Highly Efficient Recovery of Water-Soluble Polymers in Synergistic

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Highly-efficient Recovery of Water-soluble Polymers in Synergistic Kinetic Inhibition of Gas Hydrate Formation Ye Won Bae, Jeongtak Kim, Sung Hwan Ju, Kyuchul Shin, and In Woo Cheong ACS Appl. Polym. Mater., Just Accepted Manuscript • DOI: 10.1021/acsapm.8b00177 • Publication Date (Web): 16 Jan 2019 Downloaded from http://pubs.acs.org on January 22, 2019

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Applied Polymer Materials

Highly-efficient Recovery of Water-soluble Polymers in Synergistic Kinetic Inhibition of Gas Hydrate Formation Ye Won Bae,† Jeongtak Kim,† Sung Hwan Ju, Kyuchul Shin*, and In Woo Cheong* Department of Applied Chemistry, School of Applied Chemical Engineering, Kyungpook National University, 80 Daehak-ro, Buk-gu, Daegu 41566, Republic of Korea AUTHOR INFORMATION Corresponding Author *Email:

[email protected] (I.W.C.), [email protected] (K.S.)

KEYWORDS:

polymer

recovery,

thermoresponsive

polymer,

temperature, iron oxide nanoparticle, kinetic hydrate inhibitor

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ACS Paragon Plus Environment

lower

critical

solution

ACS Applied Polymer Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: The recovery of synthetic water-soluble polymers is of high environmental and economic importance. We report that these polymers are completely recovered using thermoresponsive polymer-brushed superparamagnetic nanoparticles. Thermo-responsive poly(Nisopropylacrylamide-ran-N,N-dimethylacrylamide) (RCP) and poly(N-isopropylacrylamide-ranN,N-dimethylacrylamide)-block-poly(acrylic acid) (BCP) were used as the water-soluble polymer to be recovered and as the corona layer of iron oxide nanoparticles (IONPs), respectively. Both IONPs-BCP and RCP were simply collected by a magnet after heating, and the IONP-BCP/RCP mixture allowed for good kinetic inhibition of methane hydrate formation. This work reveals that the synergistic behavior of IONPs as recovery agents is practically utilized for the recovery of synthetic water-soluble polymers.

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Although synthetic water-soluble polymers are widely used in industry as emulsifiers, stabilizers, viscosity modifiers, flocculants, films, binders, lubricants, hydrogels, etc.,1–6 they cannot be efficiently recovered and are released into the environment.7 Generally, polymers are recovered from solutions by solvent evaporation, precipitation, and centrifugation. However, evaporation and precipitation techniques are time- and energy-consuming, while centrifugation is not scalable, and the use of biodegradable substitutes is hindered by their narrow range of physical properties and high cost. Therefore, much effort has been directed at the development of alternative recovery techniques, – one of which features the grafting of water-soluble polymers onto superparamagnetic iron oxide nanoparticles (IONPs) that can be simply collected by a magnet.8–12 Stimuli-responsiveness is valuable property for polymer recovery. In particular, temperatureresponsive

polymers

such

as

poly(N-isopropylacrylamide)

(PNIPAAm),13,14

poly[2-

(dimethylamino)ethyl methacrylate],15 hydroxypropylcellulose,16 and poly(vinylcaprolactame)17 typically exhibit lower critical solution temperatures (LCST) or upper critical solution temperatures (UCST). Previously, we reported that the unexpected precipitation of PNIPAAmbased inhibitors of gas hydrate formation can be prevented by LCST and fouling control;18 however, complete polymer recovery is still impossible to achieve. Despite the fact that the possibility of LCST control–based recovery of thermoresponsive polymers has been extensively investigated, practical polymer recovery has not been systematically attempted, i.e., existing research is limited to polymer aggregation, while the industrial application of solution-phase polymer recovery features numerous limitations.

