Elsevier

Applied Energy

Volume 207, 1 December 2017, Pages 573-583
Applied Energy

Effect of guest gas on the mixed tetrahydrofuran hydrate kinetics in a quiescent system

https://doi.org/10.1016/j.apenergy.2017.06.101Get rights and content

Highlights

  • Guest gas significantly influences the mixed tetrahydrofuran hydrate formation kinetics.

  • Morphology of CO2 hydrates in different hydrate structure domains were reported.

  • Despite the higher solubility of CO2, gas uptake observed was lower than CH4.

  • Calorimetry result presents the non-coexistence of sI and sII hydrates.

Abstract

Clathrate hydrates are ‘inclusion compounds’ that have the ability to encompass multifold volumes of guest gas molecules, thus being advantageous for gas storage and gas separation applications. CO2 capture in the form of hydrates is an environmentally benign and cost-effective approach. In this work, we examine the kinetics of CO2 hydrate formation at different operating conditions that result in the formation of pure sI hydrate, pure sII hydrates and/or a mixture of sI and sII hydrates. Morphology observations of different hydrates formed are presented with the associated CO2 uptake achieved under different experimental conditions. We report strikingly contrasting morphology of mixed CO2 and mixed CH4 hydrates observed in presence of stoichiometric THF (5.6 mol%) under similar pressure diving force and operating conditions. Interesting results observed during mixed CO2 hydrates using Differential Scanning Calorimetry (DSC) are documented. Based on DSC thermograms, we report interesting observations on the effect of guest gas in the mixed THF hydrate formation and dissociation. Moreover, mixed CH4/THF hydrates were found to be more stable in comparison to mixed CO2/THF hydrates. This work highlights that the choice of guest gas plays a significant role in the associated hydrate formation kinetics in presence of THF.

Introduction

Clathrate hydrates are inclusion compounds that encompass guest molecules in host water cages. Guest molecules can be predominantly gases like methane, ethane, carbon dioxide etc., or even organic compounds like acetone, tetrahydrofuran, cyclopentane etc. Hydrates are ice-like, crystalline and non-stoichiometric compounds wherein the guest molecules are held intact only by weak Van der Waals force [1]. Due to these peculiar characteristics and other significant advantages offered including high volumetric gas storage capacity, environmental friendly nature and moderate operating conditions for formation, hydrates find applications in many areas including energy storage including methane (natural gas) [2], [3], [4], [5] and hydrogen storage [6], [7], [8], desalination [9], [10], cold storage [11], [12], carbon capture and sequestration (CCS) [13], [14], [15].

CO2 in presence of water without any additive forms a standard sI type hydrate structure. Phase equilibrium data of CO2 hydrates has been well documented in the literature [16], [17], [18], [19]. Investigation of kinetics of CO2 hydrate formation in presence of different additives has been an active research area in the last decade due the promise it offers for CO2 capture applications. Additives can be predominantly classified into kinetic and thermodynamic promoters. Thermodynamic promoters are additives that participate in hydrate formation thereby shift/alter the pure hydrate phase equilibrium curve resulting in more moderate conditions during hydrate formation (lower pressure and higher temperature than that of pure CO2 hydrates). Commonly studied thermodynamic additives for CO2 hydrate formation include tetrahydrofuran (THF) [20], [21], cyclopentane [22], [23], [24] and tetra alkyl ammonium salts [25], [26]. Recently, Lee et al. [27] evaluated the use of neohexane, a sH hydrate forming promoter for CO2 capture and sequestration. However, the inherent disadvantage is that these thermodynamic promoters preferentially occupy and stabilize the large sized cages of hydrate structure thereby compromising on the guest gas storage capacity. Kinetic promoters (commonly ‘surfactants’) on the other hand, alter the gas/liquid interfacial properties due to which increased hydrate formation rates are achieved. These kinetic promoters have no effect on the phase equilibrium curve, thus the addition of kinetic promoters does not result in any change in operating conditions of hydrate formation. Kumar et al. [28] studied the effect of different type of surfactants (including cationic, anionic and non-ionic) on the CO2 hydrate formation kinetics. Mohammadi et al. [29] reported a synergism between sodium dodecyl surfactant and silver nanoparticles during CO2 hydrate formation resulting in improved kinetics and increased CO2 storage capacity. Apart from the application of thermodynamic and kinetic promoters, enhancement in hydrate formation kinetics could also be achieved by the employment of a fixed bed reactor that enables a higher surface area of contact between gas phase and aqueous phase. Many porous fixed bed supports like silica sand [30], [31], silica gel [28], [31], polyurethane foam [32], SS-316 mesh arrangements [33] and other siliceous materials like pumice and fire hardened red clay [34] have been investigated for their ability to improve the kinetics of CO2 hydrate formation.

