Inhibition effects of polysaccharides for gas hydrate formation in methane–water system

Journal Pre-proof Inhibition effects of polysaccharides for gas hydrate formation in methane–water system

Li Wan, Na Zhang, De-Qing Liang PII:

S0167-7322(19)31558-2

DOI:

https://doi.org/10.1016/j.molliq.2019.111435

Reference:

MOLLIQ 111435

To appear in:

Journal of Molecular Liquids

Received date:

16 March 2019

Revised date:

5 July 2019

Accepted date:

23 July 2019

Please cite this article as: L. Wan, N. Zhang and D.-Q. Liang, Inhibition effects of polysaccharides for gas hydrate formation in methane–water system, Journal of Molecular Liquids(2019), https://doi.org/10.1016/j.molliq.2019.111435

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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.

© 2019 Published by Elsevier.

Journal Pre-proof

Inhibition effects of polysaccharides for gas hydrate formation in methane–water system Li Wana, b, c, d, e, f, Na Zhanga, c, d, e, f, De-Qing Lianga, c, d, e *

a

Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences,

Guangzhou, Guangdong 510640, China Hunan Institute of Technology, Hengyang, Hunan 421002, China

c

CAS Key Laboratory of Gas Hydrate, Guangzhou, Guangdong 510640, China

d

Guangdong Provincial Key Laboratory of New and Renewable Energy Research and

e-

pr

oo

f

b

e

Pr

Development, Guangzhou, Guangdong 510640, China Guangzhou Center for Gas Hydrate Research, Chinese Academy of Sciences,

University of Chinese Academy of Sciences, Beijing 100049, China

rn

f

al

Guangzhou, Guangdong 510640, China

Jo u

*Corresponding Author.

E-mail address: [email protected]

ABSTRACT: Water-soluble polymers such as polyvinylpyrrolidone (PVP), PVCap, and Gaffix VC-713 are predominantly used as kinetic hydrate inhibitors (KHIs) in the petroleum and natural gas industry to control the formation of hydrates. However, very

few

of

them

exhibit

good

biodegradability.

Therefore,

developing

environment-friendly, biodegradable inhibitors is necessary. In this study, five natural polysaccharides, gum Arabic, sodium alginate, guar gum, carboxymethyl chitosan, 1

Journal Pre-proof

and starch, were screened as potential green inhibitors. Their hydrate inhibition performances were investigated using a high-pressure stainless steel cell and methane gas and were compared with that of a typical KHI, PVP K90. The impact of the polysaccharides on the hydrate microstructure was investigated using powder X-ray diffraction and Raman spectroscopy. All polysaccharides increased the maximum

f

subcooling of hydrate formation and inhibited the hydrate growth well below their

oo

respective maximum subcooling. Guar gum exhibited the best performance, most

pr

probably owing to the presence of the side anhydroglucose group in its skeleton.

e-

Remarkably, it lowered the gas uptake by ~30% as compared with that of pure water.

Pr

The next best performance was shown by sodium alginate. Starch and carboxymethyl chitosan also performed well until the subcooling exceeded the respective maximum

al

subcooling. In contrast, gum Arabic showed weak hydrate inhibition at low

rn

subcooling and rather started to promote hydrate formation when the subcooling was

Jo u

increased to 6 °C. Moreover, its hydrate promotion activity improved as the subcooling was further increased. Finally, the hydrate inhibition mechanism of the polysaccharides is discussed.

Keywords: Gas Hydrate; Hydrate Inhibitors; Polysaccharides; Gas Uptake

2

Journal Pre-proof

1. Introduction Natural gas hydrates are non-stoichiometric crystalline compounds formed by the combination of gas and water under certain high pressure and low temperature conditions [1-3]. According to the size and type of the water lattice, these crystalline compounds are mainly divided into three types: structure I, structure II, and structure

f

H [4, 5]. The unexpected formation of gas hydrates during the natural gas

oo

transportation and processing can lead to a series of hazards, such as the blockage of

pr

gas pipelines, clogging of the blowout preventer, and damage of the deep water

Pr

e-

drilling platform, causing serious economic and safety issues [6, 7].

An effective and economic method for controlling hydrate formation is the injection

al

of additives into pipelines. These additives, which are referred to as hydrate inhibitors,

rn

contain thermodynamic hydrate inhibitors (THIs) along with low-dosage hydrate

Jo u

inhibitors (LDHIs) (less than 1 wt %). However, THIs are recently being gradually replaced by new LDHIs that are effective at lower concentrations for both economic and environmental reasons [8]. LDHIs can prolong the induction time for hydrate nucleation and thus reduce hydrate growth (kinetic hydrate inhibitors (KHIs)) or prevent the agglomeration of hydrate crystals (antiagglomerants) [9]. Numerous water-soluble polymers such as polyvinylpyrrolidone (PVP), PVCap, and Gaffix VC-713 have been studied as effective additives [10, 11]. Unfortunately, these polymers are not widely applied owing to their poor biodegradability. Therefore, it is necessary to develop environment-friendly and biodegradable inhibitors [12-16]. 3

Journal Pre-proof

Antifreeze proteins (AFP) or antifreeze glycoproteins derived from insects, plants, fungi, or bacteria are the earliest discovered natural inhibitors to retard hydrate growth [17, 18]. The AFP molecule interacts with the water on the hydrate surface through van der Waals forces and hydrogen bonding facilitated by the hydroxyl (–OH) groups

f

of its four threonine residues [19, 20]. Meanwhile, amino acids have been proposed as

oo

the second group of environment-friendly additives [21-25]. Amino acids have simple

pr

structures and can be used as molecular models to study the inhibition mechanism [21,

e-

26]. Both the amine (–NH2) and carboxylic acid (–COOH) groups of amino acids can

Pr

readily form hydrogen bonds with water molecules, affording a good inhibition effect [27]. Ionic liquids, which are considered to be environment-friendly owing to their

al

high stability and low vapour pressure, have been recently identified as alternative

rn

hydrate inhibitors. They are recognized as low-dosage and dual-function gas hydrate

Jo u

inhibitors (i.e. induce both KHI and THI effects) [28-31]. The ability to design ionic liquids with specific functionalities can render them more suitable for some specific applications. However, ionic liquids are nonpolymeric, and their performance is lower than those of oligomers or polymers that contain multiple functional groups [32]. Some synthetic polymer KHIs have been shown to be biodegradable. Musa and co-workers [33] developed a copolymer of PVCap with polyvinyl alcohol, the performance of which is comparable to that of PVCap. The copolymer also has better biodegradability (76% in 28 days) because of the hydroxyl groups. Other biodegradable polymers such as polyaspartamides [16] and poly(2-alkyl-2-oxazaline) 4

