MAXIMUM TEMPERATURE DIFFERENCE WITHOUT ICE-SCALING
IN SCRAPED SURFACE CRYSTALLIZERS DURING EUTECTIC
FREEZE CRYSTALLIZATION
P.Pronk1, C.A. Infante Ferreira1, M. Rodriguez Pascual2, G.J. Witkamp2
1
Engineering Thermodynamics Section, Delft University of Technology, Mekelweg 2, 2628
CD, Delft, The Netherlands.
2
Laboratory of Process Equipment, Delft University of Technology, Leeghwaterstraat 44,
2628 CA, Delft, The Netherlands.
E-mail: [email protected]
During eutectic freeze crystallization (EFC) in scraped surface crystallizers, ice and salt are
crystallized simultaneously around the eutectic point. Due to the strong tendency of ice to
adhere onto cold heat transfer surfaces, scale layers tend to form in crystallizers causing a
severe decrease in heat transfer rates. Prevention of ice scaling therefore plays a major role in
design and operation of freeze crystallization processes. A parallel paper reveals that both the
solute type and concentration play an important role in ice scale formation on heat exchanger
surfaces. In this work, the maximum temperature difference between the cooled surface and
the slurry without ice scaling is determined for KNO3 solutions in a continuously fed
crystallizer with four PTFE scraper blades rotating on a cooled plate. At low concentrations,
the results are in correspondence with the parallel paper showing that the maximum
temperature difference without ice scaling increases linearly with the solute concentration.
However, at solute concentrations near the eutectic point, the maximum temperature
difference without ice scaling decreases. The latter is ascribed to salt crystallization which
accelerates the ice growth rate resulting in ice scaling.
1. Introduction
Eutectic freeze crystallization (EFC) is an interesting technique to separate aqueous
electrolyte solutions into ice and salt. During eutectic freeze crystallization, a crystallizer
operates at eutectic conditions as a result of which ice and salt crystallize simultaneously.
Previous studies have shown that this separation technique is suitable for numerous
electrolyte solutions and has the potential to reduce primary energy consumption compared to
other separation techniques [HAM98]. Furthermore, experiments have shown that the purity
of the produced salt and ice crystals is very high [HAM04, VAE03].
An important aspect of eutectic freeze crystallization is ice scaling prevention, since ice
crystals have a strong tendency to adhere to cold heat exchanger surfaces. If ice scaling is not
prevented, a scale layer will form on the heat exchanging walls resulting in a strong decrease
of heat transfer rates. Ice scaling is not only of importance for eutectic freeze crystallization
but also for other ice crystallization processes, such as freeze concentration of foods [VER02]
and ice slurry production for cold thermal storage applications [BEL05].
To avoid ice scaling, most ice crystallizers are equipped with moving parts which
continuously remove ice crystals from heat transfer surfaces [STA05]. A commonly used ice
crystallizer is a scraped surface crystallizer in which rotating scraper blades prevent ice
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scaling. Experiments on ice crystallization from electrolyte solutions with a scraped surface
crystallizer revealed that the scraping rate and the ion concentration are important parameters
that determine whether ice scaling occurs or not [VAE02]. An alternative ice crystallizer is a
fluidized bed heat exchanger in which fluidized steel particles remove ice crystals from the
walls [PRO05]. Experiments have shown that the maximum temperature difference to prevent
ice scaling in this type of crystallizers linearly increases with the solute concentration.
The objective of this paper is to study ice scaling in scraped surface crystallizers during ice
crystallization from different aqueous KNO3 solutions at different rotational speeds of the
scraper blades. The results are used to obtain more understanding on ice scaling close to
eutectic conditions and to compare the ice scaling prevention ability of scraped surface
crystallizers and fluidized bed heat exchangers.
2. Experimental Set-up and Procedure
The experimental set-up mainly consists of a 10-liter scraped surface crystallizer as shown in
Figure 1. The crystallizer has a 1 mm stainless steel bottom plate with a heat transfer area of
0.031 m2 which is scraped by four rotating PTFE scraper blades that are driven by a vertical
shaft. Halfway this shaft, a turbine mixer is installed to keep the slurry homogenous. The
bottom plate is cooled by a 50 wt% potassium formate solution which follows a spiral channel
below the bottom plate of the crystallizer. The height and width of the coolant channel
measure 5 and 17 mm respectively. The coolant flow rate is 1000 l/h and its inlet temperature
is controlled within 0.1 K by a cooling machine. The crystallizer overflows to an ice melting
vessel were the produced ice crystals are melted and from which aqueous solution is pumped
back to the crystallizer.
