University of Groningen
Removal of inorganic compounds via supercritical waterLeusbrock, Ingo
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This chapter has been published as:Leusbrock, I., Metz, S. J., Rexwinkel, G., and Versteeg, G. F.; The solubility of magne-sium chloride and calcium chloride in near-critical and supercritical water ; The Journalof Supercritical Fluids 53(1-3), 17-24.
Chapter 5
The solubility of magnesium chlorideand calcium chloride in near-criticaland supercritical water
- 109 -
Chapter 5 ∥ Magnesium Chloride and Calcium Chloride
Abstract
Applications using supercritical water often encounter the presence of
inorganic compounds in feed streams, most often with a minor concentration.
These compounds can lead to damage of the equipment via erosion, scaling
and corrosion or can influence and disturb the main reaction and processes
inside the systems. In order to avoid these problems and to predict the
influence of these compounds, it is vital to posses knowledge of the properties
of the most common inorganic compounds in supercritical water.
In continuation of earlier works of the authors, the solubilities of MgCl2
and CaCl2 are investigated via a continuous flow method in the range of
660 to 690 K and 18.5 to 23.5 MPa. Contrary to earlier experiments
with single-valent salts, precipitates were found during the experiments with
MgCl2 after cleaning the setup. These precipitates were analysed via EDX
and ATF-IR. In the course of the experiments, a decrease in pH of the
samples was investigated what was caused by a parallel hydrolysis reaction.
The solubilities of both investigated salts were corrected for the hydrolysis
reaction and correlated via a semi-empirical approach based on the phase
equilibrium between the present phases.
Keywords: Calcium chloride, Magnesium chloride, Solubility, Supercriti-
cal water
- 110 -
5.1 ∥ Introduction
5.1 Introduction
The design of industrial processes on supercritical fluids highly depend on the quality
and accuracy of the property data that are - if at all - available for the relevant systems.
While a broad range of systems of supercritical CO2 plus organic compounds of all kinds
have been investigated due to advantages that came along with the usage of supercritical
CO2, this is not the case for most other fluids (e.g. Propane, Methanol). The same
applies for supercritical water.
Despite the corrosiveness and mechanical stress that supercritical water represents to
equipment and material at elevated elevated temperature and pressure (Tc = 647K, pc =22.1MPa), it has been considered as a medium of choice for reactions, polymerization,
destruction of waste components, gasification of biomass and particle formation (1–5).
Yet, the limited amount of property data on systems consisting of supercritical water
and organic / inorganic compounds is noted (6).
In many systems undergoing supercritical water processing, salts and other inorganic
compounds are present to some degree. Examples for these systems are waste streams in
supercritical water oxidation, biomass and other fuels in supercritical water gasification,
and impurities in feed water streams (7; 8). Since water in its supercritical state loses
its polar character and thereby its ability to dissolve inorganic compounds in more than
minimal quantities, salts precipitate and start to form a solid phase. The presence of
such an additional phase can have a major influence on the process and cause unwanted
and unrecognized side effects in the process itself. Such operations can be effected in the
long term by corrosion and erosion of the equipment. The salts can also act as catalysts
(e.g. alkali salts in the water-gas shift reaction during the gasification of biomass (9))
and avoid coke and tar formation (e.g. coke and tar formation in gasification processes
(9)). Another possibility of the formation of an additional phase is the option to remove
this phase from the system (e.g. by gravity, by centrifugal forces) and thereby separate
the present salts from the remaining water (10).
To analyze the behavior of inorganic compounds and to enlarge the available property
data base, the authors have investigated the solubility of mono-valent alkali nitrates
(LiNO3, NaNO3, KNO3) and alkali chlorides (LiCl, NaCl, KCl) (11; 12). The focus
of this work is on the solubility of MgCl2 and CaCl2 to extend the available data to
bivalent salts and to continue the systematic investigation of solubilities in supercritical
water. These salts have been investigated in the range of 660 to 690 K and 18.5 to
23.5 MPa. Furthermore, the formation of hydroxides will be disccused, which took
- 111 -
Chapter 5 ∥ Magnesium Chloride and Calcium Chloride
place during the experiments. These results were correlated with an approach based on
a phase equilibrium between the present phases and compared to the previous works.
