Inhibition properties of new amino acids for prevention of hydrate formation in carbon dioxide-water system: Experimental and modeling investigations

Article abstract of DOI:10.1016/j.molliq.2016.01.039

In the present work, the effect of new structures of amino acids is studied for prevention of hydrate formation in the carbon dioxide-water system. These amino acids consist of L-proline (as amino acid with nonpolar side chain), L-serine and L-glutamine (

Full text of DOI:10.1016/j.molliq.2016.01.039

Journal of Molecular Liquids 215 (2016) 656–663
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Inhibition properties of new amino acids for prevention of hydrate
formation in carbon dioxide–water system: Experimental and
modeling investigations
a
b
Hadi Roosta a, Ali Dashti a, , S. Hossein Mazloumi , Farshad Varaminian
⁎
a
Chemical Engineering Department, Faculty of Engineering, Ferdowsi University of Mashhad, Mashhad, Iran
Department of Chemical, Gas and Petroleum Engineering, Semnan University, Semnan, Iran
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 11 August 2015
Received in revised form 12 November 2015
Accepted 11 January 2016
Available online 27 January 2016
In the present work, the effect of new structures of amino acids is studied for prevention of hydrate formation in
the carbon dioxide–water system. These amino acids consist of L-proline (as amino acid with nonpolar side
chain), L-serine and L-glutamine (as amino acids with polar side chain), and L-histidine (as amino acid with
charged side chain). The inhibition effects of these amino acids were compared with glycine, L-threonine, and
poly-N-vinylpyrrolidone (PVP). Experiments were performed in the concentration range of 0.5–2 wt.%. Investi-
gation on the experimental results shows that inhibition properties of amino acids in an aqueous solution was
due to hydrophobicity, the net charge of amino acid, and electrically charge of the side chain. Based on the
experimental results, the ranking of amino acids (to decrease the hydrate growth rate) is as follows:
L-histidine N glycine N L-proline ≈ L-serine ≈ L-threonine N L-glutamine, although the inhibition effect of
some amino acids is not significant. In addition, the inhibition effects of these amino acids are quantitatively
described by the chemical affinity model. The predicted results confirm that some of the applied amino acids de-
crease CO2 hydrate formation rate.
Keywords:
CO2 hydrate
Water molecules
Kinetics
Amino acids
Chemical affinity
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
concentrations (40–60 wt.%) [1]. Thus, the use of these inhibitors
often requires significant quantities for injection. In the 1990s, low dos-
Gas hydrates are crystalline solid compounds, composed of water
and certain gas molecules (such as methane, ethane, propane, and car-
bon dioxide). The water molecules link to each other through hydrogen
bonds and form a cage-like structure around gas molecules [1].
Depending on the molecular size of the hydrate former, the water mol-
ecules in hydrate crystal can be stabilized into the three main structures
(sI, sII, and sH).
Carbon dioxide is the primary greenhouse gas, which can form hy-
drate crystals. For example, in the CO2 capture process (during CO2 in-
jection and transportation), CO2 hydrate crystals may cause blockage
in pipelines [2,3]. CO2 hydrate formation in gas reservoirs containing
CO2 is also introduced [4]. Similarly, CO2 hydrate may be formed during
CO2 injection into cold saline aquifers [5]. Thus, prevention of CO2 hy-
drate formation is of key importance in the aforementioned fields [6,7].
Inhibitor injection is the most common method to prevent gas hy-
drate formation [8,9]. Thermodynamic hydrate inhibitors (THIs) such
as salts, alcohols, and glycols are the first group of inhibitors, which
change the pressure/temperature of hydrate formation to higher
pressure and lower temperature. These inhibitors are used at high
age hydrate inhibitors (LDHIs) were introduced, which can be divided
into two groups: kinetic hydrate inhibitors (KHIs or KIs) and anti-
agglomerants (AAs). They are used at low concentrations (below
1 wt.%). KHIs retard nucleation and slow down the growth of hydrate
crystals, while AAs keep small hydrate particles dispersed and prevent
from hydrate agglomeration [1,10]. Although water-soluble polymers
such as PVP, PVCap, and Gaffix VC-713 are the most common type of
KHIs, the main problem is the poor biodegradability of these inhibitors
[1,10]. Thus, researchers focused on KHIs with more environmentally
friendly.
