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Designing Kinetic Hydrate Inhibitors – Eight Projects With Only Partial Success, But Some Lessons Learnt Malcolm A. Kelland Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 24 Apr 2017 Downloaded from http://pubs.acs.org on April 25, 2017
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Designing Kinetic Hydrate Inhibitors – Eight Projects With Only Partial Success, But Some Lessons Learnt Malcolm A. Kelland *
Department of Mathematics and Natural Science, Faculty of Science and Technology, University of Stavanger, N-4036 Stavanger, Norway
* Corresponding author: Tel.: +47 51831823; fax +47 51831750 E-mail address:
[email protected] (M.A. Kelland) Keywords: petroleum, gas hydrates, kinetic hydrate inhibitors, kinetics, mechanisms
Abstract In our quest to develop new and improved kinetic hydrate inhibitors (KHIs) for practical use in the oil and gas industry, we have carried out a number of projects that, in our hands, proved ultimately unsuccessful. However, as with most laboratory research projects, useful data has been obtained and some important lessons have been learned. These lessons can be helpful in several ways. Firstly, to understand the scope and limitations of chemicals that could be used as KHIs. Secondly, to highlight mechanistic aspects of KHI theory. Finally, the work may help inspire others to develop related but more successful research projects. In this paper we present the results of eight partially successful KHI research projects and explain why each of these projects was undertaken and the results obtained. Seven of the projects concern new classes of polymers, and the other project describes what we hoped was a new class of nonpolymeric synergists for KHI polymers, but instead gave the opposite effect. All new KHI products were investigated for their performance in high pressure multiple steel rocking cells using a Structure II-forming gas mixture.
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Background Kinetic hydrate inhibitors (KHIs) are one of the chemical methods used by the upstream oil and gas industry to prevent plugging of flow lines by gas hydrates.1-5 Commercial KHIs are formulations based around a water-soluble polymer or polymers often with added synergists that might also include the carrier solvent or solvents. KHIs belong to the family of low dosage hydrate inhibitors (LDHIs) as they are deployed at much lower concentrations than thermodynamic hydrate inhibitors (THIs). Anti-agglomerants (AAs), used for dispersing gas hydrates are the other major class of LDHIs and are described elsewhere.2,6 Typical KHI dosages are often 1-3 wt.% when diluted to viscosities suitable for injection into umbilical flow lines. Several classes of KHI polymer are commercially in use, including polymers and copolymers based on the monomers N-vinyl pyrrolidone, N-vinylcaprolactam, and Nisopropylmethacrylamide as well as hyperbranched poly(ester amide)s based on diisopropanolamine and various cyclic anhydrides.6 The mechanism of KHIs is still unclear. There is evidence to suggest that they can prevent both nucleation and crystal growth of gas hydrates, i.e. both before clusters and nuclei reach the critical nuclear size, and after when the Gibbs free energy change is negative for spontaneous crystal growth.1,7-11 The corresponding author has been involved in KHI (and AA) research and development since 1991. The main goal has usually been to develop new commercial products. However, in the last 10 years our university group has tried to explore new chemistries for a variety of reasons: •
Understand the KHI mechanism(s) operating and develop structure-activity relationships.
•
Improve performance – current KHIs are often limited to applications of maximum 10-12oC subcooling for hold times of a few days, but depending on many factors.
•
Improve KHI environmental acceptability – readily and fully biodegradable KHIs (BOD28 > 60% for the polymer in OECD306 seawater tests) are available but as far as we are aware not currently in field use.6,12
•
Improve treatment cost per barrel of produced water.
•
Improve compatibility issues such as cloud and deposition points, without loss of performance.
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One area we have not addressed in our laboratory research how to optimize the polymerization procedure for best performance. Significant enhancement can be made this way, even with polymers of the same composition and molecular weight distribution.1,6 This is an area of technology we have generally left to the chemical and service companies. In addition, some laboratory procedures and reactants are difficult to scale up cost-effectively. Thus, if large scale synthesis is possible, it still may also be difficult to reproduce exactly the same polymer composition made and tested in the laboratory.