Journal Pre-proof

[34] exhibit similar performances to that of PVP and PVCap. Furthermore, natural polymers such as hydroxyethylcellulose (HEC), starch, chitosan, and pectin have shown natural gas hydrate inhibition effects. HEC is one of the first studied natural polymers that show some KHI activity, but it shows a weaker inhibition activity than do other known KHIs. Thus, the research focus on HEC has now shifted to its

f

modification [35] or blending it with other polymers [36]. Starch is normally

oo

cationised before its use in industrial applications. Different types of starch that show

pr

weak KHI activities have been studied using methane, methane/ethane, and

e-

methane/propane gas mixtures [15, 37-39]. Chitosan is a polysaccharide obtained by

Pr

the deacetylation of chitin from the shells of crabs and shrimps and has been reported to delay the nucleation of methane and ethane hydrates [40]. The degree of

al

deacetylation of chitin may be varied from 60% to 100%. Further, pectin belongs to a

rn

family of heteropolysaccharides that contain D-galacturonic acid units, and some

Jo u

studies have demonstrated that it could inhibit methane hydrate formation at a subcooling temperature of 12.5 °C, and that it shows a better inhibition effect than does PVCap [41]. It has been speculated that the anhydroglucose unit of these polysaccharides can fit into the hydrate structure in the same fashion as the hydrophilic pendant lactam group of PVP and PVCap fits in [39, 40].

The objective of the current work is to screen a number of commercially available polysaccharides for natural gas hydrate inhibition and to systematically compare their inhibition efficiencies according to the structures. Five polysaccharides, namely, gum 5

Journal Pre-proof

Arabic, sodium alginate, guar gum, carboxymethyl chitosan, and starch, were chosen for this study. Among these, the first three are usually used as stabilizers in ice-cream to prevent or inhibit the growth of ice crystals. This is the first investigation of their performance as hydrate inhibitors in the petroleum and natural gas industry. In this study, we investigated the inhibition performances of the polysaccharides and

f

compared them with that of PVP. Further, powder X-ray diffraction (PXRD) and

oo

Raman spectroscopy were used to examine the influence of the polymers on the

e-

pr

microstructure of hydrates to further analyse the inhibition mechanism.

Pr

2. Experimental section 2.1. Materials

al

Analytical grade methane (99.99%) supplied by Guangzhou Yuejia Gases company

rn

was used for hydrate formation. Carboxymethyl chitosan with a degree of

Jo u

deacetylation of 80% or more was procured from Shanghai Macklin Biochemical Co. Ltd. Guar gum containing ≥83% galactomannan was supplied by Aladdin Chemicals, 5000–5500 cps, 200 meshes. Starch with 60%–100% solubility in cold water prepared from corn starch was supplied by Aladdin Chemicals. The starch, sodium alginate, and gum Arabic samples are of pharmaceutical grade. All the materials used in the present study are listed in Table 1, and the structures of the polysaccharides are shown in Figure 1.

6

Journal Pre-proof

Table 1. Chemicals used in the presented study Purity (%) Supplier

Methane

99.9

Guangzhou Yuejia Gases Co.

Gum Arabic

99

Yuanye Biochemical Co.

PVPK90

>95

TCI

Sodium alginate

>99

Tianjin Fuchen Chemical Reagents Factory

Starch

>98.0

Aladdin Chemicals

oo

f

Component

Carboxymethyl chitosan >99.5

Shanghai Macklin Biochemical Co. Ltd

≥99.0 Deionized/

Water

pr

Aladdin Chemicals

e-

Guar gum

Produced in the laboratory

Pr

distilled OH

*

O

*

NaOOC

HN

NaOOC OH O

O

al

O

HO

*

O

HO

O

Jo u

(a) OH

y

OH

rn

OH

* O

x HO

(b)

CH2OH

O

HO HOH2C *

OH O

HO

OH O

O

HOH2C *

OH O

* O

HO

O HO

n

(c)

* O

n

OH

(d)

O

N

n

(e) Fig. 1. Structures of various polysaccharides and PVP: (a) Carboxymethyl chitosan; (b) Sodium alginate; (c) Guar gum; (d) Starch; and (e) PVP. 7

Journal Pre-proof

2.2. Apparatus The schematic of the experimental device for hydrate formation is shown in Figure 2. The apparatus mainly consists of a high-pressure stainless steel cell with a total effective volume of 150 mL that can withstand a maximum working pressure of 20 MPa, pressure transducers (CYB-20S) with an uncertainty of ±0.02 MPa, a resistance

f

thermometer (PT100) with a maximum precision of ±0.1 K, a constant-temperature

oo

water-glycol bath (filled with 50/50 v/v of ethylene glycol and water), a buffer tank

pr

for precooling gas, a magnetic stirrer, a data acquisition system (to collect data once

Jo u

rn

al

Pr

e-

every 10 s), and a computer for recording the data.

Fig. 2. Schematic of the experimental apparatus used for hydrate formation. 2.3. Hydrate formation The hydrate formation at different subcooling was investigated by the isothermal cooling method, the details of which are available elsewhere [9, 42]. The cell was loaded with a 1 wt% solution of the polymer (50 mL) and evacuated. Then, the cell 8

Journal Pre-proof

was cooled to the experimental temperature (4 °C). Next, a certain amount of methane gas was injected into the cell. The magnetic stirrer was started immediately after the stabilization of the temperature and pressure. This point was set as time zero for the experiments.[43] The formation of the hydrate crystal could be inferred from the spike in the temperature because of the exothermic hydrate formation process. All the

f

experiments performed were of batch type, and the uptake of methane gas by the

oo

hydrates was calculated from the observed pressure drop. The subcooling (ΔTsub) is

pr

defined as the difference between the phase equilibrium temperature (Te) at the

e-

operating pressure and the experimental temperature. It is in fact the driving force for

Pr

hydrate formation expressed in terms of temperature.[43] As the hydrate grows, the driving force would become smaller, until the system reaches equilibrium. The

al

different subcoolings were determined by changing the amount of methane gas

rn

injected into the reactor (corresponding to different Te) and maintaining the

Jo u

experimental temperature at 4 °C. The typical curve of the pressure and temperature vs. time was shown in Figure S1. The gas consumption was calculated using the following equation [44, 45]: 𝑃0 𝑉

n=𝑍

0 𝑅𝑇0

𝑃𝑉

− 𝑍𝑅𝑇

(1)

where V is the volume of the gas phase in the reactor, which is assumed to be constant during the experiments. P and T are the measured pressure (vapor phase) and temperature at time t in the reactor, respectively. R is the gas constant. Z is the compressibility factor calculated by the Peng-Robinson equation of state. The maximum subcooling of the system was determined by a constant cooling 9

Journal Pre-proof

method [41, 46]. The specific procedure of this method is described in our previous report [9]. Methane gas at 10 MPa pressure was cooled under stirring at the rate of 1 °C/h, from +20 °C to −10 °C. Each experiment was repeated thrice to obtain the average value.