Antifreeze proteins (AFPs) and antifreeze glycoproteins (AFGPs)
were the main group of green inhibitors [1]. Al-Adel et al. [11] investi-
gated the effect of type I antifreeze proteins (AFPs) in comparison to
poly(VP/VC). They reported that these inhibitors decrease methane hy-
drate formation rate, while their performance was similar. Daraboina
et al. [12] also tested the effect of two antifreeze proteins (type I and
type III) on the formation kinetics of methane/ethane/propane hydrate.
Their results showed that the hydrate growth was delayed in the pres-
ence of these inhibitors. Gordienko et al. [13] tested the effects of AFPs
on tetrahydrofuran (THF) hydrate and natural gas hydrates. They
found that AFPs are superior inhibitors compared to PVP. Bagherzadeh
et al. [14] presented the mechanism of the winter flounder AFP
⁎
Corresponding author.
E-mail address: dashti@um.ac.ir (A. Dashti).
http://dx.doi.org/10.1016/j.molliq.2016.01.039
0167-7322/© 2016 Elsevier B.V. All rights reserved.

H. Roosta et al. / Journal of Molecular Liquids 215 (2016) 656–663
657
(wf-AFP) to prevent methane hydrate formation by molecular dynam-
2. Experimental
ics simulation. They observed that wf-AFP was bound to the water mol-
ecules of methane hydrate by the methyl side chain of L-threonine and
L-alanine. In fact, their results demonstrated that the L-threonine and
L-alanine (as amino acid) play a significant role to prevent hydrate
growth, although the experimental results of Perfeldt et al. [15] showed
that L-threonine is ineffective on methane hydrate formation.
2.1. Materials
CO2 (99.9 vol% purity) is utilized for hydrate formation with double
distilled water or aqueous solution of amino acids. The amino acids
(including glycine, L-proline, L-serine, L-threonine, L-glutamine, and
L-histidine) are supplied by Merck. The structures and properties of
these amino acids are tabulated in Table 1. Also PVP with a molecular
weight of 10 000 g/gmol is provided from Sigma Aldrich.
It should be noted that kinetic hydrate inhibitors are more used for
natural gas or a mixture of hydrocarbons, but for prevention of CO2 hy-
drate formation, some types of structures were tested as KHIs. The most
important of these structures are ionic liquids and some amino acids.
For example, Chun and Jaafar [16] showed that the tested ionic liquid
(1-ethyl-3-methylimidazolium tetrafluoroborate) works as a kinetic
inhibitor for CO2 hydrate formation. Also, Sa et al. [17] tested the effects
of five hydrophobic amino acids (including glycine, L-alanine, L-valine,
L-leucine, and L-isoleucine) on CO2 hydrate formation and found that
amino acids with shorter alkyl side chains (glycine and L-alanine)
were better KHIs. On the other hand, some amino acids were known
as environmentally friendly corrosion inhibitors in acidic solutions
such as the water-CO2 system [18,19]. Thus, the study of the potential
of these amino acids for prevention of CO2 hydrate formation can be
useful in evaluation of their dual application in the water–CO2 system.
Amino acids were also investigated on THF hydrate formation. Naeiji
et al. [20,21] investigated the effect of glycine and L-leucine on THF hy-
drate formation. Their results showed that glycine is more effective
KHIs.
2.2. Apparatus
The schematic of the used apparatus is shown in Fig. 1. The experi-
mental apparatus is a jacketed batch reactor with a capacity of
655 cm3 (with the uncertainty of 4 cm3) and design pressures of
0–60 bar. The reactor temperature is controlled by the cooling system.
It consists of a coolant bath with controllable circulator that is utilized
to circulate the coolant (water/ethylene glycol) through the jacket.
The temperature of the reactor is measured by a PT100 thermometer
with the uncertainty of 0.1 K. The reactor pressure is measured by a
pressure transmitter with an uncertainty of 0.1 bar. The pressure
and temperature data are recorded by the data acquisition system con-
nected to a computer.
