DES-10051; No of Pages 5
Desalination xxx (2010) xxx–xxx
Contents lists available at ScienceDirect
Desalination
j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / d e s a l
Calcium carbonate hardness removal by a novel electrochemical seeds system
David Hasson ⁎, Georgiy Sidorenko, Raphael Semiat
Rabin Desalination Laboratory, Grand Water Research Institute, Department of Chemical Engineering, Technion–Israel Institute of Technology, Haifa 32000, Israel
a r t i c l e
i n f o
Article history:
Received 31 May 2010
Received in revised form 16 June 2010
Accepted 17 June 2010
Available online xxxx
Keywords:
Scale control
Electrochemical precipitation
CaCO3 precipitation kinetics
Seeds crystallization
Electrode area
a b s t r a c t
Scale prevention is widely encountered in cooling water systems and is one of the main difficulties in both
thermal and membrane water desalination processes. The usual scale control method applied in water
desalination systems is based on the dosage of inhibiting compounds which are able to suppress scale
precipitation up to a certain degree. Electrochemical scale control systems are beneficially used for hardness
abatement of cooling tower waters. The main drawback hindering their use in desalination applications is the
very high electrode area requirement. The novel electrochemical system developed in this study enables drastic
reduction in the electrode area requirement. This is achieved by directing the precipitation to occur in a seeds
crystallization vessel rather than on the cathode. Results obtained in preliminary experiments have already
yielded a reduction in the specific cathode area by a factor exceeding 10 without altering the specific energy
requirement. Furthermore, the seeds system appears to be free from the restriction of an asymptotic precipitation
rate limit. The outstanding advantages of the low electrode area seeds system opens possibilities for widespread
applications of electrochemical hardness removal in diverse processes requiring scale prevention measures.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Scale deposition is a difficulty encountered in processing aqueous
solutions containing ions of sparingly soluble salts. Scale deposits can
readily form on flow surfaces when a solution is concentrated beyond
the solubility limit of a dissolved sparingly soluble salt or when a solution
containing an inverse solubility salt is in contact with a hot surface. Such
conditions are met in both thermal and membrane desalination
processes. Scale deposition cannot be tolerated because of its highly
deleterious effects on production capacity and specific energy consumption. The usual scale control method applied in water desalination is
based on the dosage of inhibiting compounds which are able to suppress
scale precipitation up to a certain degree. The maximum water recovery
level that can be achieved in brackish water desalination is governed
by the scale suppression capability of anti-scalants.
Many brackish water sources contain alkaline scale forming ions
which are prone to precipitate CaCO3 and Mg(OH)2. One of the
techniques used to control the scaling potential of water circulating in
cooling towers is by electrochemical precipitation of the hardness
components. The precipitation is induced by the generation of a high pH
environment around the cathode by the following cathodic reactions:
−
O2 þ 2H2 O þ 4e →4OH
−
−
ð1Þ
−
ð2Þ
2H2 O þ 2e →H2 þ 2OH :
⁎ Corresponding author. Tel.: +972 4 829 2936/2009; fax: +972 4 829 5672.
E-mail address: hasson@tx.technion.ac.il (D. Hasson).
2−
The high alkaline environment acts to convert the HCO−
3 ion into the CO3
form. The ensuing high supersaturation level of CaCO3 promotes its
precipitation:
2þ
Ca
−
−
þ HCO3 þ OH →CaCO3 þ H2 O:
ð3Þ
The high pH conditions also promote precipitation of magnesium
hydroxide:
2þ
Mg
−
þ 2OH →MgðOHÞ2 :
ð4Þ
Electrochemical scale removal offers many advantages: environmental compatibility, no need to handle and dose chemicals,
accessibility to automation and convenient process control [1,2]. The
main difficulty is disposal of the precipitated scale. Most of the deposit
adheres to the cathode leading to an increase in electrical resistance.
Several techniques have been used for removing the scale depositing
on the cathode including polarity reversal, periodic mechanical
scrapping and ultrasonic cleaning [3–5]. The prevalent technique is
polarity reversal. Its drawbacks are that it restricts the allowable
current density and shortens the lifetime of DSA electrodes [6].
The main factor prohibiting use of the current electrochemical
technology for scale control in desalination applications is the very
high specific electrode area requirement. For instance, in a brackish
desalination plant having a yearly output of one million cubic meter,
the flow rate of the concentrate stream is of the order 20 m3/h.
