Kinetic Hydrate Inhibition Performance of Poly(vinyl caprolactam

Aug 17, 2017 - Kinetic Hydrate Inhibition Performance of Poly(vinyl caprolactam) Modified with Corrosion Inhibitor Groups. Juwoon Park†, Hyunho Kimâ...
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Kinetic Hydrate Inhibition Performance of Poly(vinyl caprolactam) Modified with Corrosion Inhibitor Groups Juwoon Park,† Hyunho Kim,† Qi Sheng,‡,§ Colin D. Wood,*,‡ and Yutaek Seo*,† †

Department of Naval Architecture and Ocean Engineering, Research Institute of Marine Systems Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul, 08826, Republic of Korea ‡ CSIRO Australian Resources Research Centre, Kensington, WA 6152, Australia § Curtin University of Technology, Kensington, WA 6152, Australia ABSTRACT: Multifunctional polymers were designed and synthesized that contain both a kinetic hydrate inhibitor and corrosion inhibitor groups. This was achieved by modifying a copolymer of vinyl caprolactam and acrylic acid (PVCap-co-AA) where the acid groups were converted to known corrosion inhibitor groups including imidazole (APIM) and quaternary ammonium (ATCH) moieties. Therefore, all of the resulting polymers had the same molecular weight, end groups, and overall composition which allowed for accurate determination of the performance of these inhibitors. Their performance as kinetic hydrate inhibitors was evaluated by determining the hydrate onset time, growth rate, and resistance to flow using a high pressure autoclave. The experimental results show that both PVCap-co-APIM and PVCap-co-ATCH were able to delay hydrate nucleation; however, PVCap-co-APIM was better than PVCap-co-ATCH. The performance of new KHICIs was evaluated and compared with that of a commercial KHI, Luvicap, with three different cooling rates: for high and medium cooling rates, Luvicap delayed hydrate nucleation for a longer time compared to the new KHICIs. At low cooling rate, one of the KHICIs (PVCap-coAPIM) showed better performance than Luvicap. PVCap-co-APIM also performed better than Luvicap and PVCap-co-ATCH in decreasing the hydrate growth rate. Hydrate growth rate and resistance to flow were also studied during the hydrate formation to investigate the effect of the inhibitors on the growth of hydrate particles in the liquid phase. PVCap-co-APIM successfully suppressed hydrate growth in the early stage of hydrate formation, whereas PVCap-co-ATCH resulted in fast growth of the hydrate phase. For all of the studied cooling rates, PVCap-co-APIM showed stable resistance to flow even with increasing hydrate fraction in the liquid phase; however, PVCap-co-ATCH showed a torque surge in the early stage of hydrate formation when a fast cooling rate was used, which suggests that the ATCH group has a negative effect on the hydrate inhibition performance. These results provide a proof-of-concept that the modified PVCap-co-APIM, or related structures, that contain both kinetic hydrate inhibitor and corrosion inhibitor groups can be used as multifunctional inhibitors for offshore oil and gas fields.

1. INTRODUCTION Gas hydrates are crystalline compounds consisting of host water molecules encaging gas molecules via hydrogen bonding. They are formed under high pressure and low temperature conditions. Light hydrocarbon molecules such as methane, ethane, and propane are able to form gas hydrates.1 Recently, significant amounts of methane have been identified in marine sediments and permafrost regions where gas hydrates can exist.2 Although the methane hydrate deposits are considered a future energy resource, they have been a risk for the energy industry when transporting hydrocarbon fluids through subsea flowlines operating at high pressure and low temperatures for deep and remote offshore fields.1,3,4 Since Hammerschmidt5 suggested the occurrence of gas hydrates in gas transport pipelines with an equation for injecting methanol, the risk of hydrate formation in subsea flowlines was prevented by adding thermodynamic hydrate inhibitors (THIs) such as methanol and monoethylene glycol (MEG). These thermodynamic hydrate inhibitors shift the hydrate equilibrium conditions toward lower temperatures and higher pressures, thus avoiding hydrate formation under flowline conditions.1,6,7 However, the injection of THIs demands large infrastructure such as storage tanks, a complex regeneration process, injection pumps, and subsea distribution pipelines. Moreover, the presence of © XXXX American Chemical Society

electrolyte ions in the formation water may induce operational issues such as scale deposition and MEG loss.8−10 As such, the oil and gas industry is seeking alternative solutions that are more efficient and offer economic benefits. One of the possible solutions is the development of multifunctional inhibitors that avoids the need for injecting multiple chemicals and can overcome the incompatibility issues that are often encountered. Kinetic hydrate inhibitors (KHIs) have been widely studied as an alternative way for delaying the hydrate formation during the residence time of hydrocarbon fluids inside subsea flowlines. They do not prevent hydrate formation, as with thermodynamic hydrate inhibitors but instead of completely avoiding its formation. They are water-soluble polymers consisting of a hydrophobic backbone with pendant groups, which can interfere with the nucleation site of hydrate crystals.11,12 There have been extensive studies to develop new KHIs which have typically been benchmarked against the early discovered homo- and copolymers of N-vinylcaprolactam (VCap), N-vinylpyrrolidone (VP),13−15 and N-isopropylacrylamide (NIPAM).16−20 Employing KHIs in the field lowers the Received: July 7, 2017 Revised: August 15, 2017 Published: August 17, 2017 A

