Accelerated Methane Hydrate Formation by Ethylene Diamine

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Applied Chemistry

Accelerated methane hydrate formation by ethylene diamine tetraacetamide as an efficient promoter for methane storage without foam formation Abdolreza Farhadian, Mikhail A. Varfolomeev, Zeinab Abdelhay, Dmitrii Emelianov, Antoine Delaunay, and Didier Dalmazzone Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00803 • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 16, 2019

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Industrial & Engineering Chemistry Research

1

Accelerated methane hydrate formation by ethylene

2

diamine tetraacetamide as an efficient promoter for

3

methane storage without foam formation

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Abdolreza Farhadiana,b, Mikhail A. Varfolomeeva,b*, Zeinab Abdelhaya, Dmitrii Emelianova,

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Antoine Delaunayc, Didier Dalmazzonec

6

a

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420008 Kazan, Russian Federation.

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b

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420008 Kazan, Russian Federation.

Department of Physical Chemistry, Kazan Federal University, Kremlevskaya str. 18,

Department of Petroleum Engineering, Kazan Federal University, Kremlevskaya str. 18,

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c UCP,

ENSTA ParisTech, Université Paris-Saclay, 828 Boulevard des Maréchaux, 91762

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Palaiseau Cedex, France.

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* Corresponding Author E-mail address: [email protected].

13 14 15

ABSTRACT

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Reduction in the induction period of hydrate formation and enhancement of the gas hydrate

17

growth are vital parameters in the application of gas hydrates technique for methane storage. In

18

this work, ethylene diamine tetraacetamide (EDTAM) was developed as a new promoting agent

19

for methane storage. The effect of EDTAM on methane hydrate formation parameters was

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Industrial & Engineering Chemistry Research 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

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evaluated by high-pressure micro differential scanning calorimeter (HP-DSC) and high-pressure

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autoclave cell at 2 oC and 8.0 MPa as the static and dynamic conditions, respectively. The results

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demonstrated that the onset temperature of methane hydrate formation was increased from -14

23

oC

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water to hydrate was multiplied by a factor of 17.8 (from 3.5 wt. % in pure water to 62.3 wt. %

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in EDTAM solution) due to the presence of 0.5 wt. % of EDTAM. Moreover, the EDTAM

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reduced induction time of methane hydrate formation by a factor of 4 in comparison with pure

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water in both static (DSC experiments) and dynamic (autoclave experiments) conditions. Also,

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EDTAM increased considerably the amount of consumed mole of methane, especially during the

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first stage of hydrate growth. Unlike surfactants, the addition of EDTAM does not induce

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formation of large amount of foam in hydrate formation/dissociation process. All obtained

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results confirmed that the EDTAM is an efficient and prospective promoter for methane hydrate

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formation.

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Keywords: methane storage; without foam formation; gas hydrate; promoter; DSC

in the pure water system to -3 oC by adding 0.5 wt. % EDTAM. The average conversion of

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INTRODUCTION

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Undoubtedly, gas hydrates are high interest compounds in the world of chemistry because they

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cause a great nuisance in the industry by plugging the oil and gas pipelines. Therefore, they

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impose excess operating costs and may result in shutdown of onshore and offshore production.1

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On the other hand, they are a promising option for some applications such as gas separation,

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desalination of seawater, gas storage and transportation.2–6 Gas hydrates (or clathrates) are not

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chemical compounds, but they are crystalline and non-stoichiometric compounds formed by


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water and gas molecules under suitable thermodynamic conditions, i.e. high pressures and low

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temperatures.1,7,8 Compared to heavier fossil fuels, which lead to emit large amounts of

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greenhouse gases into the atmosphere and accelerate the global warming, natural gas, which

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mainly contains CH4 is considered as a much cleaner energy source. There are several procedures

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for the storage and transportation of natural gas, including adsorbed natural gas (ANG)

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compressed natural gas (CNG), solidified natural gas (SNG) and liquefied natural gas (LNG). It

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should be noted that CNG strategy faces two major drawbacks that include poor volumetric

