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International Journal of Greenhouse Gas Control 104 (2021) 103205
Available online 23 November 2020
1750-5836/© 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Measurement and prediction of oxygen solubility in post-combustion CO
2
capture solvents
Vanja Buvik
a
, Ida M. Bernhardsen
a
, Roberta V. Figueiredo
b
, Solrun J. Vevelstad
c
,
Earl Goetheer
b
, Peter van Os
b
, Hanna K. Knuutila
a
,
*
a
Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), NO-7491 Trondheim, Norway
b
TNO, Leeghwaterstraat 44, 2628 CA Delft, the Netherlands
c
SINTEF Industry, NO-7465 Trondheim, Norway
ARTICLE INFO
Keywords:
Oxygen solubility
Oxidative degradation
CO
2
absorption
Amine solvents
Dissolved oxygen sensor
Oxygen removal
ABSTRACT
This work aims to understand oxygen solubility in pure and aqueous amine solvents for CO
2
capture.
Commercially available dissolved oxygen sensors were studied to evaluate whether these can be used for
measuring oxygen solubility in the carbon capture processes. It also aims to understand the possible discrep-
ancies from realistic concentrations of oxygen when using a dissolved oxygen sensor. Two independent mea-
surement principles were used for this purpose, both electrochemical and optical. Furthermore, a Winkler
titration method was used to aid the validation of the sensors as well as understanding salting-out effects. A
simple model for predicting oxygen solubility in CO
2
-loaded ethanolamine solutions was made, which also has
potential for predicting oxygen solubility in other loaded amine solutions.
The results of the study show that dissolved oxygen sensors may be applied for measurement of oxygen
concentrations in amine solutions and that different amines and different concentrations in water only show
small variations in oxygen solubility. The sensors may also be used in CO
2
-loaded amine solutions, but here the
increased conductivity of the solution may give a higher measured concentration of oxygen, than it is in reality.
In ethanolamine, the consumption of oxygen is faster than the mass transfer of oxygen from gas to liquid phase,
giving lower concentrations of oxygen than it should be in absence of a chemical reaction between oxygen and
amine.
1. Introduction
The capture and storage of CO
2
from large emission sources (CCS)
has to play a key role for reaching the target of not exceeding 1.5 ◦C
increase of global average temperatures, concludes the Intergovern-
mental Panel on Climate Change (IPCC) in their report from 2018
(Rogelj et al., 2018). CCS allows for carbon (in form of CO
2
), that would
otherwise be released to the atmosphere and contribute to global
warming, to be returned underground to safe and permanent storage.
There are many studied technologies for CO
2
capture and of those, ue
gas scrubbing with liquid amine solvents is one of the most mature
technologies (Kohl and Nielsen, 1997; Leung et al., 2014; Rochelle,
2009). Liquids have inherent gas absorption properties and can physi-
cally absorb gases to some extent (Battino and Clever, 1966). Solvents
with amine functions are, however, also chemically reacting with some
gases, among these CO
2
. The amines can more selectively, and in higher
concentrations than physical gas absorption, bind the gas molecules. In
this process, a solvent reversibly binds CO
2
at low temperatures in an
absorber column and is released at high temperatures in a desorber
column.
Because the reaction is reversible, the amine solution is circulated
and reused continuously. The harsh operational conditions to which the
solution is subjected; contact time with all ue gas components and
construction material, as well as high temperatures in the desorber, can
lead to amine degradation over time (Gouedard et al., 2012; Mazari
et al., 2015; Meisen and Shuai, 1997; Reynolds et al., 2016). Oxidative
degradation, the degradation that occurs in the presence of oxygen is a
complex problem that can lead to corrosion, solvent and equipment
replacement costs, and interruption of operation time (Dhingra et al.,
2017; Goff and Rochelle, 2004; Rieder et al., 2017). For oxidative
degradation reactions to take place they require presence of oxygen
(O
2
), and the main source for oxygen is the gas phase molecular oxygen
* Corresponding author.
E-mail address: hanna.knuutila@ntnu.no (H.K. Knuutila).
Contents lists available at ScienceDirect
International Journal of Greenhouse Gas Control
journal homepage: www.elsevier.com/locate/ijggc
https://doi.org/10.1016/j.ijggc.2020.103205
Received 3 March 2020; Received in revised form 28 October 2020; Accepted 3 November 2020
International Journal of Greenhouse Gas Control 104 (2021) 103205
2
present in the surrounding air or in the ue gas. It is commonly assumed
that most of the oxidative degradation reactions take place in the
absorber column where the oxygen concentration is the highest, because
this is where the solvent is in contact with the ue gas and the tem-
perature is the lowest in the process (da Silva et al., 2012).
Laboratory experiments, where oxygen rich gas has been bubbled
through aqueous amine solutions, have shown that there is a correlation
between the amount of oxygen and the amount of oxidative degradation
observed; increasing the oxygen pressure leads to increased rate of
oxidative degradation (Supap et al., 2001; Vevelstad et al., 2016).
Further, post combustion CO
2
capture pilot studies have shown that the
loss of amine, caused by degradation into other compounds, increases
linearly with the concentration of oxygen in the treated ue gas
(L´
eonard et al., 2015). As a means of avoiding or limiting the extent of
oxidative degradation, addition of oxygen scavenging compounds is a
proven and commercially available method of reducing oxidative
degradation in carbon capture plants (Fytianos et al., 2016; L´
eonard
et al., 2014; Supap et al., 2011; Veldman and Trahan, 1997). These
scavengers react stoichiometrically with dissolved oxygen and reduce
the occurrence of oxidative degradation in the amine solution. However,
another issue arises when the scavenger molecules are used up and
removal of used-up scavenger, as well as addition of fresh scavenger has
to be performed (L´
eonard et al., 2014). Together with this, undesired
side effects such as foaming and cross-reactions with the solvent or other
products can also occur with the direct contact of the solvent with the
scavenger, as has been observed with corrosion inhibitors and other
additives (Chen et al., 2011; Thitakamol and Veawab, 2008). To simplify
operation of the carbon capture plant, a simpler way of eliminating
molecular oxygen and thereby avoiding oxidative degradation would be
preferred.
Because experimental observations show that oxygen pressure plays
an important role in degradation, oxygen solubility is also a parameter
in models attempting to predict oxidative solvent degradation (Pinto
et al., 2014) under the assumption that its solubility in amines is similar
to that of water.
