Synthesis of Effective Kinetic Inhibitors for Natural Gas HydratesXia Lou,*,† Ailin Ding,† Nobuo Maeda,‡ Shuo Wang,† Karen Kozielski,§ and Patrick G. Hartley‡
†Department of Chemical Engineering, Curtin University, Kent Street, Bentley, Western Australia 6102, Australia‡Materials Science and Engineering, and §Earth Science and Resource Engineering, Commonwealth Scientific andIndustrial Research Organisation (CSIRO), Ian Wark Laboratory, Bayview Avenue, Clayton, Victoria 3168, Australia
ABSTRACT: Six novel polymer-based kinetic hydrate inhibitors (KHIs) were synthesized and characterized. Their performancein inhibiting both tetrahydrofuran (THF) hydrate and the synthetic gas hydrate formation was examined using two differentinstruments: a ball-stop apparatus and a high-pressure automatic lag time apparatus (HP-ALTA). Performance was benchmarkedagainst two commercially available KHIs, Gaffix VC-713 and Luvicap EG, under the same working conditions. The test resultsfrom the ball-stop rig demonstrated that the new KHIs were as effective as Gaffix VC-713 and Luvicap EG in preventing theformation of THF hydrates in 3.5 wt % NaCl solutions. For the synthetic gases, most new polymers outperformed the referenceKHIs at a concentration of 0.05 wt %. Polymers containing a pendant THF functional group in the side chains showed asubstantial 12−16 °C decrease in the hydrate formation temperature of the gas water mixtures relative to those containing thesame amount of Gaffix VC-713 or Luvicap EG. The trend of the inhibition performance of the polymers was different in THFfrom that measured for gas mixtures. Small amounts of ethanol added to the hydrate formation mixtures were also shown to havean effect. Investigation of the inhibition mechanism associated with these new polymers is under way.
1. INTRODUCTIONHydrates are crystalline, ice-like solids that form when gasmolecules are trapped in hydrogen-bonded water cages at highpressure and low temperature,1 conditions which are oftenencountered in deepwater offshore operations. The formationof gas hydrate plugs in subsea pipelines can result in serioussafety and flow assurance issues for the oil and gas industry.2,3
Injection of thermodynamic inhibitors, such as alcohols,glycols, or aqueous electrolytes, has been a commonly usedmethod to prevent the formation of gas hydrates in productionpipelines. The method has proven to be effective, but theeconomic drawbacks are significant. Large volumes of inhibitorsare required, generally between 20 and 60% by weight. The costassociated with the use and recovery of inhibitors in suchvolumes is very high. The worldwide annual expense for themost commonly used thermodynamic inhibitor, methanol,alone was estimated at U.S. $220 million in 2003.4 Potentialenvironmental pollution by these chemicals has also been agreat concern.5 The desire to reduce the costs and environ-mental impacts associated with the use of thermodynamic inhi-bitors has led to increased research activities for the design,development, and inhibition mechanism exploration ofnovel, environmentally friendly low-dose hydrate inhibitors(LDHIs).6−9
Kinetic hydrate inhibitors (KHIs) are a class of LDHIs thathave been in commercial use in the oil and gas industry for overa decade.10 They are used at low concentrations, typically lessthan 1 wt % of the aqueous phase. These chemicals do not alterthe thermodynamics of hydrate formation, but they modify thekinetics of formation, by either preventing nucleation, hinderingthe crystal growth, or both. The nucleation time, often referredto as induction time, is a critical factor for field operations. It isdependent upon the subcooling, ΔT, the difference betweenthe thermodynamic hydrate equilibrium temperature and theoperating temperature at a given pressure for a specific gas
composition. KHIs are often ranked by the maximumsubcooling achievable in comparable systems or by comparisonof induction time as a function of the subcooling and inhibitorconcentration.11
