Article pubs.acs.org/EF
High-Throughput Testing of Kinetic Hydrate Inhibitors Nobuo Maeda,† Celesta Fong,† Qi Sheng,†,‡ Kelly C. da Silveira,†,§ Wendy Tian,† Aaron Seeber,† Wayne Ganther,† Malcolm A. Kelland,∥ Mohamed F. Mady,∥,⊥ and Colin D. Wood*,# †
CSIRO Manufacturing Flagship, Clayton, VIC 3168, Australia Curtin University of Technology, Bentley, WA 6102, Australia § Institute of Macromolecules, Federal University of Rio de Janeiro, CEP 21941-598 Rio de Janeiro, Brazil ∥ Department of Mathematics and Natural Science, Faculty of Science and Technology, University of Stavanger, N-4036 Stavanger, Norway ⊥ Department of Green Chemistry, National Research Centre, 33 El Bohouth Street, Post Office 12622, Dokki, Giza, Egypt # CSIRO Australian Resources Research Centre, Kensington, WA 6152, Australia ‡
S Supporting Information *
ABSTRACT: The formation of clathrate hydrates is considered to be a major flow assurance problem in offshore oil and gas lines. Kinetic hydrate inhibitors (KHIs) are used for hydrate prevention, with their efficiency assessed by techniques that are a bottleneck for new materials development in this area. Efficient design of high-performance advanced materials requires a thorough understanding of the structure−property relationships that is currently hindered by conventional evaluation protocols. A cost-effective method for the rapid, parallel screening of potential KHIs is desirable, which preferably does not involve handling of highly pressurized and potentially flammable/explosive fuel gases. We have developed a novel high-throughput KHI ranking method based on its inhibition performance of Structure II (sII)-forming cyclopentane (c-C5) hydrate under atmospheric pressure. Ice seeding was used to induce the nucleation of c-C5 hydrate to save time, so the method focuses on the growth inhibition performance (as opposed to the nucleation inhibition performance) of a KHI. Comparison of some commercial KHIs [Luvicap 21W (N-vinylpyrrolidone:N-vinylcaprolactam 2:1 copolymer), Luvicap 55W (N-vinylpyrrolidone:N-vinylcaprolactam 1:1 copolymer), polyvinylpyrrolidone (PVP), poly(N-isopropylacrylamide) (PNIPAM), and polyacrylamide (PAM)] was made using this new screen, which has been validated against conventional rocking cell measurements. The observed efficacy performance ranking of these KHIs was Luvicap 21W ≥ Luvicap 55W > PNIPAM ≥ PVP > PAM. This ranking was in reasonable agreement with the rocking cell data that gives the ranking Luvicap 55W > Luvicap 21W > PNIPAM > PVP > PAM. This method enabled parallel screening of multiple KHIs with major advantages in time, instrument complexity, safety, and material. We propose that this method could serve as a useful first screening method for identifying promising candidates for more rigorous testing.
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INTRODUCTION
in the unit cell; the preferred structure is dependent upon the inclusion molecules.2 It has been a common practice to use large quantities (up to 50 wt%) of thermodynamic hydrate inhibitors (THIs) such as methanol and monoethyleneglycol (MEG) in gas hydrate susceptible fields. THIs shift the thermodynamic phase boundaries of gas hydrate formation to lower temperatures and/or higher pressures and preclude formation of gas hydrates.2 However, as the production fields move farther offshore and deeper, and the concomitant tiebacks become ever longer, the costs of THIs increase rapidly. An additional strategy for the prevention of gas hydrate formation employs kinetic hydrate inhibitors (KHIs).4−7 KHIs contain hydrophilic polymers as the main active ingredient. They can be dosed at a much lower concentration than THIs (typically ∼1−3 wt% when correctly formulated in solvents for field use). KHIs do not preclude the formation of gas hydrates
Gas hydrates offer a wide range of promising applications such as gas storage, carbon dioxide sequestration, gas separation, and desalination, and hence research interest in the field has been rising sharply.1 However, for offshore oil and gas lines, hydrate formation under deep sea conditions is a major flow assurance problem, with pipeline blockage causing safety concerns as well as the temporary loss of production, leading to financial loss. Considerable investment has therefore been committed to the avoidance of pipeline shutdown and clathrate hydrate inhibition. Gas hydrates or clathrate hydrates are ice-like crystalline compounds that consist of nonpolar guest molecules (e.g., ethane, methane) that are entrapped within polyhedral cavities of the hydrogen-bonded water framework.2 These comprise 85 mol% water and 15 mol% guest when all the cavities are occupied.3 There are two main types of hydrate structure that are important to the oil and gas industry: Structure I (sI) and Structure II (sII). Both possess cubic symmetry, though they differ in the number and size of the polyhedral water framework © 2016 American Chemical Society
Received: March 2, 2016 Revised: June 13, 2016 Published: July 13, 2016 5432
DOI: 10.1021/acs.energyfuels.6b00498 Energy Fuels 2016, 30, 5432−5438
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Figure 1. Ideal combinatorial and high-throughput workflow for the discovery of new materials.
