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Kinetic study of ethylene hydrate formation in presence of graphene oxide and sodium dodecyl sulfate
Author’s Accepted Manuscript Kinetic Study of Ethylene Hydrate Formation in Presence of Graphene Oxide and Sodium Dodecyl Sulfate Erfan Rezaei, Mehrdad Manteghian, Marzieh Tamaddondar www.elsevier.com/locate/petrol
PII: DOI: Reference:
S0920-4105(16)30577-0 http://dx.doi.org/10.1016/j.petrol.2016.10.008 PETROL3668
To appear in: Journal of Petroleum Science and Engineering Received date: 27 June 2016 Revised date: 29 September 2016 Accepted date: 4 October 2016 Cite this article as: Erfan Rezaei, Mehrdad Manteghian and Marzieh Tamaddondar, Kinetic Study of Ethylene Hydrate Formation in Presence of Graphene Oxide and Sodium Dodecyl Sulfate, Journal of Petroleum Science and Engineering, http://dx.doi.org/10.1016/j.petrol.2016.10.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Kinetic Study of Ethylene Hydrate Formation in Presence of Graphene Oxide and Sodium Dodecyl Sulfate Erfan Rezaeia, Mehrdad Manteghiana,*, Marzieh Tamaddondarb a
Department of Chemical Engineering, Tarbiat Modares University, Tehran, P.O. Box 14115-311, Iran. b
School of Chemistry, University of Manchester, Manchester, M13 9PL, U.K.
Abstract The effects of synthesized graphene oxide suspension at concentrations of 50, 150 and 250 ppm and aqueous solution of sodium dodecyl sulfate (SDS) at concentrations of 50, 100, 300 and 500 ppm on kinetics of ethylene hydrate formation were studied. The induction time was measured at 4 °C and initial pressures of 14 and 16 bar, and the effective storage capacity was measured at 1.5 °C and initial pressure of 30 bar. The results indicated that although both graphene oxide suspension and SDS solution in the whole range of experimental concentrations reduced the induction time of ethylene hydrate formation, graphene oxide was more effective in decreasing the induction time. The minimum induction time was obtained when 150 ppm graphene oxide suspension was used, which indicated an average decrease of 96% compared to pure water. In addition, the effective storage capacity measurements showed that additives at low concentrations did not promote the effective storage capacity noticeably, while graphene oxide suspension at 150 and 250 ppm as well as SDS solution at 300 and 500 ppm promoted the effective storage capacity of ethylene hydrate significantly. SDS showed better performance in increasing the effective storage capacity, as the highest effective storage capacity was attained by using 300 ppm SDS solution, suggesting 259.8% improvement in comparison to pure water. Key words: Ethylene Hydrate, Graphene Oxide, Sodium Dodecyl Sulfate, Induction Time, effective Storage Capacity
1. Introduction Gas hydrates are non-stoichiometric compounds formed from water molecules (host) and gas molecules (guests) under favorable conditions of temperature and pressure. Water molecules create cage-like structures through hydrogen bonding, which can be occupied by gas molecules that have appropriate sizes and shapes (Sloan, 2003; Sloan Jr and Koh, 2007). Previously, many researches were focused on different approaches to inhibit hydrate formation (Heidaryan et al., 2010; Lee and Englezos, 2005). Such studies were required because hydrate formation in gas transporting pipe lines obstructed the flow and caused major concerns for oil and gas industries. Meanwhile, gas hydrates have great potentials in many industrial applications such as transport and storage of natural gas (Hao et al., 2008; Javanmardi et al., 2005), gas separation
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(Eslamimanesh et al., 2012), cooling cycles (Douzet et al., 2013; Shi and Zhang, 2013) and seawater desalination (Khawaji et al., 2008; Ngema et al., 2014). One of the major feed gases in polymer industry is ethylene; however, issues such as flammability limit its applications. In an attempt to explore a safe and easy method for storage and transportation of this gas, detailed study on kinetics of ethylene hydrate formation seems essential. Unfortunately, industrialization of gas hydrate formation faces two main obstacles; slow gas hydrate formation and low storage capacity of gas hydrate. Thus, promoting gas hydrate formation is of special importance. In recent years, many researchers have tried to investigate the promotion effect of additives on gas hydrate formation. Jarrahian and Heidaryan (2014) studied the effect of toluene sulfonic acid (TSA) isomers on the natural gas hydrate. They realized that 4 g/L TSA isomer promoted natural gas hydrate significantly. Li et al. (2016) investigated tetrahydrofuran (THF) and cyclopentane (CP) hydrate formation in the presence of glass particles with different hydrophobicity. The authors reported that the induction time for THF and CP hydrates decreased significantly when octadecyltrichlorosilane-coated glass particles were used. They also found that hydrophobic surfaces enhanced water structuring prior to hydrate formation. Many researchers have focused on promotion effect of surfactants on gas hydrate formation (Ganji et al., 2007b; Karimi et al., 2013; Sun et al., 2003). Du et al. (2014) studied the effect of ionic surfactants with equal carbon chain length on methane hydrate formation rate at 15 MPa and 274 K. Their research revealed that sodium dodecyl sulfate (SDS), dodecylamine hydrochloride (DAH), and Ndodecylpropane-1,3-diamine hydrochloride (DN2Cl) had remarkable promoting effect on methane hydrate formation. The Krafft temperatures of SDS, DAH, and DN2Cl are close to room temperature. The authors believed that since the experiments were done at a temperature less than the Krafft temperature, the surfactant solutions were possibly supersaturated and the surfactant crystals were formed. Formation of surfactant crystals increased the inhomogeneity of the system and promoted hydrate formation by heterogeneous nucleation. Zhang et al. (2007) investigated methane hydrate formation in presence of SDS. They realized that the promotion effect of SDS at temperatures below the normal Krafft temperature is possibly due to the adsorption of SDS on the hydrate nuclei that decreases the energy barrier to nucleation. Similarly, Aman et al. (2013) believed that surfactant adsorption on the hydrate surface decreased the interfacial tension and promoted hydrate formation. More recently, some researchers have also investigated the use of nanoparticles as gas hydrate promoters (Aliabadi et al., 2015; Ghozatloo et al., 2014; Kakati et al., 2016). Gas hydrate formation is an exothermic process and the presence of nanofluids with high heat transfer coefficient helps to 2
dissipate the heat generated in the hydrate formation process faster (Arjang et al., 2013; Kakati et al., 2016). On the other hand, inhomogeneity of the system increases in presence of nanoparticles and heterogeneous nucleation which is easier than homogenous nucleation takes place (Aliabadi et al., 2015; Pahlavanzadeh et al., 2016). Furthermore, nanoparticles have high specific surface area to volume ratio, therefore they enhance mass transfer rate (Mohammadi et al., 2014; Zhou et al., 2014) and promote hydrate formation. Park et al. (2010) studied the effect of multi-walled carbon nanotubes on methane hydrate formation. According to their results, 0.004 wt.% multi-walled carbon nanotubes in pure water enhanced the amount of the gas consumed by 300% compared to the experiments in which only pure water was used. Kakati et al. (2016) investigated the kinetics of CH4+C2H6+C3H8 hydrate formation in presence of aluminum oxide (Al2O3) and zinc oxide (ZnO) nanoparticles. The results showed that storage capacity of gas hydrate increased proportional to the concentration of nanoparticles. Additionally, some researchers studied the effects of additives on the stability of gas hydrates. Ganji et al. (2013) investigated the effects of nanoparticles, polymers and polymer/nanoparticles suspensions on the storage capacity and stability of methane hydrate. It was explained that most of these additives increased hydrate storage capacity and some of them were also effective in increasing the stability of methane hydrates. In another work, Ganji et al. (2007a) revealed that 500 ppm SDS solution increased both methane hydrate formation rate and the storage capacity effectively, but it decreased hydrate stability compared to the experiments that were done with pure water. The authors believed that the finer size and larger surface area of the hydrate crystals in presence of SDS increased the dissociation rate. Literature survey reveals that researches on ethylene hydrates are insufficient in spite of the potential advantages of these hydrates in petrochemical and polymer industries. Therefore, it has been attempted in this study to fill in the gap in the current knowledge pertaining to ethylene hydrate by facilitating its formation and increasing its storage capacity in presence of two additives, namely graphene oxide and SDS. The effects of synthesized graphene oxide suspension and SDS solutions at various concentrations on the kinetics of ethylene hydrate were experimentally studied. The induction time measurements were done at 4 °C and initial pressures of 14 and 16 bar, and gas consumption and effective storage capacity were measured at 1.5 °C and initial pressure of 30 bar. Finally, the promotion effect of these additives on the kinetics of ethylene hydrate formation was discussed. To the best of our knowledge, this is the first detailed investigation on the effect of these two promoters on ethylene hydrate formation.
