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Raman spectroscopic and kinetic analysis of hydrate shell formation on hydrogel particles containing monoethylene glycol
Accepted Manuscript Raman spectroscopic and kinetic analysis of hydrate shell formation on hydrogel particles containing monoethylene glycol Juwoon Park, Hyunho Kim, Yutaek Seo, Wendy Tian, Colin D. Wood PII:
S1875-5100(17)30115-4
DOI:
10.1016/j.jngse.2017.03.009
Reference:
JNGSE 2107
To appear in:
Journal of Natural Gas Science and Engineering
Received Date: 30 October 2016 Revised Date:
28 February 2017
Accepted Date: 1 March 2017
Please cite this article as: Park, J., Kim, H., Seo, Y., Tian, W., Wood, C.D., Raman spectroscopic and kinetic analysis of hydrate shell formation on hydrogel particles containing monoethylene glycol, Journal of Natural Gas Science & Engineering (2017), doi: 10.1016/j.jngse.2017.03.009. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical Abstract
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Raman spectroscopic and kinetic analysis of hydrate shell
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formation on hydrogel particles containing Monoethylene
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Glycol
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Juwoon Park1, Hyunho Kim1, Yutaek Seo1*, Wendy Tian2, and Colin D. Wood 2*
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Department of Naval Architecture and Ocean Engineering, RIMS, Collage of Engineering,
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Seoul National University, 1, Gwanak-ro, Gwanak-gu, Seoul, 151 - 744, Korea 2
CSIRO Energy, Australian Resources Research Centre, Kensington, WA, 6151, Australia
AUTHOR EMAIL ADDRESSES: [email protected] and [email protected]
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TELEPHONE: YS +82-2-880-7329; CDW +613-9545-8160
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FAX: YS +82-2-888-9298; CDW +618-6436-8555
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ABSTRACT
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Hydrate shell growth on hydrogel particles incorporating monoethylene glycol (MEG) was
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investigated using Raman spectroscopy and a high pressure autoclave apparatus. Surface images
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of hydrogel particles covered by a hydrate shell are presented and the obtained Raman spectra
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indicate MEG molecules were excluded from the formation of the hydrate shell. The Raman
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spectra and surface imaging of the particles demonstrate that the particles remain intact after the
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dissociation of the shell. In subsequent hydrate formation cycles, the hydrate fraction in the
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liquid phase was determined from gas consumption measurements in a high pressure autoclave.
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The hydrate fraction reached only 0.11 for hydrogels containing MEG while a high hydrate
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fraction of 0.41 was observed for a control system consisting of bulk water and decane mixture.
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The growth rate in the presence of hydrogel particles was suppressed in the early stages of
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hydrate formation, suggesting that the increasing MEG concentration inside the hydrogel core
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may limit the further inward hydrate growth. After hydrate formation the hydrogel particles were
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analyzed by thermogravimetric analysis (TGA), suggesting the possibility of recovering
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hydrogel polymer through conventional heating methodologies. These results demonstrate that
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injecting hydrogel particles containing high concentration of MEG may be feasible to manage
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the risk of hydrate plug formation as they will absorb the free water resulting in the dispersed
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hydrogel particles among the hydrocarbon liquid phase.
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KEYWORDS Flow assurance, Gas hydrate, Hydrogel, Shell formation, Anti-agglomeration,
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Plug prevention.
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INTRODUCTION
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Gas hydrates are nonstoichiometric crystalline compounds, consisting of host water and guest
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hydrocarbon molecules such as methane, ethane, and propane (Sloan and Koh, 2008; Sloan et al.,
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2009). They are formed through hydrogen bonding between water molecules which enclathrate
5
guest molecules at high pressure and low temperature, conditions which are found in subsea oil
6
and gas flowlines. Industry practice has been relying on the injection of vast quantities of
7
alcohol- or glycol-based thermodynamic hydrate inhibitors (THI) to prevent hydrate formation in
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subsea flowlines. However, handling large amount of THI in remote offshore fields significantly
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increases the capital and operational expenditure, thus adversely affecting the economics of field
10
development. As such hydrate prevention strategies are now moving toward hydrate risk
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management by either delaying the formation with polymer-based kinetic hydrate inhibitors
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(KHIs) or preventing the agglomeration of hydrate particles by surfactant-based anti-
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agglomerants (AAs), which are collectively known as low dosage hydrate inhibitors (LDHIs)
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(Kelland, 2006; Clard and Anderson, 2007; Duchateau et al., 2009; Del Villano and Kelland,
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2011)_).
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Although there have been numerous applications of LDHIs in offshore oil and gas fields,
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monoethylene glycol (MEG) is still a reliable solution for avoiding hydrate plug formation for
18
offshore fields due to its robust performance, negligible loss to gas phase, and lower Health,
19
Safety, and Environmental (HSE) issues (Sanford et al, 2011). The injection rate of MEG into
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subsea flowlines is typically based on the lowest temperature and highest pressure conditions
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that would be encountered in extended shut-in operation of the subsea production system. As the
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concentration of MEG in water may need to be up to 50 ~ 60 wt%, the industry is spending
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significant operating and capital expenditure. As such a small reduction in the MEG injection
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rate can result in significant saving. Recent studies of hydrate formation kinetics have suggested
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that the amount of hydrate formed can be controlled in the presence of MEG in water where the
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concentration of MEG is lower than the value required to fully avoid the hydrate formation under
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experimental conditions, a practice known as under-inhibition. Li et al. (2011) reported that
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under inhibition can have an adverse effect where the concentration of MEG in water was in
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range between 1.0 and 5.0 wt%. Experimental results showed that MEG enhanced hydrate
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formation kinetics when added at the lower concentration than 5.0 wt%. It has also been
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observed that the hydrate plug formation potential increased to a maximum at MEG
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concentration of 10 to 15 wt%. The under-inhibition effect might be effective with MEG
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concentration of higher than 20 wt%.
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Hydrate plug formation involves growth of hydrate crystals followed by agglomeration which
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results in a high resistance-to-flow for liquid hydrocarbon fluids. Joshi et al (2013) suggested the
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hydrate particles would suspend homogeneously in the liquid phase during the early stages of
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hydrate formation, however once the amount of hydrate was greater than a certain hydrate
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volume fraction in the aqueous phase, heterogeneous particle distribution would be followed by
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stationary bed formation before the hydrate plug formation. Akhfash et al. (2013; 2016) also
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observed the transition of homogeneous distribution to heterogeneous segregation of hydrate
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particles in liquid phase depending on the hydrate volume fraction. In terms of thermodynamics,
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the presence of MEG in the liquid phase shifts the hydrate equilibrium to lower temperature and
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higher pressure condition, thus reducing the driving force for hydrate formation. Recent studies
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also suggest MEG induces low resistance-to-flow for hydrate slurries. Kim et al. (2014)
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presented the amount of hydrate in liquid phase could be reduced less than 28% by adding 20 wt%
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MEG while maintaining stable torque. Sohn et al. (2015) also showed that the resistance-to-flow
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was minimized with under-inhibition of MEG for water and decane mixtures with water cut of
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60 % and 40%.
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In our previous works, the potential use of polyacrylamide-based hydrogel particles for
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preventing the agglomeration of hydrate particles by locking the water phase into particles which
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ensured that the torque was stable during the hydrate formation (Seo et al., 2014; Park et al.,
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2016). The use of hydrogel particles was further extended by incorporating mono ethylene glycol
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(MEG), then their performance for hydrate inhibition was tested using a high pressure autoclave
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by measuring hydrate onset time, hydrate fraction, and torque changes during hydrate formation.
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Although the MEG containing hydrogels showed the best performance for forming low hydrate
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fraction and maintaining stable torque further investigations are required on the role of MEG
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during the hydrate formation process on the surface of the hydrogel particles, especially with a
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spectroscopic method. Raman spectroscopy was used to identify the structural characteristics of
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the gas hydrates formed on the surface of the hydrogel particle. The hydrate structure type and
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distribution of guest molecules on the surface of hydrate particle was investigated from Raman
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spectroscopic analysis. The obtained Raman analysis along with the macroscopic studies on
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hydrate formation kinetics will provide comprehensive understanding on the effect of hydrate
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formation on the hydrogel particle. Moreover there has been no systematic study on the
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mechanism of swelling hydrogels in subsea flowlines and separation of hydrogel polymer from
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the absorbed liquid phase.
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This study represents the first effort to analyze the hydrate shell formed on the surface of a
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hydrogel particle using Raman spectroscopy. The hydrate structure formed on the surface of the
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hydrogel particle was investigated along with the presence of MEG on the surface of the hydrate
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shell. Then TGA analysis was carried out for the hydrogel samples to investigate the separation
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of the hydrogel polymer with conventional heating. The formation rate of gas hydrate with and
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without MEG in hydrogel particles was also investigated by measuring the gas consumption rate
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during hydrate formation. The obtained results suggests that MEG molecules were excluded
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from the hydrate shell during its formation and migrated inward, suggesting the under-inhibition
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effect was coupled with water discretization using hydrogel particles.
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Synthesis of MEG-Hydrogel All chemicals for hydrogel particle synthesis were supplied by
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Sigma-Aldrich and were used without further purification. These chemicals are as follows:
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polyacrylamide-co-acrylic acid partial sodium salt (PAM-co-AA), Mw 520 000, Mn 150 000,
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typical acrylamide level 80%; N-(3-(dimethylamino)propyl)-N′-ethylcarbodiimide hydrochloride
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(EDC, commercial grade); N-hydroxysuccinimide (NHS, 98%); and 1,2-diamino ethane (EDA
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>99%). The hydrogel synthesis has been explained in our previous work10, which involves the
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carbodiimide-mediated cross linking (CMC) reaction of PAM-co-AA in an inverse suspension.
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To incorporate MEG into the PAM hydrogel particles the dried PAM powder were swollen in 20
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wt% MEG solution with final polymer concentration in the hydrogel of 13 wt%. The obtained
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hydrogel particles were suspended in a liquid hydrocarbon such as decane where they maintained
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their spherical shape.