High-throughput techniques have been routinely employed by the pharmaceutical industry and biological research community for decades, and translation of this philosophy to materials research has occurred in recent years.21−23 Discovery and optimization typically follow several stages that progressively narrow the parameter space in question. In the initial phase, coarse characterization of the essential properties is performed to identify promising compositions, reaction conditions, or structures. In subsequent phases, the selected property is characterized more precisely and thoroughly; reiteration of this process leads to new materials discovery with a knowledge of structure−property correlations (Figure 1). The current study aims to develop a rapid screening method for ascertaining KHI efficiency based upon the observation of the inhibition of hydrate formation of sII-forming cyclopentane (c-C5) hydrate under atmospheric pressure. It has been shown that the efficacy ranking of the KHIs based on the use of c-C5 hydrate did not completely match that obtained using natural gas hydrates.24 Nevertheless, the rankings were broadly similar to each other, and hence c-C5 could still serve as a useful first screening method to identify promising candidates for more rigorous testing. Within the context of Figure 1, the screen is Phase 1 of the new materials discovery and optimization workflow. For this purpose, some commercially available KHIs were screened and ranked, and this ranking was validated against the more conventional rocking cell method, which uses natural gas which also forms sII hydrates. Thus, the comparison between the conventional techniques and the proposed method is two-fold: the equipment used for the testing and the chemicals that act as guests. Specifically, the comparative efficiencies of some commercial and well-known KHIs [Luvicap 21W (N-vinylpyrrolidone:Nvinylcaprolactam 2:1 copolymer), Luvicap 55W (N-vinylpyrrolidone:N-vinylcaprolactam 1:1 copolymer), polyvinylpyrrolidone (PVP), poly(N-isopropylacrylamide) (PNIPAM), and polyacrylamide (PAM)], as well as the common THI, MEG, were screened and ranked using the model sII-hydrate-forming c-C5 (Table 1). Natural gas as a guest also form sII hydrates. Tetrahydrofuran (THF) and c-C5, however, are among the several guests that form sII hydrates at atmospheric pressure and above 273.16 K, and these have been used as model hydrate formers.24−27 The thermodynamic equilibrium dissociation temperatures, Teq, of THF hydrate and c-C5 hydrate
but rather delay the nucleation of gas hydrates and/or slow the growth of gas hydrates while the production fluids are transported. Thus, an effective, reliable, inexpensive, and environmentally friendly KHI is highly desirable. As such, a vigorous search for one has been an important research topic in the field of gas hydrates.4 Certain water-soluble polymers have been found to be effective in kinetically hindering the formation of hydrates at low additive concentrations. These include homo- and copolymers of N-vinylpyrrolidone (PVP) and N-vinylcaprolactam (PVCap) that incorporate pendant groups with 5- and 7-membered hydrocarbon rings, respectively.4,7−10 While KHIs have been applied in the field, they are usually only considered effective at sub-coolings of up to 10−12 °C unless the residence time in the flow is short,6,11,12 whereas sub-coolings of up to 15−20 °C are often present under deep sea conditions. The relative efficacy ranking of KHIs can be derived from a range of conventional experimental techniques: autoclaves,13,14 stirred batch reactors,15 rocking cells,5,16 high-pressure automated lag time apparatus,17,18 and others. These instruments typically measure the induction times at constant subcoolings and/or the maximum