The crystal structure of the gas hydrate was determined by PXRD (PANalytical,

oo

f

X’Pert Pro MPD) performed at −50 °C and atmospheric pressure in the 2θ range of 5°

pr

to 80°. The hydrate cage occupation characteristics were determined by Raman

e-

spectrometry (Horiba, LabRAM HR) conducted at −50 °C in atmospheric pressure; a

Pr

523 nm Ar+ laser was used to record the Raman spectra. The hydrate samples used for these analyses were prepared using a hydrate forming system similar to that used

al

for determining maximum subcooling. The reactor was charged with 50 mL of the

rn

solution and 10 MPa of methane gas at 20 °C. After that, the system was cooled to

Jo u

1 °C under stirring at the rate of 1 °C/h to induce hydrate formation, and then maintained at 1 °C for 5 h. The hydrate samples were formed sufficiently during this period. Thereafter, the reactor was quenched in liquid nitrogen and the gas phase was released. Then, the hydrate crystals were taken out and ground in liquid nitrogen, and then stored in liquid nitrogen for further test.

3. Results and discussion 3.1. Inhibition effect of the polysaccharides on methane hydrate formation Typical pressure and temperature versus time curves obtained during a constant 10

Journal Pre-proof

cooling test using 1 wt% K90 (a kind of commercial inhibitor) are displayed in Figure 3. They illustrate the onset temperature (To) of the hydrate formation and maximum subcooling of the system. The rapid decrease in pressure observed in the curve indicates the onset of hydrate formation, and the maximum subcooling is defined as the difference between the phase equilibrium temperature (Te) corresponding to this

f

pressure and To. Here, Te was determined using the hydrate prediction model

oo

CSMGem [4]. Table 2 and Figure 4 show the maximum subcooling of systems with 1

pr

wt% of various polysaccharides listed in Table 1. The system without any additive is

e-

regarded as the blank test (reference), for which the maximum subcooling is found to

Pr

be 4.75 °C. The system with 1 wt% PVP K90 is also used as a reference. According to the data, all the polysaccharide solutions lead to higher maximum subcooling as

al

compared with that of pure water. Guar gum provided the highest maximum

rn

subcooling (8.50 °C), which is ~4 °C higher than that of the pure methane hydrate

Jo u

system. The maximum subcooling decreased in case of systems with other four polysaccharides and PVP in the following order, sodium alginate > PVP > starch > gum Arabic > carboxymethyl chitosan, with the corresponding values being 8.08, 7.96, 6.4, 5.71, and 5.61, respectively. The results illustrate that Guar gum and sodium alginate perform better among the screened polysaccharides, with their performances being better than that of PVP, which implies that hydrate formation required greater subcooling and a higher driving force in the presence of these two polysaccharides.

11

pr

oo

f

Journal Pre-proof

Jo u

rn

al

Pr

e-

Fig. 3. Typical curves obtained by a constant cooling test with 1.0 wt% PVP K90.

Fig. 4. The maximum subcooling of systems with various polysaccharides (1 wt%). (The error bar represents the experimental deviation).

12

Journal Pre-proof

Table 2. Onset pressure (Po) and temperature (To), related equilibrium temperature (Te), and average maximum subcooling of systems with various polysaccharides (1 wt%) Polymer

Po

To

Te

Maximum Subcooling

(MPa)

(℃)

(℃)

(℃) Average

Starch Carboxymethyl chitosan

4.75 5.71

f

4.75 4.55 4.95 5.85 5.51 5.77 8.18 7.56 8.14 8.38 7.96 7.90 6.65 6.21 6.35 5.93 5.05 5.85 8.68 8.35 8.47

7.96 8.08 6.40 5.61 8.50

rn

al

Guar gum

oo

Sodium alginate

13.19 13.20 13.13 12.91 12.84 12.92 12.74 12.79 12.75 12.71 12.76 12.76 12.87 12.89 12.88 12.90 12.99 12.91 12.70 12.72 12.71

pr

PVPK90

8.44 8.65 8.08 7.06 7.33 7.15 4.56 5.23 4.61 4.33 4.80 4.86 6.22 6.68 6.53 6.97 7.94 7.06 4.02 4.37 4.24

e-

Gum Arabic

9.49 9.51 9.44 9.24 9.27 9.25 9.08 9.13 9.09 9.06 9.10 9.10 9.20 9.22 9.21 9.23 9.31 9.24 9.05 9.07 9.06

Pr

No additives

Jo u

To further investigate the hydrate inhibition properties of these polysaccharides, the gas uptake in their presence during 1500 min were determined by computing the amount of gas in the vapor phase, according to Equation (1), and the results are compared with those of pure water and PVP system. Three different subcooling conditions were selected, i.e. 5 °C, 6 °C, and 7.5 °C, and the results are shown in Figure 5a, 5b, and 5c, respectively. Further, to quantify the rate of initial hydrate formation, Eq. (2) was used to calculate the normalized rate of hydrate formation in the first 30 min (NR30) [43]. NR 30 

n30 (mol  s  1  m  3 ) VW  t

(2) 13

Journal Pre-proof where Δn30 is the number of moles of gas consumed in the first 30 min, VW is the volume of water used in the experiment, and t is the hydrate growth time (t = 30 min). The results are listed in Table 3. The additional amount of a given polymer was too small to affect the equilibrium [17, 47]. It is clear that the NR30 values of the polysaccharides are similar to that of pure water under all subcooling conditions,

f

indicating that the polysaccharides have no or weak effect on nucleation. PVP reduced

oo

the initial rate of hydrate formation, with an NR30 value much lower than that of pure

pr

water. A higher PVP concentration resulted in a decrease in the initial hydrate growth

e-

rate; the initial hydrate growth rate of the system with 2 wt% PVP was one order of

Pr

magnitude lower than that of the system with 1 wt% PVP (see Table 3 and Figure S2). Nevertheless, catastrophic hydrate crystal growth followed, regardless of the

al

concentration of PVP, even leading to higher gas consumption than that observed with

rn

the pure water system, especially at higher subcooling (see Figure 5 and Figure S2 b).