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Raman analysis of hydrate shell on MEG-hydrogel particle For the spectroscopic analysis of
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hydrate shell on the surface of MEG-hydrogel, ethane was used to form hydrate with MEG
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hydrogel particles. The primary objective of this Raman analysis was to observe the presence of
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MEG in hydrate shell formed on the surface of MEG-hydrate particle. The synthesized MEG-
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hydrogel particles were exposed to ethane gas at 1oC for 3 days. After confirming the hydrate
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formation visually, the samples are stored in liquid nitrogen to prevent the dissociation of
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hydrate shell. Jobin-Yvon ARAMIS high resolution dispersive Raman microscope was used to
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measure the Raman spectra by an electrically cooled (203K) CCD detector. 514.53 nm from Ar-
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ion laser performed as the excitation source and the laser had 30mW intensity. The measurement
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was carried out while increasing the temperature in step from 93K to 153K, 213K, and finally
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243K using a Linkam TMS 94 temperature controller. The images of hydrate shell on the surface
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of the MEG hydrogel particles were taken using the microscope, and the qualitative analysis of
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hydrate shell composition was carried out from the obtained Raman spectra.
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High pressure autoclave experiments Hydrate formation kinetics were investigated during
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hydrate formation for four different systems including bulk water, 20 wt% MEG solution, PAM-
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hydrogel, and MEG-hydrogel particles. To simulate a liquid phase composed of water and
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hydrocarbon, distilled water, purchased from the OCI, and decane, supplied by SIGMA-
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ALDRICH Inc, were mixed with the volumetric ratio of water phase, 60%. Synthetic natural gas
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(CH4: 90 mol%, C2H6: 6 mol%, C3H8: 3 mol%, n-C4H10:1 mol%) and pure C2H6 were supplied
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from the Special Gas Company (Daejeon, Korea), with a stated minimum purity of 99.995
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mol%. The details of the high pressure autoclave is presented in our previous works. (Kim et. al.,
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2014; Park et. al., 2016; Seo, et. al., 2014; Sohn, et. al., 2015)
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To investigate the hydrate formation characteristics, a liquid phase (80 mL) was loaded into the
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autoclave with total internal volume of 360 mL Decane and distilled water were used to
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represent liquid hydrocarbon phase and water phase respectively, which is a control without
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hydrogel polymer in aqueous phase. The volume ratio of water in liquid phase was 60% for all
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experiments in this work whether it was bulk water or hydrogel particles, thus 48ml of water was
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mixed with 32 ml of decane. The concentration of MEG to investigate the effect of MEG on the
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hydrate formation characteristics was 20 wt%, thus 12g of MEG was mixed with 48g water. For
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studying hydrate formation in hydrogel particles, hydrogel particles were mixed with decane
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instead of bulk water. The reactor was pressurized to 120 bar at 24 oC without mixing, then the
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stirring was commenced at 600 rpm for 2 hours to saturate the liquid phase with natural gas. The
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cooling of the system was commenced from 24 oC to 4 oC with a cooling rate of 0.25oC/min,
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which was to simulate the cooling of hydrocarbon and water mixture in subsea flowlines. The
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cooling rate can be varied depending on overall heat transfer coefficient of the flowline surface.
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Once the temperature reached 4 oC, it was maintained at 4 oC for 10 hours to observe the hydrate
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formation characteristics for the studied system. A total of 32 cycles of hydrate formation and
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dissociation were repeated for i) bulk water, ii) 20.0 wt% MEG solution, iii) hydrogel with no
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MEG, and iv) MEG hydrogel with MEG concentration of 20.0 wt% (MEG-20 hydrogel).
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Hydrate formation rate and the volume fraction in the liquid phase was characterized by
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analyzing pressure decrease due to gas consumption during the hydrate formation.
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Thermogravimetric analysis (TGA) TGA for MEG 20% aqueous solution and recovered
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MEG-Hydrogel samples from Raman analysis were carried out using a Setsys 16/18 (France,
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Setaram). The temperature ramp went from 25 oC to 325 oC with heating rate of 5 oC per min
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under nitrogen injection of 30mL/min. Comparison of the TGA curves between the MEG 20 wt%
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solution and the MEG-20 hydrogel was made to confirm the possibility of evaporating MEG
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from the hydrogel particles using conventional heating method.
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RESULTS AND DISCUSSION
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Spectroscopic analysis of hydrate shell on hydrogel particles
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It was presumed that hydrate would form on the surface of hydrogel particles and the
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composition change on the surface of hydrogel was expected due to exclusion of MEG during
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the hydrate formation. The Raman spectroscopic study was carried out along with optical
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imaging to investigate the composition of hydrates formed on the MEG-20 hydrogel particle.
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The formation of hydrate on the MEG-20 hydrogel particle was visually confirmed (Figure 1),
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and the Raman spectroscopic observation for the region of C-C stretching for ethane and MEG
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provides evidence of the hydrate structure and its composition. The temperature of the Raman
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probe was maintained at 93K initially to ensure that there were significant amounts of crystals on
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the surface of the hydrogel particle. The melting and boiling temperatures of ethane are 90.4 K
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and 184.6 K, respectively. After the temperature was increased to 153 K (Figure 1b) there were
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minimal changes to the shape of the surface. The crystals on the surface of the MEG-20 hydrogel
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were still present at 213K as confirmed in the spectra(Figure 1c), however 243 K they were
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dissociated (Figure 1d). The obtained images in Figure 1 confirmed the formation of crystal
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structures on the surface of MEG-20 hydrogel particle, which would be structure I hydrate
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formed from ethane and water. After the dissociation of crystal structures, the MEG-20 hydrogel
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particle recovered its original sphere shape without losing water to the surrounding environment,
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confirming the performance of polymer PAM-co-AA even at cryogenic conditions. The exact
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hydrate structure and its composition was confirmed from the analysis of Raman spectra.
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Figure 1. MEG (20wt%)-PAM-hydrogel with ethane hydrate on the surface. When the temperature was increased, the hydrate shell started to dissociate and no hydrate was observed in the final image at 243K
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Figure S1 shows the Raman spectra obtained in the range of 300 and 3600 cm-1 for the MEG-20
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hydrogel particle while maintaining temperature at 93K, 153K, 213K, and 243K. The C-H
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stretching bond of MEG molecules would appear at 2888 and 2940 cm-1, while the C-H
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stretching bond of ethane would appear at 2888 and 2943 cm-1(Seo et al., 2009; Kim et al.,
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2014). They were superimposed on each other as shown in Figure S1 and were difficult to
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discriminate, although two sharp peaks seemed to disappear at 243 K. The presence of hydrate
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structure was confirmed with the C-C stretching bond of ethane molecules in Figure 2, where the
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Raman spectra obtained in the range of 300 and 1600 cm-1 were focused. The Raman peak at 999
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cm-1 presents the C-C stretching bond of ethane molecules encaged in large cage of structure I
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hydrate, which can be confirmed between 93K and 213K, but is not shown at 243K. These
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Raman spectra were well aligned with the images shown in Figure 1, suggesting the ethane
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hydrate was formed and stayed on the surface of the MEG-20 hydrogel particle between 93K and
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213K, but dissociated when the temperature was increased to 243K. This is due to the fact that
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the dissociation temperature of ethane hydrate in MEG 20 wt% solution was estimated to be
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240.4K at atmospheric pressure by CSM Gem. The presence of structure I hydrate on the surface
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of the MEG-20 hydrogel particle was confirmed from the images and Raman spectra in Figure 1
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and 2. However the intensities of the peaks for MEG changed adversely, small peaks from 93K
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to 213K but high intensity peaks at 243K, suggesting the concentration of MEG changed on the
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surface of the MEG-20 hydrogel particle during the process of hydrate formation and
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dissociation.
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Figure 2. Raman spectra obtained in a focusing area at 93K, 153K, 213K, and 243K, respectively. When the temperature increased, the hydrate shell dissociate and no hydrate was
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observed at 243K. Note the Raman spectra range is between 300 and 1600 cm-1 to focus on C-C stretching bond of ethane and MEG molecules.
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The concentration of ethane would be highest at the interface between the ethane gas and the
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MEG-20 hydrogel. Hydrate forms on the surface of the hydrogel particles, during which the
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MEG and polymer are excluded from the hydrate structure. Eventually the formation of ethane
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hydrate ceases due to mass transfer limitations through the hydrate shell while a separate solid
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phase covers the surface of the MEG-20 hydrogel particle. As the MEG molecules cannot
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participate in the hydrate structure, the molecules are excluded from the hydrate crystal structure
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during the formation process and are believed to be concentrated inside the hydrogel core. This is
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the most likely scenario because the MEG is more soluble in water within the particle than in the
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surrounding gas phase. Most of the solid phase observed in Figure 1 (a) to (c) is the ethane
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hydrate while only a small amount of MEG exists as can be seen from the Raman spectra for
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ethane and MEG in Figure 2. Ethane remains in the large cages of structure I hydrate up until
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213 K from the Raman peak at 999 cm-1. However, at 243K the Raman peak for ethane in
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hydrate cages disappears, suggesting the hydrate shell is dissociated and only dissolved MEG
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remains on the surface of the hydrogel particle. It is noted that the intensity of the Raman peaks
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associated with MEG (866, 1050-1150, 1459 cm-1) increased as the temperature was raised to
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243K. Upon complete dissociation of the hydrate, the MEG molecules is absorbed back into the
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surface of the MEG-20 hydrogel particle again while the hydrogel particle gains its original
21
sphere shape (Figure 1 (d)).
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Overall the MEG-20 hydrogel shows reversible behavior in a hydrophobic environment during
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hydrate formation and dissociation. This reversible behavior of the MEG-20 hydrogel particle
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would be beneficial to utilize the MEG-20 hydrogel for managing the hydrate formation risks in
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subsea flowlines. However more investigations on the exact process of hydrate formation on the
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surface of the MEG-20 hydrogel particle, including hydrate growth rate and volume fraction
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comparing to bulk solution, are further required in mixing condition.
