Based Kinetic Hydrate Inhibitors for Nucleation and Growth of Natural

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Performance of PNIPAM-based Kinetic Hydrate Inhibitors for Nucleation and Growth of Natural Gas Hydrates Juwoon Park, Kelly Cristine da Silveira, Qi Sheng, Colin D. Wood, and Yutaek Seo Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 20 Feb 2017 Downloaded from http://pubs.acs.org on February 21, 2017

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Energy & Fuels

1

Performance

2

Inhibitors for Nucleation and Growth of Natural Gas

3

Hydrates

PNIPAM-based

Kinetic

Hydrate

Juwoon Park,1 Kelly Cristine da Silveira,2,3 Qi Sheng,4 Colin D. Wood,4* Yutaek Seo,1*

4 5

of

1

Department of Naval Architecture and Ocean Engineering, Research Institute of Marine

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Systems Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul, 08826,

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Republic of Korea

8 9 10

2

Federal University of Rio de Janeiro, Institute of Macromolecules, Rio de Janeiro, Brazil 3

4

Polytechnic Institute, State University of Rio de Janeiro, Nova Friburgo, Brazil

CSIRO Australian Resources Research Centre, Kensington, WA 6152, Australia

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CORRESPONDING AUTHORS: Yutaek Seo and Colin D. Wood

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AUTHOR EMAIL ADRESSES: [email protected] and [email protected]

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TELEPHONE: YS +82-2-880-7329; CDW +61 8 6436 8701

15

FAX: YS +82-2-888-9298; CDW +618-6436-8555

ACS Paragon Plus Environment

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ABSTRACT

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Hydrate prevention strategies for offshore flowlines are now moving toward hydrate risk

18

management by delaying its nucleation and growth using water-soluble polymers, known as

19

kinetic hydrate inhibitor (KHI). This study investigates the natural gas hydrate inhibition

20

performance of three poly(N-isopropylacrylamide) (PNIPAM) based KHIs (PNIPAM-co-AA

21

(poly(N-isopropylacrylamide-co-acrylic acid), PNIPAM-co-Cp (poly(N-isopropylacrylamide-co-

22

cyclopentylamine), and PNIPAM-co-C4t (poly(N-isopropylacrylamide-co-tert-butylamine)) by

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determining the hydrate onset time, growth rate and resistance-to-flow using a high pressure

24

autoclave. These data are compared to three control groups (water, Luvicap solution and PVP)

25

under various cooling rates (0.25, 0.033, and 0.017 K/min). The results show that the nucleation

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of hydrate crystals was delayed in the presence of the KHI candidates as assessed using from the

27

onset time at different cooling rates. The effect of the KHI candidate on the hydrate growth

28

characteristics was also studied by determining the initial growth rate and torque changes with

29

increasing hydrate fraction in the liquid phase. The obtained results confirmed the synthesized

30

PNIPAM-based KHIs showed a high subcooling temperature, which is comparable to those of

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commercial KHIs. The modification of the base polymer (PNIPAM-co-AA), improves the

32

kinetic inhibition performance for PNIPAM-co-Cp (13.9 K, 12.5 K, and 7.8K for 0.25, 0.033,

33

and 0.017 K/min cooling rate, respectively) however results in decreased performance for the

34

PNIPAM-co-C4t (9.6 K. 9.9K, 7.6 K for 0.25, 0.033, and 0.017 K/min cooling rate, respectively).

35

After the hydrate onset, PNIPAM-co-C4t showed slow growth rate and stable torque during the

36

hydrate formation than PNIPAM-co-Cp, suggesting its potential role as a crystal growth

37

inhibitor. These results suggest that the performance of PNIPAM-based KHIs can be evaluated

38

with the holistic investigation on nucleation and growth of hydrate crystals.


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KEYWORDS Gas hydrate, flow assurance, kinetic hydrate inhibition, PNIPAM, nucleation

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delay, crystal growth inhibition

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1. Introduction

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Gas hydrates are nonstoichiometric crystalline compounds consisting of host water and guest

46

hydrocarbon molecules such as methane, ethane, and propane1. They are formed through

47

hydrogen bonding between water molecules that enclathrate guest molecules at high pressures

48

and low temperatures. These conditions are found in subsea oil and gas flowlines, as the

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production of offshore oil and gas fields have been moving into deeper and remote regions1-3.

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Industry practice has relied on the injection of vast quantities of alcohol- or glycol-based

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thermodynamic hydrate inhibitors (THIs) to completely prevent hydrate formation in subsea

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flowlines;4,5 however, handling large amounts of THI in remote offshore fields significantly

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increases the capital and operational expenditures, thereby adversely affecting the economics of

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field development. Consequently, hydrate prevention strategies are now moving toward hydrate

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risk management, through delaying nucleation of hydrate crystals, or the prevention of the

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agglomeration of the hydrate particles.

