In-Situ Viscosity Measurements of Cyclopentane Hydrate Slurry in

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In-Situ Viscosity Measurements of Cyclopentane Hydrate Slurry in Waxy Water-in-Oil Emulsions Yuchuan Chen, Bohui Shi, Yang Liu, Qianli Ma, Shangfei Song, Lin Ding, xiaofang Lv, Haihao Wu, Wei Wang, Haiyuan Yao, and Jing Gong Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 26 Mar 2019 Downloaded from http://pubs.acs.org on March 27, 2019

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Energy & Fuels

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In-Situ Viscosity Measurements of Cyclopentane

2

Hydrate Slurry in Waxy Water-in-Oil Emulsions

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Yuchuan Chen,† Bohui Shi,*,† Yang Liu,† Qianli Ma,† Shangfei Song,† Lin Ding,† Xiaofang Lv,‡

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Haihao Wu,† Wei Wang,† Haiyuan Yao,§ Jing Gong*,†

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† National Engineering Laboratory for Pipeline Safety/ MOE Key Laboratory of Petroleum

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Engineering/ Beijing Key Laboratory of Urban Oil and Gas Distribution Technology, China

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University of Petroleum-Beijing, Changping Beijing 102249, CHINA

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‡ Jiangsu Key Laboratory of Oil and Gas Storage and Transportation Technology, Changzhou

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University, Changzhou, Jiangsu 213016, CHINA

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§ Key Lab of Deepwater Engineering, CNOOC Research Institute, 100028, CHINA

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ACS Paragon Plus Environment

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ABSTRACT: With the tendency of offshore petroleum industry moving to the deep-water fields,

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there are several challenges for the exploration and development of oil and gas with higher paraffin

23

content in the deep-water severe environment, especially the complex flow assurance issues

24

including the coexistence of wax precipitation and hydrate formation. The effects of wax on

25

hydrate slurry viscosity, hydrate nucleation and growth, hydrate dissociation were investigated in

26

a rheometer. Results indicated the stage characteristics of viscosity evolution during hydrate

27

formation in waxy and wax-free emulsions were different, and two stages could be observed during

28

the hydrate growth process in the presence of wax. Hydrate slurry viscosity increased with the wax

29

content. The coupled hydrate-wax aggregates were difficult to be broken by the constant shearing

30

force. The shear-thinning property of hydrate slurry was not affected by the precipitated wax.

31

Hydrate formation was inhibited due to the wax precipitated in the oil phase. Specifically,

32

cyclopentane hydrate critical time and growth time increased with the wax content. The calculated

33

hydrate volume fraction decreased with the wax content based on the suspension viscosity model.

34

It was difficult for water bridge to form between two hydrate particles during the hydrate

35

dissociation process with 3.0 wt% and 5.0 wt% wax content, therefore, no obvious increase in the

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slurry viscosity was observed when the slurry viscosity decreased during the dissociation process.

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KEYWORDS: Flow assurance; Hydrate; Wax; Viscosity

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1. INTRODUCTION

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In deep-water petroleum production by the way of multiphase transport, conditions of low

41

temperature and high pressure which favor hydrate formation and wax precipitation can occur.1

42

Herein both hydrate formation and wax deposition are challenges faced by flow assurance

43

engineers.2, 3 Traditionally, a large amount of thermodynamic hydrate inhibitors (THIs) is injected

44

into pipelines for preventing hydrate formation and plugging, which results in economic and

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ecological challenges.4 Efforts nowadays are made on the strategy which is so-called risk

46

management, including anti-agglomerants (AAs)5-9 and kinetic hydrate inhibitors (KHIs).10-16 AAs

47

can prevent the agglomeration of hydrate particles in order to form hydrate slurry with easier

48

transportation ability. KHIs can adsorb on hydrate nucleus and inhibit the further growth of hydrate

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nucleus, resulting in the obstacle for hydrate nucleus reaching to the critical size. Besides, KHIs

50

can disturb the structure of water clusters, increasing the energy barrier for hydrate nucleation.17,

51

18

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in the presence of KHIs. The methods of risk management mentioned above reduce the cost needed

53

for flow assurance and protect the subsea ecology.19

Therefore, hydrate cannot form massively within the retention period of petroleum in pipelines

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So far, the laboratory experiments for the investigation on hydrate formation and flow ability of

55

hydrate slurry in the presence of wax can be clarified into four aspects, including hydrate

56

thermodynamics, hydrate kinetics, hydrate slurry rheological properties and flow behavior. Efforts

57

were made to figure out the influence of wax on hydrate thermodynamics by the research group of

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Tohidi.20-22 Tabatabaei et al.20 suggested that although hydrate formation significantly changed the


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phase boundary of wax precipitation, wax precipitation had little effect on hydrate thermodynamic

60

equilibrium. Mahabadian et al.21 proposed that the presence of wax did not change the

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thermodynamic equilibrium of hydrate noticeably. The researches on the influence of wax on

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hydrate formation kinetics and flow behavior of the hydrate slurry are relatively scarce. Liu et al.23

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used a high-pressure flow loop to investigate the impact of wax on hydrate agglomeration and

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plugging mechanism, which suggested that hydrate agglomeration and plugging scenarios were

65

quite different from the wax-free system. As for hydrate formation kinetics in waxy w/o emulsions,

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two perspectives can be focused including hydrate nucleation and growth. Conclusion about

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hydrate nucleation in the presence of wax was consistent, which was that the precipitated wax

68

inhibited hydrate nucleation despite that wax was able to offer heterogeneous nucleation sites for

69

hydrate nucleation.24-26 Conclusions on hydrate growth affected by the wax were relatively

70

inconsistent. According to Mohammadi et al.24 and Chen et al.,25 the presence of wax promoted

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the growth amount of hydrate, while Shi et al.26 advised that hydrate growth was inhibited by the

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presence of wax more obviously under the condition of low initial system pressure, the hindering

73

effect would be weakened due to the increased driving force under higher initial system pressure.

