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Energy & Fuels 2003, 17, 1180-1185
Experimental Studying of Additives Effects on Gas Storage in Hydrates Zhi-Gao Sun,*,† Rong-sheng Ma,† Ru-Zhu Wang,‡ Kai-Hua Guo,§ and Shuan-Shi Fa§ Department of Environment Engineering, South Campus of Jiangyang Road, Yangzhou University, Yangzhou 225009, China, Institute of Refrigeration and Cryogenics, Shanghai Jiaotong University, Shanghai 200030, China, and Guangzhou Institute of Energy Conversion, The Chinese Academy of Sciences, Guangzhou 510070, China Received September 4, 2002. Revised Manuscript Received March 24, 2003
Surfactants increase the rate of gas hydrate formation and the gas storage capacity in hydrates in a quiescent system. An anionic surfactant (sodium dodecyl sulfate) and a nonionic surfactant (dodecyl polysaccharide glycoside) were used in this work. The effect of a nonionic surfactant on hydrate formation is less pronounced, compared to that of an anionic surfactant. The presence of cyclopentane also improved the rate of hydrate formation; however, it could not improve the gas storage capacity in hydrates.
Introduction Gas hydrates are icelike crystalline inclusion compounds that are formed by the hydrogen bonding of water molecules, with the assistance of gases such as methane, ethane, or propane. There are two opposite directions in the investigation on hydrates. Studies related to the problematic side of hydrates are those which attempt to find ways of hydrate inhibition, whereas some studies are focused on determining some means for the promotion of hydrate formation (see, for example, the work of Sloan,1 Makogon,2 and Karaaslan and Parlaktuna3). Gas hydrates have attracted much attention recently, not only as a new natural energy resource but also as a new means for natural gas storage and transport. In reference to standard conditions, natural gas hydrates contain 150-180 volumes of gas/volume of hydrate (V/V). Natural gas hydrate technology is an attractive alternative for storing and transporting natural gas (see Gudmundsson and Børrehaug4). Khokhar and Sloan5 studied the storage properties of structure H hydrates. Hydrate formation for the purpose of natural gas storage and transport has been reported recently (see, * Author to whom correspondence should be addressed. E-mail:
[email protected]. † Yangzhou University. ‡ Shanghai Jiaotong University. § The Chinese Academy of Sciences. (1) Sloan, E. D. Clathrate Hydrate of Nature Gases; Marcel Dekker: New York, 1998. (2) Makogon, Y. F. Hydrates of Hydrocarbons; PennWell Publishing Company: Tulsa, OK, 1997. (3) Karaaslan, U.; Parlaktuna, M. Surfactants as Hydrate Promoters? Energy Fuels 2000, 14, 1103-1107. (4) Gudmundsson, J. S.; Børrehaug, A. Frozen Hydrate for Transport of Nature Gas. 2nd International Conference on Nature Gas Hydrate, Toulouse, France, June 2-6, 1996; pp 439-446. (5) Khokhar, A. A.; Sloan, E. D. Gas Storage in Structure H Hydrates. Fluid Phase Equilib. 1998, 150-151, 383-392.
for example, the work of Zhong and Rogers,6 Gudmundsson,7 and Guo et al.8). However, industrial applications of hydrate storage processes have been hindered by some problems, such as slow formation rates, unreacted interstitial water present as a large percentage of the hydrate mass, the reliability of hydrate storage capacity, and the economics of process scaleup. The main purpose of this work is to increase the rate of gas hydrate formation, improve the hydrate storage capacity, and reduce the energy costs of the hydrate formation process by adding additives. Gas Storage Capacity in Hydrates The volume of gas stored in a unit volume of hydrate under the hydrate formation conditions (pressure and temperature) is calculated using the following equation (from Makogon2):
VGH )
103VGF Mh
(1)
where Mh is the molecular weight of hydrate, which is defined as
Mh ) M + 18.02n
(2)
The volume of gas stored in a unit volume of hydrate, (6) Zhong, Y.; Rogers, R. E. Surfactant Effects on Gas Hydrate Formation. Chem. Eng. Sci. 2000, 55, 4175-4187. (7) Gudmundsson, J. S. Hydrate Non-Pipeline Technology. 4th International Conference on Gas Hydrates, Yokohama, Japan, May 1923, 2002; pp 997-1002. (8) Guo, Y. K.; Fan, S. S.; Guo, K. H.; Chen, Y. Storage Capacity of Methane in Hydrate Using Calcium Hypochlorite as Additive. 4th International Conference on Gas Hydrates, Yokohama, Japan, May 1923, 2002; pp 1040-1043.
