Tetrahydrofuran Hydrate Crystal Growth Inhibition with Synergistic

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Tetrahydrofuran Hydrate Crystal Growth Inhibition with Synergistic Mixtures – Insight into Gas Hydrate Inhibition Mechanisms Malcolm A. Kelland, Ade Rahma Dyah Hartanti, Walter Gabriel Zambrana Ruysschaert, and Henning Blomfeldt Thorsen Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01382 • Publication Date (Web): 11 Jul 2017 Downloaded from http://pubs.acs.org on July 16, 2017

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Tetrahydrofuran Hydrate Crystal Growth Inhibition with Synergistic Mixtures – Insight into Gas Hydrate Inhibition Mechanisms Malcolm A. Kelland,* Ade Rahma Dyah Hartanti, Walter Gabriel Zambrana Ruysschaert and Henning Blomfeldt Thorsen

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 Various mixtures of two chemicals have been tested for their ability to prevent tetrahydrofuran (THF) hydrate crystal growth, and compared to their ability as gas hydrate kinetic hydrate inhibitors (KHIs) using a Structure II-forming natural gas mixture, the same

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structure obtained with THF hydrates. The THF hydrate results for mixtures of poly(N-vinyl caprolactam)

(PVCap)

with

tetra-n-butylammonium

bromide

(TBAB),

tetra-n-

butylphosphonium bromide (TBPB) or hexa-n-butylguanidinium chloride (Bu6GuanCl) showed clear synergy effects in two different types of tests, an isothermal test at -0.5oC and a variable temperature test. In these tests the key parameters are respectively the lowest concentration of a mixture and the highest subcooling for which complete THF hydrate inhibition is observed. The synergistic similarities between the THF and previously obtained Structure II-forming gas hydrate results suggests that the dominant inhibition mechanism operating in theses mixtures in the gas hydrate system is crystal growth inhibition. Poor THF hydrate crystal growth inhibitors such as tetra(n-hexylammonium bromide) (THAB), showed poor synergy with PVCap in THF hydrate tests, but do show synergy with the gas hydrate system. These results indicate that another mechanism besides crystal growth inhibition is operating in the gas hydrate system. We suggest that this other mechanism is some form of nucleation inhibition or particle destabilization mechanism. This is discussed in light of other evidence from laboratory experiments and computer modelling. Nucleation inhibition in gas hydrate systems may be occurring not just for THAB but as a secondary mechanism for the other compounds tested in this study.

INTRODUCTION Kinetic hydrate inhibitors (KHIs) are a class of low dosage hydrate inhibitor (LDHI) that have been used in oil and gas fields for over 3 decades to prevent gas hydrate plug formation.1-8 KHIs packages are carefully designed mixtures of one or more water-soluble polymers together with other synergists and solvents to improve the performance. KHI performance is usually discussed in terms of the maximum subcooling that gives no hydrate


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formation for a given time period at the experimental or field conditions. However, the driving force for hydrate formation is more accurately defined by the chemical potential in the system, which is why studies have shown that absolute pressure also affects KHI performance.9-12 There are several postulated mechanisms for kinetic hydrate inhibitors (KHIs), but most workers agree that the two most important are crystal growth inhibition and nucleation inhibition.7-8 Both mechanisms may be occurring with many of the KHI classes. The object of this study was to help clarify this issue. One well-known class of KHI polymers is based on homo- and copolymers of N-vinyl caprolactam (VCap), first discovered by the gas hydrate research group at the Colorado School of Mines in the early 1990’s.3,13-14 (Figure 1). Around the same time, the oil companies Shell and later BP showed that VCapbased polymers exhibited synergy with tetraalkyl onium salts such as tetra(nbutyl)ammonium bromide (TBAB) and tetra-n-butylphosphonium bromide (TBPB).15-16 (Figure 2) Later studies showed that VCap polymers show synergy with a variety of other polymers and non-polymeric chemicals, including poly(N-alkylacrylamide)s.7-18 TBAB has also been shown to show KHI synergy with N-vinyl pyrrolidone polymers.19 The entry point to the use of tetraalkyl onium salts salts as synergists came from tetrahydrofuran (THF) hydrate crystal growth studies. These onium salts with the correct size and shape of alkyl groups were shown to be excellent crystal growth inhibitors but by themselves they are poor gas hydrate kinetic inhibitors. Mixtures of VCap polymers and TBAB were used in some of the first subsea field deployments.1 In the last few years, our group has expanded on the seminal work of Shell by researching other non-polymeric THF hydrate crystal growth inhibitors. This includes, monoalkyl cations and anions, bis- and polyquaternary onium salts, mono-, bis and polymeric amine oxides, cyclopropenyl salts and hexaalkylguanidinium salts.20-25 In this study we investigate for the first time, mixtures of tetrabutylonium or hexabutylguanidinium salts with poly(N-vinyl caprolactam) (PVCap) on