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Song et al.27 suggested that wax could significantly increase the hydrate growth rate, while Chen

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et al.25 argued that natural gas hydrate growth rate decreased due to the change in hydrate shell,

76

which was supposed to be affected by the precipitated wax. Shi et al.26 proposed that the viscosity

77

of natural gas hydrate increased with wax content. In addition, viscosity characteristics and

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rheological properties of hydrate slurry in the presence of wax are rare.


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In addition to the significance in flow assurance, the rheological properties of cyclopentane

80

hydrate slurry are of great importance where the cyclopentane hydrate is used as the refrigerating

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medium or cold storage material. Researchers carried out rheological experiments in the high-

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pressure flow loop for obtaining the hydrate slurry viscosity calculated from the pressure drop.28-

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31

84

torque sensor or a current-measurement device, which monitored the flow resistance of the mixing

85

system during the hydrate formation process.32-34 The data of torque or current is analogous to the

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flow resistance of the hydrate slurry in the pipeline. In addition, researchers used

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tetrabutylammonium bromide (TBAB) hydrate, tetrahydrofuran (THF) hydrate, and cyclopentane

88

(CP) hydrate to perform hydrate formation and rheological experiments in the rheometer or the

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viscometer.35-37 Moreover, gas hydrate slurry was prepared in the high-pressure reactor, and the

90

formed hydrate slurry was transferred into the high-pressure rheometer for performing the

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rheological experiments.26, 38, 39 Apart from the work mentioned above, gas hydrate slurry could

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also be prepared directly in the high-pressure rheometer, and a series of rheological properties

93

were measured, including flow curve, yield stress, thixotropy and viscoelasticity.40-42

Besides, researchers prepared the hydrate slurry in the high-pressure reactor equipped with a

94

CP hydrate was commonly used in the following laboratory experiments, including hydrate

95

adhesion/cohesion measurements,43-46 desalination,47-49 rheology,50-53 energy storage54 and so on55.

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The exploration of hydrate slurry rheology can be classified into two aspects: (i) Rheology for the

97

bulk properties of the formed hydrate slurry,50-53 and this work belongs to this perspective. (ii)

98

Interfacial rheology between two immiscible liquids.56 In this paper, we used CP hydrate to study


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on the impact of wax on the viscosity of hydrate slurry. There are two reasons for the choice of CP

100

hydrate as the model hydrate of this work: (i) CP hydrate form structure II hydrate which is usually

101

encountered in the subsea petroleum development fields. (ii) CP hydrate can form at atmospheric

102

pressure and moderate temperature.

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In this work, a systematic study of CP hydrate formation and dissociation in the presence of wax

104

were carried out in the stress-controlled rheometer, using w/o emulsions containing 30 vol%

105

deionized water and 70 vol% oil (mineral oil LP15 and CP) with 0.5 wt% Span 80 and wax at

106

different contents. Viscosity and yield stress of CP hydrate slurry at different wax contents were

107

studied. The inhibition effects of wax on CP hydrate nucleation and growth were discussed from

108

the perspectives of critical time, growth time and growth amount. Hydrate slurry viscosity during

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CP hydrate dissociation process with and without wax were presented.

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2. EXPERIMENTAL METHODS

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2.1. Apparatus and Materials

112

Experiments were carried out in the stress-controlled rheometer (MCR 101, Anton Paar), which

113

was equipped with a smooth bob and a smooth cylindrical cup. The radius and height of the bob

114

are 13.33 mm and 40.01 mm respectively. The inner radius of the cylindrical cup is 14.46 mm.

115

The cup is installed in the Peltier system, which is mounted in the rheometer. The temperature

116

inside the rheometer was controlled through the heat change with a water bath (DC-1006, Sunny

117

Heng-Ping Scientific Instrument Company, Shanghai), and the temperature range is from -20 to

118

150 °C. The maximum value of the torque is 150 mN∙m. Materials include CP (Aladdin, 96 % for


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purity), mineral oil LP15 (Yan-Chang Petrochemical Company, Beijing), deionized water, wax

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(Daqing Petrochemical Branch Company, Daqing), and sorbitan monolaurate (Span 80, Aladdin).

121

The continuous oil phase consists of 50 vol% mineral oil LP15 and 50 vol% CP. The density and

122

viscosity of mineral oil LP15 are 0.8978 g/cm3 and 0.043 Pa∙s at 20 °C respectively. The addition

123

of mineral oil LP15 is to obtain a viscosity-matched oil phase and dissolve wax into the organic

124

phase. The carbon distribution of wax is shown in Table 1. Dosages of wax and surfactant are

125

determined with the mass fraction of oil, including CP and mineral oil LP15. For studying the

126

influence of wax on hydrate formation and dissociation in terms of viscosity, all the other factors

127

were controlled to be unchanged, including shear rate (300 s-1), system target temperature (-2 °C),

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water cut (30 %), dosage of the surfactant Span 80 (0.5 wt%) and cooling rate (1 °C/min). Table 1. The Carbon Number Distribution of Wax

129

carbon number

wt%

carbon number

wt%

C26

7.23

C34

5.42

C27

12.22

C35

4.92

C28

11.34

C36

4.68

C29

11.02

C37

4.39

C30

8.57

C38

3.88

C31

7.15

C39

3.40

C32

6.41

C40

2.94

C33

6.43

130 131 132

2.2. Experimental Procedure Take 3.0 wt% wax content as an example, the specific procedure is shown as follows: Wax (3.0


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wt%) was added to the mineral oil LP15 (35 ml) firstly, then the mixture was placed in a water

134

bath, and the set temperature of the water bath was 40 °C. The beaker was heated for at least 120

135

min for dissolving the wax into the mineral oil completely. After that, the mixture in the beaker

