Article pubs.acs.org/EF
Synthesis of Effective Kinetic Inhibitors for Natural Gas Hydrates Xia 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 and Industrial 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 performance in inhibiting both tetrahydrofuran (THF) hydrate and the synthetic gas hydrate formation was examined using two different instruments: a ball-stop apparatus and a high-pressure automatic lag time apparatus (HP-ALTA). Performance was benchmarked against two commercially available KHIs, Gaffix VC-713 and Luvicap EG, under the same working conditions. The test results from the ball-stop rig demonstrated that the new KHIs were as effective as Gaffix VC-713 and Luvicap EG in preventing the formation of THF hydrates in 3.5 wt % NaCl solutions. For the synthetic gases, most new polymers outperformed the reference KHIs at a concentration of 0.05 wt %. Polymers containing a pendant THF functional group in the side chains showed a substantial 12−16 °C decrease in the hydrate formation temperature of the gas water mixtures relative to those containing the same amount of Gaffix VC-713 or Luvicap EG. The trend of the inhibition performance of the polymers was different in THF from that measured for gas mixtures. Small amounts of ethanol added to the hydrate formation mixtures were also shown to have an effect. Investigation of the inhibition mechanism associated with these new polymers is under way.
1. INTRODUCTION Hydrates are crystalline, ice-like solids that form when gas molecules are trapped in hydrogen-bonded water cages at high pressure and low temperature,1 conditions which are often encountered in deepwater offshore operations. The formation of gas hydrate plugs in subsea pipelines can result in serious safety 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 used method to prevent the formation of gas hydrates in production pipelines. The method has proven to be effective, but the economic drawbacks are significant. Large volumes of inhibitors are required, generally between 20 and 60% by weight. The cost associated with the use and recovery of inhibitors in such volumes is very high. The worldwide annual expense for the most commonly used thermodynamic inhibitor, methanol, alone was estimated at U.S. $220 million in 2003.4 Potential environmental pollution by these chemicals has also been a great concern.5 The desire to reduce the costs and environmental impacts associated with the use of thermodynamic inhibitors has led to increased research activities for the design, development, and inhibition mechanism exploration of novel, environmentally friendly low-dose hydrate inhibitors (LDHIs).6−9 Kinetic hydrate inhibitors (KHIs) are a class of LDHIs that have been in commercial use in the oil and gas industry for over a decade.10 They are used at low concentrations, typically less than 1 wt % of the aqueous phase. These chemicals do not alter the thermodynamics of hydrate formation, but they modify the kinetics of formation, by either preventing nucleation, hindering the crystal growth, or both. The nucleation time, often referred to as induction time, is a critical factor for field operations. It is dependent upon the subcooling, ΔT, the difference between the thermodynamic hydrate equilibrium temperature and the operating temperature at a given pressure for a specific gas © 2011 American Chemical Society
composition. KHIs are often ranked by the maximum subcooling achievable in comparable systems or by comparison of induction time as a function of the subcooling and inhibitor concentration.11 KHIs are generally water-soluble polymers. The earliest offshore field test of KHIs was carried out on the southern North Sea gas field in 1995, using Gaffix VC-713, a terpolymer of vinylcaprolactam (VCap), vinylpyrrolidone (VP), and dimethylaminoethyl methacrylates. It was reported to be effective at 0.5 wt % and 8−9 °C of subcooling.12 Soon after that, a KHI blend based on VCap polymers and tetrabutylammonium bromide (TBAB) successfully replaced glycol and became the first offshore field application of a KHI on BP’s southern North Sea gas field.13 Successful applications of co-polymers of VCap and vinylmethylacetamide, poly(VIMA/VCAP), have been reported in four further fields.14 This co-polymer outperformed VCap by 2−3 °C subcooling. By the end of 2005, it was estimated that there were 40−50 field applications of KHIs.6 These KHIs can be divided into two classes of polymers: one is the homo- and co-polymers of VCap, and the other is hyperbranched poly(ester amide)s.8 The former, the polyvinylcaprolactam (PVCap), and its co-polymers contain seven-membered lactam rings attached to the polymer backbone. It is believed that the hydrophilic lactam group plays an important role in inhibiting hydrate growth and that the hydrogen bonding between the functional group and water molecules leads to binding of the inhibitors on the hydrate surface, which blocks the transport of gas to the hydrate surface and disrupts the hydrate formation.15 An earlier report by Sloan et al.16 indicated that the performance of PVCap is related to the molecular weight of the Received: September 29, 2011 Revised: December 29, 2011 Published: December 29, 2011 1037
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Figure 1. Synthetic scheme of polymers.
