Elsevier

Fuel

Volume 122, 15 April 2014, Pages 206-217
Fuel

Combination of surfactants and organic compounds for boosting CO2 separation from natural gas by clathrate hydrate formation

https://doi.org/10.1016/j.fuel.2014.01.025Get rights and content

Highlights

  • Combination of surfactants and organic additives enhance hydrate-based processes.

  • Action mechanism based on the successive formations of (sII) and (sI) hydrates.

  • THF + SDS is the best association of additives among all the combinations tested.

  • Enclathration rate and selectivity of the separation remain too low for scale-up.

Abstract

This study investigates the effects of several combinations of surfactants and organic compounds on the separation of CO2 from a CO2–CH4 gas mixture by clathrate hydrate formation. Seven additives, three surfactants (SDS, SDBS, DATCl) and four organic compounds (THF, 1,3-dioxolane, 2-methyl-tetrahydrofuran and cyclopentane) were tested for various operating conditions and at different concentrations. The influence of these additives on the quantity of gas removed, the selectivity of the separation toward CO2, and the kinetics of hydrate formation were analyzed through experiments in a batch reactor. It was found that a suitable combination of a surfactant and a organic compound can, in some cases, strongly enhance the hydrate crystallization. The best results were obtained with a combination of the additives SDS and THF.

Introduction

Clathrate hydrates are non-stoichiometric, ice-like crystalline solids formed by a three-dimensional network of hydrogen-bonded water molecules (called host molecules), which can encage various species (called guest molecules) such as methane, carbon dioxide, cyclopentane, and acetone [1] in their cavities. These supra-molecular structures exist only when there are guest molecules in the cages, and are stable in precise thermodynamic conditions specific to the host–guest system considered. When the guest molecule is a gas, the clathrate hydrate is called a “gas hydrate”, or simply “hydrate” for short. The three main hydrate structures are “structure one”, “structure two”, and “structure H”, usually denoted (sI), (sII), and (sH), respectively. Structures (sI) and (sII) are of particular interest to the Oil and Gas industry as they can accommodate small gas molecules present in natural gas [2]. Many other details on these structures (geometry, suitable guests, etc.) can be found elsewhere [3].

Practical applications involving hydrates have recently been proposed in sectors such as energy, transport and storage of gases, refrigeration and gas separation [4]. The use of clathrate hydrates as a gas separation technique is currently at the laboratory proof-of-concept stage. The principle of the separation is based on the following analysis: when the gas hydrate is formed from an initial gas mixture (e.g. a binary mixture A + B), the final composition of the gas enclathrated into the hydrate phase will be different from the composition of the vapor phase remaining at equilibrium with the hydrate formed. Therefore, one of the components initially present in the gas (e.g. component A) will be predominantly included in the hydrate phase. Later, the hydrates formed can still be dissociated, both to recover the enclathrated gas (in this case, a gas richer in component A) and to recycle the water. Theoretically, the process seems attractive, but in practice, several points need closer investigation before any application at industrial scale. They include the hydrate formation rate, and the quantity of the component to remove from the gas mixture (which will be enclathrated in the hydrate phase), both of which must be as high as possible. Numerous hydrate-based separations have already been studied with different gas mixtures containing carbon dioxide (CO2), methane (CH4), hydrogen sulfide (H2S), sulfur hexafluoride (SF6) and others gases [5]. This process may prove an interesting alternative to conventional separation techniques (e.g. reactive absorption using alkanolamines) [6], and economically competitive for separating greenhouse gases such as SF6 [7] or CO2 [8].

Natural gas is considered to be one of the major energy sources for the future due to its comparative abundance on Earth, its relatively low environmental footprint compared to petroleum, and the multiple applications possible in various sectors of the economy [9]. After extraction from the reservoir, the natural gas is transported to processing units for pre-treatment. In this raw state, the gas mixture is complex as it contains hydrocarbons – primarily alkanes (paraffins), predominantly methane – and a variety of other non-hydrocarbon species such as water, nitrogen, helium and acid gases [10]. Carbon dioxide is one of the major contaminants of natural gas, and has to be separated from the methane to reach the commercial specifications. Gas reservoirs can contain a variable proportion of CO2, ranging from a few percent up to 75 vol%. Examples of gas fields with a very high concentration of CO2 are the offshore Natuna gas field in Indonesia (71% of CO2) [11] or the onshore El Trapial field in Argentina (where light oil coexists with gas containing concentrations of CO2 greater than 75%) [12].

Owing to the proximity of the CO2 and CH4 [13], [14], [15] hydrate equilibrium curves, it is fairly difficult to efficiently separate CO2 from a CO2–CH4 gas mixture using a hydrate-based process. However, recent results have shown that adding certain chemicals to the water, such as surfactants, organic molecules or salts, can significantly: (i) enhance the hydrate formation rate [16], [17], (ii) modify the position of the equilibrium curves [18], [19] and/or (iii) allow the selective enclathration of one of the gases in the hydrate phase [20].

Among the numerous additives reported in literature tested for hydrate applications, many of them can be classified in two classes, named “kinetic additives” and “thermodynamic additives”: (i) kinetic additives, generally used at low concentration (ranging from about ten to few thousands of ppm), are not included in the hydrate cavities and enhance (kinetic promoters) – or decrease (kinetic inhibitors) – the hydrate formation rate without any effect on the equilibrium conditions; (ii) “thermodynamic additives” modify the hydrate equilibrium conditions compared to the same system without additive, due to their presence into the cages of the hydrate structure or their ability to modify the activity of water. Similarly to kinetic ones, they are either promoters or inhibitors.

