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Application of Progressive Freeze Concentration for Water
Purification using Rotating Crystallizer with Anti-supercooling Holes
Farah Hanim Ab. Hamid, Zaki Yamani Zakaria, Norzita Ngadi and Mazura Jusoh
Faculty of Chemical Engineering, Universiti Teknologi Malaysia, 81310 Johor Bahru, Malaysia.
Abstract. In recent century, the world is experiencing clean water supply shortage and the severity of this
problem is increasing at an alarming rate. Introduction of new technologies for water purification is essential to
accommodate the demand for clean water supply. This paper proposed a new technology which is desalination of
seawater through freeze concentration using rotating cylindrical crystallizer with anti-supercooling holes, where
pure water is produced in the form of ice crystal block, which leaves behind a higher concentration solution. The
effect of coolant temperature and rotation speed were investigated and the efficiency of the system was reviewed
based on the effective partition constant (K), desalination rate and efficiency of concentration. The system has
achieved its best performance at intermediate coolant temperature which is -8°C and rotation speed of 300 rpm
producing K value, desalination rate and efficiency of concentration of 0.376, 35.71% and 62.38% respectively.
Keywords: freeze concentration, water purification, ice crystal, desalination.
1. Introduction
In this modern century, shortage of clean water supply is still a worrying issue and the problem is
expected to become more serious in the future. The enormous increment of human population growth has
caused the existing water resources inadequate to fulfil the water supply demand nowadays [1]. The high
demand for water supply is also due to the tremendous growth of industrialization and urbanization [2]. After
all, researchers have been working in earnest to find the best solution to address the water shortage. At the
end of 2011, it has been estimated that the desalination capacity of 71.9 million m3 has been produced daily
which represents the importance of desalination method in producing freshwater [3].
Therefore, many technologies of desalination systems have been introduced to produce freshwater like
multistage flash (MSF) which was established as the baseline technology, electro dialysis (ED), and
capacitive deionization technology (CDT) [3, 4]. With the growth of membrane science, reverse osmosis
(RO) has overtaken multi-stage flash as the leading desalination technology, and should be considered as the
baseline technology, while others as alternatives. The search for improved desalination method led to the use
of freeze concentration method [5]. In freeze concentration, the solution is physically separated by making
pure ice in solution. It involves fractional crystallization of water and subsequent removal of the ice. The
principle of freeze-concentration is based on the solidification phenomena of water. During ice crystal
formation, solutes are rejected by the nature of ice crystal lattice which is formed by pure water. Water
solidification process forming the small dimension ice crystal lattice makes the inclusion of any impurities
impossible except for fluorohydric acid and ammonia, thus there is no solute contaminants in ice [6]. One of
the methods to form a large single ice crystal is by applying progressive freeze concentration (PFC) process.
The ice crystal is formed on the surface of the conducting material where the cooling is supplied. Impurities
are separated from the ice phase during the ice crystals formation.
Corresponding author. Tel.: + 6075535535
E-mail address: mazura@cheme.utm.my
2015 5th International Conference on Environment Science and Engineering
Volume 83 of IPCBEE (2015)
DOI: 10.7763/IPCBEE. 2015. V83. 7
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According to previous reports, one of the main advantages of freeze desalination process is its energy
consumption [7]. In this method, only 420 kJ of energy is required to remove salt and produce 1 kg of fresh
water which is six times lower than the energy required by MSF [4]. However, one of the difficulties in
dealing with this system is ice handling after the process is completed [8]. Therefore, elements such as ice
sampling and visualization purpose are important in designing the apparatus. In fact, the apparatus design is
a crucial factor in influencing the system efficiency.
The improvement of progressive freeze concentration has been done particularly in its apparatus design
to obtain a better product quality. In order to amend the weaknesses of previous conventional designs and to
improve the efficiency in freeze concentration, several new designs for PFC system have been introduced,
constructed and operated under different conditions especially on solution movement such as stirring [9, 10],
ultrasonic radiation [11, 12], bubble-flow [13, 14], agitation and aeration [15] and also by oscillatory motion
[6, 16]. This study is focusing on movement of the target solution by rotating the vessel where crystallization
of ice is supposed to occur.
