Self-Preservation of Gas Hydrate Particles Suspended in Crude Oils

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Self-preservation of gas hydrate particles suspended in crude oils and liquid hydrocarbons: role of preparation method, dispersion media and hydrate former Andrey S. Stoporev, Andrey Yu. Manakov, Lubov K. Altunina, and Larisa A. Strelets Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01531 • Publication Date (Web): 30 Sep 2016 Downloaded from http://pubs.acs.org on October 2, 2016

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Self-preservation of gas hydrate particles suspended in crude oils and liquid hydrocarbons:

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role of preparation method, dispersion media and hydrate former

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Andrey S. Stoporev†, Andrey Yu. Manakov†,‡,*, Lubov’ K. Altunina§, Larisa A. Strelets§

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† Nikolaev Institute of Inorganic Chemistry SB RAS, Ac. Lavrentiev Avenue, 3, Novosibirsk, 630090,

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Russian Federation

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‡ Novosibirsk State University, Pirogova Street, 2, Novosibirsk, 630090, Russian Federation

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§ Institute of Petroleum Chemistry SB RAS, Akademichesky Avenue, 4, Tomsk, 634021, Russian

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Federation

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* Corresponding author. Andrey Yu. Manakov, Dr.Sci., Head of laboratory, Nikolaev Institute of

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Inorganic Chemistry SB RAS, Ac. Lavrentiev ave., 3, Novosibirsk, 630090, Russian Federation,

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Tel. +7 (383)-316-53-46, Fax: +7 (383)-330-94-89. E-mail: [email protected]

13 14

Keywords. gas hydrate, self-preservation, crude oil, disperse systems

15 16

ABSTRACT

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The effect of self-preservation of small (few dozens of micrometers) methane hydrate particles in

18

suspensions of a hydrate in crude oils was discussed in our previous work (Stoporev et.al., 2014). In this

19

work we present new experimental data on (1) self-preservation of methane hydrate suspensions

20

prepared with the use of different experimental methods; (2) self-preservation of methane hydrate

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suspended in different hydrocarbons and oil + hydrocarbon mixtures; (3) self-preservation of gas

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hydrate-in-oil suspensions formed by different hydrate forming gases. It has been demonstrated that the

23

efficiency of self-preservation of methane hydrate particles can be strongly increased by mixing with

24

crude oil. The powder of methane hydrate with particle sizes 50-150 µm was mixed with crude oil under

25

conditions providing the hydrate stability (1°С, pressure over 3 MPa). The oil was mixed with the

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hydrate by vigorous shaking. Further the sample of the hydrate suspension was cooled to liquid nitrogen

2

temperature and was recovered from the autoclave. Two stages of the hydrate decomposition occurred at

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atmospheric pressure. The first one started at ∼ -75°C that is close to the equilibrium temperature of the

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methane hydrate under atmospheric pressure (-80°C). The whole of remaining hydrate decomposed at

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the second stage at 0°С, i.e. at the ice melting point. Independent experiments proved that the same

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powder of methane hydrate without oil decomposed under atmospheric pressure in one stage started at ∼

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-75°C. No self-preservation occured in this case. The experiments on study of self-preservation of

8

methane hydrate suspensions in toluene, decane and mixtures of these solvents with crude oil were

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performed. These suspensions were synthesized from emulsions of water in the respective organic

10

liquids. No self-preservation occurred in the case of pure toluene and decane. At the same time, addition

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of 25-50 wt % of crude oil to these solvents resulted in clearly expressed self-preservation of methane

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hydrate in the corresponding suspensions. Additionally, it was demonstrated that the effect of self-

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preservation of hydrate-in-oil suspensions could be observed for ethane, propane, carbon dioxide, as

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well as for hydrocarbon mixture gas hydrates (in all cases the sizes of the hydrate particles did not

15

exceed few dozens of micrometers).

16 17

INTRODUCTION

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The self-preservation effect of gas hydrates is caused by formation of a dense ice shell on

19

the surface of gas hydrate particles during their decomposition below 0°C1-5. After ice shell

20

formation, further hydrate decomposition is limited by methane transport through it. The rate of

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hydrate decomposition decreases abruptly1–3. This phenomenon has been under intensive study

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since the first publications on the subject3-13. The temperature limits of the methane hydrate self-

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preservation were reported in6. It appeared that the self-preservation virtually did not occur below -

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30°C. The studies

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methane hydrate is covered by the hexagonal ice Ih. Low-temperature studies revealed formation of

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cubic ice Ic (or, probably, mixed phases Ih-Ic) which transformed to ice Ih at higher temperatures14-

10,14

have demonstrated that at moderate subzero temperatures the surface of the


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17

2

supercooled liquid water takes place at temperatures close to the ice melting point (e.g.7). The

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appearance of the metastable (supercooled) water during hydrate dissociation can occur at the

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temperatures above the continuation of monovariant curve of liquid water – hydrate – gas in the

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metastable region

6

supercooled water eventually undergoes crystallization to the dense ice shell causing self-

7

preservation of the hydrate.