KHIs are generally water-soluble polymers. The earliestoffshore field test of KHIs was carried out on the southernNorth Sea gas field in 1995, using Gaffix VC-713, a terpolymerof vinylcaprolactam (VCap), vinylpyrrolidone (VP), and dime-thylaminoethyl methacrylates. It was reported to be effectiveat 0.5 wt % and 8−9 °C of subcooling.12 Soon after that, a KHIblend based on VCap polymers and tetrabutylammoniumbromide (TBAB) successfully replaced glycol and became thefirst offshore field application of a KHI on BP’s southern NorthSea gas field.13 Successful applications of co-polymers of VCapand vinylmethylacetamide, poly(VIMA/VCAP), have beenreported in four further fields.14 This co-polymer outperformedVCap by 2−3 °C subcooling. By the end of 2005, it wasestimated that there were 40−50 field applications of KHIs.6
These KHIs can be divided into two classes of polymers: one isthe homo- and co-polymers of VCap, and the other is hyper-branched poly(ester amide)s.8 The former, the polyvinylcap-rolactam (PVCap), and its co-polymers contain seven-mem-bered lactam rings attached to the polymer backbone. It isbelieved that the hydrophilic lactam group plays an importantrole in inhibiting hydrate growth and that the hydrogen bond-ing between the functional group and water molecules leads tobinding of the inhibitors on the hydrate surface, which blocksthe transport of gas to the hydrate surface and disrupts thehydrate formation.15
An earlier report by Sloan et al.16 indicated that the per-formance of PVCap is related to the molecular weight of the
Received: September 29, 2011Revised: December 29, 2011Published: December 29, 2011
polymer. Polymers with an average molecular weight of 900 Dawere more effective than those of 1300, 2100, 9200, and 18 000Da. It is believed that low-molecular-weight PVCap-basedproducts with added synergists are probably the best KHIs fornatural gas hydrate inhibition on the market today.6 This KHIblend gave at least 48 h of hydrate inhibition at a subcooling of13 °C.17
In this paper, six new polymers were designed and synthe-sized. Their performance as a KHI was examined using a ball-stop rig and a high-pressure automatic lag time apparatus (HP-ALTA) in both tetrahydrofuran (THF) hydrate formation solu-tions and gas hydrate formation mixtures. The results werecompared to the commercially available and well-known KHIs,Gaffix VC-713 and Luvicap EG, both containing lactam rings inthe polymer backbones.Two of these polymers contain VCap moieties and a second
component, poly(ethylene oxide) (PEO) (Figure 1A). As men-tioned previously, the inhibition performance of PVCap can beimproved by various synergists, including high-molecular-weight PEO.18 Although PEO itself is not a kinetic inhibitor,the addition of PEO to a kinetic inhibitor solution was found toenhance the performance of the inhibitor by an order ofmagnitude in some cases. Therefore, we believe that incorpora-ting PEO into polymer chains may improve the inhibition
performance. The synthetic route of these polymers isillustrated in Figure 1A.The other four polymers were designed to incorporate THF,
a five-membered ring, into the polymers as a pendent group onthe side chains (Figure 1B). THF is a well-known hydrate guestmolecule. Unlike gas molecules, such as methane, ethane, orpropane, THF is hydrophilic and completely miscible withwater, indicating a strong affinity with water molecules. Becausewe know that adsorption of KHI molecules onto the surfaces ofparticles or hydrate crystals may prevent hydrate growth, theidea of incorporating THF pendent groups onto hydrophobicpolymer backbones was to use the strong affinity between THFrings and the water molecules to improve the adsorption of thepolymer, therefore making the inhibition more effective. Whilethe THF rings may adhere onto the hydrate surface moreeffectively, the hydrophobic polymer backbones form a blanketto sterically hinder further growth of hydrate.Two methods were used in this study to examine inhibition.
One was the ball-stop rig, which has been commonly used tomeasure induction time and ball-stop time of a model hydrate-forming system (THF−water) in the presence of inhibitors.19
While the induction time represents the end of nucleation,which is indicated when hydrates or cloudy points are visuallyobserved, the ball-stop time indicates a total block of the testvessel by the formed hydrates. The testing rig consists of small
Figure 1. Synthetic scheme of polymers.
