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Comparison of the Kinetic Hydrate Inhibition Performance of Block and Statistical N‑Alkylacrylamide Copolymers Lilian Ree,† Malcolm A. Kelland,*,† David Haddleton,‡ and Fehaid Alsubaie‡,§ †

Department of Mathematics and Natural Science, Faculty of Science and Technology, University of Stavanger, N-4036 Stavanger, Norway ‡ Department of Chemistry, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, U.K. S Supporting Information *

ABSTRACT: Various classes of water-soluble polymers have been used for over 2 decades as kinetic hydrate inhibitors (KHIs) to prevent the plugging of flowlines with gas hydrates. Many of these polymers are polyvinyl-based and are synthesized by radical polymerization of the corresponding vinylic monomers. When two or more co-monomers are used, this will give statistical copolymers. In this study, we compare the KHI performance of statistical and block copolymers made from Nisopropylacrylamide (NIPAM) with both N,N-dimethylacrylamide (DMA) and 2-hydroxyethylacrylamide (HEAA). The copolymers were made using similar procedures and have very similar molecular weights and PDI (dispersity) values to enable good KHI performance comparison. The copolymers were tested in high pressure rocking cells using a structure II-forming synthetic natural gas mixture using a slow constant cooling method. All of the 1:1 block copolymers, at 2500 or 7000 ppm, gave statistically significant lower average onset temperatures than the equivalent 1:1 statistical copolymers by about 1 °C. For the 3:1 NIPAM:HEAA statistical and block copolymers, the performances were more similar. These 3:1 ratios were also the best performing copolymers, probably reflecting the higher percentage of the more hydrophobic NIPAM co-monomer.

1. INTRODUCTION Kinetic hydrate inhibitors (KHIs) are a class of low dosage hydrate inhibitors (LDHIs) and have been used commercially in oilfield operations for over 2 decades to prevent plugging of flow lines with gas hydrates.1−4 KHI formulations contain a water-soluble polymer as the main active ingredient together with other chemicals and solvents that can act synergistically to improve the performance or lower the amount of polymer required. Many of these KHI polymers are polyvinyl-based and are made by free radical polymerization of the vinylic monomers. The most well-known examples are polymers and copolymers based on N-vinyl caprolactam. Copolymers with co-monomers such as N-vinylpyrrolidone, (dimethylamino)ethyl methacrylate (DMAEMA), (acrylamido)propanesulfonic acid (AMPS), N-methyl-N-vinyl acetamide, N-vinylpyridine, or vinyl acetate have been investigated as KHIs and some used in field operations.5−8 Another major class of KHI polymers are based on derivatives of various acrylamides and methacrylamides. Polymers and copolymers of N-isopropylmethacrylamide are now used in the field (Figure 1). Polymerization of all of these vinyl monomers is usually carried out with oxygen centered radical initiators such as persulfate salts, benzoyl peroxide, or di-tert-butyl peroxide, or azo chemicals such as α,α-azobis(isobutyronitrile) or 2,2′diamidinyl-2,2′-azopropane hydrochloride. Free radical polymerization of two or more vinylic co-monomers results in the formation of statistical copolymers with no particular order of the co-monomers.9,10 It is noted that there are exceptions to this general rule when reactivity ratios are sufficiently different. For example, maleic anhydride usually copolymerizes in an ABABAB alternate sequence with co-monomers such as © XXXX American Chemical Society

styrene, alkyl vinyl ethers, or vinyl acetate. This is due to strong electron donation in the maleic comonomer.11 In addition, sterically bulky monomers or polymerization transition states involving transition metals can lead to more ordered monomer sequences in the resulting copolymer.12 We have recently employed an aqueous single electron transfer living radical polymerization (SET-LRP) to synthesize multiblock homopolymers and copolymers of a range of acrylamide monomers including N-isopropylacrylamide (NIPAM), 2hydroxyethyl acrylamide (HEAA), N,N-dimethylacrylamide (DMA), and N,N-diethylacrylamide (DEA) (Figure 2).13 This allows for an entry point to study the KHI performance of block vs statistical copolymers of these acrylamides. In comparing KHI polymers from the same class, it is important that the molecular weights are as similar as possible since this factor has a large effect on the performance. A range of studies indicates that polymer molecular weights in the range 1500− 3000 g/mol are probably best for optimal KHI performance for a monomodal distribution of weights.1−3,14 Below this range the performance drops off rapidly, and above this range the performance decreases slowly and never disappears altogether. Only one specific copolymer in both the statistical and block copolymer isomeric forms has previously been studied for comparative KHI performance. This copolymer is poly(Nethyl-β-alanine)-co-N-propyl-β-alanine), a CO-alkylaziridine copolymer, with molecular weight (Mw) of 4075 g/mol.15 The statistical copolymer was shown to have a cloud point of 64 °C and the block copolymer 70 °C. The statistical isomeric Received: October 25, 2016 Revised: December 15, 2016 Published: January 3, 2017 A

