requirement)
2. Backwash pump—1.5 kg/cm² discharge pressure at 10 1/m² sec of filter bed area (backwash requirement)
3. Filter pump—3 to 4 kg/cm² discharge pressure, 1.3 to 4.1 1/m² sec of bed area (filteration rate)
4. Filter media—Fine sand: (30 cm):

   Grade—0.45 to 0.5 mm; Coarse sand: 25 cm;
   Grade—0.8 to 1.2 mm; Fine pebbles: 10 cm;
   Graee—3 to 6 mm; Medium pebbles: 10 cm;
   Grade—6 to 12 mm; Coarse pebbles: 20 cm;
   Grade—12 to 25 mm.
   

7.1.2 Active Carbon Filter

The active carbon filter, wherever desired, is to be installed after pressure sand filters which consist of active carbon packing capable for dechlorination, de-oiling and de-colouration along with removal of traces of iron and organics. The grade of active carbon to be used for the purpose shall also be mentioned. The mode of back washing these filters with filtered water is also to be mentioned. Here also a similar arrangement for backwashing as given above for the pressure sand filters is to be given. Normally arrangement for backwash of active carbon filter, and pressure sand filters are common, as at no stage simultaneously backwash of both active carbon filter and pressure filter is expected. Even, if it so happens, backwashing of the units can be easily staggered. The guarantee of the quality of treated water shall be incorporated in the specification. The quality parameters of the treated water shall conform to limits as follows: turbidity (< 1 NTU), chlorine (< 0.01 mg/l), and iron (< 0.01 mg/l). The material of construction of the body and lining, if any, is to be specified. Normally, epoxy lining is preferred on inside surface. The void space above the packed bed may be specified which is kept about 50 percent of packed bed.

a) Activated carbon bed depth	3 m minimum 12 m maximum 6 m normal
b) Contact time of water with activated carbon	15 minutes minimum 30 minutes normal

7.1.3 Exchanger Unit

The details of the exchanger units are to be clearly specified keeping in view the requirements of the client. These also include the required number of inspection windows, number of manholes and other mechanical requirements. The internal arrangement of the exchanger is to be left to the bidders as it depends upon the type of system adopted by them either header lateral system or strainer on bed plate system for proper collection and distribution of water uniformly through exchanger bed. The minimum, normal and maximum flow through exchanger may clearly be specified. The void space above the packed resin bed may clearly be mentioned, which is normally kept about 75 percent of the resin depth for cation and anion exchangers, whereas for mixed bed it is preferable to keep minimum 100 percent of the mixed bed resin depth for expansion. The quality of treated water guaranteed as coming out of each exchanger may be given in the specification based on which exchangers are to be designed. Normally for cation exchangers there shall be leakage of some sodium ions which depends upon the regeneration level of the exchanger. For cation exchangers, the leakage of hardness is considered nil and sodium leakage is being permitted normally in the range of 1 to 2 mg/l. The term regeneration level refers to the amount of regenerant chemical used for the regeneration of exchanger resin. For anion exchangers there is some leakage of chloride ion and silica ion, depending upon the type of anion exchange resin used in the system. With any leakage of sodium ion from cation exchanger, there is a resultant increase in leakage of anion thereby increasing conductivity and silica content of demineralized water. The conductivity and desired silica content of treated water coming out of anion exchanger shall be clearly defined in the specification for design, so that optimum regeneration level can be selected both for cation and anion exchanger units. The guaranteed water quality desired from mixed bed unit shall also be clearly defined in the specification so that the unit may be designed accordingly.

7.1.4 Degasser Tower

The deggasser tower requirement may also be clearly defined with respect to its flow rate, type of packing (stainless steel or glazed ceramic), etc. The guaranteed water quality coming out of degasser shall also be clearly
7given for which carbon dioxide shall be normally in the range of 4 to 8 mg/l as calcium carbonate. Necessary manhole, hand hole, etc, may be clearly spelled out to facilitate easy maintenance. Tower is normally placed on some height to give a gravity flow to degassed water tank placed below it, where degassed water coming out of the tower is collected and fed to down stream systems, and other uses in the plant. The degassed water tank inside is normally lined with acid and alkaline resistant tiles to prevent hardness and silica pick up from the walls by the acidic water. Normally, degasser water system is kept common for the streams, but sometimes installations may be required stream-wise which is to be clearly indicated in the specification. The number of air blowers required for the degasser tower shall be mentioned which is normally kept two for each tower. The number of degassed water pumps may also be clearly specified, so that the bidders are able to give the same type of system. Sometimes pumps are designed for 50 percent capacity only whereas in other case it is preferable to have pump capacity of 100 percent. The main consideration is the economic of the recurring cost of the plant.