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Alternative strategies exist such as kinetic hydrate inhibitors (KHIs) low dosage hydrate

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inhibitors which are water soluble polymers that consist of a hydrophobic backbone with pendant

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groups such as amide or imide groups which can hydrogen bond with the hydrate. KHIs can

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delay hydrate formation by increasing the energy barrier to form hydrate nuclei, or by adsorbing

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to the surface of the hydrate crystals6,7. These include the homo- and co-polymers of N-vinyl

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carprolactam (VCap) and N-isopropyl acrylamide (NIPAM). Unlike THIs these inhibitors are

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effective at lower concentrations in the aqueous phase lowering the concentration from 30 wt%

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to 60 wt% down to 0.5 wt% to 3 wt% thereby lowering the cost of injection for storage space and

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simplifying the logistics of inhibition. However, the performance of KHIs is affected by factors

66

such as the subcooling, gas and oil composition, gas-oil ratio, and salinity. Among these factors,


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the subcooling is the most important one regarding the screening the performance of KHIs8-10.

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The subcooling indicates the temperature difference between the hydrate equilibrium

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temperature and the fluid temperature, i.e., the thermal driving force for hydrate formation11.

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High driving force induces a large number of the hydrate nuclei that are created in the aqueous

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phase, and this creates a large surface area for the KHIs to act on, requiring an increased

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concentration or modified polymer structures.

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The performance test for KHIs has generally been carried out with a cooling of the KHI

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candidate solution at a constant rate, and a measurement of the hydrate onset time that indicates

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how long the hydrate formation is delayed in the presence of the KHI candidate. The cooling of

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the hydrocarbon mixture in a subsea flowline will be affected by the seawater temperature and

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the insulation materials for the subsea flowline; however, most of the performance tests for KHIs

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have been carried out with a fixed cooling rate or under isothermal conditions12-14. It is still

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challenging to transfer the laboratory test result into conditions relevant to the field operation, so

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there have been many attempts to develop advanced test protocols to screen the KHI candidates;

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however, there is scarce literature studying the effect of varying cooling rate on the performance

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of KHI.

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Moreover, recent works by Anderson et al.15 suggested that KHIs disturb the gas capture into

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hydrate cages by their adsorption into the growing hydrate crystals, and this directly affects the

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growth of the hydrate as well as its nucleation. Along with the measurement of the hydrate onset

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temperature, a catastrophic growth temperature was measured to indicate the way that the KHIs

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effectively suppressed the growth of the hydrate crystals

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crystals involves a more complex processes of hydrate particle growth and their agglomeration,

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which ultimately form the solid hydrate plug. In the proposed mechanism for hydrate particle

13,14

; however, the growth of hydrate


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agglomeration, a hydrate shell forms on the water droplets, and then the cohesive interaction

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between the hydrate shell-covered particles results in the formation of multi-particle aggregates

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16,17

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particle aggregation; however, controlling the growth rate of hydrate is challenging, as the mass

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transfer for hydrate formation is too fast to avoid the sintering and cohesion of the hydrate shell-

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covered water droplets. Joshi et al.16 suggested that hydrate particles will aggregate faster when

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the hydrate fraction in the liquid phase is larger than 10 ~ 20 vol%, which is supported by high

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pressure autoclave studies by Akhfash et al.17,18. These results indicate that the performance of a

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KHI must be investigated with the amount of hydrate and the resulting resistance-to-flow to

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evaluate its performance as a potential crystal growth inhibitor.

. It seems that the agglomeration of hydrate particles can be prevented by avoiding multi-

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Our previous work using high throughput test for KHI candidates19 suggested the modification of

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PNIPAM-co-AA

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(PNIPAM-co-C4t) results in the increased hydrate onset time than pure water. However, the

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stochastic nature of hydrate nucleation induces scattering of the experimental results, demanding

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more investigation in larger scale. In this work, the performance of the KHI candidates was

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investigated with the holistic investigation on both nucleation and growth of hydrates as follows:

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1) Measuring the hydrate onset time with a varying cooling rate in the nucleation stage, then 2)

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measuring the torque changes of the fluids as a function of the hydrate fraction in the growth

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stage. By employing a holistic investigation, a more effective characterization of the PNIPAM-

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based KHIs was possible.

by

attaching

cyclopentylamine

(PNIPAM-co-Cp)

or

tert-butylamine

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Energy & Fuels

2. Materials and method 2.1. Materials

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All reagents were used without purification. The following materials were purchased from Sigma