5.2 Experimental
The measurements of the solubilities was performed using a continous flow method. The
experimental setup and method has been described in detail elsewhere (11; 12); thus
only the most significant information are presented in the following.
The scheme of the experimental apparatus is shown in Figure 5.1, permitting mea-
surements up to 723 K and 25 MPa. Hastelloy is the material of choice for all heated
parts. The pressure in the system was established via a HPLC pump (LabAlliance Series
III, LabAlliance, USA), while a custom-made oven provided heat. An U-tube is installed
inside the oven with a length of 265 mm, an inner diameter of 4.6 mm and an outer
diameter of 6.35 mm. The temperature in the oven was measured at the inlet, at a
middle position and at the outlet via standard Type K thermocouples.
HPLC pump
Supply vessel
Preheater
Cooling
Back Pressure
Regulator Relief Valve
TI-1
Filter
2 m
Oven
Salt
column
Preheater
temperature
Oven Inlet
temperature
Temperature control
oven
Outlet
temperature
Pressure
Analysis
temperature
Analysis
and
samplingConduc-
tivity
measurement
Oven Outlet
temperature
Oven Center
temperature
TI-2
TI-3
TI-5 CI-1
PI-1
TI-4
TI-6
TC-1
Figure 5.1 ∥ Scheme of the experimental setup
Upon entering the U-tube, the feed stream can become supersaturated depending
on the temperature, pressure and feed concentration. If an oversaturation occurs, the
excess amount of salt will precipitate until the phase equilibrium between both phases
is established in the column. The exiting stream leaves the system in equilibrium and at
the solubility resulting from the temperature and pressure in the column. The stream
- 112 -
5.2 ∥ Experimental
is cooled down and depressurized to ambient conditions. Samples are taken when an
equilibrium state is verified via measurement of the conductivity of the outlet stream.
The analysis of the samples is done via an inductive coupled plasma atom emission spec-
trometer (ICP, Perkin-Elmer Optima 5300DV, Perkin-Elmer, USA, uncertainty < 2 %
for all investigated species) for the concentrations of Mg and Ca, ionic chromatography
(IC, Metrohm 741 Compact IC, Metrohm AG, Switzerland, uncertainty < 5 % for all
investigated species) was used for the chlorine concentration. The pH of all samples
was measured after the actual experimental run with a standard pH electrode (WTW
pH/Cond 340i/SET, WTW Wissenschaftlich-Technische Werkstatten GmbH, Germany,
uncertainty after calibration ± 0.01). For the calculation of the density, the outlet tem-
perature (TI-4 in Figure 5.1) and the pressure at the pressure sensor (PI-1 in Figure 5.1)
were used. The feed solution was prepared with deionized water and analytical grade
MgCl2 and CaCl2 (Boom B.V., The Netherlands).
5.2.1 Analysis of the precipitates
If precipitates were found in the column, samples of these precipitates were taken from
the inside of the U-tube. For optical analysis, the specimens of the solid material were air
dried, mounted on specimen studs, sputtered with a thin gold layer by using a sputtering
unit (Jeol JFC-1200, Jeol, Japan) and imaged with a scanning electron microscope
(Jeol JSM-6480LV, Jeol, Japan) at 6 kV . The SEM was combined with a energy
disperve X-ray spectroscopy unit (Noran System SIX, Thermo Fisher Scientific, USA).
In addition, images were taken via a standard light microscope (Leica MZ9.5 / DFC 320,
Leica Microsystems, Germany). Parts of these precipitates were found to have formed
bigger, loose particles in the bottom of the U-tube (cf. Figure 5.2). Other parts of the
precipitates were found on the walls of the tubing forming small clusters. Figure 5.2
shows a cross section of the U-tube where precipitates can be found attached to the wall
of the tubing. The clusters could easily be scrapped off the wall.
The typical EDX distribution for the precipitates can be found in Table 5.1. As can
be seen from the table, the samples consist mainly of magnesium and oxygen and only
contain traces of chloride. Other metals and carbon in the distribution result from the
surrounding tubing material which is made of Hastelloy.