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storage capacity and safety concerns. Although, LNG has been intended as the proper procedure

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for large scale and long-distance transportation of gas, it needs to be stored at extraordinarily low

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temperatures (∼-160 oC). On the other hand, the transportation of the natural gas through

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pipeline is not ever usable due to the distance, probability and availability of the delivery

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location.9–11 Among these procedures, SNG technology is the most promising alternative to store

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and transport natural gas in the form of hydrates, which is based on hydrate formation and

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dissociation cycles. The SNG technology provide several great advantages, including: the

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hydrate formation is environmentally friendly due to the use of water and a small amount of

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additives, the molecular form of gas will be maintained in the hydrate form, 1 m3 of hydrate will

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store ∼160 m3 of gas at STP condition and gas hydrates can be stored at ∼-25 oC and

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atmospheric pressure.9 Finally, the gas hydrate-storage strategy will provide a safer technology

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due to non-explosive nature of the gas hydrate.9,10,12–14 However, the most important limitation

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that hinders this technology for large-scale applications is the low rate of hydrate formation,

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which results in low final gas absorption. There are two ways to overcome this constraint: i)

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provide the highest gas-water contact through the development of novel reactors and ii) using

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some additives to enhance gas hydrate formation kinetics.15–18 The thermodynamic hydrate

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promoters like tetrahydrofuran (THF) and cyclopentane (CP) will alter the temperature/pressure



66

conditions of hydrate formation. On the other hand, these thermodynamic promoters reduce the

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gas uptake of the hydrate structure by taking the place of gas molecules in a part of the clathrate

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cavities. Low dosage kinetic hydrate promoters can decrease the onset time of hydrate

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nucleation, improve the hydrate growth rate and they are able to increase the water-to-hydrate

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conversion. As a result, utilizing the kinetic promoters is valuable in decreasing the process cost

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of the hydrate-based applications.15,19–21 During the recent years, several type of materials have

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been studied as kinetic hydrate promoters, like surfactants, such as sodium dodecyl sulfate

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(SDS), amino acids, polymers and starches.22–27 In addition, using the graphite and carbon

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nanotubes and fumed silica as hydrophobic solid particles is another feasible option to enhance

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the kinetic of gas hydrate formation. Also, the application of nanoparticles (like copper oxide and

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silver nanoparticles) and polymer nano spheres in the reagent has been also proved to be a

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promising idea in gas hydrate formation by increasing the storage capacity, water to hydrate

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conversion and decreasing the onset nucleation time.28–35 It should be noted that, an important

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disadvantage of surfactants is that, during hydrate formation/dissociation process they generate a

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large amount of foam when gas is released from the system.30,36–38 The generation of foam is a

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big problem in handling the process. Hence, the research and design of different type of

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promoters to enhance the hydrate formation kinetics without foam formation is necessary.38–40 In

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this study, we introduce a novel application of diamines to increase the methane hydrate growth

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rate and reduce the onset temperature of hydrate formation in comparison with pure water

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system. Evaluations proved that synthesized ethylene diamine tetraacetamide (EDTAM) in this

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work can promote formation of methane gas hydrates under different initial pressures and

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different concentrations without any foam formation. Autoclave and DCS experiments were

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conducted with and without promoter to clarify the difference in formation induction time and

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hydrate growth rate.


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MATERIALS AND METHODS

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Materials

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Ethylene diamine tetra acetic acid powder ( ≥ 98%), ethanol (98%), sodium chloride (99.5%),

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sodium bicarbonate (99.5%), ammonia solution (98%), and magnesium sulfate (97%) were

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purchased from Vekton Company. Diethyl ether (99%) and liquid sulfuric acid (99%) were

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provided from Tat Chim Product Company. All reagents were used without further purification.

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Methane gas of 99.95 wt.% purity was used for experiments. Distillated and further deionized

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water was used for autoclave and DSC experiments and for preparation of promoter solutions

98

with desired concentration.