10.1021/ef020191m CCC: $25.00 © 2003 American Chemical Society Published on Web 07/12/2003
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Figure 1. Schematic diagram of the experimental apparatus. Table 1. Test Materials Used in This Work component
purity (%)
supplier
methane cyclopentane, CP sodium dodecyl sulfate, SDS dodecyl polysaccharide glycoside, DPG water
99.99 98.4 g98 98
Guangzhou Gas Co. Meilong Chemical Co. Guangzhou Chemical Reagent Co. Guangzhou Chemical Reagent Co. distilled
under standard conditions, is calculated by the expression
VSC )
VGHpT0 Zp0T
(3)
The density of hydrate formed by natural gas is given by the following expression:
F)
∑Ni(Mi + 18.02ni) ∑18.02NiVini
(4)
Experimental Apparatus and Procedure The experimental apparatus is sketched in Figure 1. A cylindrical high-pressure cell, made of stainless steel, is used to produce gas hydrate. A stainless-steel flange, which has appropriate ports for access to the interior, is used to seal the cell on the top. The cell is designed to operate at pressures up to 20 MPa and temperatures in the range of 253-323 K; its available volume is 1072 cm3. The interlayer of the cell is cooled using circulated cooling water in combination with enough ethylene glycol to depress the freezing point of the water. The coolant is circulated from a refrigerated bath that is capable of maintaining the bath temperature to within (0.01 K of the set point, to a low-temperature capability of 258 K. The cell is enclosed by insulation. Two platinum resistance thermometers were used to measure the experimental temperature; the accuracy of these thermometers is (0.01 K. One thermometer extends into the bottom of the cell, and the other extends into the gas phase at the top. The pressure of the cell was measured using a 10-MPa digital gauge with an accuracy of (0.1% of full scale. A constant-pressure regulator can maintain a constant pressure in the cell, to within 0.02 MPa. A mass gas flowmeter (model D07-11M/ZM, Peking Jianzhong Instruments, Inc.) was used to measure the amount of gas added to the cell during hydrate formation. The flowmeter has a capacity of 0-1000 sccm, with an accuracy within 2% of full scale and a repeat-
ability of within 0.2% of the flow rate. There is a data logger to record the pressure and temperature of the cell, as well as outputs from the mass gas flowmeter in the process of hydrate formation, as a function of time. The test materials used in this work were summarized in Table 1. Surfactantssan anionic surfactant (sodium dodecyl sulfate, SDS) and a nonionic surfactant (dodecyl polysaccharide glycoside, DPG)swere weighed on a electronic balance with a readability of (0.1 mg. Distilled water was used in all the experiments. Water and cyclopentane (CP) were weighed on a electronic balance with a readability of (0.01 g. Twenty-one experiments were conducted, to observe the effect of additives on the hydrate formation. A typical procedure was as follows. The cell was rinsed with distilled water two times, and air was evacuated by the vacuum pump. Approximately 300 cm3 of the water solution was charged into the cell for each experiment. The hydrocarbon gas was then injected into the cell, up to a pressure of ∼1.0 MPa. The system was cooled to 274.05-277.55 K, with a pressure below the pressure for hydrate formation. The cell pressure was then increased to the experimental pressure over a span of 3-5 min by flowing hydrocarbon gas into the cell. Hydrate formation was monitored by temperature, pressure, and gas mass flow measurements that were recorded and displayed on the data acquisition system.
