polielektrolity jako substancje umożliwiające ultrafiltracyjne

Membranes and Membrane Processes
in Environmental Protection
Monographs of the Environmental Engineering Committee
Polish Academy of Sciences
2014, vol. 119, 373-380
ISBN 978-83-63714-18-5
GYPSUM LAYER GROWTH ON A CATION-EXCHANGE
MEMBRANE
Krzysztof MITKO1*, Marian TUREK1
Abstract: Scaling, a crystallization of sparingly soluble salts on the membrane surface,
remains one of the main problems in electrodialytic desalination. A crystallization of
calcium sulphate from the artificial, supersaturated solution flowing through the thin
intermembrane channel has been studied using ultrasonic time-domain reflectometry
(UTDR). The gypsum layer growth on a Neosepta CMX cation-exchange membrane caused
the shift in the arrival time of the ultrasonic signal, allowing to establish a kinetic model of
scaling. The results show that the method could be used for in-situ studies on membrane
scaling.
Keywords: scaling, calcium sulphate, ultrasonic reflectometry.
INTRODUCTION
Crystallization of the sparingly soluble salts – i.e. calcium sulphate, calcium
carbonate – at the membrane surface remains the major obstacle in achieving high
water recovery during the electrodialytic desalination of brackish waters, mine
drainages and seawater. Scaling increases the electric resistance of the membrane
stack, increasing the operating costs of the electrodialysis, and can eventually lead
to the deterioration of the membrane surface. In order to assess the scaling risk
during the electrodialytic desalination, an investigation of crystallization kinetics of
sparingly soluble salts in the membrane is required. Several methods can be applied
for the scaling investigation: general observation of membrane stack voltage drop;
electrical impedance spectroscopy (EIS) [1], which utilizes membrane voltage
response to alternating current of different frequencies; capacitance spectroscopy
[2], which is based on equilibrium capacitance measurements at no mass transfer
conditions; the optical methods, as ex-situ scale observation detectors (EXSOD) [3]
or laser sheet at grazing incidence (LSGI) [4]; observation of changes in the
1
Silesian University of Technology, Faculty of Chemistry, Department of Inorganic, Analytical Chemistry and
Electrochemistry, B. Krzywoustego 6, 44-100 Gliwice, Poland
* corresponding author: [email protected]
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Mitko K., Turek M.
hydrodynamic conditions in the membrane module [5,6]; and the ultrasonic timedomain reflectometry (UTDR) [7-10].
The principle of UTDR measurements is as follows: transducer is contacted
with the specimen under investigation. Ultrasounds, a mechanical wave generated
by the transducer, move through the tested specimen. At the interface between the
media (e.g. at the interface between the membrane and the solution), part of the
wave is reflected, generating the ultrasonic echo. Based on the sound velocity in the
media under investigation, V, and the time required for the echoed signal to return
to the transducer, Δt, a media layer thickness, s, can be calculated as [7]:
1
s  V t
2
(1)
Whenever the fouling or scaling appears, a difference in arrival time of the
ultrasonic echo should allow detection of newly grown deposit layer. UTDR has
been applied in the investigation of scaling in reverse osmosis [7-9], both in flatsheet and spiral wound modules, and fouling in microfiltration in a hollow fibre
module [10]. In case of electromembrane processes, an important advantage of
ultrasonic reflectometry is that no optical window is required [7], i.e. it can observe
the phenomena occurring behind the non-transparent electrodes, contrary to the
optical methods. The purpose of this paper is to investigate the scaling growth on an
ion-exchange membrane and to test whether the UTDR method can be applied for
an in-situ study of the electrodialytic desalination of sparingly soluble salts
solutions.