Figure 1: Experimental set-up with scraped surface crystallizer [VAE03]
During the experiments, the shaft torque, the coolant flow rate and the temperatures in the
crystallizer and at the inlet and outlet of the coolant were measured. The total amount of
transferred heat through the bottom plate was calculated from the measured flow rate and
coolant temperatures. Subsequently, the overall heat transfer coefficient was calculated from
the transferred heat and the temperature difference between the coolant and the slurry.
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3. Results and Discussion
Ice crystallization experiments were performed at different KNO3 concentrations and different
rotational speeds of the scraper blades. First of all, one experiment is described in detail, after
which the influences of KNO3 concentration and rotational speed on ice scaling are discussed.
3.1 Analysis of one experiment
Figure 2 shows the characteristic temperatures during an experiment with a 3.2 wt% KNO3
concentration and a scraping rate of 100 rpm. The wall temperature is deduced from the
measured temperatures, the overall heat transfer coefficient and the coolant heat transfer
coefficient. The latter was estimated by the correlation of Dittus and Boelter in which the
hydraulic diameter of the channel was used as characteristic length.
At the beginning of the experiment, the inlet temperature of the coolant was set below the
freezing temperature of the solution in the crystallizer. After some time of operation, the
temperature in the crystallizer decreased below the freezing temperature and ice seeds were
introduced to start the crystallization process. Just after the introduction of ice seeds, the
temperature in the crystallizer increased until the freezing temperature was reached (t=800 s).
After the onset of ice crystallization, the coolant inlet temperature was lowered every 15
minutes until ice scaling occurred.
0.0
Seeding
Temperature (°C) .
-1.0
-2.0
-3.0
-4.0
-5.0
Tslurry
-6.0
Twall
-7.0
Tcool,in
-8.0
0
1000
2000
3000
Time (s)
4000
5000
6000
Figure 2: Temperatures in the experimental set-up during an experiment with a 3.2 wt%
KNO3 solution at a rotational speed of 100 rpm
Ice scaling was recognized by a steep increase of the shaft torque and a decrease of both the
overall and slurry-side heat transfer coefficient. Figure 3 clearly shows that for this
experiment ice scaling started at t=4800 s.
Figure 3 also shows slurry-side heat transfer coefficients which were deduced from overall
and coolant heat transfer coefficients. Before the onset of ice crystallization, slurry-side heat
transfer coefficient were about 3000 W/m2K, while during ice crystallization heat transfer
coefficients up to 5000 W/m2K were measured. From this can be concluded that the presence
of ice crystals increases heat transfer coefficients in scraped surface crystallizers.
3
7000
6000
2
(W/m K)
Heat transfer coefficient
0.8
Overall heat transfer coefficient
Slurry heat transfer coefficient
Torque
0.7
0.6
5000
0.5
4000
0.4
3000
0.3
2000
0.2
1000
0.1
0
0.0
6000
0
1000
2000
3000
Time (s)
4000
5000
Torque (Nm) .
8000
Figure 3: Heat transfer coefficients and torque during an experiment with a 3.2 wt% KNO3
solution at a rotational speed of 100 rpm
3.2 Influence of KNO3 concentration
The described experiment was repeated for different KNO3 concentrations varying from 2.5 to
9.4 wt%. During these experiments the rotational speed was kept constant at 100 rpm. Figure
4 shows operating conditions without (‘stable’) and with ice scaling (‘unstable’). The figure
can be divided into two parts. At concentrations up to 4 wt%, the maximum temperature
difference at which ice scaling is just prevented increases linearly with the concentration. This
observation is in correspondence with results reported in a parallel paper about ice scaling in a
fluidized bed heat exchanger [PRO05]. In this paper, it is stated that ice scaling is only
prevented when the growth rate of ice crystals attached to the wall does not exceed the
removal rate. This ice growth rate is mass transfer limited which results in lower ice growth
rates at higher solute concentrations. Because of this effect, higher temperature differences
without scaling are possible at higher solute concentrations.
4.0
3.5
3.0
Stable
Unstable
-1.0
2.5
T wall (°C)
∆ T wall-to-slurry (K)
0.0
Stable SSC
Unstable SSC
Stability criterion FBHE
2.0
1.5
1.0
-2.0
-3.0
-4.0
0.5
-5.0
0.0
0.0
2.0
4.0
6.0
8.0
Concentration KNO3 (wt%)
0.0
10.0
Figure 4: Temperature difference between
slurry and wall for stable and unstable
operation conditions during ice
crystallization from KNO3 solutions
2.0
4.0
6.0
8.0
Concentration KNO3 (wt%)
10.0
Figure 5: Wall temperatures for stable and
unstable operating conditions during ice
crystallization from KNO3 solutions
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Figure 4 also shows the maximum temperature difference for KNO3 solutions in a fluidized
bed heat exchanger (FBHE) indicating that the ice scaling prevention ability of this scraped
surface crystallizer is about ten times higher for the conditions reported here.
