Article pubs.acs.org/EF
Investigation of the Kinetic Hydrate Inhibitor Performance of a Series of Copolymers of N‑Vinyl Azacyclooctanone on Structure II Gas Hydrate Fernando T. Reyes and Malcolm A. Kelland* Department of Mathematics and Natural Science, Faculty of Science and Technology, University of Stavanger, N-4036 Stavanger, Norway ABSTRACT: A series of copolymers of N-vinylazacyclooctanone (VACO) with more hydrophilic monomers, including the 5−7 ring N-vinyl lactams, N-vinyl-N-methyl acetamide (VIMA), and N-vinyl acetamide (NVA), have been synthesized. Their performance as kinetic hydrate inhibitors (KHIs) was compared to poly(N-vinylazacyclooctanone) (PVACO) homopolymer in high pressure rocking cells using a Structure II hydrate-forming natural gas mixture. The best 1:1 copolymer between any two of the four N-vinyl lactams was found to be for VP/VACO copolymer. 1:1 VACO/VIMA copolymers were also shown to be a superior KHI to PVACO, whereas copolymers of other ratios of these two monomers were less effective KHIs. 2-Butoxyethanol (BGE) was also found to be a good solvent and synergist for 1:1 VACO/VIMA copolymers. Finally, 1:1 N-vinyl lactam/NVA copolymers were shown to have worse KHI performance than the equivalent 1:1 N-vinyl lactam/VIMA copolymers at similar molecular weights.
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INTRODUCTION The oil and gas industry has been using kinetic hydrate inhibitors (KHIs) in commercial operations for almost two decades to prevent gas hydrate plugging of flow lines.1−4 As the name suggests, KHIs prevent gas hydrate formation for a time period dependent on the chemical potential (driving force) in the system. The chemical potential is related to the subcooling (ΔT) in the system. The absolute pressure in the system can also impact the KHI performance.5−8 Currently, all commercial KHIs are water-soluble polymers with amide groups, often with synergists added to improve their performance. A well-known series of amide polymers used in commercial KHI packages are based around the 5-ring and 7-ring N-vinyl lactams, N-vinyl pyrrolidone (VP) and N-vinyl caprolactam (VCap), respectively.9−16 Recently, we synthesized for the first time the homopolymer poly(N-vinylazacyclooctanone) (PVACO), containing the 8-ring lactam.17 This polymer was found to be a superior SII kinetic hydrate inhibitor (KHI) compared to the smaller poly(N-vinyl lactams) with 5−7 rings at similar molecular weights. In fact, the KHI performance increases with an increasing lactam ring size. The structures of the homopolymers of VP, VPip, VCap, and VACO are given in Figure 1. The cloud point of PVACO homopolymer is very low, around 15−24 °C depending on the method of VACO polymerization. Although a low cloud point might possibly be beneficial for high KHI performance, this makes PVACO unsuitable for injection into warm well streams due to potential precipitation of the polymer at the well head. This would render the polymer inactive against hydrate formation as well as restricting flow in the pipeline. Therefore, we decided to investigate the KHI performance of copolymers of VACO with more hydrophilic monomers to raise the cloud point of the polymer. Many copolymers of VCap have been investigated previously, but only a few hydrophilic monomers are known to © 2013 American Chemical Society
Figure 1. Structures of poly(N-vinyl pyrrolidone) (PVP) (top left), poly(N-vinyl piperidone) (PVPip) (top right), poly(N-vinyl caprolactam) (PVCap) (bottom left), and poly(N-vinyl azacyclooctanone) (PVACO) (bottom right).