. Deeper insight into this process has revealed that a decomposition of hydrates to gas and

7,18–21

. At the temperatures close to the melting point of ice metastable

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The size effect of the hydrate particles on self-preservation under atmospheric pressure has

9

been studied for methane8 and natural gas9 hydrates. It has been demonstrated that self-preservation

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is observed for particles of methane and natural gas hydrates with sizes above 250 and 500 µm,

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respectively. Decomposition process of the hydrates of these gases has two steps

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step is a fast decomposition resulting in formation of a thin ice shell on the hydrate surface. This

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step takes few dozens of minutes. The next step is a slower decomposition resulting in slow

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increase in the thickness of the ice layer. After 24 h of storage the thickness of the ice shell on the

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self-preserved methane hydrate is above 100 µm

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natural gas hydrate 9. In addition, an important factor influencing the self-preservation is the type of

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the guest molecules. Thus, Takeya and Ripmeester have demonstrated that in their experiments no

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self-preservation occurs for ethane and propane hydrates for the particle size used in the study at

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investigated p-T conditions 10. According to11, the agglomeration process of ice shells of CO2, C2H6

20

and CH4 hydrate particles during decomposition under their stable conditions is not the same. The

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particles of methane and carbon dioxide hydrates are strongly agglomerated in the course of time

22

while the corresponding rate for ethane hydrate is much slower. Thus, the type of guest molecules

23

influences on the morphology of the ice shell on the hydrate surface and probability of the self-

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preservation effect. A sensitivity of the self-preservation effect of methane hydrate to p-T conditions

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of the ice shell formation4 and to structural characteristics of natural and artificial micro-grains

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(perfection of structure)23,24 have been demonstrated. It appeared that hydrate decomposition rate

9,15,22

. The initial

22

, and it is 3 times thicker in the case of the


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depended on the microstructure of the initial hydrate and ice, annealing time, p-T conditions and the

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surface/volume ratio of the original hydrate particles. These factors were suggested as an instrument

3

controlling hydrate decomposition rate and them found a practical application in development of

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gas transportation techniques in the form of gas hydrates25–32. Almost all of the cited works noted

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that further studies in this area are required to improve the processing characteristics of the self-

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preserved hydrate. It is reasonable to suggest that development of methods to control the self-

7

preservation may be the key factor to industrial gas hydrate technologies.

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Recently it has been demonstrated that methane hydrate-in-oil suspensions obtained from

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water-in-oil emulsions with water content of 50 wt % undergo effective self-preservation even at

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particles sizes of few dozens of micrometers13. One should note that no self-preservation occurred

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in the case of hydrate suspensions in diesel oil with lower water cut (10 - 30 vol. %)33. Hydrate

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prevention is a topical field in current studies of gas hydrates34–36. It is established that hydrate

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formation in dispersed oil systems commonly occurs through reaction of the associated gas

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dissolved in oil with emulsified water. This is a kind of “paradise” for hydrate formation: large oil–

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water interface and high concentration of gas in the oil surrounding water particles. The past decade

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studies thrown additional light on hydrate formation in oil-based dispersed systems37–42. However,

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the studies of hydrate decomposition in such systems are still insufficient (e.g.37,38,43) and have been

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carried out at positive temperatures, i.e., under conditions excluding self-preservation. Efficient

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self-preservation of hydrates in oil suspensions13 is a novel phenomenon deserving careful

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examination. These circumstances have stimulated our interest to the subject.

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EXPERIMENTAL SECTION

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The following gases CH4, C2H6, C3H8, CO2 and gas mixture (methane– 64.6 mol%, ethane –

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25.1 mol%, propane – 10.3 mol%) were used in the current study. The purity of individual gases

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was not less than 99.98%. Also, distilled water, n-decane, toluene (reagent grade), surfactant

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Span 80 (Sigma-Aldrich), and oils from four fields were used (Table 1). We will use the following 4 ACS Paragon Plus Environment

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notations of oils: VOF – Verhnechonskoe oil field, GOF – Gerasimovskoe oil field, SOF –

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Sovetskoe oil field, MOF – Mamontovskoe oil field.

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The MOF oil and the powder of methane hydrate were mixed in the setup illustrated in

4

Figure 1. The installation comprised two identical autoclaves (internal diameter 40 mm, internal

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height 75 mm, operating pressure up to 12 MPa) (1) connected with a pipe equipped with a valve;

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each of the autoclaves has a manometer (3) and an inlet valve (4). At the beginning of an

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experiment the top autoclave was loaded with the MOF oil (5) and the bottom autoclave was cooled

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below 0°C and then loaded with ice powder (2) obtained by spraying distilled water into liquid

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nitrogen. Size of ice particles prepared with this method is about 100 µm. After loading, both

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autoclaves were flushed and pressurized with methane up to ~ 10 MPa, then the disconnected

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autoclaves were placed in a thermostat with the temperature set to 1°C. Slow ice melting insight the

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bottom autoclave was accompanied by intensive hydrate formation. The synthesis of the methane

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hydrate took place during four days. Every day the lower autoclave was cooled to the liquid

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nitrogen temperature, the hydrate was extracted, ground, loaded back and again pressurized to 10

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MPa. All operations were conducted at the liquid nitrogen temperature. No pressure drop was

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observed in the lower autoclave on the fourth day. The hydrate powder taken from the autoclave

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was ground again and divided in two portions. The former one (2 at Figure 1) was re-loaded in the

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bottom autoclave, while the second was used as comparison sample and for determination of ice-to-

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hydrate conversion degree in the prepared powder (assuming hydrate formulation CH4•6H2O).