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Hydrate formation kinetics as determined using a high pressure autoclave
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The hydrate formation experiments were carried out in a high pressure autoclave with continuous
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cooling and mixing of fluids at 600 rpm to ensure that the decane and water/hydrogel were
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completely mixed. Figure 3 shows the pressure-temperature (PT) trace during the cooling of
8
natural gas and liquid mixture along with the hydrate equilibrium conditions. Figure S2 presents
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the estimated hydrate equilibrium conditions in the presence of MEG in pressure range between
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2.0 and 14.0 MPa. The experiments were carried out in isochoric condition and the pressure
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continuously decreased with a rate that corresponds to the cooling of fluids from 281.1 K to
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271.1 K until the hydrate equilibrium for water-decane-natural gas mixture was passed. When
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the condition reached the point O1, hydrate formation was observed visually and the subcooling
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temperature, i.e. temperature difference between equilibrium condition and onset condition, was
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determined to be 5.6 K. Since the hydrate onset, fast pressure reduction was observed until the
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point C1, then the PT trace returned to its initial cooling path due to gas consumption for hydrate
17
formation. The hydrate formation in the hydrogel and decane mixture started slightly earlier in
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the point O2, resulting in a lower subcooling temperature of 4.5 K than bulk water and decane
19
however, the reduced pressure due to hydrate formation was much smaller than the bulk water
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system.
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The cooling was completed at 276.1 K and 10.0 MPa for water, decane, and natural gas mixture.
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The standard industry practice for completely avoiding hydrate formation, is in this system
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would require a MEG concentration in the aqueous phase to exceed 43.0 wt% in order to shift
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the hydrate equilibrium conditions to lower temperature and higher pressure region than the
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cooling path of the fluids, as seen in Figure 3. The addition of 20.0 wt% MEG (i.e., under
4
inhibited system) in the aqueous phase would not be able to prevent the hydrate formation with
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the cooling path and would induce the hydrate formation. However, the resulting hydrate onset
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condition for MEG 20.0 wt% bulk solution and MEG-20 hydrogel particles were shifted to a
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lower temperature region because the hydrate equilibrium condition was shifted in the presence
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of MEG as well. From the calculated equilibrium condition for MEG 20.0 wt% MEG solution
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with natural gas, the resulting subcooling temperature was 7.5 K in the hydrate onset condition
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point O3. For the MEG-20 hydrogel particles and decane mixture, the subcooling temperature
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from the onset condition in the point O4 was 5.8 K. The pressure reduction due to hydrate
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formation was relatively small for both MEG 20.0 wt% bulk solution and MEG-20 hydrogel
13
particles. The addition of MEG increased the subcooling temperature from 5.6 K to 7.5 K,
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however the MEG-20 hydrogel particles showed less subcooling temperature of 6.8 K,
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suggesting that the hydrate formation happened faster in MEG-20 hydrogel particles than in
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MEG 20.0 wt% bulk solution.
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12.0 Bulk water Hydrogel MEG 20wt% solution MEG-20 hydrogel Hydrate Onset conditions
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O2.Onset-Hydrogel O1.Onset-Bulk water
O4.Onset-MEG Hydrogel O3.Onset-MEG solution C3.Complete-MEG solution
C2.Complete-Hydrogel
C4.Complete-MEG Hydrogel C1.Complete-Bulk water
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Commence cooling
MEG 20wt%
MEG 43wt% 275
280
285
290
MEG 0 wt% 295
300
Temperature (K)
Figure 3. Pressure-Temperature profile during the hydrate formation form bulk water + decane, hydrogel + decane, MEG 20wt% bulk solution + decane, and MEG-20 hydrogel + decane. The solid lines represent the hydrate equilibrium curves for bulk water (black), MEG 20 wt% solution (blue), and MEG 43 wt% solution (red).
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The hydrate formation on the surface of hydrogel particles occurred at a faster rate than the bulk
7
water phase, possibly due to dispersion of hydrogel particles and enhance mass transfer of gas
8
molecules through the particles (Su et al., 2009). Figure 3 confirms that the hydrate formation
9
characteristics are very different when the system is under inhibited and in the presence of
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hydrogel polymer. Further analysis on hydrate volume fraction change in liquid phase was
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carried out from the pressure change in Figure 3 as a function of time.
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Table 1 and Figure 4 present the hydrate formation characteristics for bulk water, hydrogel
13
particles, MEG 20.0 wt% solution, and MEG-20 hydrogel particles. The aqueous phase was
14
mixed with decane with watercut 60% for every experiments. At least eight experiments were
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carried out for each system to determine the average values of water conversion to hydrate,
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hydrate fraction in liquid phase, and growth rates. Figure 4 shows the hydrate fraction changes in
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liquid phase as a function of time, where time zero indicates the hydrate onset point in Figure 3.
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The consumed gas molecules due to hydrate formation was estimated from the pressure
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difference between monitored moment and calculated pressure with the assumption of no hydrate
4
formation. This procedure has been suggested as a method for hydrate formation study in a flow
5
wheel and an autoclave systems (Ke et al., 2011; Hemmingsen, et al., 2008). The hydrate
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fraction, Φhyd in the liquid phase at 150 min since the hydrate onset was calculated from the
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following equation.
+
+(
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where Vw is the volume of water, Vw,conv is the volume of the water converted to hydrate, Vdecane is
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the volume of decane, and Vhyd is the volume of hydrate calculated from the molecular weight
10
and density of hydrates calculated at a given time. The hydration number was assumed 6.5. As
11
the volume fraction of decane in the liquid phase was 0.4, the maximum hydrate fraction in
12
liquid phase can be 0.6 in case of complete conversion of water to hydrate.
13
As hydrate nuclei grew while consuming the dissolved gas molecules in bulk water phase, the
14
linear growth rate appeared in early stage of hydrate growth. Figure 4 showed the initial growth
15
rate of hydrates in bulk water was 0.0082 vol.frac./min before an inflection point at around 30
16
min. Hydrates growth continued until the hydrate fraction reached 0.4 within 60 min since the
17
hydrate onset, where the water conversion into hydrate was 60.2 %. The addition of PAM-co-AA
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transformed the bulk water into discretized hydrogel particles mixed with decane and the
19
obtained hydrate growth for hydrogel particles was shown in Figure 4. The initial growth rate
20
was 0.0118 vol.frac./min, which was much faster than in bulk water. As the aqueous phase was
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already dispersed in particulate format, mass transfer of gas molecules into hydrogel particles
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was enhanced. Although fast formation rate was achieved in hydrogel particles, inward growth
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of hydrate shell on the surface of hydrogel particle results in a barrier that retards the mass
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transfer of gas molecules to the interior of the hydrogel particles. The hydrate fraction reached
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0.1 at 10 min after the onset, then continued to 0.17 at 150 min with water conversion of 24.1 %.
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For MEG 20.0 wt% bulk solution hydrate growth was slow for the first 60 mins initially after the
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hydrate onset. The slow growth rate was 0.0005 vol.frac./min, an order of magnitude less than in
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bulk water. The growth rate surged into 0.0101 vol.frac./min at 70 min after the onset, which is
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close to the rate for bulk water. Hydrate kept growing until the hydrate fraction in the liquid
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phase reached 0.2 at 90 min. The final hydrate fraction at 150 min was 0.22 with water
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conversion to hydrate of 31.3 %. These results suggested that the MEG molecules in the aqueous
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phase resulted in strong hydrogen bonding with the water molecules which retard the
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crystallization into hydrate structures.
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Table 1. Hydrate formation characteristics for bulk water and hydrogel systems. rslow: slow growth period due to inhibition of hydrate formation, rfast: fast growth period limited by mass transfer.
Bulk water
MEG 20.0 wt% solution MEG-20 hydrogel 16
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rfast (vol.frac./min)
5.6
60.2
0.41
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0.0082
4.5
24.1
0.17
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0.0118
7.5
31.3
0.22
0.0005
0.0101
5.8
15.4
0.11
0.0005
0.0051
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∆Tsub Water Hydrate volume (K) conversion (%) fraction
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0.3
0.2
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Hydrate fraction in liquid phase
0.4
0.1
0.0 0
30
90
120
150
Figure 4. Hydrate volume fraction changes in liquid phase as a function of time for bulk water, hydrogel particles, MEG 20.0 wt% solution, and MEG-20 hydrogel particles.
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For MEG-20 hydrogel particles mixed with decane, the slow growth period was also observed
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until 30 min after the onset and the growth rate was just same with the MEG 20.0 wt% bulk
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solution, 0.0005 vol.frac./min. Fast hydrate growth was followed with a rate of 0.0051
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vol.fac./min, but the rate was less than in bulk water. The hydrate growth on the surface of
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MEG-20 hydrogel particles reached 0.10 at 70 min, then slowly reached 0.11 at 150 min with
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water conversion into hydrate of 15.4 %. As we observed in Raman spectroscopic analysis, MEG
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molecules on the surface of MEG-20 hydrogel particles would migrate inside the hydrogel core
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during the inward growth of hydrate shell. As the MEG molecules migrated inside, fast hydrate
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formation would occur. However, the hydrate formation reached steady-state early because of
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the mass transfer limitation through the hydrate shell formed on the surface of the hydrogel
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particle. The resulting hydrate fraction and water conversion into hydrate was less than other
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aqueous system, i.e. 15% less than for bulk water. It can be concluded that the advantage of
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employing under-inhibition was commingled with the effect of transforming the aqueous phase
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into particulate formats, as seen in slow growth period and less hydrate fraction in liquid phase.
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Moreover, hydrates only formed on the surface of MEG-20 hydrogel particles without sintering
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or aggregation of the particles.
