ARTICLE
pubs.acs.org/EF
Natural Gas Hydrate Formation and Decomposition in the Presence
of Kinetic Inhibitors. 2. Stirred Reactor Experiments
Nagu Daraboina,† Praveen Linga,†,‡ John Ripmeester,§ Virginia K. Walker,|| and Peter Englezos*,†
†
Department of Chemical and Biological Engineering, University of British Columbia, Vancouver, British Columbia, Canada
Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore
§
Steacie Institute for Molecular Sciences, National Research Council Canada, Ottawa, Ontario, Canada
Department of Biology, Queen’s University, Kingston, Ontario, Canada
)
‡
ABSTRACT: A newly fabricated, stirred reactor was used to investigate hydrate inhibition and decomposition in the presence of
two commercial, chemical kinetic inhibitors, polyvinylpyrrolidone (PVP) and H1W85281, as well as two antifreeze proteins (AFPs),
type I and type III. The longest induction times and the slowest growth rates were observed with HIW8581, with the fastest growth
recorded for PVP. Type I AFP (AFP-I) was a more effective inhibitor, with respect to induction time and growth, than either PVP or
type III AFP (AFP-III). Complete hydrate decomposition occurred earlier in the presence of any of the inhibitors compared to water
controls. However, depending on the type of inhibitor present during crystallization, hydrate decomposition profiles were distinct,
with a longer, two-stage decomposition profile in the presence of the chemical kinetic inhibitors (PVP and H1W85281). The fastest,
single-stage decompositions were characteristic of hydrates in experiments with either of the AFPs. These results argue that thought
must be given to inhibitor-mediated decomposition kinetics in screens and designs of potential kinetic inhibitors. This is a necessary,
practical consideration for industry in cases when, because of long shut in periods, hydrate formation may be unavoidable, even when
inhibitors are utilized.
I. INTRODUCTION
The unexpected formation of gas hydrates in hydrocarbon
production facilities and transportation pipelines can lead to
blockages and shutdowns, and therefore, it is a serious economic
and safety issue.1,2 Traditionally, the prevention of hydrate
formation has been achieved with the addition of thermodynamic inhibitors, commonly methanol or glycols. However, in
the last two decades, economic and environmental factors have
motivated research and development to identify alternative low
dosage (less than 1 wt %) hydrate inhibitors (LDHIs). Some LDHIs
prolong the induction time for hydrate nucleation and reduce
growth (kinetic hydrate inhibitors, KHIs) while other LDHIs
prevent hydrate crystal agglomeration (antiagglomerants). A large
number of synthetic chemicals, mostly polymers, have been explored
as potential KHIs.3,4 Unfortunately, some of these polymers may
not readily biodegrade, and therefore, there has been some
interest in assessing the utility of biological inhibitors.4 10
Antifreeze proteins (AFPs) and antifreeze glycoproteins
(AFGPs) are best known from ocean fish that have evolved at
high latitudes; these proteins adsorb to embryonic ice crystals and
prevent serum freezing in the equilibrium crystallization gap.11,12
Some of these repetitive proteins derived from polar fish and
insects show inhibition activity not only toward ice but also toward
tetrahydrofuran (THF), propane, and methane hydrates.7 9 To our
knowledge, only two reports have documented the utility of
AFPs to inhibit hydrates formed from a natural gas mixture.
Ohno et al10 used high pressure microdifferential scanning
calorimetry (HP-μDSC) with a silica gel medium and reported
that biological inhibitors can inhibit natural gas hydrate formation. In addition, Jensen et al.13 reported that an ice structuring
r 2011 American Chemical Society
protein was found to outperform polyvinylcaprolactam (PVCap)
for both structure I (sI) and structure II (sII) hydrate inhibition.
Thus, it is evident that these biological inhibitors can inhibit
natural gas hydrate formation, but how well they perform
compared to commercial KHIs in an environment that approximates field conditions remains unknown.