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Aldrich: N-(3-dimethylaminopropyl)-N’-ethylcarbodiimide hydrochloride (EDC, commercial

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grade); N-hydroxysuccinimide (NHS, 98%); cyclopentylamine (Cp, 99%); tert-butylamine (C4t,

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99.5%). Poly(N-isopropylacrylamide-co-acrylic acid) (PNIPAM-co-AA) was synthesized in-

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house19. Luvicap and PVP were used as commercial inhibitors. Luvicap® EG HM was supported

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by BASF and polyvinylpyrrolidone (PVP K15, Mw = 9000 Da) was obtained from Ashland

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Chemical Co. An artificial natural gas (90.0 mol % CH4, 6.0 mol % C2H6, 3.0 mol % C3H8, and

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1.0 mol % n-C4H10) was supplied by Special Gas (Korea). Deionized water (purity: 99.99 mol%)

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was obtained from OCI.

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2.2. Synthesis of PNIPAM-derivative polymers

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In order to produce KHI candidates through post-synthetic modification, the base polymer

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poly(N-isopropylacrylamide-co-acrylic acid) (PNIPAM-co-AA) was synthesized by free radical

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polymerization19,20. The produced copolymer has 20 mol% of acrylic acid (AA), 80 mol% of N-

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isopropylacrylamide (NIPAM) and, as measured by gel permeation chromatography (GPC),

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ഥ௡ ) of 11.5 kDa and polydispersity PNIPAM-co-AA has a number average molecular weight (‫ܯ‬

129

index of 1.79. Through post-synthetic modification of the base polymer, similar molecular

130

weight polymers were synthesized, which allows the direct comparison of the different

131

functional groups in this study. After full activation of the acrylic acid content (20 mol%) via

132

carbodiimide mediated coupling (EDC/NHS), additional KHI candidates, PNIPAM-co-Cp and


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PNIPAM-co-C4t, were generated by adding cyclopentylamine (Cp) and tert-butylamine (C4t),

134

respectively. Synthesis protocol and characterization details can be found in our previous work17.

135 136

2.3. Hydrate formation kinetics experiments

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All the experiments were conducted using a high-pressure autoclave equipped with a

138

magnetically coupled stirrer with an anchor-type impeller. The reactor was constructed out of

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316 SUS. 80 mL of liquid was loaded into the reactor which has a total volume of 360mL, the

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total liquid consisted of a 60:40 volume ratio of water and hydrocarbon (in this work, decane).

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After the liquid was loaded, the cell was flushed three times using natural gas to remove any

142

residual air. Then, the autoclave was placed in the circulating bath (RW 2040G; Jeio Tech). For

143

the continuous stirring of the liquid in the cell, the anchor-type impeller was placed on a shaft

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and connected to a motor (BLDC 90). The cell was pressurized to 120 bar with a syringe pump

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at 24o C and the contents were stirred at 600 rpm to ensure the liquid was saturated with the

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natural gas. The fluids at the 600 rpm speed are in a turbulent regime with an approximate

147

Reynolds number of 32,000. Once the pressure and temperature became stable, the cell was

148

cooled down to the target temperature of 4 oC with various cooling rates (0.25 K/min, 0.033

149

K/min, and 0.017 K/min) using a constant cooling method under isochoric conditions. The

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temperature was maintained at 4 oC for 10 hours followed by ramping up to 28o C where the

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temperature was retained for three hours to dissociate the hydrate and eliminate any residual

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hydrate structures. It is worth noting that the dissociation temperature is below the measured

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cloud point for the polymers so they remain soluble under these conditions. The temperature,

154

pressure, and torque were measured using a PT 100 Ω (± 0.15o C), a transducer (0 bar to 200 bar,


8

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± 0.1 bar) and a torque sensor (TRD-50KC) (± 0.3 %), respectively. The temperature, pressure,

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and torque were logged using a data acquisition system. Five experiments were repeated for each

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PNIPAM-based KHIs (total 15) to average the hydrate onset time, subcooling temperature, and

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hydrate volume fraction. The other 15 experiments for the three control systems, pure water +

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decane mixture, 0.5 wt% PVP solution + decane mixture, and 0.5 wt% Luvicap solution +

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decane mixture, were carried out for a direct comparison with the 15 experiments for the

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PNIPAM-based KHI systems (0.5 wt% PNIPAM-co-AA solution + decane mixture, 0.5 wt%

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PNIPAM-co-Cp solution + decane mixture, and 0.5 wt% PNIPAM-co-C4t solution + decane

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mixture).