EXPERIMENTAL
During the experiments, a 10 MHz single crystal contact transducer with plane
wave emitting surface, provided and calibrated by Optel Co. Ltd., was used. The
transducer was connected to an OPLabBox 1v2 pulser-receiver with band pass
amplifier controlled by the OPBOX 2.0 ultrasonic box. The pulse voltage was
400 V; bandwidth was set to 30 MHz. The received signal sampling frequency was
0.05 Hz. The transducer was covered with sonic couplant and contacted with
a testing stack, which consisted of platinised titanium electrode, Neosepta CMX
cation-exchange membrane, 0.4 mm thick intermembrane spacer, second Neosepta
CMX membrane and the second electrode. A schematic view of the stack is
presented in Fig. 1. The crystallization compartment had a cuboid shape of 3 cm x
3 cm x 0.4 mm. The calcium chloride (0.2 M) and sodium sulphate (0.2 M)
solutions were introduced into the compartment by the separate nozzles, in order to
make sure crystallization does not start outside the module. The linear flow velocity
of both inlet streams was 1.5 cm/s, the CaCl2 and Na2SO4 streams were not
recycled. All of the experiments have been repeated twice. The experimental
conditions were designed to simulate conditions inside the concentrate compartment
Calcium sulfate scaling risk during electrodialytic desalination
375
of the single-pass electrodialyzer, i.e. when the supersaturated calcium sulphate
solution flow trough a narrow channel between two ion-exchange membranes.
PULSER-RECEIVER
calcium
chloride
TRANSDUCER
t1
t2
t3
t4
electrode
CMX membrane
intermembrane spacer
CMX membrane
sodium
sulphate
electrode
Fig. 1. A scheme of the experimental setup.
RESULTS
A fragment of the obtained ultrasonic signal can be seen in Fig. 2. The position
of the arrival times was estimated based on the approximate sound velocities in the
medium (ca. 6100 m/s in titanium, 2500 m/s in polymeric materials and 1500 m/s in
brine) and known membrane, electrode and intermembrane spacer thickness. The
arrival times represent, the interface between the electrode and the ion-exchange
membrane (t1), the interface between the first ion-exchange membrane and the
upside of the flowing solution (t2), the interface between the downside of the
flowing solution and the second ion-exchange membrane (t3) and the interface
between the second ion-exchange membrane and the second electrode (t4). After the
gypsum crystallization occurred, the arrival time t3 decreased, probably because the
gypsum deposition at the membrane surface. The arrival times t1 and t2 were
practically identical, which was an expected behaviour, knowing that the scale layer
would form at the bottom of the channel. Changes in arrival times throughout the
experiment are presented in Fig. 3. The occurrence of the gypsum crystallization
was confirmed during the process, when the gypsum crystals were forming in a tank
collecting the outlet stream, and after the dismantling of the module, when a layer
of a white deposit was visible on the membrane surface.
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Mitko K., Turek M.
The appearance of additional layer should cause the appearance of a new peak;
however, it can only be visible if the new layer thickness falls within the spatial
resolution capabilities of the system [7]. Although this was probably not the case in
the presented experiment, the arrival time shift [7] did appear, which suggest the
scaling happened.
After scaling
Before scaling
t1
t3
50
t2
t4
0
-50
0.0
0.1
0.2
0.3
0.4
0.5
Arrival time [ s]
Fig. 2. Positions and changes in arrival times of signals 1-4 during the experiment no. 1.
t1 – electrode/membrane interface, t2 – membrane/solution interface,
t3 – solution/membrane interface, t4 – membrane/electrode interface.
Since the experiments did not allow to directly measure the thickness of the
crystal layer, it was calculated based on the decreasing intermembrane distance:

t t   t 2 t  

s t   0.41  3
 t 3 0   t 2 0  
(2)
where s(t) is the gypsum layer thickness [mm] at time t, t3(t) and t2(t) are the arrival
times describing feed channel/membrane and membrane/electrode interfaces at time
Calcium sulfate scaling risk during electrodialytic desalination
377
t, and t3(0) and t2(0) are the initial (t = 0) arrival times describing feed
channel/membrane and membrane/electrode interfaces. The equation is based on
the assumption, that decreasing intermembrane distance [mm] is proportional to the
increasing thickness of the growing crystal layer [mm].
0.022
Arrival time t3 [ s]
Arrival time t1 [ s]
0.264
0.021
0.020
0.019
0.261
0.258
0.018
0.255
0
500
1000
1500
2000
0
Experiment time [s]
500
1000
1500
2000
Experiment time [s]
0.1175
0.376
Arrival time t4 [ s]
Arrival time t2 [ s]
0.1170
0.1165
0.1160
0.372
0.368
0.1155
0.364
0.1150
0
500
1000
1500
2000
Experiment time [s]
0
500
1000
1500
2000
Experiment time [s]
Fig. 3. Changes in arrival times of signals 1-4 during the experiment no. 1.