The majority of inhibition studies use single hydrate formers
rather than gas mixtures because of the inherit complexity of
natural gas blends with regard to the different hydrate equilibria,
structural characteristics, and diffusion constants of each of the
components.14 16 Even less understood is the decomposition kinetics of mixed gas hydrates in the presence of KHIs.
There is evidence that hydrates crystallized in the presence of
gas mixtures do not behave like those formed from single
gases.17 20 Nevertheless, understanding hydrate decomposition kinetics and predicting hydrate decomposition rates in
the presence of KHIs is important for efficient hydrate plug
removal in pipelines.
It is a challenge to model pipeline conditions in the laboratory,
but oil and gas companies favor stirred reactors, originally designed
by Vysniauskas and Bishnoi,21 with their utility demonstrated by
Bishnoi and his colleagues22,23 and subsequently modified and
used in multiple studies.9,24,25 Because many biological inhibitors
are available in limited quantities, we have fabricated a small scale
apparatus (crystallizer volume of 58 cm3) based on industryfavored reactors. Using this new equipment, we have successfully
Received: June 1, 2011
Revised:
August 13, 2011
Published: August 13, 2011
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Figure 1. Schematic of the apparatus for hydrate induction and decomposition.
compared two different commercial synthetic inhibitors and two
different AFPs for their impact on mixed gas hydrate nucleation,
growth, and decomposition.
II. EXPERIMENTAL SECTION
A. Materials. Deionized, distilled water was used to prepare all
solutions. Two commercial KHIs were used: polyvinylpyrrolidone
(PVP; average molecular weight, ∼10 kDa; from Sigma Aldrich) and
H1W85281 (average molecular weight, ∼3 kDa; a proprietary commercial product of unknown composition). Two fish AFPs purchased
from A/F Protein Inc. were used: Type I AFP (AFP-I; average molecular
weight, 3.3 4.5 kDa) purified from fish serum and type III AFP
(AFP-III; Swiss Prot Database accession number P19414; average
molecular weight, 7 kDa), purified after fermentation and secretion
from recombinant Saccharomyces cerevisiae yeast cells. The methane (93%)/
ethane (5%)/propane (2%) gas mixture (UHP grade) was supplied by
Praxair Technology Inc.
B. Apparatus. Figure 1 shows a schematic of the apparatus. It
consists of a crystallizer (CR) which is a cylindrical vessel (i.d. = 3.00 cm,
height = 7.07 cm) made of 316 stainless steel with a volume of 58 cm3.
A 150 cm3 reservoir (R) supplied gas during hydrate formation in a
semibatch operation. The crystallizer and the reservoir were immersed
in a temperature-controlled water bath, regulated by an external
refrigerator (VWR Scientific). Two Rosemount smart pressure
transmitters (model 3051, Norpac controls, Vancouver, BC) with a
maximum uncertainty of 0.075% of span 0 15 000 kPa (i.e., 11 kPa)
were employed. The temperature of the hydrate phase and the
gas phase of the crystallizer was measured using Omega (Omega
Engineering, Stamford, CT) copper-constantan thermocouples with
an uncertainty of 0.1 K. A valve (Fisher-Baumann) coupled to a PID
controller and connected between the reservoir and the crystallizer
regulated the flow of gas from the reservoir to the crystallizer and vice
versa. The data acquisition system (National Instruments) was
coupled with a computer to record the data, as well as to communicate with the control valve, and used LabView 8.0 (National
Instruments) software.
