03 Measuring Alkalinity in Monoethylene

Measuring Alkalinity in Monoethylene Glycol (MEG) Solutions
Marion Seiersten (marion.seiersten@ife.no)
Arne Dugstad (arne.dugstad@ife.no)
Institute for Energy Technology
PO Box 40
2027 Kjeller
Abstract
The paper reviews methods and practices for measuring alkalinity in aqueous
solutions. The focus is on glycol systems where the alkalinity must be controlled in
order to obtain corrosion protection and avoid scaling. Determining alkalinity by
simple acid-base titration is difficult when the glycol contains acetate or other weak
bases. More complex titration methods or combining titration with other analytical
methods can solve the problem.
Simplified methods that can be applied in field are under development. The basis for
these is to convert all strong alkalinity to bicarbonate by equilibrating the solution
with carbon dioxide. In one method the bicarbonate concentration is determined from
the pressure build up when the liquid is acidified in a closed compartment. Another
approach is to simultaneously measure the pressure and pH after pressurising the
sample with carbon dioxide.
Introduction
Monoethylene glycol (MEG) is used as hydrate inhibitor in gas/condensate flowlines.
Recycled MEG will contain ions of salts and neutral compounds picked up from
formation water and injected chemicals. The concentration of these species depends
on the source and how the MEG is recovered. Defining acceptance levels and to
monitor if the MEG quality is within the limits are mandatory in order to avoid setups and down time. The alkalinity of the MEG is a key parameter and must be closely
monitored. In most systems there will be alkalinity targets in order to:
­ Maintain a low FeCO3 solubility in case corrosion is controlled by pH
stabilisation
­ Avoid scaling downstream the MEG injection point in case of formation water
production
­ Force precipitation of carbonate and hydroxide in the MEG recovery process
The objective of this paper is to discuss analytical methods that can be used to control
alkalinity. The main focus is on how to measure the concentration of bicarbonate,
carbonate and hydroxide when formate, acetate and other carboxylate ions are present.
The formal definition of alkalinity is “the number of moles of hydrogen ion equivalent
to the excess of bases formed from weak acids with a dissociation constant K≤10-4.5
(at 25 °C and zero ionic strength)” [1]. The definition is not well suited for solutions
that contain bases of carboxylic acids. For example, formate formed from formic acid
having a dissociation constant K=10-3.7, will not be included while acetate will be
included as the dissociation constant of acetic acid is K=10-4.7 [2]. The definition
given by Oddo and Tomson which includes bases of carboxylic acids is better suited
for oilfield applications [3]:
Total Alkalinity =
[OH − ] + [HCO3− ] + 2 ⋅ [CO32 − ] + [HS− ] + [OH − ] + [CHOO − ] + [CH 3COO − ] + ..... − [H + ]
(1)
Where [] denotes molar or molal concentration. Formate, acetate and other weak
bases are included.
A further differentiation of alkalinity is required as it is important to distinguish
between the alkalinity that reacts with CO2 and gives a sufficiently high concentration
of HCO3 and subsequent CO32- to precipitate carbonate solids, and the weak bases
that hardly react with CO2. Consider the reaction between dissolved CO2 and a base,
B-:
CO 2 (aq) + B− (aq) + H 2 O(l)  BH(aq) + HCO3− (aq)
With the equilibrium constant, K:
[BH] ⋅ [HCO3− ]
=K
[CO 2 ] ⋅ [B− ]
(2)
(3)
-
The K should be larger than 1-10 to give a sufficiently high concentration of HCO3 .
The K consists of the dissociation constant of CO2, K1, and the dissociation constant
of the acid BH, Ka:
CO 2 (aq) + H 2 O(l)
= HCO3− (aq) + H + (aq)
BH(aq) =
H + (aq) + B− (aq)
K=
-
K1
Ka
K=
1
[HCO3− ] ⋅ [H + ]
[CO 2 ]
[B− ] ⋅ [H + ]
Ka =
[BH]
(4)
(5)
(6)
In order to form HCO3 the dissociation constant of the conjugate acid of the base
must be 1 to 1/10 of the CO2 dissociation constant; i.e. less than 10-7 (10-6.4-107.4
)which implies that acetate and the other carboxylate ions are too weak, while bases
like MDEA (methyl diethanol amine with Ka(MDEAH+)=10-8.6) and MEA (methyl
ethanol amine with Ka(MEAH+)=10-9.5) are sufficiently strong. Based on this, the
alkalinity in oilfield waters and recycled MEG can be divided into strong alkalinity;
2
-
the alkalinity that reacts with CO2 and gives an appreciable HCO3 concentration, and
the weak alkalinity constituted by carboxylates and equally weak bases.
