CASE STUDY:
A COOLING SYSTEM
WATER TREATMENT
OPTIMIZATION USING
THE ADIC-IONIC
SOFTWARE
An industrial refrigeration circuit based on an evaporative cooling system, with poor antifouling chemical treatment, can lead to the decrease of the thermal performance of the system due to the fouling of the heat exchange surfaces. Adiquimica presents, with an example of a
real case, the software Adic-Ionic®, an evolution of CalcSatu® software, which simulates the behavior of water and determines the best
operating conditions for industrial refrigeration installation and proper treatment preventing scale formation and development of corrosion.
Bodas, J., Cabré, X., Villanueva, O., Biurrarena, A., Ruiz, J.
ADIQUÍMICA
a
C. Albert Llanas 32, 08024 - Barcelona (Spain)
*[email protected]
technical article
INTRODUCTION
GENERAL INTRODUCTION
istics and type of materials of a plant limit
tors (fz) for each type of ions (monovalent,
recirculation water quality in terms of al-
divalent, trivalent and tetravalent) can be
lowable corrosion rates.
approximated mathematically from different models such as the equation Davies
One of the main considerations in design-
The precipitation of slightly soluble salts
(3) good through approximately 0.2 ionic
ing an industrial plant is the optimization of
listed above is sometimes more complex to
strength.
all resources in order to minimize the cost
analyze. Citing as example the calcium car-
per unit produced. In the area of industrial
bonate (CaCO3) scale formation is a phe-
cooling by evaporative cooling systems,
nomenon which depends on the following
the optimization is based on a compromise
parameters:
between the maximum possible water re-
- Water temperature. In contrast with other
Where:
use (if is viable with the heat level) in the
salts, the calcium carbonate has inverse solu-
- β is a function of the water temperature.
cooling process itself without compromis-
bility, that is, greater recirculation water tem-
- z is the charge of the ionic species con-
ing the circuit elements due to the phe-
perature leads to a lower solubility of the salt.
sidered.
nomena of corrosion and fouling of slightly
- pH of water. Key parameter in the distri-
- FI is the ionic strength of the medium as
soluble salts related to the water chemistry
bution of carbonated species in a particular
calculated from equation 1.
and the interaction with the structural ele-
water type. The higher pH of the recircula-
ments of the circuit.
tion water the greater proportion of carbon-
In the valid range of ionic strength of the
ate ion (CO32-), and therefore increased
model [0, 0.2], the activity factors of the
In all simulations throughout the water
risk of precipitation of calcium carbonate.
ionic species are less than unity. Thus and
chemistry in a process of evaporative
- Levels of calcium (Ca) in water. In this
back to the example of calcium carbonate,
cooling are considered calcium carbon-
case the relationship is direct, the higher
the solubility product of calcium carbonate
ate (CaCO3), calcium phosphate (Ca3(PO4)
concentration of calcium in the water the
is increased as the ionic strength of the
), calcium sulfate (CaSO4), magnesium
higher risk of precipitation of calcium car-
medium increases (4).
2
(3)
carbonate (MgCO3) and zinc and magne-
bonate.
sium hydroxides (Zn(OH)2 and Mg(OH)2)
- Ionic strength (Fl) of the medium as a
as susceptible of precipitation like fouling.
function of concentration (c) of all ionic spe-
Furthermore, considering the chloride ions
cies (n) and electric charge (z) in water (1).
(Cl ) and sulfate (SO ) like primarily re-
In an ideal situation it is considered that the
From the solubility product it is defined the
sponsible for the corrosion process lines
ionic strength is zero. In this case the activity
supersaturation index (ISS). It is defined as
since they interfere in the formation of pro-
of a specific ion, expressed as the product
the ratio of the ionic product in the present
tective layers of steel, either with chromium
of its concentration multiplied by the associ-
situation with the theoretical equilibrium
which forms part of the alloy itself or com-
ated activity ion factor (2), takes the same
situation (5). The solution may have differ-
plex formed from the molecules dosed
value as the concentration, that is, the activ-
ent states depending on the value of the
with anticorrosive properties.
ity factor is 1.
supersaturation index for a given salt; satu-
-
24
(4)
rated if the ISS takes a value of 1, underIn the bibliography numerous references
indicate by graphic way, which are the
(1)
and supersaturated if the ISS takes a value
maximum levels of chlorides in the water
in contact with a specific metal (typically
saturated if the value of SSI is less than 1
greater than 1.