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cells or test tubes filled with hydrate-forming solutions that areplaced in a cooling bath. Each cell or test tube contains a metalball rocking back and forth. The metal ball stops moving whenthe hydrate is formed and plugs the test tube. Ball-stop rigshave been used to screen a vast range of chemicals as gashydrate inhibitors.20 In this study, ball-stop times are taken asan indication of hydrate inhibition efficiency of KHIs. A moredetailed discussion about the method can be found in ourprevious work.21
In comparison to ball-stop rigs, the automatic lag timeapparatus (ALTA) is a more recent development. ALTA wasinitially built by Haymet’s group for studying the nucleation ofsupercooled liquids.22 After several important improvements bythe same research group, ALTA was subsequently employed forstatistical evaluation of liquid-to-crystal heterogeneous nuclea-tion by monitoring the nucleation temperature using lightscattering principles.23 Using ALTA, samples are repeatedlycooled on a linear cooling ramp until they nucleate and becomesolid. The samples are then warmed and thawed. Because thecooling ramp is linear, either the time or temperature at whichfreezing occurs is a useful parameter with which to measure theinduction time to nucleation.24 The freeze−thaw cycle can berepeated many hundreds of times in a single experiment togenerate reliable and reproducible statistics for nucleation.More recent studies on THF/water hydrates have shown thatALTA is an excellent instrument to evaluate the effectiveness ofKHIs by measuring the freezing temperature, i.e., the hydrateformation temperature, of hydrate-forming mixtures in the pre-sence of KHIs.25 Very recently, measurements of the hydrateformation temperature of hydrate-forming mixtures at elevatedgas pressures became possible using a high-pressure version ofthis instrument known as HP-ALTA.26
[PEOMA, average Mn ∼ 475 (I) and 1100 (II)], N-vinylcaprolactam(VCap, 98%), tetrahydrofurfuryl methacrylate (THFMA, 97%), andTHF (99%) were purchased from Sigma-Aldrich and used as received.Chloroform (Pronalys, 99%), hexane (Shell, technical grade), toluene(Labscan Analytical Science, 99.8%), ethanol (Scharlau Chemie, 99%),and ethylene glycol (Marck, 99.5%) were used as purchased withoutfurther purification. Luvicap EG, 40% poly(N-vinylcaprolactam)(Mw ∼ 2000) in ethylene glycol, was purchased from BASF (Germany),and Gaffix VC-713, 37% poly(vinylcaprolactam vinylpyrrolidone dim-ethylaminoethyl methacrylates) (Mw ∼ 82 700) in ethanol, was kindlydonated by International Specialty Products (ISP, Germany). 2.2′-Azobis(2-methylpropinitrile) (AIBN, Sigma-Aldrich) was recrystal-lized, and methyl methacrylate (MMA, ICI) was washed with a NaOHsolution before usage. Sodium chloride (Lab-Scan Analytical Science,99%) and deionized water were used in the preparation of sodiumchloride solutions.2.2. Synthesis of New Polymer KHIs. 2.2.1. PEO-co-VCap-I
and PEO-co-VCap-II. Co-polymers, PEO-co-VCap-I and PEO-co-VCap-II, were prepared by free-radical solution co-polymerization inthe following way: VCap (2.37 g, 17 mmol) and an initiator AIBN(65.68 mg, 2 mol % of total monomer) were dissolved in toluene(20 mL) in a three-neck round-bottom flask and flushed with oxygen-free nitrogen for at least 20 min. Under the protection of nitrogen, thereaction mixture was heated to 80 °C, followed by the addition of asecond monomer PEOMA (3 mmol) after 25 min. The polymer-ization mixture was stirred at 80 °C for another 140 min. After coolingto room temperature, the produced co-polymers were precipitatedfrom the reaction mixture by adding in an excess of hexane (200 mL).The precipitated co-polymers were separated from hexane and dis-solved in chloroform (20 mL). A reprecipitation was carried out toremove unreacted monomers and oligomers. After three reprecipitation
cycles and the removal of solvent by a rotary evaporator, the co-polymerswere dried to a constant weight in an oven at 37 °C. PEOMAs of twodifferent molecular weights were used in this prepara-tion. The molecular weights of the PEOs and the molar percentage ofPEO and VCap in these polymers can be found in Table 1.
2.2.2. PTHFMA Homo-polymer. THFMA (17.02 g) and AIBN(0.328 mg) were dissolved in toluene (200 mL) and then flushed withoxygen-free nitrogen for at least 20 min. Under the protection of nitro-gen, the reaction mixture was heated to 65 °C. The polymerizationmixture was stirred at 65 °C for 10 h. After cooling to room temper-ature, the polymer was precipitated in an excess of hexane (250 mL).Three reprecipitation cycles were carried out using chloroform andhexane to remove monomers and oligomers. PTHFMA was obtainedby drying the polymers to a constant weight in an oven at 37 °C.
2.2.3. PTHFMA Co-polymers. Co-polymers PTHFMA-co-MMA,PTHFMA-co-PEO, and PTHFMA-co-VCap were synthesized usingthe above procedure in which a co-monomer MMA, PEO, or VCap wasadded according to the chemical composition displayed in Table 1.
2.3. Characterization of Inhibitors. A Perkin-Elmer Spectrum100 spectrometer was used to obtain an infrared spectrum for eachpolymer. The spectra were collected at room temperature with aresolution of 4 cm−1. The data were taken between 650 and 4000 cm−1.