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THF hydrate crystal growth (Figure 2). The objective was to gain insight into the KHI mechanism by determining if the synergy observed with gas hydrate systems was also observed for the THF hydrate studies where crystal growth inhibition is supposedly the main inhibition mechanism that is operating. The results are also discussed in light of a recently developed KHI performance test on gas hydrate, called the CGI (Crystal Growth Inhibition) method.26-29

n N O

Figure 1. Poly(N-vinyl caprolactam) (PVCap).

Br+

N

N

Br-

Cl+

P+ N

N

Figure 2. From left to right, tetra-n-butylammonium bromide (TBAB), tetra-nbutylphosphonium bromide (TBPB) and hexa-n-butylguanidinium chloride (Bu6GuanCl),


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EXPERIMENTAL METHODS Chemicals THF (100%) GPR Rectapur and NaCl (min. 99.5%) Analar Normapur were obtained from VWR. The THF contained 0.01% water and maximum 100 ppm peroxide (H2O2). 0.0250.04% BHT (3,5-di-t-butyl-4-hydroxytoluene) is added as stabilizer in THF to avoid peroxide formation. Tetra-n-butylammonium bromide (TBAB) (≥99%) and tetra-n-butylphosphonium bromide (TBPB) (98%) were purchased from Sigma-Aldrich. Hexa-n-butylguanidinium chloride (Bu6GuanCl) was synthesized by direct butylation of guanidinium chloride in almost quantitative yield.22 The final product contained 70 wt.% active Bu6GuanCl in deionized water. The poly(N-vinyl caprolactam) (PVCap) used in this study was a low molecular weight polymer supplied by BASF, Germany (commercial name Luvicap EG), as a solution of the polymer in monoethylene glycol solvent at a concentration of 41.1 wt.% active.

THF Hydrate Crystal Growth Equipment The THF/NaCl solution is made by mixing 26.28 g of sodium chloride (NaCl) with 170 g of THF (C4H8O, 99.99 %) and diluting with deionized water until 900 ml. This gives a solution with 36000 ppm of NaCl content. The THF test method has been reported in several previous studies.20-25 A total of 80 ml THF/NaCl solution containing the dissolved chemicals at the required concentrations is placed in a 100 ml glass beaker (Figure 3). Two types of tests were carried out which we will call the “isothermal test” and the “maximum subcooling test”. In the isothermal test, the THF solutions with a given ratio of chemicals were cooled to -0.5oC (±0.02oC), giving approximately 3.8oC theoretical


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subcooling. Then a glass tube containing ice, both precooled to a constant set temperature of 10oC, was placed in the beaker to initiate THF hydrate formation. After one hour the tube was removed and inspected for any signs of THF hydrate crystals. If hydrate crystals were observed the concentration of the mixture of chemicals at the same chemical ratio was increased and retested until the minimum concentration that gives zero growth in 1 hour was found. Conversely, if no hydrates were formed with the initial concentration, then this was lowered gradually until hydrate started to form. The whole procedure was repeated for various ratios of the two chemicals. The lowest concentration at any one ratio that was observed to give no THF hydrate formation was recorded as the minimum inhibitor concentration (MIC) for that ratio. Care must be taken in this test to avoid a bubble at the bottom of the glass tube, which can lead to artificially low THF hydrate crystal growth. Conversely, it is important to check that the ice has not slid out of the tube giving an overly large surface area where THF hydrate can be initiated, leading to too much THF hydrate crystal growth. Good underneath lighting and clear plastic walls for the cooling bath were used to enable good visual observations. In the maximum subcooling test, the total concentration of a mixture of two chemicals was kept constant at 4500 ppm and the ratio varied. The cooling bath was adjusted to lower and lower temperatures until THF hydrate formation was observed after a test period of 1 hour. The lowest temperature that gave no THF hydrate formation within 1 hour was taken as the maximum subcooling for zero growth for that particular ratio of chemicals. The procedure was repeated for various ratios of the two chemicals.


