University of Groningen Removal of inorganic compounds via supercritical water Leusbrock, Ingo IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below. Document Version Publisher's PDF, also known as Version of record Publication date: 2011 Link to publication in University of Groningen/UMCG research database Citation for published version (APA): Leusbrock, I. (2011). Removal of inorganic compounds via supercritical water: fundamentals and applications [Groningen]: Rijksuniversiteit Groningen Copyright Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons). Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. Download date: 27-05-2018
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University of Groningen
Removal of inorganic compounds via supercritical waterLeusbrock, Ingo
IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.
Document VersionPublisher's PDF, also known as Version of record
Publication date:2011
Link to publication in University of Groningen/UMCG research database
Citation for published version (APA):Leusbrock, I. (2011). Removal of inorganic compounds via supercritical water: fundamentals andapplications [Groningen]: Rijksuniversiteit Groningen
CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).
Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.
Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.
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
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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
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
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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-
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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