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Unlike bulk water, there was no acceleration in hydrate growth for MEG-20 hydrogel particles,
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as shown in Figure 4. Slow initial growth was followed by fast growth, but soon reached steady-
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state. The mechanism of hydrate growth is different between bulk water and MEG-20 hydrogel
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particles. Compared to fast formation in bulk water, MEG-20 hydrogels do not show any
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evidence of hydrate plug formation. Figure 4 shows the hydrate growth on the surface of MEG-
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20 hydrogels steadily decreases with time and the hydrate fraction in liquid phase reaches only
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0.11. Therefore, the concentration of MEG in the hydrogel particles slightly increases to 24 wt%
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considering the loss of water into hydrate shell.
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Formation and regeneration of MEG hydrogel particles
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Figure 5 shows the schematic diagram of employing MEG hydrogel particles in subsea flowline
18
to manage the risk of hydrate plug formation. The MEG hydrogel particles with MEG
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concentration of 80 ~ 90 wt% are carried into the subsea tree to be mixed with produced
20
hydrocarbon and free water stream (Figure 5 A). MEG is a polar compound and has a strong
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interaction with the carbon-backbone of the polymer structure. Thus MEG is absorbed
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into the hydrogel particle in a similar way to water, which is favorable for delivering the MEG
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into a subsea flowline without loss to the surrounding environment. The swelling ratio of PAM-
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co-AA was previously measured as 25 for water, however it was about 10.0 for MEG (Sheng et
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al., 2014). Therefore, MEG hydrogels with desired MEG concentration can be produced with the
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pre-determined swelling ratio for MEG. Based on the amount of free water in the subsea flowline,
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the volume of MEG hydrogel particles can be calculated to achieve the target concentration of
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MEG when the MEG hydrogel particles are mixed with free water (Figure 5 top B), then
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absorbing the free water until the maximum swelling ratio is reached. The final concentration of
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MEG in MEG hydrogel particles could be lowered to 20.0 wt% which is an under-inhibited
9
condition. Although the temperature and pressure are suitable for hydrates formation, they are
10
formed on the surface of the MEG-20 hydrogel particles with slow growth period and low
11
hydrate fraction as discussed in the previous section (Figure 5 top C).
12
The images of MEG-20 hydrogel particles dispersed in decane phase are shown in Figure 5. In
13
this work, the MEG hydrogels were produced by swelling the dry polymer with water droplets
14
suspended in a liquid hydrocarbon phase. Several MEG hydrogel particles were sized of around
15
1000 micron, however most of the hydrogel particles shows distribution with mean diameter of
16
280 micron. The contact points between the hydrogel particles in the image show the distortion
17
of the spheres without agglomeration of the particles, indicating the mechanical integrity of the
18
particles.
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The reversible behavior observed for the MEG hydrogel particles is shown schematically in
20
Figure 5. When the MEG hydrogel particles are in contact with the gas molecules under
21
conditions where hydrate can form, a hydrate shell forms on the surface of the hydrogel particles
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as the concentration of gas is highest on the surface. The hydrate shell grows inward with
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increasing the thickness of the shell. The diffusion of gas molecules into the hydrogel core is
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limited by the hydrate shell. The decreasing driving force for hydrate formation due to increasing
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concentration of MEG inside hydrogel core also prevents further growth of the hydrate shell.
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Upon dissociation of the hydrate shell, evolved free water molecules are quickly absorbed back
4
into the PAM-co-AA polymer network and the hydrogel particle recovers its original shape.
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MEG molecules migrate back into the surface of the hydrogel particles as witnessed by Raman
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spectra. Theoretically when the MEG hydrogel particles arrive in the inlet facilities on the
7
offshore platform, they must be separated from the gas and liquid hydrocarbon stream. As the
8
MEG hydrogels retain the water phase inside the hydrogel core and can be suspended in liquid
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phase without aggregation simple filtration or gravitational methods could be used to separate
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the hydrogel particles from the bulk liquid phase. After they could be routed to a regeneration
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process to increase the MEG concentration of the hydrogel particles.
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Figure 5. (Top) Schematic of injecting MEG hydrogel into subsea flowline to manage the risk of hydrate plug formation. (Bottom-left) MEG-20 hydrogel particles suspended in decane phase. (Bottom-right) Hydrate shell formation on the surface of a MEG-PAM Hydrogel particle.
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4 There can be many methods available to regenerate the MEG hydrogel particles including
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heating, vacuum distillation, and chemical treatment. Thermogravimetric analysis (TGA) was
7
performed on the recovered MEG hydrogel particles to find out the possibility of regenerating
8
MEG hydrogel particles using a conventional heating method. The TGA for MEG 20.0 wt% bulk
9
solution was also carried out. Figure S3 shows the analysis for (a) the 20.0 wt% MEG bulk
10
solution and (b) MEG-20 hydrogel particles. The TGA pattern for 20.0 wt% MEG solution was
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quite different with that of MEG-PAM hydrogel as the water was evaporated completely at
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around 100 oC and remaining MEG was removed at 150 oC. It clearly represents the evaporation
13
of water from MEG by heating. The conventional regeneration method in offshore platforms is
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the distillation of water from MEG solution using the difference of boiling temperature between
15
water and MEG. For MEG hydrogel particles, as the polymer structure holds water in their
16
structure, 43% of water was evaporated at 100 oC and remaining water was completely
17
evaporated at around 150 oC together with MEG. The solid PAM-co-AA polymer was obtained
18
after completing temperature cycle and the initial amount of PAM-co-AA was recovered, which
19
was approximately 13 wt%. The unique evaporation pattern in the presence of PAM-co-AA
20
needs to be investigated further, however the TGA analysis strongly suggested the conventional
21
heating method can be used to regenerate the MEG hydrogel particles. By controlling the
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temperature cycle, the concentration of MEG hydrogel might be controlled. More comprehensive
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works will be carried out to find the optimized regeneration method for the MEG hydrogel
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particles.
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IV. Conclusions To investigate the structure and composition of hydrate on the surface of MEG hydrogel particles,
3
Raman spectroscopy and microscopy were used to obtain the Raman spectra and the images.
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Raman spectroscopic analysis clearly indicate that hydrate shell is formed on the surface with
5
ethane peak at 999 cm-1, indicating the formation of structure I hydrate. However this peak
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disappeared at 243 K upon dissociation of hydrates, instead the Raman peaks associated with
7
MEG increased, suggesting the migration of MEG molecules back into the surface upon the
8
dissociation of hydrate shell. The hydrate growth rate on the surface of MEG hydrogel particles
9
were investigated using a high pressure autoclave. The results revealed that due to the hydrate
10
shell formation the mass transfer of gas molecules inside the hydrogel core is retarded and low
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hydrate volume fraction of 0.11 is obtained, which is lower than bulk water, 0.41, MEG 20.0
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wt% bulk solution, 0.17. Moreover, the presence of MEG in MEG hydrogel particles induces
13
slow growth period for 30 min since hydrate onset, during which the growth rate was only
14
0.0005 vol.frac./min, but the rate picked up to 0.0051 vol.frac./min before reaching the hydrate
15
volume fraction of 0.11. The growth rate for MEG hydrogel particles were less than bulk water,
16
MEG 20.0 wt% solution, and even hydrogel particles without MEG. It is presumed that the
17
advantage of under-inhibition concept was commingled with the benefit of using hydrogel
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particles. The TGA analysis also indicate the conventional heating method can be modified to
19
regenerate the MEG hydrogel particles. The obtained results in this work provide a better
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understanding of the hydrate formation characteristics on particles, and present an insight to
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develop an advanced hydrate management strategy using the MEG-PAM hydrogel particles.
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AUTHOR INFORMATION
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Corresponding authors: Yutaek Seo and Colin D. Wood
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Notes
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The authors declare no competing financial interests.
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ACKNOWLEDGMENT
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This work was supported by the Technology Innovation Program (10060099) funded by the
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Ministry of Trade industry & Energy (MI, Korea) and also partially supported by the Industrial
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Strategic Technology Development Program (No. 10045068) of the Korea Evaluation Institute of
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Industrial Technology (KEIT) funded by MOTIE.
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System, J. Phys. Chem. B. 118, 9065-9075.
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Li, X., Hemmingsen, P.V., Kinnari, K., 2011, Use of under-inhibition in hydrate control
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strategies, Proceedings of the 7th International confernce on gas hydrates, Edinburgh, Scotland,
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Incorporating Thermodynamic and Kinetic Hydrate Inhibitors, Energy Fuels. 30, 2741–2750.
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Hydrate Agglomeration with Polymer Hydrogels, Energy Fuels. 28, 4409-4420.
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Seo, Y., Kang, S.-P., Jang, W., 2009. Structure and composition analysis of natural gas hydrates:
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Sloan, E. D., Koh, C. A., 2008. Clathrate Hydrates of Natural Gases, 3rd ed., CRC Press (Taylor
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Palermo, T., 2009. Hydrates: state-of-the-art inside and outside flowlines, J. Pet. Technol. 61,
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M., Cooper, A. I., 2009. Reversible hydrogen storage in hydrogel Clathrate hydrates, Adv.
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Mater., 21, 2382–2386.
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Supporting information
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Figure S1. Entire range (300 to 3600 cm-1) of Raman spectra of MEG 20 hydrogel + ethane
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hydrate from 93 K, 153K, 213 K, and 243 K. The blue arrows indicate the C-C stretching bonds
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of ethane and MEG as seen in Figure 2.
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14 Commence cooling
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Pressure (MPa)
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280
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Temperature (K)
Figure S2. Pressure and temperature changes during cooling of bulk water, 20.0 wt% solution,
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hydrogel particles, and MEG-20 hydrogel particles. Solid lines indicate the hydrate equilibrium
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condition for bulk water (black), MEG 10.0 wt% solution (pink), MEG 20.0 wt% solution (blue),
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MEG 30.0 wt% solution (green), and MEG 43.0 wt% solution (red).
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Figure S3. Thermogravimetric analysis (TGA) of (a) MEG 20.0 wt% bulk solution and (b)
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MEG-20 hydrogel particle.