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polyethylene backbone is linked to the pendant groups of one branch, and the pendant groups can enter the hydrate cage when the polymer structure absorbs on the hydrate crystal. The sodium carboxylate groups in the base polymer were modified with ATCH and APIM, which are among the main classes of corrosion inhibitors as they carry imidazole and quaternary ammonium groups. The hydrophobic groups of the backbone combined with the corrosion inhibitor groups helps to form a protective barrier once the polymer structure has attached onto the metal surface; this was discussed in detail in our previous publication20 where the corrosion data correlate with the proposed mechanism. For example, the inclusion of known corrosion inhibitor groups reduces corrosion because these groups are known to interact strongly with metal surfaces. The hydrophobic backbone will not interact with the metal surface, so when combined with the corrosion inhibitor groups, it will project away from the surface. The VCap provides hydrate inhibition, and the acrylic acid was used to couple on corrosion inhibitor groups including ATCH (quaternary ammonium), APIM (imidazole), taurine (sulfonic acid), and AMPA (phosphonic acid). Corrosion weight loss measurements were carried out in our previous work,25 and the results suggested that the APIM and ATCH modified polymers performed better than those modified with other functional groups. It is noted that the base polymer, PVCap-co-AA, showed the highest weight loss due to lack of corrosion inhibition group and a commercial PVCap was also low performing. By modification of the structure with APIM, PVCap-co-APIM the inhibitors can simultaneously prevent the nucleation of hydrates while inhibiting corrosion. PVCap-co-ATCH was also effective at retarding hydrate nucleation while inhibiting corrosion. Therefore, the newly designed and synthesized polymers can be termed as KHCIs, and among the evaluated groups, ATCH and APIM offered the best performance in terms of hydrate inhibition and corrosion. These results indicated that single polymer molecules incorporating both hydrate and corrosion inhibition groups are able to perform without compatibility issues if the polymer structure contains appropriate functional groups. However, the hydrate inhibition performance of the newly designed KHCIs was tested using a high-throughput (HTP) hydrate screening method at ambient pressure, where the polymer solutions are not mixed. While it is a reasonable assay for newly synthesized KHIs, it is not designed as a replacement for detailed autoclave tests at elevated pressures. This work expands the performance testing of the base polymer and modified versions by determining the subcooling conditions, growth rates, hydrate fraction, and torque changes for the promising KHCIs from our previous work.25 All of these parameters were measured while varying cooling rate of hydrocarbon fluid and polymer solution mixture. This has allowed for a more comprehensive description of the performance of KHCIs along with possible strategies for incorporating into an existing hydrate risk management strategy.

risk of hydrate formation with relatively simple infrastructure due to its low dosage rate, which reduces the operational cost and the footprint in offshore platforms. To date, the performance of KHIs has been evaluated by measuring the subcooling temperature, which represents the temperature difference between the theoretical hydrate equilibrium temperature and the measured hydrate formation temperature at the corresponding pressure. Previous research has focused on extending the subcooling temperature while improving biodegradability and compatibility with other chemicals such as corrosion inhibitors. Recent studies suggest that KHIs disrupt the nucleation of hydrate cages during the growth stage which results in a slow growth period, followed by catastrophic growth of hydrate crystals. The number of KHIs that have been explored is increasing; however, testing the performance of these inhibitors has been limited in terms of measuring their subcooling temperature along with the catastrophic growth temperature using a single cooling rate. Varying the cooling rate of hydrocarbon fluids may affect the performance of KHIs as it changes the driving force for hydrate nucleation and growth. Our previous work presented the advanced test methodology to evaluate the performance of newly developed KHIs based on PNIPAM-co-AA by measuring the hydrate onset time and resistance-to-flow at various cooling rates.20 The primary objective of this study was to evaluate newly designed KHIs that carry corrosion groups using an advanced testing methodology, where the KHIs were designed to inhibit hydrate formation as well as corrosion of steel pipelines. Corrosion inhibitors (CIs) are injected into subsea flowlines along with hydrate inhibitors to protect the surface of the steel pipeline from deterioration due to corrosion. They tend to act through a process of surface adsorption which creates a protective film on the metal surface. Corrosion is an issue due to the hydrocarbon fluids in subsea flowlines containing carbon dioxide or hydrogen sulfide. Therefore, in subsea flowlines, there is a risk of hydrate formation and corrosion which requires both hydrate and corrosion inhibitors to be injected. Recent reports from industry have demonstrated that CIs adversely affect the performance of KHIs, although the effectiveness of the CIs can be maintained.21−23 One theory to explain the incompatibility was the absorption of CI onto the polymer structure of KHI. Moore et al.24 investigated this theory through LC/MS measurement and suggested that the structure of PVCap was altered in the presence of EOphosphate ester, which resulted in a lowering of the performance of the PVCap with a 25% pass rate (compatible CI showed 100% pass rate). Other commercial CIs, such as aminoethyl fatty imidazoline, were partially associated with the PVCap structure and resulted in a lowering of the PVCap performance with 50% pass rate. These results suggested that incompatibility between KHI and CI may increase the risk of hydrate formation in subsea flowlines; thus, the design of new KHIs to incorporate corrosion inhibition performance must account for the negative interactions between the KHIs and CIs. In our previous work,25 single polymer molecules were developed to simultaneously prevent hydrate formation and inhibit corrosion, which was termed as kinetic hydrate and corrosion inhibitors (KHCIs). A systematic library of multifunctional polymers were generated by modifying a base polymer, which was a copolymer of vinylcaprolactam and acrylic acid (PVCap-co-AA). The base polymer, PVCap-co-AA, was modified to carry both kinetic hydrate inhibitor groups and corrosion inhibitor groups by postsynthetic modification. The

