DEMINERALIZATION PLANT—GUIDELINES
PREAMBLE (NOT PART OF THE STANDARD)
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END OF PREAMBLE (NOT PART OF THE STANDARD)
IS
13268:1992
Indian Standard
DEMINERALIZATION PLANT—GUIDELINES
UDC
628.165.04
©
BIS 1992
BUREAU
OF INDIAN STANDARDS
MANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARG
NEW DELHI 110002
March 1992
Price
Group 6
i
Water 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.
ii
Indian
Standard
DEMINERALIZATION
PLANT—GUIDELINES
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:
- The source of water (river water, well water, etc)
available for treatment;
- Quality of treated water;
- End-use of demineralized water;
- The availability of regenerants in the vicinity of the
proposed plant;
- Disposal of regeneration wastes; and
- 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
1
waste 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
2
is 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).
3
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).
4
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
5
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.
6
The 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:
- 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)
- Backwash pump—1.5
kg/cm2 discharge pressure at 10 1/m2 sec of filter
bed area (backwash requirement)
- Filter pump—3
to 4 kg/cm2 discharge pressure, 1.3 to 4.1 1/m2 sec
of bed area (filteration rate)
- 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.
Design specification:
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
7
given 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
8
sections 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
9
selector 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
10
limits 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
11
is 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.