3
2. Experimental 2.1. Material Analytical grade ethylene (99.95% purity) was supplied by Technical Gas Service, Iran. Sodium dodecyl sulfate (NaC12H25SO4, 98% purity), concentrated sulfuric acid (H2SO4 97%), potassium permanganate (KMnO4), hydrochloric acid (HCl 37%) and hydrogen peroxide (H2O2 30%) were all purchased from Merck, Germany. Graphite powder was provided by DAEJUNG Co, Korea. Distilled water was used during the experiments.
2.2 Apparatus The schematic diagram of ethylene hydrate formation apparatus is shown in Fig. 1. Ethylene hydrate tests were carried out in a jacketed stainless steel reactor with effective volume of 460 cm3. Two valves were located on the reactor for charging and discharging the gas and the solution. To increase the mass transfer rate and gas diffusion into the aqueous phase, an electromotor was used to rock the reactor so that the fluid could flow in the reactor as a falling film. Hydrate formation is an exothermic process which occurs at low temperatures. So, a mixture of ethylene glycol and water was used to cool down the reactor. The reactor jacket had an inlet and an outlet for the coolant and the coolant temperature was controlled by a circulation system. Hydrate formation reactor, transfer pipes for the coolant and all the fittings were well insulated to prevent heat loss. A platinum resistance thermometer (PT 100, temperature range: -50 to 150 °C, accuracy: ±0.1 °C) was inserted into the reactor to measure the temperature. The reactor pressure was measured by a pressure transducer (type 26.600G, operating range: 0 to 100 bar, accuracy ±0.1bar) provided by BD SENSORS which was placed on top of the reactor. After measuring the temperature and the pressure by these sensors, the data were collected by a computer interface.
2.3. Synthesis of graphene oxide Graphite oxide was synthesized according to Hummers' method (Hummers Jr and Offeman, 1958). Graphite powder (2.0 g) was initially added to cold concentrated sulfuric acid (H2SO4, 46 mL at 4 °C). Then, KMnO4 (6.0 g) was gradually added to this mixture and the mixture was stirred for 2 h. During this period, the temperature was maintained below 10 °C by using an ice bath. The mixture was then removed from the ice bath and was stirred for 30 min at 35 °C. In the next step, distilled water (92 mL) was slowly added to the mixture to keep the system temperature at 98 °C for 15 min. After diluting the
4
mixture again with distilled water (300 mL), H2O2 (30%, 15 mL) was added to reduce the remaining KMnO4 until the color of the mixture turned into bright yellow. Then, the sample was washed with aqueous solution of HCl (5%) to remove metal ions, and with distilled water (1.0 L) to remove the acid. Graphite oxide was obtained and was then dried at 60 °C for 24 h. 1.0 g of graphite oxide was added to 1.0 L of distilled water and the mixture was sonicated (700 W) for 30 min to prepare homogeneous graphene oxide suspension. It is necessary to note that graphene oxide suspension was stable for a long time and no precipitation was observed within 72 h after its preparation. Surface characteristics and elevation profile of graphene oxide layers were studied by Atomic Force Microscopy (AFM), and the result is shown in Fig. 2. A drop of diluted graphene oxide suspension (0.01 g/L) was poured on silicon and was allowed to dry. The reason for diluting the sample was to avoid overlapping of graphene oxide sheets. Graphene oxide nanosheets are reported to be about 0.8 nm thick (Schniepp et al., 2006) and as AFM results showed in this study, the thickness of the prepared sample was about 1.2 nm. This value is less than the results reported in literature for two layers of graphene oxide sheets and can prove successful synthesis of graphene oxide nanosheets.
2.4. Procedure 2.4.1. Induction time of ethylene hydrate formation Hydrate reactor was initially washed with distilled water and then dried completely. A vacuum pump was used to evacuate the inside air from the reactor. Then, 100 mL of the required sample (i.e., water, graphene oxide suspension or SDS solution) was injected into the reactor and the reactor temperature was set to 4 °C. When the desired temperature was reached, ethylene was injected into the reactor at either 14 bar or 16 bar and the electromotor was turned on and set to 25 rpm. 2.4.2. Storage capacity of ethylene hydrate In this section, the experimental procedure was similar to section 2.4.1, except that the amount of the sample was reduced to 50 mL to ensure complete conversion of water to hydrate. Also, the temperature was fixed at 1.5 °C and the initial pressure was set to 30 bar. Generally, if the tests finish at a pressure higher than the equilibrium pressure, it can be concluded that the water has been completely converted to hydrate. In this study, it was assumed that when the rate of the pressure change was less than 0.05 bar/h, hydrate formation would reach the final point. Ethylene hydrate equilibrium data and the initial conditions for the current study are shown in Fig. 3.