performance. The synthetic route of these polymers is illustrated in Figure 1A. The other four polymers were designed to incorporate THF, a five-membered ring, into the polymers as a pendent group on the side chains (Figure 1B). THF is a well-known hydrate guest molecule. Unlike gas molecules, such as methane, ethane, or propane, THF is hydrophilic and completely miscible with water, indicating a strong affinity with water molecules. Because we know that adsorption of KHI molecules onto the surfaces of particles or hydrate crystals may prevent hydrate growth, the idea of incorporating THF pendent groups onto hydrophobic polymer backbones was to use the strong affinity between THF rings and the water molecules to improve the adsorption of the polymer, therefore making the inhibition more effective. While the THF rings may adhere onto the hydrate surface more effectively, the hydrophobic polymer backbones form a blanket to 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 to measure induction time and ball-stop time of a model hydrateforming 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 visually observed, the ball-stop time indicates a total block of the test vessel by the formed hydrates. The testing rig consists of small
polymer. Polymers with an average molecular weight of 900 Da were more effective than those of 1300, 2100, 9200, and 18 000 Da. It is believed that low-molecular-weight PVCap-based products with added synergists are probably the best KHIs for natural gas hydrate inhibition on the market today.6 This KHI blend gave at least 48 h of hydrate inhibition at a subcooling of 13 °C.17 In this paper, six new polymers were designed and synthesized. Their performance as a KHI was examined using a ballstop rig and a high-pressure automatic lag time apparatus (HPALTA) in both tetrahydrofuran (THF) hydrate formation solutions and gas hydrate formation mixtures. The results were compared to the commercially available and well-known KHIs, Gaffix VC-713 and Luvicap EG, both containing lactam rings in the polymer backbones. Two of these polymers contain VCap moieties and a second component, poly(ethylene oxide) (PEO) (Figure 1A). As mentioned previously, the inhibition performance of PVCap can be improved by various synergists, including high-molecularweight PEO.18 Although PEO itself is not a kinetic inhibitor, the addition of PEO to a kinetic inhibitor solution was found to enhance the performance of the inhibitor by an order of magnitude in some cases. Therefore, we believe that incorporating PEO into polymer chains may improve the inhibition 1038
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cycles and the removal of solvent by a rotary evaporator, the co-polymers were dried to a constant weight in an oven at 37 °C. PEOMAs of two different molecular weights were used in this preparation. The molecular weights of the PEOs and the molar percentage of PEO and VCap in these polymers can be found in Table 1.