Results and Discussion In initial work, gas hydrate has been formed in a stirring system (the stirring velocity is ∼400 rpm) with methane (99.99%) and pure water at the appropriate pressure and temperature. The result showed that the rate of gas hydrate formation and the gas storage capacity in hydrates were each very small (Figure 2). Surfactant SDS was then added to improve the rate of gas hydrate formation and the gas storage capacity in a quiescent system. Figure 2 showed that the rate of methane hydrate formation and the gas storage capacity could be improved greatly with the presence of surfac-
1182 Energy & Fuels, Vol. 17, No. 5, 2003
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Figure 2. Plot showing that the presence of surfactant increases the rate of hydrate formation.
Figure 3. Capacity and the rate of methane hydrate formation in aqueous DPG.
tant SDS, compared to hydrate formation in a stirring pure water system. Surfactants (such as SDS and DPG) increase the natural gas solubility in water (see, for example, Zhong and Rogers6). Hydrates could form both at the interface between the gas and the liquid and in solutions. Therefore, the rate of hydrate formation was increased by the presence of SDS. Hydrate moved to the wall of the cell during the hydrate formation process in aqueous SDS solutions (see, for example, Zhong and Rogers6). The formed hydrates did not interrupt the gas-liquid interaction. Gas storage capacity in hydrates was improved by the presence of surfactants such as SDS or DPG. Hydrate formation in a quiescent system can reduce processing costs, because stirring is not needed. The following experiments in this work were performed in the presence of surfactants in a quiescent system in the pressure range of 3.80-6.18 MPa.
Figure 3 shows that the pressure affected the storage capacity and the rate of hydrate formation in a DPG + methane system. Very interestingly, at the early stage of hydrate formation at a lower pressure, the larger the capacity, the quicker the rate of hydrate formation; however, the system of higher pressures had the larger capacity and the quicker rate of hydrate formation at the latter stage of hydrate formation. A storage capacity of 112 (volume gas/volume hydrate, or V/V) was achieved at a processing pressure of 5.38 MPa, and a storage capacity of 80 V/V was observed at a processing pressure of 4.34 MPa. The experimental result of methane gas hydrate formation in aqueous SDS solutions was shown in Figures 4 and 5. The higher the pressure, the larger the storage capacity and the quicker the rate of hydrate formation. At 274.05 K, a storage capacity of 154 V/V
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Energy & Fuels, Vol. 17, No. 5, 2003 1183
Figure 4. Capacity and the rate of methane hydrate formation in aqueous SDS (274.05 K).
Figure 5. Capacity and the rate of methane hydrate formation in aqueous SDS (277.55 K).
was achieved at a processing pressure of 4.85 MPa and a storage capacity of 132 V/V was observed at a processing pressure of 3.92 MPa. At 277.55 K, a storage capacity of 153 V/V was achieved at a processing pressure of 5.76 MPa, whereas a storage capacity of 113 V/V was observed at a processing pressure of 4.85 MPa. These results showed that the lower temperature, the larger the gas storage capacity at the same experimental pressure. The result also showed that the effect of SDS on gas storage in hydrates was more pronounced, compared to that of DPG (according to Karaaslan and Parlaktuna3). Under the conditions of 4.85 MPa and 274.05 K, experiments of gas storage in hydrates were conducted three times in aqueous SDS solutions. The experimental results showed that the reproducibility was very good. (Gas storage capacity was in the range of 148-154 V/V, with the same time costs.)
Figure 6 shows results in the SDS + CP + methane system that were similar to those observed in Figure 4 for the SDS + methane system. The hydrate formation induction time was 10-30 min in the presence of surfactants (DPG and SDS) and cyclopentane (CP), compared to a range of 60-90 min in the presence of surfactants in this work. This showed that the hydrate formation induction time was reduced by the presence of CP, compared to that for hydrate formation without CP. The hydrate formation induction time was reduced because of a significant shift in the hydrate formation pressures to lower pressures in the methane + CP system, compared to the system of methane alone (see, for example, the work of Sun et al.9). CP can form structure II hydrates. The SDS + CP + methane system (9) Sun, Z. G.; Fan, S. S.; Guo, K. H.; Shi, L.; Wang, R. Z. Gas Hydrate Phase Equilibrium Data of Cyclohexane and Cyclopentane. J. Chem. Eng. Data 2002, 47, 313-315.
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Figure 6. Capacity and the rate of methane hydrate formation in aqueous SDS + CP.
