Article pubs.acs.org/EF
N,N‑Dimethylhydrazidoacrylamides. Part 1: Copolymers with N‑Isopropylacrylamide as Novel High-Cloud-Point Kinetic Hydrate Inhibitors Mohamed F. Mady†,‡ and Malcolm A. Kelland*,† †
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, Dokki, Cairo 12622, Egypt ABSTRACT: Hydrophobically modified methacryl- and acrylamide polymers are well-known kinetic hydrate inhibitors (KHIs). Polymers of N-isopropylmethacrylamide (IPMAM) are now commercially available. However, both polyIPMAM and Nisopropylacrylamide homopolymer (polyIPAM) have low cloud and deposition points, making it difficult to use them in the field because of precipitation problems. Comonomers that are more hydrophilic can be copolymerized with IPMAM or IPAM to raise the cloud point. In this work, we have synthesized and investigated a series of new copolymers of IPAM and a new dimethylhydrazidoacrylamide monomer (DMHAM) as KHIs for the first time using high-pressure gas hydrate rocker rig equipment. The novel polymers have high cloud points in deionized water and also in brine solutions compared to polyIPAM. A 1:2 copolymer of DMHAM/IPAM gave the highest KHI performance for this class of copolymer with a cloud point at 58 °C in deionized water. A 1:1 copolymer gave only a small reduction in performance but has a cloud point of 83 °C in 3.6% NaCl and no cloud point in deionized water. Tests were carried out using high-pressure slow constant cooling rocking cell experiments with a structure-II-forming natural gas mixture at approximately 80 bar.
1. INTRODUCTION In recent years, low-dosage hydrate inhibitors (LDHIs) are one of the efficient methods of prevention of gas hydrate control technology.1,2 Gas hydrates are ice-like crystalline clathrate solids that form by a hydrogen-bonded network enclosing roughly spherical cavities that are filled with gas molecules.3,4 Kinetic hydrate inhibitors (KHIs) are a subclass of LDHIs used primarily in the upstream oil and gas industry.1,3,5 The main ingredient in KHIs is a water-soluble polymer. It delays the nucleation and crystal growth of gas hydrates. The most critical factors for field operations are the delay time to hydrate nucleation (induction time) and its dependence upon subcooling.5 Various classes of water-soluble polymers are being successfully used in KHI formulations in oil and gas operations. Hydrophobically modified methacryl- and acrylamide polymers are well-known KHIs (Figure 1). Polymers of N-isopropylmethacrylamide (IPMAM) are now commercially available. However, there are some limitations to the use of both IPMAM and N-isopropylacrylamide (IPAM) polymer KHIs.6−8 One of these limitations is the lower critical solution temperature (LCST), also known as the cloud point (Tcl). It
was found that Tcl values (as well as deposition points) for both IPAM and IPMAM homopolymers are low (30−34 °C in deionized water), making it difficult to use in the field because of precipitation problems at high wellhead injection temperatures.9 The molecular reason for cloud point is related to the competitive strength of the hydrogen bonding between the water phase and the dissolved molecule and the amount of hydrophobic interactions between the polymer and the water phase. The internal hydrogen bonding in the compound changes as one varies the temperature.10 The hydrophobic groups in a KHI are considered a key structural feature for their KHI performance, as long as there are sufficient hydrophilic groups to keep the polymer water soluble. The amide group is an especially good polar group for use as the hydrophilic component in different KHI polymers.5 Copolymerization of IPMAM with more hydrophilic monomers has been used commercially to raise the polymer Tcl.20 Our idea was to modify the IPMAM and IPAM polymers by introducing a more hydrophilic comonomer, N,Ndimethylhydrazidoacrylamide (DMHAM), which is structurally closely related to IPMAM and IPAM, as illustrated in Figure 2. The only structural difference is that the −CH− group in the isopropyl groups is replaced by a tertiary nitrogen atom to give a dimethylhydrazido group. The hydrazine-based starting material used to make the side chains of the DMHAM polymers is 1,1-dimethylhydrazine, a cheap commercial chemical used in rocket fuels.