5
3. Results and discussion 3.1. Induction time In order to determine the induction time of ethylene hydrate, both pressure and temperature of the system were examined simultaneously. Variations of pressure and temperature during ethylene hydrate formation process in presence of pure water at the initial condition of 4 °C and 16 bar are presented in Fig. 4. As shown in Fig. 4, the pressure decreased when ethylene was injected into the reactor. This decrease in the pressure is due to the dissolution of ethylene in the aqueous phase as well as the reduction in temperature caused by the coolant. Subsequently, the initial equilibrium state appeared (t ≈ 6 min) and the pressure and the temperature remained constant. However, when hydrate nuclei started to form in the system (t ≈ 136 min) the pressure started to decrease while the temperature increased slightly. The reason for this change in the temperature is that hydrate nucleation is an exothermic process which increases the temperature of the system. Induction time is defined as the time period between the appearance of initial equilibrium state and the initial formation of gas hydrate. Further measurements for the induction time in presence of pure water, graphene oxide suspension and SDS solution were done according to the described method. In order to measure the induction time, each experiment was repeated four times and an average value, along with the standard error for the average is reported in Table 1 and Table 2. 3.1.1. Effect of graphene oxide on the induction time Table 1 shows the induction time data measured in presence of graphene oxide suspension at various concentrations (50 ppm, 150 ppm and 250 ppm), T= 4 ºC and two different initial pressures (P0= 14 bar and P0= 16 bar). A promising finding was that the presence of graphene oxide at all concentrations reduced the induction time compared to pure water. Graphene oxide has an excellent structure for heterogeneous nucleation. As this type of nucleation needs less work (Kashchiev and Firoozabadi, 2002) and is more likely than homogeneous nucleation, the induction time decreases in presence of graphene oxide. Also, the perfect network of graphene oxide may act as a pattern for assembly of water and ethylene molecules to finally facilitate hydrate nucleation. Additionally, it is possible that carboxyl and hydroxyl groups of graphene oxide play a positive role in reducing the induction time of ethylene hydrate. These groups incorporate hydrogen bonds and can
6
further stabilize the hydrate nuclei. Furthermore, the graphene oxide nanosheets have high specific surface area to volume ratio and improve mass transfer in the hydrate system. The results in Table 1 also showed that the minimum induction time at the initial pressure of 14 bar was 8 min and at 16 bar was 6 min, which indicated 97% and 95% decrease in the induction time, respectively, relative to the tests with pure water. At both initial pressures, the shortest induction time was obtained when 150 ppm graphene oxide suspension was used. 3.1.2. Effect of SDS on induction time The induction time data in presence of SDS solution at various concentrations (50 ppm, 100 ppm ,300 ppm and 500 ppm), two different initial pressures (P0= 14 bar and P0= 16 bar) and T= 4 ºC are tabulated in Table 2. In comparison with pure water, the induction time decreased in presence of SDS solution. At both initial pressures, the decrease in the induction time was insignificant when 50 ppm or 100 ppm SDS solution was used; however, by further increasing the SDS concentration to 300 ppm and 500 ppm the induction time decreased remarkably relative to the experiments with pure water. SDS solution decreases the interfacial tension and increases the rate of gas solubility, therefore providing more gas molecules for the formation of stable nuclei in the nucleation stage which consequently reduces the induction time. When SDS solution at 300 ppm was used, the induction time reached a minimum, suggesting 92% and 84% reduction in the induction time at 14 bar and 16 bar compared to pure water, respectively.
3.2. Gas consumption in formation of ethylene hydrate In order to determine the amount of ethylene consumed during hydrate formation, real gas law (Eq. (1)) was used:
n
V P0 ( R Z 0T0
Pt ) ZtTt
(1)
where P and T are the absolute pressure and temperature, respectively. V is the volume of the gas phase, Z is the compressibility factor and R is the universal gas constant. Subscripts 0 and t represent the reactor condition at time t=0 and time t, respectively. Compressibility factor was calculated by Peng-Robinson equation of state (Peng and Robinson, 1976). 3.2.1. Effect of graphene oxide on ethylene consumption 7
Fig. 5 shows pressure changes during formation of ethylene hydrate in presence of graphene oxide suspension. Mole of ethylene consumed during hydrate formation was calculated based on Eq. (1) and is shown in Fig. 6. In presence of pure water, it is likely that a layer of ethylene hydrate has covered the gas/liquid interface. This layer significantly reduced the interfacial area and the rate of ethylene diffusion into the aqueous phase. As shown in Fig. 5 and Fig. 6, graphene oxide suspension at 50 ppm did not promote hydrate formation significantly, but the rate of hydrate formation improved remarkably when the concentration was increased to 150 and 250 ppm. The positive effect of graphene oxide suspension on ethylene hydrate formation might be related to the high specific surface area to volume ratio provided by these nanosheets. It should be noted that for further analysis of graphene oxide stability, the hydrate was dissociated by increasing the temperature after finishing the hydrate formation, and then the solution in the reactor was analyzed to check the precipitation of graphene oxide. No precipitated particles were observed exhibiting good dispersity of this suspension during hydrate formation. So, due to the good dispersion of graphene oxide nanosheets in the suspension, formation and growth of gas hydrate nucleus might take place in the bulk of the aqueous phase. Thus, coverage of the gas/liquid interface with a layer of hydrate would be postponed, especially at higher concentrations of the graphene oxide suspension in which numerous nucleation sites were available. Moreover, it was observed that hydrate formation rate decreased after about 300 min at both concentrations of 150 and 250 ppm, which could be resulted from possible coverage of the gas/liquid interface by a layer of ethylene hydrate. 3.2.2. Effect of SDS on ethylene consumption Fig. 7 shows pressure changes during formation of ethylene hydrate in presence of SDS solution and Fig. 8 shows ethylene consumption over time. These results indicated that SDS solution at concentrations of 50 and 100 ppm did not promote ethylene hydrate consumption effectively, but the effect was prominent when 300 and 500 ppm SDS solutions were used. Luo et al. (2006) reported that interfacial tension between SDS solution and ethylene decreases upon increasing SDS concentration. This decrease in the interfacial tension enhances the rate of ethylene solubility in the aqueous phase. Moreover, SDS might be adsorbed on hydrate crystals (Lo, 2011; Paria and Khilar, 2004) and therefore it might reduce the agglomeration of hydrate crystals. As a result, fine and discrete crystals with high specific surface area to volume ratio were formed in the presence of SDS solution and the amount of unreacted water between ethylene hydrate crystals decreased. Also, 8
covering of gas/liquid interface with hydrate crystals would be postponed and thus the mass transfer remained significant for a longer time. These effects justify the observed improvement in ethylene hydrate formation by using 300 ppm and 500 ppm SDS solutions which is shown in Fig. 7 and Fig. 8. Nevertheless, as more ethylene hydrate crystals were produced over time in the presence of SDS solution at 300 and 500 ppm, it is likely that the gas/liquid interface would be eventually covered by hydrate crystals and the rate of hydrate formation decreased.