DOI: 10.1021/acs.energyfuels.6b02785 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 1. Top row: N-Vinyl caprolactam, N-vinylpyrrolidone, and N-vinyl-N-methylacetamide. Bottom row: (Acrylamido)propanesulfonic acid (AMPS), (dimethylamino)ethyl methacrylate) (DMAEMA), and N-isopropylmethacrylamide (NIPMAM).

Figure 2. Homopolymers of NIPAM (left), N,N-dimethylacrylamide (DMA; middle), and 2-hydroxyethylacrylamide (HEAA; right). mm guard column (50 mm × 7.5 mm), and autosampler. Commercial narrow linear poly(methyl methacrylate) standards in the range of 200 to 1.0 × 106 g·mol−1 were used to calibrate the system. All samples were passed through a 0.45 μm PTFE filter before analysis. All reactions were carried out under an inert atmosphere of oxygenfree nitrogen, using standard Schlenk techniques. General Procedures. General Procedure for Block Copolymerization by Aqueous SET-LRP (DPn = 10, Low Mn). To a Schlenk tube fitted with a magnetic stir bar and a rubber septum, H2O (2 mL) and Me6TREN (0.1 mmol) were charged and the mixture was bubbled with nitrogen for 15 min. CuBr (0.1 mmol) was then carefully added under slight positive pressure of nitrogen. The mixture immediately became blue Cu(II), and a purple/red precipitate Cu(0) was observed. In a separate vial fitted with a magnetic stir bar and a rubber septum monomer (2.5 mmol) was dissolved in H2O (1.0 mL) prior to addition of initiator (2,3-dihydroxypropyl 2-bromo-2-methylpropanoate, 0.25 mmol) and the resulting mixture was bubbled with nitrogen for 15 min. The degassed monomer/initiator aqueous solution was then transferred via cannula to the Schlenk tube containing Cu(0)/CuBr2/Me6TERN catalyst. The Schlenk tube was sealed, and the mixed solution was allowed to polymerize at 0 °C. In order to confirm the anticipated full conversion, prior to addition of deoxygenated aqueous solution of the second monomer (DPn eq), regular sampling was employed to identify the time required to reach full monomer conversion. This was repeated until conversion and/or molecular weight distributions were compromised by termination. Samples taken for 1H NMR were directly diluted with D2O. Catalyst residues were removed by filtering through a column of neutral alumina prior to DMF SEC analysis. Chain extension was performed by cannulation of a deoxygenated solution of HEAA or DMA (10 equiv) and H2O (1.0 mL) after 11 min of polymerization. General Procedure for Block Copolymerization by Aqueous SETLRP (DPn = 100, High Mn). To a Schlenk tube fitted with a magnetic stir bar and a rubber septum, H2O (2 mL) and Me6TREN (28 μmol)

form performed better as a KHI than the block analogue. It was suggested that the correct molecular spacing of the monomeric units is required for best kinetic hydrate inhibition for this particular copolymer.

2. MATERIALS AND METHODS N-Isopropylacrylamide (NIPAM; 97%) was purchased from a commercial supplier (Sigma-Aldrich) and was purified by recrystallization from hexane to remove the inhibitor. 2-Hydroxyethylacrylamide (HEAA; 97%, Sigma-Aldrich), and N,N-dimethylacrylamide (DMA; 99%, Sigma-Aldrich) were passed over a column filled with basic alumina to remove the inhibitor prior to use. HPLC grade water (H2O; VWR International, LLC) was used as the solvent for disproportionation and polymerizations. The water-soluble initiator 2,3-dihydroxypropyl 2-bromo-2-methylpropanoate was prepared as reported in the literature.16 Tris(2(dimethylamino)ethyl)amine (Me6TREN) was synthesized according to literature procedures and stored under nitrogen prior to use.17 Copper(I) bromide (CuBr; 98%, Sigma-Aldrich) was sequentially washed with acetic acid and ethanol and dried under vacuum. The homopolymers poly(N-vinyl caprolactam) (PVCap; Mn = 4092 g/mol, PDI = 1.51) and poly(N-isopropylacrylamide) (PNIPAM; Mn = 4300 g/mol, PDI = 1.26) were synthesized as previously described.18,19 Instrumentation. Proton nuclear magnetic resonance (1H NMR) spectra were recorded on Bruker DPX-300 and DPX-400 spectrometers using deuterated solvents obtained from Aldrich. Monomer conversion for NIPAM, HEAA, and DMA homopolymerization was determined, comparing the integral of vinyl protons with isopropyl, ethyl, and dimethyl protons, respectively14 (see also the Supporting Information). Size-exclusion chromatography (SEC) was conducted on a Varian 390-LC system using DMF as the mobile phase (5 mM NH4BF4) at 50 °C, equipped with refractive index, UV, and viscometry detectors, 2 × PLgel 5 mm mixed-D columns (300 mm × 7.5 mm), 1 × PLgel 5 B