Figure 1. Structures of poly(N-isopropylmethacrylamide) (polyIPMAM; R = CH3) and poly(N-isopropylacrylamide) (polyIPAM; R = H). © 2014 American Chemical Society
Received: June 22, 2014 Revised: August 5, 2014 Published: August 10, 2014 5714
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compared to some methods that were found in the literature. For example, pyrolysis of 1,1-dimethyl pyrazoline-3-oxide gave a poor yield of approximately 7%.14,15 2.4.1. N,N-Dimethylhydrazidoacrylamide (3a). White crystalline solid. Yield: 21%. Melting point (mp): 88−90 °C. IR νmax (cm−1): 3211 (NH), 1658 (CO), 1628 (CC), 1570 (NH). 1H NMR (DMSO-d6, 300 MHz) δ: 2.44 (s, 3H), 2.46 (s, 3H), 5.50,5.54 (dd, J = 12.0 and 3.0 Hz, 1H), 6.03, 6.05 (dd, J = 6.0 and 3.0 Hz, 1H), 6.83, 6.89 (dd, J = 18.0 and 9.0 Hz, 1H), 8.50 (s, 1H). 13C NMR (DMSOd6, 75.46 MHz) δ: 46.34, 47.90, 125.25, 130.57, 161.87. 2.4.2. N,N-Dimethylhydrazidomethacrylamide (3b). White crystalline solid. Yield: 65%. mp: 73−75 °C. IR νmax (cm−1): 3232 (NH), 1663 (CO), 1624 (CC), 1534 (NH). 1H NMR (DMSO-d6, 300 MHz) δ: (s, 3H), 2.44 (s, 3H), 2.45 (s, 3H), 5.24 (d, J = 0.9 Hz, 1H), 5.49 (d, J = 0.9 Hz, 1H), 8.82 (s, 1H). 13C NMR (DMSO-d6, 75.46 MHz) δ: 18.7, 45.9, 46.0, 118.8, 139.4, 165.4. 2.5. Synthesis of Novel Polymer KHIs. 2.5.1. N,N-Dimethylhydrazidoacrylamide (polyDMHAM) Homopolymer (5a). In a Schlenk tube equipped with a Young’s tap and a magnetic stir bar were placed N,N-dimethylhydrazidoacrylamide (3a) (0.5 g, 4.38 mmol) and 2% (w/w) 2,2′-azobis(2-methylpropionamidine)dihydrochloride (AAPH) in aqueous isopropyl alcohol (5.0 mL) and flushed with oxygen-free nitrogen for at least 10 min under the protection of nitrogen. The reaction mixture was allowed to heat stepwise from room temperature to 40 °C and to 60 °C, at which time additional 2% (w/w) AAPH was added and kept the reaction at each temperature overnight. After the specific time, the reaction medium became viscous and the stirring was stopped. The polymer was cooled at room temperature and was precipitated after removal of the solvents in vacuo to leave a white solid. The reaction was monitored by 1H NMR spectroscopy to determine the percentage of conversion of monomer. A series of polyDMHAM homopolymers with different molecular weights were synthesized using different initiator ratios of AAPH and following the same above procedure. The molecular weights of new polymers were calculated by gel permeation chromatography (GPC), as tabulated in Table 1. It should be noted that hompolymerization of the DMHAM
Figure 2. Structures of polyIPAM, polyIPMAM, and the hydrazinebased analogues poly(N,N-dimethyhydrazidoacrylamide) derivatives. R = H or CH3.
This work in this paper is a continuation of our research program on acrylamide-based KHIs.11−13 In the KHIs in this work, DMHAM has been homo- or copolymerized with IPAM to give polymers with varieties of molecular weights. The polymers were tested for their ability to prevent structure II (SII) gas hydrate formation in high-pressure multi-cell rocker rig experiments.