Pilot scale testing and
optimization of the chemical engineering for product consistency are usually essential. Many projects we have undertaken in the past decade have not been as successful as hoped, for a variety of reasons. However, as any researcher knows, negative results are still results and they are often useful. The understanding they bring can help guide the researcher to more positive results. For this reason, we present here results from eight projects that overall were not as successful as hoped or planned, although most of them do include some positive experimental results. We hope these results can be of help to the hydrate inhibitor R&D community, who maybe can find more successful routes to the products discussed here, or inspire them to develop other related technology, or prevent them going down some dead-end paths. The eight projects and the order in which they are discussed are as follows: 1. Polyphosphazenes 2. Hyperbranched polyethyleneimine derivatives - reaction with lactones to form lactam polymers 3. Attempted polymerisation of 2-methyl-vinyllactams 4. Attempted polymerisation of N-allyl lactams 5. Attempted polymerization of N-isopropylethacrylamide 6. Attempted oxidation of poly(N-vinyl caprolactam) to poly(N-vinyl adipimide) 7. Attempted polymerization of acrylolactams with and without alkylene spacer groups 8. Reversing the charge – tetraaryl- and tetraalkylborates as alternatives to tetraalkylammonium salt KHI synergists
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Experimental Methods - KHI Rocking cell slow constant cooling experiments All KHI tests described in the nine projects were carried out in high pressure steel rocking cells using the same test method, called the “Slow Constant Cooling” method.13-14 These KHI screening experiments were carried in five parallel cells, each of 40 mL volume. (Figure 1). Each cell contained a steel ball. The equipment was manufactured by PSL Systemtechnik, Germany. The cells were pressurised with a synthetic natural gas mixture with composition given in Table 1.
Figure 1. The rocker rig showing the 5 steel cells in a cooling bath.
Table 1. Composition of Synthetic Natural Gas (SNG). Component
Mole %
Methane
80.67
Ethane
10.20
Propane
4.90
Isobutane
1.53
n-butane
0.76
N2
0.10
CO2
1.84
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The slow constant cooling test procedure has been described in detail previously and shown to be a suitable method for screening and ranking new classes of KHIs as well as in comparison to systems with no additives or commercial KHIs.12 The more recent CGI (crystal growth inhibition) method developed by Total oil company, Heriot-Watt University, Scotland and Hydrafact, using partially melted hydrates, has been shown to be complementary and also to give a good understanding of the field limitations of a KHI product.8-9,15-16 A brief summary of the slow constant cooling method is given here. The cells are filled with 20ml of aqueous solution containing the desired test chemicals. After purging the cells are pressurised to 76 bar, rocked (20 rocks per minute at an angle of 40°) and cooled from 20.5oC to 2oC at a rate of 1 °C/h. The equilibrium temperature (HET) by dissociation was measured experimentally as gave 20.2 °C ± 0.05 °C at 76 bar.17-18 Figure 2 shows and example of the pressure and temperature data of all five cells for the same chemical at 2500 ppm. The pressure drops initially about 2 bar due to gas being dissolved in the aqueous phase as rocking begins. The average onset temperature when we observe a pressure drop due to the first sign of hydrate formation (To) is recorded. Nucleation of hydrates may have occurred earlier but could not be detected on the macroscopic scale. We also record the temperature at which rapid hydrate formation first occurs at its fastest (Ta). Figure 3 shows how these values are obtained from the graph for one rocking cell. In general we observe deviations of ±10-15% for To values and 10% for Ta values.
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Figure 2. Example of the graphical results from five parallel rocking cell slow constant cooling tests with the same chemical conditions.
Figure 3. Typical pressure and temperature graphical data obtained from five cells during a standard slow constant cooling KHI test.
Background, Results and Discussion 1. Polyphosphazenes Background Polyphosphazene chemistry has been around since the early 1960’s, largely due to the work of Allcock and co-workers.19-20 A wide variety of polymers are now available and some have become commercially available, for uses such as elastomers and in fire-resistant foams. Water-soluble polyphosphazenes are also biodegradable.21 The entry point to these polymers is
via
poly(dichlorophosphazene)
(PDCP)
hexachlorocyclotriphosphazene (Figure 4).
which
is
in
turn
made
from
22-23
PDCP reacts with a variety of functional groups, such as amines, alcohols and alkoxides.24-27 For example, poly(phosphazenium) polymer electrolytes can be made by reaction of secondary
methylcyclohexylamine
with
PDCP.
In
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some
cases,
water-soluble
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polyphosphazenes have been synthesized. The first water-soluble polymer to be made was [poly(dimethylamino)phosphazene]s.28 The water-solubility is believed to be due to small side-groups which allows the NH group to be exposed to hydrogen-bonding from water molecules. A large excess of amine (e.g. 10-fold) is usually required.
Figure
4.
Formation
of
poly(dichlorophosphazene)
(PDCP)
from
hexachlorocyclotriphosphazene.