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Characterization Methods

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A Vertex 70 FT-IR spectrometer (Bruker, Germany) with single reflection ZnSe crystal ATR

101

accessory (MIRacle, PIKE Technologies) was used to record infrared spectra (600–4000 cm−1)

102

and the data was processed through program OPUS 7.2 (Bruker). FT-IR spectra can be found in

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supporting information (Figure S2). NMR spectra were recorded on a Bruker Avance400-MHz.

104

The spectra were recorded with a frequency of 400 MHz by using deuterated DMSO as a

105

solvent, at room temperature. The chemical shifts were referenced to the solvent signals.

106 107 108

Synthesis of EDTAM

109

procedure: first, (19.96 g, 0.069 moles) of ethylene diamine tetra acetic acid was dissolved in the

110

hot ethanol. Then, the gaseous hydrochloric acid (HCl) was added to boiling solution, which was

111

generated from the reaction of H2SO4 and NaCl, as the following equation:

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Ethylene diamine tetraacetamide was synthesized (Figure 1) according to the following

NaCl (s) + H2SO4 (l)

HCl (g) + Na2SO4 (l)



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The experimental setup is shown in Figure S1 to generate directly gaseous HCl. This method was

114

used as a bubbler for the boiling solution to receive gaseous HCl. After 3 hours of refluxing

115

solution, solution was concentrated under vacuum by using Rotary Evaporator R-3000. While

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the product being cooled in an ice bath, 200 ml mixture of water/diethyl ether was added to

117

solution. Then, sodium bicarbonate was used to neutralize solution and it was extracted 5 times

118

with 50 ml of diethyl ether. The organic phase was washed with 20 ml of saturated NaCl

119

solution, dried over MgSO4 and the solvent was evaporated by rotary. Obtained product (15.18 g)

120

was dissolved in a mixture of 200 ml of methanol and 6 ml of NH3 (note: while adding the liquid

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NH3, the methanol was cooled to hinder boiling and loss of ammonia). The mixture was

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transferred to a round-bottom flask and left undisturbed until crystals began to appear. Crystals

123

were filtered off and the yield was 11.26 g (87 %). 1H NMR spectra (400 MHz, deuterium oxide)

124

of this product has the next signals: δ 7.10 (s, 0H) for NH2 groups, 4.71 – 4.62 (m, 2H) for

125

solvent, 3.21 (s, 2H) for methylene groups adjacent to amide and 2.64 (s, 1H) for methylene

126

groups adjacent to tertiary (3°) amine. In 13C NMR (101 MHz, Deuterium Oxide) we observed

127

several chemical shifts: δ 176.65 for carbon atoms in the amide carbonyl group, 57.76 for carbon

128

atoms adjacent to amide and 52.92 for carbon atoms adjacent to tertiary (3°) amine. Also in the

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FT-IR spectra these peaks are present: 3200-3400 cm-1 for tertiary (3°) amine and primary (1°)

130

amine in amide group and 1643 cm-1 for carbonyl in amide group. 1HNMR and 13C NMR spectra

131

are presented in Supporting Information (Figure S3-4). Both methods confirmed the structure of

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synthesized compound shown on Figure 1.


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133 134 135 136


Figure 1. Scheme of EDTAM (ethylenediamine-N, N, N’, N’-tetraacetamide) synthesis.



137

Apparatus and Methodology

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Differential Scanning Calorimetry (DSC) measurements

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A high-pressure micro differential scanning calorimeter SETARAM 7 evo (HP-DSC) equipped

140

with two 0.19 ml high pressure cells was applied to investigate the hydrate formation and

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dissociation process (Scheme S1). DSC has proven to be very accurate technique for

142

investigating hydrate formation.41,42 The principle of DCS is the heat flow measurements through

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a cell containing water and methane (or water + additive and methane), while temperature is

144

being controlled. Upon cooling cycle the hydrate forms, and then the hydrate dissociation occurs

145

during the heating cycle. A hydrate formation leads to heat release (exothermic), while

146

dissociation needs energy from the system (endothermic). The curves of heat flow are thus

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reported on a graph and gives not only the dissociation and formation temperatures, but also,