2. EXPERIMENTAL SECTION 2.1. Chemicals. All chemicals were purchased from VWR, Nippon Chemical Industrial Co., Ltd., Tokyo Chemical Industry Co., Ltd., and Sigma-Aldrich. All solvents were used as purchased without further purification. 2.2. Characterization of KHI Polymers. Nuclear magnetic resonance (NMR) spectra were recorded on a 300 MHz Varian NMR spectrometer in deuterated dimethyl sulfoxide (DMSO-d6) and CDCl3 using tetramethylsilane (TMS) as an internal standard, which was used to calculate the conversion of the monomers. The molecular weight and molecular weight distribution of the new polymers were determined by gel permeation chromatography, using polystyrene samples as molecular-weight standards. Gel permeation chromatography was performed with a HLC 8220 chromatograph (Tosoh Co., Tokyo, Japan) equipped with TSK gel super HM-H H4000/H3000/H2000 (7.8 mm diameter, 150 mm × 3), Tosoh Co. [two polymers did give high polydispersity index (PDI) values; attempts were first made to obtain good Mw data using water and PEB standards, but the results were not meaningful for interpretation]. 2.3. Cloud Point (Tcl) Measurement. Cloud point measurement was carried out by a standard procedure: an aqueous solution of 1 wt % polymer was heated carefully at about 2 °C/min with constant stirring, making visual observations throughout. The cloud point temperature was determined by the first sign of haze in the solution. 2.4. Synthesis of N,N-Dimethylhydrazidoacrylamide (Monomers 3a and 3b). In a round-bottom flask equipped with a magnetic stirring bar were placed N,N-dimethylhydrazine (1) (5.0 g, 83.2 mmol) and triethylamine (8.40 g, 83.2 mmol) in diethyl ether (150 mL). Acryloyl chloride derivatives (2a and 2b) (1.0 equiv) in 50 mL of Et2O were added dropwise at 0 °C. The reaction mixture was stirred for 1 h after the addition was completed at 0 °C, keeping the mixture stirred at room temperature overnight. The solution was filtered, and the solvent was removed under vacuum to give a pale yellow solid. Crystals of the pure products N,N-dimethylhydrazidoacrylamide (DMHAM) (3a) and N,N-dimethylhydrazidomethacrylamide (DMHMAM) (3b) (Figure 3) were grown from an ethyl acetate solution cooled to −30 °C over 12 h in comparatively high yields
Table 1. Molecular Weight of Novel Polymers According to Gel Permeation Chromatography polymer (molar ratio %)
initiator (AAPH) (%, w/w)
molecular weight (Mn)
PDI
polyDMHAM-I (100) polyDMHAM-II (100) polyDMHAM-III (100) polyIPAM 6k (100)a polyIPAM 7k (100)b DMHAM/IPAM (1:1) DMHAM/IPAM (1:2) DMHAM/IPAM (2:1)
2
900
7.53
4
1600
68.02
10
600
24.46
2 2 2
6400 7353 2300
3.41 2.01 6.01
2
2600
8.95
2
2900
9.07
a
A 2% (w/w) AAPH was used for synthesizing polyIPAM 6k. bA 2% (w/w) AIBN was used for synthesizing polyIPAM 7k.
monomer is quite difficult and only low-molecular-weight polymers could be obtained. However, the highest molecular weight homopolymer (Mn = 1600 Da) was high enough for performance comparison as a KHI to the other polymers in this study. 2.5.1.1. PolyDMHAM-I-2%. Yield: 66%. 1H NMR (D2O, 300 MHz) δ: 1.60−1.97 (br, 3H, −CH2−CH−CO−), 2.55 (s, 6H, −N(CH3)2). 2.5.1.2. PolyDMHAM-II-4%. Yield: 68%. 1H NMR (D2O, 300 MHz) δ: 1.59−1.96 (br, 3H, −CH2−CH−CO), 2.53 (s, 6H, −N(CH3)2). 2.5.1.3. PolyDMHAM-III-10%. Yield: 73%. 1H NMR (D2O, 300 MHz) δ: 1.59−1.98 (br, 3H, −CH2−CH−CO−), 2.54 (s, 6H, −N(CH3)2).
Figure 3. Synthesis of N,N-dimethylhydrazidoacrylamide (3a) and N,N-dimethylhydrazidomethacrylamide (3b) monomers. 5715
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CH−CO−)2), 2.56 (s, 6H, −N(CH3)2), 3.90 (br, 1H, −CH− (CH3)2). 2.5.3.3. DMHAM/IPAM (2:1). Yield: 70%. 1H NMR (D2O, 300 MHz) δ: 1.15 (s, 6H, −CH−(CH3)2), 1.58−1.98 (br, 6H, (−CH2− CH−CO−)2), 2.55 (s, 6H, −N(CH3)2), 3.91 (br, 1H, −CH− (CH3)2). 2.6. High-Pressure Gas Hydrate Rocker Rig Equipment Test Methods. Kinetic hydrate inhibition tests described herein were carried out by a constant cooling test method. Tests were conducted in five high-pressure 40 mL steel rocking cells each containing a steel ball, which is shown in Figure 5. The equipment was manufactured by PSL Systemtechnikk, Germany. The gas composition used was a synthetic natural gas (SNG) mixture given in Table 2.