2.3. Experimental procedure
Amino acids are organic compounds composed of amine and car-
boxylic acid with a specific side chain [22]. They are biodegradable
and environmentally friendly [17,20]. Also, some amino acids (such as
glycine and L-threonine) are the main part of antifreeze proteins
(AFPs). Thus, investigation of inhibition effects of new structures of
amino acids with different properties of the side chain can be useful
for the development of environmentally friendly inhibitors.
In this work, for preventing CO2 hydrate formation, four new struc-
tures of amino acids (including L-proline, L-serine, L-glutamine, and L-
histidine) are selected based on different properties of amino acids,
such as hydrophobicity, net charge of amino acid or electrically charge
of the side chain. The performance of amino acids in an aqueous solution
is examined and described based on these properties. Also the effects of
glycine, L-threonine and PVP are examined for preventing CO2 hydrate
formation in comparison to the presented structures. In addition, the
performance of amino acids for decreasing CO2 hydrate formation rate
is discussed and modeled by the chemical affinity model.
First, the reactor was washed and rinsed three times using distilled
water and evacuated by a vacuum pump. It was subsequently charged
with 300 cm3 of liquid sample (including aqueous solution of the
amino acids). It was pressurized up to 30 bar at 285.15 K with a stirring
rate of 300 rpm. The system was allowed to reach the equilibrium state
under these conditions and then was cooled to 275.15 K without agita-
tion. When the temperature was adjusted (at the constant temperature
of 275.15 K), the mixer was turned on at 300 rpm for CO2 hydrate for-
mation. The pressure change in reactor was recorded until equilibrium
pressure was reached. The moles of gas consumed during CO2 hydrate
formation were calculated by Eq. (1), which the Peng–Robinson equa-
tion of state was used for calculating compressibility factor [23].
ꢀ
ꢁ
ꢀ
ꢁ
PV
ZRT
PV
ZRT
nci ¼ no−ni ¼
−
ð1Þ
o
i
Table 1
The structure and properties of applied amino acids [19,29].
Amino acids
Glycine (Gly)
Molecular structure with side chain
pKa1 (−COOH)
pKa2 (−NH2)
pKa (side chain)
Hydrophobicity
2.34
9.60
–
−0.4
L-Proline (L-pro)
1.99
10.60
–
−1.6
2.21
2.09
2.17
9.15
9.10
9.13
–
–
–
−0.8
−0.7
−3.5
L-Serine (L-ser)
L-Threonine (L-thr)
L-Glutamine (L-gln)
L-Histidine (L-his)
1.82
9.17
6.04
−3.2

658
H. Roosta et al. / Journal of Molecular Liquids 215 (2016) 656–663
Fig. 1. Hydrate formation apparatus.
K2
⇔
3. Net charge of amino acids in CO2-saturated water
ð3Þ
ð4Þ
CO2ðaqÞ þ H2O
H2CO3ðaqÞ ΔHðKJ=molÞ ¼ −0:02
l
ð Þ
K3
⇔
The net charge calculation of amino acids in CO2-saturated water can
be useful for analysis of their inhibition effects. The net charge of amino
acids in CO2-saturated water is related to pH. On the other hand, the
reaction of CO2 with water and the temperature and pressure variations
of the system (in the range from 275.15 to 285.15 K and 15.8 to 30 bar,
respectively) change the pH, and consequently, change the net charge
of amino acids. So for calculation of the net charge, the reaction of CO2
with water and the temperature and pressure variations must be
considered.
H2CO3ðaqÞ
HCO3 ðaqÞ þ Hþ
ΔHðkJ=molÞ ¼ 7:66
ΔHðkJ=molÞ ¼ 14:85
ðaqÞ
ðaqÞ
K4
⇔
2−
ð5Þ
HCO3
CO3 ðaqÞ þ Hþ
ðaqÞ
Equilibrium constants for reactions were calculated by the van't Hoff
equation at 275.15 and 285.15 K. The calculated results are presented in
Table 2. Also for calculation of pH in the negligible concentration of
CO23− the following equation can be used [2]. Accordingly, obtained
pH values were variable from 3.77 (285.15 K, 30 bar) to 4.06
(275.15 K, 15.8 bar).
The characteristic equilibrium reactions for CO2 in water can be
considered according to the following equations [2].
