Efficiency of Antiscalants in Industrial
Cooling Water Systems
Ashok Kumar Popuri
Department of Chemical Engineering,
VFSTR, Vadlamudi, Dist.: Guntur (A.P.), India
*Corresponding Author E-mail:
akpopuri@gmail.com
ABSTRACT:
In many industrial operations formation of deposits on
heat exchanger surfaces and other cooling systems is a persistent problem.
These deposits contain mineral scales (CaCO3, CaSO4, Ca3
(PO4)2, CaF2 etc.), corrosion products
(Fe2O3, Fe3O4, Cuo etc.),
particulate matter (clay, silt etc.) and microbiological mass. Deposition of
these materials on heat exchange surfaces lead to loss of system efficiency,
overheating, unscheduled shutdown and ultimately failure of heat exchangers. In
cooling and boiler water systems, these deposits normally accumulate in low
circulation areas and may become immobilized during upset conditions resulting
in buildup of deposits on heat exchangers. An effective cooling water treatment
must control scale, particulate matter and corrosion. Over the years, a variety
of antiscaling agents have evolved including acid/chromate, zinc/chromate,
stabilized phosphonate, phosphate/zinc/polymer and all organic. Phosphonates
are excellent calcium carbonate inhibitors or antiscalants which reduce the
scale formation in cooling water systems. Determination of efficiency of these
antiscalants is also important before using them. Techniques (Boffardi, ONGC)
presently used for determining their efficiency do not provide consistent
results, these methods are time consuming and also difficult to execute.
Industries are facing problem in finding the optimum dosage ofantiscalant in
cooling water systemswith a peculiar composition. So, a technique based on the
theoretical considerations has been given herein, which produces consistent
results in an easy way and requires less time to perform using cooling system
water.
KEYWORDS: Antiscalant, Cooling System, TSOP, SHMP, STP,
Turbidity.
1.
INTRODUCTION:
The function of cooling systems is to remove heat from
a process or equipment. Efficient removal of heat is an economic requirement in
the design and operation of a cooling system1,2. The driving force
of heat transfer is the difference in temperature between the two media3,4.
In cooling water systems, transfer of heat from a process fluid or equipment
results in rise in temperature or even a change of state of the cooling water5.
Many of the properties of water as well as behaviour of its contaminants are
affected by temperature4. The tendency of a system to corrode, scale
or support microbiological growth is also affected by water temperature6,7.
As the level of water dissolved solids increases,
corrosion and deposition tendency will also increase. Because corrosion is an
electrochemical reaction and higher conductivity due to higher dissolved solids
increases the rate of corrosion8. Some salts have inverse
temperature solubility, i.e. they are less soluble at higher temperatures and
thus tend to form deposits on heat exchanger tubes9,10. Many salts
are less soluble at higher ranges of pH. As cooling tower water is concentrated
and pH increases, the tendency to precipitate salts and formation of scale
increases. Because it is one of the least soluble salts, calcium carbonate is a
common scale that forms on open recirculating cooling systems11,12. Calcium
and magnesium silicate, calcium sulfate and other types of scale can also
occur. In the absence of treatment there is a wide range in the relative
solubility of calcium carbonate and gypsum are found in cooling systems13.
The Langelier Saturation Index (LSI) and Ryznar Stability Index (RSI) can
predict calcium carbonate scaling qualitatively14,15.
Deposit accumulations in cooling water systems reduce
the efficiency of heat transfer and carrying capacity of water distribution
systems. In addition, the deposits cause oxygen differential cells to form16,17.
These cells accelerate corrosion and lead to process equipment failure. Deposit
formation is strongly influenced by system parameters, such as water
temperature, skin temperature, water velocity, residence time and system
metallurgy. The most severe deposition is encountered in process equipment
operating with high surface temperature and/or low water velocities18,19.
Fouling occurs when insoluble particles suspended in recirculating water and
forms deposit on surface. Fouling mechanisms are dominated by particle-particle
interactions that lead to formation of agglomerates20. Scale
formation occurs when the solubility of any low soluble salt is exceeded. The
scaling mechanism at the surface is due to concentration gradient. The factors
like temperature, pH and solubility affect scaling in cooling water systems21,22.
The most direct method of inhibiting formation of
scale deposits is operation at sub saturation conditions, where scale forming
salts are soluble23,24. For some salts, it is sufficient to operate
at low cycles of concentration and/or control pH25. However in most
cases, high blow down rates and low pH are required so that solubilities are
not exceeded at the heat transfer surface26. In addition, it is
necessary to maintain precise control of pH and concentration cycles27.