A Varian 380-LC HPLC/GPC was used to determine the molecularweight of produced polymer inhibitors. PolarGel L (Varian) was usedas the separation column, and the signal was collected by anevaporative light scattering detector. THF was used as the mobilephase, and narrow polydispersity PMMA standards (EasiVial PMMA,Polymer Laboratories) were used for GPC calibration.
2.4. Inhibition Performance Testing. 2.4.1. Ball-Stop RigTesting. A mixture of THF and 3.5 wt % sodium chloride was used asa hydrate-forming solution in the ball-stop rig testing experiments. Themolar ratio of THF and water was kept at 1:17. The hydrate-formingsolution is represented as THF−NaCl in the following sections. Toconduct the measurements, a rotating ball-stop rig was setup asreported in a previous work.21 The THF−NaCl solutions containingvarious polymer KHIs at the desired concentrations were injected intothe test tubes, each of which contained a stainless-steel ball. The testtubes were then mounted on the ball-stop rig that was kept in an ice−water bath. The temperature was monitored at regular intervals toensure that the water bath remained at 0 °C during the experiment.The tests were run at atmospheric pressure for up to 6 h. The time atwhich a ball in the test tube stopped because of the formation of THFhydrates was recorded as a measure of the hydrate inhibition perfor-mance. Three parallel tests were conducted for each of the hydrate-forming solutions. Details of the experimental setup and solutionpreparation procedures can be found in the study by Ding et al.21
2.4.2. HP-ALTA Testing. The HP-ALTA setup is displayed inFigure 2. The high-pressure sample chamber is made of stainless steeland sandwiched between two Peltier devices, which in turn are sand-wiched between two heat sinks. The apparatus allows for a smallvolume (∼150 μL) of water to be cooled at a controlled rate in apressurized gas atmosphere and the temperature of gas hydrateformation to be detected. After each formation of gas hydrate, thesample was dissociated at a temperature of about 15 K above thethermodyanamic equilibrium dissociation temperature of the gashydrate for at least 200 s. We assume that these conditions aresufficient to eliminate the so-called “structural memory effect”. We also
Table 1. Monomer Composition (mol %) for InhibitorSynthesis
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note that the addition of small concentrations (typically less than1 wt %) of KHIs is not expected to significantly alter the thermody-
namic equilibrium dissociation temperature of gas hydrates, and hence,the dissociation conditions discussed above were retained for KHI-containing samples.
Using HP-ALTA, the process of forming and dissociating gashydrates is automatically repeated for a statistically significant numberof nucleation events. The so-called “interfacial transmittanceconfiguration” of the instrument was adopted for this study. Moredetails can be found in the study by Maeda et al.26
In this study, a mixture of methane (90 mol %) and propane (10 mol %)(C1/C3 gas) was used in the presence of an aqueous solution contain-ing 0.05 wt % KHIs. The newly synthesized polymer KHIs were firstdissolved in ethanol prior to their mixing with water. The concentrationof KHIs in ethanol was 37 wt %, which is the polymer concentration inGaffix VC-713. A cooling rate of 0.025 °C/s was used. Measurementsfor samples containing KHIs were carried out at 10.5 ± 0.5 MPa for allsamples to compare their relative effectiveness under similar conditions.
3. RESULTS AND DISCUSSION
3.1. Polymer Characterization. Six novel polymers andco-polymers containing various organic moieties and pendentgroups were synthesized. These include PEO-co-VCap-I, PEO-co-VCap-II, PTHFMA, PTHFMA-co-MMA, PTHFMA-co-PEO, and PTHFMA-co-VCap. The measured molecularweights of the produced polymers ranged from 18 973 to 69310 Da and are in the range of the molecular weights ofLuvicap EG (2000 Da) and Gaffix VC-713 (82 700 Da).The produced polymers were characterized using Fourier
transform infrared (FTIR) spectroscopy to confirm the pre-sence of the co-monomers and the total removal of unreactedmonomers. An example of a comparative FTIR transmittancespectrum of a co-polymer PTHFMA-co-VCap and its twomonomers THFMA and VCap is displayed in Figure 3. In thisfigure, the peaks in the range of 2800−3000 cm−1 are attributedto the asymmetric and symmetric stretching of CH2 and CH3,
Figure 2. Schematic illustration of HP-ALTA.
Figure 3. FTIR spectra of (a) THFMA, (b) PTHFMA-co-VCap, and (c) VCap.