In addition, there are several factors that could affect the efficiency of the system and the thawed ice
quality [17] including solution flow rate, initial concentration, coolant temperature and operating time. This
present article is focusing on the effect of coolant temperature and rotation speed due to the fact that these
parameters are the two that the most significant in influencing the system performance.
2. Material and Methods
2.1. Feed samples
A saline solution was used throughout the experiments as raw material. In order to make saline solution,
sodium chloride was well-mixed with pure water. A 50% (v/v) ethylene glycol solution was used as coolant
in the water bath. The type of coolant that was used in this study is ethylene glycol based water solutions.
Ethylene glycol is commonly applied to transfer heat in very low temperature processes.
2.2. Laboratory equipment
The laboratory equipment for PFC is composed of three parts including a cooling bath, a motor, and the
newly designed rotating crystallizer. Fig. 1 illustrates the schematic view of the experimental setup for PFC
system used in this research. The rotating crystallizer with anti-supercooling holes was invented to ensure
that the initial supercooling can be prevented. This features of holes provide room for nucleation and
crystallization of pure water molecules to take place, where the water molecules are cooled down to below
freezing point earlier than the average bulk solution molecules, since it is nearly in contact with the wall
which is cooled by the coolant. In addition to the aforementioned statement, there will be higher chance of
ice nucleation with lower opportunity for the contaminant to be trapped in the ice due to the higher freezing
point of the pure water molecules compared to the solution that contains foreign solute molecule [9]. In
addition, the ice developed in these holes generates more ice crystal with high purity. The advantage of this
feature is that the ice lining process can be neglected which makes the operation easier. As explained, it can
be clarified that this process applies the same theory as seeding process. Therefore, the time consumption can
be reduced.
This crystallizer is attached to a motor to rotate the crystallizer in order to induce solution movement. A
set of baffle is also attached to the crystallizer wall which acts as a stirrer, to reduce the accumulation of
solute near the ice front. This baffle is detachable, so that it makes the sampling and cleaning process much
easier. A thermocouple was located in the middle of the solution in the crystallizer to measure the
temperature changes during crystallization process. This thermocouple is attached with Picolog recorder, and
all data are displayed on the computer. At the end of experiment, a salinometer was used to measure the
concentration of ice and also the concentrated solution.
2.3. Experimental procedure
A saline solution was first kept in the freezer at 2°C to 3°C as the initial temperature of the sample
should be near the water freezing temperature. Cubes of saline solution were mixed with the sample to
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maintain the temperature during the feeding process. The saline solution with initial concentration of 35g/L
was fed directly into crystallizer. Note that the initial concentration of saline solution is the same as that of
sea water. In order to allow the crystallization process to occur, the crystallizer then was immersed into the
cooling bath at the desired temperature and rotation speed. After the designated time, the rotation was
stopped and the crystallizer was taken out to be thawed. The concentrated solution was drained out
completely and a sample of the ice layer produced was collected. The salinity of each sample was then
measured using a salinometer.
Fig. 1: Experimental setup
3. Evaluation Methods
3.1. Effective partition constant
The exclusion of solute molecules from the moving ice front and the interface between the ice and
solution phases is the main mechanism of concentration in PFC [18]. For evaluation method, effective
partition constant (K) was reviewed to determine the system performance. The K value is related to the
quality of the ice produced which can be determined by the following equation [18]:
K=CS/CL (1)
where, CS is concentration of ice and CL is concentration of the concentrate. In this case, the concentration is
represented by salinity. Based on the equation of solute mass balance [18], the equation of effective partition
constant, K can be integrated as follows:
(1-K) log (VL/Vo) = log (Co/C) (2)
In Eq. (2), VL is defined as the concentrate volume, is the initial volume of solution, Co is the initial
concentration of the solution and CL is the concentration of concentrate. Eq. (2) was applied for evaluation
method in this research. In any condition, lower K value shows higher efficiency of the system. According to
Fujioka et al. (2013) [2], the effect of desalination increases when the K value is smaller. Apart from that, the
purity of ice produced can be determined based on its measured salinity. Lower salinity means higher salinity
reduction of ice produced. Thus, the lowest K-value and lowest salinity of ice are considered as the
favourable conditions for the system.