20 21

Figure 1. Scheme of the process of mixing the methane hydrate with oil; 1 – high pressure

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cell , 2 – fine methane hydrate powder, 3 – valve, 4 – pressure-gauge, 5 – MOF oil saturated with

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methane, 6 – methane hydrate/MOF oil suspension.

24


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Then the autoclaves were connected as presented in Figure 1. Further the bottom autoclave

2

was loaded with fine hydrate powder (2) and methane pressure in the autoclave was set to 3 MPa.

3

Before loading to the autoclave, the sample of the hydrate was powdered thoroughly and sifted

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through a 60 mesh sieve. Methane pressure in the cell containing methane-saturated MOF oil (5)

5

was increased to 10 MPa. The setup was again placed in the thermostat at the temperature +1°С.

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After opening the valve between the autoclaves the oil was transferred to the bottom one. The

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mixing of the oil and the hydrate was achieved by repeated vigorous shaking of the autoclave.

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Again, the autoclave was cooled to the liquid nitrogen temperature and the obtained sample of

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methane hydrate-in-MOF oil suspension (6) was recovered. The weight ratio for the obtained

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methane hydrate-in-oil suspension was 1 : 2. It was derived from gas content of pure and oil-mixed

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hydrate powders.

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All other investigated suspensions of hydrates in the oils were prepared from corresponding 13

13

water-in-oil emulsions in accordance with the procedure presented in Ref

. The emulsions with

14

water content 50 wt% were used. Basic characteristics of the disperse media used are listed in Table

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1. Additionally, the samples of the SOF oil diluted with decane or toluene were prepared to study

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the influence of concentration of oil components on self-preservation effect of the methane hydrate

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(SOF50D50 – 50 wt% of decane, SOF50T50 – 50 wt% of toluene, SOF75T25 – 25 wt% of toluene,

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and SOF25T75 – 75 wt% of toluene). In the case of decane or toluene 1 wt% solution of surfactant

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Span 80 (emulsifier) in water was used to stabilize water-in-decane and water-in-toluene emulsions;

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all other emulsions were prepared from distilled water. Suspensions were obtained in a high-

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pressure cell equipped with a cut-off valve and a manometer. 20-30 ml of emulsion was put into the

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cell. The cell was purged and filled with gas up to 6-9, 3-3.2, 0.4-0.5, 2.5-3, and 4,5-5 MPa in the

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case of methane, ethane, propane, the gas mixture and carbon dioxide respectively, and kept at

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~1°C. Hydrate formation was registered by the pressure drop at a constant temperature. To achieve

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a high degree of water conversion into the hydrate, the synthesis was carried out for a long time

26

(from 1 day to 2 months) with the constant gas pressure. The whole apparatus was cooled down to 6 ACS Paragon Plus Environment

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the temperature of liquid nitrogen before the sample extraction. A monolith of the quenched

2

hydrate-in-oil suspension obtained this way was crushed into fragments with characteristic sizes of

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3-4 mm and stored in liquid nitrogen. Procedure of synthesis of the propane hydrate was different

4

from ones for the hydrates of methane and ethane. In this case water-in-oil emulsion was placed in

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the stainless steel pot situated slightly above the bottom of the high-pressure cell. It was done to

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avoid mixing of oil component of the emulsion and liquefied propane during the synthesis and the

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recover procedure from the high-pressure cell. Mixing of the emulsion and liquid propane resulted

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in precipitation of asphaltenes and possibly had prevented hydrate formation. Size of water droplets

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in the emulsions used in our experiments was checked by optical microscopy in all cases (Table 1).

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It has been shown previously that there is no significant difference in size of the hydrate particles in

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the suspensions in comparison with the size of water droplets in the emulsions used for synthesis of

12

these suspensions 13.

13 14

To determine the volume of gas evolved from a sample as a function of the temperature the thermovolumetric method was used. The experimental technique was described in13.

15 16

RESULTS AND DISCUSSION

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Self-preservation of methane hydrate particles mixed with oil

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Figure 2 shows decomposition curves of the obtained pure hydrate powder, ground frozen

20

hydrate–MOF oil mixture, as well as of a frozen fragment of the hydrate–MOF oil mixture.