The aqueous phase in a multiphase flowline will be in thermodynamic equilibrium
with the gas and condensate phases. The CO2 in the gas will then assure that the
strong alkalinity is converted to HCO3 . If the gas contains H2S there may also be
appreciable amounts of HS-. The HCO3 (and HS-) will be maintained through the
separation process and constitute the main part of the strong alkalinity in rich MEG.
-
When the MEG is heated and re-concentrated CO2 is boiled off and HCO3 reacts to
CO32- and even CO32- may react to OH- according to:
2HCO3− (aq)  CO 2 (g) + CO32 − (aq)
CO32 − (aq) + H 2 O(l)  CO 2 (g) + 2OH − (aq)
(7)
The strong alkalinity in the lean MEG will thus normally be CO32-, OH- and HS-.
Strong bases like MDEA are also regained. If the MEG is not reclaimed, the alkalinity
balance between rich and lean MEG will only change if some of the bases are volatile
or if organic acids evaporate. The ionic bases will be removed by reclamation while
volatile bases can be reclaimed.
The methods for determining the alkalinity must be adapted to the type of alkalinity
and some information is needed on the type of bases that may form in the system.
When monitoring a rich MEG pre-treatment with forced precipitation of carbonates
and hydroxide it is important to determine the OH- / HCO3 / CO32- speciation while it
is only necessary to determine the strong alkalinity in order to monitor pH
stabilisation.
In some cases it can be sufficient to monitor the pH, but it should be noted that a pH
measurement alone is seldom an accurate measure of alkalinity. pH depends not only
on the alkalinity, but also on type of alkalinity and on the concentration of other
solutes (dissolved species) as illustrated in Figure 1. In rich MEG the pH is a function
of alkalinity, the weak/strong alkalinity distribution and the concentration of dissolved
CO2 and H2S. In lean MEG it is a function of the OH-/CO32- distribution in an amine
free system. In a system with amine, OH- and CO32- may still affect the pH if there are
other sources of alkalinity than the amine.
3
Speciation
OH-+CO32-
CO3
2- +
HCO3
HCO3
-
-+
CO2
12
50
Lean MEG
40
Rich MEG
Rich MEG
pre-treatment
30
11
CO32OH-
CO32- + HCO3-
HCO3- + CO2
8
CO2
10
0
OH-+CO32-
10
9
HCO3-
20
pH vs. CO2 content 50 mM alkalinity
13
pH
Concentration [mM]
60
7
0
10
20
30
40
50
CO2 content [mM]
60
6
70
0
10
20
30
40
50
CO2 content [mM]
60
70
Figure 1: Left: Alkalinity speciation as function of CO2 content (dissolved CO2 and
including CO2 in HCO3 and CO32-) in a solution where OH-, CO32- and HCO3
constitute the alkalinity. The speciation in rich and lean MEG and in a rich MEG pretreatment is indicated. Right: The resulting pH. The calculation is for 80wt% MEG
with 50 mM (10-3mol/L) alkalinity.
Titration methods
-
The best way to determine alkalinity is acid titration. When OH-, CO32- and HCO3
constitute the alkalinity titration by HCl gives one or two equivalence points and the
total alkalinity as well as the speciation of the three can be determines as outlined in
an ASTM standard [4]. However, when determining the speciation one must be
careful in order to avoid adding CO2 by the water used for dilution or otherwise.
It becomes more difficult to recognise the end points and determine the speciation
when the sample contains weaker bases. This is illustrated in Figure 2 which shows
titration curves for solutions that contain OH-, CO32-, HCO3 and acetate. The left
figure shows that the endpoint for HCO3 is visible when the solution is continuously
sparged with N2 when the acetate to strong alkalinity ratio is 1. The right figure shows
the effect of increasing acetate concentration on the titration curves. The end point for
HCO3 is at pH 4.5 when there is no acetate. Increasing acetate concentration shifts it
to higher pH and makes it less recognisable. In order to detect the end point for
acetate and other carboxylates the titration must be continued to a pH between 2 and
2.5. The carboxylate end point can be difficult to detect, but it is also a deflection
point and in most cases it can be determined from the d(pH)/dVtitrant curve.
