Elsevier

Journal of Membrane Science

Volume 487, 1 August 2015, Pages 249-255
Journal of Membrane Science

Water vapor permeation through cellulose acetate membranes and its impact upon membrane separation performance for natural gas purification

https://doi.org/10.1016/j.memsci.2015.03.074Get rights and content

Highlights

  • Free volume initially decreased and then increased again as humidity increased.

  • CO2 and CH4 permeabilities followed a similar trend to the free volume.

  • Results were related to pore blocking at low RH and plasticization at higher RH.

  • The change in mechanism occurred at around 2.5 kPa water vapor pressure.

  • Time dependent effects were observed above this water vapor pressure.

Abstract

Cellulose acetate is the predominant material used in membrane separation of acid gases from natural gas and biogas. However, the sensitivity of these membranes to water vapor is not well understood. In this work, flat-sheet membranes of two different degrees of acetylation, were exposed to both dry and humidified CH4 and CO2/CH4 mixtures. Positron Annihilation Lifetime Spectroscopy experiments showed that the number of free volume elements decreased as water concentration increased, indicating pore filling effects. The size of the free volume elements declined initially, followed by an increasing trend at vapor partial pressures greater than 2.5 kPa, indicating polymer swelling. Gas permeabilities of CH4 and CO2 followed a similar trend, with an initial decline due to hindered diffusion and competitive sorption, followed by an increase as the humidity exceeded 2.5 kPa. Water vapor permeabilities increased from 11,000 to 27,000 Barrer as the water activity increased but a change in the rate of increase was also noted at 2.5 kPa. At humidities in excess of 0.8, the extent of membrane swelling was such that equilibrium was not established even after 8 h of operation. Importantly, plasticization had significantly less impact on the polymer with a higher degree of acetylation.

Introduction

Natural gas production is estimated to be over 3300 billion cubic meters per year worldwide [1] and the removal of acid gases from the raw gas is the largest industrial market for membrane gas separation. While the composition of raw natural gas is different from reservoir to reservoir, it primarily consists of methane (70–90%), with small amounts of C1–C4 hydrocarbons such as ethane, propane, butane, along with higher molecular weight fractions. Water vapor, carbon dioxide, acidic gases, mercury, hydrogen sulfide, helium and nitrogen are also present in raw natural gas as impurities. A glycol dehydration unit is typically used for removing water from the natural gas feed streams prior to membrane processing [2]. However, some water vapor may still reach the membrane if the glycol unit is not working effectively. Further, glycol units are not always used in smaller installations such as biogas processing.

Cellulose acetate was the original membrane material used for CO2 separation from natural gas, building on the experience gained using this material for the desalination of saline water. Cynara® (part of Cameron) and UOP Separex® (part of Honeywell) are the two major membrane manufacturers currently supplying cellulose acetate based modules. Depending on the degree of acetylation, which corresponds to the extent of substitution of the hydroxyl groups in the glucoside repeating unit with acetyl groups (Fig. 1), the polymer is named as cellulose acetate, cellulose diacetate or cellulose triacetate.

Dehydration units (mostly diethylene or triethylene glycol dehydrators) are widely used upstream of the membrane unit to prevent hydrate formation [3], typically reducing the water content to ~100 ppm at around 70 bar, 60 °C [2]. It has previously been reported that the presence of water and some hydrocarbon derivatives (e.g. acetone) can damage and dissolve the membrane [4], but the sensitivity of the separation performance of cellulose acetate membranes to water vapor is not well understood. Almost all studies on the effects of water vapor on cellulose acetate membranes were conducted in the 1980 s [4], [5], [6], [7], [8]. The permeation measurements in this period often assumed ideal mixing of water vapor and gas and neglected the impact of concentration polarization upon the permeation properties.

Due to its small size and high hydrogen bonding affinity [9], [10], [11], water vapor can have a very significant effect upon membrane performance [6], [12], [13], [14]. It is argued that the clustering of water into multimolecular groups can act to ‘fill’ the free volume within the polymer structure, resulting in a loss of fractional free volume [15], [16]. It has been reported that these water clusters result in a decrease in the penetrant diffusivity as water vapor activity increases in a phenomenon referred to as ‘antiplasticization’ [17], [18], [19], [20], [21], [22], [23]. Conversely, water molecules can plasticize or swell the membrane structure, causing an increase in free volume and a corresponding increase in diffusivity with increasing vapor concentration [24], [25], [26], [27]. Depending on the hydrophobicity of the membrane and the activity of water vapor, these phenomena may coexist in a multi-component separation system, in both cases causing a loss of selectivity.

In this work., the mixed vapor/gas permeation system developed within our group to eliminate concentration polarization effects [14] combined with Positron Annihilation Lifetime Spectroscopy (PALS) [28] is applied to quantify the permeation of water/CO2 mixtures through cellulose acetate membranes more accurately, allowing for a better understanding of the behavior of water vapor in a simulated natural gas sweetening environment. Two types of cellulose acetate with different degree of acetylation are investigated during exposure to humidified methane and wetted carbon dioxide and methane mixtures.

Section snippets

Materials

Cellulose acetate with acetylation degrees of 51.6% (CDA) and 61.6% (CTA) (equivalent to degrees of substitution of 2.2 and 2.85, respectively) were kindly supplied by Daicel Corporation, Japan, in a powder state. The chemical structure of a typical cellulose acetate is presented in Fig. 1. Prior to membrane fabrication, the polymers were dried at 100 °C under vacuum overnight to ensure the complete removal of moisture.

Membrane preparation

Dense cellulose acetate membranes were prepared by a casting method using

Characterization

The physical properties of the cellulose acetate membranes are summarized in Table 1. CDA is more dense than CTA due to both the more bulky nature of the acetyl groups, which results in greater free volume as the acetylation degree increases; and the increased hydrogen bonding between hydroxyl groups at lower degrees of acetylation. The water uptake of cellulose acetate membranes was also greater at lower degrees of acetylation, further reflecting the greater hydrogen bonding with more hydroxyl

Conclusions

As gas humidity increases, the permeability of water vapor also increases for CDA and CTA, respectively. However, the rate of increase changed significantly at a water vapor partial pressure of ~2.5 kPa. The change at this point was confirmed by PALS analysis to be due to the onset of membrane swelling effects or plasticization. For vapor activities above 0.8 (a water partial pressure of 4.6 kPa), membrane swelling was severe enough to influence the gas separation performance for more than 8 h.

Acknowledgments

The authors would like to thank the Particulate Fluids Processing Centre (PFPC), a Special Research Centre of the Australian Research Council for access to equipment. Funding for this project is provided by the Cooperative Research Centres (CRC), Australian Government Department of Industry Australia. CMD and AJH acknowledge the support of CSIRO׳s Office of the Chief Executive Science Leader program and CMD would like to acknowledge the ARC DECRA support (DE140101359). The authors would also

References (42)

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