21

Numerical data are collected in Table 2. The images of pure hydrate powder and the hydrate powder

22

mixed with the oil are illustrated in Figure 3. As one can see, decomposition of the pure hydrate

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powder occurs in one stage with the onset at about -75°С, i.e., near the equilibrium temperature of

24

the methane hydrate. Less than 1% of the hydrate undergoes self-preservation, this decomposition

25

stage being practically non-observable on the curve. It may be explained by small (less than 250


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µm, see the Introduction section) size of gas hydrate particles used in this experiment. The

2

thermovolumetric curves obtained with the samples of the hydrate mixed with oil exhibit three

3

stages similar to those described in Ref.13 (Figure 2, Table 2). Stage (1) corresponds to degassing of

4

the frozen oversaturated solution of methane in oil13. Methane hydrate immersed in oil starts to

5

decompose at stage (2), at the temperature being close to the equilibrium value of the methane

6

hydrate. The volume of the gas evolved at stage (2) from the ground suspension was approximately

7

two times more than escaped from the fragment of the suspension. The remaining hydrate

8

decomposes at stage (3) at the temperature about 0°С. Gas evolution at this stage is undoubtedly

9

related to decomposition of the hydrate self-preserved at stage (2).

10 11

Figure 2. Comparison of gas evolution curves for methane hydrate; □ – hydrate powder without oil

12

(experiment MOF 1/1), ○ – ground frozen hydrate–MOF oil mixture (experiment MOF 2/1), ● –

13

frozen fragment of the hydrate–MOF oil mixture (experiment MOF 2/2). (1) – (3) are the stages of

14

gas release from the sample; additional information is given in the text.

15 16

Figure 3. Top: (a) methane hydrate powder before mixing with oil, (b) – after mixing;

17

scale bars at the picture equal to 10 mm. Bottom: schematic representation of hydrate powders

18

before (left) and after (right) mixing with oil. D – particle size of the suspension sample; d – particle

19

size of the gas hydrate.

20 21

As evident from the Table 2, the extent of water to hydrate conversion was ~0.97 for our powders

22

(assuming the composition of the methane hydrate CH4•6H2O). The sample was formed with

23

virtually pure hydrate with particle sizes less than 250 µm. Unfortunately, we failed to obtain SEM

24

images of proper quality to determine hydrate grain sizes in oil–hydrate mixtures. In any case the

25

mixture looks as fine-graded and uniform in all images. Distinct fragments occasionally observed


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are reaching 350 µm in size at maximum for one fragment while the major fraction of these

2

fragments fall within 50-150 µm.

3

When the results above are compared with those of Ref.

13

it is apparent that self-

4

preservation behavior of methane hydrate powder mixed with the MOF oil is similar to the self-

5

preservation of methane hydrate suspensions synthesized from water-in-oil emulsions. Furthermore,

6

the experiments with different kinds of crude oils gives no evidence of qualitative difference

7

between the obtained results. We believe that effective self-preservation of gas hydrates in oil

8

suspensions is determined by formation of adsorptive layer of heavy components of crude oil

9

(asphaltenes, resins) on the surface of hydrate particles. The adsorptive layer offers strong

10

possibilities for formation of dense ice layers at the hydrate surface. Formation of the adsorptive

11

layer seems to be independent on the preparation method of a hydrate suspension in oil and results

12

in similar effect of different oils on self-preservation. Some quantitative differences in the results

13

obtained may be explained by different content of heavy components in the oils used, different

14

viscosities of the oils, different diffusion coefficients of methane in the oils, different sizes of the

15

hydrate and suspension particles, etc. For comparison, let us compare the data on methane volumes

16

evolved at the stages (2) and (3) from the samples of the frozen suspensions presented in this work

17

and in Ref.13. In all cases we consider the experiments with ∼3 mm pieces of the suspensions. In

18

the work

19

hydrate particles located at the surface of the frozen suspension particles contacting to methane

20

atmosphere. The experiments with methane hydrate suspension in GOF oil has shown negligible

21

hydrate decomposition at the stage (2) 13. At the same time in current experiments with the mixture

22

of methane hydrate with the MOF oil about 16.5% of the hydrate is decomposed at stage (2) (Table

23

2). We suggest that in current experiments the hydrate particles at the surface of the suspension

24

fragments (contacting to atmosphere) completely decompose, and the hydrate particles surrounded

25

by the oil matrix undergo partial surface decomposition resulted in formation of ice shell at the

26

surface of the hydrate particles. This shell preserves the inner hydrate particles. However, lower

13

partial decomposition of the hydrate at stage (2) is explained by decomposition of the


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content of asphaltens and resins in MOF oil (Table 1) results in less effective self-preservation in

2

this case. Assuming that the fraction of the hydrate particles at the surface of a suspension fragment

3

sized ~ 3 mm is negligible as in Ref.13, and the characteristic size of the hydrate particles in the

4

hydrate-oil mixture is close to 100 µm, it is possible to make an estimate of the thickness of the

5

decomposed hydrate layer as ~ 6 µm. More work is necessary for closer examination of self-

6

preservation in the systems under investigation.