We attempted to make a series of such polymers for testing as KHIs, but encountered several difficulties. For example, it is known that the NH2-substituted polymer is hydrolytically unstable.19 Also, the methylamino substituted polymer crosslinks the chain during substitution, except when the reaction is carried out below room temperature. The nature of the hydrolytic stability is unclear, but ethyl glycinato-substituted polymers are not stable to water.29 In general, the longer the pendant alkyl group the more hydrolytically stable is the polymer, but the less water-soluble. The general synthetic method we chose, was to react PDCP with small alkylamines so as to keep the side-groups small enough for water-solubility (Figure 5). PDCP was purchased from Strem Chemicals, USA. The molecular weight given was 10,000 g/mole. However, we obtained a Mn value of 1370 g/mole (PDI = 1.01) by GPC in DMF solvent using polymethylmethacrylate calibration standards. This PDCP was then reacted with up to 16 equivalents of various amines in an inert solvent such as tetrahydrofuran or 1,4-dioxane. The large amount of amine is used to remove the generated HCl, otherwise it may react with the imine nitrogen in the polymer backbone. In our hands, addition of the first amine equivalent (both primary or secondary amine) gave an immediate visible reaction at 0oC, but it was
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unclear how long it took for second amine to react. Replacement of the second chlorine in PDCP appears to often require forcing conditions (higher temperatures or pressure) besides the use of excess amine (nucleophile). A temperature of 0oC was used to avoid cross-linking from the first-formed P–NH-R groups with unreacted P-Cl groups. After 1-3 hours reaction time the temperature was warmed to 20oC.
Figure 5. Synthesis of [poly(alkylamino)phosphazene]s.
Although we tried a range of amines we only managed to obtain two polymers that were water-soluble, one from reaction of PDCP with 2-aminocaprolactam (with a final reflux in THF) and one with ethylamine. The reason for making the polymer with caprolactam groups was to mimic the structure of poly(N-vinyl caprolactam) (PVCap). The ethylamine derivative was only sparingly soluble in water but we could test it as a KHI at 2500ppm (Figure 6).30 The n-propylamine
or
iso-propylamine
derivatives
were
insoluble
in
water. The
[poly(caprolactam-2-amino)phosphazene] polymer was also water-soluble to 2500ppm and we obtained a Mn value of 1570 g/mole (PDI = 1.02) by GPC analysis (Figure 6). The results of a series of rocking cell KHI screening tests with these polymers is given in Table 2.
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Figure 6. Structures of [poly(ethylamino)phosphazene] (left) and [poly(caprolactam-2amino)phosphazene] (right). Table 2. Results of 5-10 rocking cell tests at 2500ppm of polydichlorophosphazene (PDCP) derivatives and PVCap for comparison. Polymer
No. of
Average To
Average Ta
tests
(oC)
(oC)
No additive
10
17.5
17.1
PVCap
10
8.9
7.9
Aminocaprolactam
9
13.5 (13.3.-14.1)
13.2
Aminocaprolactam
5
16.0 (15.8-16.3)
15.7
Ethylamine
9
11.8 (10.8-12.8)
11.5
Ethylamine repeat
11
10.8 (10.1-11.6)
10.4
– after 4 days in water
synthesis
With no additive a pressure drop due to gas hydrate formation was first detected at 17.5oC (To value). With the aminocaprolactam polymer, we obtained an average To value of 13.5oC with values ranging from 13.3-14.1oC. In comparison the commercial KHI from BASF, PVCap (Mw = 2000-4000 g/mole, supplied in monoethyleneglycol), gave an average To value of 8.9oC at the same concentration (2500ppm). This means the aminocaprolactam polymer has only a weak KHI effect. Moreover, we found that testing the same solution four days later gave a worse result, now with an average To of 16.0oC. This indicates that polymer degradation, probably by hydrolysis, has occurred. Polyphosphazenes are known to hydrolyse, especially at low or high pH conditions. The pH of the initial solution was about 7 but it dropped to about 5 after the test due to adsorption of the acid gas CO2 in the natural gas mixture, setting up an equilibrium with carbonic acid. The pH may have been even lower during the test due to the increased pressure and therefore greater solubility of CO2 in the aqueous phase. Two samples of [poly(ethylamino)phosphazene] were also prepared. At 2500pppm both gave significantly better results than the aminocaprolactam polymer but not as good as PVCap. However, it appears we struggled to get consistency in synthesis for the two batches as the ACS Paragon Plus Environment
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average To value between them differs by about 1.0oC which from a t-test gives a p-value of