Assuming that the calcium content of the concentrate is around
2000 ppm as CaCO3 and that it is desired to reduce this value by one
half in order to extract additional permeate, it is necessary to
precipitate 20 kg/h CaCO3. A typical precipitation rate attained with
0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.desal.2010.06.036
Please cite this article as: D. Hasson, et al., Calcium carbonate hardness removal by a novel electrochemical seeds system, Desalination
(2010), doi:10.1016/j.desal.2010.06.036
2
D. Hasson et al. / Desalination xxx (2010) xxx–xxx
the current technology is around 50 g CaCO3/h/m2 cathode area. Thus,
the required electrode area is as high as 400 m2.
The present paper describes a novel electrochemical precipitation
concept which has the potential for drastic reductions of the required
electrode area. The preliminary results presented below have already
yielded reduction of the specific electrode area by a factor exceeding
10.
2.2. Factors governing seeded precipitation
The hydroxyl ion needed for precipitation of the alkaline scale
components (Eqs. (3) and (4)) is generated by the electric current.
According to Faraday's law the rate of OH− generation, WOH mol/s,
with a current of I Ampere is given by:
WOH =
2. Seeded electrochemical precipitation
2.1. Basic concept
In the conventional equipment currently used for hardness
reduction in cooling tower systems, the water is in contact with
both the cathode and the anode electrodes. The cathode performs two
functions: it generates alkalinity and serves as a scale deposition
surface. There is no medium separating the cathodic and anodic
environments. High pH conditions prevail only in a thin boundary
layer near the cathodic surface while the bulk of the water is at the
feed pH level. Consequently, the precipitation reaction occurs only in
the water film adjacent to the cathodic surface. Periodic removal of
the scale accumulating on the cathode is essential and the cleaning
techniques described above are rather cumbersome.
The basic concept of the novel process is separation of the anode
and cathode into two separate compartments using an appropriate
membrane (Fig. 1). In this case a high alkaline environment is
generated throughout the whole volume of the cathodic compartment and not only in the boundary layer adjacent to the cathode. By
transferring the alkaline solution to a separate reaction vessel
containing calcium carbonate particles, the precipitation surface is
now the extensive area of the crystal seeds rather than the restricted
area of the cathode. This concept also offers the advantage of flexible
designs through control of retention time, suspension seeds concentration and seeds specific area.
I
φ
F⋅
ð5Þ
where F is Faraday constant (96845 C/mol) and φ is the current
efficiency. The efficiency depends on the level of the current density
and on the leakage of hydroxyl ions through the separating
membrane. Some literature data [7,8] suggest that the maximum
current efficiency is around 10 A/m2 and that the specific precipitation rate tends to be an asymptotic value at current densities around
100 A/m2.
Design of a seeds electrochemical system can be guided by
literature data on the kinetics of calcium carbonate precipitation.
Kinetic coefficients reported in the literature are of two types:
fundamental coefficients kRS based on the actual crystallization area
and coefficients kRm based on seeds concentration. The most widely
adopted kinetic model was first proposed by Nancollas and Reddy [9].
The equations for continuous flow precipitation in a mixed vessel
according to this model are:
½Cai −½Cao
2+
2−
= kRS ⋅SCaCO3 ⋅ Ca
⋅ CO3 −kSP
τ
ð6Þ
½Cai −½Cao
2+
2−
= kRm ⋅mCaCO3 ⋅ Ca
⋅ CO3 −kSP :
τ
ð7Þ
An alternative kinetic model used by some researchers was
proposed by Davies and Jones [10]. According to this model,
continuous flow precipitation in a mixed vessel is given by:
½Cai −½Cao
= kRS ⋅SCaCO3 ⋅
τ
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffi2
2+ 2− Ca
⋅ CO3 − kSP
½Cai −½Cao
= kRm ⋅mCaCO3 ⋅
τ
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffi2
2+ 2− Ca
⋅ CO3 − kSP :
ð8Þ
ð9Þ
The terms in brackets are activities, SCaCO3 and mCaCO3 are crystals
surface area and seeds concentration respectively, τ is precipitation
time and superscripts i and o denote inlet and outlet conditions
respectively. As pointed out by Inskeep and Bloom [11] there is little
difference in the values of the kinetic coefficients obtained by data
reductions according to the two alternative kinetic expressions.
Table 1 summarizes literature values of the kinetic coefficients kRS
and kRm at room temperatures and also provides values of the
Arrhenius activation energy E [7,11–19]. The kinetic coefficients of
references [15,16] were measured in a falling film flowing over a
vertical tube. In all other cases, the coefficients were determined in
batch seeded experiments. There is some scatter in the data but most
studies report kRS values in the range of 0.5 to 1.5 L2/min mol cm2.