7

To assess the rate of gas escape from methane hydrate in the hydrate-oil mixture we paused

8

warming up of one of the samples at -20°C for 6 hours (Figure 4). During this period only 3.9% of

9

total gas content was released. Gas evolution took place at constant rate of ∼ 1.2 ml g-1 h-1 after the

10

initial period. In this experiment the rate of gas evolution appeared the same as in a similar

11

experiment with the hydrate synthesized from oil emulsion 13. The remaining gas escaped at stage

12

(3) and the temperature about 0°С. We emphasize that in distinction to self-preservation in diesel

13

oil33 our results are unaffected by content of water/hydrate in the mixture with oil and composition

14

of the mixture. It shows once again that the mechanism of self-preservation of the methane hydrate

15

mixed with oil is equivalent to that of the hydrate obtained directly from water-in-oil emulsion.

16

17

Figure 4. (a) gas evolution curve of methane hydrate/MOF oil suspension (experiment MOF 2/3);

18

(b) the volume of gas evolved at -20°С during 6 hours.

19 20

Self-preservation of methane hydrate suspensions with various surrounding fluids

21

In the next part of this work we studied self-preservation of methane hydrate dispersed in

22

pure decane, pure toluene and some mixtures of these solvents with the SOF oil (Figure 5a and 5b).

23

Numerical data are presented in Table 2. In this part all experiments were carried out with the

24

samples of hydrate-in-oil suspensions prepared from 50 wt% of water-in-oil emulsions. The

25

conversion of water to the hydrate fell within 24-76% depending on the dispersive media used. The 10 ACS Paragon Plus Environment

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smallest values of water to hydrate conversion were obtained for the pure SOF oil probably due to

2

comparatively high viscosity of this dispersive media. No correlations between the hydrate content

3

and such general features like (1), (2) or (3) stages of the gas evolution curve were observed. It is

4

evident that in pure solvents (decane and toluene) the self-preservation does not occur (Figure 5a).

5

To our opinion, this is due to the absence of asphalthenes, resins and other heavy components of oil

6

in these solvents. The same results were obtained in Ref. 13 In addition, elimination of the impact of

7

heavy oil components by using diesel oil instead of crude oil resulted in the lack of self-preservation

8

effect in suspension of methane hydrate in diesel oil system with water cut comparable with the

9

used in our experiments 33. The dispersions of the methane hydrate in solid decane (melting point -

10

30°С) and in liquid toluene (melting point -95°С) decompose at a temperature somewhat higher

11

then the equilibrium (Figure 5a). We suggest that this effect is related to slow diffusion of methane

12

in cold toluene and in solid decane. More detailed discussion on this point is beyond the scope of

13

the present work.

14

When it comes to self-preservation of methane hydrate formed in the mixtures of decane and

15

toluene with SOF oil (Figure 5b), the self-preservation behavior is analogous to pure SOF oil

16

because of presence of resins and asphaltenes in the systems. We emphasize that in the series of the

17

experiments with ∼3 mm pieces of frozen suspensions of methane hydrate in the SOF oil and

18

mixtures of the SOF oil with decane and toluene stage (2) of the sample decomposition is clearly

19

manifested only in the case of the sample with the smallest content of the crude oil (experiment

20

SOF25T75 1/2, Figure 5b). We speculate that content of heavy oil components in this mixture of the

21

oil and toluene is not sufficient to provide effective self-preservation of all hydrate particles in the

22

sample of the frozen suspension. It is noteworthy that recently the possibility to control gas hydrates

23

self-preservation by proper choice of the host medium was demonstrated 44. In the cited work hard

24

hydrophobic or hydrophilic balls were used as the host medium.

25


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Figure 5. Hydrates decomposition in different disperse media; (a) ○ – methane hydrate/toluene

2

(experiment toluene 1/2), ● – methane hydrate/decane (experiment decane 1/2); (b) ▽ – methane

3

hydrate/SOF oil (experiment SOF 1/2), △ – methane hydrate/ SOF75T25 (experiment SOF75T25

4

1/2),

5

hydrate/SOF50D50 (experiment SOF50D50 1/2), ◇ – methane hydrate/SOF25T75 (experiment

6

SOF25T75 1/2); (c) □ –ethane hydrate/GOF oil (experiment 1/1), △ – propane hydrate/VOF oil

7

(experiment 2/2), ● – hydrate of methane-ethane-propane mixture/GOF oil (experiment 1/1), ▽ –

8

carbon dioxide hydrate/VOF oil (experiment 1/1); in case of carbon dioxide hydrate stage (1) is not

9

presented for clarity.