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Figure 3. THF hydrate crystal growth inhibition equipment showing three ongoing tests.

RESULTS AND DISCUSSION Experiments to find the zero growth points (concentration or temperature) of each individual inhibitor were carried out prior to the synergist mixture tests. The zero growth point is the lowest total concentration or temperature of the mixture for which the THF/inhibitor mixture does not give any visible THF hydrate crystals in the 1 hour time period. The results are summarized in Table 1. TBAB clearly reduced THF hydrate crystal growth but even at 15000 ppm there was still some crystals present after 1 hour at 0oC. We couldn’t conduct tests above this temperature as the ice in the glass tube melted and slid out of the tube. Coincidentally, PVCap and TBPB were both able to prevent THF hydrate formation down to a concentration of 4500 ppm at -0.5oC. (NB: PVCap without the MEG present gave an identical result indicating that the MEG solvent in the PVCap does not affect the performance). It is interesting to see that a polymer with roughly 15-25 pendant caprolactam groups and a small


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cationic molecule with 4 butyl groups can give similar performances. Chemisorption of some of the lactam groups in PVCap onto the hydrate surface has been proposed to be the mechanism of THF hydrate crystal growth inhibition, since lower molecular weights polyvinyllactams give diminished performance and higher molecular weights increased performance.1,4,30-32 TBPB has just 4 butyl groups and the mechanism of crystal growth inhibition has not been proven. However, it has been suggested to be due to embedding of the butyl groups in the hydrate surface as the hydrate cages grow around these groups. The butyl “legs” are then physically stuck in the hydrate surface and the rest of the protruding molecule interferes with further Structure II hydrate crystal growth. The same mechanism is proposed for other quaternary ammonium, phosphonium or sulfonium salts with the correct length and shape of the alkyl groups. Bu6GuanCl was the best THF hydrate crystal growth inhibitor tested in this study giving zero growth in 1 hour down to 2200 ppm. For Bu6GuanCl, which has 6 pendant butyl groups, it might be expected to also inhibit crystal growth by an embedding mechanism but with the butyl groups spread over a larger crystal surface area. This may be the reason for the lower concentration that prevents total crystal growth compared to the onium salts.

Table 1. Zero growth concentration and temperature of the inhibitors.

Inhibitors

Zero Growth Point ppm

No additive

o

C

3.3

PVCap

4500

-0.5

TBAB

15000

>0

TBPB

4500

-0.5


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Bu6GuanCl

2200

-0.5

Since PVCap and the quaternary onium ions are proposed to prevent hydrate crystal growth by different mechanisms we first investigated if mixtures of the two could show synergy on THF hydrate. Here we will define “synergy” for the two types of tests, called the “isothermal test” and the “maximum subcooling test”:

Isothermal test: We measure the minimum total chemical concentration (MTCC) that gives zero THF hydrate growth at -0.5oC. The accuracy for determining the MTCC was ±50 ppm. Synergy is deemed to occur if a lower MTCC is observed at the same conditions by using the same weight concentration of a mixture of two chemicals instead of the same weight of one or other of the two chemicals.

Maximum subcooling test: The maximum subcooling (or minimum temperature) at which THF hydrate formation is totally inhibited for 1 hour at 4500pm total concentration at varying ratios of a chemical mixture. Synergy is deemed to occur if a lower minimum temperature is observed for a mixture than either chemical used alone at the same total concentration.