The solubility of oxygen and other gases in non-reactive liquids is an
inherent property and it depends on partial pressure of the gas and the
temperature, as described by Henry’s law given in Eq. 1
Hcp =Cl∙R∙T
Cg
(1)
where Henry’s law constant (Hcp) is correlated to the liquid-phase (Cl)
and gas phase concentrations (Cg) of the gas component (in this case O
2
),
as well as the ideal gas constant (R) and temperature (T) (Henry, 1832).
O2(g)⇄O2(aq)(2)
CO2(g)⇄CO2(aq)(3)
CO2(aq) + H2O(l) + MEA(aq)⇄HCO3−(aq) + MEAH+(aq)(4)
HCO3−(aq) + MEA (aq)⇄MEACO2−(aq) + H2O(l)(5)
In addition to the physical solubility of gases (Eq. 2 and 3), CO
2
will
chemically react with the amine in the solution. Depending on the type
of amine, different reaction products are formed. For instance, a primary
amine like ethanolamine (MEA) forms carbamate and protonated MEA
(MEACO
2
−
and MEAH
+
, Eq. 4 and 5) when reacting with CO
2
, while
tertiary amines form (bi)carbonate and protonated amine species
(Danckwerts, 1979; Puxty and Maeder, 2016). Bicarbonate (HCO3−, Eq.
4) is also formed in reaction with water. Carbamates, as well as bicar-
bonate, are ionic species which will change the properties of the solution
when formed from non-ionic compounds, a factor that inuences oxygen
solubility, known as a “salting in” or “salting out” effect (Schumpe et al.,
1978).
The existing techniques for the quantication of dissolved oxygen
include Winkler titration (Montgomery et al., 1964; Winkler, 1888), by
means of gas chromatography in a molecular sieve column with thermal
conductivity detection (GC-TCD) (Park and Catalfomo, 1964) and using
electrochemical (polarographic) dissolved oxygen sensors. The titration
method described by Winkler in 1888 is very accurate for aqueous
samples with low or no alkalinity but is not directly applicable for the
titration of amines. Quantication using an electrochemical sensor is a
highly desirable method for industrial applications (Rooney and Daniels,
1998; Wang et al., 2013); it offers a fast, cheap and direct measurement
that can easily be coupled online in the gas sweetening process. One of
the challenges of the dissolved oxygen sensors is however the ionic
strength of the solutions, which increases the conductivity and thereby
enhances the electrochemical signal perceived by the sensor. The effect
that the increased ionic strength has on the dissolved oxygen sensors is
experimentally investigated in this work and compared to the pre-
dictions of the Schumpe model for gas solubility in aqueous electrolytes.
Membrane transport of liquid or gas phase oxygen to the electrode of the
sensor through the oxygen selective membrane of the electrochemical
sensor is also studied in this work, to see if the presence of CO
2
impedes
oxygen transport to the electrodes, as this type of membrane also is
permeable to other small gas molecules (Bhattacharya and Hwang,
1997).
There are three main types of dissolved oxygen sensors, all designed
for quantication of dissolved oxygen in water; polarographic, galvanic
and optical. Polarographic, or “Clark” sensors (Clark, 1959), based on
the same working principle as galvanic dissolved oxygen sensors, being
selective reduction of O
2
, but the galvanic type has a faster response
time. The third type of dissolved oxygen sensor is optical, which relies
on an oxygen-sensitive uorescent dye, a light emitting diode and a
photodetector to measure oxygen concentration in the solution. In this
study sensors of both the galvanic, electrochemical and optical type
have been used and compared.
The need for quantifying dissolved oxygen is not solely interesting
for degradation modelling purposes mentioned above, but also for the
development of oxygen removal technologies (Monteiro et al., 2018),
where oxygen concentration needs to be quantied both before and after
removal. The ideal analysis method should be direct (no sample pro-
cessing), both decreasing the amount of work and sources of error. As
already described, the solubility of oxygen is dependent on partial
pressure and temperature, so that even small changes in either of these
parameters, which is likely to happen during sample processing, could
inuence the measurement greatly. A further challenge for the mea-
surements is the very low concentration range in which oxygen can be
present. Any measurement method for dissolved oxygen needs to be
sensitive enough to detect and quantify concentrations of oxygen in the
low ppm-range (<8 ppm).
This work has studied if different parameters, such as amine struc-
ture and concentration, as well as CO
2
loading, inuence oxygen solu-
bility. The methodological emphasis in this work was on
electrochemical and optical dissolved oxygen sensors, because of their
simple operation and potential for online measurements. Additionally, a
promising modelling approach was used to predict oxygen solubility in
CO
2
loaded amine solutions. The results of this study deepen the un-
derstanding of the extend of oxygen solubility in CO
2
loaded and
unloaded amine solutions and how oxygen concentrations can be
measured. The results also indicate that oxygen mass transfer is a
limiting factor in the oxidative degradation of amines. The results of the
study are an addition to the current understanding of oxidative degra-
dation in amine-based CO
2
-removal, as well as giving valuable infor-
mation to those wanting to measure and understand oxygen solubility in
the CO
2
capture process.
V. Buvik et al.
International Journal of Greenhouse Gas Control 104 (2021) 103205
3
2. Materials and methods
2.1. Chemicals
Pure oxygen (O
2
, N5.0) and carbon dioxide (CO
2
, N5.0) gas were
obtained from AGA and Linde Gas. and compressed air from in-house air
compression systems at NTNU and TNO. Deionized water was obtained
from local water deionization systems at NTNU and TNO. A further
overview of chemicals used can be found in Table 1. All solutions were
prepared gravimetrically.
2.2. Dissolved oxygen (DO) sensors
The solubility of oxygen was measured using two different electro-
chemical dissolved oxygen sensors and one optical oxygen sensor, all
designed for measurement of oxygen concentrations in water. The
electrochemical dissolved oxygen sensors used are based on galvanic
probes that give a measurable current proportional to the chemical
reduction of O
2
on a cathode. The two different electrochemical sensors
were used in this work were a HI-5421 Dissolved Oxygen and BOD Meter
from Hanna Instruments, with a HI76483 Clark-Type polarographic
probe and a handheld pHenomenal® OX 4100 H dissolved oxygen meter
with a pHenomenal® OXY-11 polarographic probe, from VWR. A redox
reaction gives a measurable current which directly correlates to the
oxygen concentration in the solution. Effectively, the dissolved oxygen
sensors measure the activity of O
2
in the solution which under ion-free
conditions equals the concentration of O
2
. Since salinity inuences the
activity coefcient of O
2
, the sensors are provided with a correction for
sodium chloride (NaCl) salinity. This correction factor is not applicable
for other salts, as all ions have different salting-in or -out effects
(Schumpe et al., 1978).
