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Rational Design and Testing of Low Dosage Hydrate Inhibitors for use with
Offshore Oil and Gas Production
Final Report
1
Background/Context
Gas hydrates are a crystalline mixture of water with a second (“guest”) compound. They form as stable
solids at the relatively low temperatures and high pressures that characterize many geological and industrial
environments. Hydrate formation is associated with important industrial and environmental problems. For
example, the amount of methane gas trapped in natural hydrate deposits is estimated to provide an energy
source that is at least twice as big as the energy of the total conventional fossil fuel reserves.1 This large
volume of methane promotes interest in developing hydrates as a new energy source—and indeed production
of methane has already commenced at the Malik field in Canada. The sheer volume of methane also has
ramifications for global warming predictions (a molecule of methane can absorb about 25 times more
infrared energy than a molecule of carbon dioxide) and may present a major geo-hazard (decomposition of
natural gas hydrates have already caused the world’s largest known submarine landslides2). There are also
exciting new technologies being developed that exploit their unusual physical properties. These include CO2
capture and storage,3 methane transport at ambient pressure, utilising the unusual thermal metastability of
methane hydrate,4 and energy-efficient methods for processes as diverse as salt extraction and purification,
and environmental temperature control.5
The focus application for this project was low dosage inhibitors for hydrate formation. Hydrate formation is
a major problem for pipeline transport. Large amounts of methanol are still used to avoid hydrate blockages,
with consequent risks for environmental damage. The last 15 years have seen much research into developing
“low dosage hydrate inhibitors” (LDHIs) to avoid hydrate formation. However, current LDHIs still tend to
be based on a chemistry similar to the polyvinylpyrrolidone (PVP) and polyvinylcapralactam (PVCap)
identified 15 years ago, now delivered in complex blends, and these are still not sufficiently active for most
applications6.
As identified in the original application, the major barrier to developing sufficiently active LDHIs is the lack
of knowledge about the inhibition mechanism. It is generally believed that the inhibitors are surface active,
i.e. they adsorb to the surface of small crystals, or pre-critical clusters, and thereby disrupt further growth or
aggregation of the crystals. However, these are complex systems and experiments that can probe the
molecular structure of the surface of these clusters and nano-crystals in situ are only just becoming feasible;
thus direct evidence for this mechanism is sketchy. Some information is available from neutron scattering
experiments,7 and from the similarities with anti-freeze proteins,8 but it is fair to say that the mechanism
remains highly conjectural. One of the central aims of this project was to address precisely this question, and
to accumulate rigorous, molecular level information about the way LDHIs work so that the development of
more active compounds can be speeded up. In this endeavour we have been successful, as will be shown
below.
We note in passing that the results obtained on LDHIs are likely to have spin-off benefits for all the
applications described in the opening paragraph.
2
Key Advances and Supporting Methodology
The fundamental aim of the work at Warwick was to develop a proven mechanistic understanding of hydrate
inhibition using LDHIs, so that this could be used to make further identification of new inhibitors much more
efficient. Achieving this goal was, in turn, predicated upon gaining a much better understanding of the
nucleation and early growth processes, since it is these processes that must be altered by LDHIs. The project
therefore proceeded in three distinct phases: simulated nucleation; simulated kinetic inhibition; and synthesis
of testing of potential new inhibitors.
2.1
NUCLEATION
A major achievement of this project has been to perform the first realistic molecular simulations of primary
nucleation in methane hydrate. Such simulations have been attempted before, but only by partially melting
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and then re-cooling a hydrate crystal in the bulk, where artificially high methane concentrations are
maintained and a strong seed crystal is present; thus these simulations did not really probe the nucleation
process. The present project differed from earlier studies in focussing on the methane/water interface: the
interface is the region where methane hydrate is observed to form experimentally, and it also removes the
severe mass transport constraints that are be present in the bulk melt.
Simulations were initiated by setting up a thin film of methane hydrate under a methane atmosphere, and
then melting this (300 K, 3 MPa) until (i) no residual hydrate order was observed in the water, and (ii) more
than half the aqueous methane had entered the gas phase. Hydrate order was analysed using a set of order
parameters developed previously to distinguish between liquid, ice and hydrate water phases.9 The system
was then cooled to 250 K, 3MPa (about 20 K subcooling for the potentials used10). Long timescale NPT
molecular dynamics (MD) simulations were then performed and analysed for hydrate formation. Hydratelike clusters were found to form within a couple of nanoseconds under these conditions, and during 50 ns
these clusters grew to contain more than half the water in the system. This is the first time crystal nucleation
in such a complex molecular system has been simulated directly. Snapshots of the growth of the hydrate
crystals are given in Figure 1, and it is clear that very substantial hydrate crystals had formed by the end of
the simulation. Since nucleation is a notoriously elusive and stochastic process, variant simulations were also
performed in which (i) the initial film was melted for longer periods of time (2 and 3 ns, instead of the 1 ns
used initially), (ii) partially formed hydrate clusters were disrupted by adding, and later removing, various
inhibitors. This lead to some variation of the induction time before nucleation was observed (in the range 1–
10 ns), but sustained growth of a hydrate crystal/cluster was observed in every case.