Water softening, lime softening of the makeup or side stream can be used to
lower the calcium and often alkalinity28. This reduces both the calcium
carbonate and calcium sulfate scaling tendencies of the water at a given number
of cycles and pH level. Side stream lime softening is also used to lower the
silica levels.
Cooling systems can be operated at higher cycles of
concentration and/or higher pH when appropriate scale inhibitors are applied.
These materials interfere with crystal growth, permitting operation at
supersaturated condition. Phosphonates or various polymeric materials can be
used to inhibit other types of scale, such as calcium sulfate and phosphate. In
this work scaling inhibitors are used to control the scale formation29.
2. MATERIALS AND METHODS:
2.1 Antiscalants:
Antiscalants, also called
inhibitors are the surface active materials or chelating agents which bind to
metal ions and prevents formation of scales. The following three antiscalants
are used in the experiments.
·
Sodium
hexametaphosphate (SHMP)
·
Tri sodium
orthophosphate (TSOP)
·
Sodium tripolyphosphate
(STP)
2.2 Experimental procedure:
Water properties like electrical conductivity, pH,
total hardness, calcium hardness, magnesium hardness, P-alkalinity,
M-alkalinity, chlorides, total dissolved solids and Langelier Saturation Index
are determined using the standard procedures. Efficiency of antiscalantsis
determined using Boffardi technique, ONGC technique and turbidity technique.
The antiscalants are made to circulate through the shell-and-tube heat
exchanger to test its efficiency in removing scales. Efficiency is estimated by
calculating the overall heat transfer coefficient.
3. RESULT AND DISCUSSION:
CW I (General tap water), CW II
(5 gm of calcium salt added to
1 lt of tap water), CW III (10 gm of calcium salt added to 1 lt of tap water)
and CW IV (15 gm of calcium salt added to 1 lt of tap water) are analyzed and
reported in table 1.
Table 1: Analysis of different water samples.
|
Parameter
|
CW I
|
CW II
|
CW III
|
CW IV
|
|
Electrical
conductivity, S/m
|
2.34
|
1.305
|
2.26
|
3.00
|
|
pH
|
8.15
|
8.08
|
8.19
|
8.34
|
|
Total
hardness, ppm
|
337.64
|
748
|
788
|
1334.3
|
|
Calcium
hardness as CaCO3 (ppm)
|
181.96
|
412.45
|
485.23
|
719.76
|
|
Magnesium
hardness as MgCO3 (ppm)
|
155.68
|
335.55
|
302.76
|
614
|
|
P-Alkalinity
(ppm)
|
150.135
|
130.11
|
100.09
|
50.04
|
|
M-Alkalinity
(ppm)
|
470.42
|
500.40
|
600.54
|
700.63
|
|
Total
alkalinity (ppm)
|
620.55
|
630.51
|
700.63
|
750.67
|
|
Total
dissolved solids (ppm)
|
490
|
4270
|
5450
|
6195
|
|
Langelier
saturation index
|
1.193
|
1.151
|
1.483
|
2.095
|
3.1 Efficiency of
antiscalant using Boffardi technique:
The efficiencies of
antiscalants (TSOP, SHMP and STP) are determined (Figure 1) using Boffardi
technique at different concentrations of antisclants using the volume of EDTA
rundown in titrations.
V1=Volume of EDTA rundown without
antiscalant
V1=Volume of EDTA rundown with antiscalant
Fig. 1: Efficiency of antiscalants using Boffardi
technique
3.2 Efficiency of
antiscalant using ONGC technique:
The efficiencies of
antiscalants (TSOP, SHMP and STP) are determined (Table 2) using ONGC technique
at different concentrations of antisclants using the volume of EDTA rundown in
titrations.
Table 2: Efficiency of
antiscalants using ONGC technique.
|
Antiscalant
|
EDTA
without AS,ml
|
EDTA
with AS,ml
|
Efficiency
of AS, η, %
|
|
TSOP
|
70.7
|
25
|
64.63
|
|
SHMP
|
70.7
|
35
|
50.49
|
|
STP
|
70.7
|
43
|
39.17
|
3.3 Efficiency of
antiscalant using turbidity technique:
The efficiencies of antiscalants
(TSOP, SHMP and STP) are determined using turbidity technique at different
concentrations of antisclants using the volume of Na2CO3
rundown in titrations. The efficiencies of antiscalants of different water
samples are shown in figures 2-5.