8 ACID HANDLING SYSTEM

8.1

The details of acid handling system and regeneration equipment desired for the system shall be mentioned in the specification. The details of acid storage tank capacity requirement has been given in 4.2.4. Normally, sulphuric acid or hydrochloric acid is used for regeneration of the cation exchange resin. The acid supply to plant is made by road tanker. In cases, where requirement is very large, provision of rail tanker is also made in addition to road tanker. As the sulphuric acid is much more dangerous, extra precautions are to be taken for its handling. Acid tankers are sometimes preferred to be placed on height, so as to get the gravity flow from the acid tanker to acid storage tank in demineralization plant. The transfer of acid from acid tanker to acid storage tank is being done normally by pumps, but sometimes this transfer is also effected by pressurizing the tanker by air. In this case, the acid tanker shall be capable of withholding that much air pressure, as otherwise it would lead to failure of tank causing a serious accident. The material of acid transfer pumps are to be suitably selected depending upon the type of acid used. Separate regeneration equipment are required for use with sulphuric acid and hydrochloric acid, respectively. Generally, polypropylene pumps are used for hydrochloric acid series and stainless steel pumps for sulphuric acid series.

8.1.1 Regeneration Equipment

8.1.1.1

In case of sulphuric acid, the acid storage tanks shall be fully guarded to avoid contact of moist air with stored acid, for which silica gel breather shall be provided. In addition, proper seal shall also be included in overflow line to act as a vacuum breaker. Acid from storage tank is withdrawn either by gravity or by pumps and sent to acid day tank, or to acid measuring tank, depending upon the requirement. The acid measuring tanks are given separately for each exchanger (cation exchanger or mixed bed exchanger) as the requirement of each is different. Sometimes, the acid is fed directly to ion exchanger units with the help of acid dosing pumps. The online dilution of acid is done by providing a mixing tee, but extra precaution is to be taken in choosing suitable material of construction of mixing tee (normally stainless steel for sulphuric acid series), to avoid frequent failures due to the corrosive action of acid and heat of dilution acid which is required to be diluted from 98 percent to desired regenerant concentration ranging from 1.5 to 5 percent. Separate acid dosing pumps are required for cation unit and mixed bed unit. The acid from each acid measuring tank which are normally put on sufficient elevation, is taken by gravity to acid dilution tanks placed at ground level where the concentration is reduced to about 20 to 30 percent. This dilute acid at the desired concentration is taken with the help of water ejector to different exchangers for further online dilution used for regeneration. Acid concentration is very important for regeneration of cation exchanger because the presence of more hardness in water leads to precipitation of calcium sulphate during regeneration, thereby leading to imperfect regeneration.

8.1.1.2

In case of hydrochloric acid storage tank, proper precautions shall be taken to avoid hydrochloric acid vapour going out of the tank to the surroundings, for which fume absorbers shall be provided. Acid from storage tank is transferred in similar fashion as in the case of sulphuric acid mentioned in 8.1.1.1. The on-line dilution is done by water ejector (normally ebonite ejector for hydrochloric acid series) for getting desired concentration of regenerant concentration to about 3 to 5 percent. Sufficient care has to be taken to control the acid fume in the plant area by providing fume absorbers wherever necessary. Here also, separate acid measuring tanks for different exchanger units are to be provided. Information is also to be provided on the material required for construction of equipment to handle acid.

9 NEUTRALIZATION SYSTEM

9.1

Neutralization system is another important aspect particularly in the perspective of pollution control measures. All the waste waters coming out during regeneration of exchangers are required to be collected in a pit which is to be neutralized before discharge. Normally two
8sections in neutralizing pit are provided, each section being capable of holding total waste water coming out of all exchangers at a time. Sometimes, the nuetralization pit is designed to take up either 12 hours or 24 hours collections of waste water coming out during regeneration of exchangers ; but this will add to the cost of plants as the pits require a suitable lining over RCC structure to handle acid/alkali. Proper pumping and recirculating arrangement for effluent mixing are also to be provided. Sometimes, additional air grid is provided in the pit for thorough mixing of alkali/acid for complete neutralization. Proper specifications are to be developed for this system suiting client’s requirement. Lime is normally used for neutralization for which lime preparation tank and feeding arrangement by gravity shall also be included in the specifications. Otherwise, proper acid/alkali mixing is to be specified in the specifications.