3.3. Effective storage capacity of ethylene hydrate Storage capacity of gas hydrate is calculated according to Eq. (2):
Storage capacity
VSTP VH
nRTSTP / PSTP VH
(2)
in which VSTP, PSTP and TSTP are the volume, the pressure and the temperature of the entrapped gas at standard conditions, respectively and VH is the volume of the gas hydrate. Pressure of the system at the end of hydrate formation process was constant and higher than the equilibrium pressure, making it rational to conclude that the water in the reactor was completely converted to hydrate. It can be assumed that the hydrate volume is equal to the volume of the empty hydrate lattice which is calculated by Eq. (3) (Klauda and Sandler, 2000): 5 6 2 3 H (T , P) (11.835 2.217 10 T 2.242 10 T ) 9
8.006 10 P 5.448 10
12
P
10
30
NA 46
(3)
2
where NA is Avogadro’s Number, P (MPa) and T (K) are the absolute pressure and temperature, respectively. VH is equal to 22.67 cm3/mol at the experimental condition. In this study, the term “effective storage capacity” has been used due to the rocking-type motion of hydrate formation reactor. The effective storage capacity of ethylene hydrate in presence of additives is shown in Fig. 9. As it can be seen in Fig. 9, graphene oxide at concentration of 50 ppm and SDS solution at concentration of 50 and 100 ppm did not promote the effective storage capacity of ethylene hydrate significantly; however, the effective storage capacity of ethylene hydrate increased remarkably in presence of both additives at higher concentration. It should be noted that the effective storage capacity of ethylene hydrate in presence of pure water was 42.01 (V/V). The maximum effective storage capacity was as high as 151.15 (V/V) which was obtained by using 300 ppm SDS solution, indicating 259.8% improvement compared to pure water.
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In a recent paper, Manteghian et al. (2013) studied the effects of 1,4-dioxan solution on ethylene hydrate formation at conditions similar to our experiments. Comparing the current study with the research done by Manteghian et al. (2013), 3.5% increase in the maximum effective storage capacity of ethylene hydrate has been observed in this study.
4. Conclusion In this survey, the effects of graphene oxide suspension and SDS solution on the induction time, gas consumption and effective storage capacity of ethylene hydrate were experimentally studied. A promising finding was that both additives decreased the induction time of ethylene hydrate formation. Successfully, graphene oxide suspension showed more pronounced effects in reducing the induction time, as the shortest induction time was obtained when graphene oxide suspension at 150 ppm was used. Also, it was shown that SDS solution and graphene oxide suspension at low concentration did not promote ethylene hydrate formation rate effectively, while the presence of these additives at high concentrations improved ethylene consumption and the effective storage capacity noticeably. SDS solution showed greater promotion effect on the effective storage capacity of ethylene hydrate. The highest effective storage capacity of ethylene hydrate was obtained when 300 ppm SDS solution was used.
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Table 1 The induction time of ethylene hydrate in presence of graphene oxide suspension; each experiment was repeated four times.
Additive
Graphene oxide
Pure water
Graphene oxide
Pure water
Concentration (ppm)
P0 (bar)
T(ºC)
Induction time (min)
50
14
4
22 3
150
14
4
8 2
250
14
4
10 2
-
14
4
303 3
50
16
4
17 3
150
16
4
6 2
250
16
4
13 2
-
16
4
130 3
13
Table 2 The induction time of ethylene hydrate in presence of SDS solution; each experiment was repeated four times.
Additive
Concentration (ppm)
P0 (bar)
T(ºC)
Induction time (min)
50
14
4
285 3
100
14
4
273 3
300
14
4
25 2
500
14
4
28 2
-
14
4
303 3
50
16
4
119 3
100
16
4
109 3
300
16
4
21 2
500
16
4
23 2
-
16
4
130 3
SDS
Pure water
SDS
Pure water
14
Gas injection Water/solution injection Coolant out
P sensor
Coolant in
Ethylene cylinder
T sensor
DAQ system
Electromotor Circulation system
Computer Fig. 1. The schematic diagram of ethylene hydrate formation apparatus
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Fig. 2. Surface characteristics and elevation profile of graphene oxide layers obtained from AFM
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32 28 24
P (bar)
20 16 12 8 4 0 0
1
2
3
4
5
6
7
8
9
10
T (°C)
Fig. 3. Ethylene hydrate equilibrium data induction time measurement in this study
(obtained from (Manteghian et al., 2013)), initial condition for the , initial condition for the storage capacity measurement in this study
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10
induction time
16
T-t
14
P (bar)
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P-t
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2
0
T (°C)
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1 0
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60
80
100
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140
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180
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t (min)
Fig. 4. Variations of pressure and temperature to measure the induction time of ethylene hydrate; pure water, P0=16 bar and T=4 °C
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32 pure water
30
graphene oxide 50 ppm graphene oxide 150 ppm
P (bar)
28
graphene oxide 250 ppm
26
24
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18 0
200
400
600
800
1000
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t (min)
Fig. 5. Pressure changes of ethylene over time in presence of graphene oxide suspension; P0= 30 bar and T= 1.5 ° C
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0.35
Gas consumption (mol)
0.3
0.25
pure water graphene oxide 50 ppm
0.2
graphene oxide 150 ppm graphene oxide 250 ppm
0.15
0.1
0.05
0 0
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400
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t (min)
Fig. 6. Gas consumption in presence of graphene oxide suspension; P0= 30 bar and T= 1.5 ° C
20
31 29
P (bar)
27 25 pure water
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SDS 50 ppm SDS 100 ppm
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SDS 300 ppm SDS 500 ppm
19 17 15 0
200
400
600
800
1000
1200
1400
1600
t (min)
Fig. 7. Pressure changes of ethylene in presence of SDS solution; P0= 30 bar and T= 1.5 °C
21
0.45 0.4 pure water
Gas consumption (mol)
0.35
SDS 50 ppm
0.3
SDS 100 ppm SDS 300 ppm
0.25
SDS 500 ppm
0.2 0.15 0.1 0.05 0 0
200
400
600
800
1000
1200
1400
t (min)
Fig. 8. Gas consumption in presence of SDS solution; P0= 30 bar and T= 1.5 ° C
22
1600
SDS 500 ppm SDS 300 ppm SDS 100 ppm SDS 50 ppm Graphene oxide 250 ppm Graphene oxide 150 ppm Graphene oxide 50 ppm Pure water 0
20
40 60 80 100 120 Effective Storgae Capacity (V/V)
140
160
Fig. 9. Effective storage capacity of ethylene hydrate in presence of different additives; P0= 30 bar and T= 1.5 °C
Highlights
Graphene oxide and SDS decrease the induction time of ethylene hydrate formation.
Graphene oxide is more effective in decreasing the induction time.