136

was stirred at the speed of 500 rpm for 10 min to homogenize the mixture, and then 0.5 wt% Span

137

80 was added into the mixture. Next, 30 ml deionized water was added into the beaker by a drop-

138

wise manner while keeping the stirring action going on. The emulsified mixture was stirred for 10

139

min. Afterwards, the emulsified mixtures were quickly transferred into an air bath at 40 °C, and

140

then 35 ml CP was added into emulsified mixtures, followed by applying the stirring using a

141

homogenizer (IKA T25 digital Ultra-Turrax) operating at the speed of 7000 rpm for 3 min. Then

142

19 ml homogenized emulsion was immediately transferred to the cup for less than 1/2 min, while

143

the temperature of the cup was previously controlled to be 40 °C. Then, the shearing test started at

144

the rate of 300 s-1 accompanied by cooling the sample to the target temperature of -2 °C at the

145

constant rate of 1 °C/min. No ice formation in the w/o emulsions is ensured at -2 °C, and most of

146

the rheological tests of CP hydrate slurry were carried out below the freezing point.50, 51, 53 When

147

the cooling process was completed, two externally prepared ice particles (around 1 mm diameter)

148

were gently seeded in the emulsion to avoid the long and stochastic nucleation time of CP

149

hydrate.50 When hydrate formation was completed, tests of flow curve were carried out at the shear

150

rate of 100, 200, 300, 400, 500, 600 s-1, or the formed hydrate was heated to the temperature of 10

151

°C at the rate of 0.5 °C/min for hydrate dissociation. Flow-curve tests and hydrate-dissociation

152

tests were carried out independently.


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3. RESULTS AND DISCUSSION

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3.1. Evolution of Slurry Viscosity During Hydrate Formation Process

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Figure 1 shows the viscosity evolution for the wax-free and waxy hydrate-forming emulsions.

156

Whether wax is present or not, the process of CP hydrate formation includes the cooling process,

157

isothermal nucleation, growth and shear stability.

158

(i) During the cooling process: The emulsion viscosity increases exponentially with the

159

decreasing temperature for wax-free emulsions. For waxy emulsions, when the temperature

160

decreases to the wax appearance temperature (WAT) during the cooling process, the dissolved

161

wax gradually precipitates out, which would lead to an increase in the emulsion viscosity as shown

162

in Figure 1. Wax precipitates out before hydrate formation for all wax contents of 1.0 wt%, 3.0

163

wt% and 5.0 wt%.

164

(ii) During the isothermal nucleation process: For wax-free emulsions, the viscosity increases

165

slightly, which is due to the settling of water droplet caused by the gravity differences.53 However,

166

no obvious increase in the viscosity of the waxy emulsions is observed during the isothermal

167

process, so we speculated that w/o emulsions are more stable due to the adsorption of wax onto

168

the water droplets via the synergistic effect57 and network stabilization.58 Besides, the viscosity of

169

the waxy w/o emulsions is slightly decreased due to the shear breakage of the spatial network

170

structure of wax aggregates.

171

(iii) During the growth process: The tendencies of viscosity change during hydrate growth are

172

different for waxy and wax-free w/o emulsions. After hydrate nucleation, hydrate grows quickly


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in wax-free emulsions until the viscosity reaches the maximum value, and then the viscosity tends

174

to decrease slightly due to the shear breakage effect. The change in the dispersed phase from water

175

to solid results in the increase of the viscosity by around two orders of magnitude. In comparison,

176

hydrate grows in waxy emulsions in a gentler way, two stages could be observed obviously during

177

the whole process. The precipitated wax is supposed to enhance the mass transfer resistance during

178

the later growth process of CP hydrate, which will hinder the further growth of CP hydrate.

179

(iv) During the shear-stability process: Hydrate slurry viscosity decreases slightly after the

180

completion of hydrate growth process. The decrease degree of CP hydrate slurry viscosity in the

181

presence of wax is smaller than that in the absence of wax. (Detailed discussion is shown in Section

182

3.2)

183 viscosity without wax/Pa.s

1

40

viscosity with wax/Pa.s temperature/C

WAT at 5 w%

0.1

wax content

20

Onset of hydrate growth 10

Phase equilibrium temperature

0.01

Temperature/C

30

Viscosity/Pa.s

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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0

Wax precipitation Seeding two ice particles

0.5

1.0

1.5

2.0

2.5

3.0

3.5

-10

184

Time/h

185

Figure 1. Trend of viscosity evolution for CP hydrate-forming w/o emulsions for 30 vol% water

186

cut. The wax content for waxy w/o emulsion is 5.0 wt%.


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3.2. Effect of Wax on Cyclopentane Hydrate Slurry Viscosity

188

The impact of wax on the viscosity of hydrate-forming emulsions can be determined from two

189

aspects, including the transient viscosity during hydrate growth process and the final viscosity after

190

that hydrate growth process is completed. Figure 2 shows the transient viscosity of hydrate slurry

191

during CP hydrate growth process in the presence and absence of wax. The transient viscosity

192

represents the viscosity of hydrate slurry during hydrate growth process with the same hydrate

193

formation amount after the hydrate nucleation process. The viscosity of the basic w/o emulsions

194

increases with the wax content, which is due to that more wax precipitated in the w/o emulsions

195

under the same target cooling temperature (-2 °C). Additionally, the final viscosity of the hydrate

196

slurry increases with the wax content. There are two mechanisms that impact the final slurry

197

viscosity as the wax content increases: (i) The basic viscosity of the w/o emulsions is higher at

198

higher wax content as elaborated above; (ii) Hydrate/wax aggregates are thought to be larger,

199

which could cause the flow to be more difficult. Figure 3 gives the degree of the viscosity decrease

200

after the peak value at different wax contents. The degree of the viscosity decrease can be used to

201

characterize the constant shear stability of the formed CP hydrate slurry. The degree of the

202

viscosity decrease for hydrate slurry formed from the wax-free emulsions is (38.2±8.4) %, while

203

for the hydrate slurry formed from the waxy emulsions is (13.0±2.0) %, (8.0±0.2) % and (5.0±0.6)

204

% for 1.0 wt%, 3.0 wt% and 5.0 wt% wax content respectively. It is speculated that the precipitated

205

wax tends to form spatial network structure during hydrate growth process, and the formed hydrate

206

tends to fill into the pore of the network structure. Therefore, the coupled hydrate-wax aggregates


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Energy & Fuels

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are difficult to be broken under the constant shear rate of 300 s-1.

