THERMODYNAMICS AND CHEMICAL ENGINEERING DATA Chinese Journal of Chemical Engineering, 18(2) 292 296 (2010)

Influence of Pore Size, Salinity and Gas Composition upon the Hydrate Formation Conditions*

YANG Mingjun ( )1, SONG Yongchen ( )1,**, LIU Yu ( )1, CHEN Yongjun ( )1 and LI Qingping ( )21 Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of Education, Dalian Univer-

sity of Technology, Dalian 116024 ,China 2 China National Offshore Oil Corporation Research Center, Beijing 100027, China

Abstract An experimental device was set up to study the hydrate formation conditions. Effects of pore size, salinity, and gas composition on the formation and dissociation of hydrates were investigated. The result indicates that the induction time for the formation of hydrates in porous media is shorter than that in pure water. The decrease in pore size, by decreasing the size of glass beads, increases the equilibrium pressure when the salinity and temperature are kept constant. In addition, higher salinity causes higher equilibrium pressure when the pore size and temperature are kept constant. It is found that the effects of pore size and salinity on the hydrate equilibrium are quite different. At lower methane concentration, the hydrate equilibrium is achieved at lower pressure and higher temperature. Keywords hydrate, equilibrium, pore size, salinity, gas composition

1 INTRODUCTION

Energy shortage has become a prominent issue in the world. More and more people are paying attention to natural gas hydrate (NGH), which is a new clean energy and widely distributed on earth. NGH is an ice-like crystalline compound in which gas molecules are held within cavities formed by water molecules [1].The mineral deposits of hydrate are large in scale with high energy density and shallow burial. To explore the hydrate, it is necessary to understand its physical characters, such as its equilibrium condition at differ-ent pore size and gas composition in the presence/ absence of aqueous solutions of salt. Some experi-ments were carried out to determine the equilibrium condition of gas hydrates containing hydrocarbon gases, which was used to investigate characteristics of their clathrate structure. Li et al. [2] considered that the variation of pressure in the three stages of forma-tion process and the cumulative gas consumption could be employed to explain the phenomenon that the process of gas hydrate formation is shortened in porous medium. Wang et al. [3] measured the forma-tion conditions of hydrate for four natural gases in pure water in a sapphire cell. Maekawa [4] determined the equilibrium condition experimentally for hydrate formation from binary mixtures of C2H6, C3H8 and noble gases such as Ar, Kr and Xe at the temperature of 273.5, 276.5 and 278.5 K. Ivanic et al. [5] presented a new experimental technique to assess the pressure- temperature equilibrium accurately. Maekawa [6] also determined the equilibrium condition experimentally for hydrates formed from methane and 1-propanol or 2-propanol aqueous solutions with different concen-

trations at temperatures of 274.0 287.1K and pres-sures up to 11.0 MPa. Mei et al. [7, 8] designed and made an apparatus for measuring the condition for gas hydrate formation/dissociation. They studied the hy-drate formation for a synthetic natural gas mixture in pure water and in aqueous solutions containing elec-trolyte(s), methanol, and a combination of electrolyte and methanol, and found that the strength of electro-lytes is affected by the addition of methanol. Moham-madi et al. [9] reported experimental dissociation data for methane, ethane, propane and carbon dioxide hy-drates in the aqueous solutions containing NaCl, KCl and CaCl2.

Although many experimental data [3 14] have been reported for gas hydrates containing different gas compositions in the presence of electrolytes, the in-formation on the formation of gas hydrates from components of natural gas in the porous medium with NaCl electrolytes is limited, which is useful for inves-tigating natural methane hydrates distributed in porous sediment. In this work, the equilibrium condition of gas hydrates in the porous media with different pore size and different NaCl concentration are investigated experimentally. The gas hydrates are formed from methane or the mixture of CH4, C2H6, and C3H8. The purpose of this study is to provide important data for understanding thermodynamic characters of gas hydrates.

2 EXPERIMENTAL

2.1 Experimental apparatus

Figure 1 shows a gas hydrate test device set up

Received 2009-03-26, accepted 2009-08-23. * Supported by the Key Program of National Natural Science Foundation of China (50736001), the National High Technology

Research and Development Program of China (2006AA09A209-5) and the Major State Basic Research Development Program of China (2009CB219507).

** To whom correspondence should be addressed. E-mail: powere@dlut.edu.cn

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for measuring the equilibrium of gas hydrate. It is used to simulate permafrost geological conditions, measure and record the temperature, pressure, and mass flux of gas. A high-pressure resistant vessel made of 316-stainless steel with a volume of 476 ml is used as the reactor. Five thermocouples and two pres-sure transducers, produced by Nagano, Japan Co., Ltd., are connected to the vessel. The estimated errors of temperature and pressure measurements are ±0.1K and ±0.1MPa, respectively. The detailed test has showed that the device is stable.

In order to obtain a stable low-temperature envi-ronment, the device, except the PC, is arranged in a low-temperature laboratory, provided and built by Da-lian Sanyo Refrigeration Co., Ltd. A large-scale and high-precision thermostat bath filled with glycol-water is used to control the temperature precisely. The hy-drate vessel is immersed in the bath, in which the temperature is regulated and controlled by a heater and a refrigerator. Temperature and pressure signals from thermocouples and pressure sensors are collected by A/D module (Advantech CO., Ltd.) and sent to the PC, and the data are processed by Monitor and Con-trol Generated System.