The optical dissolved oxygen sensor was a Memosens COS81D from
Endress+Hauser. This was mainly used to validate the galvanic sensor
working principle (electrochemical), to prove that a different principle
(uorescence quenching) also measures the same concentrations of ox-
ygen. Each experimental section species which sensors have been used
for the measurement and further details on their working principles are
given in Appendix A.
2.3. Methods
Schematics of the experimental setups used in the various experi-
ments are depicted in Fig. 1.
2.3.1. Experimental setup A: Oxygen solubility at ambient O
2
partial
pressure
A 300 mL double-jacketed glass reactor (Fig. 1A), which was con-
nected to a circulating combined heating and cooling bath, was lled
with approximately 200 mL of the solution, which was cooled or heated
to the experiments’ starting temperature. When the desired temperature
was achieved, compressed air was bubbled through the solution through
a sintered gas dispersion tube, under magnetic stirring for at least 10
min. The gas dispersion tube was thereafter removed, for gas bubbles
not to disturb the measurement. The magnetic stirring was maintained
throughout the experiment, to ensure circulation of the liquid, main-
taining the measured concentration around the probe head constant.
The concentration of dissolved oxygen was recorded after the temper-
ature stabilisation for each measurement point. Further measurements
of dissolved oxygen were made at stable temperatures up to the upper
operational temperature of the sensors.
2.3.2. Experimental setup B: Inuence of CO
2
loading on oxygen solubility
A 300 mL double-jacketed glass reactor (Fig. 1B), connected to a
circulating combined heating and cooling bath, was lled with
approximately 200 mL of the solvent. A mixture of CO
2
and O
2
, added
through two mass ow controllers and subsequently mixed in a stainless
steel tube, was suspended over the surface of the solution. A thermom-
eter was inserted into the solution, the water bath adjusted to the desired
temperature and temperature stability was awaited under a pure O
2
atmosphere, which was supplied through the tube suspended over the
liquid surface. Once temperature stability in the liquid was reached, the
dissolved oxygen sensor(s) was (were) inserted to the solution. A liquid
sample for total inorganic carbon (TIC) analysis (section 2.6) was taken
and simultaneously the rst measurement of dissolved oxygen recorded.
The gas ow from the tube above the liquid surface was adjusted to
contain the desired ratios of CO
2
and O
2
and was kept constant
throughout the whole experiment. A layer of paralm was used to make
a partial cover of the reactor, to assure that the gas phase over the liquid
surface always contained the desired partial pressures of CO
2
and O
2
.
Table 1
Short and trivial name, as well as structure, CAS number and key features of the chemicals used in the experiments.
IUPAC name Abbreviation Chemical structure CAS Purity Supplier Features
phenylmethanamine benzylamine
100−46-9 99 % Aldrich Primary aromatic amine
propane-1,2-diamine 1,2-DAP 78−90-0 99 % Aldrich Diamine
2-(2-aminoethoxy)ethanol DGA 929−06-6 98 % Acros Organics Ether of a primary alkanolamine
2-(dimethylamino)ethanol DMMEA 108−01-0 ≥99.5 % Aldrich Tertiary alkanolamine
3-(methylamino)propane-1,2-diol MAPD 40137−22-2 97% Aldrich Secondary alkane-diolamine
2,2’-(methylazanediyl)di(ethan-1-ol) MDEA 105−59-9 ≥99 % Aldrich Tertiary alkanolamine
2-aminoethan-1-ol MEA 141−43-5 ≥99.0 % Sigma-Aldrich Primary alkanolamine
ethane-1,2-diol MEG 107−21-1 ≥99.8 % Sigma-Aldrich Diol
2-(methyl)aminoethanol MMEA 109−83-1 ≥98 % Aldrich Secondary alkanolamine
V. Buvik et al.
International Journal of Greenhouse Gas Control 104 (2021) 103205
4
Liquid samples for TIC analysis were taken simultaneously as oxygen
concentration recordings were made, approximately every 20 or 30 min.
2.4. Comparison of a galvanic and an optical dissolved oxygen sensor
Experimental setup A (Fig. 1A) was used for the following experi-
ments with MEA, where oxygen solubility was measured with the
galvanic VWR pHenomenal® and the optical Endress+Hauser COS81D
dissolved oxygen sensors at room temperature with low loadings of 0.03
and 0.1 mol
CO2
mol
MEA
−1
. The solutions were sparged with compressed air
before commencing the measurement of their oxygen concentrations,
for reaching oxygen saturation at ambient pressure faster. Since MEA is
commonly known as an unstable amine which degrades rapidly oxida-
tively, a very stable tertiary amine, MDEA, was also investigated. This
was to see if a polarization of the oxygen selective membrane to inhibit
oxygen permeation could occur with CO
2
, which would be an opera-
tional problem with CO
2
present in the solutions. This experiment would
also show if degradation rates can be assessed with the commercial
sensors. A solution of 30 wt% MDEA (aq.) with a loading of 0.4 mol of
CO
2
per mol MDEA was studied using experimental setup A (section
2.3.1). The solution was sparged with compressed air for 15 min after
reaching temperature stability at 20 ◦C. Before measuring the oxygen
concentration, the gas dispersion tube was removed, and the two sensors
and a thermometer were inserted into the liquid. Temperature and ox-
ygen concentration stability was awaited, before the rst concentration
and temperature point were noted. The liquid was slowly heated up and
temperature and oxygen concentration measured by the two individual
sensors were recorded regularly. Because of the high oxygen consump-
tion rate, the same experiment could not be performed with MEA with
CO
2
loading.
2.5. Study of possible CO
2
effects on dissolved oxygen sensors
2.5.1. Solubility of oxygen in water using three different gas phase
compositions
Experimental setup B (Fig. 1B) was lled with deionized water and
partly closed using paralm. The water was then sparged with a specic
gas composition, provided by two mass ow controllers, for 10−15 min
at 20 ◦C, the temperature being kept constant at 20 ◦C using the com-
bined heating and cooling bath. The gas distribution tube was then
Fig. 1. Scheme for experimental setups A, B and C. Setup A and B are temperature controlled in a double-jecketed reactor, whereas C is only operated at room
temperature. The gas distribution tube in setup B is moveable and maintained over the liquid surface during the measurment. Both setup A and B use magntic stirring
for mixing, whilst setup C relies on a constant ow of gas though the reactor, eliminating the need for futher agitation.