Jo u

It has been suggested that PVP inhibits the nucleation of gas hydrates by disturbing the bulk water structure through PVP–water interactions, which render the formation of hydrate cages difficult. Meanwhile, the binding of PVP to the hydrate surface reduces the interfacial area between water molecules and hydrate nuclei, leading to a lesser global reaction rate of methane enclathration into hydrate cavities. All these factors result in reduced initial rate of hydrate formation [27, 48, 49]. Although PVP delays and impedes the hydrate growth at the early stage, the binding of PVP to the hydrate would lead to a decline in the PVP concentration in the liquid phase, resulting in uninhibited growth of hydrates [50]. The hydrates formed with PVP are small 14

Journal Pre-proof

crystals with higher porosity than that of those formed with pure water. Therefore, the hydrates with PVP provide a larger interfacial area between the water molecules and hydrate, thereby enhancing mass transfer from the liquid to the hydrate phase, eventually leading to the formation of a large quantity of hydrates. Meanwhile, pure water has been observed to form larger crystals containing occluded water [3, 51]. As

f

shown in the plots in Figure 5, the highest gas uptake is observed with the PVP

oo

system when the hydrate formation reached a plateau after 1500 min. A different

pr

behaviour is observed for the system with the polysaccharides. At a low subcooling

e-

temperature (ΔTsub = 5 °C), starch showed the best inhibition effect (~40% gas uptake

Pr

compared to that of a pure water system), and the effect decreased with an increase in the subcooling temperature and disappeared when the temperature exceeded its

al

maximum subcooling temperature (ΔTsub = 7.5 °C). A similar characteristic was

rn

observed for the system with carboxymethyl chitosan. By contrast, gum Arabic could

Jo u

moderately inhibit the hydrate growth when the subcooling temperature was 5 °C, while when the subcooling temperature was increased to 6 °C, it started to promote the hydrate formation, and its hydrate promoting effect enhanced with a further increase in the subcooling temperature. Guar gum showed good performance under the selected three subcooling temperature conditions; in its presence, the gas uptake lowered by ~30% compared with that of water. The next best performance was shown by sodium alginate. In conclusion, all the polysaccharides investigated in this study decreased the hydrate growth rates compared to those of PVP and pure water below their respective maximum subcooling temperature, although they did not inhibit the 15

Journal Pre-proof

hydrate nucleation. This may be attributed to their stronger adsorption to the hydrate surface, which effectively reduced the crystal growth rate [52, 53]. To verify the

Jo u

rn

al

Pr

e-

pr

oo

f

results, PXRD and Raman analyses of the formed hydrates were carried out.

16

Pr

e-

pr

oo

f

Journal Pre-proof

Fig. 5. Gas-consumption curves during the methane hydrate formation in the presence

Jo u

rn

and c) 7.5 °C.

al

of 1 wt% additives at different subcooling temperatures (ΔTsub): a) 5.0 °C, b) 6.0 °C,

Table 3. The initial rate of hydrate formation in the presence of various polymers under different subcooling temperatures Rate of hydrate formation, NR30 (mol·s−1·m−3) Polymer (1 wt%)

No additives

ΔTsub = 5.0 °C

ΔTsub = 6.0 °C

ΔTsub = 7.5 °C

4.26 × 10−7

5.53 × 10−7

6.90 × 10−7

3.46 × 10−7

6.46 × 10−7

5.66 × 10−7

6.86 × 10−7

5.69 × 10−7

7.77 × 10−7

Carboxymethyl chitosan Gum Arabic

17

Journal Pre-proof 5.86 × 10−7

5.01 × 10−7

3.53 × 10−7

Sodium alginate

6.14 × 10−7

4.04 × 10−7

4.28 × 10−7

Starch

2.33 × 10−7

6.53 × 10−7

7.77 × 10−7

PVPK90 (1 wt%)

1.66 × 10−7

1.00 × 10−7

1.56 × 10−7

PVPK90 (2 wt%)

3.78 × 10−8

1.43 × 10−8

1.05 × 10−7

f

Guar gum

oo

3.2. Effect of the KHIs on the structure of methane hydrate

pr

The crystal structure of the methane hydrate was analysed by PXRD. As expected,

e-

methane formed a hydrate with structure I without any inhibitors, and the

Pr

characteristic diffraction peaks of this hydrate are marked on the PXRD pattern (Figure 6) [9, 27, 54]. Through comparison of the PXRD patterns, it was determined

al

that the additives did not change the crystal type of the methane hydrate, implying

rn

that the factors determining the hydrate formation and hydrate structures are different.

Jo u

However, the characteristic peak intensities are much lower in the presence of all the tested inhibitors compared to those of the pure water system, suggesting that the polysaccharides inflect the hydrate growth and decrease the hydrate conversion rate. The intensity of the (321) crystal plane of the hydrate structure formed by various systems can be ranked as follows: water > PVP > gum Arabic > sodium alginate > carboxymethyl chitosan > starch > guar gum. The results indicate that all the investigated polysaccharides perform better than PVP, among which, guar gum performed the best and gum Arabic the worst, consistent with the results of hydrate formation experiments. The intensity of the (321) crystal plane of the methane hydrate 18

Journal Pre-proof

with PVP is found to be lower than that of pure water, which might be because the addition of PVP provides a higher maximum subcooling than does pure water, resulting in a longer time to induce nucleation and regulate hydrate growth during the cooling process. Comparably, the pure water system started to form hydrate at a relatively high temperature, and therefore, a large amount of methane hydrate was

Jo u

rn

al

Pr

e-

pr

oo

f

formed.

Fig. 6. PXRD patterns of methane hydrates formed in the presence of 1 wt% of various additives. Characteristic peaks corresponding to the hydrate with structure I are marked.