Further analysis of the precipitates was performed via infrared spectrography (ATR-
FTIR, Model Shimadzu 8400S, measurement range between 400 to 4000 cm−1). The
result of the IR analysis of the sample can be found in Figure 5.3. As can be seen,
- 113 -
Chapter 5 ∥ Magnesium Chloride and Calcium Chloride
(a)
(b)
Figure 5.2 ∥ Precipitates from the bottom of the U-tube; image taken via light micro-
scope (a); Precipitates on the column wall; image taken via SEM (b)
one peak at about 3690 cm−1 can be clearly distinguished. This peak results from the
hydroxyl group of magnesium hydroxide (13; 14).
Combining the EDX and the IR results, it is to conclude that the formed particles
consist of magnesium hydroxide. This conclusion is also supported by the shape of the
crystals. Magnesium hydroxide tends to form hexagonal crystals which could also be
found in the particles found here (cf. Figure 5.4) (15). Particles consisting of MgCl2
can not be found at ambient state due to the high solubility in water at these conditions.
- 114 -
5.3 ∥ Results and Discussion
Table 5.1 ∥ EDX distribution
Element wt.-%
Carbon 23.00
Oxygen 43.83
Magnesium 23.06
Chlorine 0.34
Chrome 0.08
Manganese 0.01
Nickel 9.69
Figure 5.3 ∥ IR spectra of the precipitates
5.3 Results and Discussion
5.3.1 Correlation of the experimental data
For the interpretation of the experimental results, a description on base of a phase equi-
librium is chosen (16–19). Here, it is assumed that the solid phase and the supercritical
fluid form an equilibrium depending on the state of the system (cf. Figure 5.5). The
equilibrium between the solid and fluid phase can be formulated as follows:
- 115 -
Chapter 5 ∥ Magnesium Chloride and Calcium Chloride
Figure 5.4 ∥ Image of the hexagonal crystals of Mg(OH)2
a ⋅Mec ∗ m ⋅H2O(f) + b ⋅Xd ∗ p ⋅H2O(f)⇌ MeaXb ∗ n ⋅H2O(f) (5.1)
MeaXb ∗ n ⋅H2O(f) ⇌ MeaXb(s) + n ⋅H2O(f) (5.2)
Ô⇒Ks =α
MeaXb ∗ n⋅H2O(f)
αMeaXb(s)
⋅ αnH2O(f)
(5.3)
Here, Me and X represent the salt cation respectively the salt anion with a and b as the
number of ions in the salt molecule and c and d their valency. s and f refer to the phases
solid and fluid; n, m and p are the number of water molecules. The formation of the
solid phase is assumed to occur via the associated complex and not via dissociated ions.
The phase equilibrium constant Ks on base of the activities of the present species can
be simplified with several assumptions. The interaction between the presented species
is neglected while the activity coefficient of the solid salts is assumed as unity. The fluid
phase is assumed as an ideal one, thereby allowing the usage of the density of water as
concentration. More elaborated description of the assumptions and the efficiency of this
approach can be found be elsewhere (11; 16; 17). The assumptions lead to the following
expression for the solubility of an inorganic compound in supercritical water:
K∗s ≈
mMeaXb ∗ n⋅H2O(f)
1 ⋅ ρnm, H2O(f)
(5.4)
Ô⇒ mMeaXb ∗ n⋅H2O(f)
=K∗s ⋅ ρnm, H2O(f)
(5.5)
- 116 -
5.3 ∥ Results and Discussion
Figure 5.5 ∥ Equilibrium curve of NaCl in supercritical water (11)
This expression can be extended further by substituting the equilibrium constant under
usage of a van’t Hoff-like expression:
K∗s ≈
mMeaXb ∗ n⋅H2O(f)
1 ⋅ ρnm, H2O(f)
(5.6)
Ô⇒ mMeaXb ∗ n⋅H2O(f)
=K∗s ⋅ ρnm, H2O(f)
(5.7)
Ô⇒ log mMeaXb ∗ n⋅H2O
= logK∗s + n ⋅ log ρ
m, H2O(5.8)
= −∆solvH
R ⋅ T + ∆solvS
R+ n ⋅ log ρ
m, H2O(5.9)
R is the universal gas constant, T the system temperature, m the solution molality, ρ
the density. Ks respectively K∗s are the equilibrium constant and the equilibrium constant
including the simplifications. The Gibbs energy of solvation, ∆solvG, the enthalpy of
solvation, ∆solvH, and the entropy of solvation, ∆solvS are assumed as independent of
the system parameters temperature, pressure and density; n is later on referred to as
the coordination number. The molalities and densities are expressed on an amount of
substance base. The density of pure water is calculated via the IAPWS95 equation of
state (20). The experimental data were fitted to the parameters ∆solvH, ∆solvS and n.