V. Buvik et al.
International Journal of Greenhouse Gas Control 104 (2021) 103205
5
placed above the liquid surface with gas still being distributed into the
gas phase of the partly closed system and the Endress+Hauser COS81D
dissolved oxygen sensor submerged in the liquid. Stability of the signal
was awaited for 10−15 min and the temperature and concentration of
oxygen measurement was noted. Three gas mixtures were studied using
this setup; N
2
with air, CO
2
with air and pure air.
2.5.2. Gas phase oxygen measurement with the optical sensor
In experimental setup C (Fig. 1C), the 1.3 L glass reactor was added
gas mixtures of different compositions, which were investigated using
the Endress+Hauser COS81D dissolved oxygen sensor. Gas ows were
controlled using two Bronkhorst® mass ow controllers, which supplied
the gas at the top of the reactor and could escape through a small
opening in the bottom of the reactor. The constant ow of the gas
through the reactor ensured agitation around the sensor head, ensuring
reliable oxygen solubility measurements. Stability of the signal was
awaited for 10−15 min and the temperature and concentration of oxy-
gen measurement was noted. Three compositions of air, CO
2
and N
2
were investigated, all at a room temperature of about 20 ◦C.
2.6. Analytical methods
A Shimadzu TOC-L
CPH
in Total Inorganic Carbon (TIC) mode was
used for the determination of the amount of CO
2
(as the only inorganic
carbon species) in the solution. Amine titration with H
2
SO
4
determined
the exact amine concentration and from this, the exact CO
2
loading (
α
)
of the solution was determined (Ma’mun et al., 2006). All samples were
analysed twice, yielding a relative deviation of ≤2% for both methods,
both comparing two parallel analyses and analysing standards of known
concentrations.
Winkler titrations were performed using a HI-3810 Chemical test
Dissolved Oxygen kit from Hanna Instruments. The quantication of
dissolved oxygen relies on the reactions given in Eqs. 6–8.
2Mn2++O2+4OH−→ 2MnO2↓+2H2O→2MnO(OH)2(6)
MnO(OH)2+2I−+4H+→I2+Mn2++3H2O(7)
I2+2S3O2−
3→2S4O2−
6+2I−(8)
An Agilent 7890A GC–MS was used in Electron Spray Impact ioni-
zation mode (ESI) with an Equity™ – 1701 Fused silica capillary column
(30 m ×0.25 mm lm thickness) and helium as the carrier gas. The
quantication of dissolved oxygen was performed by a triple quadrupole
mass spectrometer, by Single Ion Monitoring (SIM) of the fragment and
molecular ions of oxygen, m/z =16 and m/z =32. These ions appear in
the very start of the chromatogram, before any other compounds elute.
2.7. Modelling
The solubility of O
2
in CO
2
-loaded and unloaded 30 wt% MEA so-
lutions and concentrated NaCl solutions was estimated using the model
of Weisenberger and Schumpe (1996). The model is suitable for pre-
dicting gas solubility into electrolyte solutions with concentrations up to
2–5 kmol m
−3
and has been widely used in the literature. (Chatenet
et al., 2000; Haug et al., 2017; Knuutila et al., 2010). The model
expression is given in Eq. 9.
log(CG,0
CG)=Σ(hi+-hG)ci(9)
In the equation C
G,0
and C
G
are the gas solubility in water and in the
electrolyte solution, respectively, h
i
is the ion-specic parameter, h
G
is
the gas-specic parameter and c
i
is the concentration the ion. The
temperature dependence of the gas-specic parameter is given in Eq. 10.
hG=hG,0+hT(T−298.15 K)(10)
here h
G,0
is a gas-specic parameter, h
T
is a gas-specic parameter for
the temperature effect and T is temperature.
In this work, the solubility of O
2
in water, CO2,0 with unit mol dm
−3
,
was determined using the correlation given in Eq. 11 proposed by Xing
et al. (2014). In the equation pO2 is the partial pressure of O
2
above the
solution,
CO2,0=55.56pO2
exp(3.71814 +5596.17
T−1049668
T2)−pO2
(11)
The ion concentrations in CO
2
loaded 30 wt% MEA (aq.) solutions
were determined from NMR speciation data reported by B¨
ottinger et al.
(2008) and density data needed to convert mole fractions to molar
concentrations were taken from Hartono et al. (2014).
The gas-specic parameter for oxygen in Eq. 9 are reported in liter-
ature. However, since the ion-specic parameters for protonated MEA
(hMEAH+) and carbamate (hMEACOO−) were unknown, they were deter-
mined by tting Eq. 9 to experimental nitrous oxide (N
2
O) solubility
data for CO
2
loaded 30 wt% MEA (aq.) solutions at 40 ◦C reported by
Hartono et al. (2014). Since N
2
O does not react chemically with the
amine, it offers data for only the physical absorption in the liquid phase,
which is then used to calculate the ion specic parameters. The Henry’s
law constant for N
2
O in water (H
N2O,W
) was determined using the cor-
relation given in Eq. 12, provided by Penttil¨
a et al. (2011), and the
partition coefcient expressed in Eq. 13 was calculated to be used as the
C
G,0
value for N
2
O at 40 ◦C.
HN2O,W=exp(158.245 - 9048.596
T- 20.860lnT- 0.00252T)(12)
m=RT
H(13)
3. Results and discussion
3.1. Schumpe model and parameter tting for estimation of oxygen
solubility
The Schumpe model was used to represent the solubility of oxygen
into water and salt solutions using parameters from literature. However,
the model can also be used to predict oxygen solubility of aqueous amine
solutions, using data for physical solubility of N
2
O and speciation in CO
2
loaded solution. A theoretical prediction of oxygen solubility in loaded
amine solutions, based on the effects of the ionic species in the solution
has not yet been made for MEA. Using the literature and tted ion
specic constants given in Table 2, the predicted shown in Fig. 2 were
calculated, proving that the model can accurately represent the N
2
O
solubility in loaded 30 wt% MEA. The constants needed for calculating
the O
2
and N
2
O solubility into CO
2
loaded 30 wt% MEA (aq.) solutions
and in concentrated NaCl solutions are given in Fig. 2. The performance
of the model to predict O
2
-solubility using the tted parameters will be
Table 2
Parameters for Eqs. 1 and 2. The h
T,i
value for O
2
is valid from 273 K to 353 K and
that of N
2
O is valid from 273 K to 313 K.