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which are present in both monomers and the producedpolymer (spectra a, b, and c). The peak at 3103 cm−1 of theTHFMA spectrum a and that at 3109 cm−1 of VCap spectrum c,both arising from the polymerizing vinyl group CC−Hstretching, have disappeared in the produced polymerPTHFMA-co-VCap spectrum b. The peaks at 1638 cm−1 ofspectrum a and at 1654 cm−1 of spectrum c, attributed to CCdouble bond stretching, also disappeared in the PTHFMA-co-VCap (spectrum b). These indicated complete polymerizationand/or total removal of the monomers.In addition, the presence of the amide CO stretching
vibration at 1636 at cm−1 and the ester CO stretching at1716 cm−1 in spectrum b have confirmed the presence of boththe caprolactam ring from VCap and the COOCH2THF groupfrom THFMA. For both amide CO and ester CO, therewas a slight shift of the signal to a higher wavenumber incomparison to those of the monomers. This is a result ofpolymerization that has converted all CC double bonds intoC−C single bonds; therefore, the CO bonds are no longerconjugated with CC.3.2. Ball-Stop Rig Testing. The measured ball-stop times
for THF hydrate-forming solutions are summarized in Table 2.
In this table, the concentration of an inhibitor was calculatedon the basis of the ratio of the polymer mass and the mass ofthe THF−NaCl solution. The reported ball-stop time wasbased on the observations of three parallel experiments. For aspecific polymer KHI, all three test tubes must demonstratezero hydrate formation over a 6 h period to have a resultrecored as >360 min. Observations other than that are reportedas <360 min. It should be noted that the observations recordedwithin a period of 360 min have been found to be reliable andreproducible for a comparison of the inhibitor performance.20,22
The ball-stop time can be extended to greater than 12 h in ourstudy21 or 24 h in the work of others19 once the inhibitionperformance passes the 6 h (360 min) mark.Most rig testing experiments were carried out on the THF−
NaCl hydrate formation solutions that were directly mixed withcalculated amounts of dried inhibitor polymers. However,Luvicap EG is a 40 wt % solution of poly(N-vinylcaprolactam)in ethylene glycol, and Gaffix VC-713 is a 37% solution ofpoly(vinylcaprolactam vinylpyrrolidone dimethylaminoethylmethacrylates) in ethanol. Both ethylene glycol and ethanolare thermodynamic inhibitors for gas hydrates. The apparentsynergy effect of these solvents to both inhibitors has been well-demonstrated and studied in our previous work.21 To make the
test results of the new polymers directly comparable to those ofLuvicap EG and Gaffix VC-713, one testing solution was madeof THF−NaCl and the synthesized co-polymer KHI, PEO-co-VCap-I, after it was dissolved in 37% ethanol.At a concentration of 0.25 wt %, the ball-stop time was
greater than 360 min when Luvicap EG and Gaffix VC-713were used in the hydrate-forming solutions. The ball-stop timewas generally shorter than 360 min when the newly poly-merized inhibitors were used. When the inhibitor concentrationwas increased to 3.5 wt % (3.0 wt % for PTHFMA), the ball-stop time of all THF−NaCl hydrate formation solutionsexceeded 360 min, except for that containing PEO-co-VCap-II(PTHFMA-co-PEO and PTHFMA-co-VCap were not testedbecause of the limited solubility). When 37% ethanol (equalamount to that in Gaffix VC-713) was added to 2.5 wt % PEO-co-VCap-I, the ball-stop time changed from below 360 min toover 360 min. The results demonstrated that the new polymerKHIs alone are as effective as Gaffix VC-713 and Luvicap EG ata concentration of 3.5 wt %. When ethanol is added to thepolymers, they might outperform Gaffix VC-713 and LuvicapEG. More systematic studies are required to understand theimpact of ethanol on the inhibition of these KHIs.When the chemical structures of PEO-co-VCap-I and PEO-
co-VCap-II are compared, the only difference is that the lengthof the PEO pendent groups in PEO-co-VCap-I is 7−8CH2CH2O repeating units, much shorter than that in PEO-co-VCap-II, which contains 22−23 CH2CH2O repeating units(Figure 1A). Co-polymers of PEO-co-VCap containing a higherPEO ratio were also synthesized and tested (data not shown).None of them showed longer ball-stop times than PEO-co-VCap-I. These observations agree with the previous work thatPEO itself is a weak hydrate inhibitor27 and that the lowermolecular-weight PEO chain has a better synergistic effect withPVCap-type inhibitors.18
These ball-stop rig testing results indicated that the polymerssynthesized are as effective as the reference inhibitors LuvicapEG and Gaffix VC-713 in preventing THF hydrate forma-tion over a 6 h time period. The addition of small amountsof ethanol can further improve the performance of theseinhibitors.