ꢀ
ꢁ
K2 ꢀ K3
pH ¼ −0:5 log 10−14
þ
ꢀ PCO
ð6Þ
K1
⇔
2
CO2ðgÞ
CO2ðaqÞ ΔHðkJ=molÞ ¼ ‐20:29
ð2Þ
K1

H. Roosta et al. / Journal of Molecular Liquids 215 (2016) 656–663
659
Table 2
Equilibrium constants of the characteristic reactions for CO2 in water.
Equilibrium constant
At 298.15 K
At 285.15 K
At 275.15 K
K1
K2
K3
K4
3.4 × 10−2
1.7 × 10−3
4.5 × 10−7
4.7 × 10−11
4.9 × 10−2
1.7 × 10−3
3.9 × 10−7
3.6 × 10−11
6.7 × 10−2
1.7 × 10−3
3.5 × 10−7
2.8 × 10−11
The net charge of applied amino acids at different pH values can be
calculated by Eq. (7) [24]. This equation demonstrates that the net
charge of amino acids is obtained by summing the charge of each
group. The first term of this equation is considered for negative charge
groups of amino acids (\\COO−) and the second term is for positive
charge groups (\\NH+3 or charged side chain of L-histidine) [24]. The
obtained results from the net charge of amino acids are tabulated in
Table 3.
!
m
X
−1
pH−pK
þ1
Net Z ¼
þ
ð7Þ
pH−pK
10−
þ 1 10
þ 1
ð
jÞ
ð
jÞ
j¼1
4. Modeling
In current work, the chemical affinity was used for the kinetic study
of CO2 hydrate formation with amino acids. For obtaining kinetic
parameters of this model (tk and −Ar/RT) the following equation was
derived in previous work [25].
ꢂ
ꢀ
ꢀ
ꢁꢁꢃ
Ai
RT
Ar
RT
ti
tk
ti
tk
¼ −
− ln
exp 1−
ð8Þ
where
Ai
RT
nci
ncf
Fig. 2. The algorithm for calculation of kinetic parameters (tk and −Ar/RT).
¼ − ln
:
ð9Þ
fact, the rate of gas consumption reflects the growth rate of CO2 hydrate
[26]. On the other hand, the growth rate of hydrate decreases with
increasing of concentration from 0.5 to 1.5 wt.%, while the inhibitory ef-
fects are almost the same in concentrations of 1.5 and 2 wt.%. Fig. 4
shows the effect of L-serine on CO2 hydrate formation kinetics. It also
decreases the gas consumption rate. The experimental results for
L-glutamine and L-histidine are shown in Figs. 5 and 6, respectively.
The results demonstrate that the effect of L-glutamine on decreasing
of CO2 hydrate formation rate is negligible, while the effect of
L-histidine is more significant. In fact, it seems that L-histidine is more
effective than other amino acids for preventing CO2 hydrate formation,
although the inhibition effect of these new green structures must be
compared together. In addition, the performance of these amino acids
The presented algorithm in Fig. 2 can be used to obtain kinetic pa-
rameters (tk and −Ar/RT) of Eq. (8). Also, for prediction of gas consump-
tion (nci) the following equation can be used [25]:
−A
r
ꢂꢀ
ꢀ
ꢁꢁꢃ
RT
nci
ncf
ti
tk
ti
tk
¼
exp 1−
:
ð10Þ
5. Results and discussion
5.1. Experimental results and discussion
The experiments for investigation of the effect of amino acids on CO2
hydrate formation kinetics were performed at concentrations of
0.5–2 wt.%. The first test was performed with L-proline. Fig. 3 shows
the effect of L-proline on growth rate of CO2 hydrate. The gas consump-
tion rate was decreased with L-proline in comparison to pure water. In
Table 3
The net charge of amino acids under experimental conditions.
Amino acids
Net charge at pH = 3.77
(285.15 K, 30 bar)
Net charge at pH = 4.06
(275.15 K, 15.8 bar)
Glycine (Gly)
L-Proline (L-pro)
L-Serine (L-ser)
L-Threonine (L-thr)
L-Glutamine (L-gln)
L-Histidine (L-his)
0.036
0.016
0.026
0.020
0.024
1.005
0.019
0.008
0.014
0.011
0.013
0.995
Fig. 3. The effect of L-proline on CO2 hydrate formation rate.