208

Viscosity/Pa.s

1

0.1

0 wt% wax content 1 wt% wax content 3 wt% wax content 5 wt% wax content 0.01

1

2

3

4

Time after the cooling process/h

209 210

Figure 2. Viscosity of hydrate-forming emulsions against time after the cooling process at

211

different wax contents.

212 50

Degree of viscosity decrease/%

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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40

30

20

10

0

213

0

1

2

3

4

5

Wax content/wt%


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Energy & Fuels

Figure 3. Degree of the viscosity decrease after the peak value at different wax contents.

215 216

Figure 4 shows flow curves of hydrate slurry formed from w/o emulsions at different wax

217

contents at -2 °C, which imply that the formed hydrate slurry exhibits shear-shinning property

218

whether wax is present in w/o emulsions or not. That is to say, the precipitated wax has no

219

influence on the shear-thinning property of hydrate slurry,35, 38, 41, 51 which indicates that the formed

220

hydrate/wax slurry could still be pumped through pipelines by increasing the pump power. Besides,

221

Figure 4 indicates that there is yield stress for CP hydrate slurry, the yield stresses of CP hydrate

222

slurry with different wax contents are measured using the method of linear stress sweep which

223

plots the shear strain versus shear stress after four hours of annealing time, as shown in Figure 5.

224

The measured yield stresses of CP hydrate slurry are 42, 51, 74 and 212 Pa for 0 wt%, 1.0 wt%,

225

3.0 wt% and 5.0 wt% wax content respectively. Future research should be focused on the effect of

226

wax on the yield stress of CP hydrate slurry and the impact of hydrate formation on the yield stress

227

of waxy w/o emulsions independently.


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Energy & Fuels

0 wt% wax content 1 wt% wax content 3 wt% wax content 5 wt% wax content power law model

2.5

Viscosity/Pa.s

2.0

1.5

1.0

0.5 100

200

300

400

500

600

Shear rate/s-1

228 229

Figure 4. Flow curves of hydrate slurry at different wax contents. (Symbols represent the

230

viscosity data from 100 to 600 s-1; lines represent the fitting results of the power law model).

231 6E+05

5E+05

0 wt% wax content 1 wt% wax content 3 wt% wax content 5 wt% wax content

4E+05

Shear strain/%

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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3E+05

2E+05

1E+05

0E+00 1E+00

232 233

1E+01

1E+02

1E+03

Shear stress/Pa

Figure 5. Linear stress sweep experiments for CP hydrate slurry with different wax contents.

234


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Energy & Fuels

The simplest viscosity model for describing the shear-shinning property is the power law model as shown in eq 1, which is suitable for the medium range of shear rate. 𝜇 = 𝐾𝛾𝑛 ― 1

(1)

238

Where 𝜇 is hydrate slurry viscosity (Pa∙s), K is the consistency coefficient (Pa∙sn), n is the flow

239

behavior index. Table 2 shows the fitting results of parameters for slurry flow curves at different

240

wax contents using the power law model, which shows that the dependence of hydrate slurry

241

viscosity on the shear rate can be well determined as the power law function. The consistency

242

coefficient increases with the wax content as shown in Figure 6. The strong impact of wax on the

243

consistency coefficient can be observed. The flow behavior index decreases with the wax content,

244

and all values of the flow behavior index at different wax contents are close to 0.5 as shown in

245

Figure 6, which indicates a strongly shear-thinning slurry. The flow behavior index n and the

246

consistency coefficient K were obtained by linearly fitting the flow behavior index and consistency

247

coefficient with wax content, thus hydrate slurry viscosity model at different wax contents can be

248

simply established, which depends on shear rate and wax content as shown in eq 2.

249 250

(

𝜇 = (12.73 + 3.65𝐶𝑤𝑎𝑥)𝛾 0.51 ― 0.01𝐶𝑤𝑎𝑥

)

(2)

Where Cwax is the wax content in waxy w/o emulsions.

251


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Energy & Fuels

consistency coefficient/Pa.sn flow behavior index

30

0.51 0.50

25 0.49 20

0.48

n  0.51  0.01Cwax 0.47

R 2  0.99

15

K  12.73  3.65Cwax

Flow behavior index

Consistency coefficient/Pa.sn

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

Page 16 of 36

0.46

R 2  0.98

10

0.45 0

1

2

3

4

5

252

Wax content/wt%

253

Figure 6. Fitting results of the flow behavior index and consistency coefficient with wax content.