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Energy & Fuels were charged and the mixture was bubbled with nitrogen for 15 min. CuBr (56 μmol) was then carefully added under slight positive pressure of nitrogen. The mixture immediately became blue Cu(II), and a purple/red precipitate Cu(0) was observed. In a separate vial fitted with a magnetic stir bar and a rubber septum monomer (7.0 mmol) was dissolved in H2O (4.0 mL) prior to addition of initiator (2,3-dihydroxypropyl 2-bromo-2-methylpropanoate, 0.07 mmol), and the resulting mixture was bubbled with nitrogen for 15 min. The degassed monomer/initiator aqueous solution was then transferred via cannula to the Schlenk tube containing Cu (0)/CuBr2/Me6TREN catalyst. The Schlenk tube was sealed, and the mixed solution was allowed to polymerize at 0 °C. Samples of the reaction mixture were then removed for analysis. The sample for 1H NMR spectroscopy was directly diluted with D2O. Catalyst residues were removed by filtering through a column of neutral alumina prior to DMF SEC analysis. Chain extension was performed by cannulation of a deoxygenated solution of HEAA (100 equiv) and H20 (4.0 mL) after 30 min of polymerization. General Procedure for Statistical Copolymerization by Aqueous SET-LRP. The general procedure for copolymerizations by aqueous SET-LRP was followed.20−22 After full disproportionation of CuBr, a degassed solution of the initiator, NIPAM, and HEAA was transferred to the Schlenk tube containing Cu(0)/CuBr2/Me6TREN catalyst. Samples of the reaction mixture were then removed for analysis. A summary of the copolymers synthesized is given in Table 1 together with their cloud points as 1 wt % solutions in deionized water.

Figure 3. UV−vis spectroscopy cloud point measurement for block copolymer, NIPAM:HEAA 3:1.

Table 1. Homopolymers and N-Isopropylacrylamide Copolymers Synthesized and Their Cloud Points (Tcl) composn

molar ratio

copolymer

PNIPAM

−

−

PHEAA PDMA NIPAM:HEAA

− − 1:1

NIPAM:HEAA

1:1

NIPAM:HEAA

3:1

NIPAM:DMA

1:1

− − block statistical block statistical block statistical block statistical

Mn (g/mol)

PDI

TCl (°C)

5000 35000 4500 5000 30000 29300 5500 5400 3700 3700 4200 4100

1.10 1.15 1.12 1.09 1.13 1.20 1.12 1.13 1.09 1.07 1.07 1.09

32 31 >95 >95 >95 >95 >95 >95 87 87 >95 >95

Figure 4. Rocker rig showing the five steel cells in a cooling bath.

Table 2. Composition of Synthetic Natural Gas (SNG)

Cloud points were measured using UV−vis spectroscopy.23 Only two of the copolymers, 3:1 NIPAM:HEAA (mole ratio) statistical and block copolymers, showed a cloud point below 95 °C, giving a value of 87 °C in both cases (Figure 3). High Pressure Kinetic Hydrate Inhibitor Experimental Methods. KHI tests were carried in a set of five 40 mL steel rocking cells each containing a steel ball. (Figure 4). The equipment was manufactured by PSL Systemtechnik GmbH, Oesterode am Harz, Germany. The gas composition used was a synthetic natural gas mixture given in Table 2. The equilibrium temperature (Teq) at the initial pressure of 76 bar was determined to be 20.2 ± 0.05 °C by standard laboratory dissociation experiments warming at 0.025 °C/h for the last 3−4 °C.24,25 This value agrees very well with a calculated Teq value of 20.5 °C at 76 bar using Calsep’s PVTSim software. The KHI test procedure was a constant cooling test method over 18.5 h. This allows for a new set of experiments to be carried out each day while giving a slow enough cooling to separate out the performances and rankings of the KHIs. The constant cooling test procedure is as follows: (1) A 20 mL aliquot of aqueous solution of the test chemical at the desired active polymer concentration is added to each of the steel cells. (2) Air in the cells is removed with a combination of vacuum pumping and filling with SNG to 2 bar, and then the procedure is repeated.