4
100 mmol/kg acetate, 100 mmol/kg strong
alkalinity
N2 sparge
70
No gas
10
ERC N2
10
ERC
pH
40
30
4
140
120
8
50
6
160
10 mmol/kg SA, 10
mmol/kg acetate
30 mmol/kg SA, 10
mmol/kg acetate
30 mmol/kg, no acetate
60
ERC No gas
8
pH
Effect of acetate
12
80
100
6
80
60
4
40
20
2
0
2
10
0
0.5
1
1.5
2
2.5
ERC
12
0
0
20
0
0.5
1
1.5
2
2.5
0
Normalised volume
Normalised volume
Figure 2: Titration curve (pH versus acid volume normalised to strong alkalinity) for
samples with different concentration of strong and weak bases. Left: Effect of N2
sparging to reveal HCO3 end point. Right: Effect of increasing acetate:strong
alkalinity ratio on HCO3 end point. The strong alkalinity is 50/50 OH-/CO32- in all
cases.
-
When the HCO3 end point is unrecognisable, the strong alkalinity can be determined
from the total alkalinity when the concentration of carboxylic acids is known. The
carboxylic acid concentration can be analysed independently by e.g. ion
chromatography (IC), but it can also be determined by titration. Tomson et al. outlines
a method where both the strong alkalinity and the carboxylate (anions of carboxylic
acids) are determined by fitting a measured curve to a calculated curve [5]. It should
be noted that the method is most accurate when the solution is sparged with CO2 to
convert the strong alkalinity to HCO3 prior to the titration and the CO2 partial
pressure is fixed during the titration. That is obtained by sparging the titration vessel
with 1% CO2 in N2. Kaasa and Østvold describe a method where the carboxylic acid
concentration is determined by titrating the solution with NaOH after the HCl titration
[6]. This method requires that the CO2 formed when the sample is titrated with HCl is
completely removed either by boiling or by vigorous sparging with N2 for sufficiently
long time.
The two titration methods mentioned are suitable for determining the strong alkalinity
and the carboxylic acid concentration in formation water samples, in produced water
and in rich MEG. Both can also be used for lean MEG when the OH- / CO32- / HCO3
speciation is not required, but only acid titration and back titration with NaOH is
suitable when the speciation is required. When CO32- and/or OH- are present in
addition to carboxylates, the acid titration curve will have at least two end points. The
first is for CO32- + OH- and the last for the total alkalinity. This is exemplified in
Figure 3. The figure shows the curves obtained by titration with acid (blue) and back
titration with NaOH (green). The back titration curve is plotted as acid volume (total
volume HCl minus volume NaOH). The volumes to the end points gives the total
5
alkalinity (A), the OH- + CO32- concentration (B), the acetic acid (acetate)
concentration (C) and the strong alkalinity (D).
S7
80
pH NaOH
10
B
8
pH
90
pH HCl
D
6
ERC HCl
70
ERC NaOH
60
50
C
40
4
30
20
A
2
0
ERC
12
0
10
10
5
15
0
volume [ml]
Figure 3: Titration curves of a lean MEG sample containing OH-, CO32- and acetate.
The blue solid curve is the pH measured during acid titration (0.1N HCl), the blue
dotted curve is a normalised derivative of the acid titration curve, the green curve is
the pH measured during titration with NaOH (0.1 N) plotted as acid volume (Total
volume HCl minus volume NaOH) and the green dotted curve is a normalised
derivative of the NaOH titration curve.
Bicarbonate and total inorganic carbon methods
The titration methods described above requires a good titrator and they take time.
When the solutions contain surfactants as is often the case for MEG, sparging can be a
challenge due to the foaming. Alternative methods that are faster, but still give
reliable results are thus demanded for field applications. The most popular of these
methods determine the strong alkalinity based on the amount CO2 released when the
solution is acidified. It should be noted that when such methods are applied on
solutions where the strong alkalinity is constituted by bases other than HCO3 , these
bases must be converted to HCO3 by sparging the solution with CO2. Necessary
precautions must then be taken in order to remove the excess dissolved CO2 prior to
the analyses and assure that the measured CO2 comes from HCO3 only.