C. Hydrate Formation. The hydrate formation procedure is
available in detail in the literature.26 The crystallizer was loaded with
KHI solution (10 mL), pressurized with the gas mixture, and then
depressurized (at a pressure below the equilibrium hydrate formation
pressure) three times in order to remove air from the system. Subsequently, the crystallizer temperature and pressure were set to the desired
level, and when this was achieved (approximately 5 min), the stirrer was
started. This was set as time zero for the experiments. The aqueous
solution in the crystallizer was stirred using a magnetic stirrer at a
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Table 1. Experimental Conditions, Induction Times, and Mean Gas Consumption for Methane/Ethane/Propane Gas Hydrate
Formation at 275.15 K and 8.1 MPa
sample
induction
moles consumed
mean induction
mean gas consumption,
no.
state
time (min)
after 10 h
time, t ind (min)
n (mol)
1
2
fresh
memory
gas mixture CH4 (93%) C2H6 (5%)
C3H8 (2%)
water
water + 0.5 wt % AFP I
water + 0.5 wt % AFP III
PV
zRT
G, t
þ
t ind = 6.2
t ind,fresh = 7.3
n = 0.0173
nfresh = 0.0174
t ind,memory = 5
nmemory = 0.0172
3
fresh
6.7
0.0180
4
memory
5.7
0.0178
5
fresh
19
0.0157
t ind = 13.5
n = 0.0151
memory
12
0.0151
t ind,fresh = 17.3
nfresh = 0.0153
7
fresh
15.7
0.0149
t ind,memory = 9.7
nmemory = 0.0149
8
memory
7.4
0.0146
9
10
fresh
memory
0.0093
0.0088
t ind = 208.3
t ind,fresh = 182.8
n = 0.0079
nfresh = 0.0085
11
fresh
73.7
0.0076
t ind,memory = 233.8
nmemory = 0.0081
12
memory
121.4
0.0073
292
346
13
fresh
32.0
0.0117
t ind = 19.6
n = 0.0121
14
memory
15.3
0.0109
t ind,fresh = 23.0
nfresh = 0.0109
15
fresh
14.0
0.0101
t ind,memory = 16.2
nmemory = 0.0107
16
memory
17.0
0.0104
17
18
fresh
memory
8.7
8.0
0.0156
0.0151
t ind = 8.9
t ind,fresh = 10.2
n = 0.0149
nfresh = 0.0152
19
fresh
11.7
0.0147
t ind,memory = 7.7
nmemory = 0.0147
20
memory
7.3
0.0143
constant speed (400 rpm). Experiments were routinely conducted at
275.15 K and 8.1 MPa. The equilibrium hydrate formation (structure II)
pressure for the gas mixture at 275.15 K is 1.06 MPa.1 All hydrate
formation experiments with and without inhibitors were carried out in a
semibatch manner (constant pressure and temperature, with a fixed
amount of aqueous solution and continuous supply of gas). The
nucleation point or induction time was identified on the basis of a
sudden temperature rise or increased gas consumption. Hydrate formation is associated with the incorporation of gas and a consequent drop in
crystallizer pressure. Here, constant pressure was maintained with the
PID controller. Pressure (P) and temperature (T) measurements were
used to calculate the number of moles of gas consumed (gas uptake) by
the following equation:26
ΔnH ¼ nH, t nH, 0
PV
¼
zRT G, 0
0.0167
0.0166
6
water + 0.5 wt % PVP
water + 0.5 wt % HIW85281
8.0
4.3
PV
zRT
SV, 0
PV
zRT
SV, t
where nH is the number of moles consumed to form hydrate (H)
or dissolved in water at time t and time 0, z is the compressibility
factor calculated by Pitzer’s correlation, and V is the volume of the
crystallizer.