component

mol %

methane ethane propane isobutane n-butane N2 CO2

80.67 10.20 4.90 1.53 0.76 0.10 1.84

(3) The cells are pressurized to 76 bar with SNG 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 to 2 °C while logging the pressure and temperature for each individual cell, as well as the cooling bath. A typical graph of pressure and temperature data vs time from one of the five cells is shown in Figure 5. At the start of rocking the cell, the pressure drops about 1−2 bar due to gas being dissolved in the aqueous phase. The temperature drops at a constant rate until the minimum of 2 °C after 1120 min. During this time the pressure also drops at a constant rate, as it is a closed system, until the rate of pressure drop increases due to hydrate formation after about 720 min. The first deviation from the pressure drop not due to the temperature drop is taken as the time for onset of hydrate formation, although nucleation on an undetectable scale may have occurred earlier. The onset temperature, To, at this time is determined, which in Figure 5 is 8.8 °C. Within a set of 10 experiments the degree of scattering for To values is in the range but no higher than 10−15% and reflects the C

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Figure 5. Determination of To and Ta values from the graph of pressure and temperature vs time. stochastic nature of gas hydrate formation. A more thorough investigation of the reproducibility at various test conditions in this multicell rocker rig has been carried out.26 To determine if there is a significant difference between sets of To values for two polymers we use the t test and find the p-value. If the p-value is less than 0.05, we consider that there is a significant difference between the To values and thus the performance of the KHIs at the 95% confidence level.27 The temperature, at which rapid hydrate formation first reaches its fastest rate (i.e., when the pressure drop first reaches its maximum value) is called the Ta value. In the experiment in Figure 5 this occurs at 8.1 °C. Generally, we find that there is less scattering in the Ta values ( 35 °C. This manifests itself in a critical micelle concentration which varies as a function of the block lengths and weight percent of one block. In solution we would speculate that the reason is related to the percentage of NIPAM monomer and the negligible effect of HEAA monomer. HEAA has a pendant hydroxyl group which is not helpful for kinetic hydrate inhibition as a number of hydroxylated polymers have all shown poor KHI performance.2 The homopolymer of the monomer DMA (PDMA) was found to be a poor KHI as the pendant hydrophobic group is small. Therefore, it is the NIPAM monomer in the copolymers with its larger hydrophobic isopropyl group that we assume is contributing the bulk of the KHI effect. This claim is reinforced by PNIPAM which gave the lowest average To value at 2500 ppm of all the polymers tested, and the average To values obtained by the copolymer 3:1 NIPAM:HEAA with low Mn, which gave lower

Table 3. Average Values of a Minimum of Eight Experiments of the Onset Temperature (To) and Fast Hydrate Growth (Ta) for All KHI Copolymer Tests polymer no additive PVCap PNIPAM PNIPAM PDMA PHEAA 1:1 NIPAM:HEAA 1:1 NIPAM:HEAA 3:1 NIPAM:HEAA NIPAM:DMA 1:1 1:1 NIPAM:DMA 1:1 NIPAM:HEAA

polymer type

Mn (g/mol)

concn (ppm)

av To (°C)

av Ta (°C)

− statistical statistical statistical statistical statistical statistical block statistical block statistical block statistical block statistical block statistical block

4092 5000 35000 5000 4500 5400 5500 29300 30000 3700 3700 4100 4200 4100 4200 5400 5500

− 2500 2500 2500 2500 2500 2500 2500 2500 2500 2500 2500 2500 2500 7000 7000 7000 7000

17.9 10.6 9.6 11.5 17.2 17.8 12.2 11.2 12.0 11.2 10.5 10.3 11.2 10.4 9.7 8.3 11.3 10.2

17.6 9.1 7.3 10.2 16.4 16.9 10.1 9.3 10.1 10.3 8.7 8.4 9.4 8.6 7.8 6.3 8.9 8.1

can be useful to compare the crystal growth inhibition, but this is only true if the To values are similar; otherwise, the driving force is not the same. The defining result from Table 3 is that each 1:1 block copolymer gave a lower average To (and Ta) value than the equivalent statistical 1:1 copolymer, at both concentrations and all molecular weights investigated. The average To values for the block copolymers were found to be 0.6−1.0 °C lower. Statistical t tests gave p-values of under 0.05 for each pair of copolymers compared, indicating a statistically significant result in each case. For the 3:1 NIPAM:HEAA statistical and block copolymers, the performances were more similar; the difference of 0.2 °C in the average To value is not

