Elsevier

Chemical Engineering Journal

Volume 358, 15 February 2019, Pages 598-605
Chemical Engineering Journal

Thermodynamic and kinetic influences of NaCl on HFC-125a hydrates and their significance in gas hydrate-based desalination

https://doi.org/10.1016/j.cej.2018.10.032Get rights and content

Highlights

  • Thermodynamic and kinetic influences of NaCl on HFC-125a hydrate were investigated.

  • NaCl enrichment in the unconverted solution resulted in a lower conversion.

  • The presence of NaCl had little effect on the ΔH of HFC-125a hydrate.

  • The hydrate dissociation was retarded due to the formation of NaCl⋅2H2O.

Abstract

In this study, HFC-125a was selected as a hydrate-forming guest for gas hydrate-based desalination. The thermodynamic and kinetic effects of NaCl on HFC-125a hydrates were investigated with a primary focus on phase equilibria, gas uptake, dissociation enthalpy, and dissociation behavior. The equilibrium curve of HFC-125a hydrate shifted to higher pressure regions at any given temperature depending on the concentration of NaCl. The presence of NaCl also reduced the gas uptake and conversion to hydrates, because of the enrichment of NaCl in the solution during gas-hydrate formation. Even though NaCl did not affect the dissociation enthalpy of the HFC-125a hydrate, the thermograms obtained using a high-pressure micro-differential scanning calorimeter (HP μ-DSC) demonstrated that HFC-125a + NaCl hydrates started to dissociate at lower temperatures due to NaCl in unconverted solutions. Rietveld refinement of powder X-ray diffraction (PXRD) patterns indicated that the HFC-125a hydrate (sII) was transformed into Ih as it dissociated. The dissociation of HFC-125a + NaCl hydrates was retarded and completely ended at higher temperatures compared to the pure HFC-125a hydrate by the sodium chloride dihydrate (NaCl⋅2H2O). Overall, these results could facilitate a better understanding of HFC-125a hydrates in the presence of NaCl; further, they might also be useful in the design and operation of hydrate-based desalination plants using HFC-125a.

Introduction

When water molecules come into contact with pressurized gas at low temperatures, they can be crystallized into gas hydrates, which can trap gas molecules, called “guests”, in their hydrogen-bonded water frameworks, called “hosts” [1]. These non-stoichiometric crystalline compounds, called gas hydrates, have been actively studied in various science and engineering fields. Gas hydrates exist in the form of three different structures (structure I (sI), structure II (sII), and structure H (sH)) in nature and each structure consists of differently sized and shaped cages stabilized by van der Waals forces [2]. Although there are many factors that influence the structure of gas hydrates, the most crucial element to determine the structure is the size of the guest molecules. sI hydrates consists of 2 small (512) and 6 large (51262) cages in a unit cell and can capture only small molecules, such as methane (CH4) and carbon dioxide (CO2), whereas sII hydrates, composed of 16 small (512) and 8 large (51264) cages in a unit cell, can entrap larger molecules, such as propane, i-butane, hydrofluorocarbon (HFC) gases, and chlorofluorocarbon (CFC) gases. Much larger liquid hydrocarbons, such as methylcyclopentane and neohexane form sH hydrates, composed of 3 small (512), 2 medium (435663), and 1 large (51268) cages in a unit cell, in the presence of help gases for stabilization of crystal structure [1], [2], [3], [4], [5].

In the past, gas hydrates were considered as a nuisance in the oil and gas industry as they can undergo agglomeration into solid gas hydrates, leading to serious problems of flowline rupture and explosion during oil and gas production and transportation [6]. Therefore, thermodynamic and kinetic hydrate inhibitors have been actively investigated to avoid pipeline plugging caused by gas hydrate formation [7], [8], [9]. However, naturally occurring gas hydrates deposited in the deep-ocean sediments of continental margins or in permafrost regions are regarded as future energy sources because their total carbon content is estimated to be twice as high as the amount found in conventional fossil fuels, including coal, petroleum, and natural gas [10]. Recently, gas hydrates have also been used for CO2 capture and storage [11], [12], [13], [14], [15], [16], [17], [18]. In particular, the storage of CO2 in natural gas hydrates linked with natural gas production has been studied for both energy and environmental purposes [19], [20], [21], [22], [23], [24]. In contrast to previously used simple production methods, such as depressurization and thermal stimulation [25], [26], the CH4-CO2 replacement method has several significant advantages over conventional dissociation-based methods. Firstly, it does not induce the geological hazards associated with seafloor subsidence, because CH4 production and CO2 hydrate formation take place simultaneously in the sediment. Further, it works double duty, sequestering CO2 for global warming mitigation and exploiting CH4 for clean energy production [27], [28]. On the other hand, various greenhouse gases, including HFCs and SF6, can be subjected to gas hydrate-based separation, because these gases can be selectively captured in hydrate cages due to their lower hydrate equilibrium pressure at any given temperature [29], [30], [31], [32], [33].