Parameter Unit Value Reference
hMEAH+m
3
kmol
−1
0.0133 This work
hMEACOO−m
3
kmol
−1
0.1284 This work
hHCO−
3 m
3
kmol
−1
0.0967 (Weisenberger and Schumpe, 1996)
hNa+m
3
kmol
−1
0.1143 (Weisenberger and Schumpe, 1996)
hCl−m
3
kmol
−1
0.0318 (Weisenberger and Schumpe, 1996)
hG,0,N2O m
3
kmol
−1
−0.0085 (Weisenberger and Schumpe, 1996)
hG,0,O2 m
3
kmol
−1
0 (Weisenberger and Schumpe, 1996)
hT,O2 m
3
kmol
−1
K
−1
−0.000334 (Weisenberger and Schumpe, 1996)
hT,N2O m
3
kmol
−1
K
−1
−0.000479 (Weisenberger and Schumpe, 1996)
V. Buvik et al.
International Journal of Greenhouse Gas Control 104 (2021) 103205
6
discussed in section 3.4.3 and Fig. 6.
3.2. Validation of the oxygen sensors with CO
2
unloaded solutions
Solubility of oxygen was measured in water (results in Appendix) and
in 30 wt% (aq.) MEA by the three dissolved oxygen sensors was
compared to available literature data. The results of this validation
experiment can be found in Fig. 3. In MEA 30 wt% (aq.), the maximum
absolute deviation between the dissolved oxygen sensors in this work
was ±0.6 mg L
-1
(13 %, in the higher temperature range) and the
maximum absolute deviation comparing the measured oxygen solubility
to literature values from Wang et al. was 1.4 mg L
−1
(13 %, in the higher
temperature range). It should be noted that Rooney and Daniels (1998)
and Wang et al. (2013) used polarographic dissolved oxygen probes for
their dissolved oxygen measurements. Polarographic dissolved oxygen
sensors use the same principle of detection as galvanic probes, therefore,
the same oxygen selective cathode reactions apply.
All oxygen selective membranes are also selective towards CO
2
, the
effect of varying the gas phase composition between O
2
, N
2
and CO
2
was
studied in both water (described in section 2.5.1, results in Supple-
mentary table 4) and gas phase (described in section 2.5.2, results in
Supplementary table 5). No signicant difference could be observed
between measurements in mixtures of air and N
2
, and air and CO
2
.
Replacing half the air with either of the two other gases, gave half the
concentration of oxygen measured with only air, although a small dif-
ference can be seen in the gas phase experiment, which can potentially
be explained either by the sensor being slightly less accurate in air, or
the experimental setup not being ideal for this type of experiment. Either
way, dissolved or chemically bound CO
2
does not appear to inuence
the dissolved oxygen sensor.
Overall, the validation experiments and comparison with literature
data for oxygen solubility in amine solutions and water, measured with
comparable sensors, show a good agreement. This means that the sen-
sors can be assumed to work in the same manner as other sensors from
other brands and with slightly different working principles.
3.3. Winkler titration and GC–MS analyses
Oxygen solubility in selected solutions was also measured using
Winkler titration. The quantication principles of dissolved oxygen
using an electrochemical sensor and by performing a Winkler titration
are different, but both are species-specic, measuring only dissolved
oxygen. The electrochemical sensor gives a signal based on the current
created from the cathodic reduction of O
2
to H
2
O., while the titration
relies on reduction of O
2
to form a MnO
2
salt. These independent
methods show the same concentration of oxygen in pure water, but upon
addition of salt, they deviate from one another. As Winkler titration
method is not suitable for measuring the concentration of oxygen in
amine solutions with a high pH, only aqueous solutions with NaCl or
MEG because of their neutral pH, were analysed. Other disadvantages of
a titration method in this case are the challenges related to sampling.
Sampling will involve change of temperature and oxygen pressure,
which in turn inuences the oxygen solubility of the sample, addition-
ally the amine solutions used for CO
2
capture are alkaline, a trait that is
not compatible with the necessary acidication of the sample. All in all,
these disadvantages make Winkler titration a bad alternative in indus-
trial applications.
Further, as an attempt to nd an additional independent validation
method, a GC-MS study was performed for pure amine solutions, satu-
rated with oxygen at normal atmospheric oxygen pressure (p
O2
≈0.21
atm) by sparging with air. The possibility of using a GC-MS quantifying
the molecule ions with m/z 16 and 32 in the pre-elution peak of a fused
silica capillary column, was also tested in this work. Since the dissolved
gas molecules are not retained in the column, the assumption that all the
gas would elute before the rest of the sample was made, and indeed a
pre-chromatogram peak containing only the m/z 14 and 28 (N
+
and N
2
+
)
in addition to 16 and 32 (O
+
and O
2
+
) was seen. In different solvents, this
peak’s size varied, sometimes in orders of magnitude, in different
amines and solvents and the reproducibility was poor (repeated analyses
gave standard deviations >30 %). This validation attempt was therefore
abandoned. A MS method for oxygen quantication could possibly
become useful if it would be possible to retain the analytes in the
chromatographic column in the future. If a GC method should be used,
the results suggest that a molecular sieve (for gas separation) with a pre-
column (for removal of solvent/liquid) GC-TCD method (Park and Cat-
alfomo, 1964) would be recommended, although this equipment is not
very common. Because of this, we were not able to access an instrument
for testing the GC-TCD method in this work. It is also unsure whether
this detection method would be able to quantify such low concentrations
of oxygen.
3.4. Oxygen solubility in amine solutions
3.4.1. Oxygen solubility in different concentration of MEA (aq.)
The concentration of amine in water changes the physical properties
of the liquid, such as viscosity and density. To study whether varying the
concentration of amine also inuences the oxygen solubility of the sol-
vent, aqueous solutions of ethanolamine (MEA) were prepared and their
oxygen solubility measured at different temperatures and compared to
pure MEA and water.