3. Experimental
Fig. 1. Electrochemical cell with separate compartments.
Electrochemical precipitation of CaCO3 was studied in the
continuous flow system shown in Fig. 2. Flow of the feed solution
through the alkaline cathodic compartment was in the once-through
mode while flow of a solution in the acidic anodic compartment was
in recycling mode.
Two electrochemical cells were used. The first cell had a total
volume of 900 mL and was separated into two compartments by a
cationic ion-exchange membrane (Nafion N-966, DuPont). The
Please cite this article as: D. Hasson, et al., Calcium carbonate hardness removal by a novel electrochemical seeds system, Desalination
(2010), doi:10.1016/j.desal.2010.06.036
D. Hasson et al. / Desalination xxx (2010) xxx–xxx
3
Table 1
Literature values of the kinetic coefficients in CaCO3 precipitation.
Reference
Temp.
kRS
kRm
SCaCO3
mCaCO3
E
°C
L2/min mol cm2
L2/min mol mg
m2/L
mg/L
J/mol
33
25
0.9
0.62–0.8
1.5–3.0a
0.37
4.3–5.6
13.1–26.1a
0.8–1.9
0.04–0.01
0.1–0.4
0.09–0.17
0.004–0.456
1000–2500
140–570
100–200
20–2000
48,100
39,200
Lisitsin et al., 2009 [12]
Inskeep and Bloom, 1985 [11]
Kazmierczak et al., 1982 [13]
Reddy and Gaillard, 1981 [14]
Hasson et al., 1981 [15]
Hasson et al., 1978 [16]
Benjamin et al., 1977 [17]
Sturrock et al., 1976 [18]
25
25–30
25–30
20
20
Wiechers et al., 1975 [19]
Nancollas and Reddy, 1971 [9]
25
25
a
0.48–0.78
0.3–0.54
0.1–0.16
72,210
86,250
5.7a
1.1–2.8
3.0–4.5a
2.2–2.5
0.3–0.5
900–2000
300–3000
0.1–0.6
100–1000
350–2000
43,100
46,000
Kinetic coefficients evaluated by the Davies and Jones model.
solution leaving the cathodic compartment with an augmented pH
flowed into a 1 L stirred vessel in which the main crystallization
process took place. The anode consisted of a 100 × 100 mm DSA plate
while the cathode consisted of stainless steel plate of the same
dimensions. This cell enabled operation at current densities in the
range of 40 to 120 A/m2. The second electrochemical cell was
designed to provide higher current densities. The cell had a total
volume of 50 mL. The anode consisted of a 100 × 25 mm DSA plate
while the cathode consisted of a stainless steel plate of the same
dimensions. This cell enabled operation at current densities up to
600 A/m2. The experimental systems enabled feed flow rates through
the cathodic compartment in the range of 50–150 mL/min. Flow in the
cell was laminar with Reynolds numbers below 50.
Test solutions were prepared by dissolving technical grade salts
CaCl2 and NaHCO3 in a solution containing 50 mM of NaCl. The
salinities of the solutions flowing in both cathodic and anodic
compartments were identical. Solution conductivity was around
7.5 mS/cm. The pH of the solution in the feed tank was maintained
constant by controlled bubbling of CO2 actuated by a pH controller
(Mettler Toledo—pH 2050e). The calcium concentration was varied in
the range of 400 to 800 ppm as CaCO3, the total alkalinity in the range
of 250 to 500 ppm as CaCO3 and the inlet pH was in the range of 6.8 to
8.1.
The calcium removal rate was evaluated from the difference in
calcium concentration between the feed and the solution leaving the
crystallizer. Calcium concentrations were determined by the EDTA
titrimetric method (Standard method 3500-Ca). Alkalinity was
Fig. 2. Flowsheet of the continuous flow precipitation system.
measured by potentiometric titration to the end point of pH = 4.3
(Standard method 2320). Each experiment lasted at least 6 residence
times; steady state conditions were reached after 2–3 residence times.
Each experiment was repeated several times and the reproducibility
of results was satisfactory.
4. Results
4.1. Electrode area
The improvement in electrolyzer performance obtained by shifting
the role of precipitation surface to the seeds crystallizer was
investigated in a series of experiments carried out at varying current
densities in the two seeds systems. The data obtained were compared
with results measured in a conventional electrolyzer in which
precipitation mainly occurs on the cathode [20,21].