Isothermal Experiments The results for the isothermal tests for PVCap mixtures with TBAB and TBPB are given in Figures 4 and 5 respectively. They show the same general trend, whereby mixtures of two chemicals perform better than the individual chemicals, i.e. there is a MTCC, which is lower than either individual chemical.


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The best synergistic performance at -0.5oC was a 1750 ppm of a 9:1 mixture of TBPB and PVCap, which was the ratio that gave the lowest concentration for complete THF hydrate inhibition for any mixture in this study. This is an extremely powerful crystal growth inhibitor mixture, which very few individual chemicals or blends are expected to surpass. A n-butylated linear polyethyleneimine oxide might be able to give a better performance than this synergistic mixture. This polyimine oxide was able to totally inhibit THF hydrate formation at 2000 ppm at equivalent test conditions but a meagre 0.01g growth in 1 hour was observed at 1000 ppm.25 Mixtures of PVCap with the very best tetraalkylammonium salts, TiHexAB or tri-t-heptylpentylammonium bromide, might also achieve greater performance than the 9:1 TBPB:PVCap mixture, although they have not been investigated. Optimizing the molecular weight of PVCap may also improve the synergistic performance.

Figure 4. Zero growth concentration of PVCap-TBAB mixture on THF hydrate crystals.


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Figure 5. Zero growth concentration of PVCap-TBPB mixture on THF hydrate crystals.

We know from previous studies in the 1990’s by Shell and BP that TBAB and TBPB are synergists for PVCap in gas hydrate studies with a Structure II-forming gas mixture.15-16,33 Therefore, we would expect similar shape graphs as those shown in Figures 2 and 3 for gas hydrate studies. This has been carried out recently by our group for PVCap/tetra-npentylphosphonium bromide (PVCap/TPPB) mixtures.24 The graph is reproduced in Figure 6. PVCap/TBPB mixtures are assumed to give similar results. The similarities between the THF and gas hydrate results for mixtures of PVCap with tetraalkylonium salts suggests that the dominant inhibition mechanism operating in these synergistic mixtures is crystal growth inhibition. The fact that synergy is observed also suggests that PVCap and TBAB or TBPB interact with the hydrate crystal surface in non-competitive ways but rather complement each other.


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Crystal growth inhibition is clearly an operating mechanism for KHI polymers such as PVCap but has also been proposed for other polymer classes.34 Unidirectional crystal growth inhibition of THF hydrate has been proposed as a method to rank the performance of KHIs.35 This study used poly(N-vinylpyrrolidone) (PVP). However, the method has obvious limitations as good THF hydrate growth inhibitors such as TBAB, TBPB or Bu6GuanCl are known to be poor gas hydrate kinetic inhibitors when used by themselves, as discussed earlier.

Figure 6. Synergism of TPPB with PVCap at 3000 ppm total concentration of active components in gas hydrate constant cooling tests; error bars represent standard deviation [Reproduced from reference 24].

The results with mixtures of PVCap and Bu6GuanCl gave a similar synergism trend as for TBAB and TBPB (Figure 7). However, when used alone, Bu6GuanCl was a significantly


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better THF hydrate crystal growth inhibitor than PVCap preventing crystal growth down to about 2200 ppm. Further, due to the greater performance of Bu6GuanCl, this limited the range of mixtures that showed synergy. Thus, true synergy, giving an even lower combined concentrations of PVCap and Bu6GuanCl than either chemical separately, was only seen for the 1:9 mixture. This mixture which prevented THF hydrate formation down to 1900 ppm, lower than either PVCap alone or Bu6GuanCl alone. For the 1:2 and 2:1 PVCap:Bu6GuanCl mixtures the minimum concentration was 2500 ppm and 2900 ppm respectively, which is now higher than pure Bu6GuanCl. PVCap and Bu6GuanCl mixtures also show synergy in a Structure II-forming gas hydrate system. These previously published results are shown in Figure 8.22,36

Figure 7. Zero growth concentration of PVCap-Bu6GuanCl mixture on THF hydrate crystals.