Graphene oxide and SDS improve the effective storage capacity of ethylene hydrate.
SDS is more effective in increasing the effective storage capacity.
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PII: DOI: Reference:
S0920-4105(16)30577-0 http://dx.doi.org/10.1016/j.petrol.2016.10.008 PETROL3668
To appear in: Journal of Petroleum Science and Engineering Received date: 27 June 2016 Revised date: 29 September 2016 Accepted date: 4 October 2016 Cite this article as: Erfan Rezaei, Mehrdad Manteghian and Marzieh Tamaddondar, Kinetic Study of Ethylene Hydrate Formation in Presence of Graphene Oxide and Sodium Dodecyl Sulfate, Journal of Petroleum Science and Engineering, http://dx.doi.org/10.1016/j.petrol.2016.10.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Kinetic Study of Ethylene Hydrate Formation in Presence of Graphene Oxide and Sodium Dodecyl Sulfate Erfan Rezaeia, Mehrdad Manteghiana,*, Marzieh Tamaddondarb a
Department of Chemical Engineering, Tarbiat Modares University, Tehran, P.O. Box 14115-311, Iran. b
School of Chemistry, University of Manchester, Manchester, M13 9PL, U.K.
Abstract The effects of synthesized graphene oxide suspension at concentrations of 50, 150 and 250 ppm and aqueous solution of sodium dodecyl sulfate (SDS) at concentrations of 50, 100, 300 and 500 ppm on kinetics of ethylene hydrate formation were studied. The induction time was measured at 4 °C and initial pressures of 14 and 16 bar, and the effective storage capacity was measured at 1.5 °C and initial pressure of 30 bar. The results indicated that although both graphene oxide suspension and SDS solution in the whole range of experimental concentrations reduced the induction time of ethylene hydrate formation, graphene oxide was more effective in decreasing the induction time. The minimum induction time was obtained when 150 ppm graphene oxide suspension was used, which indicated an average decrease of 96% compared to pure water. In addition, the effective storage capacity measurements showed that additives at low concentrations did not promote the effective storage capacity noticeably, while graphene oxide suspension at 150 and 250 ppm as well as SDS solution at 300 and 500 ppm promoted the effective storage capacity of ethylene hydrate significantly. SDS showed better performance in increasing the effective storage capacity, as the highest effective storage capacity was attained by using 300 ppm SDS solution, suggesting 259.8% improvement in comparison to pure water. Key words: Ethylene Hydrate, Graphene Oxide, Sodium Dodecyl Sulfate, Induction Time, effective Storage Capacity
1. Introduction Gas hydrates are non-stoichiometric compounds formed from water molecules (host) and gas molecules (guests) under favorable conditions of temperature and pressure. Water molecules create cage-like structures through hydrogen bonding, which can be occupied by gas molecules that have appropriate sizes and shapes (Sloan, 2003; Sloan Jr and Koh, 2007). Previously, many researches were focused on different approaches to inhibit hydrate formation (Heidaryan et al., 2010; Lee and Englezos, 2005). Such studies were required because hydrate formation in gas transporting pipe lines obstructed the flow and caused major concerns for oil and gas industries. Meanwhile, gas hydrates have great potentials in many industrial applications such as transport and storage of natural gas (Hao et al., 2008; Javanmardi et al., 2005), gas separation
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(Eslamimanesh et al., 2012), cooling cycles (Douzet et al., 2013; Shi and Zhang, 2013) and seawater desalination (Khawaji et al., 2008; Ngema et al., 2014). One of the major feed gases in polymer industry is ethylene; however, issues such as flammability limit its applications. In an attempt to explore a safe and easy method for storage and transportation of this gas, detailed study on kinetics of ethylene hydrate formation seems essential. Unfortunately, industrialization of gas hydrate formation faces two main obstacles; slow gas hydrate formation and low storage capacity of gas hydrate. Thus, promoting gas hydrate formation is of special importance. In recent years, many researchers have tried to investigate the promotion effect of additives on gas hydrate formation. Jarrahian and Heidaryan (2014) studied the effect of toluene sulfonic acid (TSA) isomers on the natural gas hydrate. They realized that 4 g/L TSA isomer promoted natural gas hydrate significantly. Li et al. (2016) investigated tetrahydrofuran (THF) and cyclopentane (CP) hydrate formation in the presence of glass particles with different hydrophobicity. The authors reported that the induction time for THF and CP hydrates decreased significantly when octadecyltrichlorosilane-coated glass particles were used. They also found that hydrophobic surfaces enhanced water structuring prior to hydrate formation. Many researchers have focused on promotion effect of surfactants on gas hydrate formation (Ganji et al., 2007b; Karimi et al., 2013; Sun et al., 2003). Du et al. (2014) studied the effect of ionic surfactants with equal carbon chain length on methane hydrate formation rate at 15 MPa and 274 K. Their research revealed that sodium dodecyl sulfate (SDS), dodecylamine hydrochloride (DAH), and Ndodecylpropane-1,3-diamine hydrochloride (DN2Cl) had remarkable promoting effect on methane hydrate formation. The Krafft temperatures of SDS, DAH, and DN2Cl are close to room temperature. The authors believed that since the experiments were done at a temperature less than the Krafft temperature, the surfactant solutions were possibly supersaturated and the surfactant crystals were formed. Formation of surfactant crystals increased the inhomogeneity of the system and promoted hydrate formation by heterogeneous nucleation. Zhang et al. (2007) investigated methane hydrate formation in presence of SDS. They realized that the promotion effect of SDS at temperatures below the normal Krafft temperature is possibly due to the adsorption of SDS on the hydrate nuclei that decreases the energy barrier to nucleation. Similarly, Aman et al. (2013) believed that surfactant adsorption on the hydrate surface decreased the interfacial tension and promoted hydrate formation. More recently, some researchers have also investigated the use of nanoparticles as gas hydrate promoters (Aliabadi et al., 2015; Ghozatloo et al., 2014; Kakati et al., 2016). Gas hydrate formation is an exothermic process and the presence of nanofluids with high heat transfer coefficient helps to 2
dissipate the heat generated in the hydrate formation process faster (Arjang et al., 2013; Kakati et al., 2016). On the other hand, inhomogeneity of the system increases in presence of nanoparticles and heterogeneous nucleation which is easier than homogenous nucleation takes place (Aliabadi et al., 2015; Pahlavanzadeh et al., 2016). Furthermore, nanoparticles have high specific surface area to volume ratio, therefore they enhance mass transfer rate (Mohammadi et al., 2014; Zhou et al., 2014) and promote hydrate formation. Park et al. (2010) studied the effect of multi-walled carbon nanotubes on methane hydrate formation. According to their results, 0.004 wt.% multi-walled carbon nanotubes in pure water enhanced the amount of the gas consumed by 300% compared to the experiments in which only pure water was used. Kakati et al. (2016) investigated the kinetics of CH4+C2H6+C3H8 hydrate formation in presence of aluminum oxide (Al2O3) and zinc oxide (ZnO) nanoparticles. The results showed that storage capacity of gas hydrate increased proportional to the concentration of nanoparticles. Additionally, some researchers studied the effects of additives on the stability of gas hydrates. Ganji et al. (2013) investigated the effects of nanoparticles, polymers and polymer/nanoparticles suspensions on the storage capacity and stability of methane hydrate. It was explained that most of these additives increased hydrate storage capacity and some of them were also effective in increasing the stability of methane hydrates. In another work, Ganji et al. (2007a) revealed that 500 ppm SDS solution increased both methane hydrate formation rate and the storage capacity effectively, but it decreased hydrate stability compared to the experiments that were done with pure water. The authors believed that the finer size and larger surface area of the hydrate crystals in presence of SDS increased the dissociation rate. Literature survey reveals that researches on ethylene hydrates are insufficient in spite of the potential advantages of these hydrates in petrochemical and polymer industries. Therefore, it has been attempted in this study to fill in the gap in the current knowledge pertaining to ethylene hydrate by facilitating its formation and increasing its storage capacity in presence of two additives, namely graphene oxide and SDS. The effects of synthesized graphene oxide suspension and SDS solutions at various concentrations on the kinetics of ethylene hydrate were experimentally studied. The induction time measurements were done at 4 °C and initial pressures of 14 and 16 bar, and gas consumption and effective storage capacity were measured at 1.5 °C and initial pressure of 30 bar. Finally, the promotion effect of these additives on the kinetics of ethylene hydrate formation was discussed. To the best of our knowledge, this is the first detailed investigation on the effect of these two promoters on ethylene hydrate formation.