The memory experiments were conducted using the same procedure explained above except that the memory experiments were
started 4 h after the complete decomposition of hydrates formed in
fresh solutions.26
D. Hydrate Decomposition. After hydrate formation, the crystals
were decomposed by heating the water bath from 275 to 295 K at
8.1 MPa at the start of the decomposition experiment, with similar
heating profiles for each experiment. Briefly, the procedure is as follows:
after the end of the formation experiment, the heater was turned on
(time zero for the decomposition experiment) to heat the reactor from
275 to 295 K; the stirrer speed of 400 rpm used for the formation experiment was continued, and the data was recorded for every 20 s. The
hydrates start to decompose once the temperature crosses the equilibrium
phase boundary, concomitant with an increase in crystallizer pressure. The
expansion of gas due to the temperature-mediated increase was calculated
by conducting a control experiment with no hydrate formation. The
procedure for the control experiment is as follows: water (10 mL) was
introduced into the crystallizer, the pressure was set to 8.1 MPa, and the
temperature was increased from 275 to 295 K without any mixing. The
temperature and pressure were monitored for the control experiments.
The difference between the hydrate experiments (gas expansion due to
temperature rise and gas released due to hydrate decomposition) and the
no-hydrate experiment corresponded to the gas release attributed to
hydrate decomposition.
The normalized gas release is calculated as follows:
normalized gas release ¼ n=nt
where n is the number of moles of gas released at any given time in the
experiment and nt is the total number of moles of gas recovered at the
end of the experiment.
III. RESULTS AND DISCUSSION
A. Hydrate Nucleation and Growth in the Presence of
Inhibitors. As expected, there was a significant delay in the onset
of hydrate nucleation, as determined by the longer induction
time in the presence of any of the KHIs (Table 1). When all
experiments were compared, the commercial inhibitor H1W85281
was the most effective in prolonging the period before nucleation. AFP-I was modestly more effective than PVP and AFP-III
(Figure 2). When compared to water controls, hydrate nucleation was delayed by a factor of 1.4 in the presence of AFP-III, 2.2
in the presence of PVP, 3.2 in the presence of AFP-I, and 33.6 in
the presence of H1W85281. Overall, average hydrate nucleation
times were as follows: water (6.2 min) < AFP-III (8.9 min) <
PVP (13.5 min) < AFP-I (19.6 min) < H1W85281 (208.3 min).
It is noted that the average hydrate nucleation times in our
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Figure 2. Induction time for hydrate formation in the presence of
inhibitors at 8.1 MPa. The scale for the induction time is logarithmic as a
result of large differences in the induction times between the inhibitors.
HP-DSC work27 were in the order water (114 min) < AFP-III (188
min) < PVP (204 min) < H1W85281 (252). The order of the
delay of induction in the presence of inhibitors correlates well
between HP-DSC (1 μL) and stirred tank reactor (10 mL)
results.
The average induction times for recrystallization were always
less than with fresh solutions for water controls, PVP, AFP-I, and
AFP-III (Table 1). HIW85281 samples did not show this
“memory effect”.
It is not yet clear what processes govern the hydrate nucleation
rate. It has been suggested that KHIs act by adsorbing to
heterogeneous nucleators such as impurities in the water phase
and the walls of the crystallizer.6,10 If correct, then H1W85281 is
more likely, by an order of magnitude, to interact with such
foreign materials, thereby minimizing nucleation sites in order to
delay the induction time.
All the inhibitors reduced overall hydrate growth or the moles
of gas consumed 10 h after the initial nucleation (Table 1). In the
presence of AFP-III and PVP, gas consumption was reduced by
13 14% compared to that seen in water controls. AFP-I reduced
gas uptake by 30%, and H1W8528 reduced it by 54%. Thus, the
commercial KHI H1W85281 was also the most effective in
reducing hydrate growth, concomitant with the superior delay
in the onset of hydrate formation.
The impact of the various inhibitors on hydrate formation can
be best visualized on a time profile (Figure 3). During the first
5 min, the rate of hydrate growth in the presence of any of the
inhibitors was similar to and slower than that of the water
controls. Subsequently, the profile changed, and after 120 min,
the slowest growth rates were seen with HIW85281 and AFP-I
(Figure 3). Hydrate formation in the absence of KHIs reached a
plateau in 5 6 h, as expected in a stirred vessel where the
nucleation and growth are followed by the state of crystal
agglomeration.28 When inhibitors were present, hydrate formation
continued to increase slowly for the duration of the experiment,
except for AFP-I, which, similarly to the water samples,
appeared to plateau, although at a significantly lower level than
the controls.