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Figure 8. Synergism of Bu6GuanCl with PVCap at 3000 ppm total concentration of active components in sII-forming gas hydrate constant cooling tests; error bars represent standard deviation [reproduced from ref 36].

So far these studies indicate that both the polymer KHI and the small molecule ionic synergist are crystal growth inhibitors. But, a nucleation inhibition mechanism has also been proposed for some KHIs although the exact details are unclear.33,36-38 An example of the possibility of nucleation inhibition concerns the good synergistic ability of tetra-nhexylammonium bromide (THexAB) with PVCap in a Structure II-forming gas hydrate system despite THexAB being a very poor THF hydrate crystal growth inhibitor.33 (Figure 9) It was suggested that the four n-hexyl groups in THexAB caused enhanced water perturbation relative to other quaternary ammonium salts that were better THF hydrate crystal growth inhibitors. This water perturbation is assumed to inhibit the formation of critical nuclear size gas hydrate particles, i.e. nucleation inhibition. Conversely, some polymers are poor THF hydrate crystal growth inhibitors but show good performance as Structure II-forming gas hydrate

inhibitors.

This

includes

polyesteramides,

polyaspartamides


and

poly(N-

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alkyl(meth)acrylamide)s.7-8,39-40 Presumably, nucleation inhibition is also operating here to some extent. Computer modelling evidence for nucleation inhibition has also been reviewed.8

Figure 9. Tetra-n-hexylammonium bromide (THexAB).

To support the earlier work that THexAB is a synergist with PVCap for gas hydrate systems, despite THexAB being a very poor THF hydrate crystal inhibitor, we conducted some THF hydrate experiments. We found that even at 20,000 ppm THexAB did not prevent THF hydrate crystal growth at -0.5oC. Hydrate growth began immediately when the ice-filled glass tube was lowered into the solution. Also, 1:2, 1:4 and 1:9 mixtures of PVCap and THexAB required more total dosage than PVCap alone in order to totally inhibit THF hydrate formation for 1 hour. These results suggest that THexAB is not a synergist with PVCap for THF hydrate crystal growth inhibition, supporting the conclusion that another mechanism must be operating for gas hydrate systems where synergy is observed. This other mechanism is probably some form of nucleation inhibition. Two plausible (and possibly related) nucleation inhibition theories have been proposed and have been briefly reviewed.8 The first is perturbation of the bulk water phase, disrupting the


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bulk water phase and preventing sufficient gas and water molecules to cluster together, thus leading to nucleation inhibition. This mechanism was first suggested by Exxon Mobil in their work on poly(N-alkylmethacrylamide)s, where the surface-to-volume ratio of the polymer is one of the critical factors.1,7,41 Clearly, small molecules with hydrophobic groups can also disrupt the water phase, which is maybe why THexAB is a good gas hydrate synergist in blends with PVCap. Likewise, TBAB, TBPB and Bu6GuanCl could also prevent nucleation by this mechanism in addition to their ability to prevent hydrate crystal growth. The second proposed nucleation inhibition mechanism comes from computer molecular simulation evidence on KHI polymers. For example, Moon et al. showed that poly(Nvinylpyrrolidone) molecules maintained a distance of 5−10 Å from the hydrate cluster surface, instead of adsorbing directly onto the growing surfaces, but was still able to destabilize and lead to dissociation of the hydrate particle.42 Kvamme et al. evaluated the effects of N-vinyl lactam polymers on Structure I and II hydrates with molecular dynamics simulations.43 The results showed that the KHIs could interact with the hydrate water clusters and trigger nuclei dissolution without having direct contact. Further computer simulation evidence has been reviewed. Note that these mechanisms could be occurring for both subcritical hydrate particles as well as stable hydrate crystals that have growth beyond the critical nuclear size. These studies showing how KHI polymers destabilize gas hydrate particles might explain how both good and bad THF hydrate crystal inhibitors can give good results in the CGI gas hydrate test method.26-29 In this method, gas hydrates are formed in the presence of the KHI by cooling the system beyond the inhibition ability of the KHI. Then the system is warmed until most not but all the hydrates have melted. Finally, the system is cooled again and subcooling regions determined that prevent further hydrate crystal formation, as well as