The hydrate samples were further analysed using Raman spectroscopy to investigate the cage occupation characteristics and localization. The Raman spectrum of the methane hydrate formed in the pure water system (Figure 7a) shows clear peaks in the 19

Journal Pre-proof C–H region from 2800 to 3000 cm−1 and the O–H region from 3000 to 3400 cm−1. The Raman shifts at ~2906 and 2916 cm−1 correspond to the C–H stretching vibrations of methane molecules encaged in large (51262) and small (512) hydrate cages, respectively [17, 55]. The structural information is usually analysed using the IL/IS value, the ratio of the integrated peak intensities for the large (IL) and small (IS)

f

hydrate cages, and the theoretical IL/IS value of hydrate for pure water is 3.0. As

oo

shown in Figure 7a, the experimental IL/IS ratio for the pure water system is 3.09,

pr

which is very close to the theoretical value. Some researchers have proposed that a

e-

key feature of the inhibition performance of KHIs is that some large cages are bonded

Pr

with the small functional groups of KHIs, such as the lactam ring [47, 49, 50], carboxyl group [27, 54], or hydroxyl group [9], resulting in the reduction of the IL/IS

al

ratio. The IL/IS ratio is 2.92 for the system with 1 wt% PVP, as shown in Figure 7b,

rn

consistent with the conclusions of previous studies [27, 47]. It is noteworthy that the

Jo u

IL/IS ratio also decreased in the presence of polysaccharides (see in Figure 7b), with the values being lower than that of PVP. This result indicates that the addition of either of these polysaccharides into the system results in less number of large cages occupied by methane compared to that of a system with PVP for some positions in the hydrate during the hydrate growth stage, thereby implying better inhibition performance of the polysaccharides. According to the results of hydrate formation experiments, guar gum shows good KHI performance while gum Arabic shows relatively poor performance, which is confirmed by the corresponding IL/IS values. The former affords the smallest IL/IS value of 2.62, while the latter provides the 20

Journal Pre-proof

Jo u

rn

al

Pr

e-

pr

oo

f

largest value of 2.78, amongst the polysaccharides.

Fig. 7. a) Analysis of the Raman spectra of methane hydrates; b) Raman spectra of methane hydrates in the presence of different polysaccharides.

21

Journal Pre-proof

3.3. Viscosity Further, it is necessary to examine how the hydrate inhibition depends upon the rheological properties of the system. Viscosity is an important parameter determining the rheological properties of polymeric materials. Therefore, the viscosities of 1 wt% solutions of the polysaccharides and PVP at the experimental temperature (4 °C) were

f

measured using an NDJ-8S rotating viscometer. The rotor and rotation speeds were

oo

maintained constant during the experiment. The temperature of the solutions was

pr

maintained using a cooling bath. The results are listed in Table 4. According to the

e-

literature [56], 600 cP is marked as the viscosity limit for the transition of the system

Pr

to a heterogeneous one leading to blockage. Therefore the viscosity of the polysaccharides listed in Table 4 would not restrict their application in pipelines. The

al

viscosity of a polymer solution mainly depends on its relative molecular weight,

rn

molecular dimension, and molecular geometry [57], which are also the factors that

Jo u

determine their inhibition properties. Seo et al. reported that the viscosity of the aqueous solution of PVCap (a well-known KHI) increased with an increase in its molecular weight; the viscosity was found to be greater than 100 cP when the molecular weight was above 1×104 g/mol [58]. In fact, larger molecules are found to be more effective at inhibiting hydrate growth, whereas smaller molecules are more effective in inhibiting nucleation [58, 59]. As shown in Table 3, all the polysaccharides have much higher molecular weights than that of PVP except for starch, which might lead to higher solution viscosity. The sodium alginate solution has the highest viscosity, mostly due to its molecular geometry. Similarly, the large 22

Journal Pre-proof

molecular dimension of guar gum might be the reason for its relatively high viscosity. The data in Table 3 and Figure 5 indicate that starch and PVP provide lower initial growth rates at ΔTsub = 5.0 °C (below their respective maximum subcooling) while large polysaccharides such as guar gum and sodium alginate inhibit the growth rate at a later stage, consistent with the previous conclusions [58, 59]. The complex

f

molecular structure of gum Arabic might be the reason for its low viscosity and poor

oo

KHI performance. According to these results, a high solution viscosity could synergy

pr

KHI performance at the later growth stage by decreasing the diffusion of gas into the

e-

bulk liquid phase, but no positive correlation was observed for this hypothesis.

Pr

Although the viscosity of the 1 wt% sodium alginate solution is higher than that of 1 wt% guar gum solution (as shown in Table 3), they show inverse KHI effects.

Jo u

Polymer

rn

al

Table 4. Viscosities of 1 wt% polysaccharide solutions Apparent viscositya

Mn b (g/mol) (cP)

Gum Arabic

5.02 × 105

3.9

PVPK90

9.00 × 104

6.7

Sodium alginate

1.45 × 105

89.8

Starch

4.19 × 103

3.5

Carboxymethyl chitosan 3.57 × 106

10.4

5.38 × 105

59.2

Guar gum No additives a

0.85

Average value from five measurements. bDetermined by GPC. 23

Journal Pre-proof

3.4. Discussion Previous experimental studies [39, 52, 53] and simulations [60, 61] have indicated that both commercial and biological inhibitors most likely inhibit the hydrate growth by an adsorption-inhibition mechanism. The hydrophilic pendant lactam ring of the KHIs (such as PVP and PVCap) is thought to play a key role in the adsorption of the

f

inhibitors onto the hydrate crystal, which interacted with molecular cages on the

oo

hydrate surface via hydrogen bonding and van der Waals interaction with water. It

pr

was demonstrated that the concentration of PVP in the liquid phase decreases with

e-

hydrate growth, while the methane concentration in the liquid phase increases,

Pr

consistent with the anticipated conclusion [17, 49, 50]. However, it is known that the addition of a small amount (0.1–2.0 wt%) of a polymeric KHI does not affect the

al

equilibrium condition [17, 47]. The selected polysaccharides are highly hydrophilic

rn

and easily combine with water in the solution. By combining the experimental results

Jo u

with Raman analysis, the reason for the KHI properties of these polysaccharides possessing can be attributed to the binding with the open cavities of the hydrate structure by the anhydroglucose unit of the polysaccharides similar to the hydrophilic pendant lactam group, which prevents the entry of gas molecules, leading to the inhibition of further hydrate crystal growth and reduction in gas uptake. Figure 8 shows a schematic to help visualize these concepts. The hydroxyl group in the anhydroglucose unit of the polysaccharide structure (shown in Figure 1) has better ability to form hydrogen bonds with water molecules as compared to that of the lactam group of PVP. Thus, the inhibition performance of the former is enhanced 24

Journal Pre-proof

compared to that of the latter. Furthermore, the difference between the inhibition performances of polysaccharides is most probably due to their different side chains. The side anhydroglucose group on the guar gum skeletal structure (see in Figure 1c) could increase its adsorption onto the hydrate surface, which effectively decreases the hydrate growth rate. Therefore, it performs better than other polysaccharides. The

f

sodiation of the side carboxyl group of sodium alginate significantly improved its

oo

water solubility, and it therefore showed relatively better inhibition effect. However,

pr

gum Arabic is mostly composed of polysaccharides with a small fraction of proteins,

e-

and the multi-branched complex molecular structure is dominated by arabinogalactan

Pr

[62], which might be the reason why it showed the poorest hydrate inhibition

Jo u

rn

al

performance.