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Herein, we report the synergistic recovery of a synthetic water-soluble polymer in the presence of the IONPs decorated with the same polymer by making use of its LCST behavior. For this purpose,

thermoresponsive

poly(N-isopropylacrylamide-ran-N,N-dimethylacrylamide)

[P(NIPAAm-ran-DMAAm)] random copolymer (RCP) was synthesized as the synthetic watersoluble polymer, and P(NIPAAm-ran-DMAAm)-block-PAA (block copolymer, BCP)-coated IONPs (BCP-IONPs) were prepared as an RCP recovery agent, respectively, by using reversibleaddition fragmentation chain-transfer (RAFT) polymerization. Furthermore, the synthesized thermoresponsive RCP and IONPs-BCP were used to kinetically inhibit methane hydrate formation in subsea methane production flowlines. The formation of RCP and BCP was confirmed by 1H NMR, with the corresponding spectra shown in Fig. S1. Based on the relative areas of two dimethyl group peaks (6H, d, 2.66–3.07 ppm, DMAAm) and the isopropyl group peak (1H, e, 3.7–3.97 ppm, NIPAAm), the molar ratio of NIPAAm to DMAAm was determined as 2:1. The Mn and ÐM values of RCP were measured by size-exclusion chromatography as 3.7  104 g mol−1 and 1.2, respectively. The block length of PAA in BCP was estimated as 2.3  103 g mol−1 using the Mn of RCP and the relative areas of COOH (1H, g, 11.64–12.84 ppm, AA) and dimethyl group (6H, d, 2.66–3.07 ppm, DMAAm) peaks. As a result, the Mn of BCP was determined as 3.93  104 g mol−1. TEM imaging revealed that as-synthesized IONPs-BCP were smaller, more stable, and better dispersed than BCP-free IONPs (IONPs) (Figs. 1(a) and (b)), and the mean sizes of IONPs and IONPs-BCP were measured using ImageJ software as 11.4 and 10.0 nm, respectively. The inset of Fig. 1(b) demonstrates that BCP formed a corona layer around IONP (as indicated by a white arrow), and XRD (Fig. S2) revealed that both IONPs-BCP and IONPs contained spinel4


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structured magnetite (Fe3O4; JCDPS No. 01-071-4918),19 as confirmed by the observation of characteristic (220), (311), (400), (511), (422), and (440) peaks. The mean sizes of IONPs and IONPs-BCP calculated using the Scherrer equation equaled 11.8 and 10.8 nm, respectively, and were in good agreement with those determined by TEM, while the respective mean hydrodynamic diameters were measured by DLS as 276 and 89 nm. The size difference between DLS and TEM is mainly attributed to the hydrated corona layer of the BCP of IONPs-BCP. The hydration of BCP layer causes significant friction drag and reduced diffusion rate, and which affects the hydrodynamic size measured in DLS. Therefore, the size of IONPs-BCP determined by DLS is much larger than the size by TEM, because DLS measures the hydrodynamic size associated with Brownian diffusion, while TEM measures size in dried state.20,21 Moreover, the particle size distribution of IONPs-BCP was narrower than that of IONPs (Fig. 1(c)), which was attributed to the fact that the BCP corona layer prevented particle aggregation. The formation of the above corona layer was also confirmed by ATR-FTIR spectroscopy (Fig. S3). In particular, typical Fe–O (600 cm−1) and –OH (3430 cm−1) peaks were observed in the spectrum of IONPs, whereas the spectrum of IONPs-BCP featured C=O (1620 cm−1, PNIPAAm), N–H (3270, 1540 cm−1) and C=O (1710 cm−1, PAA) peaks along with low-intensity Fe–O peaks and C=O (1710 cm−1) peaks of PAA units. The results confirmed that BCPs bind to the IONP surface via coordination bond by the carboxylic acid group of PAA in BCP.22,23 Based on the results of TGA, the content of BCP in IONPs-BCP was determined as ~56 wt%, while an only insignificant weight loss was observed for IONPs (Fig. 1(d)). The saturation magnetizations (Ms) of IONPs and IONPs-BCP (Fig. S4) were determined by VSM as 67.7 and 29.1 emu/g, respectively, which was consistent with the TGA-determined content of Fe3O4 (44 wt%). The 5



insets of Fig. S4 indicate that coercivity and residual magnetization showed no hysteresis, implying that both IONPs and IONPs-BCP were superparamagnetic. Both RCP and BCP were thermoresponsive and featured LCSTs of 46 and 42 C, respectively, as indicated by their UV-visible transmittance spectra (Fig. S5). The incorporation of hydrophilic DMAAm increased the LCST of RCP from 35 C (LCST value for pure PNIPAAm13) to 46 C, whereas the introduction of the PAA block resulted in extensive hydrogen bonding between the carboxylic acid moieties of AA and the amide groups of NIPAAm, decreasing the LCST of BCP from 46 to 42 C.24 Although no significant change in transmittance with temperature was observed for IONPs-BCP, their LCST could be estimated as 34 C, as can be seen from the inset of the magnified transmittance curve presented in Fig. S5.