Figure 6. Effect of concentration on the average To values for the statistical and block copolymers 1:1 NIPAM:HEAA at low Mn and 1:1 NIPAM:DMA at low Mn. E

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and block copolymers, the performances were statistically undistinguishable. The 3:1 copolymers were also the best performing copolymers, probably reflecting the higher percentage of the more hydrophobic NIPAM co-monomer allowing for aggregation above the cloud point/LCST. The performance enhancement of raising the copolymer concentration was surprisingly low compared to other KHI polymer classes. We are currently attempting to expand our studies on block and statistical copolymers to those based on other (meth)acrylamides as well as on N-vinyl lactams.

values than the 1:1 copolymers. This copolymer contains a higher molar percentage of NIPAM monomer than the other copolymers. Thus, it appears when the composition of the block copolymer is more than 50 wt % NIPAM, the aggregation leads to improved KHI. We further speculate that it is necessary to have a long chain of NIPAM monomers as found in the block copolymers for good kinetic hydrate inhibition. This is not available in the copolymers where the polymerization rates of the monomers is fairly similar, leading to good mixing of the two monomers along the chain.29 A computer molecular simulation may help determine what is really going on in solution that might affect the KHI performance of the polymers. Since the cloud points of these copolymers are either very high or non-existent, we assume the hydrogenbonding inter- and intramolecularly, is fairly weak and that the polymers are initially well uncoiled and strongly hydrogenbonded to water molecules. The performance of the two sets of 1:1 NIPAM:HEAA copolymers of molecular weights (Mn) of about 30000 and 5500 g/mol did not show a significant difference in performance. Generally, KHI performance diminishes gradually as the average molecular weight increases above about 1500− 2000 g/mol. In the case of these NIPAM:HEAA copolymers we presume the polymers with Mn values of about 5500 g/mol have already lost some performance ability compared to lower molecular weights and are on par with the higher molecular weight copolymers. The KHI performance of 1:1 NIPAM:HEAA with low Mn copolymer was found to be only a little better at 7000 ppm than 2500 ppm for both block and statistical copolymers, with the average To values dropping by about 1 °C due to the increased concentration (Figure 6). We have previously seen that raising the active polymer concentration from 2500 to 7000 ppm usually gives a more substantial lowering of the To values, i.e., increase in KHI performance, so the result for the NIPAM:HEAA copolymer was a little surprising.30 For example, for the n-propylamine derivative of the homopolymer of 2-vinyl-4,4-dimethylazlactone (polyVDMA), under identical test conditions, the average To value drops from 10.1 to 8.2 °C (a difference of 1.9 °C) as the concentration is increased from 2500 to 7000 ppm. For poly(N-ethylglycine)-ran-poly(N-isobutylglycine) (PNEG17-rPNiBG5) the average To value dropped from 8.1 to 4.3 °C (a difference of 3.8 °C) as the concentration was increased from 2500 to 7000 ppm.31 For the 1:1 NIPAM:DMA with low Mn copolymer the drop in average To values was from 1.5 and 2.1 °C for the statistical and block copolymers respectively on increasing the test concentration from 2500 to 7000 ppm. This drop is more in line with previous studies on other KHI polymer classes, such as those cited in this paragraph.

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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.6b02785. Analyses of copolymers of acrylamides, plots of log MW vs dw/(d log M), and NMR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +47 51831823. Fax +47 51831750. E-mail: malcolm. [email protected]. ORCID

Malcolm A. Kelland: 0000-0003-2295-5804 David Haddleton: 0000-0002-4965-0827 Present Address §

King Abdulaziz City for Science and Technology (KACST), Riyadh 12371, Saudi Arabia. Notes

The authors declare no competing financial interest.

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REFERENCES

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4. CONCLUSION We have carried out a study to compare the KHI performance of statistical and block copolymers. A series of copolymers has been made from the monomer N-isopropylacrylamide (NIPAM) with either the monomer N,N-dimethylacrylamide (DMA) or N-hydroxyethylacrylamide (HEAA). The copolymers were made in identical procedures and have very similar molecular weights and PDI values. The copolymers were tested in high pressure rocking cells using a structure II-forming synthetic natural gas mixture using a slow constant cooling method. For all 1:1 copolymers, the block copolymers gave statistically significant lower average onset temperatures than the statistical copolymers. For the 3:1 NIPAM:HEAA statistical F

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DOI: 10.1021/acs.energyfuels.6b02785 Energy Fuels XXXX, XXX, XXX−XXX