164


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3. Results and Discussion 3.1. Hydrate onset characteristics at different cooling rate

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Once the fluid temperature becomes lower than the hydrate equilibrium temperature, hydrate

168

nuclei would be formed instantly from the thermodynamic point of view; however, there needs

169

to be a driving force to initiate the nucleation. The primary function of kinetic hydrate inhibitors

170

is to delay the nucleation of hydrate crystals under the given subcooling condition. Therefore, the

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hydrate onset characteristics were analyzed according to the hydrate onset time and subcooling

172

temperature. In the constant cooling experiments, the hydrate nucleation point was determined

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by the rapid gas decrease from the hydrate formation. Table 1 presents the mean value and

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standard deviation over five repeat trials for the subcooling temperature (∆Tsub) and the hydrate

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onset time (tonset) at different cooling rate.

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The subcooling temperature is a significant factor presenting the required thermal driving force

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for the nucleation of hydrate crystals; thus, it can be a criterion for the estimation of the kinetic

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inhibition performance of the potential KHI candidate. The concentration of KHI candidate was

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0.5 wt% in aqueous phase, as in the control groups. The increase of the cooling rate indicates

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high thermal energy was applied over a certain period, resulting in a different hydrate nucleation

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point. For pure water, the subcooling temperature changes from 3.3 to 4.7 K with increasing

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cooling rate, while the onset time decreases 200.8 to 20.4 min. The addition of Luvicap and PVP

183

increases the subcooling temperature as well as the onset time, clearly indicating the polymer

184

structure acts on the hydrate nucleation process. The synthesized PNIPAM-base KHIs also

185

increased the subcooling temprature and onset time compared to pure water.

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Energy & Fuels

Table 1. Hydrate onset conditions for 0.5 wt% PNIPAM-based KHIs and control systems. Cooling rate

Subcooling temperature

Onset time

(K/min)

(K)

(min)

0.25

14.1 (0.1)

80.4 (2.0)

0.033

7.6 (0.1)

241.6 (0.9)

0.017

7.1 (0.3)

436.2 (21.0)

0.25

13.9 (0.9)

71.0 (8.6)

0.033

12.5 (0.1)

389.4 (2.4)

0.017

7.8 (0.8)

481.6 (47.8)

0.25

9.6 (0.6)

54.5 (5.4)

0.033

9.9 (1.0)

311.4 (1.1)

0.017

7.6 (1.1)

468.8 (65.6)

0.25

4.7 (0.6)

20.4 (2.1)

0.033

3.3 (0.1)

104.4 (3.9)

0.017

3.3 (0.1)

200.8 (4.7)

0.25

11.6 (0.2)

83.8 (5.2)

0.033

11.4 (0.9)

353.2 (26.3)

0.017

7.9 (0.9)

484.8 (56.2)

0.25

12.5 (1.2)

73.9(10.3)

0.033

10.2 (0.5)

323.0 (11.9)

0.017

8.5 (0.1)

548.5 (2.6)

PNIPAM derivatives

PNIPAM-co-AA

PNIPAM-co-Cp

PNIPAM-co-C4t

Pure water

Luvicap

PVP

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The standard deviation of subcooling temperature and onset time is shown in parenthesis.


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Figure 1 and Figure 2 showed the comparison of the hydrate onset time and subcooling

191

temperature between the PNIPAM-based KHIs and the control group. The PNIPAM-co-AA

192

solution showed subcooling temperatures of 14.1 K, 7.6 K, and 7.1 K with the cooling rates of

193

0.25 K/min, 0.033 K/min, and 0.017 K/min, respectively; furthurmore, the onset times were 80.4

194

min, 241.6 min, and 436.2 min. For the PNIPAM-co-Cp system the subcooling temperatures of

195

13.9 K, 12.5 K, and 7.8 K with the cooling rates of 0.25 K/min, 0.033 K/min, and 0.017 K/min,

196

respectively; moreover, the onset times are 71.0 min, 389.4 min, and 481.6 min. The

197

modification of the base polymer structure, PNIPAM-co-AA, with cyclopentylamine enhanced

198

the kinetic inhibition performance at the cooling rate of 0.033 K/min. For the PNIPAM-co-C4t,

199

the subcooling temperatures were 9.6 K, 9.9 K, and 7.6 K and the onset times were 54.5 min

200

311.4 min, and 468.8 min, respectively. We also included a PNIPAM-co-AA modified with tert-

201

butylamine PNIPAM-co-C4t which adversely affected the performance of PNIPAM-based KHI,

202

where the subcooling was higher than in pure water but lower than in PNIPAM-co-AA solution.