660
H. Roosta et al. / Journal of Molecular Liquids 215 (2016) 656–663
Fig. 4. The effect of L-serine on CO2 hydrate formation rate.
Fig. 7. The effect of used amino acids at concentration of 1.0 wt.%.
can be compared to glycine and L-threonine. Thus, experiments were
also performed with glycine and L-threonine. It should be noted that
glycine was selected as the best introduced inhibitor of amino acids in
literatures [17,20]. On the other hand, some literatures reported that
L-threonine has a significant role in the inhibition mechanism of some
antifreeze proteins [14]. Also, the inhibition effect of L-threonine on
CO2 hydrate formation is not reported in references.
Figs. 7 and 8 show the effects of new structures of amino acids in
comparison to glycine and L-threonine at concentrations of 1 and
1.5 wt.%. The results demonstrated that the inhibition effect of L-
histidine is higher than glycine, while the effects of L-proline, L-serine,
and L-threonine are less than glycine. Also, L-glutamine has the lowest
inhibition effect on CO2 hydrate formation rate.
L-proline ≈ L-serine ≈ L-threonine N L-glutamine. In fact, the inhibition
effect of L-histidine with charged side chain is more than uncharged
side chain amino acids with polar or nonpolar side chain. On the other
hand, the inhibition effect of glycine is more than other amino acids
with polar or nonpolar side chain (uncharged side chain), while
L-glutamine is the least effective amino acid. Also the inhibition effects
of L-proline, L-serine, and L-threonine are almost the same. It should
be noted that L-threonine decreased CO2 hydrate formation rate,
while according to some reports, it was ineffective on methane hydrate
formation [15]. For analysis of these results the inhibition mechanism
and inhibition properties of amino acids must be investigated.
The inhibition mechanism of amino acids is probably related to two
main reasons. First, the oxygen atoms of carbonyl in amino acids form
hydrogen bonds with water molecules of hydrate surface. Second, the
side chain of amino acid (usually hydrophobic group or charged side
chain) forms a van der Waals interaction or electrostatic interactions
with crystal surface. Therefore, they interrupt nucleation and disrupt
further growth of the hydrate crystals [10,27,28].
Based on experimental results, the ranking of amino acids (to de-
crease the hydrate growth rate) is as follows: L-histidine N glycine N
According to these notifications, the inhibition properties of amino
acids for preventing CO2 hydrate formation can be analyzed. One of
the main properties of amino acids is hydrophobicity of side chain. Hy-
drophobicity values of applied amino acids are presented in Table 1. The
comparison of these values with experimental results shows that the in-
hibition effect of uncharged side chain amino acids is decreased with re-
ducing hydrophobicity values, thus L-glutamine with the minimum
value of hydrophobicity is the least effective amino acid and glycine
with maximum hydrophobicity is the most effective one. However,
with reducing hydrophobicity value of L-proline in comparison to
L-serine and L-threonine the inhibition effect is not varied. The effect
of the hydrophobic group was also investigated in some literatures.
Recently Bagherzadeh et al. [14] reported that wf-AFP can be bound to
the water molecules of methane hydrate by the hydrophobic methyl
Fig. 5. The effect of L-glutamine on CO2 hydrate formation rate.
Fig. 6. The effect of L-histidine on CO2 hydrate formation rate.
Fig. 8. The effect of used amino acids at concentration of 1.5 wt.%.

H. Roosta et al. / Journal of Molecular Liquids 215 (2016) 656–663
661
Table 4
side chain of L-threonine and L-alanine. In fact, they found that hydro-
phobic groups block the empty hydrate half cages, while the hydrate
surface is stabilized via hydrophobic interaction of the side chain with
the surrounding water molecules. So the inhibition effects of applied
amino acids are increased with increasing the values of hydrophobicity.
The net charge of amino acids is also one of the main properties of
amino acids for prevention of CO2 hydrate formation. The net charges
of amino acids were calculated under experimental conditions. The ob-
tained results are presented in Table 3. Based on the obtained values the
net charge of L-histidine is very high in comparison to other applied
amino acids. Thus the inhibition effect of L-histidine is more than
other applied amino acids. In fact, high net charge (charged side
chain) of L-histidine causes the stronger interactions with water mole-
cules of hydrate surface. In addition, the polar water molecules around
charged molecules or charged side chain of L-histidine are less ice-
like. Therefore, the rate of nucleation and growth of CO2 hydrate is de-
creased. Also it can be seen that the net charge of glycine is more in com-
parison to L-proline, L-serine, L-threonine, and L-glutamine. Thus more
inhibition effect of glycine in comparison to these amino acids may be
due to the net charge.
