The impact of ethylene glycol and hydrogen sulphide on the performance of cellulose triacetate membranes in natural gas sweetening
Introduction
Natural gas is a primary energy resource that will occupy over 25% of the global electricity market in the next decades, as well as acting as a transport fuel and direct heating resource [1]. The composition of raw natural gas varies widely but typically contains impurities such as nitrogen (N2), carbon dioxide (CO2), water (H2O) and hydrogen sulphide (H2S) that require removal to meet pipeline specifications. Membrane separation has been used for many decades for acid gas removal, known as natural gas sweetening, with advantages in energy efficiency, land footprint and a lack of chemical consumption [2]. Although many new membrane materials have been developed, cellulose triacetate (CTA) membranes still retain the bulk of this separation market because of their high CO2 – methane (CH4) selectivity, commercial readiness and acceptance as a low risk option by the industry [3], [4].
Raw natural gas is usually saturated with water which is generally removed upstream of the membrane unit to avoid pipeline corrosion and hydrate formation [5], [6], [7]. Glycols such as monoethylene glycol (MEG), diethylene glycol (DEG) and triethylene glycol (TEG) are the most common solvents utilised for this purpose [8], [9]. Due to the extremely low vapour pressure of glycols (28 Pa for monoethylene glycol [10] and < 1 Pa for triethylene glycol [11] at 35 °C), carryover of these solvents in the vapour state is usually limited. However, carryover of entrained glycol droplets can occur [7]. This is a significant issue, because glycol is known to plasticise polymers [12], and the entrainment of the glycol solution into the membrane unit can thus alter the permselectivity of the membrane [13]. A study on the effect of MEG and TEG vapours on CO2/CH4 separation across a facilitated transport membrane has been reported [14]. However, to the best knowledge of the authors, there is no study on the effect of glycol solutions on the gas separation performance of CTA membranes.
Hydrogen sulphide is a common species in natural gas with concentration varying from 4 to 10,000 ppm [15] that will enter the membrane unit with the natural gas. Many studies on CO2 removal from natural gas by cellulose acetate membranes have observed the co-permeation of H2S with CO2 in the membrane unit [15], [16], [17]. Li et al. reports the performance of cellulose acetate membranes in the presence of H2S – H2O mixtures [18]. However, the impact of temperature on H2S permeation through the CTA membrane has not been well studied. Heilman et al. [19] reported the permeability and sorption of H2S into several polymer films including a cellulose acetate film manufactured by Polaroid but the results presented were limited. Data on the long term effect of H2S on CTA gas separation performance is also limited.
In this investigation, the effect of two standard glycols, MEG and TEG, on the gas separation of CTA membranes over a 2000 h period is investigated. The permeation of H2S through CTA membrane at different partial pressures (0.2–0.75 kPa) and temperatures (22–80 °C) is also reported, as is the long term impact of H2S on the membrane performance over a 7200 h period.
Section snippets
Membrane fabrication
The polymer utilised in the investigation was a commercial cellulose triacetate powder supplied by Cellulose Company – Daicel Corporation, Japan. The degree of substitution of acetyl groups on the polymer is 2.85, corresponding to a degree of acetylation of 61.6%.
Dense CTA membranes were fabricated by a solvent casting method. The polymer powder was dried under vacuum at 100 °C overnight prior to membrane fabrication. The dried powder was then dissolved in dicholoromethane (ChemSupply,
Sorption uptake of glycol liquids in cellulose triacetate
The sorption uptake of both MEG and TEG (Fig. 1) shows a sigmoidal shape which indicates that while the glycols initially swell the CTA membrane by Fickian diffusion, polymer relaxation or plasticisation occurs as the solute concentration increases [27], [28], [29], [30]. The transition from Fickian diffusion to non-Fickian polymer relaxation occurs at roughly 30 h of exposure of these 70 µm membranes to the solution. It should be noted that the transition time would be of the order of seconds
Conclusions
The impact of glycols on dense CTA membranes has been studied for up to 2000 h. The absorption of ethylene glycol and triethylene glycol into the CTA membrane enhanced the permeation of CH4 and CO2 through the wet membranes but resulted in a gradual decline in the permeation of He. This reflected relaxation of the polymer structure. In particular, WAXD analysis confirmed that a significant loss of crystallinity occurred during exposure to these glycols, providing more accessible free volume in
Acknowledgements
The authors would like to acknowledge the funding support for this research project from The University of Melbourne, Particulate and Fluid Processing Centre (PFPC), the Peter Cook Centre for Carbon Capture and Storage Research at the University of Melbourne and Brown Coal Innovation Australia (BCIA). The X-ray diffraction analysis was performed within the Materials Characterisation and Fabrication Platform (MCFP) at the University of Melbourne and the Victorian Node of the Australian National
References (70)
Post-combustion CO2 capture with chemical absorption: a state-of-the-art review
Chem. Eng. Res. Des.