●

–

methane

hydrate/SOF50T50

(experiment

SOF50T50

1/2),

□

–

methane

10 11

Self-preservation of gas hydrates formed by different gases

12

The results of the experiments on self-preservation behavior of the hydrates formed by

13

different gases are collected in Table 3 and Figure 5c. The experiments were performed with frozen

14

oil suspensions of ethane, propane, methane-ethane-propane mixture, and CO2 hydrates (size of

15

suspension particles ∼3 mm). Equilibrium formation/decomposition temperatures of the respective

16

gas hydrates at 0.1 MPa are -33.5°C for ethane hydrate, -14°C for propane hydrate, -57.5°C for

17

carbon dioxide hydrate and -38.4°C for the hydrate of the gas mixture. These values were calculated

18

from the data presented in Ref. 45. Obtained experimental thermovolumetric curves (Figure 5c) are

19

similar in appearance and resemble the curves discussed in previous paragraphs. The curves exhibit

20

three or two stages of gas evolution. Low-temperature stage (1) starts at temperatures below -80°C

21

that is below the decomposition temperatures of these hydrates. By analogy with the experiments

22

discussed above, this stage corresponds to degassing of the frozen oversaturated solution of the

23

respective gases in oil. Evaporation of small quantities of liquid (ethane, propane) or solid (CO2)

24

hydrate formers also contribute to this stage. Condensation of these hydrate formers could happens

25

in the course of cooling of the experimental autoclave. Stage (3) corresponds to decomposition of


Page 13 of 31

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Energy & Fuels

1

self-preserved gas hydrate and holds at temperatures close to 0°C. This stage occurs at all

2

thermovolumetric curves under discussion (Figure 5c). Thermovolumetric curves obtained for

3

ethane, propane and gas mixture hydrates lack inflections that can be clearly attributed to

4

decomposition of gas hydrates starting at equilibrium temperatures. Weak inflections marked Stage

5

(2) in the Table 3 are manifested at somewhat higher temperatures. Additional work is necessary for

6

clear interpretation of these effects. In the case of CO2 equilibrium decomposition temperature of

7

the hydrate coincides with the interval of temperatures where evaporation of solid CO2 and

8

degassing of the frozen oversaturated CO2 solution occurs in our experiments. These processes are

9

undistinguishable. The data discussed above made it apparent that self-preservation of gas hydrates

10

immersed in crude oils is not dependent on the type of hydrate former. In our experiments we

11

observed effective self-preservations of gas hydrates formed by hydrate formers with significantly

12

different chemical and physical properties. We speculate that heavy components of crude oil

13

adsorbed on the hydrate surface are of primary importance for formation of a dense ice shell on this

14

surface and, consequently, define effective self-preservation of the hydrate. The impact of all other

15

factors on the process of self-preservation appears insignificant in this case.

16 17

CONCLUSIONS

18

The results of the present work revealed that self-preservation of gas hydrate particles may

19

be enhanced by immersing them into crude oil or mixture of crude oil with some organic solvents.

20

For example, the powder of pure hydrate with particle sizes 50-150 µm almost completely

21

decomposes around the equilibrium temperature while the suspension of the same powder in oils

22

undergoes self-preservation and can be stored at ∼ -20°C for dozens of hours. The performance of

23

self-preservation appeared to be independent on method of the suspension preparation and the type

24

of the hydrate former. At the same time it was demonstrated that dilution of crude oil with decane or

25

toluene results in the decrease of the self-preservation efficacy at least at high extent of dilution. No

26

self-preservation was detected for suspensions of methane hydrate in pure toluene and decane. The 13 ACS Paragon Plus Environment

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Page 14 of 31

1

most probable cause of effective self-preservation is adsorption of heavy components of oil on gas

2

hydrate surface that is favorable for formation of dense ice shell on the hydrate surface.

3 4

ACKNOWLEDGMENT. We sincerely grateful to the reviewers for their help in improving the

5

text.

6 7

REFERENCES

8

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(3) Istomin, V. A.; Yakushev, V. S. Gas–Hydrates Self-Preservation Effect. In Physics and

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Kuhs, W. F.; Genov, G.; Staykova, D. K.; etc. Ice perfection and onset of anomalous

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Conference on Gas Hydrates (ICGH 2011); Edinburgh, Scotland, United Kingdom, 17–21

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Thermal History on the Gas Content of Natural Gas Hydrate Pellets under Ambient

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hydrate storage and transportation processes. Energ. Convers. Manage. 2008, 49 (10), 2546–

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Gas Hydrates (ICGH8-2014); Beijing, China, 28 July–1 August, 2014.

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on Emulsion Stability Using DSC and Visual Techniques. Chem. Eng. Sci. 2008, 63, 3942–

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Takeya, S.; Fujihisa, H.; Gotoh, Y.; etc. Methane Clathrate Hydrates Formed within

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Hydrophilic and Hydrophobic Porous Media: Kinetics of Dissociation and Distortion of

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Host Structure. J. Phys. Chem. C 2013, 117, 7081–7085.