KHIs are thought to reduce the transport of guest molecules to
the hydrate surface.29,30 This may be mediated by their adsorption
onto the hydrate surface, which effectively decreases the crystal
ARTICLE
Figure 3. Effect of inhibitors on hydrate growth for experiments
conducted at 8.1 MPa and 275.15 K (experiments 1, 5, 9, 13, and 17
from Table 1). Time zero in the graph corresponds to the nucleation
point (induction time given in Table 1) for the experiments.
Figure 4. Temperature profiles for decomposition experiments
(experiments 1, 2, and 3 from Table 1).
growth rate.31 34 Gordienko et al.35 similarly demonstrated that
AFPs adsorb to THF hydrate, suggesting that both commercial and biological inhibitors likely inhibit hydrate growth by
an adsorption inhibition mechanism. Although Kvamme
et al. 36 correlated hydrate inhibitor interactions to binding
strengths, it is not clear how these could differ because the
molecules should either bind and be incorporated into the
growing hydrate crystal at higher driving forces or over longer
periods of time or not bind. Alternatively, we suggest that
very effective KHIs, once close to the hydrate surface, have a
high probability of being “anchored”, using a chemical group
that fills an empty cage, not only resulting in the inhibition of
further crystal growth but also partially explaining the reduction in gas uptake. Such a model has been developed in silico
for PVP on hydrates.37 In such a model, H1W85281 would
have the highest probability of incorporation into the hydrate
crystal.
B. Hydrate Decomposition in the Presence of Inhibitors.
Hydrates formed in the presence of inhibitors were decomposed
by heating, which resulted in consistent melting profiles (Figure 4).
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Figure 5. Pressure profiles for the no-hydrate experiment (NHE, gas
expansion due to temperature increase), the hydrate experiment (HE,
gas expansion due to temperature increase, and gas release due to
decomposition of hydrate), and the difference (HE NHE, gas release
due to decomposition of hydrate).
Figure 6. Typical gas hydrate decomposition, along with heating
profile, for experiment 1 in Table 1.
The heating profiles for the other experiments are not shown.38 The
increase in the pressure generated by decomposition was also
consistent, as demonstrated by Figure 5, which shows the pressure
profile of two experiments conducted under the same experimental
conditions. In the absence of formed hydrate, increased pressure was
solely due to gas expansion as temperature increased, with a much
higher pressure associated with decomposition of hydrates. A typical
decomposition curve obtained after accounting for the gas expansion is shown in Figure 6. The normalized hydrate decomposition
profiles for all the systems investigated are shown in Figure 7. It is
clearly evident from the figure that decomposition started later in the
absence of inhibitors and was consistent between experiments
(Figure 8). The complete decomposition of the hydrates
formed in the presence of the commercial KHIs, PVP and
HIW85281, appeared to be in two stages and was so protracted that maximum pressures were not achieved until after
the control samples (water only) had decomposed (Figure 8b
and c). These observations are not surprising, as it has
been previously reported that kinetic inhibitors can increase
the temperature39,40 or the time19,41 required for complete
hydrate decomposition.
ARTICLE
Figure 7. Normalized gas release profiles in the presence of inhibitors
(experiments 1, 7, 9, 13, and 17).