-
A recent publication describes a simple method for determining HCO3 alkalinity and
the basis for the acidifying methods [7]. The equipment is a sealable vessel that can be
pressurised, a pressure gauge connected to it and means to inject acid and bicarbonate
solution. The sample is added to the vessel and the vessel is closed and shaken. Then
6
an aliquot of acid is added before the vessel is shaken again. The procedure is
repeated with an aliquot of NaHCO3. The pressure is read before the injection of acid,
after injection of acid and thorough shaking and after the injection of NaHCO3. The
HCO3 alkalinity can then be calculated based on the three pressure readings and a
constant which incorporates the Henry’s law constant for CO2 and other constants for
the apparatus and conditions. The beauty of the method is the first pressure reading
which reduces the uncertainty due to dissolved CO2, and the final addition of HCO3
which calibrates for activity variation due to the sample composition. By doing this,
the method eliminates the need for knowing or calculating activity/fugacity
coefficients. Wang et al. show that the method is fairly accurate even for samples that
contain considerable amounts of acetic acid as long as the pH of the original sample is
above 5.8 [7].
The principle of acidifying and measure the released CO2 is also the basis for many
commercial instruments which measure total inorganic carbon (TIC), process
alkalinity etc. They can be used, but some precautions must be taken. They are often
intended to measure CO32-, HCO3 and (for TIC also dissolved CO2) as a sum not the
alkalinity which implies that if the strong alkalinity is more than HCO3 it must be
converted to HCO3 . Furthermore one must assure that any excess dissolved CO2
which is not bounded as HCO3 , is removed prior to the measurement.
These methods are readily applicable for rich MEG. If they are used for lean MEG,
one must assure that all strong alkalinity is converted to HCO3 prior to the
measurement. This can be done by sparging with CO2 prior to the measurements. The
method cannot give the OH-/CO32- speciation which is needed in order to monitor
precipitation of carbonates and hydroxides in a rich MEG pre-treatment or a MEG
reclaimer.
Methods based on pH measurements at fixed CO2 partial pressure
In general the pH does not give the alkalinity. However, it is a good measure when the
dissolved CO2 concentration is fixed by sparging with a gas with known CO2 content
at a given pressure or by other means. Figure 4 illustrates this; the pH is a linear
function of the logarithm of the strong alkalinity for a large range of alkalinity and
CO2 partial pressures.
However, the presence of carboxylates is not easy to overcome. As shown in Figure 5,
the acetate buffers the pH when the acetate to strong alkalinity ratio is larger than ca.
1. The range of buffering depends slightly on the CO2 partial pressure. If it is
calibrated for the acetate content, the method can be used as long as the strong
alkalinity constitute more than 30% of the total alkalinity.
7
pH vs. CO2 partial pressure and alkalinity
9
0.1 bar CO2
pH
8
1 bar CO2
10 bar CO2
7
6
5
4
3
0.1
1
10
100
Strong alkalinity [mM]
1000
Figure 4: pH as function of strong alkalinity in 50wt% MEG.
pH vs. alkalinity
9
100 mM acetate
50 mM Acetate
10 mM acetate
10 mM acetate, 0.3 bara CO2
0 acetate
pH
8
7
6
5
4
1
10
100
Strong alkalinity [mM]
1000
Figure 5: pH as function of strong alkalinity in 50wt% MEG with varying acetate
concentrations. The main calculations are for 3 bara CO2, one is for 0.3 bara CO2 as
indicated in the legend.
The method as outlined above is an alternative for determining the total strong
alkalinity. It does not give any speciation. However, it is possible if one is also able to
measure the amount of CO2 that dissolves in the liquid and reacts to HCO3 as
described in a recent patent [8]. The method uses a vessel that can be pressurised, a
pressure gauge, a pH meter and a device to add a controlled amount of CO2. An
outline of the apparatus and a picture of a prototype are shown in Figure 6. The
sample is added to the vessel and degassed before a known amount of CO2 is added
from a cartridge or otherwise. The pH and pressure are read when the liquid and the
gas are equilibrated, and the alkalinity is determined from an algorithm or from
calibration curves.