2. MATERIALS AND METHOD 2.1. Materials and Synthesis of PVCap-Based Polymers. The distilled water and decane (purity: 99%) were purchased from OCI (Korea) and Sigma-Aldrich, respectively. The water and decane were used without further purification. The synthetic natural gas (CH4: 90 mol %, C2H6: 6 mol %, C3H8: 3 mol %, n-C4H10: 1 mol %) was supplied by Special Gas (Korea). Luvicap EG HM was supplied by BASF. PVCap-based KHIs were synthesized from the following B

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Figure 1. Structures of PVCap-based KHIs: (from left) PVCap-co-AA, PVCap-co-ATCH, and PVCap-co-APIM. materials from Sigma-Aldrich: N-vinylcaprolactam (VCap; 98%); acrylic acid (AA, contains 200 ppm, MEHQ as inhibitor, 99%) was used after being filtered through a neutral activated alumina column; (2-aminoethyl)trimethylammonium chloride hydrochloride (ATCH, 99%); 1-(3-aminopropyl)imidazole (APIM, ≥97%). The detailed procedure for the synthesis of PVCap-based KHIs was described in our previous work.25 The polymer was synthesized by free radical copolymerization of VCap with AA in the presence of AIBN as an inhibitor, which was used then as a base polymer onto which corrosion inhibition groups are coupled using a chemical modification via the AA group. Among the synthesized KHCIs, PVCap-co-APIM (20) and PVCap-co-ATCH (20) were selected to further evaluate via the advanced inhibitor test methodology as they showed low threshold concentration from HTP screening as well as low weight loss from the corrosion coupon test. Figure 1 presents the structures of PVCap based KHIs synthesized in this work. 2.2. Hydrate Inhibition Performance Evaluation. All experiments were conducted using a high pressure autoclave made of 316 stainless steel equipped with magnetic stirrer coupling and anchor type impeller. The temperature and pressure were measured by a PT 100Ω (±0.15 °C) and transducer (0−200 bar, ± 0.1 bar) and were logged using a data acquisition system. For comparison, pure water and Luvicap solution were selected as control groups. The concentration of PVCap derivatives (PVCap-co-AA, PVCap-co-ATCH (20), and PVCap-co-APIM (20)) was 0.5 wt % based on the aqueous phase for all experiments. A 80 mL aliquot of liquid was loaded into the autoclave having a 360 mL inner volume. The volume fraction of water against decane was 60%. The autoclave was immersed in a water bath connected to an external circulator (Jeiotech, 2025G), and pressurized to 120 bar with synthetic natural gas at 24 °C while stirring at 600 rpm for gas saturation. At this stirring speed, the fluid is in a turbulent regime. Once the liquid phase was saturated with gas and the pressure stabilized, the temperature was decreased from 24 to 4 °C while varying the cooling rates (0.25, 0.033, and 0.017 K/min). After achieving the target temperature of 4 °C, the temperature was maintained for 10 h to observe the hydrate formation process in the liquid phase. After this time, the temperature was increased to 28 °C to completely dissociate the hydrate phase before running the next cycle. Five experiments were repeated for each PVCap-based KHIs (total 15) in order to obtain an average subcooling temperature and hydrate volume fraction. The other 10 experiments for the two control systems, pure water + decane mixture and 0.5 wt % Luvicap solution + decane mixture, were carried out for a direct comparison with the 15 experiments for the PVCap-based KHI systems (0.5 wt % PVCap-coAA solution + decane mixture, 0.5 wt % PVCap-co-ATCH (20) solution + decane mixture, and 0.5 wt % PVCap-co-APIM (20) solution + decane mixture). Performance of PVCap-based KHIs was evaluated by measuring the subcooling temperature. Three different cooling rates were applied to simulate the different insulation status of subsea flowlines. Upon hydrate onset, the increase of hydrate fraction was monitored in the liquid phase along with the torque changes during mixing. The hydrate fraction was estimated from the pressure difference between the experimental pressure and the postulated pressure with no hydrate formation. If the KHI molecules interact with the growing hydrate crystals, the hydrate fraction would increase slowly and the increase in torque due to hydrate agglomeration/deposition might be delayed.

Thus, the role of PVCap-based KHIs during the hydrate growth stage was also investigated to evaluate their performance.

3. RESULTS AND DISCUSSION 3.1. Kinetic Inhibition Performance of PVCap-Based KHIs. The mean value and standard deviation over three repeat trials for the subcooling temperature (ΔTsub) and the hydrate onset time (tonset) of PVCap-based KHIs are presented in Table 1. Hydrocarbon fluids traveling through subsea flowlines will Table 1. Subcooling Temperature and Onset Time from Hydrate Formation Experiments with 0.5 wt % PVCapBased KHIs Solutiona PVCap-based KHIs PVCap-co-AA

PVCap-co-ATCH

PVCap-co-APIM

pure water

Luvicap

cooling rate (°C/min)

ΔTsub (K)

tonset (min)

0.017 0.033 0.25 0.017 0.033 0.25 0.017 0.033 0.25 0.017 0.033 0.25 0.017 0.033 0.25

8.5 (0.1) 8.4 (0.2) 9.5 (1.1) 7.7 (0.3) 7.6 (0.2) 8.4 (0.6) 8.3 (1.1) 9.0 (0.3) 9.7 (0.1) 3.3 (0.1) 3.3 (0.1) 4.7 (0.6) 7.9 (0.9) 11.4 (0.9) 11.6 (0.2)