More information on this approach can be found in the previous works of the authors
(11; 12).
- 117 -
Chapter 5 ∥ Magnesium Chloride and Calcium Chloride
5.3.2 Experimental results on Magnesium Chloride
The solubility of MgCl2 was investigated in the range of 660 to 670 K and 19 to 22.5
MPa.
Contrary to previous experiments (11; 12), precipitates were found inside the column
after rinsing and cleaning the setup. These precipitates are assumed to result from
a parallel hydrolysis reaction. Additionally, a drop in pH from appr. 6.7 for the feed
solution to between 2.5 and 2.9 depending on temperature and pressure was recorded
(cf. Table C.1).
Hydrolysis mechanism
The mechanism that is assumed to lead to the formation of the precipitates and to be
responsible for the pH decrease, is depicted below:
MgCl2 + 2 H2O ⇋ Mg(OH)2(s) ↓ +2 HCl (5.10)
(5.11)
The formed magnesium hydroxide precipitates and remains in the column while the
formed HCl remains in solution and leaves the U-tube, whereby the pH of the effluent
stream decreases. This is supported by the fact that all investigated samples had a
pH between 2.4 and 2.9 while the feed solution had a pH of 6.8. The occurrence of a
hydrolysis reaction was also found by other authors (17; 21).
Solubility of Magnesium Chloride
Resulting from the composition of MgCl2, the ratio between the magnesium and chlorine
concentration in the samples should be two. In Figure 5.6, the measured anion and cation
solubilities can be seen. While the anion concentration shows a steadily increasing trend
with increasing density, the cation concentration shows deviations from the expected
behavior as could be assumed after former investigations of other salts. Also, the ratio
between both concentrations is not two, but higher and does not have a constant value.
It is assumed that these deviations result from the parallel hydrolysis reaction.
In order to correct the measured solubilities for this parallel reaction, the following
correction for the magnesium concentration can be made via the formulation of a com-
ponent balance, the via the ICP measured magnesium concentrations and pH values and
Eq. 5.11:
- 118 -
5.3 ∥ Results and Discussion
Figure 5.6 ∥ Uncorrected results of the magnesium and chlorine composition as a
function of water density; ○, chlorine concentration; ▽, magnesium concentration, △,
double magnesium concentration
cH3O+ = cHCl = 2 ⋅ cMg(OH)2 (5.12)
ctotal(Mg) = cICP (Mg) + cHydrolysis(Mg)= cICP (Mg) + c(Mg(OH)2)= cICP (Mg) + 0.5 ⋅ c(HCl)= cICP (Mg) + 0.5 ⋅ 10−pH (5.13)
Figure 5.7 shows the original magnesium and chlorine concentration, the corrected
magnesium concentration and - in order to compare the ratios between the chlorine con-
centration and the corrected magnesium concentration - twice the corrected magnesium
concentration. As can be seen, the corrected magnesium concentration now shows a
consistent trend. Furthermore, the ratio between the corrected magnesium concentra-
tion and the chlorine concentration is approximately two for the samples presented here.
Therefore, a correction of the magnesium concentration for the hydrolysis reaction is
assumed as necessary to evaluate the experimental data.
The experimental results of MgCl2 including the corrections can be found in Figure
5.8. In order to correlate the experimental data with the equilibrium approach mentioned
above (cf. Eq. 6.3), the parameters ∆solvH, ∆solvS and n were fitted via a minimization
- 119 -
Chapter 5 ∥ Magnesium Chloride and Calcium Chloride
Figure 5.7 ∥ Corrected results of the magnesium and chlorine composition as a function
of water density; ○, chlorine concentration; ◻, magnesium concentration; △, corrected
magnesium concentration; ▽, double magnesium concentration
routine included in MATLAB to the experimental data. As can be seen, the experimental
data is in good agreement with the correlation.