Figure 7. Comparison of the effect of SDS and SDS + CP on the rate of methane hydrate formation.
formed a mixture of structure I and structure II hydrates in this work, because only 1.0 wt % of CP was involved in the experimental system. A storage capacity of 151 V/V was achieved at a processing pressure of 4.85 MPa, and a storage capacity of 129 V/V was observed at a processing pressure of 3.80 MPa. Figure 7 shows a comparison of the effect of SDS and SDS + CP on hydrate formation. The results show that the presence of CP improved the rate of hydrate formation but had no effect on the gas storage capacity in hydrates. That is to say, the presence of CP shortened the time costs of hydrate formation. This work showed that the rate of hydrate formation and the storage capacity could be improved by the presence of surfactants. Figure 8 presents the relationship of methane gas storage capacity in hydrates and
surfactant concentration. The result showed that the greatest storage capacity was observed when the DPG concentration was ∼500 ppm. The result in Figure 8 also shows that a best surfactant solution concentration for gas storage in hydrates does exist. The experimental results of gas storage in hydrates with the presence of additives in this work were tabulated in Table 2. The results in Table 2 show that CP can reduce experimental time costs for gas storage in hydrates. Conclusions Two different surfactantssnamely, an anionic surfactant (sodium dodecyl sulfate, SDS) and a nonionic surfactant (dodecyl polysaccharide glycoside, DPG)sand
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Energy & Fuels, Vol. 17, No. 5, 2003 1185
Figure 8. Gas storage capacity in hydrates in aqueous DPG. Table 2. Experimental Results of Methane Storage in Hydrates Using Various Additives and Temperaturesa experiment no.
pressure (MPa)
experimental time costs (min) DPG, 274.05 K 455 370 285
storage capacity (V/V)
1 2 3
5.38 4.96 4.34
4 5 6 7
5.54 4.85 4.34 3.92
SDS, 274.05 K 330 425 460 765
163 154 147 132
8 9 10
5.76 5.33 4.85
SDS, 277.55 K 665 705 760
153 146 113
6.18 4.85 3.80
SDS + CP, 274.05 K 315 340 540
169 151 129
5.80 5.43 4.85
SDS + CP, 277.55 K 480 595 590
152 149 115
11 12 13 14 15 16
of 151 V/V was observed at 4.85 MPa and 274.05 K within 340 min in the presence of SDS and CP.
112 101 80
Acknowledgment. The authors acknowledge the financial support by Chinese Natural Science Foundation (under Grant No. 50176051) and Chinese Jiangsu Province Education Committee Program (under Grant No. G0109199). Nomenclature M ) molecular weight of hydrate former gas
a DPG ) dodecyl polysaccharide glycoside, 500 ppm; SDS ) sodium dodecyl sulfate, 300 ppm; CP ) cyclopentane, 1.0 wt %.
one liquid hydrocarbon (cyclopentane, CP) were used to investigate the influence of additives on the gas storage capacity of hydrates and the rate of hydrate formation. The tests showed that gas storage in hydrates could be conducted in a quiescent system in the presence of SDS or DPG. Surfactants are hydrate formation promoters and can improve the rate of hydrate formation and the gas storage capacity. The effect of a nonionic surfactant (DPG) on hydrate formation is less pronounced, compared to that of an anionic surfactant (SDS). The presence of CP could also increase the rate of hydrate formation and reduce the hydrate formation induction time. A methane storage capacity of 154 (volume gas/ volume hydrate, or V/V) was achieved at a processing pressure of 4.85 MPa and 274.05 K within 425 min in the presence of SDS, and a methane storage capacity
Mh ) molecular weight of hydrate n ) hydrate number Ni ) molecular fraction of the ith hydrate former gas component p ) pressure (MPa) T ) temperature (K) VG ) volume of mole of gas (m3) VGH ) volume of gas stored in a unit volume of hydrate Vi ) specific volume of water in hydrate formed by component i (m3/kg) VSC ) volume of gas stored in a unit volume of hydrate under standard conditions Z ) compression factor F ) density (kg/m3) Subscripts 0 ) standard conditions i ) component of natural gas G ) gas GH ) gas in hydrates h ) hydrate EF020191M