Fig. 2. The Henry’s law constant for N
2
O at 40 ◦C in MEA 30 wt% (aq.) so-
lutions at different loadings. (○) Hartono et al. (2014) and (x) the model of
Weisenberger and Schumpe (1996).
Fig. 3. Oxygen solubility of 30 wt% (aq.) MEA measured with three different
dissolved oxygen sensors and compared to data from Wang et al. (2013). Sol-
ubility measured at p
O2
=0.21 atm.
V. Buvik et al.
International Journal of Greenhouse Gas Control 104 (2021) 103205
7
The experiments were performed using the two electrochemical
dissolved oxygen sensors and experimental setup A (section 2.3.1). The
results given in Fig. 4 show a small deviation (17 % deviation and ±1.6
mg L
−1
in the lower temperature range, 11 % and 0.6 mg L
−1
above 20
◦C) from the oxygen solubility in water. These observations agree with
those of Wang et al. (2013), who concluded that the presence of MEA in
an aqueous solutions does not signicantly inuence the concentration
of dissolved oxygen compared to pure water. For further comparison,
Henry’s law constant (H
cp
) was calculated from the measured solubility
and pressure and these results show no signicant difference from the
Henry’s law constants of water reported in literature (Supplementary
Fig. 6 and Supplementary table 6).
3.4.2. Oxygen solubility in pure amine solvents
The structural variations in different amines suggest that the oxygen
solubility could vary, as other chemical and physical properties of the
solvents do. For the purpose of investigating this, amines with a rela-
tively wide array of structural variation were subjected to the same
conditions.
Oxygen solubility was measured and compared in structurally
different amines at varying temperatures, using experimental setup A
(2.3.1) and both electrochemical dissolved oxygen sensors. A primary
(MEA), a secondary (2-methylaminoethanol, MMEA) and a tertiary
alkanolamine (2-(dimethylamino) ethanol, DMMEA), as well as an aro-
matic amine (benzylamine), a diamine (1,2-diaminopropane, 1,2-DAP)
and an ether of a primary alkanolamine (2-(2-aminoethoxy) ethanol,
DGA) were studied, and the measured oxygen solubility of the amines
are shown in Fig. 5. Pure amine solutions also show a very similar ox-
ygen solubility to water, with very little variation. A study of amine
viscosity and density showed that for alkanolamines, the oxygen solu-
bility seems to decrease with increasing viscosity and density, an effect
that could be related to the strength of hydrogen bonding in the solution.
Detailed information about this can been found in the Appendix.
3.4.3. Inuence of CO
2
loading on oxygen solubility
An increase in ionic strength, such as achieved by loading an amine
with CO
2
, leads to decreasing oxygen solubility. This effect is described
by the ability of the ionic species to inuence the dissolved oxygen ac-
tivity coefcient and is commonly known as the “salting out effect”, vice
versa, ions that decrease the dissolved gas’ activity coefcient can cause
a “salting in effect” (Battino and Clever, 1966).
When an amine is used for CO
2
capture, the solution always contains
CO
2
, even at “lean” loadings. Therefore, pre-loaded solutions of MEA
were studied at different temperatures. Aqueous solutions of MEA were
loaded with pure CO
2
to obtain loadings in the range of 0.07−0.4 mol
CO
2
per mol amine. The amine and CO
2
concentrations were determined
by amine titration and TIC analysis (section 2.6). The ability of the so-
lutions to dissolve oxygen was investigated at different temperatures
using experimental setup A (section 2.3.1) and both electrochemical
dissolved oxygen sensors. The results given in Fig. 6 show decreasing
oxygen solubility with increasing concentration of CO
2
in the solution.
Oxygen concentration was also measured at the temperatures 30, 35 and
45 ◦C and these follow the same trend. For illustrational purposes, only
the data for the temperatures which have been modelled are depicted in
Fig. 6, whereas the other datasets are given in Supplementary Fig. 5. The
oxygen concentration reaches a similar sudden drop in the data series
recorded at 35 and 45 ◦C. Whether a drop or a linear decay in oxygen
concentration is taking place already at 30 ◦C is unclear. These ndings
do, however, indicate that at realistic process conditions, which are
above 35 ◦C, the oxygen concentration is likely to be severely inuenced
by rapid oxidative degradation of MEA already at typical lean loadings.
While the modelling approach predicts the oxygen solubility into CO
2
-
loaded MEA solutions, what is measured here, is not representative of
the physical solubilty of the solvent because oxygen is chemically
consumed through degradation reactions with loaded MEA. It can be
assumed that oxidative degradation already takes place at lower load-
ings, but that it seems that the degradation reactions are slow enough for
the measured dissolved O
2
and the model predictions to be close to each
other.
After observing the drastic drops in oxygen concentration with
increasing CO
2
loading, a series of experiments where oxygen partial
pressure was varied were performed. MEA solutions of 30 wt% (aq.)
Fig. 4. Oxygen solubility in various concentrations of MEA at varying tem-
peratures at p
O2
=0.21 atm. The shown oxygen solubility is the average of the
measured solubility from the two different probes.
Fig. 5. Oxygen solubility of pure solvents as a function of temperature, all
measured at p
O2
=0.21 atm.
Fig. 6. With the adjusted ion parameters for MEAH
+
and MEACOO
−
, as well as
known values from Weisenberger and Schumpe (1996), the solubility oxygen in
loaded MEA-solutions were predicted. The predicted solubilities are here
compared to the measured solubilities at different temperatures, using the VWR
pHenomenal® dissolved oxygen sensor.
V. Buvik et al.
International Journal of Greenhouse Gas Control 104 (2021) 103205
8
were subjected to an O
2
/CO
2
atmosphere of different, but in each
experiment constant, ratios of the two gases, using experimental setup B
(Fig. 1B and section 2.3.2) and the VWR pHenomenal® sensor. This led
to the CO
2
-loading increasing over time. All mixing ratios of O
2
and CO
2
gave a decreasing measured oxygen solubility in the amine solution with
increasing loading (and time). Fig. 7 shows how the amount of oxygen in
the surrounding atmosphere directly inuences the solubility of oxygen
in the solutions, but with increasing CO
2
loading the oxygen concen-
tration in all cases suddenly drops. This drop indicates the point at which
chemical reaction between amine and oxygen takes place faster than
oxygen is transferred from the gas to the liquid phase.