(2)
steel) according to its temperature and
composition (in% molybdenum regarding
In a real case the ionic strength takes a
to the total alloy). Therefore, the character-
value other than zero and the activity fac-
2
(5)
technical article
OPERATION OF AN EVAPORATIVE
From the operation described a mass balance
COOLING INSTALLATION
is established which is represented in fig.1
At a basic level, an evaporative cooling
device (cooling tower or evaporative condenser) cools water from two main mechanisms:
- Transfer of sensible heat due to contact
established between water and air. This
mechanism may involve about 25% of total
calories of removed water in the process.
- Evaporation. As a consequence of the
physical phase equilibrium laws (in this
Figure 1. Diagram of basic operation of an
installation of evaporative cooling.
case liquid - gas), when the relative humidity of the air is below 100%, the air
Value
pH
7,94
Conductivity
645 µS/cm
AlkM
2,3 meq/L
Calcium
38 mg Ca/L
Magnesium
18 mg Mg/L
Chlorides
129 mg Cl/L
Sulphates
92 mg SO4/L
Phosphates
0,01 mg PO4/L
Nitrates
35 mg NO3/L
Table 1. Parameters / feed water composition
to the evaporative condenser.
STUDY CASE
is not saturated with water at a given
temperature, and consequently a certain
Parameter
terial of construction is basically stainless
DESCRIPTION OF THE SYSTEM
amount of water evaporates due to the
steel AISI 304, with an approximate level of
chlorides tolerance of 700-800 mg / L at
concentration gradient of it. This evapo-
The case study is based on a system of
the process conditions.
ration removes a certain quantity of water
compression - expansion of ammonia in
from the overall mass and consequently
order to maintain a refrigerated room at
its latent heat, resulting in a global cool-
5°C for the preservation of foods requiring
ing of the water mass. The percentage of
cold storage. This food distribution center
Initially the circuit described worked as ba-
this cooling due to evaporation, although
is located in the area of Vallès (Catalonia).
sic operating parameters described in Table
it depends on the season or the year and
In the circuit, the ammonia is compressed
2 without antiscaling chemical treatment.
from geographic location of the facility, it
isothermally by a dual compressor system
is used to estimate 75% of total calories
with heat exchange between stages of
removed.
BACKGROUND
Parameter
Value
compression. Subsequently, ammonia (in
Concentration Factor
2,25
liquid phase) expands adiabatically absorb-
pH of the recirculation water
8,87 (free)
Following evaporation, the water recircula-
ing by the ambient (and thereby cooling) the
Temperature swing
7,5 ºC
tion salt concentration increases, which may
latent heat required for vaporization. The
increase the risk of precipitation of slightly
heat exchange in the compression step is
soluble salts and increased levels of ionic
performed through an evaporative cooling
species with related corrosion phenomena
system which removes the latent heat of
Under these operating conditions the pres-
properties. For this reason is established an
condensation of ammonia and the suitable
ence of scaling was detected in the higher
operation maintenance system based on
sensible heat compression associated.
temperature circuit points, which when
pseudo steady state conditions that they
Table 2. Operating parameters in the evaporative cooling circuit.
analyzed, revealed a majority percentage of
consist in applying a purge flow in the sys-
The water supply of the evaporative cool-
calcium carbonate (CaCO3). These inlays
tem, added to a not concentrated flow rate
ing circuit has the analytical characteristics
caused losses in the thermal performance
of makeup water, and consequently they
shown in Table 1.
of the system in some cases leading to
help to maintain the supersaturation index
unacceptable temperature increases in the
of a given salt (limiting specie) below a maxi-
The circuit has points of maximum water
freezers that generated alarms in the con-
mum permissible value.
temperature of around 50°C, and the ma-
trol room of the distribution center (Figure 2)
3
technical article
with an average of 30.4 alarms per month.
tant updates in bookstores of the equilibrium
when operates at a constant pH by adding
constants governing the reaction chemistry
sulfuric acid.
between species; all based on a new data
- Simulation of free pH tower; in this case is
recently published in the scientific literature.
determined the distribution of chemical species in a plant of evaporative cooling without
addition of acid to control the pH of the recirculation water. In this mode of addition calculation, the pH at which water evolves circuit
is calculated taking into account the laws of
physical equilibrium with the atmosphere and
Fig 2. Number of alarms generated by high
temperature in the cold room under study during
2011. The dashed line indicates the mean of
incidents per month.
To solve these problems described it is defined a study of operating alternatives:
-The optimization of water replacement
the concentration of species present due to
evaporation as the main cooling mechanism.
Fig 3. Interface principale du logiciel Adic-ionic ;
Introduction des données de départ et sélection
du module de calcul.
contribution to the circuit.