cells or test tubes filled with hydrate-forming solutions that are placed in a cooling bath. Each cell or test tube contains a metal ball rocking back and forth. The metal ball stops moving when the hydrate is formed and plugs the test tube. Ball-stop rigs have been used to screen a vast range of chemicals as gas hydrate inhibitors.20 In this study, ball-stop times are taken as an indication of hydrate inhibition efficiency of KHIs. A more detailed discussion about the method can be found in our previous work.21 In comparison to ball-stop rigs, the automatic lag time apparatus (ALTA) is a more recent development. ALTA was initially built by Haymet’s group for studying the nucleation of supercooled liquids.22 After several important improvements by the same research group, ALTA was subsequently employed for statistical evaluation of liquid-to-crystal heterogeneous nucleation by monitoring the nucleation temperature using light scattering principles.23 Using ALTA, samples are repeatedly cooled on a linear cooling ramp until they nucleate and become solid. The samples are then warmed and thawed. Because the cooling ramp is linear, either the time or temperature at which freezing occurs is a useful parameter with which to measure the induction time to nucleation.24 The freeze−thaw cycle can be repeated many hundreds of times in a single experiment to generate reliable and reproducible statistics for nucleation. More recent studies on THF/water hydrates have shown that ALTA is an excellent instrument to evaluate the effectiveness of KHIs by measuring the freezing temperature, i.e., the hydrate formation temperature, of hydrate-forming mixtures in the presence of KHIs.25 Very recently, measurements of the hydrate formation temperature of hydrate-forming mixtures at elevated gas pressures became possible using a high-pressure version of this instrument known as HP-ALTA.26
Table 1. Monomer Composition (mol %) for Inhibitor Synthesis samples
PEOMA
VCap
THFMA
MMA
PEO-co-VCap-I PEO-co-VCap-II PTHFMA PTHFMA-co-MMA PTHFMA-co-PEO PTHFMA-co-VCap
15 (Mn = 475) 15 (Mn = 1100) 0 0 50 0
85 85 0 0 0 50
0 0 100 50 50 50
0 0 0 50 0 0
2.2.2. PTHFMA Homo-polymer. THFMA (17.02 g) and AIBN (0.328 mg) were dissolved in toluene (200 mL) and then flushed with oxygen-free nitrogen for at least 20 min. Under the protection of nitrogen, the reaction mixture was heated to 65 °C. The polymerization mixture was stirred at 65 °C for 10 h. After cooling to room temperature, the polymer was precipitated in an excess of hexane (250 mL). Three reprecipitation cycles were carried out using chloroform and hexane to remove monomers and oligomers. PTHFMA was obtained by 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 using the above procedure in which a co-monomer MMA, PEO, or VCap was added according to the chemical composition displayed in Table 1. 2.3. Characterization of Inhibitors. A Perkin-Elmer Spectrum 100 spectrometer was used to obtain an infrared spectrum for each polymer. The spectra were collected at room temperature with a resolution 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 molecular weight of produced polymer inhibitors. PolarGel L (Varian) was used as the separation column, and the signal was collected by an evaporative light scattering detector. THF was used as the mobile phase, and narrow polydispersity PMMA standards (EasiVial PMMA, Polymer Laboratories) were used for GPC calibration. 2.4. Inhibition Performance Testing. 2.4.1. Ball-Stop Rig Testing. A mixture of THF and 3.5 wt % sodium chloride was used as a hydrate-forming solution in the ball-stop rig testing experiments. The molar ratio of THF and water was kept at 1:17. The hydrate-forming solution is represented as THF−NaCl in the following sections. To conduct the measurements, a rotating ball-stop rig was setup as reported in a previous work.21 The THF−NaCl solutions containing various polymer KHIs at the desired concentrations were injected into the test tubes, each of which contained a stainless-steel ball. The test tubes were then mounted on the ball-stop rig that was kept in an ice− water bath. The temperature was monitored at regular intervals to ensure 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 at which a ball in the test tube stopped because of the formation of THF hydrates was recorded as a measure of the hydrate inhibition performance. Three parallel tests were conducted for each of the hydrateforming solutions. Details of the experimental setup and solution preparation procedures can be found in the study by Ding et al.21 2.4.2. HP-ALTA