X Conference Chances and possibilities of chemical industry in EU
The Metastable Zone of aqueous solutions
Wojciech BOGACZ*, Janusz WÓJCIK – Department of Chemical and Process Engineering, Silesian
University of Technology, Gliwice, Poland
Please cite as: CHEMIK 2014, 68, 3, 198–201
Introduction
The Metastable Zone (MSZ) is one of the most essential notions
in industrial crystallization. However, its definitions have still some
vagueness, and models to estimate the metastable limit (MSL) have
not been matured enough to be used in practical operation. The MSZ
is an area between concentration of solubility (It is a thermodynamic
equilibrium between a solid phase and a liquid phase) and concentration
of detection the first nuclei (supersolubility). MSZ could be also
defined as the supercooling at which first crystals appear when the
solution is cooled. The solubility curve is well defined and determined
uniquely through experiments for each system. On the other hand,
the metastable limit is not. The experimental evaluation of MSZW
can be carried out by isothermal or polythermal methods. The first
one involves attaining the supersaturation as fast as possible, followed
by the measurement of the time lag for the appearance of the first
detectable nuclei in the solution. Where the latter is presented in
Figure 1 – a solution of known concentration C2 at the saturation
temperature T2 (point B) is cooled at constant cooling rate R from
a temperature above T2 (point A) to a temperature T1 at which first
crystals are detected in the solution (point C) [1÷5]. The MSZW can
be considered as a characteristic property of crystallization for any
system [4] however, it is not fixed quantity. It depends on a number
of variables e.g. cooling rate, impurities, solution thermal history,
fluid dynamics and volume of solution [1÷16]. It is usually observed
that the higher cooling rate results in a lower temperature for crystal
appearance and in the same time wider MSZW. It is important to stress
that the measured MSZW depends on the real cooling rate and the
cooling strategy have an influence on the MSZW [3, 8, 9, 11, 14].
Foreign particles suspended in a solution (heterogeneous nucleation),
crystal seeds or created from attrition inside crystallizing system
(secondary nucleation) cause reducing barriers of free energy. The
various mechanisms of nucleation have different metastable limits – the
narrowest for secondary nucleation, next for primary heterogeneous
and the widest for primary homogeneous [1÷3, 7, 8, 12]. The MSZW
and therewith the starting point of nucleation also depends on other
process conditions such as impurities. Foreign particles introduced
from the outside of system (i.e. dust from air) to pure solution are
a prerequisite for heterogeneous primary nucleation [8].
Fig. 1. Determination of the maximum supercooling ΔTmax in the
polythermal method of MSZW.
Corresponding author:
Wojciech BOGACZ – M.Sc., e-mail: [email protected]
200 •
The MSZW decrease as the stirrer speed increases. This feature
was explained qualitatively in [3] by assuming the higher rate of
secondary nucleation at higher stirrer speeds – the decrease in MSZW
with an increase in stirrer speed by the mechanism of ‘‘washingaway’’ of nuclei generated on the stirrer blade by surface-induced
heterogeneous nucleation. Attrition by crystal impeller collision has
been recognized by many researchers at the most common secondary
nucleation mechanism. There could be another explanation that the
rate of secondary nucleation from grown nuclei becomes high as the
stirrer speed is increased [3, 9]. The MSZW has been traditionally
treated as a volume independent reproducible property. Contrary
to the conventional understanding of the MSZW shown in [12], where
Kadm et. al. assumed that the MSZW is a volume dependent stochastic
property. This nature of MSZWs at different volumes and their interrelationship can be explained based on the Single Nucleus Mechanism
(SNM). Nucleation can result in the formation of just a single nucleus
in the clear supersaturated solution, which after growing to certain size
causes secondary nucleation. This single nucleus might be formed at
different time instances in isolated experiments due to the stochastic
nature of nucleation. It expresses itself strongly at smaller volumes
during MSZW measurement, because the probability of forming the
single nucleus is then smaller. Stochastic modeling could predict the
effect of volume on the MSZW with reasonable accuracy and could
be used for identification the transition volume at which the MSZW
become reproducible [12].
