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

41

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

42

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

43

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

44

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

45

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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