9.2

The details of alkali handling system and regeneration equipment desired for the system shall be clearly mentioned in the specifications. Details of alkali storage tank are given in 4.2.4. Normally only caustic soda is used for regeneration of anion exchange resin in anion and mixed bed exchanger units, but sometimes ammonia is also used for regeneration of weak base anion exchange resin specially, in the nitrogenous fertilizer plant producing ammonia.

9.2.1

Ammonia solution (10 percent) is preferred to be stored in the storage tank, which shall be properly sealed to avoid any vapour of ammonia escaping into atmosphere. This solution is fed to exchanger with the help of pump or water ejector to get the final concentration of ammonia (about 4 percent) required for regeneration of exchanger.

9.2.2

The caustic soda solution tanks details have been given in 4.2.4, which shall be part of the specification, but the tanks shall be provided with air breather to avoid carbon dioxde intake from atmosphere which could lead to formation of sodium carbonate. Proper sealing, therefore, is also to be provided.
The alkali tank normally stores caustic lye solution (about 40-47 percent) coming by tanker (road or rail) depending upon the requirement of alkali in the plant. The alkali pumps are used for transfer of alkali solution from the tanker to storage tank, from where, it is transferred to alkali day or alkali measuring tank by gravity or by alkali transfer pumps depending upon the elevation of the tank. The alkali day tank is designed for storing alkali required for regeneration of various exchangers in a day. The alkali measuring tanks are separately provided for anion exchange resin of anion and mixed bed exchanger units. The transfer of alkali from measuring tank to anion exchanger can be done either by alkali dosing pumps with on-line dilution by alkali ejector to achieve desired concentration of alkali solution for regeneration which normally ranges from 2 to 5 percent. But for regeneration of alkali to mixed bed unit the alkali ejector is used to get the desired alkali solution concentration in the range of 4 to 5 percent. Use of alkali dosing pumps is also preferred specially in case where pressure drop expected is high, such as in thoroughfare regeneration system.
Sometimes, it becomes difficult to get lye solution in the vicinity of the plant, then alternate arrangement of preparing alkali solution is to be made at the site by getting solid alkali in the form of flakes or solid. For this purpose, a separate alkali solution preparation tank has to be provided equipped with proper stirring arrangement. In addition, the alkali transfer pumps are required for transferring alkali solution prepared in the tank, which shall also be used for recirculation of alkali solution in the tank for proper mixing of solid to prepare the solution. At least one caustic preparation solution tank shall be included in the specification to take care of any extreme emergency, in case lye solution is not made available due to some reasons beyond control.

10 GENERAL AND CONSTRUCTIONAL FEATURES

It is essential to describe general and constructional features of various equipment in the specification, including the mode of their operation, location of the plant, etc.

10.1 General Features

These cover (a) mode of operation (b) location of the plant, (c) type of instrumentation desired, and (d) electrical system requirement, etc.

10.1.1 Mode of Operation

10.1.1.1

For small plants, manual operation is preferred, as the operation of the small size valves does not pose any problem. Further, with the instruction, the total cost of the plant goes up, which discourages recourse to sophistication in small plants. Nowadays, due to operational difficulties and to minimize the recurring cost, the labour cost is to be reduced, which encourages one to go in for semi-auto and auto operation of the plant. The mode of operation, therefore, has to be clearly specified.

10.1.1.2

Semi-auto operation includes the operation of various valves through selector switches located in the control panel so that the operators can operate the plant from the control panel. Sometimes semi-auto operation includes stopping of the plant during service run by the
9selector switch, and thereafter the regeneration is to be carried out by means of sequence timer or programme logic controller. This system requires a lot of precision, maintenance, workmanship, reliability, and smoothness in operation of various instruments and valves. Although, such kind of system is becoming popular, one has to consider before hand the factors mentioned earlier. Complete auto-operation is not at all desirable in India because of the large variation in night and day temperature. However, still, some client prefer to go in for automatic plants. Naturally, success will depend upon the regular maintenance of various instruments and auto valves in operation.