be useful for incorporation into VCap copolymers without significant loss of KHI performance relative to PVCap homopolymer. For example, VCap/VP copolymers still retain good KHI activity and are commercially available.1−4 VCap/Nvinyl-N-methyl acetamide (VCap/VIMA) copolymers have also been studied in detail and shown to be superior SII hydrate KHIs to PVCap when the copolymer ratio is approximately 1:1.18 VCap/VIMA based KHI products have been used in the field for several years, although they are currently not used due to the increased cost of the VIMA monomer.19−22 High Received: December 11, 2012 Revised: January 31, 2013 Published: February 4, 2013 1314
dx.doi.org/10.1021/ef302054a | Energy Fuels 2013, 27, 1314−1320
Energy & Fuels
Article
performance VCap/Vinyl alcohol (VCap/VOH) copolymer KHIs are also commercially available and can be made by selective hydrolysis of VCap/Vinyl acetate copolymers.23,24 This paper investigated the SII gas hydrate KHI performance of VACO copolymers with the 5−7 ring N-vinyl lactams, with VIMA and with the related monomer N-vinyl acetamide (NVA) (Figure 2). KHI test results of the three possible 1:1 copolymers using just the 5−7 ring N-vinyl lactams are also included in this study.
Table 1. Molecular Weights for Polymers Synthesized in This Study
Figure 2. Poly(N-vinyl-N-methyl acetamide) (PVIMA) (left) and poly(N-vinyl acetamide) (PNVA) (right).
Polymer Synthesis. Solvents, polymerization initiators, and the monomers VIMA, NVA, VP, and VCap were supplied by Sigma-Aldrich chemical company. The VACO monomer was generously made by BASF, Germany, by vinylation of azacyclooctanone with ethyne under high pressure. VPip was also supplied by BASF, as previously reported.25,26 All other chemicals were obtained from commercial sources. Copolymers of the N-vinyl lactams were synthesized using the following general procedure: The monomer was mixed with 1−20 wt % of the initiator 2,2-azobis(2-methylpropionitrile) (AIBN) and 4 times its weight of 2-propanol in a Schlenk tube. The resulting solution was degassed on a high vacuum line and sealed under nitrogen. The reaction mixture was then stirred and allowed to polymerize at 80 °C for typically 16 h. Solvents were removed under reduced pressure to leave a white solid. 1H and 13C nuclear magnetic resonance (NMR) spectroscopy indicated greater than 99% monomer conversion. One 1:1 VIMA/VACO sample was also made in 2-butoxyethanol (BGE) solvent, and the solvent was not removed. A summary of all polymers used in this study is given in Table 1. The number-average molecular weights and their distribution were measured by gel permeation chromatography (Tosoh System HLC-8120GPC at 40 °C or PL GPC 50 integrated GPC system at 50 °C) with PMMA standards using DMF as an eluent (Table 1). For PVACO the highest molecular weight sample we have been able to synthesize has Mn of 1700 Da and Mw 4700 Da. The cloud points (Tcl) of the polymers as 1.0 wt % solutions in fresh water and with increasing sodium chloride salinity were determined and are depicted in Figures 3−6.25 The data clearly shows that increasing the salt concentration leads to a lower cloud point. The cloud points for 1:1 VIMA and 1:1 NVA copolymers with the N-vinyl lactams show fairly similar trends. This is to be expected, as the difference between NVA and VIMA is only an extra methyl group in VIMA. High Pressure Gas Hydrate Rocker Rig Equipment Test Methods. Kinetic hydrate inhibition experiments were conducted in five high pressure 40 mL steel rocking cells, each containing a steel ball (Figure 7). The equipment was supplied by PSL Systemtechnik, Germany, and has been described previously.29 The gas composition used was a synthetic natural gas mixture given in Table 2.
polymer
Mn (Da)
PDI
PVACO VACO/VP 1:1 VACO/VPip 1:1 VACO/VCap 1:1 VP/VIMA 1:1 VPip/VIMA 1:1 VCap/VIMA 1:1 VACO/VIMA 1:1 VACO/VIMA 1:1; low Mw in BGEa VACO/VIMA 2:1 VACO/VIMA 1:2 VACO/VIMA 1:4 VACO/VIMA 1:9 PVIMA VP/NVA 1:1 VPip/NVA 1:1 VCap/NVA 1:1 VACO/NVA 1:1 PNVA
1700 8524 7221 5480 8978 6081 7645 6141