Adic-ionic simulates the behavior of a par-
- Minimize the risk of scaling.
ticular type of water in an evaporative cooling circuit, predicting the risk of scaling of
slightly soluble salts in the system. Thus,
ADIC-IONIC
from an iterative calculation, Adic-ionic determines the optimum concentration factor
Adic-ionic is a powerful software tool that
of operation that minimizes consumption of
combines an algorithm for solving the sys-
feed water in the system and maintains the
tem of equations describing the chemi-
supersaturation indexes below the maxi-
cal and ionic equilibrium in a given type
mum permissible limits. Similarly, and de-
of water, and an expert system that col-
pending on the type of metal more restric-
lects Adiquímica’s experience in the area
tive in the circuit, Adic-ionic also includes
of treatment of water. The application aims
the presence of species with corrosive ef-
to recommend, industrial cooling circuits,
fect on the calculation of the optimal factor
the product (s) and most suitable dose for
working concentration.
inhibiting scale formation sparingly soluble
Fig 4. Quality indicators for simulated water
determined by Adic-ionic.
For the last two calculation modes Adicionic provides the ability to simulate the
salts in the system, control the phenomena
Adic-ionic offers the possibility of three types
behavior of the cooling system to a cer-
corrosion in the water circulation line, and
of simulation based on the characterization
tain concentration factor or to optimize that
prevents development of microbial popula-
of the water of a specific facility (Figure 3):
value based on the following criteria:
tions in risk areas.
- Water supply. Allows obtaining the
- Minimal makeup water consumption in
chemical species distribution to the start-
the circuit.
Adic-ionic born from the evolution of soft-
ing water. Also, supersaturation indexes are
- Avoiding the risk of precipitation of slight-
ware CalcSatu [6], developed by the late
calculated for each of the considered salts.
ly soluble salts considered keeping super-
Adiquimica 90s This new version presents a
- Simulation of cooling tower with con-
saturation indexes below the maximum
major improvement in the calculation engine
trolled pH. In this first simulation of the be-
permissible values.
used to solve chemical and ionic equilibrium
havior of water in evaporative cooling facility
- Maintaining the level of corrosive spe-
among the species considered, and impor-
chemical species distribution is determined
cies below the acceptable maximum limit
4
technical article
Salt
% Reached max.
allowable SSI
CaCO3
5937 %
Finally, having determined the optimal con-
Mg(OH)2
0.4 %
ware ADIC-ionic are expressed based on
centration factor in the system (from the
Zn(OH)2
0%
the following points:
calculation of chemical and ionic equilib-
CA3(PO4)2
3.9 %
- Supersaturation indexes of various salts
rium), and solving the material balances
CASO4
3.7 %
considered in terms of percentage of the
in the system, Adic-ionic launches expert
MgCO3
0.5 %
maximum permissible values (see graph)
system that integrates to propose to the
- Main indicators of water quality, such as the
specific case study the best solution in
Paramètre
% atteint du
maximum admissible
Ryznar Index or Langelier Index, which deter-
terms of water treatment. The expert sys-
IRC
80.5 %
mine its corrosive or scale (Fig.4) character.
tem from the main indicators calculated
- Distribution of chemical species as a re-
by the program and, together issues re-
sult of the resolution of the system equations
lated to the nature of the circuit and the
characteristic of each system (Figure 5).
limitations on the use of certain products,
- Calculation of the relative corrosion index
guides to the user to formulate a proposal
The simulation result confirms that the
(RCI) developed by Adiquímica is included
for an optimized treatment.
higher risk associated with the installation
depending on the materials of construction
removed calories from the circulating water
of the circuit.
during different year seasons.
The results of all simulation using the soft-
Table 3. Achieved percentages of maximum
supersaturation allowable indexes considered of
slightly soluble salts, and relative corrosion rate
(IRC) calculated with the Adic-ionic software.
in the program. The RCI is based on a list
in the current regime of operation is the
of variables that directly influence the cor-
precipitation of calcium carbonate, with a
rosive nature of the water in the system.
rate of supersaturation index of 236x respect to the saturation conditions. Other
salts considered and the relative corrosion
rate (indicator developed by Adiquímica for
assessing the risk of corrosion in the system) is maintained at acceptable levels.
The objective of this study is the evaluation
of the viability of a possible alternative opFig 6. Calculation of the mass global balance in
the process of evaporative cooling from assistant
Adic-ionic.
eration and treatment based on Adic - Ionic
software projections for the system under
consideration.