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after further calculations, the amount of ice or hydrate being obtained. It was published

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previously41 that in standard DSC cell in static conditions (there is no mixing in the system) there

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are some limitations due to low mass transfer. Capillary tubes (in our case four) with a diameter

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2.3 mm and length 9 mm were used to overcome this limitation by increasing the surface area of

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the liquid-gas interface. These tubes were filled by studied solution (volume 2 μl) and placed in

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the cell. Afterwards, the pressure inside the cell was fixed to a desired value with methane (99.5

154

wt. % purity) and temperature program was started. An isothermal and temperature ramping

155

method at constant cooling/heating rate were used in this study as temperature programs. In the

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isothermal method, once the pressure reached to 80 MPa, the temperature was dropped quickly

157

from 25 °C to −12 °C at 1 °C/min and kept constant at −12 °C for 10 hours. The hydrate

158

nucleation event was recorded by onset time of exothermic peak appearance at this constant

159

temperature. In the constant rate temperature ramping, the temperature was decreased from 20 to

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−35 °C at different rates (0.2, 0.5, 0.75 and 1.0 oC/min) then raised from −35 to 20 °C at a rate of


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0.25 °C/min. Several initial pressures have been tested out: 3, 5, 8.5, 11 and 14 MPa for

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methane. The sample contained 100 or 200 mg of solution with different concentration of

163

additive: pure water; water with 0.5 wt.% of additive.

164

High-pressure autoclave cell

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A high-pressure autoclave cell (Scheme S2) was applied to determine induction time and growth

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rate of methane hydrate with and without of EDTAM. During the formation of gas hydrate,

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methane is encaged into the hydrate structure, therefore pressure inside the system decreases. A

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sudden pressure drop and temperature rise indicates the formation of gas hydrates. Thus, from

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the beginning of the experiment, the temperature and pressure were continuously monitored

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while gas hydrate formation occurred. A volume of 30 ml of solution (pure water or 0.1, 0.3 and

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0.5 wt.% of EDTAM) was charged in a 50 ml autoclave cell that was kept under vacuum to

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avoid the possibility of remaining gases. Autoclave was purged by methane gas with a nominal

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purity of 99.95% at least 3 times. After that, methane was injected into the cell at a pressure of 8

174

MPa. The high-pressure autoclave cell was maintained at 2 oC and the solution was stirred at 400

175

rpm during the whole experiments. The time zero was designated at the point when the desired

176

autoclave cell temperature and pressure had been reached. The pressure of the gas and

177

temperature of the solution were recorded while stirring the solution. Once the formation of the

178

hydrate began, a sudden drop in pressure was observed.

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RESULTS AND DISCUSSION

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Hydrate formation in DSC

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Hydrates formation and dissociation process depend not only on temperature, pressure and gas

182

composition, but also on the presence of inhibitors or promoters.43 The results of different

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experiments at 8 MPa, with different cooling rates in the absence and presence of EDTAM gave

184

various onset temperatures of methane hydrate formation and heat quantities that are presented in



185

Table 1. As seen in Figure 2, onset temperature of methane hydrate formation in pure water

186

system at different cooling rates is about -14 oC. Figure 3 illustrates the heat flow signals

187

obtained during the hydrate formation in a 0.5 wt. % EDTAM solution at different cooling rates.

188

In EDTAM systems onset temperature of methane hydrate formation has increased to -3 oC,

189

depending on the cooling rate. Also, by comparing the peak area in DSC thermograms, it can be

190

concluded that more hydrate was formed in the presence of EDTAM in comparison with pure

191

water system. Figure 4 shows a comparison of the heat flow curves obtained with these two

192

systems for two different cooling rates. This comparison shows well that the intensity of DSC

193

thermograms in the presence of EDTAM is much higher than water. These results clearly show

194

that, the presence of small amounts of EDTAM in water can remarkably increase the onset

195

temperature of hydrate formation and increase the amount of hydrates formed. Both these

196

findings confirm that EDTAM can be an efficient promoter for methane hydrate formation. By