In contrast, DMHMAM monomer is very difficult to polymerize under a variety of conditions using different initiators with a variety of concentrations. The initiators that we investigated included AAPH, azobis(isobutyronitrile) (AIBN), potassium persulfate (KPS), and ditert-butyl peroxide (DTBP). The viscosity of the mixtures remained unchanged after 24 h at elevated temperatures, indicating little or no polymerization. The 1H NMR spectrum confirmed that polymerization had not occurred. The reason is probably due to the strong interaction between monomer molecules because of hydrogen bonding.16 Later, it was discovered that polyDMHMAM can be produced from the hydrochloride monomer at low pH. We will report our findings on the synthesis and performance of KHIs based on DMHMAM polymers in future publications. 2.5.2. N-Isopropylacrylamide (PolyIPAM) Homopolymer (5b). To a 100 mL round-bottom flask equipped with a magnetic stirring bar was added N-isopropylacrylamide (4) (0.7 g, 6.19 mmol) and aqueous isopropyl alcohol (10 mL). A 2% (w/w) AAPH or AIBN was added, and nitrogen was bubbled through the mixture for 10 min. The mixture was then heated to 60 °C with stirring for 12 h. After completion of the reaction (monitored by 1H NMR), the solution was allowed to cool to room temperature. The solid was collected under vacuum to give a high yield of the polyIPAM product. 2.5.2.1. PolyIPAM-2%. Yield: 87%. 1H NMR (D2O, 300 MHz) δ: 1.14 (s, 6H, −CH−(CH3)2), 1.58−2.0 (br, 3H, −CH2−CH−CO−), 3.89 (br, 1H, −CH−(CH3)2). 2.5.3. Synthesis of Copolymers of DMHAM and IPAM (6). The example shown here is for a 1:1 ratio. In a Schlenk tube equipped with a Young’s tap and a magnetic stirring bar were placed DMHAM (3a) (0.4 g, 3.51 mmol), IPAM (4) (0.39 g, 3.44 mmol), and 2% (w/w) AAPH (15.8 mg, 2 mol % of total monomer) in aqueous isopropyl alcohol (10.0 mL), flushed with nitrogen for at least 10 min. Under the protection of nitrogen, the reaction mixture was allowed to heat stepwise from room temperature to 40 °C and to 60 °C, at which time additional 2% (w/w) AAPH was added and kept the reaction at each temperature overnight. After the specific time, the reaction medium became viscous and the stirring was stopped. The polymer solution was cooled to room temperature. The solvents were removed under vacuo to leave a white solid. The reaction was monitored by 1H NMR spectroscopy to determine the percentage of conversion of monomer. It was found that the copolymer yield is at least 98% by NMR and that the ratios in the copolymer are therefore roughly the same as the ratio of monomers added. Three DMHAM/IPAM copolymers with different molecular weights were synthesized by the above procedure. The molecular weights of new polymers are summarized in Table 1. The route for synthesis of all new polymers is shown in Figure 4. 2.5.3.1. DMHAM/IPAM (1:1). Yield: 67%. 1H NMR (D2O, 300 MHz) δ: 1.15 (s, 6H, −CH−(CH3)2), 1.59−2.02 (br, 6H, (−CH2− CH−CO−)2), 2.55 (s, 6H, −N(CH3)2), 3.91 (br, 1H, −CH-(CH3)2). 2.5.3.2. DMHAM/IPAM (1:2). Yield: 73%. 1H NMR (D2O, 300 MHz) δ: 1.15 (s, 6H, −CH−(CH3)2), 1.59−2.01 (br, 6H, (−CH2−
Figure 5. Rocker rig showing the five steel cells in a cooling bath.
