Facilitated oxygen transport in liquid membranes: review and new concepts
Introduction
In this article, an overview is given on the current status of liquid membranes for the production of oxygen enriched air. In the second part, we introduce preparation routes for a new class of membrane, the so-called micro-encapsulated liquid membrane, and show preliminary results.
The separation of gas mixtures is a major operation in the (petro)chemical industry, whereby the separation of oxygen/nitrogen presents one of the main applications. Oxygen enriched air is used in many industrial processes which do not require pure oxygen, e.g. combustion of natural gas, coal gasification and liquifying, as well as in the production of peroxides, in sewage treatment, in welding and in the glass production. Standard methods are cryogenic distillation and pressure swing adsorption [1]. Since these techniques are still highly energy intensive, the number of current applications is limited and a less costly process would be desirable. As an alternative approach, gas separation membranes for the production of oxygen enriched air have been developed over the last 30 years based on the selective oxygen permeability of polymeric membrane materials and later on carrier mediated transport in liquid membranes. Polymeric membranes systems, which have proven to be less cost intensive to operate, are presently still not suitable to produce highly oxygen enriched air, i.e. air with an oxygen content in excess of 50–60 vol.% and for commercial large scale production [2], [3].
Improvement of polymeric membranes for gas separation can only be achieved by increasing both permeability and perm-selectivity. Polymeric membrane materials with relatively high selectivities used so far show generally low permeabilities, which is referred to as trade-off or ‘upper bound’ relationship for specific gas pairs [4]. For commercial production of oxygen enriched air, the upper bound relationship presents the major disadvantage in the utilisation of polymeric membranes. To improve single bulk material (polymer) properties, facilitated transport of a specific gas molecule through modified polymeric membranes or liquid membranes containing mobile carrier molecules has been investigated since the first paper of Scholander [5] in 1960.
Facilitated or carrier mediated transport is a coupled transport process that combines a (chemical) coupling reaction with a diffusion process. The solute has first to react with the carrier to form a solute-carrier complex, which then diffuses through the membrane to finally release the solute at the permeate side. The overall process can be considered as a passive transport since the solute molecule is transported from a high to a low chemical potential. In the case of polymeric membranes, the carrier can be chemically or physically bound to the solid matrix (fixed carrier system), whereby the solute hops from one site to the other. Mobile carrier molecules have been incorporated in liquid membranes, which consist of a solid polymer matrix (support) and a liquid phase containing the carrier molecules [6], see Fig. 1.
For both types of facilitated transport systems, mediated solute transport by fixed or mobile carriers, two modes of solute transport can be distinguished, see Fig. 1. This so-called dual-mode transport mechanism describes the combined total oxygen flux through the membrane. It was first proposed to explain the transport behaviour of gases, such as carbon dioxide in glassy polymers. The first mode refers to the solution-diffusion of the solute, e.g. oxygen and nitrogen, through the polymer matrix of the membrane. Characteristic for this mode is a low oxygen selectivity and a low transport rate (diffusivity), determined by a Henry-type sorption. The second mode concerns the facilitated transport provided by the carrier. It is highly sensitive for oxygen and can be described by a Langmuir-type adsorption. Due to the dual mechanism, the total flux is not proportional to the driving force. Therefore, even at very low concentrations of oxygen in the feed phase still appreciable oxygen fluxes can be obtained [7], [8].
General advantages of facilitated transport membranes are improved selectivity, increased flux and, especially if compared with membrane contactors, the possibility to use expensive carriers. The specific pre-requisites, advantages and disadvantages connected to both types of carrier systems, the fixed and the mobile carrier, are listed in Table 1. So far, mainly conventional liquid membranes have been loaded with different mobile carrier systems to obtain facilitated transport properties [9]. Problems encountered are (evaporative) loss of solvent and carrier, temperature limitations, a too large membrane thickness and, therefore, too low permeabilities as well as a limited solubility of the carrier in the liquid medium. The low fluxes achieved have, until now, limited their application in industrial separation processes. In particular for oxygen carrier systems, a major problem is the instability of the carrier against irreversible oxidation. Improvements necessary for (large scale) commercial applications involve, therefore, the development of new membrane morphologies and stable carrier systems.
Section snippets
Background
In this section, an overview is given on facilitated oxygen transport in liquid membranes, whereby we will lay our main emphasis on oxygen/nitrogen separation.
The concept of a molecular carrier transport involving a reversible chemical combination between permanent and mobile species was pursued and developed by Osterhout and colleagues in the early 1930s, although the principle has been demonstrated much earlier by Pfeffer in 1910 and Freudlich and Gann in 1915 [10]. The model experiments of
Micro-encapsulated membranes
The concept of micro-encapsulated membranes, as already mentioned above, was introduced by Bauer et al. [21]. They developed an asymmetric membrane by a dry/wet phase inversion process whereby a carrier solution was encapsulated in a closed-cell morphology within the ultrathin selective top-layer of only 0.1–0.5 μm thickness. The porous support layer gave good mechanical properties to the membrane in order to withstand mechanical stress from high pressures, which in turn could affect the thin
Conclusion and outlook
Commercial membrane systems developed for the production of oxygen enriched air are not yet mature enough to be used for large-scale industrial applications requiring oxygen contents of 60–70 vol.%. Existing polymeric membranes show, due to the upper bound relationship between permeability and selectivity, a selectivity which is too low to obtain the required oxygen purity in a commercially feasible single stage process. Of the materials studied so far, none shows selectivity in excess of 10 and
Acknowledgements
The authors are grateful to The Netherlands Foundation for Chemical Research (NWO — CW) in collaboration with The Netherlands Technology Foundation (STW) for financial support. A. Figoli would like to thank Prof. H. Strathmann for introducing him to membrane science. M.P. de Jong and B. Folkers are thanked for their help in performing part of the experiments and R. Fiammengo for stimulating discussion on oxygen carrier systems.
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