197

closely examining Figure 4, as well as data from Table 1, it is observed that at faster cooling

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rates, methane hydrates are formed at a lower temperature and the temperature changes are more

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significant in the water system. Since hydrate formation is governed by a crystallization process,

200

at fast cooling rates water molecules do not have enough time to orient themselves for

201

crystallization, so a higher driving force is required to crystallization. It is related to the time

202

required for the molecular organization before nucleation of hydrates. Thus, at higher cooling

203

rates the onset of nucleation shifted to a lower temperature than lower cooling rate. A slow

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cooling rate provides sufficient time for molecular organization and crystallization occur higher

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temperature. It also has been reported in the literature the crystallization exotherm was shifted

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towards lower temperatures as the cooling rate was increased.44

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208 209

Figure 2. Typical DSC thermograms for methane hydrate formation in pure water at the

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different cooling rates (the first cooling cycle at 8 MPa, standard uncertainties u are u(T)=0.1

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oC,

u(P)=0.002 MPa).

212 213 214



215 216

Figure 3. Typical DSC thermograms of methane hydrate formation in the presence of 0.5 wt. %

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EDTAM at the different cooling rates (the first cooling cycle at 8 MPa, standard uncertainties

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u are u(T)=0.1 oC, u(P)=0.002 MPa).

219 220


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Table 1. Summarized data of onset temperature and heat release of methane hydrate formation at

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different cooling rates with and without promoter (0.5 wt. %) at 8 MPa with three

223

cooling/heating cycles (standard uncertainties u are u(T)=0.1 oC, u(P)=0.002 MPa).

Pure water

EDTAM

0.5 0.75 1.0 0.5 0.75 1.0

-14.13 -14.26 -14.53 -3.26 -3.71 -6.03

7.152 6.428 5.450 210.1 219.6 223.7

0.5 0.75 1.0 0.5 0.75 1.0

-12.9 -13.5 -11.3 -3.4 -4.2 -4.2

7.321 6.124 5.496 234.7 225.4 237.7

224 225

226


0.5 0.75 1.0 0.5 0.75 1.0

-14.2 -14.2 -14.5 -3.3 -3.2 -5.7

Quantity of heat (J ∕ g)

Onset temperature (◦c)

3rd cooling cycle Cooling rate (oC ∕min)



Cooling rate (oC ∕min)

2nd cooling cycle



1st cooling cycle Cooling rate (oC ∕min)

Sample Sample

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


5.965 5.471 6.397 162 174.5 154.5


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Figure 4. Comparison of DSC thermograms of methane hydrate formation in the pure water and

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0.5 wt. % EDTAM system at different cooling rates (the first cooling cycle at 8 MPa, standard

229

uncertainties u are u(T)=0.1 oC, u(P)=0.002 MPa).

230

The enthalpy of fusion of any chemical compound is equal to the heat effect of the phase

231

transition of solid to liquid state at constant pressure, as following equation:

232

Q = mLfusion

(1)

233

Q represents the energy released (crystallization) or absorbed (fusion) by the substance, m is the

234

mass of substance used in DSC experiments and Lfusion is the latent heat of melting of the

235

substance. The obtained energy release (absorption) and transition temperatures provide a way

236

for distinguishing the heat amount associated with the formation or dissociation of ice and

237

hydrate. After integrating the corresponding heat flows it is possible to calculate the quantity of

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ice and hydrates formed. The value of the latent heat of fusion of ice was taken from CRC

239

handbook and is equal to 333.6 J∕g.45 The latent heat of dissociation of methane hydrate at 10 oC

240

and 7.8 MPa is equal to 424.6 J∕g according to Voronov's work 46. From Eq. (1) we can get Eq.

