Experimental study on gas hydrate formation from natural gas mixture

https://doi.org/10.1016/j.jngse.2014.01.016Get rights and content

Highlights

  • Isochoric and isothermal experiments were conducted on the mixture of natural gas.

  • Rate of natural-gas consumption by hydrate is determined based on experiments.

  • It is shown that the rate of hydrate formation is exponentially proportional to pressure driving force.

Abstract

In this work, gas consumption by clathrate hydrate from mixture of natural gas at different pressures and temperatures (275.15–287.25 K and 1.1–5.4 MPa) are reported. The experiments were conducted in a stirred-reactor tank in isothermal and isochoric conditions. The amount of consumed gas by the hydrate phase was determined considering the pressure reduction during the experiments. Results show the rate of gas consumption is exponentially related to the over-pressurizing of the system.

Introduction

Gas hydrates are ice-like compounds, which may form when water and light gases such as methane and ethane are present together at low temperature and relatively high pressures (Skovborg and Rasmussen, 1993). Although hydrate formation has some negative connotations in the petroleum and gas processing industry, but it has the potential for many applications. Carbon dioxide capture and sequestration, gas storage, air-conditioning systems, water desalination and treatment technology, concentration of dilute aqueous solutions, and separation of different gases from flue gas streams are some applications of this phenomena (Eslamimanesh et al., 2012, Chatti et al., 2005, Makogon, 2010).

There has been major progress in thermodynamic modeling of gas hydrate since 1934 (Hammerschmidt, 1934) but the most challenging and questions about hydrate concerns how hydrates from, dissociate, and inhibit with time – kinetics (Sloan and Koh, 2008). The strong influence of heat and mass transfer on the hydrate formation makes the hydrate kinetics to be difficult to predict.

Several studies on the hydrate formation from single and two components can be found in the literature (Vysniauskas and Bishnoi, 1983, Vysniauskas and Bishnoi, 1985, Englezos et al., 1987a, Englezos et al., 1987b, Clarke and Bishnoi, 2001, Bergeron and Servio, 2008, Bergeron and Servio, 2009, Verrett and Servio, 2012) but there is few study about multi-component mixture (Mork, 2002). In this work, we used a typical mixture of natural-gas (methane, ethane, propane and iso-butane) that are guest molecules of gas hydrates. Several experiments were conducted at different temperatures and pressures in a stirred tank reactor at constant volume to determine the rate of gas consumption. We proposed a relation between the rate of hydrate formation and over-pressurizing. Results show that gas consumption by the hydrate phase is exponentially proportional to over-pressurizing (super-saturation) of the mixture.

Section snippets

Materials

Table 1 reports the material purities and suppliers. Highly purified water was prepared from Ramin Power Plant (Ahwaz).

Experimental apparatus

The detailed of the apparatus is shown in Fig. 1. The unit consists of a visual double-wall stirred cell made of SS316. A 4 cm coated magnetic bar is located in the cell and is driven by an external magnetic stirrer (Labinco L-71). The stirrer was calibrated using Smart Sensor Digital Tachometer (AR926). The heat is removed from the cell by a bath circulator (Lauda R8A) that

Rate of gas consumption

The experimental study is based on the determination of the amount of gas phase with time. Thus, any change in the moles of the gas phase is considered as the consumption by the hydrate phase. The cell temperature is kept constant and the pressure is recording. It is assumed that the gas phase volume has no change during each run. Therefore, the total moles of the gas phase can be calculated if the composition of vapor is available. SRK EOS as an appropriate equation of state was used to

Results and discussion

12 experiments were completed at four temperature levels (275.15–287.25 K). Table 5 shows the experiments conditions, including cell temperature (T), pressures (P) and compositions of the gas phase at initial and final states. Structure II is predicted due to the presence of propane and iso-butane in the gas phase.

Fig. 2, Fig. 3, Fig. 4, Fig. 5 show the consumption curves. Each figure belongs to an operating temperature. As it was predictable, higher pressure greater consumption rate. Based on

Conclusion

In this work, we provided new experimental data on hydrate formation from the natural gas mixture in a batch stirred tank reactor. The mixture consists of methane, ethane, propane and iso-butane mixture as they contribute during hydrate formation. Temperature and pressure were changed from 275.15 K to 287.25 K and 1.1 MPa to 5.4 MPa respectively. As all experiments were carried under isothermal conditions, we used the difference between the operating pressure and equilibrium pressure as the

Acknowledgment

Authors would like to thank all members of the Petroleum University of Technology Research Center for their technical cooperation.

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