Gas hydrate-based desalination is a viable technology in water treatment and purification, because salts and other impurities are excluded during hydrate formation [34], [35], [36]. Hydrate-based desalination is not significantly affected by the salt concentrations of the target solution because it is based on the phase transition between liquid and solid phases [37]. Therefore, it can be applied to the desalination or purification of not only sea water but also highly concentrated waste water and produced water. Owing to its capacity to remove highly concentrated salts, hydrate-based desalination can potentially overcome the limitations of conventional desalination methods, such as thermal distillation and reverse osmosis [38], [39]. Appropriate hydrate-forming substances, which can be readily incorporated in hydrate cages at milder pressure and temperature conditions and have lower solubility in water, should be selected for hydrate-based desalination for this technology to become more competitive [40]. Several possible guests, such as cyclopentane, SF6, and some HFCs have been studied for hydrate-based desalination with respect to their thermodynamic and kinetic aspects [34], [41], [42], [43]. In this study, HFC-125a (pentafluoroethane) was selected as a promising hydrate guest for desalination, because HFC-125a has a very low solubility in water and forms gas hydrates at low pressures. However, the hydrate phase equilibria, formation and dissociation behavior, and thermal properties of HFC-125a in the presence of salts are not very well known.

In this study, the hydrate phase equilibria of HFC-125a + NaCl (0, 3.5, and 8.0 wt%) + water mixtures were evaluated to verify the influence of NaCl on the thermodynamic stability of HFC-125a hydrates. In addition, the gas uptake and conversion of water to gas hydrates were investigated to elucidate the effect of NaCl on the hydrate-formation behavior. The dissociation enthalpy of HFC-125a hydrates in the presence of NaCl was measured using a high-pressure micro-differential scanning calorimeter (HP μ-DSC). Structural identification and quantitative analysis of HFC-125a + NaCl hydrates were carried out by powder X-ray diffraction (PXRD) analysis. In particular, the dissociation behavior of HFC-125a hydrates in the presence of NaCl was observed by collecting temperature-dependent PXRD patterns at atmospheric pressure.

Section snippets

Materials

HFC-125a (pentafluoroethane) gas (purity of 99.9%) was supplied by RIGAS Co. (Republic of Korea). NaCl (purity of 99.5%) was purchased from Sigma-Aldrich Co. (USA). Double-distilled and deionized water was used for gas-hydrate formation. All the materials were used as received without further purification.

Phase equilibria

A specially designed high-pressure equilibrium cell with an internal volume of 250 cm3 was used to measure the three-phase (hydrate (H)-liquid water (LW)-vapor (V)) equilibria of

Phase equilibria of HFC 125a hydrates in the presence of NaCl

The thermodynamic stability of HFC-125a hydrates in the presence of NaCl is fundamental to understanding this process and setting up the operating conditions for a HFC-125a hydrate-based desalination process. As shown in Fig. 2 and Table 1, the three-phase equilibria (H-LW-V) of HFC-125a + NaCl (0, 3.5, and 8.0 wt%) hydrates were measured using a conventional isochoric (pVT) method to examine the thermodynamic influence of NaCl on HFC-125a hydrates. The phase equilibrium data of pure HFC-125a

Conclusions

The thermodynamic stability, gas uptake, dissociation enthalpy, structural identification, and dissociation behavior of HFC-125a + NaCl (0, 3.5, and 8.0 wt%) hydrates were comprehensively evaluated for their potential application in hydrate-based desalination processes. The three-phase (H-LW-V) equilibria of HFC-125a hydrates in the presence of NaCl (0, 3.5, and 8.0 wt%) measured by a conventional isochoric method indicated that the thermodynamic stability of HFC-125a hydrates is significantly

Acknowledgements

This research was supported by the Mid-career Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Science and ICT (2017R1A2B4005455) and also by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) through “Human Resources Program in Energy Technology” (No. 20164030201010) funded by the Ministry of Trade, Industry and Energy, Republic of Korea.

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