Finally, to better understand the results, the concentrations of dis-
solved oxygen were measured in unloaded and loaded MDEA. The re-
sults are presented in Fig. 8. When compared to data presented in Figs. 6
and 7 for MEA loaded with CO
2
, the same unexpected drop at higher
loading was not seen in MDEA. Instead the apparent oxygen solubility
increases from 7.6 mg L
−1
in CO
2
free MDEA to 8.0 mg L
−1
when the
solution was loaded to 0.4 mol CO
2
per mol MDEA at 20 ◦C. The same
behaviour was seen with both the optical and a galvanic sensor.
The fact that CO
2
loaded MDEA shows a measurable concentration of
oxygen and loaded MEA not, supports the explanation of rapid con-
sumption of dissolved oxygen in rapidly degrading amine solutions such
as MEA. It can be assumed that dissolved oxygen sensors are able to give
information about oxygen consumption rates, which again could give
indications about the amines’ oxidative degradation rates. For this to be
fully understood, mass transfer rates of oxygen from gas to liquid phase
must be known.
Furthermore, the solubility of oxygen in loaded MDEA is comparable
to that of water or unloaded MEA. Since the dissolved oxygen sensor also
measures the same concentration of oxygen regardless of N
2
or CO
2
being present in the gas phase in addition to O
2
, it can be clear that CO
2
does not inuence the measurement or causes membrane concentration
polarization. As both the electrochemical and optical sensor measure the
same oxygen concentrations in loaded solutions, we regard that both
sensors are free from inuence of other gases.
Finally, Fig. 6 also shows the modelling results based on the
measured physical solubility of inert N
2
O in loaded solutions of MEA by
Hartono et al. (2014) and NMR speciation data of loaded MEA solutions
(B¨
ottinger et al., 2008). The oxygen solubility of solutions of MEA with
CO
2
loading could be predicted using the model parameters given in
Table 2 and Eqs. 9–11. As it can be seen in Fig. 6, where oxygen con-
centrations at different temperatures and CO
2
concentrations are
compared to the concentrations predicted by the model, a slight devia-
tion is observed for temperatures at 20 ◦C, where the measured con-
centration of oxygen is slightly higher than predicted. This effect can be
explained by the increased conductivity and therefore amplication of
the probe signal. At 40 ◦C, the deviation is however high, due to rapid
oxygen consumption at this temperature.
Despite the signal being inuenced by the ionic strength of the so-
lution, the dissolved oxygen sensor method for determining oxygen
concentrations of the amine solutions is the simplest, and only direct, of
the tested methods. Of the few other existing options, the Winkler
titration method is not suitable for use in alkaline solutions in addition
to being indirect and involving sampling. Equilibrium cell experiments
for measurement of dissolved oxygen requires high pressure, due to the
low solubility of O
2
. If the amines degrade rapidly in the presence of
oxygen, the dissolved oxygen sensors do not actually measure the sol-
ubility of oxygen in the solution, as it is limited by the mass transfer from
gas to liquid phase and the degradation rate of the amine. This goes
especially for CO
2
-loaded solutions of degradable amines like MEA and
puts a large limitation to the applicability of dissolved oxygen sensors
for determining the presence of liquid phase oxygen. Considering the
results of this study, MEA appears to consume oxygen faster than fresh
oxygen dissolves from the gas to the liquid phase, although there is a
theortical inherent physical oxygen solubility in the solution, this is not
measurable. This is not due to the measurement principle, but rather
MEA’s fast degradation or reaction rate in the presence of oxygen.
3.4.4. O
2
solubility in a degraded amine solution
A degraded amine solution contains a mixture which can contain
alkaline amine, acidic and alkaline degradation products, heat stable
salts, carbamate, dissolved metals and other ionic components. To study
the effect of degradation products on oxygen solubility in an amine so-
lution, a highly degraded amine solution was studied in the same
manner as lean and loaded amines were studied in experimental section
3.4.1 to 3.4.3, with varying temperature of the solution. The degraded
solution which was studied had been submitted to a three-week
campaign of laboratory scale oxidative degradation and contained less
than 1% of the original alkalinity (amine concentration). The degraded
solution contained a total anionic HSS concentration of 0.75 mol kg
−1
.
The oxygen solubility in 30 wt% (aq.) MAPD (CO
2
free) was
measured with the VWR pHenomenal® dissolved oxygen sensor in
experimental setup A (2.3.1) before and after strong oxidative degra-
dation (CO
2
concentration <1 g/kg). A complete loss of amine was
found in the degraded solution, meaning that the solution primarily
consisted of water and degradation compounds. The measurement re-
sults in Table 3 oxygen solubility neither increases nor decreases
signicantly with amine loss and increased concentration of degradation
products.
Since there is no signicant difference in the concentration of an
unloaded (CO
2
-free) amine solution and a highly degraded solution (also
Fig. 7. The solubility of oxygen decreasing with increasing CO
2
loading, when
left in a gas atmosphere containing solely O
2
and CO
2
vol%) at 30 ◦C and 1 atm.
Fig. 8. The oxygen solubility of a solution of aqueous 30 wt% MDEA (aq.) with
0.4 mol of CO
2
per mol MDEA, measured with VWR pHenomenal® galvanic
and the Endress+Hauser optical dissolved oxygen sensor at different
temperatures.
V. Buvik et al.
International Journal of Greenhouse Gas Control 104 (2021) 103205
9
as good as CO
2
-free), it can be speculated that the presence of degra-
dation compounds in the form of heat stable salts have much less in-
uence on the oxygen solubility than carbamate and protonated amine
formation.
3.5. Signicance of the results for the capture process
In an absorber column where CO
2
is absorbed from a ue gas, the
liquid temperatures typically vary from 40 to 80 ◦C and as the solvent
absorbs CO
2
, the ionic strength of the solution increases. Furthermore,
typical ue gas contains generally between 5 and 14 mol% oxygen
(Feron et al., 2014; Hjelmaas et al., 2017; Lombardo et al., 2014; Moser
et al., 2011a, b; Rieder et al., 2017). These factors lead to lower oxygen
concentrations compared to those seen in this study, making the quan-
tication of dissolved oxygen challenging, both because of upper oper-
ating temperature limits of the sensors and their limits of detection and
quantication. The use of commercially available dissolved oxygen
sensors made for water testing purposes can, however, be considered in
industrial applications in amine solutions, if it has a relatively low ox-
ygen consumption rate and the measurement takes place below the
upper operating temperature of the sensor. Of the tested sensors, the
optical dissolved oxygen sensor seems more capable of withstanding the
alkalinity of the amine solutions than the other two, not suffering from
neither corrosion nor other damage in the process, and may therefore be
more suitable for dissolved oxygen measurements in a carbon capture
facility.