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Figure 1. TEM micrographs of (a) IONPs and (b) IONPs-BCP, with high-magnification images presented as insets. The white arrow in the inset of (b) indicates the BCP corona layer. (c) Particle size distributions of IONPs (dh = 276 nm) and IONPs-BCP (dh = 89 nm) measured by DLS. (d) TGA curves of IONPs and IONPs-BCP.

Fig. 2(a) illustrates the thermoresponsive aggregation behavior of IONPs-BCP in the presence of RCP. Since P(NIPAAm-ran-DMAAm) is hydrophilic at room temperature, both RCP and IONPs-BCP are dispersible in water under these conditions. However, both RCP and BCP on IONPs become hydrophobic at 60 °C (i.e., above LCST). Consequently, IONPs-BCP engage in hydrophobic interactions and form aggregates, while RCP undergoes a coil-to-globule transition and adheres onto the surface of IONPs-BCP. Larger aggregates of IONPs-BCP with RCP are more sensitive to the magnetic field than individual IONP-BCP particles and are easily attached to the magnetic stirring bar (Fig. 2(b)). This synergistic effect facilitates the complete recovery of both RCP and IONPs-BCP. The video clips demonstrating the aggregation and re-dispersion of RCP (1 wt%) and IONPs-BCP (2 wt%) are provided in the Supporting Information. To investigate the effect of concentration on the aggregation between IONPs-BCP and RCP, the above reagents were dispersed in water at concentrations of 0–2 wt%, and the obtained dispersions were heated at 60 °C in the presence of a magnetic bar. Vials containing RCP and IONPs-BCP were photographed after 0, 2, and 24 h of stirring at 1000 rpm, and the supernatant solutions were collected after 24 h. Fig. 2(b) presents the photographs of 1 wt% RCP – 2, 1, 0 wt% IONPs-BCP samples. When RCP is used as a low-dosage hydrate inhibitor (LDHI), it is typically injected into oil and gas flowlines at concentrations below 1 wt%.25 At an IONPs-BCP 7



concentration of 2 wt%, both IONPs-BCP and RCP were almost completely recovered, whereas when a 1 wt% loading of IONPs-BCP was used, the dispersion became clear but yellowish, which indicated that IONPs were not completely removed. When no IONPs-BCP were added to the RCP solution (0 wt% IONPs-BCP in Fig. 2(b)), RCP remained in a dispersed state and could not be easily recovered, since some RCP aggregates became attached to the vial wall, which can cause unexpected fouling problems during the practical usage of water-soluble RCPs.26 Furthermore, pure IONPs without the BCP brush could not grab RCP, as shown in Fig. S6. Recovery percentages were calculated based on the amounts of IONPs-BCP and RCP remaining in supernatants. After supernatant collection, the residual concentrations of IONPsBCP were determined by UV-visible spectrophotometry using the calibration curve obtained by analyzing reference solutions (Fig. S6). In addition, the recovery of RCP was quantitatively determined by subtracting the amount of IONPs-BCP calculated above from the amounts of IONPs-BCP and RCP remaining in the supernatant after freeze-drying. The recoveries of IONPsBCP and RCP are presented in Fig. 2(c). When a 2 wt% loading of IONPs-BCP was used, both IONPs-BCP and RCP were almost completely recovered (recovery = 100% for IONPs-BCP and 99.3% for RCP). At an IONPs-BCP loading of 1 wt%, the recoveries of IONPs-BCP and RCP slightly decreased (to 98.5 and 95.1%, respectively), while a recovery of only 45% was observed for pure RCP solution (1 wt%) in the absence of IONPs-BCP, with the remaining 55% corresponding to aggregates attached to the vessel wall. Photographs and recoveries corresponding to various concentrations of IONPs-BCP and RCP are shown in Figs. S6 and S7. For pure RCP solutions, when the RCP concentration increased from 1 to 2 wt%, RCP recovery increased from 45 to 93.8%; however, substantial removal of RCP aggregates was impossible, 8


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since most of them adhered to the vial wall. In contrast, at an IONPs-BCP loading of 2 wt%, RCP and IONPs-BCP recoveries were determined as >99% and 100%, respectively.