0.6 ns
0.9 ns
10.5 ns
5.4 ns
40.2 ns
10.8 ns
Figure 1: simulated nucleation of methane hydrate. Snapshots of the network of water molecules showing
hydrate structure, taken at different times from a simulation without (top) and with inhibitor added at the
beginning (bottom)
The nucleation was validated through various different analyses. The order parameter analysis showed
consistent growth in the fraction of water contained in the hydrate phase. Radial distribution functions
(RDFs) confirmed the disappearance of aqueous methane (seen in the decreasing height of the initial peak at
4 Å in the methane-methane RDF) and exhibited formation then growth in peaks at those distances that
characterise methane hydrate; new peaks also formed at longer distances (typically 5 peaks, out to beyond 10
Å), indicating the onset of long-range order. Further validation came from an analysis of the cage structures
in the crystal nucleus. These clearly showed the signature of a type II hydrate structure, whereas methane
hydrate forms as a type I structure: recent time-dependent diffraction experiments have shown that methane
initially forms metastable type II structures which subsequently change to the thermodynamically stable type
I phase only after significant maturation,11 in conformity with Oswald’s step rule for nucleation. This finding
is also important in that if confirms that the simulated nucleation is not merely a reversal of the initial melt,
since the simulations were set up with a type I (i.e. methane) hydrate film.
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As well as the achievement of simulating nucleation, this work has also been valuable in clarifying the
processes that lead to hydrate nucleation. The prevailing model for nucleation — the labile cluster theory —
envisages nucleation and growth as the aggregation of solvated methane molecules with clathrate-like
solvation cages. Analysis of the simulations showed no evidence for clathrate-like solvation cages, and the
initial hydrate network was actually a convoluted 2-D surface without any closed (3-D) cages. Further, as the
clusters grew they showed very large fluctuations in the instantaneous cluster size, with fluctuations of more
than 60% occurring on a sub-picosecond timescale. Analysis of the cluster shapes showed no evidence that
this was due to the transient bridging links between separate clusters. Instead, the growth process appears to
be a highly collective phenomenon, with small correlated motions in many water molecules creating
flickering hydrogen-bond networks that gradually get captured into the hydrate crystal. This has major
ramifications for the effect of LDHIs, as will be discussed below.
2.2
INHIBITION
Having established the feasibility of simulating spontaneous hydrate nucleation, it became possible to
simulate the effect of LDHIs explicitly. LDHIs, by their nature, are used at concentrations that are too small
to perturb the thermodynamic equilibrium significantly; thus they must act by changing the kinetics of the
system. It is convenient to define two classes of LDHI: anti-agglomerants prevent growth into large, hydrate
deposits, while kinetic inhibitors (KIs) delay the first appearance of stable hydrate crystals. Some compounds
can do both. It was the KIs that were the focus of this project.
Polyvinylpyrrolidone (PVP) was adopted as an archetypal KI. It was the first identified and, together with
polyvinylcapralactam (PVCap), is the best characterised KI in the literature. Experiments have shown that
PVP works best as a low molecular weight oligomer, with greatest activity found for the smallest polymers
that have been tested (currently about 1000 u mean molecular weight). This corresponds to 8–10 monomer
units. kinetic inhibition was therefore simulated by introducing an octamer of PVP into the nucleation
studies. Care was taken to do this with the least possible disruption of the water. Thus, the KI was inserted
into the hydrocarbon phase (typically 5 Å from the water surface), and the water held immobile while the
hydrocarbon phase equilibrated to relieve any stress caused by the insertion. Only then was the fully mobile
system simulated with long timescale MD.