Technologies for oxygen removal and oxidation inhibition in the
amine solvent are being developed, to reduce the problems and costs
related to oxidative degradation in amine scrubbers. These techniques
generally base on oxygen removal by addition of oxygen scavengers,
either by direct addition to the amine solution or indirect contact
through a membrane barrier (Monteiro et al., 2018; Supap et al., 2011;
Veldman and Trahan, 1997). Additionally a “salting out” method could
potentially be applied, where an intentional increase of ionic strength of
a solution is performed by addition of salts to decrease the overall ox-
ygen solubility (L´
eonard et al., 2014). This approach will need careful
testing, as the increase of salinity also may inuence other solvent
properties, like corrosivity, viscosity, density, cyclic capacity and heat of
absorption.
Regardless of which of these techniques are being investigated or
applied in industry, there is a need for a reliable method for quantifying
the amount of oxygen in the solution before and after the removal
operation, to evaluate the efciency of the technology and to detect
operational problems in the removal process if they occur. Ideally, the
means of measurement should be direct, and preferably also online, to
avoid unnecessary amounts of work and errors during sampling, in
addition to giving a possibility for automated analyses. A dissolved ox-
ygen sensor would be an ideal solution for direct measurement, but it
would require the amine solution of choice to not consume dissolved
oxygen faster than it is being transferred from the gas to the liquid phase,
to make any sense to measure at all. It also requires a sufciently low
detection and quantication limit of the sensor. Oxygen concentrations
expected to be found in an amine scrubbing facility, where the tem-
perature generally is high and the pressure of ue gas oxygen is low, are
in the lower ppm range (at least <6 ppm, probably lower) and if the
solvent is readily degradable, maybe even in the ppb-range. Since the
apparent solubility given by the dissolved oxygen sensors are higher
than reality, given the increased conductivity of the solution, it is
possible that the sensor’s sensitivity is increased and may be used in
lower concentration ranges than expected. This effect will, however,
need to be further studied and understood.
4. Conclusions
The results of the study show that commercially available dissolved
oxygen sensors may be used to measure oxygen concentrations in amine
solutions both in the presence and absence of CO
2
. The increased con-
ductivity of the solution when the amine has chemically bound CO
2
gives a slight amplication of the signal, which means that the actual
concentration of oxygen is lower than measured. Oxygen solubility does
not vary much in different solutions with and without amines. The
factors inuencing oxygen solubility the most are temperature, oxygen
pressure and also the CO
2
loading. Amines with rapid oxidative degra-
dation rates, such as ethanolamine, will consume oxygen from the so-
lution faster than the oxygen transfer rate from gas to liquid phase.
Measurement of oxygen concentrations in rapidly degrading amine so-
lutions is therefore not useful. The actual oxygen concentration in these
solvents will be very low, likely below the detection limit of any
commercially available dissolved oxygen sensors (<<1 ppm). For
amines which are stable under oxidative conditions, the sensors are t
for the purpose of measuring their oxygen concentrations.
The tested modelling approach seems both promising and realistic.
However, for adjusting the Schumpe model, data on physical solubility
of an inert gas in loaded solutions as well as ionic speciation are needed.
Comparing model predictions to measured oxygen concentrations in
solutions give indications of the solvent’s degradation rate. The
modelling approach can be a helpful tool when using oxygen solubility
as a parameter for degradation modelling or when experimental deter-
mination is not possible.
The commercially available dissolved oxygen sensors may nd an
application as a fast screening method for the evaluation of oxidative
stability of novel solvents, in addition to measurement of dissolved ox-
ygen concentrations in chemically stable solvents.
CRediT authorship contribution statement
Vanja Buvik: Conceptualization, Methodology, Investigation,
Writing - original draft, Writing - review & editing. Ida M. Bernhard-
sen: Investigation, Writing - original draft, Writing - review & editing.
Roberta V. Figueiredo: Investigation, Writing - original draft, Writing -
review & editing. Solrun J. Vevelstad: Conceptualization, Methodol-
ogy, Writing - review & editing. Earl Goetheer: Conceptualization,
Methodology, Writing - review & editing. Peter van Os: Writing - re-
view & editing, Funding acquisition. Hanna K. Knuutila: Conceptual-
ization, Methodology, Writing - review & editing, Supervision, Project
administration, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing nancial
interests or personal relationships that could have appeared to inuence
the work reported in this paper.
Acknowledgements
The authors would like to thank Eirini Skylogianni and Sigrid Steinsli
Table 3
Oxygen solubility of 3-methylamino-1,2-propanediol (MAPD,
α
=0) before and
after laboratory scale oxidative degradation over a three-week period. Oxygen
solubility was measured at pO
2
=0.21 atm.
MAPD 30 wt% (aq.)
fresh
MAPD 30 wt% (aq.)
degraded
Amine conc. [mol
kg
−1
]
2.85 Amine conc. [mol kg
−1
] 0.0161
T (◦C) c
O2
[mg
L
−1
]
T (◦C) c
O2
[mg
L
−1
]
11.0 10.3 10.4 10.3
20.4 7.8 20.5 8.2
30.3 6.7 30.3 6.8
39.8 5.7 39.8 5.7
49.2 4.5 49.8 4.8
V. Buvik et al.
International Journal of Greenhouse Gas Control 104 (2021) 103205
10
Austad for performing the viscosity and density measurements, Profes-
sor Rudolf Schmid and Dr Susana Villa Gonzalez for the advice and help
planning and running the GS-MS analyses and Dr Saravanan Janakiram
and Dr Arne Lindbråthen for valuable help and advice about oxygen
selective membranes.
This publication has been produced with support from the NCCS
Centre, performed under the Norwegian research program Centres for
Environment-friendly Energy Research (FME). The authors acknowl-
edge the following partners for their contributions: Aker Solutions,
Ansaldo Energia, CoorsTek Membrane Sciences, Emgs, Equinor, Gassco,
Krohne, Larvik Shipping, Lundin, Norcem, Norwegian Oil and Gas, Quad
Geometrics, Total, and the Research Council of Norway (257579/E20).
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.ijggc.2020.103205.
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