(2011)The performance of carbon membranes in the presence of condensable and non-condensable impurities
J. Membr. Sci.
(2011)- et al.
Change in transport properties of anion-exchange membranes in the presence of ethylene glycols in electrodialysis
J. Colloid Interface Sci.
(1998) - et al.
Effect of monoethylene glycol and triethylene glycol contamination on CO2/CH4 separation of a facilitated transport membrane for natural gas sweetening
J. Membr. Sci.
(2012) - et al.
Membrane processes for the removal of acid gases from natural gas. I. Process configurations and optimization of operating conditions
J. Membr. Sci.
(1993) Water vapor permeation in polyimide membranes
J. Membr. Sci.
(2011)The potential for use of cellulose triacetate membranes in post combustion capture
Int. J. Greenh. Gas. Control
(2016)Operating temperature effects on the plasticization of polyimide gas separation membranes
J. Membr. Sci.
(2007)Evaluation of semi permeable membranes for determination of organic contaminants in drinking water
Water Res.
(1975)Water vapor permeation through cellulose acetate membranes and its impact upon membrane separation performance for natural gas purification
J. Membr. Sci.
(2015)
The solution-diffusion model: a review
J. Membr. Sci.
Contributions of diffusion and solubility selectivity to the upper bound analysis for glassy gas separation membranes
J. Membr. Sci.
Analysis of pervaporation of methanol-MTBE mixtures through cellulose acetate and cellulose triacetate membranes
Polymer
Permeation behavior of carbon dioxide-methane mixtures in cellulose acetate membranes
J. Membr. Sci.
Synthesis and characterization of cellulose acetate from rice husk: eco-friendly condition
Carbohydr. Polym.
Application of cellulose acetate to the selective adsorption and recovery of Au(III)
Carbohydr. Polym.
The effect of degree of acetylation on gas sorption and transport behavior in cellulose acetate
J. Membr. Sci.
Membrane processes for the removal of acid gases from natural gas. II. Effects of operating conditions, economic parameters, and membrane properties
J. Membr. Sci.
Gas permeability, diffusivity, solubility, and aging characteristics of 6FDA-durene polyimide membranes
J. Membr. Sci.
Physical aging of blends of cellulose acetate polymers with dyes and plasticizers
Polymer
Plasticization of ultra-thin polysulfone membranes by carbon dioxide
J. Membr. Sci.
Solubility of methane and carbon dioxide in ethylene glycol at pressures up to 14 MPa and temperatures ranging from (303 to 423) K
J. Chem. Thermodyn.
Solubilities of inert gases in ethylene glycol
J. Chem. Thermodyn.
Vapor-liquid equilibria for acid gases and lower alkanes in triethylene glycol
Fluid Phase Equilibria
Poly(ether urethane) and poly(ether urethane urea) membranes with high H2S/CH4 selectivity
J. Membr. Sci.
An empirical correlation of gas permeability and permselectivity in polymers and its theoretical basis
J. Membr. Sci.
Revisiting the experimental and theoretical upper bounds of light pure gas selectivity-permeability for polymeric membranes
J. Membr. Sci.
Membrane Technology and Applications
A novel approach to gas separation using composite hollow fiber membranes
Sep. Sci. Technol.
Sour Gas Treatment Process
Cited by (26)
Highly permeable and selective polymer inclusion membrane for Li<sup>+</sup> recovery and underlying enhanced mechanism
2024, Journal of Membrane SciencePerformance and stability of cellulose triacetate membranes in humid high H<inf>2</inf>S natural gas feed streams
2024, Journal of Membrane ScienceBio-based nonporous membranes: Evolution and benchmarking review
2023, Journal of Industrial and Engineering ChemistryThe influence of propane and n-butane on the structure and separation performance of cellulose acetate membranes
2021, Journal of Membrane ScienceCitation Excerpt :This agrees well with the sorption results in Fig. 1. As a comparison, Lu et al. reported that the CTA membrane density decreased from 1.299 g/cm3 to 1.270 g/cm3 after the membrane was soaked in triethylene glycol [20]. XRD analysis was used to investigate the influence of propane or n-butane on the polymer crystallinity.