241

(2) for determination of ice and hydrate masses: Q L fus

242

m

243

In clathrate hydrate structure I, there are 46 water molecules for 8 cavities. So, the mass of water

244

in one mole of hydrate is 18 × 46 = 828 g. The cage occupancy of non-stoichiometric methane

245

hydrate was taken from the work of Kang et al.47 who determined a composition of

246

CH4∙6.38H2O, thus according to this result 7.21 CH4 molecules will be distributed in 46 H2O

247

molecules. So, the water content in hydrates is 87.8 wt. %. Values of hydrate mass, wt. % of

248

water converted into hydrate and ∆H of hydrate formation were summarized in Table 2 for pure


(2)

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water and EDTAM systems at pressures 5, 8.5, 11, and 14 MPa. As seen in Table 2, in pure

250

water system (at 5, 8.5, 11, and 14 MPa) the average conversion of water to hydrate is 3.1 wt. %.

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Compared to that, in 0.5 wt. % EDTAM solution the average conversion rate of water to hydrate

252

is 64 wt. %, which is 18 times higher than in pure water. A good reproducibility of results was

253

obtained even at different pressures. The maximum mass of hydrate and wt.% of water converted

254

into hydrate in pure water are equal to 3.8 mg and 4.0 wt. %, whereas these values for EDTAM

255

solution achieve 80.5 mg and 70 wt. %, respectively. These results clearly show that, EDTAM

256

has a promoting effect on the amount of methane hydrate formation.

257 258

Table 2. DSC data for pure water and 0.5 wt. % EDTAM aqueous solution (average of the three

259

tests) in 100 mg sample (standard uncertainties u are u(T)=0.1 oC, u(P)=0.002 MPa).

260

Sample

Pure water

EDTAM

Initial pressure (MPa) 5 8.5 11 14 5 8.5 11 14

Mass of hydrate (mg)

∆Hhydrate (J)

3.0 3.2 3.1 3.8 67 80.5 72 76

1.31 1.4 1.3 1.6 29.1 34.3 31.3 32.1

Wt. % of water converted into hydrate 3.1 2.9 2.7 4.0 57.7 70 63.5 65

261 262 263

Table 3. DSC data for 0.5 wt. % EDTAM aqueous solution (average of the three tests) in 200

264

mg sample (standard uncertainties u are u(T)=0.1 oC, u(P)=0.002 MPa).

265

Initial pressure (MPa)

Mean mass of hydrate (mg)

∆Hhydrate (J)

8.5

141

60


Mean wt. % of water converted into hydrate 61


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11

125.5

53.3

54

14

124

52.6

53

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We did some experiments with 200 mg solutions in addition to measurements with 100 mg to

269

investigate the possibility of hydrate formation on the wall of autoclave cell, because usually this

270

behavior observes in the presence of surfactant promoters and hydrate sticks to the walls by

271

capillarity effect. In experiments with 100 mg EDTAM the cell was 2/3 empty and so, there is

272

the probability of occurrence of this phenomenon. But in experiments with 200 mg of EDTAM

273

less than 1/3 of the cell’s surface was available. In accordance with the results of Tables 2 and 3,

274

increasing the sample mass from 100 to 200 mg led only to small decrease in the fraction of

275

water converted to hydrate. Thus, it shows that the surfactant-like mechanism did not play an

276

important role in EDTAM solution.

277

Hydrate nucleation

278

Hydrate nucleation process was studied by DSC technique in isothermal conditions for kinetics

279

evaluation. The promotion effect can be evaluated in terms of reduction in nucleation time for a

280

given temperature. The comparison of isothermal runs for pure water and 0.5 wt. % EDTAM

281

solutions are shown in Figure 5. The hydrate nucleation is detected by sharp exothermic peaks as

282

time progresses at constant temperature −12 °C. As expected, nucleation was promoted

283

remarkably in the EDTAM system and the mean of hydrate nucleation time in the presence of

284

EDTAM (~1 hour) decreased 4 times compared to pure water system (~4 hours). In addition, the

285

Figure 6 show the melting behavior of methane hydrates in the pure water and EDTAM systems.