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2. Experimental 2.1. Material Analytical grade ethylene (99.95% purity) was supplied by Technical Gas Service, Iran. Sodium dodecyl sulfate (NaC12H25SO4, 98% purity), concentrated sulfuric acid (H2SO4 97%), potassium permanganate (KMnO4), hydrochloric acid (HCl 37%) and hydrogen peroxide (H2O2 30%) were all purchased from Merck, Germany. Graphite powder was provided by DAEJUNG Co, Korea. Distilled water was used during the experiments.
2.2 Apparatus The schematic diagram of ethylene hydrate formation apparatus is shown in Fig. 1. Ethylene hydrate tests were carried out in a jacketed stainless steel reactor with effective volume of 460 cm3. Two valves were located on the reactor for charging and discharging the gas and the solution. To increase the mass transfer rate and gas diffusion into the aqueous phase, an electromotor was used to rock the reactor so that the fluid could flow in the reactor as a falling film. Hydrate formation is an exothermic process which occurs at low temperatures. So, a mixture of ethylene glycol and water was used to cool down the reactor. The reactor jacket had an inlet and an outlet for the coolant and the coolant temperature was controlled by a circulation system. Hydrate formation reactor, transfer pipes for the coolant and all the fittings were well insulated to prevent heat loss. A platinum resistance thermometer (PT 100, temperature range: -50 to 150 °C, accuracy: ±0.1 °C) was inserted into the reactor to measure the temperature. The reactor pressure was measured by a pressure transducer (type 26.600G, operating range: 0 to 100 bar, accuracy ±0.1bar) provided by BD SENSORS which was placed on top of the reactor. After measuring the temperature and the pressure by these sensors, the data were collected by a computer interface.
2.3. Synthesis of graphene oxide Graphite oxide was synthesized according to Hummers' method (Hummers Jr and Offeman, 1958). Graphite powder (2.0 g) was initially added to cold concentrated sulfuric acid (H2SO4, 46 mL at 4 °C). Then, KMnO4 (6.0 g) was gradually added to this mixture and the mixture was stirred for 2 h. During this period, the temperature was maintained below 10 °C by using an ice bath. The mixture was then removed from the ice bath and was stirred for 30 min at 35 °C. In the next step, distilled water (92 mL) was slowly added to the mixture to keep the system temperature at 98 °C for 15 min. After diluting the
4
mixture again with distilled water (300 mL), H2O2 (30%, 15 mL) was added to reduce the remaining KMnO4 until the color of the mixture turned into bright yellow. Then, the sample was washed with aqueous solution of HCl (5%) to remove metal ions, and with distilled water (1.0 L) to remove the acid. Graphite oxide was obtained and was then dried at 60 °C for 24 h. 1.0 g of graphite oxide was added to 1.0 L of distilled water and the mixture was sonicated (700 W) for 30 min to prepare homogeneous graphene oxide suspension. It is necessary to note that graphene oxide suspension was stable for a long time and no precipitation was observed within 72 h after its preparation. Surface characteristics and elevation profile of graphene oxide layers were studied by Atomic Force Microscopy (AFM), and the result is shown in Fig. 2. A drop of diluted graphene oxide suspension (0.01 g/L) was poured on silicon and was allowed to dry. The reason for diluting the sample was to avoid overlapping of graphene oxide sheets. Graphene oxide nanosheets are reported to be about 0.8 nm thick (Schniepp et al., 2006) and as AFM results showed in this study, the thickness of the prepared sample was about 1.2 nm. This value is less than the results reported in literature for two layers of graphene oxide sheets and can prove successful synthesis of graphene oxide nanosheets.
2.4. Procedure 2.4.1. Induction time of ethylene hydrate formation Hydrate reactor was initially washed with distilled water and then dried completely. A vacuum pump was used to evacuate the inside air from the reactor. Then, 100 mL of the required sample (i.e., water, graphene oxide suspension or SDS solution) was injected into the reactor and the reactor temperature was set to 4 °C. When the desired temperature was reached, ethylene was injected into the reactor at either 14 bar or 16 bar and the electromotor was turned on and set to 25 rpm. 2.4.2. Storage capacity of ethylene hydrate In this section, the experimental procedure was similar to section 2.4.1, except that the amount of the sample was reduced to 50 mL to ensure complete conversion of water to hydrate. Also, the temperature was fixed at 1.5 °C and the initial pressure was set to 30 bar. Generally, if the tests finish at a pressure higher than the equilibrium pressure, it can be concluded that the water has been completely converted to hydrate. In this study, it was assumed that when the rate of the pressure change was less than 0.05 bar/h, hydrate formation would reach the final point. Ethylene hydrate equilibrium data and the initial conditions for the current study are shown in Fig. 3.