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ACCEPTED MANUSCRIPT Highlights for Manuscript entitled “Raman spectroscopic and kinetic analysis of hydrate shell formation on hydrogel particles containing Monoethylene Glycol”
1. This work investigate the formation of hydrate shell on the surface of hydrogels.
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2. Raman spectra indicates MEG molecules are excluded during hydrate shell formation. 3. The presence of MEG in hydrogel particles reduce the hydrate fraction less than 0.11.
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4. Reversible behaviour of MEG hydrogels can manage the risk of hydrate plug formation.
S1875-5100(17)30115-4
DOI:
10.1016/j.jngse.2017.03.009
Reference:
JNGSE 2107
To appear in:
Journal of Natural Gas Science and Engineering
Received Date: 30 October 2016 Revised Date:
28 February 2017
Accepted Date: 1 March 2017
Please cite this article as: Park, J., Kim, H., Seo, Y., Tian, W., Wood, C.D., Raman spectroscopic and kinetic analysis of hydrate shell formation on hydrogel particles containing monoethylene glycol, Journal of Natural Gas Science & Engineering (2017), doi: 10.1016/j.jngse.2017.03.009. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical Abstract
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Raman spectroscopic and kinetic analysis of hydrate shell
2
formation on hydrogel particles containing Monoethylene
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Glycol
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Juwoon Park1, Hyunho Kim1, Yutaek Seo1*, Wendy Tian2, and Colin D. Wood 2*
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Department of Naval Architecture and Ocean Engineering, RIMS, Collage of Engineering,
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Seoul National University, 1, Gwanak-ro, Gwanak-gu, Seoul, 151 - 744, Korea 2
CSIRO Energy, Australian Resources Research Centre, Kensington, WA, 6151, Australia
AUTHOR EMAIL ADDRESSES: [email protected] and [email protected]
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TELEPHONE: YS +82-2-880-7329; CDW +613-9545-8160
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FAX: YS +82-2-888-9298; CDW +618-6436-8555
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ABSTRACT
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Hydrate shell growth on hydrogel particles incorporating monoethylene glycol (MEG) was
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investigated using Raman spectroscopy and a high pressure autoclave apparatus. Surface images
4
of hydrogel particles covered by a hydrate shell are presented and the obtained Raman spectra
5
indicate MEG molecules were excluded from the formation of the hydrate shell. The Raman
6
spectra and surface imaging of the particles demonstrate that the particles remain intact after the
7
dissociation of the shell. In subsequent hydrate formation cycles, the hydrate fraction in the
8
liquid phase was determined from gas consumption measurements in a high pressure autoclave.
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The hydrate fraction reached only 0.11 for hydrogels containing MEG while a high hydrate
10
fraction of 0.41 was observed for a control system consisting of bulk water and decane mixture.
11
The growth rate in the presence of hydrogel particles was suppressed in the early stages of
12
hydrate formation, suggesting that the increasing MEG concentration inside the hydrogel core
13
may limit the further inward hydrate growth. After hydrate formation the hydrogel particles were
14
analyzed by thermogravimetric analysis (TGA), suggesting the possibility of recovering
15
hydrogel polymer through conventional heating methodologies. These results demonstrate that
16
injecting hydrogel particles containing high concentration of MEG may be feasible to manage
17
the risk of hydrate plug formation as they will absorb the free water resulting in the dispersed
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hydrogel particles among the hydrocarbon liquid phase.
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KEYWORDS Flow assurance, Gas hydrate, Hydrogel, Shell formation, Anti-agglomeration,
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Plug prevention.
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INTRODUCTION
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Gas hydrates are nonstoichiometric crystalline compounds, consisting of host water and guest
3
hydrocarbon molecules such as methane, ethane, and propane (Sloan and Koh, 2008; Sloan et al.,
4
2009). They are formed through hydrogen bonding between water molecules which enclathrate
5
guest molecules at high pressure and low temperature, conditions which are found in subsea oil
6
and gas flowlines. Industry practice has been relying on the injection of vast quantities of
7
alcohol- or glycol-based thermodynamic hydrate inhibitors (THI) to prevent hydrate formation in
8
subsea flowlines. However, handling large amount of THI in remote offshore fields significantly
9
increases the capital and operational expenditure, thus adversely affecting the economics of field
10
development. As such hydrate prevention strategies are now moving toward hydrate risk
11
management by either delaying the formation with polymer-based kinetic hydrate inhibitors
12
(KHIs) or preventing the agglomeration of hydrate particles by surfactant-based anti-
13
agglomerants (AAs), which are collectively known as low dosage hydrate inhibitors (LDHIs)
14
(Kelland, 2006; Clard and Anderson, 2007; Duchateau et al., 2009; Del Villano and Kelland,
15
2011)_).
16
Although there have been numerous applications of LDHIs in offshore oil and gas fields,
17
monoethylene glycol (MEG) is still a reliable solution for avoiding hydrate plug formation for
18
offshore fields due to its robust performance, negligible loss to gas phase, and lower Health,
19
Safety, and Environmental (HSE) issues (Sanford et al, 2011). The injection rate of MEG into
20
subsea flowlines is typically based on the lowest temperature and highest pressure conditions
21
that would be encountered in extended shut-in operation of the subsea production system. As the
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concentration of MEG in water may need to be up to 50 ~ 60 wt%, the industry is spending
23
significant operating and capital expenditure. As such a small reduction in the MEG injection
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rate can result in significant saving. Recent studies of hydrate formation kinetics have suggested
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that the amount of hydrate formed can be controlled in the presence of MEG in water where the
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concentration of MEG is lower than the value required to fully avoid the hydrate formation under
4
experimental conditions, a practice known as under-inhibition. Li et al. (2011) reported that
5
under inhibition can have an adverse effect where the concentration of MEG in water was in
6
range between 1.0 and 5.0 wt%. Experimental results showed that MEG enhanced hydrate
7
formation kinetics when added at the lower concentration than 5.0 wt%. It has also been
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observed that the hydrate plug formation potential increased to a maximum at MEG
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concentration of 10 to 15 wt%. The under-inhibition effect might be effective with MEG
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concentration of higher than 20 wt%.
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Hydrate plug formation involves growth of hydrate crystals followed by agglomeration which
12
results in a high resistance-to-flow for liquid hydrocarbon fluids. Joshi et al (2013) suggested the
13
hydrate particles would suspend homogeneously in the liquid phase during the early stages of
14
hydrate formation, however once the amount of hydrate was greater than a certain hydrate
15
volume fraction in the aqueous phase, heterogeneous particle distribution would be followed by
16
stationary bed formation before the hydrate plug formation. Akhfash et al. (2013; 2016) also
17
observed the transition of homogeneous distribution to heterogeneous segregation of hydrate
18
particles in liquid phase depending on the hydrate volume fraction. In terms of thermodynamics,
19
the presence of MEG in the liquid phase shifts the hydrate equilibrium to lower temperature and
20
higher pressure condition, thus reducing the driving force for hydrate formation. Recent studies
21
also suggest MEG induces low resistance-to-flow for hydrate slurries. Kim et al. (2014)
22
presented the amount of hydrate in liquid phase could be reduced less than 28% by adding 20 wt%
23
MEG while maintaining stable torque. Sohn et al. (2015) also showed that the resistance-to-flow
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was minimized with under-inhibition of MEG for water and decane mixtures with water cut of
2
60 % and 40%.
3
In our previous works, the potential use of polyacrylamide-based hydrogel particles for
4
preventing the agglomeration of hydrate particles by locking the water phase into particles which
5
ensured that the torque was stable during the hydrate formation (Seo et al., 2014; Park et al.,
6
2016). The use of hydrogel particles was further extended by incorporating mono ethylene glycol
7
(MEG), then their performance for hydrate inhibition was tested using a high pressure autoclave
8
by measuring hydrate onset time, hydrate fraction, and torque changes during hydrate formation.
9
Although the MEG containing hydrogels showed the best performance for forming low hydrate
10
fraction and maintaining stable torque further investigations are required on the role of MEG
11
during the hydrate formation process on the surface of the hydrogel particles, especially with a
12
spectroscopic method. Raman spectroscopy was used to identify the structural characteristics of
13
the gas hydrates formed on the surface of the hydrogel particle. The hydrate structure type and
14
distribution of guest molecules on the surface of hydrate particle was investigated from Raman
15
spectroscopic analysis. The obtained Raman analysis along with the macroscopic studies on
16
hydrate formation kinetics will provide comprehensive understanding on the effect of hydrate
17
formation on the hydrogel particle. Moreover there has been no systematic study on the
18
mechanism of swelling hydrogels in subsea flowlines and separation of hydrogel polymer from
19
the absorbed liquid phase.
20
This study represents the first effort to analyze the hydrate shell formed on the surface of a
21
hydrogel particle using Raman spectroscopy. The hydrate structure formed on the surface of the
22
hydrogel particle was investigated along with the presence of MEG on the surface of the hydrate
23
shell. Then TGA analysis was carried out for the hydrogel samples to investigate the separation
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of the hydrogel polymer with conventional heating. The formation rate of gas hydrate with and
2
without MEG in hydrogel particles was also investigated by measuring the gas consumption rate
3
during hydrate formation. The obtained results suggests that MEG molecules were excluded
4
from the hydrate shell during its formation and migrated inward, suggesting the under-inhibition
5
effect was coupled with water discretization using hydrogel particles.
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8
Synthesis of MEG-Hydrogel All chemicals for hydrogel particle synthesis were supplied by
9
Sigma-Aldrich and were used without further purification. These chemicals are as follows:
10
polyacrylamide-co-acrylic acid partial sodium salt (PAM-co-AA), Mw 520 000, Mn 150 000,
11
typical acrylamide level 80%; N-(3-(dimethylamino)propyl)-N′-ethylcarbodiimide hydrochloride
12
(EDC, commercial grade); N-hydroxysuccinimide (NHS, 98%); and 1,2-diamino ethane (EDA
13
>99%). The hydrogel synthesis has been explained in our previous work10, which involves the
14
carbodiimide-mediated cross linking (CMC) reaction of PAM-co-AA in an inverse suspension.