Table 2. Composition of SNG component
mol %
methane ethane propane isobutane n-butane N2 CO2
80.67 10.20 4.90 1.53 0.76 0.10 1.84
Details of the KHI test procedure are given below. At the start of each constant cooling experiment, the pressure was approximately 75 bar. The equilibrium temperature (Teq) at this pressure was determined by standard laboratory dissociation experiments warming at 0.025 °C/h for the last 3−4 °C.17,18 Five repeat equilibrium tests were carried out, which gave 20.2 ± 0.05 °C as Teq. This value agrees very well with a calculated Teq value of 20.5 °C at 75 bar using Calsep’s PVTSim software. The constant cooling KHI test procedure was as follows: (1) Each cell was filled with 20 mL of distilled water or solution of a dissolved additive. (2) Air in the cells was removed with a combination of vacuum pumping and filling with SNG to 2 bar and then the procedure was repeated. (3) The cell was pressurized to 75 bar and rocked at 20 rocks/min at an angle of 40°. (4) The cells were cooled from 20.5 °C at a rate of 1 °C/h down to 2 °C. If rapid hydrate formation had not occurred during this time, as judged by a large fast pressure drop, the temperature was held at 2 °C until it had occurred. (5) The pressure and temperature for each individual cell as well as the cooling bath were logged on a computer. Figure 6 shows the pressure and temperature of all five cells for polyDMHAM-I (Mn = 900 Da) under standard constant cooling KHI tests in the multi-cell rocker rig. For the pressure and temperature
Figure 4. Synthetic routes for the preparation of homo- and copolymer KHIs. 5716
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Figure 6. Pressure and temperature logged from five cells during standard constant cooling KHI tests in the multi-cell rocker rig.
Figure 7. Pressure and temperature curves versus time during a standard constant cooling test using 2500 ppm polyDMHAM-I (Mn = 900 Da). The graph in Figure 7 illustrates the determination of the onset temperature (To) and the temperature of fast hydrate formation (Ta) for one of a series of 8−10 tests. In this example, the KHI used is polyDMHAM-I at 2500 ppm. The overall results show that the temperatures are homogeneous for all cells in the water bath and none of the cells contains any systematic errors that lead to consistently better or worse results.
curves versus time, the pressure drops about 2 bar because of gas being dissolved in the aqueous phase. The temperature drops at a constant rate until the minimum of 2 °C after 1120 min. According to the graph, the pressure decreases constantly because of the decrease in the temperature. In addition, exotherms are sometimes seen around the time of the fastest rate of hydrate formation; for example, we see this in experiments with no KHI. However, the example given is for a reasonably powerful KHI, which also slows the growth of hydrate crystals sufficiently that the exotherm is not easily seen as the produced heat is dissipated by the cooling effect of the cell materials and surrounding water bath.
3. RESULTS AND DISCUSSION 3.1. Study of the Cloud Point of the New DMHAM Polymers. Table 3 summarizes the cloud points of the novel 5717