2.2 Materials and procedure

The glass beads used in the study were produced by As-One Co., Ltd., Japan. Methane (purity: 99.9%), ethane (purity: 99.5%) and propane (purity: 99.5%) were provided by Dalian Guangming Special Gas Co., Ltd.

Dry glass beads were packed into the vessel tightly with pure water or electrolytic solution with certain concentration of NaCl. The vessel was kept vertically with both side valves opening to discharge the solution until there was no liquid droplet. Three kinds of sediment with different pore sizes were ob-tained by changing the size of the glass beads. Be-cause these glass beads were made of the same mate-rial and spherical (as shown in Table 1), the effects of the surface structure and mineral composition on the phase equilibrium of methane hydrate were ignored. After the vessel was connected to the system, N2 was

injected into the vessel to make sure there was no leakage. A vacuum pump was used to discharge the gas in the vessel. The hydrocarbon gases were then injected slowly into the vessel with a pressure higher than the equilibrium pressure at the working tempera-ture. The amounts of injected solution and gas were recorded. The vessel was then closed and kept at a steady environmental temperature, which must be high enough to prevent hydrate formation in 24 hours. The temperature of the bath was set to a certain value and the module was connected to the power for pre-heating, so as to prevent any possible leakage in the system and make the gases dissolved sufficiently in water. After that, the temperature of the bath was de-creased to the working temperature, usually lower than the predicted temperature, and the data acquisi-tion unit began to work. Gas hydrates began to form in the vessel some time later, and the formation of gas hydrate was considered to be finished when the pres-sure did not change in the vessel. As shown in Fig. 2, the temperature has a sudden increase in 25 min due to the formation of hydrate. If the hydrate does not form for a long time, the temperature may be lower than the ice point because ice crystals provide nucleation sites for the subsequent hydrate formation. In the experi-ment, the formation of gas hydrate is quick and the temperature presents two sudden increases, indicating two steps for the formation of the gas hydrate. The decrease in the pressure during hydrate formation is

Figure 1 Scheme of a gas hydrate test device 1 distilled water; 2 constant flow pump; 3 needle valve; 4 high pressure cell; 5 glass beads; 6 glycol-water bath; 7 relief valve;8 thermocouple; 9 pressure sensor; 10 A/D module

Figure 2 Change of P and T with time temperature; pressure

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more than 1.4 MPa in Fig. 2. After the pressure is kept constant for 100 min, the temperature of the bath is increased and the hydrate begins to decompose. The temperature is constant from 900 to 1050 min, and the heat in the system is absorbed by the hydrate for its decomposition. The intersection of P-T curve is the equi-librium condition of the gas hydrate, as shown in Fig. 3.

Figure 3 A typical P-T curve

3 RESULTS AND DISCUSSION

The equilibrium conditions for the gas hydrate formed from the mixture of methane, ethane, and propane in the porous medium with different pore sizes and solutions are summarized as follows.

The equilibrium curve of hydrate at different pore size shows the effects of glass bead size on the formation and dissociation of gas hydrate in Fig. 4. The decrease in the pore size increases the equilibrium pressure when the salinity and the temperature are kept constant. At small pore size, the condition for the formation of gas hydrates corresponds to low tem-perature and high pressure, mainly due to an addi-tional resistance effect of capillary surface tension that leads to lower water activity and affects the hydrate equilibrium [15]. Therefore, it is necessary to account for the additional forces that result from interactions with the media, mainly the capillary forces. This, in turn, causes a depression of the freezing point of water in the pore [16].

Figure 4 shows that the equilibrium curves of hydrate in glass beads BZ-04 and bulk hydrate are nearly identical, so that the pore size among glass beads BZ-04 affects hardly on hydrate equilibrium.

This phenomenon is explained by Turner et al [17].Since the size of a stable nucleating crystal is de-pendent upon temperature and a low temperature sta-bilizes small crystals, a nucleating crystal cannot form in small pores unless the temperature is low enough for the crystal to be smaller than the pore size. The equilibrium condition for hydrate in porous media can be calculated when the pore size is given, and the suppressed temperature caused by porous media will decrease with the increase of pore size. When the pore radii are large enough, the suppressed temperature becomes very small. Turner et al. [17] reported that any shift in pores larger than 60 nm in radius cannot be distinguished from the errors of thermocouples in their equilibrium apparatus.

Seo et al. [18] found from NMR (nuclear mag-netic resonance) spectroscopy that the structure of CH4 hydrate in silica gel pores with diameter of 6.0 nm is identical with that of bulk CH4 hydrate. Since the interfacial tension between vapor CH4 and liquid water is greater than that between hydrate and water, hydrate particles in pores contact with continuous wa-ter phase and methane gas preferentially fills the lar-ger pores to form bulk phase. Therefore, the effect of capillary force on the chemical potential of methane at equilibrium can be ignored.