The values for the three parameters ∆H, ∆S and n can be found in Table 5.2.
The experimental data including temperature and pressure as well as their standard
deviations, the density, the composition and the pH for the measurements on MgCl2
can be found in Table C.1.
5.3.3 Experimental results on Calcium Chloride
The solubility of CaCl2 was investigated in the range of 660 to 690 K and 18.5 to 23.5
MPa.
During the experiments with CaCl2, no precipitates were found after rinsing and cleaning
the setup in contrast to the experiments with MgCl2. The non-presence of any precipi-
tates of calcium compounds is assumed to result from the different solubility products as
discussed below. A drop in pH however from appr. 6.7 for the feed solution to between
3.4 and 4 was found indicating nevertheless the occurence of the parallel hydrolysis re-
action (cf. Tbl. C.2 and Fig. 5.9). Therefore, the experimental data of CaCl2 was
corrected in the same manner as described above for MgCl2.
- 120 -
5.3 ∥ Results and Discussion
Figure 5.8 ∥ Solubility of MgCl2 as a function of water density; ○, this work; solid line
represents the description of the experimental data with Eq. 6.3
Figure 5.9 ∥ Measured pH of the CaCl2 samples as a function of water density; ▽,
measured pH values
The presence of precipitates at ambient state after rinsing
Crystals of MgCl2 and CaCl2 were not found after the experiments due to their higher
solubilities at ambient conditions. For the presence of the products of the hydrolysis
- 121 -
Chapter 5 ∥ Magnesium Chloride and Calcium Chloride
Table 5.2 ∥Model parameters of the salts MgCl2 and CaCl2 for Eq. 5.9
Salt ∆H/J ⋅mol−1 ∆S/J ⋅mol−1 ⋅K−1 n / - Molecule radius (22) / 10−12m
MgCl2 2 -107.42 3.44 419
CaCl2 8436 -82.57 2.52 468
reaction, the solubility product of Mg(OH)2 and Ca(OH)2 at 298 K, 0.1 MPa can be
compared. Ca(OH)2 has a solubility product of 5.02 ⋅ 10−6, while the solubility product
of Mg(OH)2 is with 5.61 ⋅ 10−12, several orders of magnitude smaller (15). Therefore,
no particles of Ca(OH)2 can be found while Mg(OH)2 is still present at ambient state
after rinsing with water.
Solubility of Calcium chloride
The experimental results of CaCl2 including the corrections can be found in Figure 5.10.
As can be seen, the experimental data is in good agreement with the correlation.
The values for the three parameters ∆H, ∆S and n can be found in Table 5.2. The
experimental data including temperature and pressure as well as their respective standard
deviations, the density, the composition and the pH for the measurements on CaCl2 can
be found in Table C.2.
5.3.4 Possible correlation between salt properties and model pa-
rameters
As described in the previous work of the authors (12), a correlation between the parame-
ters derived from Eq. 5.9 and the radius of the molecules is investigated. The radii of the
molecules is calculated as the sum of the corresponding crystal radii. The parameters for
three alkali nitrates (LiNO3, NaNO3, KNO3) respectively chlorides (LiCl, NaCl, KCl)
plus the parameters of three additional monovalent salts (CuO,PbO,KOH) combined
with the parameters presented in the section before are used for investigating this pos-
sible correlation. The parameters for CuO,PbO and KOH were derived from property
data available in literature (19; 23; 24). The parameters for these salts and the alkali
chloride and nitrate salts can be found in Table 5.3 (12). The radii of the salt molecules
were obtained from the corresponding crystal radii and can be found in Table 5.3 (22).
The figures 5.11, 5.12 and 5.13 contain the respective parameter of the salts as a
- 122 -
5.3 ∥ Results and Discussion
Figure 5.10 ∥ Solubility of CaCl2 as a function of density; ○, this work; dashed line
represents the description of the experimental data with Eq. 6.3
function of the radius. Two classes of salts can be distinguished for the parameter n
depending on their bonding character (12). The results for ∆H and ∆S do not allow
such a clear conclusion due to experimental errors and the limited range of experimental
data. Yet, the authors assume a comparable correlation between these parameters and
the property of the respective salt molecule.