254 255

Table 2. Parameters of the Power Law Model Fitting for Hydrate Slurry Flow Curves at

256

Different Wax Contents. expt. 1 2 3 4

257

wax content (wt%) 0 1.0 3.0 5.0

K (Pa∙sn) 11.33 17.88 24.20 30.39

n 0.51 0.50 0.47 0.45

3.3. Influence of Wax on Cyclopentane Hydrate Nucleation and Growth

258

Nucleation and growth are two important components of hydrate formation.4 Hydrate induction

259

time is the key index for hydrate nucleation. Hydrate growth rate, time and amount are the

260

significant macro-information for hydrate growth. Generally, hydrate induction time is referred as

261

the time elapsed until the consumption of a detectable number of moles of hydrate-forming gas.4

262

The system target temperature and the cooling rate remain constants as -2 °C and 1 °C/min for all


16

Page 17 of 36

263

experiments, since hydrate induction time was affected by subcooling and cooling rate.59

264

Considering that CP hydrate nucleation is promoted by the addition of the external ice particles,

265

the concept of critical time was proposed by researchers instead of induction time under the

266

specific experimental procedure.60 Specifically, critical time is defined as the time after the

267

external ice particles adding to the emulsions until the onset of hydrate growth as shown in Figure

268

7. It is hard to distinguish the accurate ending of CP hydrate nucleation or beginning of CP hydrate

269

growth in the rheometer, which will bring about the disadvantages in analyzing the results of

270

hydrate nucleation and growth. Moreover, no obvious increase in the viscosity after CP hydrate

271

nucleation can be observed, which makes the determination of CP hydrate critical time difficult.

272

Herein, the ending point of hydrate nucleation or the onset point of hydrate growth can be

273

evaluated by the steepest rising rate in the slope of the hydrate slurry viscosity against time, as

274

shown in Figure 7.

275 1

0.6 0.4 0.2

Critical time

0.1

0.0 -0.2

Onset of hydrate growth

-0.4 0.0

276

0.4

0.8

1.2

1.6

2.0

Gradient of the viscosity versus time/Pa/3600

1.0 0.8

Viscosity/Pa.s

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

Energy & Fuels

2.4

Time after the cooling process/h


17

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

277

Page 18 of 36

Figure 7. Illustration of the determination of CP hydrate critical time at 1.0 wt% wax content.

278

Figure 8 shows that CP hydrate critical time increases with the wax content, the critical time

279

for wax-free emulsions is (1.32±0.15) hours. While the critical times for 1.0 wt%, 3.0 wt% and 5.0

280

wt% wax content are (1.96±0.18), (2.02±0.15), (2.15±0.25) hours respectively. The stochasticity

281

of CP hydrate critical time is enhanced with the wax content increased. The critical time is

282

prolonged obviously with wax precipitated in w/o emulsions. However, the increase in hydrate

283

critical time is smaller as the wax content increases from 1.0 wt% to 5.0 wt%. It is presumed that

284

the convergence effect of wax content on prolonging critical time is due to the interface coverage

285

ratio of wax on water droplets within 5.0 wt% wax content used in the experiments. As wax content

286

increases, the adsorption capacity of wax on water droplets may reach the limit. Besides, wax

287

precipitated in w/o emulsions is the impurity, which may provide extra two-dimensional surface

288

and reduce the interface energy,61, 62 whereby heterogeneous nucleation rate tends to increase with

289

the wax content in the system. Further analysis is needed to figure out the kinetic inhibition or

290

promotion of CP hydrate nucleation in the presence of wax.

291


18

Page 19 of 36

2.8 2.6 2.4

Critical time/h

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

Energy & Fuels

2.2 2.0 1.8 1.6 1.4 1.2 1.0

0

2

3

4

5

Wax content/wt%

292 293

1

Figure 8. CP hydrate critical time at different wax contents.

294 295 296 297 298 299 300 301 302 303 304 305

Kashchiev and Firoozabadi61, 62 suggested that the nucleation rate for one-component hydrate can be formulated by eq 3: J  Ae  / kT  exp  4c 3vh2 ef3  /  27 kT  2  

(3)

Where A is the kinetic parameter (m-2/s-1), which represents the nucleation type and the way of the building unit attaching to the hydrate nuclei surface. Δμ is the driving force for hydrate nucleation (J), k is the Boltzmann constant (J/K), T is the system target temperature (K), c is the shape factor, vh is the volume of a hydrate building unit (m3), σef is the effective specific surface energy (J/m2). The driving force for hydrate formation at the isobaric regime is given by eq 4.61, 62 The system target temperature for CP hydrate formation is constant for all wax contents, thus the driving force during hydrate nucleation process is deemed to be unchanged.

  se T

(4)


19

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

306 307 308

Page 20 of 36

Where Δμ is the driving force for hydrate nucleation, Δse is the entropy of hydrate dissociation for per hydrate building unit at equilibrium temperature, ΔT is the subcooling. The kinetic parameter A can be expressed by eq 5:61, 62

309

A  zf eC0

310

Where z is the Zeldovich factor, f e is the frequency of hydrate building unit attaching to the

311

hydrate nuclei, C0 is the concentration of nucleation sites in the system.

(5)

312

It is assumed that both z and f e are independent of the wax in the system.61, 62 For wax-free

313

emulsions, the nucleation sites are suggested to be the interface of the water/oil interface. While

314

for waxy emulsions, the nucleation sites not only include the water/oil interface, but also include

315

the wax particles existed in w/o emulsions. Thus, on the one hand, the concentration of the

316

nucleation sites decreases due to the wax adsorption at the oil-water interface. Therefore, the value

317

of the kinetic parameter A decreases with the increasing wax content. However, on the other hand,

318

wax as one kind of impurities can provide new nucleation sites after blocking the exist nucleation

319

sites, whereby the value of the kinetic parameter A increases with the wax content. The second

320

parameter that may be affected by the wax content is the effective specific surface energy σef. With

321

the precipitation of wax into the emulsion system, the effective specific surface energy will

322

decrease due to the adsorption of wax at the nucleus/oil interface.61, 62 Therefore, the nucleation

323

rate will increase with the wax content. To conclude, the influence of wax content on the kinetic

324

parameter A and specific surface energy contributes to the convergence effect as shown in Figure

325

8, but the inhibition influence of wax on CP hydrate nucleation dominates within the wax content


20

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326

Energy & Fuels

used in this paper.