15
To incorporate MEG into the PAM hydrogel particles the dried PAM powder were swollen in 20
16
wt% MEG solution with final polymer concentration in the hydrogel of 13 wt%. The obtained
17
hydrogel particles were suspended in a liquid hydrocarbon such as decane where they maintained
18
their spherical shape.
19
Raman analysis of hydrate shell on MEG-hydrogel particle For the spectroscopic analysis of
20
hydrate shell on the surface of MEG-hydrogel, ethane was used to form hydrate with MEG
21
hydrogel particles. The primary objective of this Raman analysis was to observe the presence of
22
MEG in hydrate shell formed on the surface of MEG-hydrate particle. The synthesized MEG-
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hydrogel particles were exposed to ethane gas at 1oC for 3 days. After confirming the hydrate
2
formation visually, the samples are stored in liquid nitrogen to prevent the dissociation of
3
hydrate shell. Jobin-Yvon ARAMIS high resolution dispersive Raman microscope was used to
4
measure the Raman spectra by an electrically cooled (203K) CCD detector. 514.53 nm from Ar-
5
ion laser performed as the excitation source and the laser had 30mW intensity. The measurement
6
was carried out while increasing the temperature in step from 93K to 153K, 213K, and finally
7
243K using a Linkam TMS 94 temperature controller. The images of hydrate shell on the surface
8
of the MEG hydrogel particles were taken using the microscope, and the qualitative analysis of
9
hydrate shell composition was carried out from the obtained Raman spectra.
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High pressure autoclave experiments Hydrate formation kinetics were investigated during
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hydrate formation for four different systems including bulk water, 20 wt% MEG solution, PAM-
12
hydrogel, and MEG-hydrogel particles. To simulate a liquid phase composed of water and
13
hydrocarbon, distilled water, purchased from the OCI, and decane, supplied by SIGMA-
14
ALDRICH Inc, were mixed with the volumetric ratio of water phase, 60%. Synthetic natural gas
15
(CH4: 90 mol%, C2H6: 6 mol%, C3H8: 3 mol%, n-C4H10:1 mol%) and pure C2H6 were supplied
16
from the Special Gas Company (Daejeon, Korea), with a stated minimum purity of 99.995
17
mol%. The details of the high pressure autoclave is presented in our previous works. (Kim et. al.,
18
2014; Park et. al., 2016; Seo, et. al., 2014; Sohn, et. al., 2015)
19
To investigate the hydrate formation characteristics, a liquid phase (80 mL) was loaded into the
20
autoclave with total internal volume of 360 mL Decane and distilled water were used to
21
represent liquid hydrocarbon phase and water phase respectively, which is a control without
22
hydrogel polymer in aqueous phase. The volume ratio of water in liquid phase was 60% for all
23
experiments in this work whether it was bulk water or hydrogel particles, thus 48ml of water was
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mixed with 32 ml of decane. The concentration of MEG to investigate the effect of MEG on the
2
hydrate formation characteristics was 20 wt%, thus 12g of MEG was mixed with 48g water. For
3
studying hydrate formation in hydrogel particles, hydrogel particles were mixed with decane
4
instead of bulk water. The reactor was pressurized to 120 bar at 24 oC without mixing, then the
5
stirring was commenced at 600 rpm for 2 hours to saturate the liquid phase with natural gas. The
6
cooling of the system was commenced from 24 oC to 4 oC with a cooling rate of 0.25oC/min,
7
which was to simulate the cooling of hydrocarbon and water mixture in subsea flowlines. The
8
cooling rate can be varied depending on overall heat transfer coefficient of the flowline surface.
9
Once the temperature reached 4 oC, it was maintained at 4 oC for 10 hours to observe the hydrate
10
formation characteristics for the studied system. A total of 32 cycles of hydrate formation and
11
dissociation were repeated for i) bulk water, ii) 20.0 wt% MEG solution, iii) hydrogel with no
12
MEG, and iv) MEG hydrogel with MEG concentration of 20.0 wt% (MEG-20 hydrogel).
13
Hydrate formation rate and the volume fraction in the liquid phase was characterized by
14
analyzing pressure decrease due to gas consumption during the hydrate formation.
15
Thermogravimetric analysis (TGA) TGA for MEG 20% aqueous solution and recovered
16
MEG-Hydrogel samples from Raman analysis were carried out using a Setsys 16/18 (France,
17
Setaram). The temperature ramp went from 25 oC to 325 oC with heating rate of 5 oC per min
18
under nitrogen injection of 30mL/min. Comparison of the TGA curves between the MEG 20 wt%
19
solution and the MEG-20 hydrogel was made to confirm the possibility of evaporating MEG
20
from the hydrogel particles using conventional heating method.
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RESULTS AND DISCUSSION
2
Spectroscopic analysis of hydrate shell on hydrogel particles
3
It was presumed that hydrate would form on the surface of hydrogel particles and the
4
composition change on the surface of hydrogel was expected due to exclusion of MEG during
5
the hydrate formation. The Raman spectroscopic study was carried out along with optical
6
imaging to investigate the composition of hydrates formed on the MEG-20 hydrogel particle.
7
The formation of hydrate on the MEG-20 hydrogel particle was visually confirmed (Figure 1),
8
and the Raman spectroscopic observation for the region of C-C stretching for ethane and MEG
9
provides evidence of the hydrate structure and its composition. The temperature of the Raman
10
probe was maintained at 93K initially to ensure that there were significant amounts of crystals on
11
the surface of the hydrogel particle. The melting and boiling temperatures of ethane are 90.4 K
12
and 184.6 K, respectively. After the temperature was increased to 153 K (Figure 1b) there were
13
minimal changes to the shape of the surface. The crystals on the surface of the MEG-20 hydrogel
14
were still present at 213K as confirmed in the spectra(Figure 1c), however 243 K they were
15
dissociated (Figure 1d). The obtained images in Figure 1 confirmed the formation of crystal
16
structures on the surface of MEG-20 hydrogel particle, which would be structure I hydrate
17
formed from ethane and water. After the dissociation of crystal structures, the MEG-20 hydrogel
18
particle recovered its original sphere shape without losing water to the surrounding environment,
19
confirming the performance of polymer PAM-co-AA even at cryogenic conditions. The exact
20
hydrate structure and its composition was confirmed from the analysis of Raman spectra.
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Figure 1. MEG (20wt%)-PAM-hydrogel with ethane hydrate on the surface. When the temperature was increased, the hydrate shell started to dissociate and no hydrate was observed in the final image at 243K
6
Figure S1 shows the Raman spectra obtained in the range of 300 and 3600 cm-1 for the MEG-20
7
hydrogel particle while maintaining temperature at 93K, 153K, 213K, and 243K. The C-H
8
stretching bond of MEG molecules would appear at 2888 and 2940 cm-1, while the C-H
9
stretching bond of ethane would appear at 2888 and 2943 cm-1(Seo et al., 2009; Kim et al.,
10
2014). They were superimposed on each other as shown in Figure S1 and were difficult to
11
discriminate, although two sharp peaks seemed to disappear at 243 K. The presence of hydrate
12
structure was confirmed with the C-C stretching bond of ethane molecules in Figure 2, where the
13
Raman spectra obtained in the range of 300 and 1600 cm-1 were focused. The Raman peak at 999
14
cm-1 presents the C-C stretching bond of ethane molecules encaged in large cage of structure I
15
hydrate, which can be confirmed between 93K and 213K, but is not shown at 243K. These
16
Raman spectra were well aligned with the images shown in Figure 1, suggesting the ethane
17
hydrate was formed and stayed on the surface of the MEG-20 hydrogel particle between 93K and
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213K, but dissociated when the temperature was increased to 243K. This is due to the fact that
2
the dissociation temperature of ethane hydrate in MEG 20 wt% solution was estimated to be
3
240.4K at atmospheric pressure by CSM Gem. The presence of structure I hydrate on the surface
4
of the MEG-20 hydrogel particle was confirmed from the images and Raman spectra in Figure 1
5
and 2. However the intensities of the peaks for MEG changed adversely, small peaks from 93K
6
to 213K but high intensity peaks at 243K, suggesting the concentration of MEG changed on the
7
surface of the MEG-20 hydrogel particle during the process of hydrate formation and
8
dissociation.
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Figure 2. Raman spectra obtained in a focusing area at 93K, 153K, 213K, and 243K, respectively. When the temperature increased, the hydrate shell dissociate and no hydrate was
12
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observed at 243K. Note the Raman spectra range is between 300 and 1600 cm-1 to focus on C-C stretching bond of ethane and MEG molecules.
4
The concentration of ethane would be highest at the interface between the ethane gas and the
5
MEG-20 hydrogel. Hydrate forms on the surface of the hydrogel particles, during which the
6
MEG and polymer are excluded from the hydrate structure. Eventually the formation of ethane
7
hydrate ceases due to mass transfer limitations through the hydrate shell while a separate solid
8
phase covers the surface of the MEG-20 hydrogel particle. As the MEG molecules cannot
9
participate in the hydrate structure, the molecules are excluded from the hydrate crystal structure
10
during the formation process and are believed to be concentrated inside the hydrogel core. This is
11
the most likely scenario because the MEG is more soluble in water within the particle than in the
12
surrounding gas phase. Most of the solid phase observed in Figure 1 (a) to (c) is the ethane
13
hydrate while only a small amount of MEG exists as can be seen from the Raman spectra for
14
ethane and MEG in Figure 2. Ethane remains in the large cages of structure I hydrate up until
15
213 K from the Raman peak at 999 cm-1. However, at 243K the Raman peak for ethane in
16
hydrate cages disappears, suggesting the hydrate shell is dissociated and only dissolved MEG
17
remains on the surface of the hydrogel particle. It is noted that the intensity of the Raman peaks
18
associated with MEG (866, 1050-1150, 1459 cm-1) increased as the temperature was raised to
19
243K. Upon complete dissociation of the hydrate, the MEG molecules is absorbed back into the
20
surface of the MEG-20 hydrogel particle again while the hydrogel particle gains its original
21
sphere shape (Figure 1 (d)).