The model presented by Pronk et al. [PRO05] is not applicable for KNO3 concentrations
above 4 wt% since the maximum temperature without ice scaling decreases from that
concentration on. This behavior can be explained by considering the wall temperatures of
stable and unstable experiments as shown in Figure 5. This figure indicates that for higher
KNO3 concentrations, ice scaling started when the wall temperature was lower than
approximately –3.7°C. This phenomenon is explained by the hypothesis that at this wall
temperature KNO3 crystals spontaneously nucleate near the ice crystals on the bottom plate
and that therefore the eutectic freeze crystallization process locally takes place. Although the
eutectic temperature of the KNO3-H2O system is –2.9°C, it is plausible that nucleation of
KNO3 crystals only occurs when a certain degree of supersaturation is reached. The salt
crystallization process takes up the salt supersaturation that exists near the growing ice
interfaces on the bottom plate. As a result of this, the ice growth rate is no longer mass
transfer limited and the ice growth suddenly increases strongly. This strongly increased ice
growth rate exceeds the removal rate resulting in ice scaling.
3.3 Influence of rotational speed of scraper blades
Figure 6 shows stable and unstable operating conditions for different rotational speeds of the
scraper blades in a 2.5 wt% KNO3 solution. Eventually, it was expected that higher
temperature differences without ice scaling could be achieved at higher scraping rates.
However, the results show the opposite. The observed phenomenon might be explained by the
fact that mass transfer coefficients strongly increase at higher rotational speeds and that
therefore the ice growth rate increases strongly resulting in ice scaling. More experiments
especially at lower rotational are however needed to verify this hypothesis.
3.0
Stable
Unstable
∆ T wall-to-slurry (K)
2.5
2.0
1.5
1.0
0.5
0.0
0
20 40 60 80 100 120 140 160 180
Rotational speed of scraper blades (rpm)
Figure 6: Temperature difference between slurry and wall for stable and unstable conditions
at different rotational speeds during ice crystallization from a 2.5 wt% KNO3 solution
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4. Conclusions
During ice crystallization at low KNO3 concentrations, the maximum temperature difference
without ice scaling linearly depends on the solute concentration. However, at concentrations
near the eutectic point, this maximum temperature difference is determined by a minimum
wall temperature, which is slightly below the eutectic temperature. The latter phenomenon is
ascribed to salt crystallization which accelerates the ice growth rate resulting in ice scaling.
Furthermore it was shown that the scaling prevention of scraped surface heat exchangers is
significantly higher compared to fluidized bed heat exchangers.
5. References
[BEL05]
J. Bellas, S.A. Tassou, Present and future applications of ice slurries, Int. J.
Refrig., 28, 115-121, 2005.
[HAM98] F. van der Ham, G.J. Witkamp, J. de Graauw, G.M. van Rosmalen, Eutectic
freeze crystallization: Application to process streams and waste water purification,
Chem. Eng. Proc., 37, 207–213. 1998.
[HAM04] F. van der Ham, M.M. Seckler, G.J. Witkamp, Eutectic freeze crystallization in a
new apparatus: the cooled disk column crystallizer, Chem. Eng. Proc., 43, 161–
167, 2004.
[PRO05] P. Pronk, C.A. Infante Ferreira, G.J. Witkamp, Ice scaling prevention with a
fluidized bed heat exchanger, submitted to the 16th International Symposium on
Industrial Crystallization to be held in Dresden, 2005.
[STA05] E. Stamatiou, J.W. Meewisse, M. Kawaji, Ice slurry generation involving moving
parts, Int. J. Refrig., 28, 60-72, 2005.
[VAE02] R.J.C. Vaessen, C. Himawan, G.J. Witkamp, Scale formation of ice from
electrolyte solutions on a scraped surface heat exchanger plate, J. Cryst. Growth,
237-239, 2172-2177, 2002.
[VAE03] R.J.C. Vaessen, Development of scraped eutectic crystallizers, Ph.D. thesis, Delft
University of Technology, 2003.
[VER02] R.J. Verschuur, R. Scholz, M. van Nistelrooij, B. Schreurs, Innovation in freeze
concentration technology, Proceedings of the 15th International Symposium on
Industrial Crystallization, 15-18 September 2002, Sorrento, 1035-1040, 2002.
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