3.3. HP-ALTA Testing Results. The HP-ALTA test resultson the gas mixture were displayed in Figure 4. The concen-tration of the KHIs tested by HP-ALTA was kept at 0.05 wt %for all samples. The hydrate formation temperature shown inthe figure was the median value of over 100 measurements formost of the samples (except for PTHFMA-co-PEO andPTHFMA-co-VCap, which were the median value of ≈50measurements). The scatter shown in the figure covers 100%scatter range of each data set and is a measure of “stochasticity”.It should be noted that higher subcooling was often observed
during the first few cooling cycles, following which super-cooling became smaller and remained more constant over therest of the cycles. Given the dissociation conditions described insection 2.4, we believe that this is resulted from increasing gassaturation in the solutions over the initial cycles, resulting inless necessity for gas transport in hydrate formation and lesssupercooling being observed in the later measurements. Thecollection of a large number of statistics using HP-ALTA meansthat the median values of the hydrate formation temperatureare insensitive to the contribution from the first few cycles.The hydrate formation temperature of the mixed gas in the
solutions containing new inhibitors PEO-co-VCap-I and PEO-co-VCap-II is not significantly different from those without a
Table 2. Molecular Weight and Ball-Stop Time of the NewPolymeric KHIs in Comparison to Luvicap EG and GaffixVC-713
aThe results were obtained from hydrate formation solutionscontaining inhibitors with added ethanol or ethylene glycol.
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KHI (control) or those containing Gaffix VC-713 and LuvicapEG. However, at a concentration as low as 0.05 wt %, polymerscontaining THF pendent groups, PTHFMA, PTHFMA-co-MMA, PTHFMA-co-PEO, and PTHFMA-co-Vcap, have led toa substantial decrease in the hydrate formation temperature. Upto 16 °C reduction was observed in gas hydrate formationliquid that contains polymers made of THF rings and a hydro-philic co-monomer, such as PEO and Vcap, in comparison tothose containing Gaffix VC-713 and Luvicap EG. A smallerchange was shown by PTHFMA-co-MMA, which is probablydue to the hydrophobic co-monomer unit contained in thepolymer main chain. The significantly decreased hydrate forma-tion temperature by the THF-containing polymers is a demon-stration that the five element ring of THF, a hydrate promoter,might indeed enhance the affinity of the polymer for thehydrate crystals as we hypothesized, therefore leading to a moreefficient inhibition of gas hydrates. It is also interesting tonotice that, when these polymers were used in the THF−NaClhydrate-forming solutions (in the ball-stop rig testing experi-ment), PEO-co-VCap-I and PEO-co-VCap-II seemed as effec-tive as other new polymers. This indicates that the inhibitionperformance of these inhibitors is dependent upon the type ofhydrate guest molecules. In particular, THF hydrates form inthe presence of a high concentration of THF in the water phasewithout the concern for mass-transfer effects, whereas constantgas diffusion into the water phase is necessary for synthetic gashydrates to form. Further investigations are under way toexamine this observation in greater detail.
4. CONCLUSION
In this study, six novel polymer-based KHIs were synthesizedand characterized. Preliminary examinations using a ball-stoprig and a HP-ALTA have shown that the new KHIs are as effec-tive as two commercially available hydrate inhibitors, LuvicapEG and Gaffix VC-713, in preventing the formation of THFhydrate over a 6 h period. At a very low concentration of 0.05wt %, they can significantly reduce the hydrate formation tem-perature of a methane−propane gas mixture. The polymer inhi-bitors containing pendant THF functional groups have shown asignificant decrease of 12−16 °C in the gas hydrate formationtemperature, in comparison to the control-gas-forming solution
and those containing the same amount of Gaffix VC-713 andLuvicap EG, indicating a very effective inhibition in natural gashydrate formation. The different inhibition performances ofthese inhibitors in gas hydrate formation solutions and THFhydrate formation mixtures and in relation to the chemicalstructures are a topic of ongoing study.
The authors acknowledge financial support for this work byCSIRO in Australia through the Wealth from Oceans FlagshipCollaborative Research Fund. In addition, Nobuo Maedaacknowledges the support of an Australian Research CouncilFuture Fellowship (FT0991892). We are also grateful to theISP in Germany for their kind donation of Gaffix VC-713 andto Shaun Howard for the artwork of Figure 2.
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