540.2 (5.6) 284.5 (2.0) 52.1 (7.4) 473.5 (13.7) 246.0 (1.9) 41.9 (3.8) 517.4 (69.3) 290.5 (5.9) 42.5 (0.8) 200.8 (4.7) 104.4 (3.9) 20.4 (2.1) 484.8 (56.2) 353.2 (26.3) 83.8 (5.2)

a

The standard deviation of the subcooling temperature and onset time is shown in parentheses. Data for pure water and Luvicap solution were from our previous work.20

experience cooling due to heat exchange with the surrounding seawater through the flowline wall. This can vary depending on whether insulation materials are used, which would change the cooling rate of the fluid. The cooling rate was varied from 0.017 to 0.25 °C/min, thus from slow to fast cooling of the hydrocarbon fluids. Since the subcooling temperature presents the thermal driving force for the nucleation of hydrate crystals, a fast cooling rate results in a high driving force over a short period. For pure water, ΔTsub was 3.3 °C, while tonset was 200.8 min at a slow cooling rate of 0.017 °C/min. With increasing cooling rate (0.033 °C/min), the ΔTsub was maintained 3.3 °C and tonset was decreased to 104.4 min. For the highest cooling rate of 0.25 °C/min, ΔTsub was 4.7 °C and tonset was 20.4 min. These results suggested that the subcooling temperature was less affected by changing the cooling rate, whereas the hydrate onset time was highly affected. Figure 2 shows the comparison of the subcooling temperature between the PVCap-based KHIs and the control groups C

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Figure 2. Subcooling temperature of PVCap-co-AA, PVCap-co-ATCH, and PVCap-co-APIM. Pure water and Luvicap solution were studied to provide comparison. Error bars indicate the standard deviation.

Figure 3. Hydrate onset time as a function of the subcooling temperature for PVCap-based KHIs with varying cooling rates: 0.25, 0.033, and 0.017 K/min. Solid lines indicate linear regression of the measurements.

Figure 3 shows the tonset as a function of ΔTsub with varying cooling rates. For a fast cooling rate of 0.25 K/min, Luvicap showed the best performance with a difference in subcooling of 1.9 °C. PVCap-APIM and PVCap-co-AA were close, while PVCap-co-ATCH showed the lowest performance (Luvicap > PVCap-co-APIM ≈ PVCap-co-AA > PVCap-co-ATCH). When the cooling rate was reduced to 0.033 °C/min, Luvicap again showed the best performance when compared to the base PVCap-co-AA and modified polymers. PVCap-co-APIM was better than others, but the subcooling difference between PVCap-co-APIM and Luvicap was 2.4 °C, which was calculated to 72.7 min from the cooling rate. PVCap-co-ATCH showed the lowest subcooling temperature (Luvicap > PVCap-co-APIM > PVCap-co-AA > PVCap-co-ATCH). For a slow cooling rate of 0.017 °C/min, PVCap-co-AA showed the best performance,

including pure water and a Luvicap solution. The 0.5 wt % PVCap-co-AA showed ΔTsub of 9.8, 8.4, and 8.5 °C with the cooling rates of 0.25, 0.033, and 0.017 °C/min, respectively, indicating the synthesized PVCap-co-AA performed well as a kinetic hydrate inhibitor. The commercial KHI, Luvicap, showed better performance than PVCap-co-AA at cooling rates of 0.25 and 0.033 °C/min; however, it showed similar performance at 0.017 °C/min. When the base polymer was modified with corrosion groups, PVCap-co-APIM, a slightly enhanced performance was observed as seen from the increased ΔTsub to 9.7, 9.0, and 8.3 °C with the cooling rates of 0.25, 0.033, and 0.017 °C/min, respectively. However, for PVCap-coATCH, ΔTsub was decreased slightly compared to the base polymer at every cooling rate studied in this work. D

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Energy & Fuels Table 2. Hydrate Formation Characteristics in 0.5 wt % PVCap-Based KHI Solutions while Varying Cooling Rate cooling rate (°C/min)

system

rini (min−1)

rlate (min−1)

τmax (N cm)

Φtrans

Φfinal

xconv (%)

0.017

PVCap-co-AA PVCap-co-ATCH PVCap-co-APIM pure water Luvicap PVCap-co-AA PVCap-co-ATCH PVCap-co-APIM pure water Luvicap PVCap-co-AA PVCap-co-ATCH PVCap-co-APIM pure water Luvicap

0.0034 0.0071 0.0017 0.0063 0.0003 0.0014 0.0101 0.0025 0.0164 0.0037 0.0020 0.0121 0.0077 0.0103 0.0051

0.0001 0.0004 0.0004 0.0003 0.0005 0.0004 0.0003 0.0004 0.0004 0.0005 0.0004 0.0010 0.0006 0.0003 0.0007

8.4 9.1 8.1 9.2 11.7 7.9 8.9 8.4 10.3 9.9 7.8 17.8 7.4 13.9 9.3

0.04 0.12 0.13 0.42 0.23 0.03 0.13 0.11 0.07 0.12 0.03 0.07 0.15 0.25 0.09

0.27 0.38 0.31 0.45 0.30 0.22 0.34 0.33 0.49 0.36 0.27 0.42 0.43 0.43 0.45

38.94 54.93 44.62 66.53 43.65 31.37 49.93 47.79 72.50 52.39 38.25 62.56 62.99 64.04 66.87