203

Compared to the performance of PNIPAM-based KHIs, the commercial KHIs, both Luvicap and

204

PVP, showed high subcooling temperature and long onset time. For fast cooling rate of 0.25

205

K/min, the subcooling temperature of PVP solution was 12.5 K and slightly less than that of

206

PNIPAM-co-AA and PNIPAM-co-Cp. When reducing the cooling rate to 0.033 K/min, Luvicap

207

solution showed the subcooling temperature of 11.4K, however the value was still less than that

208

of PNIPAM-co-Cp. For slow cooling rate of 0.017 K/min, the subcooling temperatures for KHI

209

candidates were obtained at around 8.0 K, except PNIPAM-co-AA solution. As seen in Figure 1

210

and Figure 3, the variation of subcooling temperature was less than 50% at the studied cooling

211

rate ranges, however the onset time increased almost seven times with decreasing the cooling

212

rate. Thus the subcooling temperature might need to be evaluated along with the onset time.


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214 215 216

Figure 1. Hydrate onset time comparison between the PNIPAM-based KHIs, commercial KHIs (Luvicap and PVP) 0.5 wt %, and water system. Error bars indicate the standard deviation.

217 218 219 220

Figure 2. Hydrate subcooling temperature comparison between the PNIPAM-based KHIs, commercial KHIs (Luvicap and PVP) 0.5 wt %, and water system. Error bars indicate the standard deviation.


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Figure 3 shows the hydrate onset time as a function of subcooling temperature with various

222

cooling rates. The leading KHI would result in a high subcooling temperature and long hydrate

223

onset time. For the application of KHI for offshore fields, the maximum value for the subcooling

224

temperature is the maximum difference between sea water temperature and hydrate equilibrium

225

temperature of hydrocarbon fluid, while the maximum value for the onset time is the longest

226

duration of hydrocarbon fluid inside the subsea flowlines. In this work, the maximum subcooling

227

temperature came from the target cooling temperature, 277.15K and the hydrate equilibrium

228

temperature, 293.15K. The maximum onset time was 600 min from the duration of experiment.

229

For the cooling rate of 0.25 K/min, the obtained onset time showed linear increase with

230

increasing the subcooling temperature depending on the added KHIs. Pure water showed the

231

shortest onset time 20.4 min and the lowest subcooling temperature 4.7 K while PNIPAM-co-

232

AA showed the longest onset time 80.4 min at the highest subcooling temperature 14.1 K. The

233

performance of PNIPAM-co-Cp was close to the base polymer and Luvicap was next. When

234

reducing the cooling rate to 0.033 K/min, the linear relationship shifted to a lower hydrate risk

235

area due to increased onset time. The performance of PNIPAM-co-Cp was the best in this

236

cooling rate and Luvicap followed. For the cooling rate of 0.017 K/min, commercial KHIs,

237

Luvicap and PVP, performed better than PNIPAM-based KHIs with slight difference. As seen in

238

Figure 3, the performance of KHIs can not be simply judged from the measured subcooling

239

temperature and would be better evaluated from the relationship between the subcooling

240

temperature vs. the onset time. Moreover, the cooling rate also affects the relationship. Figure 3

241

suggests that the cooling rate of 0.033 K/min maintains high subcooling temperature and long

242

onset time, thus the performance of KHIs placed in the low risk plane. Fast cooling rate 0.25

243

K/min induces short onset time whereas slow cooling rate 0.017 K/min results in low subcooling


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244

temperature. These results suggest that the insulation design for offshore flowlines may affect the

245

performance of the deployed KHI which must be considered in pipeline design.

246

Our previous work adopting High Throughput hydate test showed stochastic hydrate formation

247

for PNIPAM-co-AA, however confirmed an indicative inhibition peformance. Modification of

248

the polymers contributed to the inhibition performance by increasing the hydrophobic content in

249

the polymer structure. In this work, we again confirmed the perfomance of the PNIPAM-based

250

KHIs with quatitative analysis of hydrate onset time and subcooling temperature. The PNIPAM-

251

based KHIs and the commercial KHI have different functional groups and the structure and

252

percentage of each group varies the adsorbing behaviors on the hydrate cage, thereby inducing a

253

delayed hydrate nucleation. These phenomena would come from different functional groups in

254

each PNIPAM-based KHI. It is well known that commercial KHIs such as PVCap and PVP have

255

a polyethylene backbone that is linked to the pendant groups of one branch (five- or seven-

256

membered ring), including the amide group, and that the pendant groups enter the hydrate cage

257

when the KHI absorbs on the hydrate crystal with the polymer backbone stretches along the

258

hydrate-crystal surface. By modifying the structure of PNIPAM-co-AA the surface area of the

259

pendant group would change which can result in steric hinderance in terms of hydrate formation.