Interestingly, it is apparent that: (i) several surfactants are able to enhance the hydrate formation kinetics at very low dosage (such as a few hundred of ppm in water) [21], [22]; and (ii) several organic molecules, particularly those which have a 5-membered cyclic structure derived from the cyclopentane, have also been reported to act as efficient thermodynamic promoters [23], [24]. However, very few works address combinations of additives. Even the two most commonly cited hydrate promoters, sodium dodecyl sulfate (SDS) – a well-known anionic surfactant – and tetrahydrofuran (THF) – a cyclic ether – have rarely been used together. The authors have recently shown that a combination of these two additives can work very well in the presence of pure carbon dioxide [25] and with a CO2 + CH4 gas mixture [26].

This paper presents hydrate formation studies performed using various combinations of two types of additives: kinetic and thermodynamic. The objective is to analyze the effect of these combinations on the enclathration kinetics, process capacity and efficiency of the CO2 separation from a CO2–CH4 gas mixture. Three surfactants (anionic and cationic) and four organic compounds (water-soluble or not) were tested at different concentrations, in various process operating conditions.

Section snippets

Parameters for process performance

Five parameters were defined for this study to analyze, compare the results, and quantify the process performance:

  • The quantity of gas removed is the total mol number of gas removed from the gas phase, the reference being the total mol number of gas initially loaded in the reactor. This quantity can be calculated with the following equation:

CO2r+CH4r=CO2init+CH4init-(CO2hyd+CH4hyd+CO2aq+CH4aq)where superscripts r, init, hyd and aq correspond to the gas removed from the gas phase, the gas

Additives and materials used

Seven hydrate promoters were chosen: two anionic and one cationic surfactants: sodium dodecyl sulfate (SDS), sodium dodecyl benzene sulfonate (SDBS) and dodecyl trimethyl ammonium chloride (DATCl); and four 5-membered cyclic organic compounds: tetrahydrofuran (THF), 1,3-dioxolane (DIOX), 2-methyl-tetrahydrofuran (m-THF) and cyclopentane (CP). Information on these additives is provided in Table 1.

THF, DIOX and CP are well known to form hydrates of structure (sII), where the 512 cavities are

Detail of a typical experiment

A typical experiment, carried out with a combination of SDS and DIOX is shown in Fig. 2, and briefly discussed in this section.

From the beginning of the experiment to point A, the reactor pressure decreased as the gas (mainly the CO2) solubilized into the solution, and then stabilized (at point A) when the solubility equilibrium was reached. From point A to B, the reactor was cooled until the target temperature Ttarg was reached. In some cases, the solution which was initially transparent (see

Concluding remarks

This study investigates the effect of a combination of thermodynamic additives and kinetic additives on hydrate formation, where three ionic surfactants (SDS, SBBS and DATCl) and four cyclic organic compounds (DIOX, CP, THF and m-THF) were tested, in various combinations.

The global action mechanism is identical for most of the combinations tested. At low concentrations of the thermodynamic promoter (i.e. 4 wt%), the system first forms a mixed hydrate containing the organic compound which, when

Acknowledgements

Joseph Diaz is particularly acknowledged for his work and assistance on the experimental rigs. The staff of the “Atelier de Physique” of the University of Pau is also thanked. The authors are grateful to Total E&P (“Gas Solutions” R&D Project), Fundayacucho from Venezuela and CG64 (Conseil Général des Pyrénées Atlantiques) for financial support of this work.

References (47)

  • T.A. Strobel et al.

    Thermodynamic predictions of various tetrahydrofuran and hydrogen clathrate hydrates

    Fluid Phase Equilib

    (2009)
  • J.-P. Torré et al.

    CO2 enclathration in the presence of water-soluble hydrate promoters: hydrate phase equilibria and kinetic studies in quiescent conditions

    Chem Eng Sci

    (2012)
  • J.S. Zhang et al.

    Does SDS micellize under methane hydrate-forming conditions below the normal krafft point?

    J Colloid Interface Sci

    (2007)
  • K. Watanabe et al.

    Surfactant effects on hydrate formation in an unstirred gas/liquid system: an experimental study using HFC-32 and sodium dodecyl sulphate

    Chem Eng Sci

    (2005)
  • C. Noik et al.

    Anionic surfactant precipitación in hard water

    J Colloid Interface Sci

    (1987)
  • M.D. Jager et al.

    Experimental determination and modeling of structure II hydrates in mixtures of methane + water + 1,4-dioxane

    Fluid Phase Equil

    (1999)
  • J.-P. Torré et al.

    CO2 capture by hydrate formation in quiescent conditions: in search of efficient kinetic additives

    Energy Proc

    (2011)
  • P. Di Profio et al.

    Surfactant promoting effects on clathrate hydrate formation: are micelles really involved ?

    Chem Eng Sci

    (2005)
  • J.G. Beltran et al.

    Gas hydrate measurement techniques and phase rule considerations

    J Chem Thermidyn

    (2012)
  • M.J. Lazzaroni et al.

    High-pressure phase equilibria of some carbon dioxide–organic–water systems

    Fluid Phase Equilib

    (2004)
  • A. Gennaro et al.

    Solubility and electrochemical determination of CO2 in some dipolar aprotic solvents

    J Electroanal Chem

    (1990)
  • E.D. Sloan

    Fundamental principles and applications of natural gas hydrates

    Nature

    (2003)
  • C.A. Koh

    Towards a fundamental understanding of natural gas hydrates

    Chem Soc Rev

    (2002)
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