Fig. 3 compares calcium carbonate removal rates per unit cathode
area measured in the 900 mL seeds electrolyzer with data obtained in
conventional equipment. The figure displays a phenomenon observed
in several studies [7,8,20,21] that increase in current density initially
augments the precipitation but that at sufficiently high current
densities the precipitation rate tends to an asymptotic limit. There is
no clear explanation for this phenomenon. According to Faimon et al.
Fig. 3. Rates of CaCO3 precipitation per unit cathode area in the 900 mL seeds system
compared with data measured in conventional systems.
Please cite this article as: D. Hasson, et al., Calcium carbonate hardness removal by a novel electrochemical seeds system, Desalination
(2010), doi:10.1016/j.desal.2010.06.036
4
D. Hasson et al. / Desalination xxx (2010) xxx–xxx
[22], the decreased precipitation rate at high chlorine release rates is
due to CaCO3 dissolution by the following acidifying effect:
2þ
2CaCO3 þ Cl2 þ H2 O→2Ca
−
−
−
þ 2HCO3 þ Cl þ ClO :
ð10Þ
Gabrielli et al. [7] suggest that limitations in the mass transfer rate
of Ca2+ and HCO−
3 from the solution bulk to the reaction zone are
responsible for the asymptotic precipitation tendency. This hypothesis finds support in a previous study [20] which provides theoretical
and experimental evidence that CaCO3 precipitation on a cathodic
surface is mass transfer controlled. The most likely explanation for the
linearity relationship between precipitation rate and current density
in the seeds system is that the overwhelming proportion of the
precipitation reaction occurs on the seeds and the process is kinetic
rather than mass transfer limited.
The maximum precipitation rates achieved in conventional
systems are seen in Fig. 3 to fall below 100 g CaCO3/h m2. The data
obtained with the seeds system showed no asymptotic limitation. The
precipitation rate increased linearly with the current density and a
rate of 300 g CaCO3/h m2 was attained at the maximum allowable
current density of 120 A/m2 in the 900 mL seeds system.
Data measured with the improved 50 mL seeds system (Fig. 4)
confirmed the linear increase of precipitation rate with current
density. At the maximum allowable current density of 600 A/m2 the
measured precipitation rate represents a reduction in specific cathode
area by a factor exceeding 10. Another outstanding advantage of the
seeds system is that it appears to be free from the restriction of an
asymptotic precipitation rate limit.
4.2. Energy consumption
An additional major parameter influencing the economics of
electrochemical scale precipitation is the specific energy consumption. Analysis of the electrolytic CaCO3 precipitation system shows
that the main parameters governing the specific energy consumption
are the electrical resistances of the solution, of the electrodes and of
the wiring connections [20]. The solution resistance depends on the
solution conductivity and on the distance between the electrodes.
Fig. 5 displays specific energy results obtained in the electrolytic
system described in references [20,21] and energy data measured in
the present study. It is seen that the energy consumption in both
electrochemical systems is of the same order of magnitude and is
Fig. 5. Energy consumption in seeds systems compared with data measured in
conventional systems.
typically in the range of 4 to 6 kWh/kg CaCO3. The reason for the low
energy levels observed in the seeds system is that it allowed
narrowing the electrodes gap, thus enabling low energy consumptions at high current densities.
4.3. Kinetics coefficients
The design of an electrochemical seeds system is closely related to
the kinetics of the precipitation in the crystallization vessel. Kinetic
coefficients based on seeds concentration in the 12 experiments
carried out in this study were in the range of 6 to 12 for Eqs. (7) and
10 to 18 L2/min mol mg seeds, for Eq. (9). These values are somewhat
higher than literature data and might be due to the small particle sizes
formed in the self nucleating crystallizer of the experimental system.
5. Conclusions
The major restriction barring application of the electrochemical
hardness reduction technology in desalination applications is the
requirement for very high electrode area. The aim of the novel system
developed in this study was to achieve a drastic reduction in the
required electrode area. The exploratory experiments carried out have
already shown the possibility of electrode area reduction by a factor
exceeding 10. The new concept opens the possibility of integrating
electrochemical hardness removal in desalination processes and
implementing significant improvements in the electrochemical
technology currently used in cooling water practice. It could also
find widespread use in diverse processes requiring scale control
measures.
Acknowledgement
Thanks are due to Albert Musafir, Project Manager of “Dead Sea
Bromine Co.”, for providing the ion-exchange membrane Nafion N-966
used in this work.
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Fig. 4. Rates of CaCO3 precipitation per unit cathode area in the two seeds systems
compared with data measured in conventional systems.
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(2010), doi:10.1016/j.desal.2010.06.036