Obtained kinetic parameters of the chemical affinity model for applied amino acids in the
concentration range of 0.5–2 wt.%.
Amino acids
Concentration
(wt.%)
Model parameters
tk(s)
−Ar/RT
Pure
Glycine
Glycine
Glycine
4562
4649
5484
7016
7174
4848
5100
5142
5984
5089
5411
5445
5588
4585
4643
6182
6249
4235
4876
5070
5004
5660
7780
0.81
0.77
0.67
0.65
0.64
0.79
0.77
0.74
0.72
0.80
0.77
0.75
0.75
0.79
0.74
0.74
0.73
0.81
0.77
0.77
0.76
0.67
0.60
0.5
1.0
1.5
2.0
0.5
1.0
1.5
2.0
0.5
1.0
1.5
2.0
0.5
1.0
1.5
2.0
0.5
1.0
1.5
0.5
1.0
1.5
Glycine
L-Proline
L-Proline
L-Proline
L-Proline
L-Serine
L-Serine
L-Serine
L-Serine
L-Threonine
L-Threonine
L-Threonine
L-Threonine
L-Glutamine
L-Glutamine
L-Glutamine
L-Histidine
L-Histidine
L-Histidine
5.2. Modeling results and discussion
The performance of applied amino acids to decrease the hydrate
growth rate can be described quantitatively by the chemical affinity
model. In fact, the main purpose is the calculation of kinetic parameters
for performance evaluation of amino acids as KHIs for the development
of environmentally friendly inhibitors. According to the presented algo-
rithm in Fig. 2 the kinetic parameters (the values of −Ar/RT and tK) can
be calculated. For example, Ai/RT versus −ln[(ti/tk) exp(1 − (ti/tk))] is
plotted in Fig. 9 for CO2 hydrate formation with L-histidine. The linear
relation with zero intercept confirms that the value of tk is assumed
correctly. Also, the slope of this line can be considered as the value of
−Ar/RT. The values of −Ar/RT and tK for other experiments were calcu-
lated similarly, as listed in Table 4. On increasing the amino acid concen-
tration from 0.5 to 2.0 wt.%, the values of −Ar/RT are decreased. These
results confirm that the rate of CO2 hydrate formation is decreased
with increasing the amino acid concentration. The minimum value of
−Ar/RT is obtained for L-histidine at a concentration of 1.5 wt.%, while
this value is higher for glycine and the other amino acids at all concen-
trations (even at concentration of 2 wt.% of glycine). These values con-
firm that the inhibition effect of L-histidine is more than glycine. Also,
these values for glycine are less than amino acids with uncharged side
chain. On the other hand, these values for L-glutamine are close to
pure water and show that the inhibition effect of L-glutamine is almost
negligible. Also Fig. 10 shows that the average values of −Ar/RT in con-
centration of 0.5, 1, and 1.5 wt.% are decreased with increasing hydro-
phobicity values. Thus the results of model also confirmed that the
rate of CO2 hydrate formation is reduced with increasing hydrophobic-
ity of uncharged side chain amino acids. The obtained values of tk are
also presented in Table 4. These values show that CO2 hydrate formation
is slower in the presence of amino acids rather than pure water.
The obtained kinetic parameters (tk and −Ar/RT) can be used for cal-
culation of gas consumption (nci) by Eq. (10). Fig. 11 shows the results of
the chemical affinity model for prediction of gas consumption during
CO2 hydrate formation in the presence of amino acids (in concentration
of 1 wt.%). The results are in good agreement with experimental data
and confirm that the chemical affinity model can be applied for investi-
gation of the inhibition effect of amino acids on CO2 hydrate formation.