Testing. The HP-ALTA setup is displayed in Figure 2. The high-pressure sample chamber is made of stainless steel and sandwiched between two Peltier devices, which in turn are sandwiched between two heat sinks. The apparatus allows for a small volume (∼150 μL) of water to be cooled at a controlled rate in a pressurized gas atmosphere and the temperature of gas hydrate formation to be detected. After each formation of gas hydrate, the sample was dissociated at a temperature of about 15 K above the thermodyanamic equilibrium dissociation temperature of the gas hydrate for at least 200 s. We assume that these conditions are sufficient to eliminate the so-called “structural memory effect”. We also
2. EXPERIMENTAL SECTION 2.1. Chemicals. Poly(ethylene glycol) methyl ether methacrylate [PEOMA, average Mn ∼ 475 (I) and 1100 (II)], N-vinylcaprolactam (VCap, 98%), tetrahydrofurfuryl methacrylate (THFMA, 97%), and THF (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 without further 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 dimethylaminoethyl methacrylates) (Mw ∼ 82 700) in ethanol, was kindly donated by International Specialty Products (ISP, Germany). 2.2′Azobis(2-methylpropinitrile) (AIBN, Sigma-Aldrich) was recrystallized, and methyl methacrylate (MMA, ICI) was washed with a NaOH solution before usage. Sodium chloride (Lab-Scan Analytical Science, 99%) and deionized water were used in the preparation of sodium chloride 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-coVCap-II, were prepared by free-radical solution co-polymerization in the 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 oxygenfree nitrogen for at least 20 min. Under the protection of nitrogen, the reaction mixture was heated to 80 °C, followed by the addition of a second monomer PEOMA (3 mmol) after 25 min. The polymerization mixture was stirred at 80 °C for another 140 min. After cooling to room temperature, the produced co-polymers were precipitated from the reaction mixture by adding in an excess of hexane (200 mL). The precipitated co-polymers were separated from hexane and dissolved in chloroform (20 mL). A reprecipitation was carried out to remove unreacted monomers and oligomers. After three reprecipitation 1039
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namic equilibrium dissociation temperature of gas hydrates, and hence, the dissociation conditions discussed above were retained for KHIcontaining samples. Using HP-ALTA, the process of forming and dissociating gas hydrates is automatically repeated for a statistically significant number of nucleation events. The so-called “interfacial transmittance configuration” of the instrument was adopted for this study. More details 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 containing 0.05 wt % KHIs. The newly synthesized polymer KHIs were first dissolved in ethanol prior to their mixing with water. The concentration of KHIs in ethanol was 37 wt %, which is the polymer concentration in Gaffix VC-713. A cooling rate of 0.025 °C/s was used. Measurements for samples containing KHIs were carried out at 10.5 ± 0.5 MPa for all samples to compare their relative effectiveness under similar conditions.
3. RESULTS AND DISCUSSION 3.1. Polymer Characterization. Six novel polymers and co-polymers containing various organic moieties and pendent groups were synthesized. These include PEO-co-VCap-I, PEOco-VCap-II, PTHFMA, PTHFMA-co-MMA, PTHFMA-coPEO, and PTHFMA-co-VCap. The measured molecular weights of the produced polymers ranged from 18 973 to 69 310 Da and are in the range of the molecular weights of Luvicap EG (2000 Da) and Gaffix VC-713 (82 700 Da). The produced polymers were characterized using Fourier transform infrared (FTIR) spectroscopy to confirm the presence of the co-monomers and the total removal of unreacted monomers. An example of a comparative FTIR transmittance spectrum of a co-polymer PTHFMA-co-VCap and its two monomers THFMA and VCap is displayed in Figure 3. In this figure, the peaks in the range of 2800−3000 cm−1 are attributed to the asymmetric and symmetric stretching of CH2 and CH3,
Figure 2. Schematic illustration of HP-ALTA. note that the addition of small concentrations (typically less than 1 wt %) of KHIs is not expected to significantly alter the thermody-
Figure 3. FTIR spectra of (a) THFMA, (b) PTHFMA-co-VCap, and (c) VCap. 1040