Fig. 4 presents the calculated thickness of the gypsum layer growing on the ionexchange membrane surface. Following model was assumed for the description of
gypsum layer growth:
s t   a t  t ind 
b
(3)
where s is the scale layer thickness [mm] at time t [s], a and b are the empirical
kinetic coefficients and tind is the induction time of scale layer growth [s]. In the
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Mitko K., Turek M.
experimental conditions, the gypsum supersaturation was 6.52, which corresponded
to 9.34 s of gypsum nucleation induction time [11]. The model was fitted to the
experimental data – the estimated parameters are presented in Tab. 1. Although
there is a difference in a gypsum nucleation induction time and the scale layer
growth induction time, these two parameters should not be directly compared. In
a single-pass system, the scaling risk is strongly affected by the hydrodynamic
conditions inside a module. When a calcium chloride stream is mixed with the
sodium sulphate stream, a supersaturated calcium sulphate is created; the
crystallization can be observed after the nucleation induction time passes. However,
the supersaturated solution is not stationary – after the nucleation induction time,
some fraction of the solution has already flowed to the different place, possibly
even outside the module. The broader is the residence time distribution, the higher
is the risk that enough growing crystal nuclei are not washed outside the module
and stay inside to start the crystallization [12]. The calculated induction time of the
scale layer growth means that at the given hydrodynamic conditions (flow velocity,
residence time distribution, and spacer geometry) it took 39 s and 34 s for
experiments no. 1 and 2, respectively, to accumulate enough crystal nuclei for the
scale layer to be detectable.
Scale layer thickness [mm]
0.03
0.02
0.01
0.00
0
500
1000
1500
2000
Experiment time [s]
Fig. 4. Gypsum layer growth during the experiment no. 1. Points represent the
experimentally-obtained values; solid line represents the fitted model.
Calcium sulfate scaling risk during electrodialytic desalination
379
Table 1. Estimated values of scale layer growth model.
Parameter Experiment 1 Experiment 2 Mean value
9.2 ± 0.7
10.0 ± 0.2
9.6
B
0.44 ± 0.02
0.40 ± 0.02
0.42
tind
39 ± 4
34 ± 10
36.5
A∙104
CONCLUSIONS
The results show that the UTDR method can detect scale layer formation in
a narrow channel between the ion-exchange membranes, which suggest the method
could be utilized for scaling investigation during the electrodialytic desalination;
moreover, the possibility of UTDR measurements behind the non-transparent
electrode, a must-have in case of experiments with electromembrane processes, was
confirmed. Although the previous studies do not address this issue, a more thorough
investigation of the applied ultrasonic signal on the gypsum crystallization process
is required.
ACKNOWLEDGEMENTS
This work was financed by the Polish National Science Centre upon decision
no. DEC-2012/05/N/ST8/02951.
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NARASTANIE WARSTWY GIPSU NA MEMBRANIE
KATIONOWYMIENNEJ
Krzysztof MITKO, Marian TUREK
Streszczenie: Krystalizacja trudno rozpuszczalnych soli na powierzchni membrany (scaling)
pozostaje głównym problemem podczas elektrodialitycznego odsalania. Krystalizacja
siarczanu wapnia z przesyconego roztworu modelowego, płynącego w cienkim kanale
pomiędzy membranami jonowymiennymi, została zbadana z zastosowaniem reflektometrii
ultradźwiękowej w dziedzinie czasu (UTDR). Narastanie warstwy gipsu na membranie
Neosepta CMX spowodowało zmniejszenie czasu przybycia sygnału ultradźwiękowego,
pozwalając na opracowanie modelu kinetyki scalingu. Wyniki pozwalają sądzić, że metoda
UTDR może być zastosowana do badań in situ nad blokowaniem membran.
Słowa kluczowe: blokowanie membran, reflektometria ultradźwiękowa, siarczan wapnia.

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