327

To the best of our knowledge, few attempts have been made to calculate the growth amount of

328

the CP hydrate. Corak et al.47 calculated the experimental hydrate number to quantify the mass of

329

the formed CP hydrate. Raman et al.63 used the model from Toda et al.64 extended from the

330

Einstein’s viscosity model65 to predict the growth amount of CP hydrate.

331

In this paper, two methods can be used to calculate the hydrate volume fraction at the end of the

332

growth process. One of which is based on the measurement of the temperature change considering

333

the exothermic nature of hydrate formation. However, because we do not have a tightly controlled

334

heat flow, calorimetry does not work. The other method for quantitatively determining the CP

335

hydrate growth amount is based on the suspension viscosity model proposed by Camargo and

336

Palermo,66 as shown in eqs 6 and 7. It is of importance to figure out the influence of wax on the

337

growth amount of CP hydrate based on the viscosity model, although there are some simplified

338 339

conditions for the viscosity model.

r 

340

eff 341

1  eff  eff  1    max 

d   A  d   p

2

(6)

 3 f 

(7)

Where μr is the relative viscosity, ϕeff is the effective hydrate volume fraction, ϕmax is the maximum 342

hydrate volume fraction, which is equal to be 4/7, ϕ is the actual hydrate volume fraction, dA is the 343

diameter of the aggregate (μm), and is assumed to be a constant (200 μm),23 dp is the diameter of 344

the hydrate particle (μm), and is assumed to be a constant (40 μm),23 f is the fractal dimension,


21

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Page 22 of 36

345

which is equal to be 2.5. 346

According to eq 6, the effective hydrate volume fraction can be expressed by eq 8: 347

eff 

3.5 r  5.25 r  1 6.125 r

1/ 2

1

(8)

348

Combined with eq 7, the actual hydrate volume fraction can be defined as eq 9: 349

 3.5 r  5.25 r  11/ 2  1   0.4472  6.125 r  

(9)

350

As shown in Figure 9, CP hydrate volume fraction in the presence and absence of wax after the 351

completion of CP hydrate growth process can be determined. The calculated hydrate volume 352

fraction in wax-free emulsions is 22.5 %. For 1.0 wt%, 3.0 wt% and 5.0 wt% wax content, the 353

calculated hydrate volume fractions are 22.2 %, 22.0 % and 21.7 % respectively. According to the 354

investigation carried out by Ahuja et al.,35 all the water was converted to CP hydrate in w/o 355

emulsions with 30% water cut and without salt or any other thermodynamic hydrate inhibitors. 356

Karanjkar et al.67 reported that the water droplet immersed in the CP was supposed to convert into 357

hydrate fully, although Span 80 could lead to the change in the morphology of CP hydrate. It is 358

noteworthy that the calculated hydrate volume fractions are used herein for qualitative comparison 359

of CP hydrate formation amount in wax-free and waxy w/o emulsions, because the amount of 360

formed CP hydrate cannot be easily determined like the way of gas hydrates. So, water droplets 361

dispersed in the bulk organic phase may convert into CP hydrate as much as possible despite of 362

mass transfer limitations, but surfactant Span 80 in w/o emulsions could result in the hairy or 363

mushy morphology of CP hydrate.67


22

Page 23 of 36

364

The calculated hydrate volume fraction decreases a little with the increase of wax content. There 365

are two reasons for the decrease of hydrate growth amount as the wax content increases: (i) The 366

interfacial adsorption of wax on the water droplets reduces the contact area for the water droplets 367

and CP droplets. (ii) The suspended wax in the bulk oil phase will block the path of further mass 368

transfer of CP to the oil-water interface, making the mass transfer resistance to be larger, but the 369

suspended wax may not affect this motion of action significantly according to the calculation 370

results. 371

22.6

Calculated hydrate volume fraction/%

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

Energy & Fuels

22.4

22.2

22.0

21.8

21.6

372

0

1

2

3

4

5

Wax content/wt%

373

Figure 9. Calculated CP hydrate volume fraction at different wax contents after hydrate growth 374

process. 375

376

Except the growth amount, hydrate growth time is another significant kinetic parameter for

377

hydrate growth process. Here, CP hydrate growth time is defined as the time from the onset of


23

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Page 24 of 36

378

hydrate growth as shown in Figure 7 to the point where the viscosity reaches the maximum value,

379

which is slightly different from the definition by the previous researchers.52 The elapsed time

380

needed for the value of the viscosity reaching up to 90 % of the final viscosity after the onset of

381

hydrate growth is defined as the growth time or evolution time by the previous researchers.52

382

Figure 10 shows the growth time for different wax contents. Like the effect of wax content on

383

hydrate critical time, CP hydrate growth time is prolonged when the wax content ranges from 0 wt%

384

to 5.0 wt%. As mentioned in Section 3.1., two stages could be observed obviously in hydrate

385

growth process, which would lead to the delay in hydrate growth time. As shown in Figure 10,

386

more time is needed for CP hydrate growth in waxy emulsions than wax-free emulsions, despite

387

that the growth amount in waxy emulsions is less than that in wax-free emulsions as indicated

388

from Figure 9. Therefore, in the qualitative perspective, CP hydrate growth rate is decreased in

389

w/o emulsions with the presence of wax. In conclusion, CP hydrate formation in waxy w/o

390

emulsions is inhibited by the presence of wax in terms of the critical time, growth time and growth

391

amount.

392


24

Page 25 of 36

1.2

1.0

Growth time/h

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

Energy & Fuels

0.8

0.6

0.4

0.2 0

393 394

1

2

3

4

5

Wax content/wt%

Figure 10. CP hydrate growth time at different wax contents.