260

The change in polymer structure also affects the cloud point of the polymer as shown in our

261

previous work so using this synthetic method the performance of the KHIs can be modulated in

262

terms of hydrate peroformance and cloud point.


16

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263 264 265 266

Figure 3. Hydrate onset time as a function of subcooling temperature of PNIPAM-based KHIs with various cooling rates: 0.25 K/min (yellow region), 0.033 K/min (green region), and 0.017 K/min (blue region).

267 268


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Page 18 of 32

3.2. Hydrate growth characteristics at different cooling rate

270

After the hydrate onset, hydrate particles quickly consumed dissolved gas molecules in liquid

271

phase and grow further with mass transfer of gas molecules from vapor to liquid phase. The

272

hydrate plug formation would occur when the hydrate fraction in liquid phase was large enough

273

to accompany the agglomeration and bedding of the hydrate particles. Our previous work and

274

literature indicated that the resistance-to-flow due to the agglomeration and bedding of hydrate

275

particles became evident with hydrate fraction in liquid phase more than 0.10. KHI molecules

276

may inhibit the hydrate growth by attaching on the surface, thus reducing the growth rate in early

277

stage of hydrate formation process.

278

In this work the performance of crystal growth inhibition for the PNIPAM-based KHIs

279

(PNIPAM-co-AA, PNIPAM-co-Cp, and PNIPAM-co-C4t was investigated at different cooling

280

rates and compared to the control groups (pure water, Luvicap solution, and PVP solution). The

281

hydrate volume fraction was estimated from the gas consumption during the formation process,

282

and the amount of gas consumed for hydrate formation was calculated from the pressure

283

difference between the experimental and calculated pressures with the postulation that no hydrate

284

is formed 21,22. This procedure has been suggested as a method for the hydrate formation studies

285

in a flow wheel23 and in autoclave systems24-27. The hydration number was assumed 6.5 22. The

286

growth rate was estimated from the consumed amount of gas while the formation proceeds

287

linearly in early stage. The hydrate fraction in the liquid phase at 100 min after the hydrate onset

288

and the value at the end of each cycle are calculated to present the hydrate fraction in early and

289

last stage of the experiment. Table 2 presents the average values and the standard deviation of

290

the growth rate, hydrate fraction at 100 min, hydrate fraction at 600 min, and water conversion to

291

hydrate at 600 min.


18

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292

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Table 2. Hydrate formation kinetics characteristics under 0.5 wt% PNIPAM-based KHIs. Cooling rate (K/min)

PNIPAM-coAA

PNIPAM-coCp

PNIPAM-coC4 t

Pure water

Luvicap

PVP

Hydrate Hydrate Growth rate (vol. frac. / volume fraction volume fraction min) at 100 min at 600 min

Water conversion (%)

0.25

1.2×10-3

0.11 (0.02)

0.32 (0.03)

45.8 (3.9)

0.033

0.3×10-3

0.02 (0.00)

0.18 (0.02)

25.6 (2.4)

0.017

0.2×10-3

0.02 (0.01)

0.18 (0.01)

25.7 (1.5)

0.25

6.0×10-3

0.25 (0.01)

0.39 (0.03)

56.9 (4.1)

0.033

3.8×10-3

0.20 (0.01)

0.33 (0.03)

47.2 (5.3)

0.017

0.2×10-3

0.02 (0.01)

0.28 (0.02)

40.2 (3.1)

0.25

7.7×10-3

0.24 (0.02)

0.42 (0.02)

62.3 (3.9)

0.033

2.4×10-3

0.24 (0.05)

0.54 (0.08)

81.3 (13.5)

0.017

3.6×10-3

0.14 (0.15)

0.47 (0.06)

69.2 (10.2)

0.25

8.9×10-3

0.43 (0.02)

0.50 (0.02)

74.0 (3.9)

0.033

14.7×10-3

0.38 (0.03)

0.49 (0.02)

72.5 (3.7)

0.017

6.5×10-3

0.35 (0.01)

0.45 (0.02)

66.5 (2.8)

0.25

4.9×10-3

0.20 (0.06)

0.40 (0.06)

58.9 (9.8)

0.033

3.8×10-3

0.13 (0.01)

0.38 (0.03)

55.8 (4.2)

0.017

0.2×10-3

0.03 (0.02)

0.38 (0.04)

55.2 (6.5)

0.25

6.4×10-3

0.29 (0.03)

0.38 (0.02)

55.7 (3.8)

0.033

7.8×10-3

0.32 (0.02)

0.44 (0.02)

64.9 (2.6)

0.017

10.1×10-3

0.31 (0.01)

0.43 (0.04)

63.3 (6.5)

293

The standard deviation of the hydrate volume fraction at 100 min and 600 min, and water

294

conversion is shown in parenthesis.