0.033

0.25

delay time until the formation of a hydrate plug.14−20 In order to study the role of PVCap-based KHIs during the hydrate growth stage, the growth rate and resistance-to-flow were investigated by estimating the hydrate fraction from the change in pressure and torque. Table 2 presents the initial growth rate (rini), late growth rate (rlate), maximum torque (τmax), hydrate fraction at maximum torque (Φtrans), hydrate fraction at 600 min (Φfinal), and water conversion to hydrate at 600 min (xconv). As seen in Figures 4−6, the hydrate fraction increased rapidly in the early stage of hydrate formation, but soon reached an inflection point from which the hydrate fraction increases slowly. We presented the hydrate growth rate in the early stage of hydrate formation as the initial growth rate; then the late growth rate indicated the hydrate growth rate after reaching the inflection point. The change in torque with respect to hydrate fraction was used to locate the maximum torque and the corresponding hydrate fraction. Recent studies suggest that torque changes indicate resistance to the hydrocarbon flow due to the agglomeration and bedding of hydrate particles.26,27 In this work, decane was mixed with water in the liquid phase at 600 rpm, where decane droplets were dispersed homogeneously. As the volume of water in the liquid phase was 60%, the continuous phase was water in this work. The decane droplets would be dispersed in water phase while bulk hydrate particles were formed and grew; this might form a liquid bridge which accelerates the agglomeration of hydrate particles. The PVCap-based KHIs would be absorbed to the surface of growing hydrate particles as they are dissolved in the water phase. Their ability to inhibit hydrate growth would result in a low hydrate fraction as well as dispersed hydrate particles without causing high resistance to flow. Figure 4 shows the increase of hydrate volume fraction as a function of time for 300 min after the hydrate onset for each system when the cooling rate was 0.017 °C/min. For pure water, hydrate formation can be observed when the subcooling temperature was 3.3 °C and the initial growth rate was 0.0063 min−1. As seen in Figure 4a, the hydrate growth curve showed an inflection point at approximately 100 min when the hydrate fraction was close to 0.4. Then, the hydrate fraction kept increasing to 0.42, where the torque started to rise and showed a maximum of 9.2 N cm. After the inflection point, the late growth rate was 0.0003 min−1 and the final hydrate fraction was 0.45, suggesting that the growth rate was very slow after the

closely followed by PVCap-co-APIM. The performance of PVCap-co-ATCH and Luvicap was less than the above (PVCap-co-AA > PVCap-co-APIM > Luvicap ≈ PVCap-coATCH). The difference between the highest performing (PVCap-co-AA) and the lowest (PVCap-co-ATCH) KHIs was only 0.8 °C. Overall, the synthesized polymers performed less than the commercial polymer under fast and median cooling rates; however, two of them showed slightly better performance under slow cooling. The commercial KHI polymer does not carry any corrosion groups, so it is unlikely to act as a corrosion inhibitor.25 In our previous work,25 a high-throughput hydrate test showed the low threshold concentration, 0.5 wt % for the base polymer and 0.1 wt % for the modified polymer with ATCH and APIM. This work expands the test for kinetic hydrate inhibition by measuring hydrate onset time and subcooling temperature under mixing, at elevated pressure with natural gas, and while varying the cooling rate. As discussed in Figure 3, PVCap-co-APIM showed better inhibition performance than the base polymer and PVCap-co-ATCH at fast and slow cooling. However, for the slow cooling rate, the base polymer, PVCap-co-AA, performed better than both modified polymers. PVCap-co-ATCH showed the lowest performance among the tested KHIs. These results suggest that the performance of the PVCap-based KHIs obtained from subcooling temperatures and onset times can vary from that of the high-throughput hydrate test. Previous work by Dirdal et al. also found the results of testing a range of KHIs using cyclopentane at atmospheric pressure correlated fairly well with tests carried out by trichlorofluoromethane in high pressure stirred autoclaves, but there were some notable exceptions.29 KHI performance tests using cyclopentane at atmospheric pressure can be carried out in relatively short duration and for many KHI samples. However, for the qualified KHI candidates, comprehensive investigation must be carried out using the reliable experimental apparatus and conditions that can simulate the production of hydrocarbon fluids in subsea flowlines. 3.2. Hydrate Growth Characteristics in the Presence of KHIs and KHCIs. Measurement of the subcooling temperatures clearly indicated that the modified polymers structure can display both kinetic hydrate inhibition and corrosion inhibition performance during the nucleation stage of hydrate crystals. Recent studies suggest that the KHIs also affect the growth of hydrate crystals and provide an additional E

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Figure 4. Hydrate growth and corresponding torque changes for PVCap-based KHIs at a cooling rate of 0.017 °C/min. (a) Hydrate volume fraction in liquid phase as a function of time. (b) Torque changes for pure water and Luvicap solution. (c) Torque changes for PVCap-based KHIs.

hydrate particles and inhibited the agglomeration of hydrate particles. Addition of the ATCH group to the base polymer, PVCap-co-ATCH, showed the initial growth rate of 0.0071 min−1 and maximum torque of 9.1 at a hydrate fraction of 0.12. The late growth rate was 0.0004 min−1, and the final hydrate fraction was 0.38. For PVCap-co-APIM, the initial growth rate was 0.0017 min−1 and the maximum torque was 8.1 N cm at a hydrate fraction of 0.13. The late growth rate was 0.0004 min−1, and the final hydrate fraction was 0.31. These results suggest that both ATCH and APIM groups showed good compatibility with the VCap group in preventing high resistance to flow when the cooling rate was 0.017 °C/min. However, the ATCH showed higher initial growth rate and hydrate fraction than that of APIM, indicating better inhibition performance of APIM.