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3. Results and discussion 3.1. Induction time In order to determine the induction time of ethylene hydrate, both pressure and temperature of the system were examined simultaneously. Variations of pressure and temperature during ethylene hydrate formation process in presence of pure water at the initial condition of 4 °C and 16 bar are presented in Fig. 4. As shown in Fig. 4, the pressure decreased when ethylene was injected into the reactor. This decrease in the pressure is due to the dissolution of ethylene in the aqueous phase as well as the reduction in temperature caused by the coolant. Subsequently, the initial equilibrium state appeared (t ≈ 6 min) and the pressure and the temperature remained constant. However, when hydrate nuclei started to form in the system (t ≈ 136 min) the pressure started to decrease while the temperature increased slightly. The reason for this change in the temperature is that hydrate nucleation is an exothermic process which increases the temperature of the system. Induction time is defined as the time period between the appearance of initial equilibrium state and the initial formation of gas hydrate. Further measurements for the induction time in presence of pure water, graphene oxide suspension and SDS solution were done according to the described method. In order to measure the induction time, each experiment was repeated four times and an average value, along with the standard error for the average is reported in Table 1 and Table 2. 3.1.1. Effect of graphene oxide on the induction time Table 1 shows the induction time data measured in presence of graphene oxide suspension at various concentrations (50 ppm, 150 ppm and 250 ppm), T= 4 ºC and two different initial pressures (P0= 14 bar and P0= 16 bar). A promising finding was that the presence of graphene oxide at all concentrations reduced the induction time compared to pure water. Graphene oxide has an excellent structure for heterogeneous nucleation. As this type of nucleation needs less work (Kashchiev and Firoozabadi, 2002) and is more likely than homogeneous nucleation, the induction time decreases in presence of graphene oxide. Also, the perfect network of graphene oxide may act as a pattern for assembly of water and ethylene molecules to finally facilitate hydrate nucleation. Additionally, it is possible that carboxyl and hydroxyl groups of graphene oxide play a positive role in reducing the induction time of ethylene hydrate. These groups incorporate hydrogen bonds and can
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further stabilize the hydrate nuclei. Furthermore, the graphene oxide nanosheets have high specific surface area to volume ratio and improve mass transfer in the hydrate system. The results in Table 1 also showed that the minimum induction time at the initial pressure of 14 bar was 8 min and at 16 bar was 6 min, which indicated 97% and 95% decrease in the induction time, respectively, relative to the tests with pure water. At both initial pressures, the shortest induction time was obtained when 150 ppm graphene oxide suspension was used. 3.1.2. Effect of SDS on induction time The induction time data in presence of SDS solution at various concentrations (50 ppm, 100 ppm ,300 ppm and 500 ppm), two different initial pressures (P0= 14 bar and P0= 16 bar) and T= 4 ºC are tabulated in Table 2. In comparison with pure water, the induction time decreased in presence of SDS solution. At both initial pressures, the decrease in the induction time was insignificant when 50 ppm or 100 ppm SDS solution was used; however, by further increasing the SDS concentration to 300 ppm and 500 ppm the induction time decreased remarkably relative to the experiments with pure water. SDS solution decreases the interfacial tension and increases the rate of gas solubility, therefore providing more gas molecules for the formation of stable nuclei in the nucleation stage which consequently reduces the induction time. When SDS solution at 300 ppm was used, the induction time reached a minimum, suggesting 92% and 84% reduction in the induction time at 14 bar and 16 bar compared to pure water, respectively.
3.2. Gas consumption in formation of ethylene hydrate In order to determine the amount of ethylene consumed during hydrate formation, real gas law (Eq. (1)) was used:
n
V P0 ( R Z 0T0
Pt ) ZtTt
(1)
where P and T are the absolute pressure and temperature, respectively. V is the volume of the gas phase, Z is the compressibility factor and R is the universal gas constant. Subscripts 0 and t represent the reactor condition at time t=0 and time t, respectively. Compressibility factor was calculated by Peng-Robinson equation of state (Peng and Robinson, 1976). 3.2.1. Effect of graphene oxide on ethylene consumption 7
Fig. 5 shows pressure changes during formation of ethylene hydrate in presence of graphene oxide suspension. Mole of ethylene consumed during hydrate formation was calculated based on Eq. (1) and is shown in Fig. 6. In presence of pure water, it is likely that a layer of ethylene hydrate has covered the gas/liquid interface. This layer significantly reduced the interfacial area and the rate of ethylene diffusion into the aqueous phase. As shown in Fig. 5 and Fig. 6, graphene oxide suspension at 50 ppm did not promote hydrate formation significantly, but the rate of hydrate formation improved remarkably when the concentration was increased to 150 and 250 ppm. The positive effect of graphene oxide suspension on ethylene hydrate formation might be related to the high specific surface area to volume ratio provided by these nanosheets. It should be noted that for further analysis of graphene oxide stability, the hydrate was dissociated by increasing the temperature after finishing the hydrate formation, and then the solution in the reactor was analyzed to check the precipitation of graphene oxide. No precipitated particles were observed exhibiting good dispersity of this suspension during hydrate formation. So, due to the good dispersion of graphene oxide nanosheets in the suspension, formation and growth of gas hydrate nucleus might take place in the bulk of the aqueous phase. Thus, coverage of the gas/liquid interface with a layer of hydrate would be postponed, especially at higher concentrations of the graphene oxide suspension in which numerous nucleation sites were available. Moreover, it was observed that hydrate formation rate decreased after about 300 min at both concentrations of 150 and 250 ppm, which could be resulted from possible coverage of the gas/liquid interface by a layer of ethylene hydrate. 3.2.2. Effect of SDS on ethylene consumption Fig. 7 shows pressure changes during formation of ethylene hydrate in presence of SDS solution and Fig. 8 shows ethylene consumption over time. These results indicated that SDS solution at concentrations of 50 and 100 ppm did not promote ethylene hydrate consumption effectively, but the effect was prominent when 300 and 500 ppm SDS solutions were used. Luo et al. (2006) reported that interfacial tension between SDS solution and ethylene decreases upon increasing SDS concentration. This decrease in the interfacial tension enhances the rate of ethylene solubility in the aqueous phase. Moreover, SDS might be adsorbed on hydrate crystals (Lo, 2011; Paria and Khilar, 2004) and therefore it might reduce the agglomeration of hydrate crystals. As a result, fine and discrete crystals with high specific surface area to volume ratio were formed in the presence of SDS solution and the amount of unreacted water between ethylene hydrate crystals decreased. Also, 8
covering of gas/liquid interface with hydrate crystals would be postponed and thus the mass transfer remained significant for a longer time. These effects justify the observed improvement in ethylene hydrate formation by using 300 ppm and 500 ppm SDS solutions which is shown in Fig. 7 and Fig. 8. Nevertheless, as more ethylene hydrate crystals were produced over time in the presence of SDS solution at 300 and 500 ppm, it is likely that the gas/liquid interface would be eventually covered by hydrate crystals and the rate of hydrate formation decreased.