10.1.2 Location of the Plant

This becomes an important factor for the total cost of the plant. In power sector it is normally preferable to go in for completely covered plant but in other chemical industries, including fertilizers, trends have set in to go in for open plants. The open plant is more economical compared to a covered plant, but there are some operational hazards which may have to be faced by the operators during monsoon, winters, and summers. In case, the plant is made semi-auto type it really becomes advantageous to go in for open plant, as frequent visit of operators to field is avoided. However, ultimately the choice between open and covered plant remains with the client. But the type (open or covered) must be included in the specification.

10.1.3 Instrumentation

Nowadays, more and more on-line instruments are included in the plants. This gives instantaneous analysis of water at various stages of the plant. More and more instruments are there in semi-auto and auto plants to control the regeneration and service run.

10.1.3.1

In filters, most of the time manual operation is preferred but in some cases auto operation is selected, in case of auto operation, any high pressure drop across the bed leads to automatic backwashing followed by rinsing of the filter before putting for service run. For detection of pressure drop across the bed, differential pressure indicator alarm is used, which is connected with operation of service and backwash valves of the filter. Further, flow indicator integrator on individual filters are required besides recorder. Sometimes, in manual plant, use of only water meter in the feed line is preferred. In case of auto or semi-auto operation, the valves included for operation are pneumatically operated gate valves and only in some cases, where instrument air is not available, motorized valves are used. Inlet and outlet of the individual filters have pressure gauges which depend upon client’s requirement. However, outlet pressure gauge is definitely required to assess the pressure drop across the bed, but inlet pressure gauge can be avoided.

10.1.3.2

The exchangers are required to have more instrumentation particularly in auto and semi-auto plants. It is desirable to have flow indicator in the inlet. Sometimes, only one flow indicator is provided in the inlet with a water meter in the outlet to assess the total quantity of water coming out during service run or in between two regenerations. Sometimes, differential pressure indicator is also provided to assess the pressure drop across the bed, otherwise only pressure gauges are provided in inlet and outlet pipes. The cation exchange unit is required to have sodium in indicating meter to indicate the leakage of cations, for which one probe is put in the outlet of the exchanger. The anion exchanger unit is provided with conductivity indicator to assess the conductivity, and even sometimes with silica analyser to estimate the silica content in the treated water. In some cases, even on-line, pH meter is provided in the outlet of the unit, but normally pH is measured in the laboratory only. The mixed bed exchangers have conductivity meter, silica analyser, pH meter to give the indications of quality of treated water. Sometimes, only a conductivity indicator is provided and rest of the measurements are carried in the laboratory. Besides above, pneumatic control valves are provided for each operation on the exchangers, sometimes motor operated valves are selected in place of pneumatic type specially at the place where instrument air is not available for use. In case, auto or semi-auto operation of the plant is not envisaged, most of the instruments on exchangers can be discarded, except flow indicator, and integrator which are needed in all circumstances.

10.1.3.3

On regeneration side, handling of acid, alkali tanks and their feeding systems are involved. Here also, a good instrumentation is required for auto and semi-auto plants. The instrumentation depends upon the type of operation for feeding regenerant to exchangers. The tanks shall have level switches, so that proper levels in various tanks can be maintained. The system shall be provided with sufficient pneumatic or motorized control valves, besides the auto operation of pumps or blowers wherever necessary. Various stages of operation are better controlled by the timers which have to be set in advance during operation of the plant. Nowadays, use of programme logic control is also adopted for these systems, where the programme of operation is set in advance. The basic requirements of the instruments in the system shall be given in the specification, based on which the system shall be designed for operation.

10.1.3.4

Some instruments are required for inlet and outlet pipelines to and from the battery
10limits of the plant. Normally, on water incoming lines instruments contain pressure indicators, recorders, pressure switch, etc. Sometimes use of water meter is made only for recording total water flow to the plant, specially in case of manual plants. For treated water line, many online instruments are needed, such as pressure indicator, flow indicator/recorder, conductivity indicator, pH indicator/recorder, silica analyser recorder, sodium analyser, recorder, chloride analyser recorder, etc, depending upon the requirement. In normal plants, even the outgoing line has minimum flow indicator and pressure indicator. For the neutral effluent line, it becomes essential to provide at least a pH indicator to assess the pH of the effluent of the plant, which has to be neutral before disposal. However, specific instruments on various lines cannot be listed their installation varies from plant to plant. All the same, requirements of these instruments are to be clearly mentioned in the specification.

10.1.3.5 Instrument control panel

The details of instrument control panel may clearly be given in the specification so that all necessary instruments are provided on the panel. The requirement of enunciations for alarms may clearly be mentioned in the specification. The control panel varies according to the system adopted. But this leads to requirement of airconditioned room for satisfactory performance of instruments incorporating integrated circuits, relays and solid state system provided in control circuits.