8
P
Constant volume of CO2
v1
v8
P-4
P-1
E-6
v5
v7
V-9
v4
Inert gas
CO2
v2
sample
v3
pH
CO2
CO2
Pressurized sample
measurement container
v6
Figure 1
Figure 6: Outline (left) and picture of apparatus for measuring alkalinity based on
CO2 partial pressure and pH.
The advantage of adding a controlled amount of CO2 and measure the resulting CO2
pressure is that the alkalinity speciation can be determined. The CO2 that dissolves in
the liquid will react to HCO3 if the part of the alkalinity is CO32- and/or OH- and
based on the stoichiometry of the reactions (8), the speciation can be determined when
the total strong alkalinity is derived from the pH measurement and the pH is measured
both prior to and after the CO2 is added. The method requires that Henry’s law
constant for CO2 is known at the given conditions. The pH alkalinity relationship is
normally determined by calibration. The calibration can be simplified to addition of a
constant amount of alkalinity.
CO 2 (g)  CO 2 (aq)
CO 2 (aq) + CO32 − (aq) + H 2 O(l)  2HCO3− (aq)
−
(8)
−
3
CO 2 (aq) + OH (aq)  HCO (aq)
This method is also suited for solutions where some of the alkalinity is HS- as it is a
closed system. The limitations with organic acids are the same as for the sparging CO2
method; i.e. the strong alkalinity must constitute at least 30% of the total alkalinity.
Conclusions
The best methods for accurately determining the alkalinity of MEG solutions are
titration. Precautions must be taken when organic acids are present in the sample. The
titration method must be set-up so that the true end point can be read in order to
determine the total alkalinity. To measure strong alkalinity, the concentration of
carboxylate acids must be known from other analyses or be determined by a
subsequent NaOH titration. Alternatively the strong alkalinity and the concentration
of carboxylates can be determined by an algorithm that fits the dissociation constants
to the measured curve.
9
There are alternatives to titration that is faster and easier to use in field. When using
these one must take into account that most of them are developed for measuring the
alkalinity as HCO3 . Other strong alkalinity must thus be converted to HCO3 by
adding CO2. For methods designed to determine the inorganic carbon any dissolved
CO2 must be removed prior to the measurements in order to obtain the correct HCO3
concentration.
A novel method makes it possible to determine the alkalinity from combined
measurements of pH and CO2 partial pressure. It has the advantage of being little
affected by foaming and it is able to measure the CO32-/OH- speciation and can thus
be used to monitor carbonate and hydroxide precipitation in a rich MEG pre-treatment
or in a MEG reclaimer.
References
1.
2.
3.
4.
5.
6.
7.
8.
Dickson, A.G., An exact definition of total alkalinity and a procedure for the
estimation of alkalinity and total inorganic carbon from titration data. DeepSea Res., Part A, 28(6A) (1981) p: 609-23.
Partanen, J.I., Calculation of Stoichiometric Dissociation Constants of Formic,
Acetic, Glycolic and Lactic Acids in Dilute Aqueous Potassium, Sodium or
Lithium Chloride Solutions at 298.15 K. Acta Chemica Scandinavia, 52 (1998)
p: 985-994.
Oddo, J.E. and M.B. Tomson, Simplified Calculation of CaCO3 Saturation at
High Temperatures and Pressures in Brine Solutions. Journal of Petroleum
Technology, 34 (1982) p: 1583-1590.
ASTM D3875-08 Standard Test Method for Alkalinity in Brackish Water,
Seawater, and Brines. 2008, ASTM,
Tomson, M.B., et al., Measurement of Total Alkalinity and Carboxylic Acid
and Their Relation to Scaling and Corrosion. SPE Journal, (2006) p: 103110.
Kaasa, B. and T. Østvold, Alkalinity in Oil Field Waters. What Alkalinity is
and How it is Measured. SPE International Conference on Oilfield Chemistry.
SPE-37277, 1997
Wang, L., et al., Field Method for Determination of Bicarbonate Alkalinity.
SPE International Oilfield Scale Conference and Exhibition. SPE-169758-MS,
2014
Dugstad, A. and M. Seiersten, Method and Apparatus for Analyzing Alkalinity
Conditions in Aqueous Liquids. W.I.P. Organisation, WO 2012/152935 A1,
2014.
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