3.2. Efficiency of concentration
The concentration increment in solution relative to the quantity of NaCl remaining in the ice fraction is
defined as efficiency (E %) [19-21]. In theory, the lower the NaCl content remaining in the ice fraction, the
more concentrated the solution will be. The efficiency can be calculated using Eq. (3) as follows:
L i
L
C CEfficiency(%) 100
C
(3)
where CL and Ci are the concentration of NaCl in the concentrated solution and ice fraction, respectively.
3.3. Desalination rate
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In order to ascertain the performance of desalination process, desalination rate, Rd was employed into
calculation using Eq. (4) as follows [17]:
o i
d
o
C CR 100%
C
(4)
where Co and Ci are the initial concentration of the solution and the concentration of ice produced
respectively. The desalination rate also can be an indicator to discover the efficiency of the system. The
higher value of Rd indicates a better performance for the system.
4. Results and Discussions
4.1 Effect of coolant temperature
A series of preliminary experiments were conducted to ascertain the range of operating parameters. The
studied range of coolant temperature was chosen based on the freezing point of pure water and saline water.
It was found that the freezing point of saline water decreased as salt concentration increased. Pure water will
obviously become ice at the studied temperature as the freezing point of water is 0°C, leaving behind saline
water which has lower freezing point.
The coolant temperature is an important parameter that influences the process, because it is closely
related to freezing rate [17]. An investigation on coolant temperature effect is crucially needed in order to
determine the best temperature for the system. Fig. 2 shows the relationship between coolant temperature and
K value, Rd and E%. From the observation of plotted graph, it is clearly indicated that a change can be seen
even when the coolant temperature was shifted for only -1°C. The highest K value is noted at -7°C, where
the ice nucleation is not perfectly shaped, thus the chance for dendritic ice to be formed on the ice surface is
higher. As a result, this dendritic structure enlarges the chance for solutes to be trapped into the ice, resulting
in highly impure ice layer.
It is also worth noting that the intermediate temperature of -8°C is considered as the best temperature for
this system due to its lowest value of K which is 0.376. There are decreasing changes of K value from -7°C
to -8°C showing that lower coolant temperature resulted in lower K, which means higher efficiency for the
system [15]. However, as the coolant temperature was decreased to -9°C and below, the K value started to
increase. This means that the efficiency of the system would also decrease if the coolant temperature is too
low. At -9°C, there is a possibility that the saline water would also start to freeze and the salt would get
trapped into the ice layer formed, resulting in higher value in salinity of ice. Thus, K value would also be
higher, yielding low efficiency of the system.
Coolant temperature,°C
-12 -11 -10 -9 -8 -7 -6
K v
alu
e
0.36
0.38
0.40
0.42
0.44
0.46
0.48
0.50
Eff
icie
ncy
%
50
52
54
56
58
60
62
64
Des
alin
atio
n r
ate,
Rd
24
26
28
30
32
34
36
38
K value
Efficiency %
Desalination rate, Rd
Fig. 2: Relationship between coolant temperature and K value, Rd and E %.
In fact, coolant temperature will influence the ice growth rate, whereby low growth rate will give high
purity of ice produced. According to Flesland (1995) [22], when the difference between the entering solution
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and the surface temperature increases, the ice growth rate will increase as well. In the same way, when the
coolant temperature is decreased, higher growth rate of ice front will be observed. This situation is
undesirable in producing a low K for the system. The higher the ice growth rate, the more impurities would
be entrained in the ice. In addition, the solute outward movement will be blocked by the high speed of
moving front, resulting in promotion of solute inclusion in the ice crystals [18]. Therefore, the salinity of ice
would increase as the coolant temperature is reduced to -11°C because of the high amount of ice impurities
present at the low temperature applied, resulting in faster freezing rate, hence, the impurities are easily
trapped into the ice phase.