3.3. Effective storage capacity of ethylene hydrate Storage capacity of gas hydrate is calculated according to Eq. (2):
Storage capacity
VSTP VH
nRTSTP / PSTP VH
(2)
in which VSTP, PSTP and TSTP are the volume, the pressure and the temperature of the entrapped gas at standard conditions, respectively and VH is the volume of the gas hydrate. Pressure of the system at the end of hydrate formation process was constant and higher than the equilibrium pressure, making it rational to conclude that the water in the reactor was completely converted to hydrate. It can be assumed that the hydrate volume is equal to the volume of the empty hydrate lattice which is calculated by Eq. (3) (Klauda and Sandler, 2000): 5 6 2 3 H (T , P) (11.835 2.217 10 T 2.242 10 T ) 9
8.006 10 P 5.448 10
12
P
10
30
NA 46
(3)
2
where NA is Avogadro’s Number, P (MPa) and T (K) are the absolute pressure and temperature, respectively. VH is equal to 22.67 cm3/mol at the experimental condition. In this study, the term “effective storage capacity” has been used due to the rocking-type motion of hydrate formation reactor. The effective storage capacity of ethylene hydrate in presence of additives is shown in Fig. 9. As it can be seen in Fig. 9, graphene oxide at concentration of 50 ppm and SDS solution at concentration of 50 and 100 ppm did not promote the effective storage capacity of ethylene hydrate significantly; however, the effective storage capacity of ethylene hydrate increased remarkably in presence of both additives at higher concentration. It should be noted that the effective storage capacity of ethylene hydrate in presence of pure water was 42.01 (V/V). The maximum effective storage capacity was as high as 151.15 (V/V) which was obtained by using 300 ppm SDS solution, indicating 259.8% improvement compared to pure water.
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In a recent paper, Manteghian et al. (2013) studied the effects of 1,4-dioxan solution on ethylene hydrate formation at conditions similar to our experiments. Comparing the current study with the research done by Manteghian et al. (2013), 3.5% increase in the maximum effective storage capacity of ethylene hydrate has been observed in this study.
4. Conclusion In this survey, the effects of graphene oxide suspension and SDS solution on the induction time, gas consumption and effective storage capacity of ethylene hydrate were experimentally studied. A promising finding was that both additives decreased the induction time of ethylene hydrate formation. Successfully, graphene oxide suspension showed more pronounced effects in reducing the induction time, as the shortest induction time was obtained when graphene oxide suspension at 150 ppm was used. Also, it was shown that SDS solution and graphene oxide suspension at low concentration did not promote ethylene hydrate formation rate effectively, while the presence of these additives at high concentrations improved ethylene consumption and the effective storage capacity noticeably. SDS solution showed greater promotion effect on the effective storage capacity of ethylene hydrate. The highest effective storage capacity of ethylene hydrate was obtained when 300 ppm SDS solution was used.
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Table 1 The induction time of ethylene hydrate in presence of graphene oxide suspension; each experiment was repeated four times.
Additive
Graphene oxide
Pure water
Graphene oxide
Pure water
Concentration (ppm)
P0 (bar)
T(ºC)
Induction time (min)
50
14
4
22 3
150
14
4
8 2
250
14
4
10 2
-
14
4
303 3
50
16
4
17 3
150
16
4
6 2
250
16
4
13 2
-
16
4
130 3
13
Table 2 The induction time of ethylene hydrate in presence of SDS solution; each experiment was repeated four times.
Additive
Concentration (ppm)
P0 (bar)
T(ºC)
Induction time (min)
50
14
4
285 3
100
14
4
273 3
300
14
4
25 2
500
14
4
28 2
-
14
4
303 3
50
16
4
119 3
100
16
4
109 3
300
16
4
21 2
500
16
4
23 2
-
16
4
130 3
SDS
Pure water
SDS
Pure water
14
Gas injection Water/solution injection Coolant out
P sensor
Coolant in
Ethylene cylinder
T sensor
DAQ system
Electromotor Circulation system
Computer Fig. 1. The schematic diagram of ethylene hydrate formation apparatus
15
Fig. 2. Surface characteristics and elevation profile of graphene oxide layers obtained from AFM
16
32 28 24
P (bar)
20 16 12 8 4 0 0
1
2
3
4
5
6
7
8
9
10
T (°C)
Fig. 3. Ethylene hydrate equilibrium data induction time measurement in this study
(obtained from (Manteghian et al., 2013)), initial condition for the , initial condition for the storage capacity measurement in this study
17
10
induction time
16
T-t
14
P (bar)
9
P-t
8
12
7
10
6
8
5
6
4
4
3
2
2
0
T (°C)
18
1 0
20
40
60
80
100
120
140
160
180
200
t (min)
Fig. 4. Variations of pressure and temperature to measure the induction time of ethylene hydrate; pure water, P0=16 bar and T=4 °C
18
32 pure water
30
graphene oxide 50 ppm graphene oxide 150 ppm
P (bar)
28
graphene oxide 250 ppm
26
24
22
20
18 0
200
400
600
800
1000
1200
1400
1600
t (min)
Fig. 5. Pressure changes of ethylene over time in presence of graphene oxide suspension; P0= 30 bar and T= 1.5 ° C
19
0.35
Gas consumption (mol)
0.3
0.25
pure water graphene oxide 50 ppm
0.2
graphene oxide 150 ppm graphene oxide 250 ppm
0.15
0.1
0.05
0 0
200
400
600
800
1000
1200
1400
1600
t (min)
Fig. 6. Gas consumption in presence of graphene oxide suspension; P0= 30 bar and T= 1.5 ° C
20
31 29
P (bar)
27 25 pure water
23
SDS 50 ppm SDS 100 ppm
21
SDS 300 ppm SDS 500 ppm
19 17 15 0
200
400
600
800
1000
1200
1400
1600
t (min)
Fig. 7. Pressure changes of ethylene in presence of SDS solution; P0= 30 bar and T= 1.5 °C
21
0.45 0.4 pure water
Gas consumption (mol)
0.35
SDS 50 ppm
0.3
SDS 100 ppm SDS 300 ppm
0.25
SDS 500 ppm
0.2 0.15 0.1 0.05 0 0
200
400
600
800
1000
1200
1400
t (min)
Fig. 8. Gas consumption in presence of SDS solution; P0= 30 bar and T= 1.5 ° C
22
1600
SDS 500 ppm SDS 300 ppm SDS 100 ppm SDS 50 ppm Graphene oxide 250 ppm Graphene oxide 150 ppm Graphene oxide 50 ppm Pure water 0
20
40 60 80 100 120 Effective Storgae Capacity (V/V)
140
160
Fig. 9. Effective storage capacity of ethylene hydrate in presence of different additives; P0= 30 bar and T= 1.5 °C
Highlights
Graphene oxide and SDS decrease the induction time of ethylene hydrate formation.
Graphene oxide is more effective in decreasing the induction time.
Graphene oxide and SDS improve the effective storage capacity of ethylene hydrate.
SDS is more effective in increasing the effective storage capacity.
23