19

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Page 20 of 32

295

Figure 4 shows the growth of hydrate for 100 min after the onset at each cooling rate. For every

296

cooling rate, pure water showed fast hydrate growth than other KHI-added systems. The hydrate

297

fraction reached 0.43 at 100 min with initial growth rate of 0.0089 vol. frac./min and increased

298

slightly more to 0.50 at 600 min at fast cooling 0.25 K/min. The addition of PVP reduces the

299

initial growth rate 0.0064 vol. frac./min and the hydrate fraction at 100 min was 0.29. Luvicap

300

performed better with the initial growth rate 0.0049 vol. frac./min and the hydrate fraction 0.13

301

at 100 min. The base polymer PNIPAM-co-AA out-performed Luvicap showing the initial

302

growth rate 0.0012 vol. frac./min and the hydrate fraction 0.11 at 100 min. However, by

303

modifying the structure into PNIPAM-co-Cp and PNIPAM-do-C4t showed a growth rate 0.0060

304

and 0.0077 vol. frac./min, and the resulting hydrate fraction was 0.24 and 0.25, respectively. As

305

seen in Figure 3 (a), the growth curve for PNIPAM-co-Cp and PNIPAM-co-C4t solutions placed

306

between PVP and Luvicap. PNIPAM-co-C4t showed very slow growth period during the first 15

307

min, however fast growth was followed soon after. It can be concluded that the base polymer

308

PNIPAM-co-AA outperformed commercial KHIs in terms of crystal growth inhibition, however,

309

further modification adversely affect its performance. This is important because it highlights

310

which functional groups can improve KHIs performance. The trends are more accurate than

311

using other synthetic routes (eg., free-radical co-polymerization) because this approach generates

312

all polymers with similar Mw, same composition and end groups.

313

Similar results were obtained for cooling rate of 0.033 K/min and 0.017 K/min. For the initial

314

growth after the onset, PVP has the fastest growth rate at slow cooling 0.017 K/min, and is even

315

faster than the pure water until the volume fraction of 0.3. The hydrate growth rate of the other

316

systems is slower than that of the pure water. The modified polymers, PNIPAM-co-Cp and

317

PNIPAM-co-C4t, performed better than PVP and close to Luvicap. The base polymer PNIPAM-


20

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Energy & Fuels

318

co-AA was the best. The initial growth rate was 0.003 and 0.002 vol. frac./min at cooling rate

319

0.033 and 0.017 K/min, respectively. The hydrate fraction was 0.02 at 100 min and reached 0.18

320

at 600 min, which was about half of the fraction obtained in Luvicap solution. The modification

321

of the base polymer increased the cloud point and enhanced the nucleation inhibition

322

performance, however it was not straightforward to the performance for the crystal growth

323

inhibition.

324

Figure 4 suggested the cooling rate also affected the performance of KHIs significantly during

325

the formation process. The rapid cooling down of the system would induce relatively higher

326

subcooling temperatures, leading to an excess energy supply for the hydrate formation; therefore,

327

the larger temperature difference of the solution under faster cooling rates was more favorable to

328

the fast hydrate growth than under slower cooling rates. Insulation of subsea flowlines would

329

decrease the cooling rate and would help to reduce the hydrate growth as seen in Figure 4 (c).

330

This was true for PNIPAM-based KHIs and Luvicap, however, PVP showed the increase of

331

initial growth rate from 0.0064 to 0.0101 vol.frac./min when reducing the cooling rate from 0.25

332

K/min to 0.017 K/min. As showed in previous section, PVP performed well to inhibit the hydrate

333

onset, however once the hydrate crystals started to growth, its performance as a crystal growth

334

inhibitor was not as effective as others. More investigation must be carried out to understand the

335

role of functional groups in KHIs during the formation process.

336 337


21

Energy & Fuels

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Page 22 of 32

(a)

(b)

(c)


22

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339 340 341

Energy & Fuels

Figure 4. Comparison of hydrate growth for PNIPAM-based KHIs, commercial KHIs (Luvicap and PVP) 0.5 wt % solutions, and pure water under cooling rate, (a) 0.25 K/min, (b) 0.033 K/min, (c) 0.017 K/min


23

Energy & Fuels

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Page 24 of 32

342

To investigate the relation between the hydrate volume fraction and the resistance-to-flow, the

343

torque with the hydrate growth was measured. Figure 5 (a) – (c) showed the measured torque

344

changes during the hydrate formation for PNIPAM-based KHIs and control groups at different

345

cooling rates. The resistance-to-flow is a qualitative parameter to estimate the flowability of

346

hydrocarbon fluid to keep transporting the hydrate particles without forming the hydrate plug. As

347

the sintering and bedding of hydrate particles occurs, lower flowability of the hydrocarbon fluid

348

would results in the torque increases. As seen for the pure water in Figure 5 (a) – (c), torque

349

started to increase when hydrate fraction increased more than 0.10 in liquid phase, suggesting the

350

agglomeration of hydrate particles would occur for larger hydrate fraction in liquid phase than

351

0.10. The growth rate of the pure water became faster with an increased torque after the 0.15

352

hydrate fraction, as seen in Figure 5 (a) and (b), which means that the hydrate that had formed in

353

the system changed from homogeneous to heterogeneous.