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varying hydrate growth rates at higher subcoolings. Currently, it appears that all openly reported CGI studies have considered VCap-based KHIs, which are known to be excellent hydrate crystal growth inhibitors from studies with THF hydrate. However, other good gas hydrate KHI polymers, but much less effective THF crystal growth inhibitors have also been shown to give excellent performance results in the CGI test method. An example is polymers based on N-isopropylmethacrylamide which are used in commercial KHI formulations.6 A possible explanation is a KHI mechanism in which such polymers prevent further hydrate crystal growth, or even destabilize preformed hydrates, by acting at a distance from the hydrate crystal surface by the computer modelled methods described above.

Maximum Subcooling Experiments For further evidence of the synergistic effects in THF hydrate crystal growth studies, we carried out a second type of test, called the “maximum subcooling test”. The results for 4500 ppm mixtures of PVCap with TBAB, TBPB and Bu6GuanCl are given in Figures 8-10. We observed the same general trends with the three mixtures as was seen using the isothermal test method at -0.5oC. Thus, mixtures of TBAB, TBPB or Bu6GuanCl with PVCap showed synergy giving greater MTCC values for some mixtures than either of the chemicals alone. With 4500 ppm TBAB alone (the far right hand side of the graph in Figure 10) we could not fully stop hydrate formation at any temperature below 0.0oC. Above this temperature the ice in the glass tube would melt. Therefore the graph begins on the right with the 1:9 mixture. Interestingly, in this series of tests the best mixture is the 1:4 mixture giving complete inhibition down to -2.8oC or 6.1oC subcooling, whereas in the isothermal tests at -0.5oC it was the 1:9 mixture that performed best. This result has been checked twice by different researchers. We are not sure why there is this discrepancy between the two tests. However,


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for the PVCap/TBPB and PVCap/Bu6GuanCl mixtures the best mixture in both types of test was the 1:9 mixture (Figures 11-12). In the maximum subcooling test with the PVCap/TBPB mixture we also used a 1:19 mixture. This mixture did not perform as well as the 1:9 mixture indicating that now there is too little polymer present for optimal performance.

Figure 10. Maximum subcooling of PVCap and TBAB mixtures for zero THF hydrate crystal growth.


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Figure 11. Maximum subcooling degree of PVCap and TBPB mixtures for zero THF hydrate crystal growth.

Figure 12. Maximum subcooling of PVCap and Bu6GuanCl mixtures for zero THF hydrate crystal growth.


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CONCLUSION

We have carried out a series of THF hydrate crystal growth experiments with PVCap blended with three small ionic organic compounds, TBAB, TBPB and Bu6GuanCl. Two types of test were employed, an isothermal test at -0.5oC in which the concentration of inhibitors was varied, and the maximum subcooling test in which the total concentration remains constant. Synergy was observed in both tests for PVCap blended with all three compounds. A 9:1 blend of TBPB and PVCap prevented complete THF hydrate growth at -0.5oC down to a concentration as low as 1750 ppm. A 1:9 blend of PVCap with Bu6GuanCl showed the next best synergy, preventing THF hydrate growth at -0.5oC down to a concentration of 1900 ppm. These results are on a par with the best previously reported THF hydrate crystal growth inhibitors. Blends of PVCap with TBAB, TBPB or Bu6GuanCl have previously been shown to have synergistic KHI properties in a Structure-II forming gas hydrate system. The THF hydrate results correlate well with the gas hydrate results suggesting that crystal growth inhibition is a dominant mechanism occurring in the gas hydrate system. However, THAB showed poor synergy with PVCap in THF hydrate crystal growth inhibition tests, but gave strong synergy in our Structure II-forming gas hydrate system. These results suggest another mechanism besides crystal growth inhibition is operating in the gas hydrate system. We propose that this other mechanism is some kind of nucleation inhibition or particle destabilization mechanism, which may also be occurring for the PVCap blends with TBAB, TBPB and Bu6GuanCl as a secondary mechanism.

References


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