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test results of the new polymers directly comparable to those of Luvicap EG and Gaffix VC-713, one testing solution was made of THF−NaCl and the synthesized co-polymer KHI, PEO-coVCap-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-713 were used in the hydrate-forming solutions. The ball-stop time was generally shorter than 360 min when the newly polymerized inhibitors were used. When the inhibitor concentration was increased to 3.5 wt % (3.0 wt % for PTHFMA), the ballstop time of all THF−NaCl hydrate formation solutions exceeded 360 min, except for that containing PEO-co-VCap-II (PTHFMA-co-PEO and PTHFMA-co-VCap were not tested because of the limited solubility). When 37% ethanol (equal amount to that in Gaffix VC-713) was added to 2.5 wt % PEOco-VCap-I, the ball-stop time changed from below 360 min to over 360 min. The results demonstrated that the new polymer KHIs alone are as effective as Gaffix VC-713 and Luvicap EG at a concentration of 3.5 wt %. When ethanol is added to the polymers, they might outperform Gaffix VC-713 and Luvicap EG. More systematic studies are required to understand the impact of ethanol on the inhibition of these KHIs. When the chemical structures of PEO-co-VCap-I and PEOco-VCap-II are compared, the only difference is that the length of the PEO pendent groups in PEO-co-VCap-I is 7−8 CH2CH2O repeating units, much shorter than that in PEOco-VCap-II, which contains 22−23 CH2CH2O repeating units (Figure 1A). Co-polymers of PEO-co-VCap containing a higher PEO ratio were also synthesized and tested (data not shown). None of them showed longer ball-stop times than PEO-coVCap-I. These observations agree with the previous work that PEO itself is a weak hydrate inhibitor27 and that the lower molecular-weight PEO chain has a better synergistic effect with PVCap-type inhibitors.18 These ball-stop rig testing results indicated that the polymers synthesized are as effective as the reference inhibitors Luvicap EG and Gaffix VC-713 in preventing THF hydrate formation over a 6 h time period. The addition of small amounts of ethanol can further improve the performance of these inhibitors. 3.3. HP-ALTA Testing Results. The HP-ALTA test results on the gas mixture were displayed in Figure 4. The concentration of the KHIs tested by HP-ALTA was kept at 0.05 wt % for all samples. The hydrate formation temperature shown in the figure was the median value of over 100 measurements for most of the samples (except for PTHFMA-co-PEO and PTHFMA-co-VCap, which were the median value of ≈50 measurements). 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 supercooling became smaller and remained more constant over the rest of the cycles. Given the dissociation conditions described in section 2.4, we believe that this is resulted from increasing gas saturation in the solutions over the initial cycles, resulting in less necessity for gas transport in hydrate formation and less supercooling being observed in the later measurements. The collection of a large number of statistics using HP-ALTA means that the median values of the hydrate formation temperature are 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 PEOco-VCap-II is not significantly different from those without a
which are present in both monomers and the produced polymer (spectra a, b, and c). The peak at 3103 cm−1 of the THFMA spectrum a and that at 3109 cm−1 of VCap spectrum c, both arising from the polymerizing vinyl group CC−H stretching, have disappeared in the produced polymer PTHFMA-co-VCap spectrum b. The peaks at 1638 cm−1 of spectrum a and at 1654 cm−1 of spectrum c, attributed to CC double bond stretching, also disappeared in the PTHFMA-coVCap (spectrum b). These indicated complete polymerization and/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 at 1716 cm−1 in spectrum b have confirmed the presence of both the caprolactam ring from VCap and the COOCH2THF group from THFMA. For both amide CO and ester CO, there was a slight shift of the signal to a higher wavenumber in comparison to those of the monomers. This is a result of polymerization that has converted all CC double bonds into C−C single bonds; therefore, the CO bonds are no longer conjugated with CC. 3.2. Ball-Stop Rig Testing. The measured ball-stop times for THF hydrate-forming solutions are summarized in Table 2. Table 2. Molecular Weight and Ball-Stop Time of the New Polymeric KHIs in Comparison to Luvicap EG and Gaffix VC-713 ball-stop time (min) inhibitors
Mw (Da)
0.25 wt %
0.35 wt %
Luvicap EG Gaffix VC-713 PEO-co-VCap-I PEO-co-VCap-II PTHFMA PTHFMA-co-MMA PTHFMA-co-PEO PTHFMA-co-VCap
2000 82700 27212 63919 42239 18973 22921 69310
>360a >360a 360a 360 min. Observations other than that are reported as