A two-stage hydrate decomposition profile may be due to
inhomogeneous hydrate formation, composed of both structure I
and II hydrate, as has been previously reported in mixed gases of
methane and ethane as well as in more complex natural gas
mixtures.15,16,42,43 Makogon and Holditch39,40 reported that the
presence of KHIs increased the temperature at which hydrates
completely decomposed. It is noteworthy, too, that Schicks
et al.43 reported that the hydrate decomposition line shifted
toward higher temperatures with increasing concentrations of
ethane and propane in a gas mixture. Thus, it is possible that
heavier hydrocarbons participate to a greater extent in hydrate
formation, which likely has a heterogeneous crystal structure, in
the presence of the commercial KHIs. In an alternative but
related explanation, a new hydrate, possibly of a different
composition, could form subsequent to the temperature drop
induced by the ongoing dissociation of the existing hydrate and/
or the changing gas mixture composition. This newly formed
hydrate may even act as an impurity in hydrates of a different
composition. Also, because melting temperatures can vary depending on the hydrate dimension (<1 μm), commercial KHIs
could also have an impact on crystal size.
In contrast to the two-stage decomposition observed with the
commercial KHIs, the hydrate melting profiles with the two
biological inhibitors were strikingly simpler. Single-stage decompositions, more similar to the water controls, albeit at lower
temperatures, were seen in the presence of AFP-I and AFP-III
(Figure 8d and e). As well, unlike the case with PVP or
H1W85281, there was such a rapid decomposition that the
overall melting period was even less than that required for
hydrates formed in the absence of any inhibitors (Figure 7).
These observations are consistent with our previous study on
hydrate formation and decomposition using high pressure differential scanning calorimetry (HP-DSC), in that multiple endothermic peaks were observed with the commercial KHIs in
contrast to the less complex profiles seen with AFP-III.27
Although the reasons for this distinct behavior in the two types
of inhibitors are unknown, we speculate that crystals formed in
the AFP-containing solutions were more homogeneous, consistent with a sharp melting profile, and that the incorporation of
the protein resulted in less stable crystals, which decomposed
at lower temperatures compared to those formed in the presence
of the commercial KHIs. For a better understanding of these
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Figure 8. Normalized gas release profiles in the presence of (a) water, (b) PVP, (c) H1W85281, (d) AFP-I, and (e) AFP-III.
complex observations, the compositional and structural analyses
using GC, XRD, and Raman and NMR spectroscopy are
presented as Part 3 of this series.44
IV. CONCLUSIONS
A newly fabricated stirred reactor operated at a constant
pressure and temperature allowed a direct comparison of the
formation and dissociation of mixed gas hydrates in the presence
of commercial and biologically based kinetic hydrate inhibitors
(KHIs). All inhibitors significantly delayed hydrate nucleation
and reduced the hydrate growth. The new commercial inhibitor
H1W85281 was the most effective in prolonging the induction
time (by a factor of 33.6) and reducing growth (by a factor of 2.2)
compared to the other tested inhibitors. These analyses confirm
that KHIs perform both as nucleation inhibitors and growth
inhibitors, and such properties may reflect different mechanisms
of action. A two-stage decomposition of hydrates formed in the
presence of the two commercial inhibitors is suggestive of
heterogeneous hydrate crystals, consistent with our previous
study using calorimetry as well as the observations of other
researchers. In contrast, hydrates formed in the presence of either
AFP type decomposed in a manner similar to that observed for
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no KHI controls but faster. Although the probability of hydrate
formation can be reduced with KHIs, unusual circumstances,
such as long shut-in periods, can nevertheless result in hydrate
formation and plugging of pipelines. In these cases, decomposition kinetics in the presence of inhibitors is an important factor
for consideration. Use of biological inhibitors not only delays
nucleation and inhibits hydrate growth, but when conditions
change, hydrates formed in the presence of AFPs show complete
decomposition at an earlier time, an advantageous and valuable
attribute for any KHI.
’ AUTHOR INFORMATION
Corresponding Author
*Phone: +1 604-822-6184. Fax: +1604-822-6003. E-mail: englezos@
interchange.ubc.ca.
’ ACKNOWLEDGMENT
We thank Shell Global Solutions for their suggestions and
encouragement. The financial support from the Natural Sciences
and Engineering Research Council of Canada (NSERC) is greatly
appreciated.
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