inflection point. For Luvicap solution, the initial growth rate was 0.0003 min−1 and the late growth rate was 0.0005 min−1. The torque quickly increased with increasing hydrate fraction unlike pure water; then it showed severe fluctuation when the hydrate fraction was larger than 0.20. The maximum torque was 11.7 N cm at a hydrate fraction of 0.23. The final hydrate fraction was 0.30, which is significantly lower than that of pure water; however, the torque changes, i.e., resistance to flow, suggests a high risk of hydrate blockage formation for Luvicap solution, although the hydrate growth rate was low. The base polymer (PVCap-co-AA) showed an initial growth rate of 0.0034 min−1, but in 50 min, the growth rate became 0.0001 min−1. The maximum torque was 8.4 N cm at a hydrate fraction of 0.04; however, the baseline torque was at around 7 N cm. The base polymer successfully retarded the growth of F

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Figure 5. Hydrate growth and corresponding torque changes for PVCap-based KHIs at a cooling rate of 0.033 °C/min. (a) Hydrate volume fraction in liquid phase as a function of time. (b) Torque changes for pure water and Luvicap solution. (c) Torque changes for PVCap-based KHIs.

ATCH, and PVCap-co-APIM, respectively. As the baseline torque value was about 7 N cm, only a slight increase was observed during hydrate growth. For PVCap-co-ATCH, the initial growth rate was 0.0101 min−1, which was close to that of pure water. Due to early occurrence of the inflection point, a lower hydrate fraction was obtained in the presence of PVCapco-ATCH than in pure water. However, its growth rate was faster than that of PVCap-co-AA and PVCap-co-APIM, thus there may be a risk of hydrate blockage formation if the functional group for hydrate inhibition became ineffective. Figure 6 shows hydrate growth curves and torque changes for PVCap-co-AA based KHIs at a cooling rate of 0.25 °C/min. Similar results were obtained in that the initial growth rate was faster for PVCap-co-ATCH, 0.0121 min−1, than the growth rate of PVCap-co-APIM, which was 0.0077 min−1. PVCap-co-AA showed the lowest initial growth rate, 0.0020 min−1. The late growth rates after the inflection points were about 0.0004

Figure 5 presents the hydrate growth curves as a function of time and the torque changes with hydrate fraction for each system at a cooling rate of 0.033 °C/min. The initial growth rate increased for most systems due to increased subcooling temperature. For pure water, the initial growth rate was 0.0164 min−1, which was a 2.6 times higher rate than that of 0.017 °C/ min. Interestingly, the maximum torque was 10.3 N cm in the early stages of hydrate formation when the hydrate fraction was 0.07. The torque fluctuated and involved a stepwise increase of hydrate phase as seen in Figure 4a until the inflection point was observed at around 90 min and hydrate fraction of 0.35. Since the inflection point, the late growth rate decreased to 0.0004 min−1 and the torque fluctuated slightly. As seen in Figure 5c, addition of PVCap-co-AA based KHIs showed stable torque along with increase of hydrate fraction. The maximum torque was 7.9, 8.9, and 8.4 N cm at hydrate fractions of 0.03, 0.13, and 0.11 for PVCap-co-AA, PVCap-coG

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Figure 6. Hydrate growth and corresponding torque changes for PVCap-based KHIs at a cooling rate of 0.25 °C/min. (a) Hydrate volume fraction in liquid phase as a function of time. (b) Torque changes for pure water and Luvicap solution. (c) Torque changes for PVCap-based KHIs.

Table 3. Performance of PVCap-Based KHI Solutions while Varying Cooling Rate variables Initial growth rate

max. torque

hydrate fraction at transition

Hydrate fraction at 600 min

cooling rate (°C/min)

ranking

0.017 0.033 0.25 0.017 0.033 0.25 0.017 0.033 0.25 0.017 0.033 0.25

PVCap-co-ATCH ≫ PVCap-co-AA > PVCap-APIM > Luvicap PVCap-co-ATCH ≫ Luvicap > PVCap-APIM > PVCap-co-AA PVCap-co-ATCH ≫ PVCap-co-APIM > Luvicap > PVCap-co-AA Luvicap > PVCap-co-ATCH > PVCap-co-AA ≈ PVCap-co-APIM Luvicap > PVCap-co-ATCH > PVCap-co-APIM > PVCap-co-AA PVCap-co-ATCH > Luvicap > PVCap-co-AA > PVCap-co-APIM Luvicap > PVCap-co-APIM ≈ PVCap-co-ATCH ≫ PVCap-co-AA PVCap-co-ATCH ≈ Luvicap ≈ PVCap-co-APIM > PVCap-co-AA PVCap-co-APIM > Luvicap > PVCap-co-ATCH > PVCap-co-AA PVCap-co-ATCH > PVCap-co-APIM ≈ Luvicap > PVCap-co-AA Luvicap > PVCap-co-ATCH ≈ PVCap-co-APIM > PVCap-co-AA Luvicap ≈ PVCap-co-APIM ≈ PVCap-co-ATCH ≫ PVCap-co-AA

H

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Figure 7. Hydrate fraction increase and torque changes as a function of time for PVCap-co-APIM solution in early stage of hydrate formation.