10.1.4 Electrical System

Requirements of electrical system are to be broadly converted in the specification so that the plant can accordingly be designed. Sometimes, only high voltage power is available which calls for provision of a step down transformer in the system which shall be mentioned in the specification. The specification of control centres, switches, and location of push buttons may clearly be mentioned in detail so that these items are provided in the plant accordingly.

10.2 Constructional Features

The important constructional features of the various tanks, exchangers may be mentioned in the specification, so that the system is designed accordingly. The aspects covered include the material of construction, and general features of the tanks, exchangers, etc.

10.2.1 Filters

As the filters contain feed water and no acid/alkali is coming in contact, only mild steel vessels are required. Sometimes epoxy or bitumen lining is desired so that iron pick up from the vessel is minimized. The filters have proper arrangements for collection and distribution of water so that no channelling occurs inside the packed bed. The basic details may only be mentioned to enable the designer of the plant to meet the proper performance and guaranteed requirements.

10.2.2 Exchanger Vessels

All the exchangers require some kind of protective inner lining over mild steel to protect it from corrosive liquids. Normally, rubber lining or ebonite lining is suggested, but now, use of FRP or polymethane is also recommended. Proper distribution of water and regenerants are to be provided inside the vessel so that, it is uniformly discributed all over the packed bed. In smaller diameter vessels, only one distributor is enough for putting both water and regenerant but in other cases it is preferable to have separate water and regenerant distributors. For collection of the treated water, good system is required for which lateral header system or bed support system is provided. As the collector system works as distribution system during back-washing of the vessels, it requires special design. In case of counter-current regeneration, the regenerant flows from bottom, so the collection system which requires a good arrangement for uniform distribution throughout the bed. In this case, a middle collector is also provided for discharging the water coming during the regeneration in mixed bed exchangers also, similar kind of arrangement is made. However, the details of constructional features are to be left to the system designer to be worked out on the basic requirement and treated water guarantee at different stages.

10.2.2.1 Back-wash outlet strainers

Strainers of stainless steel or PVC construction are installed in the back-wash outlet of exchanger to prevent resin loss during back-wash operation.

10.2.2.2 Resin traps

Resin traps are installed at the treated water outlet of each exchanger to cater the resin leaks through leaky strainers or loosened strainers in the collecting system during service runs.

10.2.3 Water Storage Tanks

RCC tanks are used with bitumen lining for feed water storage. In case of degassed water tank, acidic water from degasser tower is stored, so proper lining is desired from inside. Normally, rubber lining is suggested for this purpose but nowadays, FRP lining is also suggested. In case of demineralized water tank or polished water tank, lining becomes essential from inside to avoid corrosion and iron pick up in the treated water. In some cases, only epoxy lining
11is suggested from inside, but in other cases rubber lining is also recommended. The neutralization pit for storing waste water during regeneration is normally made of RCC with suitable lining from inside. Epoxy, acid-alkali resistant titles and bricks suitably joined are also used. The details of such lining are to be clearly marked in the specification.

10.2.4 Regenerant Tanks

For concentrated sulphuric acid only mild steel tanks are to be used, whereas dilute sulphuric acid shall require stainless steel tanks or some special lined tanks particularly where it is getting diluted as heat evolved in those tanks is quite high; suitable corrosive resistance lining material is used for sustaining the temperature rise. Teflon lining for such tanks shall be ideal, but due to its limited availability, special rubber lining is suggested, which is to be properly maintained during operation. The temperature of the solution is never allowed to go beyond 70—80°C to protect the lining.
In case of hydrochloric acid FRP lining or rubber lining for inside surface is quite useful. In case of caustic soda or sodium carbonate sometimes no lining for inside surface is provided, but it is advisable to give some lining to avoid ingress of extra iron from the vessel. These linings may be of epoxy, rubber or FRP, which is to be clearly spelt out in the specification.

10.2.5 Others

For pressure vessels, dished ends are to be provided; for some atmospheric tanks also dished end bottoms are required. For horizontal cylindrical tank like degassed water tank, large size acid/alkali tanks, etc, also dished ends are required. Proper breather seals, etc, are required in various tanks to check the harmful vapours escaping out, and also to check the ingress of moisture. Suitable on-line traps are also desired to check the loss of various packings in case of any damage in collection system during operation.
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