Table 5.3 ∥Model parameters of the salts CuO, PbO, and KOH for Eq. 5.9 (12)
Salt ∆H/J ⋅mol−1 ∆S/J ⋅mol−1 ⋅K−1 n / - Molecule radius (22) / 10−12m
CuO 23749 -103.68 1.34 238
PbO 27901 -57.97 1.97 197
KOH 13369 -81.10 3.24 277
LiCl 5199 -69.61 2.48 240
NaCl 18784 -86.74 4.88 280
KCl 13123 -95.66 4.65 318
LiNO3 15594 -81.91 4.33 352
NaNO3 5149 -93.06 3.93 392
KNO3 -7793 -111.91 3.72 430
- 123 -
Chapter 5 ∥ Magnesium Chloride and Calcium Chloride
Figure 5.11 ∥∆H (cf. Eq. 5.9) as a function of the molecule radius; ○, CuO; ▽, PbO;
◇, LiCl; ◻, KOH; △, NaCl; 7, KCl; ⊕, LiNO3; ⋆, NaNO3; +, MgCl2; ⊞, KNO3; |,
CaCl2
Figure 5.12 ∥∆S (cf. Eq. 5.9) as a function of the molecule radius; ○, CuO; ▽, PbO;
◇, LiCl; ◻, KOH; △, NaCl; 7, KCl; ⊕, LiNO3; ⋆, NaNO3; +, MgCl2; ⊞, KNO3; |,
CaCl2
- 124 -
5.4 ∥ Conclusions
Figure 5.13 ∥ n (cf. Eq. 5.9) as a function of the molecule radius; ○, CuO; ▽, PbO;
◇, LiCl; ◻, KOH; △, NaCl; 7, KCl; ⊕, LiNO3; ⋆, NaNO3; +, MgCl2; ⊞, KNO3; |,
CaCl2
5.4 Conclusions
In the work presented here, the solubilities of MgCl2 and CaCl2 were studied in de-
pendence of the parameters density, temperature and pressure. The investigated range
was 660 to 690 K and 18.5 to 23.5 MPa. The measurements were performed using a
continuous flow method.
For all experiments, a decrease in pH was found that was caused by a parallel hydrol-
ysis reaction. An approach to correct this parallel reaction was presented and applied
successfully to interpret the experimental results. For the experiments with MgCl2, pre-
cipitates were found after rinsing the setup in contrary to any former experiments. These
precipitates were to found to be consisting of Mg(OH)2, thereby proving the occurrence
of the parallel hydrolysis reaction. The reason for the presence of these precipitates at
ambient state after rinsing the setup in comparison to any other salt investigated so far
is assumed to result from the low solubility product of Mg(OH)2.
The corrected experimental results could be correlated in good agreement with Eq.
5.9. The parameters derived from this correlation agree to the trends for these param-
eters presented in the earlier studies of the authors, where a dependency between the
parameters and the radius of the salt molecule was found (12).
- 125 -
Chapter 5 ∥ Magnesium Chloride and Calcium Chloride
Further it is to conclude that parallel reactions like presented in this work have to be
kept in mind for further investigations and applications. Although this might be consid-
ered as a minor problem for certain systems with less severe hydrolysis and thereby lower
pH variations like NaCl, it can lead to errors in measurement and evaluation of solubil-
ities if not addressed properly. Also, the presence of precipitates even at ambient state
must be taken into account in order to avoid damage to the equipment and disturbance
of measurements and system behavior.
Acknowledgements
The authors would like to thank Thibaut Garcia de Changy for his contribution to the
experimental part of this work. Additionally the authors would like to thank Kamuran
Yasadi, Arie Zwijnenburg, Janneke Tempel and Jelmer Dijkstra for their contribution in
the analysis of the samples.
This work was performed in the TTIW-cooperation framework of Wetsus, centre of
excellence for sustainable water technology (www.wetsus.nl). Wetsus is funded by the
Dutch Ministry of Economic Affairs, the European Union Regional Development Fund,
the Province of Fryslan, the City of Leeuwarden, and the EZ/Kompas program of the
’Samenwerkingsverband Noord-Nederland’. The authors like to thank the participants
of the research theme Salt for their financial support.
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5.5 ∥ References
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