Fig. 8. Schematic of the possible hydrate growth inhibition mechanisms of the polysaccharides.

25

Journal Pre-proof

4. Conclusions Five

polysaccharides,

namely,

gum

Arabic,

sodium alginate,

guar

gum,

carboxymethyl chitosan, and starch, were investigated as potential green inhibitors of natural gas hydrate formation, and their performances were compared with that of a commercial KHI, PVP K90. KHI tests were carried out using a high-pressure stainless

f

steel cell with methane gas. All the polysaccharides significantly increased the

oo

maximum subcooling for hydrate formation and reduced the hydrate growth rate

pr

within their respective maximum subcooling. The system with guar gum showed the

e-

highest maximum subcooling and was the most effective in reducing the hydrate

Pr

growth. It lowered the gas uptake by ~30% compared with that of pure water. Sodium alginate showed the next best performance. Starch and carboxymethyl chitosan

al

showed good hydrate inhibition performance until the temperature exceeded their

rn

respective maximum subcooling. In contrast, gum Arabic started to promote hydrate

Jo u

formation when the subcooling was increased to 6 °C, and its hydrate promoting effect improved as the subcooling was increased further. Further, the results of PXRD and Raman spectral analysis of the hydrate microstructure are consistent with these results. The polysaccharides added into the system did not change the hydrate crystal structure but only decreased the formation of hydrates by retarding the hydrate growth rate. The ratio of the Raman peak intensities of the large (51262) and small (512) hydrate cages decreased in the presence of polysaccharides; that is, less number of large cages were occupied by methane in the presence of the polysaccharides, which is probably due to the retarded hydrate growth rate. Further, it is possible that the 26

Journal Pre-proof

anhydroglucose unit of the polysaccharides filled the empty cavity of the hydrate structure the same way as the hydrophilic pendant lactam group of the commercial KHIs, and the hydroxyl group in the anhydroglucose unit facilitated a strong interaction between the polymer and water molecules, thus preventing the entry of gas molecules into the hydrate structure, which leads to a further inhibition of the hydrate

f

crystal growth and reduction of the gas uptake. The difference between the inhibition

oo

performances of the polysaccharides is most probably due to their different side

pr

chains. The side anhydroglucose group on the guar gum skeletal structure might

e-

enhance its adsorption onto the hydrate surface, thereby leading to the lowest IL/IS

Pr

ratio compared to those of other polysaccharides. It therefore showed the best inhibition performance. The sodiated side carboxyl group of sodium alginate greatly

al

improved its water solubility, leading to its relatively better hydrate inhibition

rn

performance. The multi-branched complex molecular structure of gum Arabic might

Jo u

be the reason for its poorest performance among the tested materials in inhibiting the hydrate formation. These results can serve as guidelines for future attempts to introduce glucose side groups into commercial inhibitors to improve both their inhibition and biodegradation performance.

Acknowledgements This work was supported by National Natural Science Foundation of China (41603062) ， National

key

research

(2017YFC0307306). 27

and

development

plan

of

China

Journal Pre-proof

Notes The authors declare no competing financial interest.

References

oo

of the American Chemical Society 126 (2004) 1569-1576.

f

[1] M.T. Storr, P.C. Taylor, J.P. Monfort, P.M. Rodger, Kinetic inhibitor of hydrate crystallization, Journal

pr

[2] N. Daraboina, C. Malmos, N. von Solms, Synergistic kinetic inhibition of natural gas hydrate

e-

formation, Fuel 108 (2013) 749-757.

Pr

[3] H. Sharifi, J. Ripmeester, V.K. Walker, P. Englezos, Kinetic inhibition of natural gas hydrates in saline solutions and heptane, Fuel 117 (2014) 109-117.

al

[4] E.D. Sloan, C.A. Koh, Clathrate Hydrates of Natural Gases, Crc Press (2007).

rn

[5] A. Perrin, O.M. Musa, J.W. Steed, The chemistry of low dosage clathrate hydrate inhibitors, Chem

Jo u

Soc Rev 42 (2013) 1996-2015.

[6] E.D. Sloan, Fundamental principles and applications of natural gas hydrates, Nature 426 (2003) 353-363.

[7] N. Daraboina, P. Linga, J.A. Ripmeester, V.K. Walker, P. Englezos, Natural Gas Hydrate Formation and Decomposition in the Presence of Kinetic Inhibitors. 2. Stirred Reactor Experiments, Energy & Fuels 25 (2011) 4384-4391. [8] J.J. Carroll, Natural gas hydrates : a guide for engineers, Gulf Professional Publishing (2009). [9] L. Wan, D.Q. Liang, Q. Ding, G. Hou, Investigation into the inhibition of methane hydrate formation in the presence of hydroxy-terminated poly(N-vinylcaprolactam), Fuel 239 (2019) 173-179. 28

Journal Pre-proof

[10] N.A. Cortez-Lemus, A. Licea-Claverie, Poly(N-vinylcaprolactam), a comprehensive review on a thermoresponsive polymer becoming popular, Progress in Polymer Science 53 (2016) 1-51. [11] M.S. Kamal, I.A. Hussein, A.S. Sultan, N. von Solms, Application of various water soluble polymers in gas hydrate inhibition, Renewable and Sustainable Energy Reviews 60 (2016) 206-225. [12] N. Daraboina, J.A. Ripmeester, V.K. Walker, P. Englezos, Natural Gas Hydrate Formation and

f

Decomposition in the Presence of Kinetic Inhibitors. 1. High Pressure Calorimetry, Energy & Fuels 25

oo

(2011) 4392-4397.

pr

[13] H. Zeng, L.D. Wilson, V.K. Walker, J.A. Ripmeester, The inhibition of tetrahydrofuran

e-

clathrate-hydrate formation with antifreeze protein, Canadian Journal of Physics 81 (2003) 17-24.