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1

Energy & Fuels

Tables Table 1. Composition and Properties of the Crude Oils and Emulsions Used in This Work. VOF

GOF

SOF

MOF

decane

toluene

Content of asphaltenes (wt %)

0,1

2,2

1.5

0.5

absent

absent

Content of paraffins (wt %)

2,3

5,1

1.9

-

absent

absent

Content of resins (wt %)

19,7

5,1

10.7

4.0

absent

absent

Solidification temperature (°C)

-43

+6

-16

-15

-30

-95

Density (kg/m3)

858

863

867

841

730

867

Viscosity (mPa s)

19.3

25.1

-

-

0.85

0.56

Density of the emulsion (kg/m3)

919

931

890

890

-

-

Viscosity of the emulsion (mPa·s)

184.9

130.8

25.2

45.5

-

-

the emulsion (µm) (standard

20 (9)/

16 (4)/

37 (23)/

22 (7)/

72 (23)/

31 (13)/

deviation of the average size)/size

6-69

6-25

12-155

13-41

19-196

10-88

0.26

0.34

0.08

0.22

0.07

0.14

Average size of water droplets in

range (µm) Oil – water interface area per gram of water, m2.

2


Energy & Fuels

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Page 20 of 31

Table 2. Summary of the Data Obtained in Thermovolumetric Experiments and Some Characteristics of the Samples with the Methane Hydrate Disperse №a

t b, day

D c, mm

αd

V1 e, ml/g

V2 e, ml/g

V3 e, ml/g

MOF

1/1 f 2/1 g 2/2 g 2/3 g

4 4 4 4

< 0.3 < 0.5 ≈3 < 0.5

0.97 0.97 0.97 0.97

45.4 49.8 37.3

215.2 79.6 36.4 82.2

2.8 137.5 185.7 134.4

Decane

1/1 1/2

1 1

< 0.5 ≈3

0.58 0.67

-

126.6 145.6

-

Toluene

1/1

1

≈3

0.50

4.7

109.6

-

1/2

1

≈3

0.57

5.1

122.0

-

SOF50D50

1/1 1/2

1 1

≈3 ≈3

0.67 0.76

26.8 29.6

50.8 56.8

95.8 107.6

SOF50T50

1/1

1

≈3

0.60

30.0

9.6

121.8

1/2

1

≈3

0.64

19.0

11.0

128.2

1/1

1

< 0.5

0.67

28.9

44.6

105.9

1/2

1

≈3

0.76

28.2

23.9

149.5

SOF75T25

1/1 1/2

1 1

≈3 ≈3

0.48 0.50

20.0 15.6

5.8 11.4

99.4 99.6

SOF

1/1

1

≈3

0.24

20.0

6.4

47.4

1/2

1

≈3

0.30

19.8

7.6

60.0

medium

SOF25T75

a

N/M is a number of an experiment, where N is a number of the synthesis and M is a number of the experiment with

the given type of oil. b Time of synthesis. c Particle size of the suspension sample. d α is a fraction of water to hydrate conversion determined from thermovolumetric data. e Volume of methane emitted in stages 1, 2, and 3, respectively. Experimental values not redused to normal conditions are presented.

f

Pure methane hydrate before mixing with

MOF oil. g The suspension was prepared by pure methane hydrate and MOF oil mixing.

1


Page 21 of 31

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Energy & Fuels

Table 3. Summary of the Data Obtained in Thermovolumetric Experiments and Some Characteristics of the Samples with the Hydrates of Different Gases Oil

Gas

C2H6

GOF

№a

t b, day

D c, mm

T1 (∆T1) d, °C

T2 (∆T2) d, °C

T3 (∆T3) d, °C

1/1

14

≈3

-72.5 (16.8)

-8.3 (9.3)

+1 (5.4)

1/2

14

≈3

-72.5 (21.1)

-8.8 (7.1)

+2.7 (4.9)

1/3

14

≈3

-77.7 (17.7)

-9.4 (11.5)

+1.8 (5.4)

1/1

14

≈3

-32.4 (15.0)

-

-

1/2

14

≈3

-29.8 (20.5)

-

-

1/1

22

≈3

-

-2.6 (4.0)

+1.5 (4.2)

1/2

22

< 0.5

-

-4.1 (4.1)

+0.3 (4.7)

1/3 1/1

22 14

< 0.5 ≈3

-39.1 (10.2)

-4.3 (5.4) -

+1.2 (5.5) -

2/1

14

≈3

-

-12.4 (9.2)

-0.2 (3.9)

2/2

14

≈3

-

-9.7 (5.4)

-0.3 (5.5)

1/1

31

≈3

-60.6 (30.6)

-15 (4.4)

-1.4 (9.3)

C3H8

Mixture

C3H8 VOF CO2 a

N/M is a number of an experiment, where N is a number of the synthesis and M is a number of the experiment with

the given type of oil. b Time of synthesis. c Particle size of the suspension sample. d Temperature at the half-width (full width of temperature range at half maximum) of gas emitted peak in stages 1, 2, and 3, respectively.

1


Energy & Fuels

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Page 22 of 31

1

Figure captions

2

Figure 1. Scheme of the process of mixing the methane hydrate with oil; 1 – high pressure cell , 2 –

3

fine methane hydrate powder, 3 – valve, 4 – pressure-gauge, 5 – MOF oil saturated with methane, 6

4

– methane hydrate/MOF oil suspension.