395 396

3.4. Influence of Wax on Cyclopentane Hydrate Dissociation

397

Figure 11 shows the trend of viscosity during CP hydrate dissociation in the absence of wax. 398

The CP hydrate slurry is heated from -2 °C to 10 °C at a constant rate of 0.5 °C/min. The slurry 399

viscosity keeps constant until there is a catastrophic decrease in the viscosity as the temperature 400

increases linearly. There is an abrupt increase in the slurry viscosity around (5.2±0.2) °C, which is 401

deemed to be the equilibrium temperature of the CP hydrate. According to Nakajima et al.,68 402

Sefidroodi et al.,69 Sloan et al.,4 three-phase equilibrium temperature of CP and water was between 403

7.7 °C and 7.9 °C. However, the dissociation temperature will decrease when the other liquid 404

hydrocarbon is present in the system according to the experiments carried out by Zylyftari et al.,52 405

which suggested that the dissociation temperature of CP hydrate was 5.4 °C when the volume ratio 406

of the CP in the organic hydrocarbon is 50 %. In addition, Abojaladi et al.70 found that the melting


25

Energy & Fuels

407

temperature of CP hydrate was 5.3 °C with 10 ml CP and 10 ml white spirit presented in the 408

experimental system. So, the profile of viscosity-temperature in the rheometer might be another 409

method to measure CP hydrate equilibrium temperature. The reason for the abrupt increase in the 410

viscosity is that the water released due to the dissociation of the CP hydrate will adhere onto the 411

hydrate surface and will contribute to the formation of the water bridge between hydrate particles, 412

thereby causing hydrate particles tend to aggregate.41 After the sharp increase in the viscosity, the 413

slurry viscosity decreases to a low value. Additionally, it indicates that hydrate starts dissociating 414

even if the temperature is still within the equilibrium-stability zone, because hydrate slurry 415

viscosity decreases a little even if the system temperature is still lower than the equilibrium 416

temperature. 417

1.0

run 1 run 2 run 3

0.8

Viscosity/Pa.s

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

Page 26 of 36

0.6

0.4

0.2

0.0 -2

418 419

0

2

4

6

8

10

Temperature/C

Figure 11. CP hydrate slurry viscosity in the absence of wax during CP hydrate dissociation. 420


26

Page 27 of 36

421

As shown in Figure 12, for the wax content of 1.0 wt%, the same behavior of the viscosity 422

change tendency can be observed. The equilibrium temperature of CP hydrate at 1.0 wt% wax 423

content is equal to (5.4±0.2) °C, which indicates that the presence of wax did not alter the 424

thermodynamic equilibrium of hydrate noticeably.21 However, with the wax content of 3.0 wt% 425

and 5.0 wt%, no obvious viscosity peak can be observed when the system temperature approaches 426

to the CP hydrate equilibrium temperature during the heating process, which is thought to be 427

affected by the increased amount of precipitated wax. 428

1.6

0 wt% wax content 1 wt% wax content 3 wt% wax content 5 wt% wax content

1.4 1.2 1.0 0.8 1.1 1.0

0.6 Viscosity/Pa.s

Viscosity/Pa.s

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

Energy & Fuels

0.4 0.2

0.9 0.8 0.7 0.6

0.4 0.3

0.0

4.6

4.8

5.0

5.2

5.4

5.6

5.8

6.0

Temperature/C

-2

429 430

0 wt% wax content 1 wt% wax content

0.5

0

2

4

6

8

10

Temperature/C

Figure 12. Viscosity of CP hydrate slurry during CP hydrate dissociation at different wax 431

contents. 432 433

As shown in Figure 13, the left and middle panels show the aggregation of hydrate particles 434

due to the water bridge during hydrate dissociation process; the right panel shows the hypothesized


27

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

Page 28 of 36

435

mechanism of CP hydrate dissociation with higher wax content of 3.0 wt% and 5.0 wt%. There 436

are two types of wax precipitated in the oil phase, one of which tends to adsorb onto the surface 437

of hydrate particles with the help of the surfactant; while the other type of wax suspends in the oil 438

phase.25 Therefore, as wax content increases, the amount of the adsorbed wax and the suspended 439

wax increases as shown in Figure 13. According to the adhesion force measurements carried out 440

by Aman et al.,44 a stable quasi-water layer which adhered on the surface of CP hydrate tended to 441

form the water bridge between two CP hydrate particles. Besides, the quasi-water layer tended to 442

develop into the water bridge. Considering that wax is hydrophobic, and the wetting property of 443

the CP hydrate is hydrophilic,44, 45 there is more hydrophobic wax existed between two hydrate 444

particles at 3.0 wt% and 5.0 wt% wax content, which would make the released water from the 445

dissociated hydrate much more difficult to migrate towards the gap between two hydrate particles. 446

Therefore, it is hard for the water bridge to form between two hydrate particles during hydrate 447

dissociation process when wax contents are 3.0 wt% and 5.0 wt%. Consequently, the phenomenon 448

of the abrupt increase in the slurry viscosity barely occurs when the wax contents are 3.0 wt% and 449

5.0 wt%. 450


28

Page 29 of 36 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

Energy & Fuels

1 wt% wax content

without wax

3 wt% and 5 wt% wax content

Oil

Hydrate shell Liquid bridge Liquid bridge

Aggregation of two hydrate particles

Aggregation of two hydrate particles

water droplet

451 452

slice-like wax

Two hydrate particles without liquid bridge

needle-like wax

Span 80

Figure 13. Illustration of the hypothesized mechanism for the observed difference in the 453

viscosity change at different wax contents during CP hydrate dissociation process. 454

455

4. CONCLUSIONS

456

The impact of wax on the viscosity of CP hydrate slurry was investigated using the rheometer. The

457

following conclusions can be obtained: (i) The evolution patterns of the slurry viscosity during

458

hydrate growth in waxy and wax-free emulsions were different, where hydrate grew more slowly

459

in waxy emulsions and two stages could be obviously observed compared to the one-stage growth

460

in wax-free emulsions. (ii) The final hydrate slurry viscosity and yield stress increased with wax

461

content. The coupled hydrate-wax aggregates were difficult to be broken by the constant shearing

462

force after the hydrate growth process. The formed hydrate slurry exhibited shear-shinning

463

property whether wax was present or not, and the consistency coefficient index increased with wax


29

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

Page 30 of 36

464

content, while the flow behavior index decreased with wax content using the power law model.