min−1 for PVCap-co-AA, 0.0010 min−1 for PVCap-co-ATCH, and 0.0006 min−1 for PVCap-co-APIM. The initial growth rate for pure water was 0.0103 min−1, and the late growth rate was 0.0003 min−1, while the addition of Luvicap reduced the initial growth rate to 0.0051 min−1, and the late growth rate of 0.0007 min−1. PVCap-co-ATCH showed almost no hydrate inhibition performance from the hydrate growth rate and the hydrate fraction. For pure water, the maximum torque was 13.9 N cm at a hydrate fraction of 0.25, whereas PVCap-co-ATCH solution showed a maximum torque of 17.8 N cm at a hydrate fraction of 0.07. PVCap-co-APIM showed similar hydrate growth rate and hydrate fraction to those of Luvicap, and the associated torque changes indicated a minimum resistance to flow compared to other KHIs. It clearly showed the hydrate growth became fast and more hydrate was formed in the presence of the ATCH group, suggesting the incompatibility of the ATCH group against the VCap group for hydrate inhibition. Table 3 summarizes the performance of KHI polymers on hydrate growth at each cooling rate. Hydrate blockage formation would occur when the hydrate particles agglomerate and segregate from the continuous phase to deposit on the wall. Without hydrate inhibitors, increasing cooling rate resulted in a rapid increase in the hydrate fraction, which will increase the possibility of agglomeration and bedding of hydrate particles. In Figures 4−6, increasing hydrate fraction involves a steady increase or fluctuation of torque, i.e., high resistance to flow. Therefore, the role of the KHI during the hydrate formation process would be to suppress the hydrate growth rate, which results in a low hydrate fraction in the liquid phase for extended periods. In this work, the performance of PVCap-co-AA, the base polymer before modification, could reduce the hydrate growth to maintain stable torque with increasing hydrate fraction. By modifying the AA group with APIM, PVCap-co-APIM showed a slight increase of hydrate growth rate compared to PVCap-co-AA, but stable torque was observed for all cooling rates. The modification with ATCH, PVCap-co-ATCH, resulted in rapid hydrate growth compared to other PVCap-based KHIs, and a surge in torque was observed in the early stages of hydrate formation. Figure 7 shows the torque surge in PVCap-co-ATCH solution. After the

hydrate onset, the hydrate fraction reached only 0.03 when torque started to rise; then a maximum torque of 17.8 N cm was observed when the hydrate fraction reached 0.09. Recent studies suggest that the resistance to flow resulted from agglomeration and bedding of hydrate particles which became evident with the hydrate fraction in the liquid phase was more than 0.10.27,28 However, the modification to PVCap-co-ATCH resulted in high resistance to flow in the early stage of hydrate formation, which would increase the risk of hydrate blockage when hydrate formation begins. Unlike PVCap-co-APIM, PVCap-co-ATCH was only effective to delay the hydrate onset and was not able to prevent the hydrate plug formation as seen from fast hydrate growth and high resistance to flow. Our previous work suggested that the modification of PVCap-co-AA with APIM or ATCH groups showed effective corrosion inhibition performance. From the high pressure autoclave tests in this work, the PVCap-co-APIM showed the best combination of lowering hydrate nucleation while suppressing the growth rate while maintaining the torque during hydrate formation process. Its modification with ATCH showed adverse effects on hydrate inhibition performance, suggesting the incompatibility between this hydrate inhibitor group and this corrosion group. More investigation will be carried out to test the performance of PVCap-co-APIM under simulated field conditions. However, the obtained results in this work clearly suggest that it is possible to design and synthesize the multifunctional polymers containing both kinetic hydrate and corrosion inhibition performance by synergistic incorporation of the APIM group into VCap chains.

4. CONCLUSIONS This study investigates the hydrate inhibition performance of PVCap-based KHIs (PVCap-co-AA, PVCap-co-ATCH, and PVCap-co-APIM) by determining the hydrate onset time, growth rate, and resistance-to-flow using a high pressure autoclave. First, the subcooling conditions were determined for PVCap-based KHIs solutions and the results were compared to those of pure water and Luvicap solution at various cooling rates (0.25, 0.033, and 0.017 °C/min). PVCap-APIM and PVCap-co-AA were close, while PVCap-co-ATCH showed the I