It is not unusual that process water in pipelines contains electrolytes, which inhibit the formation of hydrates. The information of thermodynamic stability of hydrates is important for the flow system containing

Table 1 Property and supplier of materials

Material Purity or composition Glass beads size/mm Supplier

methane 99.9% Dalian Guangming Special Gas Co., Ltd., China

ethane 99.5% Dalian Guangming Special Gas Co., Ltd., China

propane 99.5% Dalian Guangming Special Gas Co., Ltd., China

NaCl 99.0% Shenyang Xinxing Reagent Factory, China

BZ-01 soda glass 0.105 0.125 As-One Co., Ltd., Japan

BZ-02 soda glass 0.177 0.250 As-One Co., Ltd., Japan

BZ-04 soda glass 0.350 0.500 As-One Co., Ltd., Japan

Figure 4 Equilibrium of methane hydrate for different glass beads

BZ-01 glass beads with pure water; BZ-02 glass beads withpure water; BZ-04 glass beads with pure water; date of Deaton, W.M.

Chin. J. Chem. Eng., Vol. 18, No. 2, April 2010 295

electrolyte and methanol, so there is a strong interest in measuring the phase equilibrium data of hydrate [19]. The effects of concentration of NaCl on the methane hydrate formation and decomposition are investigated in this work. NaCl is added into distilled water to make the solution. Fig. 5 shows the equilib-rium curves of methane hydrate at different concentra-tion of NaCl with glass beads BZ-01 and BZ-02. For a comparison, the experimental result of methane hy-drate equilibrium without glass beads, obtained by Deaton et al. [1] is also shown in Fig. 5. For the same glass sand, in the presence of NaCl in the solution, the equilibrium curve shifts to the left. The increase of salt concentration also increases the shift of equilibrium curve. This is mainly due to the “ion effect” caused by salt ionization in water. The “ion effect” destroys the ionization balance and changes the balance of “hydra-tion ion constant”. As a result, the temperature for the methane hydrate formation is reduced. On the other hand, the water activity coefficient deviates from unity in the presence of electrolytes or alcohol. Therefore, the methane hydrate stability is decreased [20 23].

Figure 5 Equilibrium of methane hydrate in BZ-01 and BZ-02

BZ-01 with pure water; BZ-01 with 0.2 mol·L 1 NaCl; BZ-01 with 0.4 mol·L 1 NaCl; BZ-02 with pure water;

BZ-02 with 0.2 mol·L 1 NaCl; BZ-02 with 0.4 mol·L 1

NaCl; + date from Deaton, W.M.

The effects of gas composition (see Table 2) on the hydrate equilibrium are shown in Fig. 6. With the experiment of formation and dissociation of gas hy-drate in the simulated sediment, the phase equilibrium of gas hydrate composed of methane, ethane and pro-pane is studied. Lower methane concentration leads to a phase equilibrium at lower pressure and higher tem-perature. The solid line in Fig. 6 is an equilibrium

curve of pure methane in BZ-01 bed filled with pure water, which is far away from other data. It can be concluded that ethane and propane have great influ-ence on the phase equilibrium of methane hydrate. Thus the addition of a small amount of hydrocarbon with larger molecular weight (e.g., propane) in meth-ane gas has a dramatic effect on the hydrate formation pressure. This has been reported in literature. The data obtained by Deaton and Frost [1] indicates that, at 280.4 K, the hydrate forms from water and pure CH4at 5.35 MPa, while from the mixture of 99% CH4 and 1% C3H8 at 3.12 MPa. Sloan has reported that the dramatic decrease in the hydrate pressure with the addition of a small amount of propane in the methane gas is caused by the structure change, from s I to s II [All common natural gas hydrates belong to the three crystal structures, cubic structure I (s I), cubic struc-ture II (s II), or hexagonal structure H (s H)] [1].

4 CONCLUSIONS

The experimental result indicates that the induc-tion time of gas hydrate formation in porous media is shorter than that in pure water. The decrease in the pore size, by decreasing glass bead size, increases the equilibrium pressure when the salinity and tempera-ture are kept the constant, that is, lower temperature and higher pressure are required for the formation of methane hydrate in the glass bead bed. The increase of salinity also increases the equilibrium pressure when the pore size and temperature are kept constant. With the experiment of formation and dissociation of gas hydrate in the simulated sediment, the phase equilib-rium of gas hydrate composed of methane, ethane and propane are obtained. The equilibrium at lower meth-ane concentration needs a lower pressure and a higher temperature. The experimental data are in good agreement with the data in literature.

REFERENCES

1 Sloan, E.D., Clathrate Hydrates of Natural Gases, 2nd edition, Marcel

Figure 6 Effects of gas composition on hydrate equilib-rium

01# gas in BZ-01; 02# gas in BZ-01; 03# gas in BZ-01; 04# gas in BZ-01; 05# gas in BZ-01; pure methane in

BZ-01

Table 2 Gas composition

Composition Gas

Methane Ethane Propane

01# 0.868 0.060 0.072

02# 0.901 0.046 0.053

03# 0.937 0.034 0.029

04# 0.948 0.024 0.028

05# 0.958 0.021 0.021

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