465

(iii) The wax precipitated in the oil phase had an inhibition effect on the hydrate formation in terms

466

of hydrate critical time, growth time and growth amount. CP hydrate critical time and growth time

467

increased with wax content. There was a convergence effect of wax content on the critical time,

468

and the impact of wax content on the kinetic parameter and specific surface energy was assumed

469

to contribute to the convergence effect. The final hydrate volume fraction decreased with wax

470

content based on the calculated results using the suspension viscosity model. (iv) There was no

471

abrupt increase in the viscosity during the dissociation process of hydrate at wax contents of 3.0

472

wt% and 5.0 wt%, it was hard for the water bridge to form between two hydrate particles during

473

the hydrate dissociation process in the presence of wax especially for high wax contents. More

474

efforts should be made on the effect of wax on the other rheological properties of hydrate slurry

475

for the flow assurance in the development fields of subsea waxy petroleum, including thixotropy,

476

yield stress and viscoelasticity, considering that both hydrate slurry and waxy emulsion can exhibit

477

the non-Newtonian properties.

478 479

AUTHOR INFORMATION

480

Corresponding author

481

*(Bohui

482

*(Jing

483

ORCID

Shi) Phone: +86-010-89733804. Email: [email protected]

Gong) Phone: +86-010-89732156. Email: [email protected]


30

Page 31 of 36 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

Energy & Fuels

484

Yuchuan Chen: 0000-0001-9919-0067

485

Bohui Shi: 0000-0003-2683-6984

486

Yang Liu: 0000-0002-8556-8775

487

Lin Ding: 0000-0002-3722-5778

488

Notes

489

The authors declare no competing financial interests.

490

ACKNOWLEDGEMENTS

491

This work was supported by the Beijing Municipal Natural Science Foundation (No. 3192027),

492

the National Natural Science Foundation of China (No. 51874323, No. 51534323, & No.

493

51774303), the National Key Research and Development Plan (No. 2016YFC0303704), the

494

National Science and Technology Major Project of China (No. 2016ZX05028004-001,

495

2016ZX05066005-001), and the 111 Project (No. B18054), all of which are gratefully

496

acknowledged.

497

NOMENCLATURE 𝜇

hydrate slurry viscosity (Pa∙s)

𝛾

shear rate (s-1)

K

consistency coefficient (Pa∙sn)

n

flow behavior index

Cwax

wax content in waxy emulsions (wt%)

A

kinetic parameter (m-2/s-1)

Δμ

driving force for hydrate nucleation (J)

k

Boltzmann constant (J/K)

T

system target temperature (K)


31

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

c

shape factor

vh

volume of a hydrate building unit (m3)

σef

effective specific surface energy (J/m2)

z

Zeldovich factor

f

 e

Page 32 of 36

frequency of hydrate building unit attaching to the hydrate nuclei

C0

concentration of nucleation sites in the system

μr

relative viscosity

ϕeff

effective hydrate volume fraction

ϕmax

maximum hydrate volume fraction

ϕ

actual hydrate volume fraction

dA

diameter of the aggregate (m)

dp

diameter of the hydrate particle (m)

f

fractal dimension

498 499

REFERENCES

500

(1) Sloan, E. D.; Koh, C. A.; Sum, A. K.; Ballard, A. L.; Creek, J.; Eaton, M.; Lachance, J.; Mcmullen, N.;

501

Palermo, T.; Shoup, G. Natural Gas Hydrates in Flow Assurance. 2010.

502

(2) Zheng, S.; Saidoun, M.; Palermo, T.; Mateen, K.; Fogler, H. S. Wax Deposition Modeling with

503

Considerations of Non-Newtonian Characteristics: Application on Field-Scale Pipeline. Energy Fuels 2017, 31

504

(5), 5011-5023.

505

(3) Zheng, S.; Haji-Akbari, A.; Fogler, H. S. Entrapment of Water Droplets in Wax Deposits from Water-in-

506

Oil Dispersion and Its Impact on Deposit Build-up. Energy Fuels 2017, 31 (1), 340-350.

507

(4) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases, 3rd ed.; CRC Press: Boca Raton, FL, 2008.

508

(5) Yan, K. L.; Sun, C. Y.; Chen, J.; Chen, L. T.; Shen, D. J.; Liu, B.; Jia, M. L.; Niu, M.; Lv, Y. N.; Li, N.

509

Flow Characteristics and Rheological Properties of Natural Gas Hydrate Slurry in the Presence of Anti-

510

Agglomerant in a Flow Loop Apparatus. Chem. Eng. Sci. 2014, 106 (6), 99-108.

511

(6) Gao, S. Hydrate Risk Management at High Watercuts with Anti-Agglomerant Hydrate Inhibitors. Energy

512

Fuels 2009, 23 (4), 2118-2121.

513

(7) Pei, C. C.; Kelland, M. A. Study of the Gas Hydrate Anti-agglomerant Performance of a Series of n-Alkyl-

514

tri(n-butyl)ammonium Bromides. Energy Fuels 2013, 27 (3), 1285-1292.

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(8) Zhang, H.; Du, J. W.; Wang Y. H. Investigation into THF Hydrate Slurry Flow Behaviour and Inhibition

516

by an Anti-Agglomerant. RSC Adv. 2018, 8 (22), 11946-11956.

517

(9) Dong, S. B.; Firoozabadi, A. Hydrate Anti-Agglomeration and Synergy Effect in Normal Octane at Varying

518

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In-Situ Viscosity Measurements of Cyclopentane Hydrate Slurry in

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