Pr

[14] P. Naeiji, A. Arjomandi, F. Varaminian, Amino acids as kinetic inhibitors for tetrahydrofuran hydrate formation: Experimental study and kinetic modeling, Journal of Natural Gas Science and

al

Engineering 21 (2014) 64-70.

Jo u

59 (2005) 417-423.

rn

[15] S. Pal, D. Mal, R.P. Singh, Cationic starch: an effective flocculating agent, Carbohydrate Polymers

[16] L.D. Villano, R. Kommedal, M.A. Kelland, Class of Kinetic Hydrate Inhibitors with Good Biodegradability, Energy & Fuels 22 (2008) 3143-3149. [17] M.A. Kelland, History of the Development of Low Dosage Hydrate Inhibitors, Energy & Fuels 20 (2006) 825-847. [18] C.M. Perfeldt, P.C. Chua, N. Daraboina, D. Friis, E. Kristiansen, H. Ramløv, J.M. Woodley, M.A. Kelland, N. von Solms, Inhibition of Gas Hydrate Nucleation and Growth: Efficacy of an Antifreeze Protein from the Longhorn BeetleRhagium mordax, Energy & Fuels 28 (2014) 3666-3672. [19] P.L. Davies, J. Baardsnes, M.J. Kuiper, V.K. Walker, Structure and function of antifreeze proteins, 29

Journal Pre-proof

Philos Trans R Soc Lond B Biol Sci 357 (2002) 927-935. [20] S.A. Bagherzadeh, S. Alavi, J.A. Ripmeester, P. Englezos, Why ice-binding type I antifreeze protein acts as a gas hydrate crystal inhibitor, Phys Chem Chem Phys 17 (2015) 9984-9990. [21] J. Sa, B.R. Lee, D. Park, K. Han, H. Chun, K.H. Lee, Amino acids as natural inhibitors for hydrate formation in CO2 sequestration, Environmental Science & Technology 45 (2011) 5885-5891.

f

[22] Y. Jin, M. Kida, Y. Konno, J. Nagao, Clathrate Hydrate Equilibrium in Methane–Water Systems with

oo

the Addition of Monosaccharide and Sugar Alcohol, Journal of Chemical & Engineering Data 62 (2017)

pr

440-444.

e-

[23] C.B. Bavoh, B. Partoon, B. Lal, L. Kok Keong, Methane hydrate-liquid-vapour-equilibrium phase

Pr

condition measurements in the presence of natural amino acids, Journal of Natural Gas Science and Engineering 37 (2017) 425-434.

al

[24] C.B. Bavoh, B. Partoon, B. Lal, G. Gonfa, S. Foo Khor, A.M. Sharif, Inhibition effect of amino acids

rn

on carbon dioxide hydrate, Chemical Engineering Science 171 (2017) 331-339.

Jo u

[25] H. Roosta, A. Dashti, S.H. Mazloumi, F. Varaminian, Inhibition properties of new amino acids for prevention of hydrate formation in carbon dioxide–water system: Experimental and modeling investigations, Journal of Molecular Liquids 215 (2016) 656-663. [26] M. Maddah, M. Maddah, K. Peyvandi, Molecular dynamics simulation of methane hydrate formation in presence and absence of amino acid inhibitors, Journal of Molecular Liquids 269 (2018) 721-732. [27] J.H. Sa, G.H. Kwak, K. Han, D. Ahn, S.J. Cho, J.D. Lee, K.H. Lee, Inhibition of methane and natural gas hydrate formation by altering the structure of water with amino acids, Sci Rep 6 (2016) 31582. [28] C. Xiao, H. Adidharma, Dual function inhibitors for methane hydrate, Chemical Engineering 30

Journal Pre-proof

Science 64 (2009) 1522-1527. [29] C. Xiao, N. Wibisono, H. Adidharma, Dialkylimidazolium halide ionic liquids as dual function inhibitors for methane hydrate, Chemical Engineering Science 65 (2010) 3080-3087. [30] M. Sina, K. Nazari, M. Mohammad-Taheri, M.R. Moradi, Dual Effect of Low-Dosage Poly(Ethylene Oxides) on Methane Hydrate Formation, Chemical Engineering & Technology 36 (2013) 1117-1124.

f

[31] M. Tariq, D. Rooney, E. Othman, S. Aparicio, M. Atilhan, M. Khraisheh, Gas Hydrate Inhibition: A

oo

Review of the Role of Ionic Liquids, Industrial & Engineering Chemistry Research 53 (2014)

pr

17855-17868.

e-

[32] M.A. Kelland, A Review of Kinetic Hydrate Inhibitors from an Environmental Perspective, Energy &

Pr

Fuels 32 (2018) 12001-12012.

[33] O.M. Musa, L. Cuiyue, Polymers Having N-vinyl Amide and Hydroxyl Moieties, Isp Investments Inc.

al

(1011 Centre Road, Suite 315, Wilmington, DE 19805, US), 2012.

rn

[34] L.D. Villano, R. Kommedal, M.W.M. Fijten, U.S. Schubert, R. Hoogenboom, M.A. Kelland, A Study

Jo u

of the Kinetic Hydrate Inhibitor Performance and Seawater Biodegradability of a Series of Poly(2-alkyl-2-oxazoline)s, Energy & Fuels 23 (2009) 3665-3673. [35] H. Roosta, A. Dashti, S. Hossein Mazloumi, F. Varaminian, Inhibition and promotion effects of modified HECs and modified starches on the growth rate of hydrate in methane-propane-water system, Journal of Molecular Liquids (2017). [36] E.F. Jokandan, P. Naeiji, F. Varaminian, The synergism of the binary and ternary solutions of polyethylene glycol, polyacrylamide and Hydroxyethyl cellulose to methane hydrate kinetic inhibitor, Journal of Natural Gas Science and Engineering 29 (2016) 15-20. [37] M.R. Talaghat, Experimental investigation of double gas hydrate formation in the presence of 31

Journal Pre-proof

modified starch as a kinetic inhibitor in a flow mini-loop apparatus, The Canadian Journal of Chemical Engineering 90 (2012) 429-436. [38] M.R. Talaghat, Enhancement of the performance of modified starch as a kinetic hydrate inhibitor in the presence of polyoxides for simple gas hydrate formation in a flow mini-loop apparatus, Journal of Natural Gas Science and Engineering 18 (2014) 7-12.

f

[39] J. Dong Lee, H. Wu, P. Englezos, Cationic starches as gas hydrate kinetic inhibitors, Chemical

oo

Engineering Science 62 (2007) 6548-6555.

pr

[40] Y. Xu, M. Yang, X. Yang, Chitosan as green kinetic inhibitors for gas hydrate formation, Journal of

e-

Natural Gas Chemistry 19 (2010) 431-435.