achievable sub-coolings during constant cooling ramps. They require flammable gases, high pressures, and long data accumulation times. The amount of sample required for testing is also large, which renders the use of the conventional technique challenging for precious inhibitors such as antifreeze proteins.19,20 These conventional protocols have proven to be a bottleneck for new materials development in this area, in that a large number of expensive instruments are required for parallel screening of multiple inhibitors. Structure is central to the development of a knowledge base that will lead to the understanding required for control and prediction of hydrate formation. Efficient design of high-performance advanced materials requires a thorough understanding of structure− property relationships that is currently hindered by the conventional evaluation protocols. As astronomical numbers of combinations are theoretically possible in the design of a polymeric KHI, and recent advances in high-throughput synthesis allow this broad experimental space to be explored, a cost-effective method for the rapid, parallel screening of potential KHIs is desirable, which preferably does not involve handling of highly pressurized and potentially flammable/ explosive fuel gases. 5433
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The so-called “memory effect” is a commonly used strategy to increase the reproducibility of the measurements related to intrinsically stochastic nucleation events.28 The memory effect refers to observations that gas hydrates form easier or faster from water obtained by the dissociation of gas hydrates or melting of ice.2,29,30 In this instance this would require that each KHI solution be heated to a few degrees above the thermodynamic equilibrium dissociation temperature, Teq, of the c-C5 hydrate after seeding with ice, and then cooling the solution back to below Teq. However, such a protocol would introduce additional experimental parameters to be controlled, and given that the purpose of our protocol was a fast and crude screening method of a large number of KHIs, we decided to adopt the direct ice seeding method.31−33 Since the ice seeding effectively ensures the nucleation of c-C5 hydrate, there is a risk that we may accidentally rule out a promising nucleation inhibitor based on its poor performance as a growth inhibitor. Powder Diffraction. To verify that the hydrate was being formed in these experiments, X-ray diffraction was performed on the powder diffraction beamline at the Australian Synchrotron,34 at an X-ray energy of ∼13 keV. Each sample was placed in a 1.0 mm diameter borosilicate capillary, and data were collected over the 2θ range of ∼2−82°. Pawley analyses were performed on the data using the Bruker TOPAS V5 program. The background signal was described using a combination of Chebyshev polynomial linear interpolation function, 1/x function, and a broad pseudo-Voight function to describe the background due to the capillary itself. Cell parameters, peak full width at half-maximum, and peak scale factors were all refined. A representative XRD pattern is shown in Figure 3 for c-C5 hydrate formed, in this case in the presence of PVP. The crystal structure has
Table 1. List of KHIs Used in This Study KHI name Luvicap 55W Luvicap 21W polyvinylpyrrolidone (PVP) K15 poly(N-isopropylacrylamide) (PNIPAM) polyacrylamide (PAM)
MW (g/mol) 2000−4000 21 000 9000 8500 5 000 000
supplier BASF BASF Ashland Chemical Co. synthesized inhouse16 Sigma-Aldrich
are 277.5 and 280.8 K, respectively.24,25 c-C5 hydrate thus offers more room for sub-cooling above the ice point; it is also sparingly soluble in water and so is a more appropriate analogue for natural gas than THF, which is miscible with water at all proportions.