Figure 2. (a) Schematic illustration of processes occurring in IONPs-BCP aqueous dispersion in the presence of RCP upon heating. (b) Photographic images illustrating polymer recoveries in IONPs-BCP + 1 wt% RCP mixtures. (c) Recoveries (% weight) of IONPs-BCP and RCP determined by spectroscopic and gravimetric methods, respectively. 9



Recently, we have synthesized a series of LCST-modulated copolymers and applied their synergistic effect to combinational kinetic and thermodynamic hydrate inhibition systems.18 In view of the heavy dosages of thermodynamic hydrate inhibitors (THIs) required for effective operation, kinetic hydrate inhibitors (KHIs), a sub-class of LDHIs, have drawn increased attention as a means of risk management, allowing hydrates to form in flowlines but delaying hydrate nucleation or preventing the agglomeration of hydrate particles. However, the recovery or recycling of KHIs is usually challenging, and their release into the environment places an unnecessary burden on the same. Some current methods for KHI regeneration or reuse have limitations; it is only applicable to an aqueous system without thermodynamic inhibitors such as glycols27 or the KHIs are reused only as a form of glycol-KHI mixtures, which are separated from the aqueous phase streams at the previous stage.28 The synergistic recovery of RCP/IONPsBCP described in this work not only enables easy and complete recovery of water-soluble RCPs, but also is applicable to any fluid mixtures containing glycols or alcohols. Thus, it can be an adequate solution to this environmental problem of flow assurance engineering, since most KHIs are water-soluble polymers. Here, we further investigated the effect of IONPs-BCP on the performance of RCP as a kinetic inhibitor of methane hydrate formation and compared the obtained results with those obtained for pure water and RCP systems under the same conditions. The synergistic recovery of RCP/IONPs-BCP described in this work enables easy and complete recovery of water-soluble RCPs and can thus be an adequate solution to this environmental problem of flow assurance engineering, since most KHIs are water-soluble polymers. Therefore, we further investigated the effect of IONPs-BCP on the performance of RCP as a kinetic

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inhibitor of methane hydrate formation and compared the obtained results with those obtained for pure water and RCP systems under the same conditions. Fig. 3 shows the time profile of hydrate formation–caused methane consumption, revealing that the highest consumption was observed for pure water. Interestingly, for 1 wt% IONPs-BCP, the onset of hydrate formation occurred earlier (after 7.3 min) than for pure water (25.5 min, Table S1), i.e., hydrate formation was faster in the former case. This finding implies that the corona layer of P(NIPAAm-ran-DMAAm) anchored at the IONP surface could not effectively inhibit hydrate formation; nevertheless, the amount of consumed methane gas was nearly half of that consumed in pure water, although IONPs-BCP themselves could not kinetically inhibit hydrate formation. This behavior was attributed to the considerable amount of water present in the hydrated corona layer, since the content of BCP in IONPs-BCP equals 56 wt%. On the other hand, the mixed systems of 1 wt% IONPs-BCP + 1 wt% RCP and 2 wt% IONPs-BCP + 1 wt% RCP showed better inhibition performance than 1 wt% RCP, with the respective onset times determined as 131.5, 111.0, and 60.0 min. The onset times, 10% hydrate conversion times, and sub-cooling temperatures for all systems in Fig. 3 are tabulated in Table S1. Four experiments were conducted for each system to obtain reproducible hydrate formation trends (Fig. S8). The results shown in Fig. 3 and Table S1 confirm that IONPs-BCP can well promote the inhibitory action of KHIs (RCP in this work) despite not being able to act as a hydrate inhibitor. This can be rationalized by the interaction between RCP and IONPs-BCP. The polymer brushes anchored at the IONP surface interact with RCPs around the IONPs-BCP particle, and which can improve the inhibition of RCP. However, it is difficult to explain exactly how IONPs-BCP affect hydrate formation kinetics at this stage because even the inhibition mechanism of KHI polymers has not 11