INITIAL SIMULATION AND OBJECTIVES
PROPOSED SOLUTION
Once determined the calculation mode
From the water supply whose composition
The proposal is to continue operating the
from which the system of characteris-
is detailed in Table 1, a first simulation with
circuit without control of the pH because
tic equations will be solved, Adic-ionic
the Adic-ionic software is performed in order
the dosage of sulfuric acid is not possible
launches a wizard to calculate the overall
to identify the main problems associated with
in the installation in order to maintain the
mass balance of the process (Figure 6).
the initial operating system. The results of this
pH controlled. With the Adic-Ionic software,
For this calculation they are considered
simulation are shown in Table 3.
and the feed water composition described
Fig 5. Distribution of chemical species of simulated water by Adic-ionic.
the differences between the percentages
in Table 1, a projection is performed, with
of cooling due to evaporation with the total
pH not controlled, in order to determine the
5
technical article
optimal concentration cycles of work (Ta-
ration index of calcium carbonate which
were due to power outages. During main-
ble 4), the appropriate dosage of inhibitor,
considerably restricts the possible product
tenance shutdowns state of water circula-
which allows to maintain the rate of calcium
treatment to be used. The Adiclene 526
tion tubes were inspected at the point of
carbonate supersaturation index below the
not only effectively inhibits the precipitation
maximum temperature, revealing the ab-
maximum permissible value.
of calcium carbonate on the exchange sur-
sence of calcium carbonate scale.
faces, and also includes in its formulation
Parameter
Value
Concentration factor
3
pH of water recirculation
8,94 (free)
Among these, a specific corrosion inhibitor
Table 4. Operating conditions determined by
Adic-ionic for the proposed solution.
for copper in this heat system exchangers.
Adic-ionic simulates the water recirculation
By comparing the initial results (Table 3)
water. Applied to real situations, Adic-ionic
behavior in the installation at the operating
and those obtained by the simulation treat-
can simulates the behavior of the water
conditions proposed by the software, in or-
ment obtained by Adic-ionic software (Ta-
in evaporative cooling circuits in order to
der to optimize the inhibitor dosage. Table 5
bles 4 and 5), it can be seen that the use
predict the optimum operating conditions
shows the Adic-ionic proposed treatment ef-
of Adiclene 526 not only will reduce the
of the installation and the proper chemical
fects on the percentages achieved of maxi-
percentage reached for the maximum su-
treatment to minimize the risk of scaling
mum permissible supersaturation indexes.
persaturation index of calcium carbonate
and to promote the absence of corrosion
phenomena to the largest concentration
also achieves and even increases by 33%
factor possible.
agents suitable for the corrosion protection
for the materials in the installation study.
Adic-ionic is a rigorous calculation tool for
chemical and ionic equilibriums for a given
Salt
% Reached max.
allowable SSI
compared with the previous situation, but
CaCO3
83.8 %
the concentration factor of work.
Mg(OH)2
0.2 %
Zn(OH)2
0%
CA3(PO4)2
0.1 %
CASO4
1.3 %
MgCO3
0,2 %
Paramètre
% atteint du
maximum admissible
IRC
7.2 %
Table 5. Achieved percentages of the maximum allowable supersaturation indexes slightly
soluble salts considered and corrosion relative
index to the proposal solution without addition
of inhibitor.
CONCLUSIONS
BIBLIOGRAPHY
[1] Sastri, V.S., Ghali, E., Elboujdaini, M.,
Corrosion Prevention and Protection. John
Wiley and Sons 2007.
[2] Fontana and Greene: Corrosion Engineering 2nd ed. McGraw-Hill Series in Materials Science and Engineering 1978
Fig 7. Number of high temperature alarms generated per month during 2012 in the refrigerated
room under study. The red dashed line indicates
the average of alarms per month of 2011, while
the black dashed line indicates the average of
2012.
[3] ASM INTERNATIONAL: Metals Hand-
determines that: at the conditions to which
The proposed treatment with the Adic-
waters 3rd ed. John Wiley and Sons 1996.
the recirculation water is maintained, the
ionic software was implemented in early
[5] McCabe, W., Smith, J,. Harriot, P.: Unit
optimal treatment product is Adiclene 526
2012 leading to a considerable reduction
Operations of Chemical Engineering (7th
(formulated product from Adiquimica, SA).
in the average high temperature alarms in
ed).McGraw Hill Chemical Engineering
Hardness levels calculated in the stream of
the room per month, from 30 to 3-4 mes-
Series 2005.
concentrated water (> 500 ppm CaCO3),
sages (Figure 7). Note that, as a result of
[6] Adroer, M. Coma, J.: Cálculo de Equi-
together with the fact that the pH is not
expansion of the center tasks, in 2012 the
librios Iónicos en Soluciones Acuosas. Ing-
kept under control, lead to a supersatu-
majority of alarms caused by thermal failure
eniería Química 30, 342 (203-210) 1998.
The product recommendation module or
Expert System integrated in the program
6
book Volume 13 Corrosion. 9th ed. ASM
INTERNATIONAL 1990.
[4] Stumm, W. Morgan, J.: Aquatic Chemistry, Chemical Equilibria and Rates in Natural