The PVP was found to migrate to the aqueous side of the interface in less than 1 ns, but thereafter remained
near the interface. The effect of the PVP was dependent on the time of addition. When added from the start
of an otherwise nucleating simulation, no nucleation was observed (see Figure 1). When inserted after about
12 ns, by which stage a hydrate cluster containing about 300 water molecules had formed, the hydrate cluster
was found to degrade completely in about 2 ns. When inserted after 25 ns (by which time the cluster
contained more than 600 water molecules and appeared to be above the critical cluster size) no effect was
observed following the addition of the PVP. This is consistent with the behaviour observed experimentally,
whence PVP will delay the onset of nucleation, but once hydrate crystals have formed the PVP does nothing
to prevent the catastrophic growth of methane hydrate. Simulations were repeated by adding PVP in different
locations and at different concentrations (3% and 6% by weight of water), and showed a repeatable effect on
the stability of the hydrate clusters. Further, simulations were performed in which PVP was added, partial
degradation of the hydrate allowed to occur, and then the PVP was removed; these showed immediate regrowth of the hydrate crystal. We conclude that the destabilisation of the hydrate was a real effect of the
PVP, and not simply an artefact of the chaotic/stochastic nature of nucleation. This was further confirmed by
performing simulations with polyvinylstyrene (an inactive polymer) as a control.
Analysis of the KI simulations revealed that no contact was needed between the hydrate crystal and the PVP
for degradation to occur. Indeed, throughout the simulations the PVP remained within 5 Å of the
water/methane interface, while the hydrate crystal remained near the centre of the water film (typically 10–
15 Å from the PVP). Yet, even at this distance, the PVP was effective in destabilising small hydrate crystals.
Snapshots of several conformations taken about 0.5 ns after addition of PVP to a 300-water hydrate cluster
are shown in Figure 2. Transient hydrate-like networks were seen to form between the PVP and the main
cluster, but had the effect of destabilising the cluster. These results clearly imply that surface adsorption is
not essential for LDHI activity, and that a better understanding of the apparent inhibition-at-distance may
help identify completely new chemical motifs for active LDHIs. Detailed analysis of these results is still in
progress, but based on these results we conjecture that both the cluster and the PVP generate flickering
hydrogen bond networks with clathrate-like order—in a kind of halo; however these networks are
incommensurate with each other, and so tend to destabilise sub-critical hydrate nuclei.
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Figure 2: snapshots taken from the middle of a simulation in which PVP was added after the formation of a
300-water hydrate cluster.
The KI simulations indicated that the response of the hydrate to the presence of the KI was relatively rapid,
so that indicative results of activity could be obtained from simulations of 2–3 ns duration. This has enabled
us to perform the first detailed study of the effect of polydispersity within polymeric KIs on their activity. An
ensemble of simulations was performed in which polymers of different weight (4–12 monomers) tacticity
and initial conformation were introduced into the system with partially formed hydrate crystal.* Although
noisy, these results do show a weak mass effect (suggesting that peak activity should be at about 10
monomers). However, there greatest import is in the proof of concept, showing that very detailed simulations
of realistically complex KI systems is now feasible.
2.3
DEVELOPMENT OF NEW COMPOUNDS
During the course of this grant, 13 new compounds have been identified and synthesised at Warwick, and
then tested for LDHI activity at Heriot-Watt. Due to confidentiality issues and preparation of a patent
application, the chemical composition of these additives is not specified in this final report, and compounds
are referred to by an identification tag. Early compounds were based on the sulfo-betaines we have identified
elsewhere 1.12 Two literature routes were adapted for the synthesis of the novel betaines. For betaines with C3
and C4 chains between the ammonium and sulfonate groups, nucleophilic ring-opening of the appropriate
sultone with a range of amines was used.13 For C2 linked betaines, reaction of the amine with an ester of
vinyl sulfonic acid proved most effective.14 We found that judicious choices of ester group R’ and of reaction
conditions are necessary for each amine substrate. Control of the rate of the ester cleavage appears to be
crucial; the ester must remain intact until the nucleophilic addition is complete (since vinyl sulfonic acid is
insufficiently reactive), but if cleavage is too slow the desired product is not obtained.
n
SO 2 + R 3N
SO 3
n
R 3N
O
O2
S
*
R 3N
OR'
R 3N
O2
S
R3N
OR'
R 3N
SO 3
These simulations were performed using the time on the HPCx facility obtained through the MPP Materials
Consortium (headed by C.R.A. Catlow). Since the consortium obtained the time under a separate EPSRC grant
(itemising a number of projects such as this LDHI study), the use of this central facility will be itemised on the final
report of the consortium grant, and so has not been itemised in the IGR form accompanying the present report.
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A full explanation of the LDHI testing procedures will be provided by the Heriot-Watt group in their final
report. Activity was measured in terms of the induction time (tind): the time between reaching a specified
subcooling (usually taken to be 10 °C) and the first appearance of hydrates.
The first 10 compounds tested produced only mild activity, with the best (KI-7 and KI-8) producing
induction times that were about 1/3 that induced by a commercial KI blend based on PVCap. Interestingly,
KI–7 and KI–8 did seem to perform comparatively better with methane hydrate (structure I) than with
natural gas hydrate (structure II). In contrast, the latest compounds (KI-12 and KI-13) appear to be extremely
active, giving induction times that are up to an order of magnitude longer than those observed with the
commercial PVCap blend. Patent applications are being prepared for these latest compounds. A summary of
the induction times is given in Table 1.
