Effects of cyclopentane on CO2 hydrate formation and dissociation as a co-guest molecule for desalination

https://doi.org/10.1016/j.jct.2016.09.006Get rights and content

Highlights

  • CP decreases CO2 hydrate phase equilibrium pressure by forming CO2-CP hydrates.

  • The increase of CP can’t decrease hydrates phase equilibrium pressure unlimitedly.

  • Higher CP concentration lowers CO2 hydrate gas uptake.

  • The optimal CP molar ratio is 0.01 based on hydrate phase equilibrium and gas uptake.

Abstract

Cyclopentane (CP) is considered to be a potential co-guest molecule in carbon dioxide (CO2) hydrate-based desalination. The experimental thermodynamic data of CO2-CP hydrates were measured for a salt solution, where CP was chosen as a hydrate promoter. Seven experimental cases (62 cycles) were studied with different molar ratios of CP/water (0, 0.0025, 0.005, 0.0075, 0.01, 0.02, and 0.03). Hydrate phase equilibrium data were generated using an isochoric method, and the hydrate saturations were calculated based on gas uptake. The results indicated that the increase in CP concentration significantly decreased the CO2 hydrate equilibrium pressure to a certain limit; the hydrate saturation also decreased during this process. Also, it was determined that CP encouraged the formation of s-II double CO2-CP hydrates, which are different from s-I simple CO2 hydrate. The CO2-CP guest provides a strengthened stability and moderate hydrate phase equilibrium conditions for hydrate-based desalination. The recommended optimal molar ratio of CP is 0.01 when the increase in equilibrium was more than 10 K, and the decrease in hydrate saturation was less than 2%.

Introduction

Clathrate hydrates are solid compounds in which water molecules are linked through hydrogen bonding and create cavities to enclose a large variety of guest molecules [1]. CO2 hydrate is known to form a structure I (s-I) hydrate, which is composed of two pentagonal dodecahedra (512) and six tetrakaidecahedra (51262) cages. Because the concentration of CO2 has gradually increased and reached a high level in today’s atmosphere, which has led to significant climate change and is responsible for approximately 64% of the enhanced “greenhouse effect”, the disposal of CO2 has become an issue of worldwide concern [2], [3]. One of the proposed schemes of mitigating CO2 emission is to sequester it as gas hydrates in ocean and marine sediments [4], [5], [6]. The principle of capturing CO2 is that CO2 molecules are enclathrated into hydrate cages formed by water molecules [7], [8]. Thus, many studies of CO2 hydrate have been performed regarding the separation of a CO2-containing gas mixture [9], [10]. The hydrate equilibrium conditions for CO2 capture from a flue or fuel as CO2 hydrate have been widely investigated [11], [12], [13]. There are also studies of the CO2 hydrate phase equilibrium in water with different salts [14], [15], [16], [17], which provide basic information for the development of hydrate-based desalination.

The guest component of a hydrate is critical for hydrate-based technologies and also affects the efficiency, safety and energy consumption of hydrate-based desalination technology. Additives are often used to mitigate the hydrate formation pressure and improve the hydrate formation rate and gas capacity [18], [19], [20], [21]. The most common hydrate formation promoters are tetrahydrofuran (THF) [19], tetra-butyl ammonium bromide (TBAB) [22], and sodium dodecyl sulphate (SDS); however, their solubilities in water cannot be ignored particularly for hydrate-based desalination [23], [24], [25]. CP is another good promoter and is similarly immiscible with water. Additionally, its promotion effect on hydrates was reported to be larger than that of THF [26]. Corak et al. suggested that the formation of a simple CP hydrate could be used for seawater desalination [27]. Cha et al. [28] also investigated CP hydrates that were formed with co-guest CO2 molecules at elevated temperatures for the desalination of water with high salinity. However, the equilibrium data of CO2 hydrate in the presence of CP in a saline solution are limited [29], [30], [31], [32]. Recently, Ngema et al. [33] conducted experimental measurements and thermodynamic modelling of the dissociation condition of CP-CO2 hydrates in different saline solution. It provided some new methods and data for this topic, and still need to be clarified furtherly. With more favourable hydrate formation conditions, CP hydrate has been considered to be a potential media for H2 storage and CO2 separation from pre- and post-combustion gases [34]. Additionally, Li et al. [35] combined CP and TBAB as a mixed promoter to separate CO2 from IGCC fuel gas to increase the gas uptake and separation efficiency. To date, certain studies of the effects of CP hydrate promoters have also been conducted [36], [37], [38], [39]; however, useful data of CP-CO2 hydrate phase equilibrium are still limited.

CP is a potential co-guest for CO2 hydrate-based desalination. The thermodynamic characteristics of CP-CO2 hydrate formation and the associated dissociation process must be further clarified, particularly with regard to the hydrate phase equilibrium conditions and saturation. In this study, the phase equilibrium conditions for CO2 hydrate with CP as the co-guest molecule in porous media were investigated experimentally. The hydrate saturation is calculated based on CO2 gas uptake to analyse the cost and efficiency of the hydrate formation process. The data obtained in this hydrate investigation is significant for future research in this field of study.

Section snippets

Experimental apparatus and materials

The experimental apparatus used in this study is shown in Fig. 1, and detailed illustrations are provided in prior publications [40], [41]. A high-pressure-resistant vessel made of 316 stainless steel with a volume of 750 cm3 was used as the hydrate formation and dissociation reactor. Six thermocouples with an accuracy of 0.1 K in a hexagonal distribution and one pressure transducer with an accuracy of ±25 kPa were connected to the vessel. The materials used in these experiments are listed in

Results and discussion

CO2 hydrate formation and dissociation in porous media and salt solutions in the presence of liquid CP were experimentally investigated. Seven experimental cases (62 cycles) were conducted initially at 0.6–4.5 MPa with CP molar ratios ranging from 0 to 0.03. To assess the uncertainties in these experiments, we conducted three repeated measurements of the hydrate phase equilibrium, and each point was repeated twice. All hydrate phase equilibrium data and the primary measurement uncertainties are

Conclusions

The characteristics of CO2 hydrate formation with CP in porous media were investigated in the presence of a salt solution using a high-pressure-resistant vessel. Seven experimental cases (62 cycles) with different CP molar ratios, ranging from 0 to 0.03, were investigated. Along with the presence and increase in CP, the hydrate phase equilibrium pressure was found to decrease markedly until the ultimate conditions as the hydrate saturation decreased; this resulted from the transition of the

Acknowledgments

This study was financially supported by grants from the National Natural Science Foundation of China (51576025 and 51436003), the High-Tech Research and Development Program of China (2013AA09250302), the Program for Liaoning Excellent Talents in University (LJQ2014007) and the Fundamental Research Funds for the Central Universities of China.

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