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such as CO2 and H2S. At these conditions, partial protonation and, therefore, quaternization of the hydrazine groups may have occurred. This would give even more hydrophilic dimethylhydrazinium ionic groups, which would increase water solubility further. In 3.6 wt % NaCl aqueous solution, the Tcl of polyDMHAM homopolymer still remains over 100 °C; however, the Tcl of 1:1 DMHAM/IPAM copolymer is now 83 °C, and the Tcl of 1:2 of DMHAM/IPAM copolymer drops to 45 °C. The above results show that polyDMHAM has much higher hydrophilicity than polyIPAM, despite the only difference in structure being the replacement of a CH group with a N atom. The high Tcl values indicate that DMHAM polymers will have much improvement for injection compatibility and salinity compared to polyIPAM and undoubtedly other commercial polyvinyl-based KHIs, such as N-vinyl caprolactam (VCap) polymers. 3.2. High-Pressure KHI Experiments. 3.2.1. Comparison of PolyDMHAM to PolyIPAM. The polyDMHAMs were first tested using a standard constant cooling test at 2500 ppm dissolved in fresh water cooling at 75 bar and 20.5−2.0 °C over 18.5 h with 600 rpm stirring. The onset temperature (To) and fast hydrate formation temperature (Ta) are shown in Table 4 and are also presented in Figure 8. The difference between the To and Ta values gives some indication of the ability to retard the gas hydrate crystal growth process. In our experience, the new polymers in this class are poor hydrate crystal growth inhibitors, and therefore, their primary KHI mechanism is nucleation inhibition. Figures 9 and 10 show the degree of scattering in the results from the standard constant cooling tests for all of the novel KHI polymers. The statistical differences in the performances between the polymers were determined by comparing To and Ta values between polymers using independent sample t tests with equal variances for each polymer. When the p value is less than the predetermined significance level, which is often 0.05, this indicates that the observed result would be highly statistically significant.19 The results from 8 to 10 experiments on each polymer are compared to deionized water (i.e., no additive) as well as using
Table 3. Cloud Points (Tcl) and Molecular Weights of Polymers at pH 5.0 polymer (molar ratio %) polyDMHAM-I (100) polyDMHAM-II (100) polyDMHAM-III (100) polyIPAM 6k (100) polyIPAM 7k (100) DMHAM/IPAM (1:1) DMHAM/IPAM (1:2) DMHAM/IPAM (2:1)
Mn
Tcl (°C) in DI H2O
Tcl (°C) in 3.6% NaCl
900 1600 600
>100 >100 >100
>100 >100 >100
6400 7353 2300 2600 2900
30 29 >100 58 >100
23 22 83 45 >100
Table 4. Constant Cooling KHI Tests in the High-Pressure Multi-cell Rocker Riga
a
polymer (molar ratio %)
To(av) (°C)
Ta(av) (°C)
To(av) − Ta(av) (°C)
no polymer polyDMHAM-I (100) polyDMHAM-II (100) polyDMHAM-III (100) polyIPAM 6k (100) polyIPAM 7k (100) DMHAM/IPAM (1:1) DMHAM/IPAM (1:2) DMHAM/IPAM (2:1)
17.9 12.8 13.5 13.6 11.5 10.9 11.6 10.7 12.1
17.7 10.9 10.7 12.0 8.6 7.5 8.4 8.6 9.1
0.2 1.9 2.8 1.6 2.9 3.4 3.2 2.1 3.0
All polymer concentrations are 2500 ppm.
polymers that were investigated as KHIs. PolyIPAM is known to have a LCST of about 30−34 °C in distilled water depending upon the polymerization methods and the molecular weight.9 For polyDMHAM-I, polyDMHAM-II, and polyDMHAM-III homopolymers, no cloud point was observed in deionized water up to 100 °C. In addition, both 1:1 and 2:1 molar ratio DMHAM/IPAM copolymers also show no cloud point up to 100 °C in deionized water. However, the cloud point of 1:2 molar ratio DMHAM/IPAM copolymer is 58 °C. It should be noted that all of the Tcl values in this study were measured at pH, a typical pH for produced fluids in contact with acid gases,
Figure 8. Average values from 8 to 10 experiments for the onset temperature (To) and fast hydrate formation temperature (Ta) in constant cooling KHI experiments at 2500 ppm. 5718
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Figure 9. To and Ta values from the standard constant cooling tests for polyDMHAM homopolymers compared to no additive at 2500 ppm.
Figure 10. To and Ta values from the standard constant cooling tests.