Fig. 2: Efficiency of
antiscalant on CW I
Fig. 3: Efficiency of
antiscalant on CW II
Fig. 4: Efficiency of
antiscalant on CW III
Fig. 5: Efficiency of
antiscalant on CW IV
3.4 Experiments with
antiscalants in shell-and-tube heat exchanger:
The antiscalants are circulated
through a shell-and-tube heat exchanger and calculated the efficiency in
removing scales. This is known by calculating the overall heat transfer
coefficient, U. The calculated overall heat transfer coefficients with TSOP,
STP and SHMP at different concentrations of antiscalants and at different time
periods are reported in tables 3-11. The overall heat transfer coefficients at
different times using TSOP, STP and SHMP are plotted in figures 6-8.
Table 3: Temperatures and
overall heat transfer coefficients with TSOP antiscalant at 1 ppm.
|
Time (min)
|
Concentration of antiscalant (ppm)
|
Cold fluid temperatures (°C)
|
Hot fluid temperatures (°C)
|
U, W/m2°C
|
|
Inlet
|
Outlet
|
Inlet
|
Outlet
|
|
15
|
1
|
33.7
|
32.8
|
40.7
|
36.7
|
617.91
|
|
30
|
1
|
35.9
|
38.2
|
44.9
|
41.4
|
872.49
|
|
45
|
1
|
36.9
|
38.7
|
45.3
|
42.4
|
770.87
|
Table 4: Temperatures and
overall heat transfer coefficients with TSOP antiscalant at 5 ppm.
|
Time (min)
|
Concentration of antiscalant (ppm)
|
Cold fluid temperatures (°C)
|
Hot fluid temperatures (°C)
|
U, W/m2°C
|
|
Inlet
|
Outlet
|
Inlet
|
Outlet
|
|
15
|
5
|
28.9
|
17.4
|
31.2
|
28.5
|
915.85
|
|
30
|
5
|
30.1
|
31.1
|
37.3
|
34.8
|
646.10
|
|
45
|
5
|
31
|
32
|
38.6
|
35.6
|
723.67
|
Table 5: Temperatures and
overall heat transfer coefficientswith TSOP antiscalant at 10 ppm.
|
Time
(min)
|
Concentration of antiscalant (ppm)
|
Cold fluid temperatures (°C)
|
Hot fluid temperatures (°C)
|
U, W/m2°C
|
|
Inlet
|
Outlet
|
Inlet
|
Outlet
|
|
15
|
10
|
35.9
|
34.3
|
42.8
|
38.3
|
650.88
|
|
30
|
10
|
37.0
|
38.5
|
45
|
42.1
|
1544.13
|
|
45
|
10
|
37.1
|
40
|
47.1
|
42.8
|
1114.81
|
Table 6: Temperatures and
overall heat transfer coefficients with STP antiscalant at 1 ppm.
|
Time (min)
|
Concentration of antiscalant (ppm)
|
Cold fluid temperatures (°C)
|
Hot fluid temperatures (°C)
|
U, W/m2°C
|
|
Inlet
|
Outlet
|
Inlet
|
Outlet
|
|
15
|
1
|
39.1
|
40.1
|
45.7
|
43.3
|
694.12
|
|
30
|
1
|
38.8
|
40
|
45.2
|
43
|
717.13
|
|
45
|
1
|
38.6
|
40.4
|
46.3
|
43.7
|
786.97
|
Table 7: Temperatures and
overall heat transfer coefficients with STPantiscalant at 5 ppm.
|
Time (min)
|
Concentration of antiscalant (ppm)
|
Cold fluid temperatures (°C)
|
Hot fluid temperatures (°C)
|
U, W/m2°C
|
|
Inlet
|
Outlet
|
Inlet
|
Outlet
|
|
15
|
5
|
39.4
|
40.2
|
46.6
|
43.7
|
701.59
|
|
30
|
5
|
39.3
|
40.5
|
46.4
|
43.8
|
728.94
|
|
45
|
5
|
39.3
|
40.4
|
46.3
|
43.8
|
691.21
|
Table 8: Temperatures and
overall heat transfer coefficients with STPantiscalant at 10 ppm.
|
Time (min)
|
Concentration of antiscalant (ppm)
|
Cold fluid temperatures (°C)
|
Hot fluid temperatures (°C)
|
U, W/m2°C
|
|
Inlet
|
Outlet
|
Inlet
|
Outlet
|
|
15
|
10
|
34.6
|
34.9
|
41.2
|
38.5
|
607.20
|
|
30
|
10
|
35.7
|
36.4
|
42.7
|
40.1
|
624.46
|
|
45
|
10
|
37
|
37.9
|
44.1
|
41.3
|
711.55
|
Fig. 6: Overall heat transfer
coefficients at different antiscalant concentrations using TSOP
Fig. 7: Overall
heat transfer coefficients at different antiscalant concentrations using STP
Table 9: Temperatures and
overall heat transfer coefficients with SHMP antiscalant at 1 ppm.