286

As seen in Figure 6, the area of melting peaks show the amount of ice/hydrate formed during the

287

experiment. By comparing the melting peak in the two systems it can be understood that, in the

288

presence of EDTAM, the amount of hydrate is higher than in pure water system. This indicates


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289

that EDTAM significantly promotes hydrate growth after initial nucleation. The remarkable

290

point to note here is that EDTAM does not influence the thermodynamics of methane hydrate

291

formation and the dissociation temperature of methane hydrates was the same in the presence

292

and absence of EDTAM. This means that the EDTAM acts as a kinetic promoter and does not

293

affect the hydrate phase equilibrium curve. Both ramping and isothermal experiments revealed

294

that EDTAM is an effective promoter for hydrate formation process.

295 296



297

Figure 5. Typical DSC heat flow and temperature signal in pure water and 0.5 wt.% EDTAM

298

aqueous solution samples obtained during hydrate formation step using an isothermal program.

299 300

Figure 6. The melting behavior of methane hydrates in pure water and 0.5 wt. % EDTAM

301

aqueous solution samples obtained during hydrate decomposition step using an isothermal

302

program.

303 304

Hydrate growth rate

305

Figure 7 shows the methane hydrates formed in the EDTAM system are ice-like and no foam

306

formation was observed during hydrate formation and dissociation. Analyzing data from high-

307

pressure autoclave cell experiments show significant difference in methane nucleation and

308

growth rate in the EDTAM system in comparison with pure water (Table 4 and Figure 8). The


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309

time period before significant pressure drop indicating the growth of hydrate crystals and was

310

defined as induction time. The results show that the EDTAM is effective kinetic promoter since

311

it promoted hydrate nucleation by an average of around 2 minutes (in 0.5 wt. % sample), as

312

compared to pure water, in which hydrates nucleated in ∼8 minutes. It was reported,36 that at 1

313

oC

314

enhanced nucleation time by a factor of 4.1 and 2.9 in comparison with pure water. But one of

315

the major problems with using SDS is the formation of foam during the hydrate

316

formation/decomposition steps. It is clear, that the onset of hydrate nucleation in the presence of

317

EDTAM is significantly promoted compared to the pure water. Also, no foam formation was

318

observed during the formation and dissociation of methane hydrates in the presence of EDTAM.

and 6 MPa sodium dodecyl sulfate (SDS) and cetyltrimethylammoniumbromide (CTAB)

319 320

Table 4. Experimental conditions and mean induction time for methane gas hydrate formation at

321

(a and b are the results of high-pressure autoclave and HP-μDSC experiments, respectively), the

322

standard uncertainties u are u(T)=0.1 oC, u(P)=0.005 MPa. Mean induction

Mean induction

Methane consumption

time, t̅ind (min) a

time, t̅ind (h) b

in the autoclave (wt. %)

Sample

Concentration

Water

-

8

4.5

41

EDTAM

0.1

5.9

3.1

53

EDTAM

0.3

4.2

2.6

61

EDTAM

0.5

2

1

75

323



324 325

Figure 7. The methane hydrate formed in the EDTAM system without foam formation ((a) and

326

(b) during hydrate formation and (c) after hydrate dissociation).

327 328 329

Figure 8. Hydrate yield and gas uptake curves during methane hydrate formation with different

330

concentrations of EDTAM at 2 oC and 8 MPa.


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331 332 333

To determine the promoter efficiency, the methane uptake and hydrate yield were calculated in

334

the high-pressure autoclave cell and the results are shown in Figure 8. After 350 minutes of onset

335

of hydrate formation, 41 wt. % of methane consumed in the autoclave (3.3 MPa of pressure

336

drop) in pure water and 75 wt. % of methane consumed (6MPa of pressure dropped) in the

337

presence of 0.5 wt. % of promoter. As shown in Figure 8, the addition of EDTAM leads to a

338

significant increase in the amount of consumed mole of methane, especially during the first 100

339

minutes of hydrate growth. The methane hydrate yield for water system is 35 wt. %, while by

340

adding 0.3 and 0.5 wt. % of EDTAM the hydrate yield increase up to 63 and 75 wt. %,

341

respectively. Figure 8 confirm that hydrate growth was significantly increased in the presence of