Experimental
The main goal of this work is to reexamine and determine the
nucleation point and the MSZW of potassium chloride, potassium
nitrate and potassium sulphate solutions. Deionized and degassed
water and analytically pure salts (Avantor S.A) were used in the
preparation procedure. The solution was first heated to 15°C
above the saturation point and filtered twice then introduced
to Reaction Calorimeter Mettler Toledo RC1e. The temperature
and concentration of solubility were determined with a precision of
1•10–3 g. Concentration of KCl, KNO3 and K2SO4 was monitored in
order to keep it constant during the test. The sample in the vessel was
heated 5°C approximately above the saturation point and stabilized
over 120 minutes. Then, the solution was cooled down at constant
rate (30°C/h). The nucleation point was determined by two different
techniques. The first method was to detect the visible grown nuclei
by using naked eye and turbidity meter. The increase in the turbidity
(purple line Fig. 2) was caused by the sudden appearance of several
small crystals. In the second case the energetic effects of nucleation
were measured.
Figure 2 shows sudden increase heat flow rate (blue line)
between the solution and the jacket of vessel caused by nucleation.
The rapid change of the jacket (red line) and the solution (green line)
temperatures could determine the nucleation point. The solution was
heated up after the measurement until the dissolution of all crystals.
The determination of the nucleation point was repeated following
the same procedure.
In Figures 2a and 2b stabilization time period is not shown in
order to preserve the clarity of presented data. In Figure 2c were
presented only one stage of experiment – cooling because heat effect
of crystallization and change of turbidity and temperature was too
weak, and invisible on graph which shown all period of measurement.
nr 3/2014 • tom 68
Table 1 shows parameter set up and results of conducted
experiments.
Translation into English by the Author
Table 1
Results of the experiments
Salt
Cooling
rate
Volume of
solution
Speed of
stirrer
Solubility Nucleation
temperature
point
R, oC/h
V, cm3
N, RPM
T*, °C
Tn , °C
Supercooling
ΔTm=T*-Tn
°C
KCl
30
350
300
36.6
30.5
6.1
KNO3
30
350
300
37.7
35.5
2.2
K2SO4
30
350
300
41.6
19.5
21.1
Summary
Nucleation point and MSZW was investigated for three solutions
of inorganic salts – KCl, KNO3, K2SO4 by used Reaction Calorimeter
Mettler Toledo RC1e. Polythermal method was chosen to determine
nucleation point. The beginning of crystallization was indicated
by measure turbidity and heat flow. The existing MSZW models have
limitations in that they are valid only for constant rate cooling under
constant solution concentration. The MSZW is reproducible under
identical experimental conditions.
Literature
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nr 3/2014 • tom 68
* Wojciech BOGACZ – M.Sc., graduated in Chemical and Process
Engineering and received his Master’s degree from the Silesian University of
Technology in Gliwice (2012). He is currently pursuing a Ph. D. in chemical
engineering at the Silesian University of Technology under the guidance of
Associated Professor Janusz Wójcik. He is the author of 1 presentation and
1 poster at the national conferences.
e-mail: [email protected]; phone: +48 32 237 29 92
Janusz WÓJCIK – (D.Sc., Eng), Associated Professor, is a graduate of
the Faculty of Technology and Chemical Engineering, Silesian Technical
University (1978). He completed an internship at BP Prosynchem. PhD
at the Faculty of Chemistry, Silesian University of Technology (1987) and
habilitation at the Faculty of Chemistry, Silesian University of Technology
(2011). He was at the DAAD scholarship at the University of Karlsruhe,
worked a full-time researcher at University College London. Research
interests: chemical engineering, methods of separation, crystallization,
sedimentation, numerical methods, CFD. He is the author of monograph,
48 articles in the scientific press, author or co-author of 53 presentations
at national and international conferences, 7 academic textbooks and
5 patents.
e-mail: [email protected]; phone: +48 32 237 14 61
• 201
X Conference Chances and possibilities of chemical industry in EU
Fig. 2 .Calorimetry, turbidimetry and temperature progress
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