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(9) Vahedi, S.; Wood, T. Practicalities of Thermodynamic Hydrate Inhibitor Distribution Within 179 Hydrocarbon Systems Under Steady State and Dynamic Operations. In 16th International Conference on Multiphase Production Technology, Cannes, France, June 2013; BHR Group: Cranfield, U.K., 2013. (10) Yang, J.; Vajari, S. M.; Chapoy, A.; Tohidi, B. Minimizing Hydrate Inhibitor Injection Rates. In International Petroleum Technology Conference, Kuala Lumpur, Malaysia, December 2014; IPTC: Kuala Lumpur, Malaysia, 2014. (11) Larsen, R.; Knight, C. A.; Sloan, E. D. Clathrate hydrate growth and inhibition. Fluid Phase Equilib. 1998, 150−151, 353−360. (12) Kelland, M. A. History of the development of low dosage hydrate inhibitors. Energy Fuels 2006, 20, 825−847. (13) Mady, M. F.; Kelland, M. A. N, N-dimethylhydrazidoacrylamides. Part 2: High-cloud-point kinetic hydrate inhibitor copolymers with n-vinylcaprolactam and effect of pH on performance. Energy Fuels 2015, 29, 678−685. (14) O’Reilly, R.; Ieong, N. S.; Chua, P. C.; Kelland, M. A. Crystal growth inhibition of tetrahydrofuran hydrate with poly(N-vinyl piperidone) and other poly(N-vinyl lactam) homopolymers. Chem. Eng. Sci. 2011, 66, 6555−6560. (15) Lucas, E. F.; Spinelli, L. S.; Khalil, C. N. Polymers applications in petroleum production. In Encyclopedia of Polymer Science and Technology; John Wiley & Sons, Inc: Hoboken, NJ, 2002. (16) Chua, P. C.; Kelland, M. A.; Hirano, T.; Yamamoto, H. Kinetic hydrate inhibition of poly(N-isopropylacrylamide)s with different tacticities. Energy Fuels 2012, 26, 4961−4967. (17) Perrin, A.; Musa, O. M.; Steed, J. W. The chemistry of low dosage clathrate hydrate inhibitors. Chem. Soc. Rev. 2013, 42, 1996− 2015. (18) Maeda, N.; Fong, C.; Sheng, Q.; da Silveira, K. C.; Tian, W.; Seeber, A.; Ganther, W.; Kelland, M. A.; Mady, M. F.; Wood, C. D.High-Throughput Testing of Kinetic Hydrate Inhibitors. Energy Fuels 2016, 30, 5432−5438. (19) da Silveira, K. C.; Sheng, Q.; Tian, W.; Fong, C.; Maeda, N.; Lucas, E. F.; Wood, C. D. High throughput synthesis and characterization of PNIPAM-based kinetic hydrate inhibitors. Fuel 2017, 188, 522−529. (20) Park, J.; da Silveira, K. C.; Sheng, Q.; Wood, C. D.; Seo, Y. Performance of Poly(N-isopropylacrylamide)-Based Kinetic Hydrate Inhibitors for Nucleation and Growth of Natural Gas Hydrates. Energy Fuels 2017, 31, 2697−2704. (21) Menendez, C. M.; Jardine, J.; Mok, W. Y.; Ramachandran, S.; Jovancicevic, V.; Bhattacharya, A. New Sour Gas Corrosion Inhibitor Compatible with Kinetic Hydrate Inhibitor. In IPTC 2014: International Petroleum Technology Conference, Doha, Qatar, Jan 19−22, 2014; IPTC: Kuala Lumpur, Malaysia, 2014. (22) Moloney, J.; Mok, W. Y.; Gamble, C. G. Compatible Corrosion and Kinetic Hydrate Inhibitors for Wet Sour Gas Transmission Lines. In CORROSION 2009, Atlanta, GA, March 22−26, 2009; NACE International, 2009; Document no. NACE-09350. (23) Obanijesu, E. O.; Gubner, R.; Barifcani, A.; Pareek, V.; Tade, M. O.The influence of corrosion inhibitors on hydrate formation temperature along the subsea natural gas pipelines. J. Pet. Sci. Eng. 2014, 120, 239−252. (24) Moore, J. A. Understanding Kinetic Hydrate Inhibitor and Corrosion Inhibitor Interactions. In Proceedings of the Offshore Technology Conference, Houston, TX, May 4−7, 2009; OTC: Richardson, TX, 2009. (25) Sheng, Q.; da Silveira, K. C.; Tian, W.; Fong, C.; Maeda, N.; Gubner, R.; Wood, C. D. Simultaneous hydrate and corrosion inhibition with modified Poly(vinyl caprolactam) polymers. Energy Fuels 2017, 31, 6724−6731. (26) Kim, J.; Shin, K.; Seo, Y.; Cho, S. J.; Lee, J. D. Synergistic Hydrate Inhibition of Monoethylene Glycol with Poly(vinylcaprolactam) in Thermodynamically Underinhibited System. J. Phys. Chem. B 2014, 118, 9065−9075.

lowest performance when the cooling rate was increased from 0.017 to 0.25 °C/min. For delaying hydrate formation, Luvicap again showed better performance than the base PVCap-co-AA and modified polymers, although the performance of Luvicap diminished than the modified polymers at 0.017 °C/min (Luvicap > PVCap-co-APIM ≈ PVCap-co-AA > PVCap-coATCH). For the growth stages, the modification of the base polymer with APIM, PVCap-co-APIM, enhanced the kinetic hydrate inhibition performance. The initial growth rate was less than that in pure water and PVCap-co-ATCH solution. The hydrate fraction increased slowly while maintaining a stable torque. The modification with ATCH adversely affected the kinetic inhibition performance of PVC-based KHI, where the initial growth rate was faster than that of other PVCap-based KHIs and a high hydrate fraction was obtained. For a fast cooling rate, 0.25 °C/min, a surge in torque was observed in the early stage of hydrate formation, suggesting a high risk of hydrate blockage formation upon hydrate onset. These results suggest that the multifunctional polymer was successfully designed and synthesized by incorporating corrosion inhibitor groups, APIM, onto a known kinetic hydrate inhibitor, VCap. Their efficacy was able to be evaluated with the holistic investigation on nucleation and growth of hydrate phase in the liquid phase.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +82-2-880-7329. Fax: +82-2-888-9298 (Y.S.). *E-mail: [email protected]. Phone: +61 8 6436 8701. Fax: +618-6436-8555 (C.D.W.). ORCID

Colin D. Wood: 0000-0001-6160-0112 Yutaek Seo: 0000-0001-8537-579X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Technology Innovation Program (10060099) funded by the Ministry of Trade, Industry & Energy (MI, Korea).

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REFERENCES

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DOI: 10.1021/acs.energyfuels.7b01956 Energy Fuels XXXX, XXX, XXX−XXX