342

EDTAM. These results clearly show that, EDTAM is able to reduce the induction time and

343

increase hydrate growth rate in comparison with pure water system, significantly. A possible

344

explanation for this promotion effect is that the EDTAM by forming new hydrogen bonds with

345

water molecules will increase the number of cavities to incorporate gas molecules. Since the gas

346

hydrate is formed by a process based on formation of hydrogen bonds among water molecules,

347

each promoter can have good performance when they able to affect these hydrogen bonds. It can

348

be affected in two ways: first, increasing the number of hydrogen bonds by functional groups

349

such as OH, NH2, C=O, P and etc. Second, the molecule does not disturb the structure of the

350

water through groups such as aromatic or cyclic structures and alkyl chains, because of these

351

groups can create distance among hydrogen bonds. Thus, EDTAM is an efficient promoter for

352

methane hydrate formation because it has no cyclic groups or long alkyl chains and also has

353

ability to form additional hydrogen bonds with water molecules. This can be confirmed by the

354

DCS results in which the temperature changes due to the high cooling rate are more significant in

355

the EDTAM system. In fact, EDTAM with its ability to form additional hydrogen bonds enhance



356

the molecular organization of water molecules by cooperativity effect before nucleation of

357

hydrates. Further experiments are needed to evaluate the promotion performance of EDTAM and

358

to investigate the performance of EDTAM in systems with synthetic gases, it will be the purpose

359

of future work. The point to be noted here is that although EDTAM similar ethylene diamine

360

tetraacetate (EDTA) is poorly biodegradable,48,49 it has a good promotion effect and is also

361

readily synthesized in mild conditions.

362 363

CONCLUSIONS

364

Solidification technology can be one of the prospective methods for methane or natural gas

365

storage. However, the practical application of this technology can be improved by using of

366

chemical promoters of gas hydrates formation. In this work, we have proposed ethylene diamine

367

tetraacetamide (EDTAM) as a new type of methane gas hydrate promoter. Efficiency of

368

developed promoter was tested in static and dynamic conditions using differential-scanning

369

calorimetry and high-pressure autoclave cell. In both case EDTAM showed excellent promoting

370

properties:

371 372 373 374 375 376 377 378

- EDTAM increases the temperature of methane hydrate formation in aqueous solution from -14 oC in pure water to -3.2 oC at 0.5 wt. % concentration. - EDTAM decreases the induction time in both static and dynamic conditions on four times. - EDTAM improves the hydrate growth rate and total amount hydrate formed at ramping and isothermal experiments in comparison with pure water. - EDTAM does not induce foam formation in hydrate formation and dissociation process like previously proposed promoters based on surfactants.


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379

Obtained results confirmed that EDTAM possesses multiple promoting activities and has

380

prospective for practical application. However, the mechanism of gas hydrate formation in

381

presence of EDTAM should be studied in more details as well as the effect of water quality and

382

natural gas composition on its efficiency.

383 384

ACKNOWLEDGMENTS

385

This work has been carried out on the basis of the Russian Government Program of Competitive

386

Growth of Kazan Federal University and was supported by the Russian Foundation for Basic

387

Research Project N 18-05-70121.

388 389

Supplementary information is available free of charge on the

390

ACS Publications website.

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In this work, we have proposed a new type of methane gas hydrate promoter ethylene diamine tetraacetamide (EDTAM). Efficiency of developed promoter was tested in static and dynamic conditions using differential-scanning calorimetry and high-pressure autoclave cell. In both case EDTAM showed excellent promoting properties: increases the temperature of methane hydrate formation in aqueous solution from 14 oC in pure water to -3.2 oC at 0.5 wt.% concentration; decreases the induction time in both static and dynamic conditions on four times; improves the hydrate grow rate and total amount hydrate formed at ramping and isothermal experiments in comparison with pure water; does not induce foam formation in hydrate decomposition stage like previously proposed promoters based on surfactants. 784x447mm (120 x 120 DPI)


Accelerated Methane Hydrate Formation by Ethylene Diamine

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