been clearly elucidated yet.18,29,30 It should also be noted that IONPs-BCP allow KHIs such as RCP to be completely recovered using a magnet and can be reused by manipulating temperature as follows: both RCP and IONPs-BCP are first aggregated by heating at 60 °C. Then RCP and IONPs-BCP can be separated by lowering the temperature between the LCST (46 °C) of RCP and the LCST (34 °C) of IONPs-BCP, since RCP is hydrophilic below 46 °C. Therefore, the polymer-brushed nanoparticles synthesized herein can be applied in flow assurance engineering as dual-function materials for enhancing the hydrate inhibition performance of KHIs and enabling their easy recovery after the flow arrives onshore.

Figure 3. Average cumulative moles of methane consumed for hydrate formation in pure water, 1 wt% RCP, 1 wt% IONPs-BCP, 1 wt% IONPs-BCP + 1 wt% RCP, and 2 wt% IONPs-BCP + 1 wt% RCP systems. The time of equilibrium establishment was set to zero.

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In summary, we herein synthesized temperature-responsive RCP and the corresponding BCP by stepwise RAFT polymerization and employed BCP to synthesize thermoresponsive IONPsBCP. Upon heating, IONPs-BCP behaved as a synergistic collector that could capture RCP and allowed for 100% recoveries of IONPs-BCP and RCP, as confirmed by recovery tests conducted for various concentrations and combinations of RCP and IONPs-BCP solutions. Finally, IONPsBCP + RCP solutions were used to inhibit methane hydrate formation, since one of the practical applications of RCP synthesized in this work is the inhibition of gas hydrate formation. IONPsBCP did not show excellent inhibition performance when used on their own; however, excellent inhibition performance was observed when they were used in combination with RCP. These results suggest that IONPs-BCP can be used as a good model collector for the recovery of LCSTmodulated KHIs in methane gas flowlines and a KHI synergist. Polymers with a relatively low LCST can be unexpectedly precipitated at the inhibitor injection point because these sites are usually hot, so careful LCST-modulation is essential for the practical application of the synergistic recovery concept proposed in this work. The use of IONPs on an industrial scale is also an important issue to overcome in near future. Nevertheless, our strategy is to replace conventional KHIs, such as poly(vinyllactam)s or hyperbranched poly(esteramide)s, which causes serious environmental pollution by securing recovery and reusability.

ASSOCIATED CONTENT Supporting Information. Experimental details; representative 1H NMR spectra and structures of RCP and BCP; XRD patterns of IONPs and IONPs-BCP; ATR-FTIR spectra of IONPs, BCP, and IONPs-BCP; field-dependent magnetization curves of IONPs and IONPs-BCP; UV-vis 13



transmittances (at 500 nm) of 0.5 wt% aqueous RCP, BCP, and IONPs-BCP as functions of temperature; videos and photographs of IONPs-BCP and RCP in aqueous solutions; recoveries of IONPs-BCP and RCP; and time-dependent amounts of methane transformed into the corresponding hydrate in different systems. These data (PDF) are available free of charge at XXX.

AUTHOR INFORMATION Notes The authors declare no competing financial interests. †Y. W. Bae and J. Kim contributed equally. ACKNOWLEDGMENT This study was supported by the Ministry of Trade, Industry and Energy of Korea (Grant Nos. 10070241 and 10067082) and the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Grant No. NRF-2018R1D1A1B07040575). REFERENCES (1) Bolto, B.; Gregory, J. Organic Polyelectrolytes in Water Treatment. Water Res. 2007, 41, 2301–2324. (2) Zahrim, A. Y.; Tizaoui, C.; Hilal, N. Coagulation with Polymers for Nanofiltration Pretreatment of Highly Concentrated Dyes: A Review. Desalination 2011, 266, 1–16. (3) Dai, S.; Ravi, P.; Tam, K. C. pH-Responsive Polymers: Synthesis, Properties and Applications. Soft Matter 2008, 4, 435–449. 14


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Highly Efficient Recovery of Water-Soluble Polymers in Synergistic

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