System
tind / hrs
Commercial (PVCap)
PVP
KI-7
KI-12
KI-13
Methane
(structure I)
5
1
2
–
150
Natural Gas
(structure II)
16
13
6
40
150
Table 1: induction times for various LDHIs at subcoolings of 10 °C
3
Project Plan Review
Essentially all the key objectives stated in the original report have been achieved. In particular, of those
objectives that were to be led by the Warwick group, we have:
•
characterised the microscopic structural changes associated with hydrate nucleation and growth;
•
elucidated the microscopic mechanisms by which LDHIs inhibit nucleation of hydrate crystals;
•
designed new LDHIs that can out-perform current commercial LDHI blends;
•
established an active collaboration with the hydrate research centre at Heriot-Watt
We note that this was a collaborative project with the Gas Hydrate research group at Heriot-Watt, and the
achievement of some of the stated objectives will be described in their final report, once their grant
terminates.
There were some small changes in the details of the work plan, but these were adopted because they
enhanced the achievement of the above goals. In particular, rather than simulating a series of different
inhibitor archetypes, as originally proposed, we chose instead to undertake the first molecular level study of
polydispersity in LDHIs. Since most LDHIs are polymeric/oligomeric systems, the active component will be
present with a mixture of molecular weights, tacticities and conformations; the activity of PVP is known to
depend on the average molecular weight of the compound used, but the effect of the other properties is
unknown. The success of our research project, combined with the advent of the HPCx facility, gave us the
chance to perform simulations with a large range of PVP oligomers, and thus to obtain the first molecularlevel information about variations in activity between the different oligomers. Simulations with different
LDHI archetypes are continuing through an industrial CASE award from ICI.
4
Research Impact and Benefits to Society
The results of this research have created considerable excitement in the hydrate community, and are likely to
change the way in which the oil industry develops its inhibitors. When this work was presented at the RSC
“Chemistry in the Oil Industry” conference (Manchester, November 2003), at least four separate companies
requested formal consultations on whether these calculations could incorporated into their hydrate research
programmes. This was in addition to the pre-existent collaboration we had established with ICI/Uniqema
during the life of this project. Negotiations are ongoing with these companies, and a list of contacts can be
provided if requested.
In addition, the research performed in this project has established an authoritative collaboration between the
Warwick and Heriot-Watt groups, which has enabled us to lead in preparing and submitting an application
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for a Marie-Curie Research Training Network (also involving groups in Norway, Germany, Holland and
Hungary and totalling 2.7 million €) that will take the understanding of hydrate growth and inhibition
developed in this project and use it to underpin some exciting new hydrate-based technologies for gas
transport, CO2 capture and storage, and energy management.
5
Explanation of Expenditure
Expenditure was incurred essentially as proposed. The one minor variation is that we chose to perform the
synthetic work by employing PDRAs who were already at Warwick on a series of short-term contracts,
rather than employing a technician to help with the synthesis. This was felt to be more productive and to
provide a better time frame for the project: short, intensive periods of synthesis could provide several new
compounds, and then allow time for these to be tested and the results digested before commencing the next
session of synthesis.
6
Further Research or Dissemination Activities
A number of activities are planned to maximize the transfer of knowledge from this project to potential
beneficiaries. Discussion and consultation with the oil companies mentioned above will continue to ensure
there is maximum uptake of our new methods and compounds in their hydrate research programs. We are
also in the process of preparing a patent application for the new compounds developed in this project, and
will seek suitable partners to develop applications for these compounds once patents are issued. A further
four publications arising from this work—on top of the 7 already published/accepted—are already in draft
form, and will be submitted to a mixture of Chemistry and Chemical Engineering journals so as to maximize
the breadth readership background. Finally, this material will continue to be presented at conferences and
seminars. It has already been presented orally at the 4th International Conference on Gas Hydrates
(Yokohama, May 2002), at the RSC Chemistry in the Oil Industry conference (Manchester, November 2003)
and as a plenary talk at the CCP5 annual conference (Cardiff, September, 2003). It will also form the basis of
a plenary talk at the RSC Nanoclusters, Nucleation and Crystal Growth conference (Oxford, April 2004).
Further, the expertise gained in this project has led the PI to become coordinator on a Marie Curie Research
Training Network Application involving research groups in the UK, Norway, Germany, Holland and
Hungary together with 6 European industrial companies and a separate INTAS proposal involving groups in
the UK, Russia and Slovenia. If successful, these will make an ideal forum for further dissemination, as well
as providing fertile collaborations to extend the work.
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