5.3. The inhibition effect of PVP in comparison to applied amino acids and
analyzing on potential of applied structure as KHIs
The experiments were also performed with PVP. Fig. 12 shows the
effect of PVP in comparison to glycine and L-histidine (which had the
best inhibitory effect among applied amino acids). In concentrations of
1 and 1.5 wt.% the effect of PVP is more than glycine while in concentra-
tion of 1 wt.% the effect of PVP and L-histidine is almost alike. Also in
Fig. 10. Average values of −Ar/RT in concentrations of 0.5, 1, and 1.5 wt.% versus
hydrophobicity (for CO2 hydrate formation in the presence of uncharged side chain
amino acids).
Fig. 9. Ai/RT versus −ln[(ti/tk) exp(1 − (ti/tk))] for CO2 hydrate formation with L-histidine
at concentration of 1.5 wt.%.

662
H. Roosta et al. / Journal of Molecular Liquids 215 (2016) 656–663
Fig. 11. Comparison between the calculated and the experimental gas consumption (nci) in the presence of amino acids (in concentration of 1 wt.%).
concentration of 1.5 wt.% the effect of L-histidine is more than PVP. A
characterization of CO2 hydrate formation in the presence of additives
is introduced which can be useful for better understanding the effect
of amino acids on CO2 hydrate growth. Zhang et al. [30] showed that
under high carbonate (CO2) concentrations, the effect of additives on
formation rate of CO2 hydrate may be insignificant. In fact, additive
(promoters or inhibitors) for appropriate performance should be
adsorbed on the crystal surface of hydrate, but in high concentration
of CO2 a competitive adsorption between additives and CO2 will affect
the performance of additive. This is more effective in initial times of
CO2 hydrate formation, because the concentration of CO2 is higher and
adsorption of the applied structure on crystals of CO2 hydrate is insignif-
icant. Thus, the inhibition effect of amino acids or PVP on the initial rate
of CO2 hydrate formation is insignificant. While, CO2 hydrate formation
Fig. 12. Comparison between PVP, glycine, and L-histidine in concentrations of 1 and 1.5 wt.%.

H. Roosta et al. / Journal of Molecular Liquids 215 (2016) 656–663
663
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is continued the concentration of CO2 (around the crystal surface of CO2
hydrate) is decreased and amino acids and PVP can be more adsorbed
on the crystal surface. Therefore, the inhibition effect of these structures
is more significant with time. Consequently, charged and ionic struc-
tures as promoter or inhibitor (such as ionic liquid) are more effective
on kinetics of CO2 hydrate formation. The reason may be due to more
adsorption of these structures on crystals of CO2 hydrate in a competi-
tive adsorption with CO2 and thus for the same reason, the effect of
L-histidine may be more in comparison to PVP (in concentrations of
1.5 wt.%). In fact, according to Table 3 the net charge of L-histidine is
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crease CO2 hydrate formation rate, and their inhibition effect is more
at higher concentrations. L-Histidine was more effective than glycine,
PVP, and other amino acids. In fact, the effect of L-histidine with charged
side chain and high net charge is more than uncharged side chain amino
acids with low net charge. Furthermore, the experimental results dem-
onstrate that the inhibition effects of uncharged side chain amino acids
increase with increasing hydrophobicity values. Consequently, the inhi-
bition effect of glycine is more than L-proline, L-serine, and L-threonine,
while L-glutamine has the lowest inhibition effect on CO2 hydrate
formation rate. Also, the obtained kinetic parameters of the chemical
affinity model (− Ar/RT and tK) confirm that the rate of CO2 hydrate
formation is reduced in the presence of amino acids.
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Nomenclature
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1998.
Ai
Ar
the chemical affinity at state i
proportionality constant in Eq. (8) which denotes the affinity
rate constant
K
equilibrium constant
nci
ncf
no
ni
P
PCO2
R
T
t
ti
tk
V
Z
moles of gas consumed up to time ti
total moles of consumed gas
initial moles of gas in the reactor
moles of gas at time ti in the reactor
pressure
partial pressure of CO2
universal gas constant
temperature
time
time required to reach state i
time required to reach equilibrium conditions
volume of gas in the reactor
compressibility factor
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Biochem. Educ. 13 (1) (1985) 10–11.
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mation kinetics with or without additives and modeling based on chemical affinity,
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with bis- and polyquaternary ammonium salts, Chem. Eng. Sci. 69 (2012) 483–491.
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Subscripts
o
i
j
initial value
state i
arbitrary component
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