THF is a prominent sII hydrate structure forming guest studied as a thermodynamic promoter with different guest gases like methane [2], [3], hydrogen [35], [36] and CO2 [37], [38], [39] during hydrate formation. THF is miscible with water and has the ability form hydrates without any guest gas below 4.4 °C at atmospheric pressure [35]. Torre et al. [20] investigated CO2 hydrates formation kinetics in presence of kinetic promoter (SDS) and thermodynamic promoter (THF). The concentration of THF and SDS used were 4.0 wt% (equivalent to 1.0 mol%) and 0.3 wt% respectively. Both batch type and semi-continuous mode of operation were employed and the study was performed in quiescent reactor mode (stirred till the nucleation after which the stirrer was switched off). They report enhanced kinetics with higher water to hydrate conversions with the combination of two types of additives in comparison to the usage of individual additives and pure CO2 hydrates (without any additive). Lirio et al. [21] also studied the kinetics of CO2 hydrate formation in presence of both these additives but at a higher concentration of THF(5 mol%) and lower concentration of SDS (0.05 wt%). They have studied kinetics at two different pressures of 3.0 and 5.0 MPa and two temperatures of 274.15 and 277.15 K using a stirred tank reactor configuration. Most optimal conditions for mixed hydrate formation were reported to be 3.0 MPa and 274.15 K in presence of 5.0 mol% THF and 0.05 wt% SDS. Recent study by Kim et al. [40] reports mixed CO2/THF hydrate formation at three different concentrations of 0.5, 1.0 and 1.5 mol% THF at three different pressures of 0.5, 1.5 and 2.5 MPa. All experiments were performed at 274.15 K. Optimal conditions of hydrate formation were reported to be at 1.5 MPa using 1.5 mol% THF at 274.15 K and authors envisage the advantage of using mixed hydrate slurry for a district cooling purpose. All above listed studies report experiments that were performed under conditions that resulted in the formation of mixed CO2/THF hydrates in regions of overlapping sI and sII hydrate domains. To our knowledge, there are no documented studies in literature examining mixed CO2/THF hydrate formation in only sII hydrate domain. So, we chose experimental conditions for the present study to be 3.0 MPa and 283.2 K. It is not possible to form pure sI CO2 hydrates at the chosen conditions as the equilibrium pressure of CO2 hydrate formation at 283.2 K is 6.3 MPa [17]. Table 1 summarizes the experimental conditions employed in the current study in comparison to that of experimental conditions reported in the literature. Examination of the kinetics of mixed CO2/THF hydrates is of practical relevance for several applications pertaining to gas hydrate technology for gas separations involving CO2 streams like CO2 capture from flue, fuel, land fill and bio gas streams. Further, studies by Sun et al. [41], [42] examine the formation and dissociation characteristics of CO2/THF mixed hydrates. Reduction of CO2 emission (utilizing CO2 during hydrate formation) coupled with the application of cold energy (during hydrate dissociation) suited for district-cooling application is envisaged through CO2/THF mixed hydrates [40], [41], [42]. Kim et al. [40] estimated the coefficient of performance (COP) of CO2 + THF hydrate cooling system to be 11.55.

Recently, Veluswamy et al. [3] reported enhanced methane hydrate formation kinetics in presence of THF promoter at lower pressures of 3.0 MPa and 283.2 K with high methane storage capacity in unstirred tank reactor (UTR). The objective of the current study was to evaluate if such similar enhancement of hydrate formation kinetics could be observed for CO2 hydrates in presence of THF in UTR under similar operating conditions. Experiments were performed at same temperature and pressure. Equilibrium pressure for mixed CO2/THF hydrates at 283.2 K is interpolated to be about 0.6 MPa [43], very close to the equilibrium pressure of 0.5 MPa for mixed CH4/THF hydrates at 283.2 K [44] thus ensuring similar pressure driving force for the hydrate formation for both systems. Gas uptake and kinetics achieved under different hydrate structure forming experimental conditions were documented with associated morphology observations. In the current study, we also chose experimental conditions for hydrate formation such that it is possible to form CO2 hydrates of sI structure and/or a combination of both sI and sII structures for comparison with hydrates formed solely in sII domain.

Section snippets

Materials

CO2 gas cylinder of 99.8% purity purchased from SOXAL Pte Ltd, Tetrahydrofuran (THF) of 99.7% purity obtained from Fisher Chemicals, 99% pure sodium dodecyl sulphate (SDS) from AMRESCO and deionised water obtained from Elga micromeg deionization apparatus were used in experiments.

Experimental apparatus

Experimental apparatus used is shown in Fig. 1 and is similar to that detailed in the study by Veluswamy et al. [3]. The reactor has an internal volume of 142 ml and fitted with two marine type viewing windows (at the

Gas uptake and Visual observations of hydrate formation in UTR configuration

Fig. 3 presents equilibrium curves for pure CO2 and CO2 + THF systems along with the indication of experimental conditions used for the current study. As mentioned in the introduction, we chose the experimental temperature and pressure for the current study to be 283.2 K and 3.0 MPa in order to form CO2/THF mixed hydrates only in sII domain. Thus, CO2 hydrate formation in presence of stoichiometric 5.6 mol% THF was studied at 3.0 MPa and 283.2 K. Visual observations during the hydrate formation

Conclusion

We present the visual observations during hydrate formation observed for CO2 hydrates with and without thermodynamic (THF) and kinetic (SDS) promoters. Experimental conditions were chosen such that it is possible to form pure sI, pure sII and/or a mixture of sI/sII hydrates. THF in stoichiometric concentration did not result in the improvement of kinetics/CO2 uptake unlike the synergistic effect associated with rapid hydrate formation kinetics achieved for CH4 study. DSC observations also

Acknowledgements

The work was funded in part under the Energy Innovation Research Programme (EIRP, Award No. NRF2015EWTEIRP002-002), administrated by the Energy Market Authority (EMA) and funded by the National Research Foundation (NRF) of Singapore.

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