5 6

Figure 2. Comparison of gas evolution curves for methane hydrate; □ – hydrate powder without oil

7

(experiment MOF 1/1), ○ – ground frozen hydrate–MOF oil mixture (experiment MOF 2/1), ● –

8

frozen fragment of the hydrate–MOF oil mixture (experiment MOF 2/2). (1) – (3) are stage of gas

9

release from sample; more additional information is given in the text

10 11

Figure 3. Top: (a) methane hydrate powder before mixing with oil, (b) – after mixing;

12

scale bars at the picture equal to 10 mm. Bottom: schematical representation of hydrate powders

13

before (left) and after (right) mixing with oil. D – particle size of the suspension sample; d – particle

14

size of the gas hydrate.

15 16

Figure 4. (a) gas evolution curve of methane hydrate/MOF oil suspension (experiment MOF 2/3);

17

(b) the volume of gas emitted at -20°С during 6 hours.

18 19

Figure 5. Hydrates decomposition in different disperse media; (a) ○ – methane hydrate/toluene

20

(experiment toluene 1/2), ● – methane hydrate/decane (experiment decane 1/2); (b) ▽ – methane

21

hydrate/SOF oil (experiment SOF 1/2), △ – methane hydrate/ SOF75T25 (experiment SOF75T25

22

1/2),

23

hydrate/SOF50D50 (experiment SOF50D50 1/2), ◇ – methane hydrate/SOF25T75 (experiment

24

SOF25T75 1/2); (c) □ –ethane hydrate/GOF oil (experiment 1/1), △ – propane hydrate/VOF oil

25

(experiment 2/2), ● – hydrate of methane-ethane-propane mixture/GOF oil (experiment 1/1), ▽ –

●

–

methane

hydrate/SOF50T50

(experiment

SOF50T50

1/2),

□

–

methane


Page 23 of 31

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Energy & Fuels

1

carbon dioxide hydrate/VOF oil (experiment 1/1); in case of carbon dioxide hydrate stage (1) is not

2

presented for clarity.

3


Energy & Fuels

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

Figure 1. Scheme of the process of mixing the methane hydrate with oil; 1 – high pressure cell , 2 – fine methane hydrate powder, 3 – valve, 4 – pressure-gauge, 5 – MOF oil saturated with methane, 6 – methane hydrate/MOF oil suspension. 202x280mm (300 x 300 DPI)

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Energy & Fuels

Figure 2. Comparison of gas evolution curves for methane hydrate; □ – hydrate powder without oil (experiment MOF 1/1), ○ – ground frozen hydrate–MOF oil mixture (experiment MOF 2/1), ● – frozen fragment of the hydrate–MOF oil mixture (experiment MOF 2/2). (1) – (3) are stage of gas release from sample; more additional information is given in the text 89x57mm (300 x 300 DPI)


Energy & Fuels

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Figure 3. Top: (a) methane hydrate powder before mixing with oil, (b) – after mixing; scale bars at the picture equal to 10 mm. Bottom: schematic representation of hydrate powders before (left) and after (right) mixing with oil. D – particle size of the suspension sample; d – particle size of the gas hydrate.

121x130mm (300 x 300 DPI)


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Energy & Fuels

Figure 4. (a) gas evolution curve of methane hydrate/MOF oil suspension (experiment MOF 2/3); (b) the volume of gas emitted at -20°С during 6 hours.

124x93mm (300 x 300 DPI)


Energy & Fuels

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Figure 4. (a) gas evolution curve of methane hydrate/MOF oil suspension (experiment MOF 2/3); (b) the volume of gas emitted at -20°С during 6 hours.

122x96mm (300 x 300 DPI)


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Energy & Fuels

Figure 5. Hydrates decomposition in different disperse media; (a) ○ – methane hydrate/toluene (experiment toluene 1/2), ● – methane hydrate/decane (experiment decane 1/2); (b) ▽ – methane hydrate/SOF oil (experiment SOF 1/2), △ – methane hydrate/ SOF75T25 (experiment SOF75T25 1/2), ● – methane hydrate/SOF50T50 (experiment SOF50T50 1/2), □ – methane hydrate/SOF50D50 (experiment SOF50D50 1/2), ◇ – methane hydrate/SOF25T75 (experiment SOF25T75 1/2); (c) □ –ethane hydrate/GOF oil (experiment 1/1), △ – propane hydrate/VOF oil (experiment 2/2), ● – hydrate of methane-ethane-propane mixture/GOF oil (experiment 1/1), ▽ – carbon dioxide hydrate/VOF oil (experiment 1/1); in case of carbon dioxide hydrate stage (1) is not presented for clarity. 276x195mm (299 x 299 DPI)


Energy & Fuels

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Energy & Fuels



Self-Preservation of Gas Hydrate Particles Suspended in Crude Oils

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