|
Time (min)
|
Concentration of antiscalant (ppm)
|
Cold fluid temperatures (°C)
|
Hot fluid temperatures (°C)
|
U, W/m2°C
|
|
Inlet
|
Outlet
|
Inlet
|
Outlet
|
|
15
|
1
|
38.6
|
39.2
|
44.3
|
42
|
704.31
|
|
30
|
1
|
40.7
|
41.8
|
47.2
|
44.6
|
795.25
|
|
45
|
1
|
40.3
|
41.6
|
47
|
44.3
|
849.57
|
Table 10: Temperatures and
overall heat transfer coefficients with SHMP antiscalant at 2 ppm.
|
Time (min)
|
Concentration of antiscalant (ppm)
|
Cold fluid temperatures (°C)
|
Hot fluid temperatures (°C)
|
U, W/m2°C
|
|
Inlet
|
Outlet
|
Inlet
|
Outlet
|
|
15
|
2
|
39.5
|
40.5
|
46.4
|
43.7
|
735.01
|
|
30
|
2
|
40.2
|
41.2
|
46.9
|
44.3
|
735.90
|
|
45
|
2
|
40.7
|
41.8
|
47.1
|
44.7
|
750.29
|
Table 11: Temperatures and
overall heat transfer coefficients with SHMP antiscalant at 5 ppm.
|
Time (min)
|
Concentration of antiscalant (ppm)
|
Cold fluid temperatures (°C)
|
Hot fluid temperatures (°C)
|
U, W/m2°C
|
|
Inlet
|
Outlet
|
Inlet
|
Outlet
|
|
15
|
5
|
39.7
|
41.5
|
46.6
|
43.9
|
950.91
|
|
30
|
5
|
41.1
|
42.2
|
48.3
|
45.1
|
834.22
|
|
45
|
5
|
42.6
|
43.3
|
50.1
|
46.4
|
858.30
|
Fig.8: Overall heat transfer coefficients at
different antiscalant concentrations using SHMP
4. CONCLUSION:
From the experimental results it is observed that STP
provides minimum anti-scaling efficiency in all the cooling waters. When total
hardness in cooling water is more than 500 ppm, TSOP and SHMP are good
antiscalants. In almost all the cases it seems that the efficiency initially
increases with increase in concentration of antiscalant and becomes almost
constant after a certain optimum concentration.
Both Boffardi and ONGC techniques are cumbersome and
they do not provide reproducible results and do not allow the measurement of
antiscalant efficiency in the cooling water of a particular composition. Using
these techniques it is not possible to conduct experiments and find out the
exact concentration of a particular antiscalant to achieve maximum efficiency
in the cooling water of specific composition. These techniques are highly time
consuming and not possible to quickly monitor the dose of antiscalant in the
cooling water systems.
The turbidity technique is simple, easy to carry out
and is based on actual theoretical considerations in the precipitation of CaCO3.
This technique provides highly reproducible results and any change either in
quality of the water or the concentration of antiscalant becomes evident in the
results of antiscalant efficiency as is clear from the data recorded. It can be
helpful in determining the efficiency of branded antiscalants and finding their
optimum concentration in actual cooling water systems.
NOMENCLATURE:
|
AS
|
Antiscalant
|
|
CW
|
Cooling water
|
|
CW I
|
General tap water
|
|
CW II
|
5 gm of calcium
salt added to 1 lt of tap water
|
|
CW III
|
10 gm of calcium salt
added to 1 lt of tap water
|
|
CW IV
|
15 gm of calcium
salt added to 1 lt of tap water
|
|
EDTA
|
Ethylenediaminetetraacetic
acid
|
|
ONGC
|
Oil and natural
gas corporation
|
|
ppm
|
Parts per million
|
|
SHMP
|
Sodium
hexametaphosphate
|
|
STP
|
Sodium
tripolyphosphate
|
|
STMP
|
Sodium
trimetaphosphate
|
|
TSOP
|
Tri sodium
orthophospahate
|
|
TSPP
|
Tetra sodium
pyrophosphate
|
|
η
|
Efficiency
|
CONFLICT OF INTEREST:
The author
declare no conflict of interest.
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