354

The hydrate growth and torque changes for PNIPAM-based KHIs showed different patterns than

355

commercial KHIs. PNIPAM-co-Cp and PNIPAM-co-C4t indicated torque increases once the

356

hydrate volume fraction increased more than 0.10. When the cooling rate was 0.017 K/min, the

357

PNIPAM-co-AA system represented the smallest growth rate and torque value with the least

358

hydrate volume fraction, followed by the PNIPAM-co-Cp system, as presented in Figure 5 (c).

359

The PNIPAM-co-C4t system showed a plateau, but a sudden increase during the hydrate fraction

360

reached the moment of 0.10. Overall, the base polymer PNIPAM-co-AA was the best to suppress

361

the torque increase during the hydrate formation, but its modification into PNIPAM-co-CP and

362

PNIPAM-co-C4t did not help to control the increase of resistance-to-flow. More investigation

363

will be carried out near future to understand the role of KHI during the agglomeration and

364

bedding of hydrate particles during the formation process.


24

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Energy & Fuels

365


25

Energy & Fuels

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Page 26 of 32

366 367 368 369 370 371

Figure 5. Relation between hydrate fraction and torque in representative cycle for PNIPAMbased KHIs, commercial KHIs (Luvicap and PVP) 0.5 wt % solutions, and pure water under the cooling rate, (a) 0.25 K/min, (b) 0.033 K/min, (c) 0.017 K/min.


26

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372

4. Conclusions

373

This study investigates the hydrate inhibition performance of three PNIPAM-based KHIs

374

(PNIPAM-co-AA, PNIPAM-co-Cp, and PNIPAM-co-C4t) by determining the hydrate onset

375

time, its growth rate, and resistance-to-flow using a high pressure autoclave. Comparison to three

376

control groups (water, Luvicap solution, and PVP solution) is also performed at various cooling

377

rates (0.25, 0.033, and 0.017 K/min). The modification of the base polymer structure, PNIPAM-

378

co-AA, with cyclopentylamine enhanced the kinetic inhibition performance at the cooling rate of

379

0.033 K/min. For the PNIPAM-co-C4t, the subcooling temperatures were 9.6 K, 9.9 K, and 7.6 K

380

and the onset times were 54.5 min 311.4 min, and 468.8 min, respectively. The modification

381

with tert-butylamine adversely affected the performance of PNIPAM-based KHI, where the

382

subcooling was higher than in pure water but lower than in PNIPAM-co-AA solution. The

383

obtained results confirmed the synthesized PNIPAM-based KHIs presented high subcooling

384

temperature, which is comparable to those of commercial KHIs. The obtained subcooling

385

temperature was plotted with the hydrate onset time, which suggested the cooling rate of 0.033

386

K/min maintains high subcooling temperature and long onset time, thus the performance of KHIs

387

placed in the low risk plane. Using the cooling rate to determine the performance of KHI

388

candidates is a new approach. After the hydrate onset, PNIPAM-co-C4t showed slow growth

389

rate, but stable torque during the hydrate formation was achieved with PNIPAM-co-Cp,

390

suggesting the base polymer PNIPAM-co-AA showed best performance as a crystal growth

391

inhibitor. These results suggested that the performance of PNIPAM-based KHIs were able to be

392

evaluated with the holistic investigation on nucleation and growth of hydrate crystals.

393


27

Energy & Fuels

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394

AUTHOR INFORMATION

395

Corresponding authors: Yutaek Seo and Colin D. Wood

396

Notes

397

The authors declare no competing financial interests.

Page 28 of 32

398 399

ACKNOWLEDGMENT

400

This work was supported by the Technology Innovation Program (10060099) funded by the

401

Ministry of Trade, Industry & Energy (MI, Korea) and also supported by the Global Leading

402

Technology Program of the Office of Strategic R&D Planning (OSP) funded by the Ministry of

403

Trade, Industry & Energy (10042424).

404 405


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Based Kinetic Hydrate Inhibitors for Nucleation and Growth of Natural

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