22
Overall the MEG-20 hydrogel shows reversible behavior in a hydrophobic environment during
23
hydrate formation and dissociation. This reversible behavior of the MEG-20 hydrogel particle
24
would be beneficial to utilize the MEG-20 hydrogel for managing the hydrate formation risks in
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subsea flowlines. However more investigations on the exact process of hydrate formation on the
2
surface of the MEG-20 hydrogel particle, including hydrate growth rate and volume fraction
3
comparing to bulk solution, are further required in mixing condition.
4
Hydrate formation kinetics as determined using a high pressure autoclave
5
The hydrate formation experiments were carried out in a high pressure autoclave with continuous
6
cooling and mixing of fluids at 600 rpm to ensure that the decane and water/hydrogel were
7
completely mixed. Figure 3 shows the pressure-temperature (PT) trace during the cooling of
8
natural gas and liquid mixture along with the hydrate equilibrium conditions. Figure S2 presents
9
the estimated hydrate equilibrium conditions in the presence of MEG in pressure range between
10
2.0 and 14.0 MPa. The experiments were carried out in isochoric condition and the pressure
11
continuously decreased with a rate that corresponds to the cooling of fluids from 281.1 K to
12
271.1 K until the hydrate equilibrium for water-decane-natural gas mixture was passed. When
13
the condition reached the point O1, hydrate formation was observed visually and the subcooling
14
temperature, i.e. temperature difference between equilibrium condition and onset condition, was
15
determined to be 5.6 K. Since the hydrate onset, fast pressure reduction was observed until the
16
point C1, then the PT trace returned to its initial cooling path due to gas consumption for hydrate
17
formation. The hydrate formation in the hydrogel and decane mixture started slightly earlier in
18
the point O2, resulting in a lower subcooling temperature of 4.5 K than bulk water and decane
19
however, the reduced pressure due to hydrate formation was much smaller than the bulk water
20
system.
21
The cooling was completed at 276.1 K and 10.0 MPa for water, decane, and natural gas mixture.
22
The standard industry practice for completely avoiding hydrate formation, is in this system
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would require a MEG concentration in the aqueous phase to exceed 43.0 wt% in order to shift
2
the hydrate equilibrium conditions to lower temperature and higher pressure region than the
3
cooling path of the fluids, as seen in Figure 3. The addition of 20.0 wt% MEG (i.e., under
4
inhibited system) in the aqueous phase would not be able to prevent the hydrate formation with
5
the cooling path and would induce the hydrate formation. However, the resulting hydrate onset
6
condition for MEG 20.0 wt% bulk solution and MEG-20 hydrogel particles were shifted to a
7
lower temperature region because the hydrate equilibrium condition was shifted in the presence
8
of MEG as well. From the calculated equilibrium condition for MEG 20.0 wt% MEG solution
9
with natural gas, the resulting subcooling temperature was 7.5 K in the hydrate onset condition
10
point O3. For the MEG-20 hydrogel particles and decane mixture, the subcooling temperature
11
from the onset condition in the point O4 was 5.8 K. The pressure reduction due to hydrate
12
formation was relatively small for both MEG 20.0 wt% bulk solution and MEG-20 hydrogel
13
particles. The addition of MEG increased the subcooling temperature from 5.6 K to 7.5 K,
14
however the MEG-20 hydrogel particles showed less subcooling temperature of 6.8 K,
15
suggesting that the hydrate formation happened faster in MEG-20 hydrogel particles than in
16
MEG 20.0 wt% bulk solution.
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12.0 Bulk water Hydrogel MEG 20wt% solution MEG-20 hydrogel Hydrate Onset conditions
11.0
O2.Onset-Hydrogel O1.Onset-Bulk water
O4.Onset-MEG Hydrogel O3.Onset-MEG solution C3.Complete-MEG solution
C2.Complete-Hydrogel
C4.Complete-MEG Hydrogel C1.Complete-Bulk water
10.0
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11.5
Commence cooling
MEG 20wt%
MEG 43wt% 275
280
285
290
MEG 0 wt% 295
300
Temperature (K)
Figure 3. Pressure-Temperature profile during the hydrate formation form bulk water + decane, hydrogel + decane, MEG 20wt% bulk solution + decane, and MEG-20 hydrogel + decane. The solid lines represent the hydrate equilibrium curves for bulk water (black), MEG 20 wt% solution (blue), and MEG 43 wt% solution (red).
6
The hydrate formation on the surface of hydrogel particles occurred at a faster rate than the bulk
7
water phase, possibly due to dispersion of hydrogel particles and enhance mass transfer of gas
8
molecules through the particles (Su et al., 2009). Figure 3 confirms that the hydrate formation
9
characteristics are very different when the system is under inhibited and in the presence of
10
hydrogel polymer. Further analysis on hydrate volume fraction change in liquid phase was
11
carried out from the pressure change in Figure 3 as a function of time.
12
Table 1 and Figure 4 present the hydrate formation characteristics for bulk water, hydrogel
13
particles, MEG 20.0 wt% solution, and MEG-20 hydrogel particles. The aqueous phase was
14
mixed with decane with watercut 60% for every experiments. At least eight experiments were
15
carried out for each system to determine the average values of water conversion to hydrate,
16
hydrate fraction in liquid phase, and growth rates. Figure 4 shows the hydrate fraction changes in
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liquid phase as a function of time, where time zero indicates the hydrate onset point in Figure 3.
2
The consumed gas molecules due to hydrate formation was estimated from the pressure
3
difference between monitored moment and calculated pressure with the assumption of no hydrate
4
formation. This procedure has been suggested as a method for hydrate formation study in a flow
5
wheel and an autoclave systems (Ke et al., 2011; Hemmingsen, et al., 2008). The hydrate
6
fraction, Φhyd in the liquid phase at 150 min since the hydrate onset was calculated from the
7
following equation.
+
+(
−
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where Vw is the volume of water, Vw,conv is the volume of the water converted to hydrate, Vdecane is
9
the volume of decane, and Vhyd is the volume of hydrate calculated from the molecular weight
10
and density of hydrates calculated at a given time. The hydration number was assumed 6.5. As
11
the volume fraction of decane in the liquid phase was 0.4, the maximum hydrate fraction in
12
liquid phase can be 0.6 in case of complete conversion of water to hydrate.
13
As hydrate nuclei grew while consuming the dissolved gas molecules in bulk water phase, the
14
linear growth rate appeared in early stage of hydrate growth. Figure 4 showed the initial growth
15
rate of hydrates in bulk water was 0.0082 vol.frac./min before an inflection point at around 30
16
min. Hydrates growth continued until the hydrate fraction reached 0.4 within 60 min since the
17
hydrate onset, where the water conversion into hydrate was 60.2 %. The addition of PAM-co-AA
18
transformed the bulk water into discretized hydrogel particles mixed with decane and the
19
obtained hydrate growth for hydrogel particles was shown in Figure 4. The initial growth rate
20
was 0.0118 vol.frac./min, which was much faster than in bulk water. As the aqueous phase was
21
already dispersed in particulate format, mass transfer of gas molecules into hydrogel particles
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was enhanced. Although fast formation rate was achieved in hydrogel particles, inward growth
2
of hydrate shell on the surface of hydrogel particle results in a barrier that retards the mass
3
transfer of gas molecules to the interior of the hydrogel particles. The hydrate fraction reached
4
0.1 at 10 min after the onset, then continued to 0.17 at 150 min with water conversion of 24.1 %.
5
For MEG 20.0 wt% bulk solution hydrate growth was slow for the first 60 mins initially after the
6
hydrate onset. The slow growth rate was 0.0005 vol.frac./min, an order of magnitude less than in
7
bulk water. The growth rate surged into 0.0101 vol.frac./min at 70 min after the onset, which is
8
close to the rate for bulk water. Hydrate kept growing until the hydrate fraction in the liquid
9
phase reached 0.2 at 90 min. The final hydrate fraction at 150 min was 0.22 with water
10
conversion to hydrate of 31.3 %. These results suggested that the MEG molecules in the aqueous
11
phase resulted in strong hydrogen bonding with the water molecules which retard the
12
crystallization into hydrate structures.
13 14 15
Table 1. Hydrate formation characteristics for bulk water and hydrogel systems. rslow: slow growth period due to inhibition of hydrate formation, rfast: fast growth period limited by mass transfer.
Bulk water
MEG 20.0 wt% solution MEG-20 hydrogel 16
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rfast (vol.frac./min)
5.6
60.2
0.41
-
0.0082
4.5
24.1
0.17
-
0.0118
7.5
31.3
0.22
0.0005
0.0101
5.8
15.4
0.11
0.0005
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0.3
0.2
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Hydrate fraction in liquid phase
0.4
0.1
0.0 0
30
90
120
150
Figure 4. Hydrate volume fraction changes in liquid phase as a function of time for bulk water, hydrogel particles, MEG 20.0 wt% solution, and MEG-20 hydrogel particles.
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For MEG-20 hydrogel particles mixed with decane, the slow growth period was also observed
6
until 30 min after the onset and the growth rate was just same with the MEG 20.0 wt% bulk
7
solution, 0.0005 vol.frac./min. Fast hydrate growth was followed with a rate of 0.0051
8
vol.fac./min, but the rate was less than in bulk water. The hydrate growth on the surface of
9
MEG-20 hydrogel particles reached 0.10 at 70 min, then slowly reached 0.11 at 150 min with
10
water conversion into hydrate of 15.4 %. As we observed in Raman spectroscopic analysis, MEG
11
molecules on the surface of MEG-20 hydrogel particles would migrate inside the hydrogel core
12
during the inward growth of hydrate shell. As the MEG molecules migrated inside, fast hydrate
13
formation would occur. However, the hydrate formation reached steady-state early because of
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the mass transfer limitation through the hydrate shell formed on the surface of the hydrogel
2
particle. The resulting hydrate fraction and water conversion into hydrate was less than other
3
aqueous system, i.e. 15% less than for bulk water. It can be concluded that the advantage of
4
employing under-inhibition was commingled with the effect of transforming the aqueous phase
5
into particulate formats, as seen in slow growth period and less hydrate fraction in liquid phase.