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MATERIALS AND METHODS
Chemicals. The KHIs tested were (1) Luvicap 21W, 34.6 wt% VP:VCap 2:1 copolymer in water (from BASF), low MW = 2000− 4000 Da; (2) Luvicap 55W, 53.8 wt% VP:VCap 1:1 copolymer in water (from BASF), MW = 21 000 Da; (3) polyvinylpyrrolidone (PVP) K15 (from Ashland Chemical Co.), MW = 9000 Da; (4) poly(N-isopropylacrylamide) (PNIPAM), MW = 8500 Da (synthesized in-house16); and (5) polyacrylamide-co-acrylic acid (PAM), MW = 5 000 000 Da (Sigma-Aldrich). Monoethylene-glycol (MEG) was purchased from Sigma-Aldrich (purity >99.8%) and used as received. Cyclopentane (c-C5) was purchased from Merck (purity >98%) and used as received. For each KHI to be tested, a dilution series of the KHI aqueous solutions was prepared using water from a Millipore unit (>18.2 MΩ resistivity). The range of concentrations of the dilution series was at least 2 orders of magnitude and covers the typical KHI dosage of 0.5 wt%. High-Throughput Hydrate Testing of Kinetic Hydrate Inhibitors. First, 0.5 mL of each KHI solution was placed in a separate vial (2 mL) and frozen using dry ice, and 0.1 mL of liquid cC5 was added to each frozen KHI solution. The mixed solution was then placed inside a constant-temperature water bath, which was in turn housed in a refrigerator (3−3.5 °C; Figure 2). Typically it took 1 wt% (10 000 ppmw) for PVP (1 wt% PVP solution did not always inhibit the formation of c-C5 hydrate at 1 wt%, as the top and bottom panels show). For PAM, even the highest concentration of 10 wt% (100 000 ppmw) was not sufficient. These results are summarized in Table 3. On the basis of these results, we conclude that the efficacy ranking of these five KHIs is Luvicap 21W ≥ Luvicap 55W > PNIPAM ≥ PVP > PAM
Figure 4. Example temperature−pressure profile which shows the onset temperature, To, and the temperature at which rapid hydrate formation takes place, Ta, of a rocking cell measurement. 5435
DOI: 10.1021/acs.energyfuels.6b00498 Energy Fuels 2016, 30, 5432−5438
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Energy & Fuels Table 3. Threshold Concentration Ct of KHIs above Which the KHI Inhibited Cyclopentane Hydrate Formation, after the Cutoff Time of 20 h at 4 °C KHI
Ct (wt%)
Luvicap 21W Luvicap 55W poly(N-isopropylacrylamide) (PNIPAM) polyvinylpyrrolidone (PVP) K15 polyacrylamide (PAM)
0.3 0.5 1 2 >10
Luvicap 21W and Luvicap 55W are both highly effective KHIs which are structurally similar to each other. It is not surprising that the rapid c-C5 hydrate screening method was not sensitive enough to discern the two. This limitation that the method is not capable of discriminating the performance of structurally similar KHIs suggests that it can only serve as the first screening of a family of mutually similar KHIs from other polymer groups. The rapid c-C5 hydrate screening method is an optical method and does not detect any microscopic c-C5 hydrate formation, let alone nucleation. Therefore, that the efficacy ranking from this method was in broad agreement with the rocking cell method suggests that a large part of the inhibition effect of the KHIs comes from the growth inhibition of c-C5 hydrate. We note that unlike in rocking cells/stirred autoclaves, the samples in the rapid c-C5 hydrate screening method are quiescent. Stirring could facilitate the growth of gas hydrate by at least two mechanisms: (1) create new aqueous−guest interfaces at which gas hydrate can form, or (2) dissipate the latent heat that arises from the formation of gas hydrate. Thus, the quiescent samples in the new method are expected to deter the growth of c-C5hydrates compared with stirred systems. It follows that any new KHIs that show poor inhibition performance from the rapid c-C5 hydrate screening method are not worth subjecting to further, more rigorous testing. Even though the use of ice seeding is convenient in inducing c-C5 hydrate formation with certainty, the nucleation and growth mechanisms of c-C5 hydrates may not be the same as in the absence of any ice. Nevertheless, our results suggest that the difference is not drastic enough to significantly alter the efficacy ranking of KHIs. Despite the coarse nature of the screen, this method has enabled fast parallel screening of KHIs.