For polyIPAM homopolymer (Mn = 7353 Da), the average To value is 10.9 °C and the average Ta value is 7.5 °C. The performance is undoubtedly higher for more optimized polymerization procedures and molecular weight distributions. This homopolymer was made in a similar way as the polyDMHAM homopolymers for comparison. Recently, we also studied the performance of polyIPAM (Mn = 6000 Da) made in a similar procedure but at a higher concentration of 5000 ppm at the same experimental conditions. It was found that the average To value was 9.4 °C and average Ta value was 6.4 °C.13 This improved performance in the average To and Ta values is typically observed when increasing the KHI concentration. The To and Ta values of different ratio DMHAM/IPAM copolymers show that the best copolymer tested was the 1:2 DMHAM/IPAM copolymer. For this copolymer, the average To and Ta values for 10 experiments were 10.7 and 8.6 °C. The
different molecular weights of DMHAM/IPAM copolymers. According to the results of standard constant cooling tests, we observed that the average value of the onset temperature (To) for polyDMHAM homopolymer varied between 12.8 and 13.6 °C with a rapid hydrate formation temperature (Ta) in the range of 10.9−12.0 °C. The results of independent sample t test for differences in To shows that polyDMHAM-I (Mn = 900 Da) and polyDMHAM-II (Mn = 1600 Da) gave a p value of 0.013; there is an efficient significant difference in the onset temperatures for two polymer. The average To values for polyDMHAM-I and polyDMHAM-II are not statistically different. The average To and Ta values of deionized water with no additive are 17.9 and 17.7 °C, respectively. These results show that polyDMHAM has significant activity as a SII gas hydrate KHI even as oligomers at very low molecular weights. Further optimization of the polymerization process could probably improve the KHI performance. 5719
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(11) Chua, P. C.; Kelland, M. A.; Yamamoto, H.; Hirano, T. Energy Fuels 2012, 26, 4961. (12) Chua, P. C.; Kelland, M. A.; Ishitake, K.; Satoh, K.; Kamigaito, M.; Okamoto, Y. Energy Fuels 2012, 26, 3577. (13) Chua, P. C.; Kelland, M. A.; Ajiro, H.; Sugihara, F.; Akashi, M. Energy Fuels 2013, 27, 183. (14) Ovsyannikova, L. A.; Sokolova, T. A.; Zapevalova, N. P. Zh. Org. Khim. 1968, 4, 455. (15) Moore, D. R.; Tesoro G. C. U.S. Patent 3,441,606, 1969. (16) Sokolova, T. A.; Ovsyannikova, L. A.; Zapevalova, N. P. Polym. Sci. USSR 1968, 10, 1113. (17) Gjertsen, L. H.; Fadnes, F. H. Ann. N. Y. Acad. Sci. 2000, 912, 722. (18) Tohidi, B.; Burgass, R. W.; Danesh, A.; Ostergaard, K. K.; Todd, A. C. Ann. N. Y. Acad. Sci. 2000, 912, 924. (19) Myers, R. H.; Myers, S. L.; Walpole, R. E.; Ye, K. Probability and Statistics for Engineers and Scientists; Pearson Education International: Upper Saddle River, NJ, 2007. (20) Conrad, P. G.; Acosta, E. J.; McNamee, K. P.; Bennett, B. M.; Lindeman, O. E. S.; Carlise, J. R. WO Patent 2010045523, 2010.
average To value is statistically better than that of polyIPAM, suggesting that the performance of IPAM polymers can at least be maintained even at high Tcl values using DMHAM copolymers. However, it should be noted that the molecular weight of polyIPAM is higher than that of the 1:2 DMHAM/ IPAM copolymer. The 1:1 DMHAM/IPAM polymer also gave a useful performance as a KHI compared to polyIPAM, even with no Tcl at up to 100 °C in deionized water. PolyIPAM was shown to have a Tcl of only 30 °C in deionized water. The p value between the To values for the two polymers polyIPAM 7k and 1:2 DMHAM/IPAM copolymer (Mn = 2300 Da) is 0.01, showing a statistically significant difference in the KHI performance.
4. CONCLUSION We have synthesized and investigated a series of new copolymers of IPAM and a new dimethylhydrazidoacrylamide monomer (DMHAM) as KHIs for the first time using highpressure gas hydrate rocker rig equipment. The novel polymers give high cloud points in deionized water and also in brine solutions compared to polyIPAM. KHI tests were carried out using high-pressure slow constant cooling rocking cell experiments with a SII-forming natural gas mixture at approximately 80 bar. The DMHAM homopolymers gave useful KHI performance, but the performance was improved further when DMHAM was copolymerized with IPAM. A 1:2 DMHAM/IPAM copolymer gave the highest KHI performance for this class of copolymer and gave a cloud point of 58 °C in deionized water. A 1:1 copolymer gave only a small reduction in performance compared to the 1:2 copolymer and has a cloud point of 83 °C in 3.6% NaCl and no cloud point in deionized water. We are currently studying copolymers of VCap with the DMHAM monomer as well as polymers of the DMHMAM monomer.
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The authors declare no competing financial interest.
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REFERENCES
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dx.doi.org/10.1021/ef501391g | Energy Fuels 2014, 28, 5714−5720