Pr

[41] S. Xu, S. Fan, S. Fang, X. Lang, Y. Wang, J. Chen, Pectin as an Extraordinary Natural Kinetic Hydrate Inhibitor, Sci Rep 6 (2016) 23220.

al

[42] S. Xu, S. Fan, S. Fang, Y. Wang, X. Lang, Excellent synergy effect on preventing CH4 hydrate

rn

formation when glycine meets polyvinylcaprolactam, Fuel 206 (2017) 19-26.

Jo u

[43] H.P. Veluswamy, W.J. Ang, D. Zhao, P. Linga, Influence of cationic and non-ionic surfactants on the kinetics of mixed hydrogen/tetrahydrofuran hydrates, Chemical Engineering Science 132 (2015) 186-199.

[44] J. Liu, J.X. Ding, D.Q. Liang, Experimental study on hydrate-based gas separation of mixed CH 4 /CO 2 using unstable ice in a silica gel bed, Energy 157 (2018) 54-64. [45] M. Zare, A. Haghtalab, A.N. Ahmadi, K. Nazari, A. Mehdizadeh, Effect of imidazolium based ionic liquids and ethylene glycol monoethyl ether solutions on the kinetic of methane hydrate formation, Journal of Molecular Liquids 204 (2015) 236-242. [46] P.S.R. Prasad, B.S. Kiran, Are the amino acids thermodynamic inhibitors or kinetic promoters for 32

Journal Pre-proof

carbon dioxide hydrates?, Journal of Natural Gas Science and Engineering 52 (2018) 461-466. [47] E.D. Sloan, S. Subramanian, P.N. Matthews, J.P. Lederhos, A.A. Khokhar, Quantifying Hydrate Formation and Kinetic Inhibition, Industrial & Engineering Chemistry Research 37 (1998) 3124-3132. [48] S.P. Kang, J.Y. Shin, J.S. Lim, S. Lee, Experimental measurement of the induction time of natural gas Hydrate and its prediction with polymeric kinetic inhibitor, Chemical Engineering Science 116 (2014)

f

817-823.

oo

[49] D. Posteraro, J. Ivall, M. Maric, P. Servio, New insights into the effect of polyvinylpyrrolidone (PVP)

pr

concentration on methane hydrate growth. 2. Liquid phase methane mole fraction, Chemical

e-

Engineering Science 126 (2015) 91-98.

Pr

[50] J. Ivall, J. Pasieka, D. Posteraro, P. Servio, Profiling the Concentration of the Kinetic Inhibitor Polyvinylpyrrolidone throughout the Methane Hydrate Formation Process, Energy & Fuels 29 (2015)

al

2329-2335.

rn

[51] J.P. Lederhos, J.P. Long, A. Sum, R.L. Christiansen, E.D.S. Jr, Effective kinetic inhibitors for natural

Jo u

gas hydrates, Chemical Engineering Science 51 (1996) 1221-1229. [52] R. Gordienko, H. Ohno, V.K. Singh, Z. Jia, J.A. Ripmeester, V.K. Walker, Towards a Green Hydrate Inhibitor: Imaging Antifreeze Proteins on Clathrates, PLOS ONE 5 (2010). [53] R. Kumar, J.D. Lee, M. Song, P. Englezos, Kinetic inhibitor effects on methane/propane clathrate hydrate-crystal growth at the gas/water and water/n-heptane interfaces, Journal of Crystal Growth 310 (2008) 1154-1166. [54] Q. Zhang, X. Shen, X. Zhou, D. Liang, Inhibition Effect Study of Carboxyl-Terminated Polyvinyl Caprolactam on Methane Hydrate Formation, Energy & Fuels 31 (2017) 839-846. [55] N. Daraboina, J. Ripmeester, V. Walker, P. Englezos, Natural Gas Hydrate Formation and 33

Journal Pre-proof

Decomposition in the Presence of Kinetic Inhibitors. 3. Structural and Compositional Changes, 2011. [56] H. Moradpour, A. Chapoy, B. Tohidi, Controlling Hydrate Slurry Transportability by Optimizing Anti-Agglomerant Usage in High Water Cut Systems, 1 (2011). [57] H. Zhang, J.K. Wang, R. Wang, Y.C. Dong, Microwave irradiated synthesis of grafted cationic starch: Synthesis, characterization, application, and biodegradation, Journal of Applied Polymer Science 130

f

(2013) 1645-1652.

oo

[58] S.D. Seo, H.J. Paik, D.H. Lim, J.D. Lee, Effects of Poly(N-vinylcaprolactam) Molecular Weight and

pr

Molecular Weight Distribution on Methane Hydrate Formation, Energy & Fuels 31 (2017) 6358-6363.

e-

[59] D. Posteraro, J. Verrett, M. Maric, P. Servio, New insights into the effect of polyvinylpyrrolidone

Pr

(PVP) concentration on methane hydrate growth. 1. Growth rate, Chemical Engineering Science 126 (2015) 99-105.

al

[60] M.A. Bellucci, M.R. Walsh, B.L. Trout, Molecular Dynamics Analysis of Anti-Agglomerant Surface

rn

Adsorption in Natural Gas Hydrates, The Journal of Physical Chemistry C 122 (2018) 2673-2683.

Jo u

[61] H. Ji, D. Chen, C. Zhao, G. Wu, Molecular Dynamics Simulation of Methane Hydrate Formation and Dissociation in the Clay Pores with Fatty Acids, The Journal of Physical Chemistry C 122 (2018) 1318-1325.

[62] D. Renard, C. Garnier, A. Lapp, C. Schmitt, C. Sanchez, Structure of arabinogalactan-protein from Acacia gum: From porous ellipsoids to supramolecular architectures, Carbohydrate Polymers 90 (2012) 322-332.

Highlights 1.

Five natural polysaccharides were selected as potential inhibitors, most for the first time.

2.

All the polysaccharides increased the maximum subcooling and reduced the hydrate growth rate 34

Journal Pre-proof

although did not inhibit the nucleation. 3.

Guar gum and sodium alginate had a better performance than commercial KHI PVP K90.

4.

Difference in inhibition performance of the polysaccharides depended on their side chains, and

Jo u

rn

al

Pr

e-

pr

oo

f

glycoside group was the optimum.

35

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8