6
Moreover, hydrates only formed on the surface of MEG-20 hydrogel particles without sintering
7
or aggregation of the particles.
8
Unlike bulk water, there was no acceleration in hydrate growth for MEG-20 hydrogel particles,
9
as shown in Figure 4. Slow initial growth was followed by fast growth, but soon reached steady-
10
state. The mechanism of hydrate growth is different between bulk water and MEG-20 hydrogel
11
particles. Compared to fast formation in bulk water, MEG-20 hydrogels do not show any
12
evidence of hydrate plug formation. Figure 4 shows the hydrate growth on the surface of MEG-
13
20 hydrogels steadily decreases with time and the hydrate fraction in liquid phase reaches only
14
0.11. Therefore, the concentration of MEG in the hydrogel particles slightly increases to 24 wt%
15
considering the loss of water into hydrate shell.
16
Formation and regeneration of MEG hydrogel particles
17
Figure 5 shows the schematic diagram of employing MEG hydrogel particles in subsea flowline
18
to manage the risk of hydrate plug formation. The MEG hydrogel particles with MEG
19
concentration of 80 ~ 90 wt% are carried into the subsea tree to be mixed with produced
20
hydrocarbon and free water stream (Figure 5 A). MEG is a polar compound and has a strong
21
interaction with the carbon-backbone of the polymer structure. Thus MEG is absorbed
22
into the hydrogel particle in a similar way to water, which is favorable for delivering the MEG
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into a subsea flowline without loss to the surrounding environment. The swelling ratio of PAM-
2
co-AA was previously measured as 25 for water, however it was about 10.0 for MEG (Sheng et
3
al., 2014). Therefore, MEG hydrogels with desired MEG concentration can be produced with the
4
pre-determined swelling ratio for MEG. Based on the amount of free water in the subsea flowline,
5
the volume of MEG hydrogel particles can be calculated to achieve the target concentration of
6
MEG when the MEG hydrogel particles are mixed with free water (Figure 5 top B), then
7
absorbing the free water until the maximum swelling ratio is reached. The final concentration of
8
MEG in MEG hydrogel particles could be lowered to 20.0 wt% which is an under-inhibited
9
condition. Although the temperature and pressure are suitable for hydrates formation, they are
10
formed on the surface of the MEG-20 hydrogel particles with slow growth period and low
11
hydrate fraction as discussed in the previous section (Figure 5 top C).
12
The images of MEG-20 hydrogel particles dispersed in decane phase are shown in Figure 5. In
13
this work, the MEG hydrogels were produced by swelling the dry polymer with water droplets
14
suspended in a liquid hydrocarbon phase. Several MEG hydrogel particles were sized of around
15
1000 micron, however most of the hydrogel particles shows distribution with mean diameter of
16
280 micron. The contact points between the hydrogel particles in the image show the distortion
17
of the spheres without agglomeration of the particles, indicating the mechanical integrity of the
18
particles.
19
The reversible behavior observed for the MEG hydrogel particles is shown schematically in
20
Figure 5. When the MEG hydrogel particles are in contact with the gas molecules under
21
conditions where hydrate can form, a hydrate shell forms on the surface of the hydrogel particles
22
as the concentration of gas is highest on the surface. The hydrate shell grows inward with
23
increasing the thickness of the shell. The diffusion of gas molecules into the hydrogel core is
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limited by the hydrate shell. The decreasing driving force for hydrate formation due to increasing
2
concentration of MEG inside hydrogel core also prevents further growth of the hydrate shell.
3
Upon dissociation of the hydrate shell, evolved free water molecules are quickly absorbed back
4
into the PAM-co-AA polymer network and the hydrogel particle recovers its original shape.
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MEG molecules migrate back into the surface of the hydrogel particles as witnessed by Raman
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spectra. Theoretically when the MEG hydrogel particles arrive in the inlet facilities on the
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offshore platform, they must be separated from the gas and liquid hydrocarbon stream. As the
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MEG hydrogels retain the water phase inside the hydrogel core and can be suspended in liquid
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phase without aggregation simple filtration or gravitational methods could be used to separate
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the hydrogel particles from the bulk liquid phase. After they could be routed to a regeneration
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process to increase the MEG concentration of the hydrogel particles.
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Figure 5. (Top) Schematic of injecting MEG hydrogel into subsea flowline to manage the risk of hydrate plug formation. (Bottom-left) MEG-20 hydrogel particles suspended in decane phase. (Bottom-right) Hydrate shell formation on the surface of a MEG-PAM Hydrogel particle.
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4 There can be many methods available to regenerate the MEG hydrogel particles including
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heating, vacuum distillation, and chemical treatment. Thermogravimetric analysis (TGA) was
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performed on the recovered MEG hydrogel particles to find out the possibility of regenerating
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MEG hydrogel particles using a conventional heating method. The TGA for MEG 20.0 wt% bulk
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solution was also carried out. Figure S3 shows the analysis for (a) the 20.0 wt% MEG bulk
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solution and (b) MEG-20 hydrogel particles. The TGA pattern for 20.0 wt% MEG solution was
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quite different with that of MEG-PAM hydrogel as the water was evaporated completely at
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around 100 oC and remaining MEG was removed at 150 oC. It clearly represents the evaporation
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of water from MEG by heating. The conventional regeneration method in offshore platforms is
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the distillation of water from MEG solution using the difference of boiling temperature between
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water and MEG. For MEG hydrogel particles, as the polymer structure holds water in their
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structure, 43% of water was evaporated at 100 oC and remaining water was completely
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evaporated at around 150 oC together with MEG. The solid PAM-co-AA polymer was obtained
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after completing temperature cycle and the initial amount of PAM-co-AA was recovered, which
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was approximately 13 wt%. The unique evaporation pattern in the presence of PAM-co-AA
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needs to be investigated further, however the TGA analysis strongly suggested the conventional
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heating method can be used to regenerate the MEG hydrogel particles. By controlling the
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temperature cycle, the concentration of MEG hydrogel might be controlled. More comprehensive
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works will be carried out to find the optimized regeneration method for the MEG hydrogel
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particles.
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IV. Conclusions To investigate the structure and composition of hydrate on the surface of MEG hydrogel particles,
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Raman spectroscopy and microscopy were used to obtain the Raman spectra and the images.
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Raman spectroscopic analysis clearly indicate that hydrate shell is formed on the surface with
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ethane peak at 999 cm-1, indicating the formation of structure I hydrate. However this peak
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disappeared at 243 K upon dissociation of hydrates, instead the Raman peaks associated with
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MEG increased, suggesting the migration of MEG molecules back into the surface upon the
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dissociation of hydrate shell. The hydrate growth rate on the surface of MEG hydrogel particles
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were investigated using a high pressure autoclave. The results revealed that due to the hydrate
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shell formation the mass transfer of gas molecules inside the hydrogel core is retarded and low
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hydrate volume fraction of 0.11 is obtained, which is lower than bulk water, 0.41, MEG 20.0
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wt% bulk solution, 0.17. Moreover, the presence of MEG in MEG hydrogel particles induces
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slow growth period for 30 min since hydrate onset, during which the growth rate was only
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0.0005 vol.frac./min, but the rate picked up to 0.0051 vol.frac./min before reaching the hydrate
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volume fraction of 0.11. The growth rate for MEG hydrogel particles were less than bulk water,
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MEG 20.0 wt% solution, and even hydrogel particles without MEG. It is presumed that the
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advantage of under-inhibition concept was commingled with the benefit of using hydrogel
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particles. The TGA analysis also indicate the conventional heating method can be modified to
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regenerate the MEG hydrogel particles. The obtained results in this work provide a better
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understanding of the hydrate formation characteristics on particles, and present an insight to
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develop an advanced hydrate management strategy using the MEG-PAM hydrogel particles.
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AUTHOR INFORMATION
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Corresponding authors: Yutaek Seo and Colin D. Wood
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Notes
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The authors declare no competing financial interests.
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ACKNOWLEDGMENT
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This work was supported by the Technology Innovation Program (10060099) funded by the
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Ministry of Trade industry & Energy (MI, Korea) and also partially supported by the Industrial
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Strategic Technology Development Program (No. 10045068) of the Korea Evaluation Institute of
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Industrial Technology (KEIT) funded by MOTIE.
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Supporting information
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Figure S1. Entire range (300 to 3600 cm-1) of Raman spectra of MEG 20 hydrogel + ethane
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hydrate from 93 K, 153K, 213 K, and 243 K. The blue arrows indicate the C-C stretching bonds
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of ethane and MEG as seen in Figure 2.
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14 Commence cooling
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Pressure (MPa)
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Temperature (K)
Figure S2. Pressure and temperature changes during cooling of bulk water, 20.0 wt% solution,
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hydrogel particles, and MEG-20 hydrogel particles. Solid lines indicate the hydrate equilibrium
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condition for bulk water (black), MEG 10.0 wt% solution (pink), MEG 20.0 wt% solution (blue),
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MEG 30.0 wt% solution (green), and MEG 43.0 wt% solution (red).
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Figure S3. Thermogravimetric analysis (TGA) of (a) MEG 20.0 wt% bulk solution and (b)
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MEG-20 hydrogel particle.
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ACCEPTED MANUSCRIPT Highlights for Manuscript entitled “Raman spectroscopic and kinetic analysis of hydrate shell formation on hydrogel particles containing Monoethylene Glycol”
1. This work investigate the formation of hydrate shell on the surface of hydrogels.
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2. Raman spectra indicates MEG molecules are excluded during hydrate shell formation. 3. The presence of MEG in hydrogel particles reduce the hydrate fraction less than 0.11.
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4. Reversible behaviour of MEG hydrogels can manage the risk of hydrate plug formation.