To further demonstrate that the rapid c-C5 hydrate screening method is capable of detecting hydrate inhibition, we also measured a dilution series of MEG (Figure S5). It can be seen that only concentrations of 25 wt% or higher inhibited c-C5 hydrate formation for 20 h at 3−3.5 °C. This MEG result is for the sole purpose of demonstrating that the proposed method can detect hydrate inhibition at sufficiently high concentrations of MEG, as expected. No quantitative and/or industrial implications should be inferred from the threshold concentrations of the KHIs or from that of MEG. Rocking Cell Method. The results for the same five KHIs, but now tested in the rocking cells, are summarized in Table 4. Table 4. Results of the Standard Constant Cooling Ramp Method (1 K/h) in Steel Rocking Cellsa KHI
To (°C)
Ta (°C)
blank Luvicap 55W Luvicap 21W poly(N-isopropylacrylamide) (PNIPAM) polyvinylpyrrolidone (PVP) K15 polyacrylamide (PAM)
17.2 4.3 5.5 6.9 11.0 17.1
16.5 3.8 4.7 6.1 10.9 16.5
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a
The concentration of each KHI was 5000 ppm. The average onset temperature, To, and the average temperature at which rapid hydrate formation takes place, Ta, were measured from eight tests for each KHI. The standard deviation was in the range of 5−7%.
CONCLUSIONS There is a demand for new kinetic hydrate inhibitors that are cost-effective and efficient. To date, the techniques available for evaluating the efficiency of KHIs are a bottleneck for new materials development. We have adopted a high-throughput philosophy to screen, in parallel, the KHIs so that we can begin to build a more comprehensive understanding of structure− property relationships. The method utilizes cyclopentane hydrate to screen KHIs under atmospheric pressure. Ice seeding was used to enable rapid screening, which could accidentally rule out a promising nucleation inhibitor on the basis of its poor performance as a growth inhibitor. Nevertheless, the observed efficacy performance ranking of the selected KHIs compares well with rocking cell data, as described above. Despite its simplicity, this method has sufficiently replicated the same broad trends identified by more rigorous testing using conventional techniques. The great advantage of this method lies in its significant improvement with respect to the following important points: • timemultiple samples can be screened in parallel, with a typical experimental duration of 24 h • safetythe experimental setup does not require elevated pressures or noxious gases, as it uses cyclopentane at subcoolings of 3−4 degrees • materialsa small amount of material required, viz 0.5 mL of KHI solution
As expected, the polyacrylamide showed no activity as a KHI. Although it is water-soluble and contains strong hydrogenbonding amide groups, it has no hydrophobic functionality, which is essential for good KHI performance. PVP K15 is the next worse KHI tested. It is water-soluble at all temperatures 0−100 °C at the test concentration of 5000 ppm and thus has no cloud point. The ring methylene groups in PVP do impart some hydrophobicity, enabling the polymer to have some activity as a KHI.4,38 PNIPAM and the VP:VCap copolymers are all more hydrophobic than PVP, exhibiting cloud points in solution.39 Luvicap 21W has a higher proportion of the VP monomer than Luvicap 55W and was therefore expected to perform worse as a KHI. As can be seen, the ranking from To and that from Ta matched with each other. In either case, the efficacy ranking of these five KHIs is Luvicap55W > Luvicap21W > PNIPAM > PVP > PAM
Comparison of the Two Methods. There is a rather minor difference between the two rankings in that the rapid cC5 hydrate screening method ranked Luvicap 21W ahead of Luvicap 55W in contrast to the rocking cell method. The rest of the efficacy ranking was the same from both methods. 5436
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Energy & Fuels This method could serve as a useful first screening method which can be used to identify promising candidates for more rigorous testing.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b00498. Figures S1−S5, showing photographs of a dilution series of Luvicap 55W, PNIPAM, PVP, PAM, and MEG after a cutoff time of 20 h (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to thank Dr. Qinfen Gu of the Australian Synchrotron for his assistance on the Powder Diffraction beamline. We would also like to thank Mr. Barry Halstead, CSIRO Minerals Resources, for his assistance with Xray powder patterns taken from a laboratory source.
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REFERENCES
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