In other perspective, the desalination rate, Rd was also investigated in order to discover the effectiveness
of the desalination process in this system. The plotted graph shows the effect of coolant temperature towards
the desalination rate, Rd. The best condition was achieved when the Rd was higher and the ice salinity in ice
was lower. Based on that standard, the temperature of -8°C gives the highest Rd which is 35.71% and the
lowest ice salinity with 22.5 g/L. The highest Rd means the desalination process is excellently done and the
lowest salinity shows less impurity in ice.
The lowest temperature which is at -11°C recorded the lowest Rd. This explains that, the desalination
process is not completely working at this point. This is because when the temperature is too low, the solution
could not be desalinated due to the fact that almost the whole solution has turned into ice form, hence no
separation process between pure water and saline solution occurred. In addition, according to Zhang and
Hartel (1996) [23], lower temperatures resulted in higher ice crystal growth rates, hence poor purity of ice
produced.
4.2 Effect of rotation speed
For this part, the determination of the effect of rotation speed towards K value, Rd and E% has been done.
Rotation speed represents the flow rate of the solution. This solution movement was introduced to provide a
uniform distribution of flow, hence reducing the accumulation of solute near the liquid-ice interface. Fig. 3
clearly indicates a trend of decreasing K value with increasing flow rate.
Rotation speed, rpm
100 150 200 250 300 350 400
K v
alu
e
0.36
0.38
0.40
0.42
0.44
0.46
0.48
0.50
Eff
icie
ncy
%
50
52
54
56
58
60
62
64
Des
alin
atio
n r
ate,
Rd
26
28
30
32
34
36
38
K value
Efficiency %
Desalination rate, Rd
Fig. 3: Effect of rotation speed on K value, Rd and E %
Theoretically, higher flow rate will result in lower value of K. Miyawaki et al.(2005) [24] stated that the
increase in flow rate will decrease the advance rate of the ice front. Therefore, the value of K will be
decreased giving the higher purity of ice. The lower value of K means it has better efficiency and resulted in
a highly pure ice crystal layer. This theory applied the same agreement with a previous study by Okawa et
al.(2009) [25]. According to Okawa et al.(2009) [25], the higher flow rate promotes slower solidification rate,
resulting in less concentration captured in ice. However, if the rotation speed applied is too high, the solution
flow might have a potential to erode the ice layer which has been formed on the crystallizer wall, thus
reducing the solution concentration in liquid phase due to the increment of pure water volume in the solution
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after the erosion. This explains the higher K value obtained when the maximum speed of 350 rpm was
applied.
The desalination rate was observed and it has an increasing trend as the rotation speed was increased as
shown in the plotted graph. It has been explained that increasing the flow rate of solution promotes heat
transfer where it will produce more ice crystals. This means that the separation process between pure water
and seawater is effectively worked. In addition, the shear force of fluid flow is capable to carry away the
solute which is entrapped between the dendrite structure in the ice formed [26]. Therefore, higher flow rate
will result in ice layer with higher purity.
5. Conclusions
This study has successfully proven that the newly designed progressive freeze concentration system has
a splendid potential to be applied for desalination process. The effect of coolant temperature and rotation
speed were magnificently investigated by employing three determinant factors which are effective partition
constant (K), desalination rate (Rd) and efficiency of concentration (E%). From the results, the system has
achieved its best performance at intermediate coolant temperature which is -8°C and rotation speed of 300
rpm with the K value, desalination rate and efficiency of concentration of 0.376, 35.71% and 62.38%
respectively.
6. Acknowledgements
The financial supports of Research University Grant (04H46) and Fundamental Research Grant Scheme
(4F224) from Universiti Teknologi Malaysia and Ministry of Education (MOE) are gratefully acknowledged.
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