LITMIR.BIZ

Figure 3.2 Diffusion and solubility coefficient for different polyimides.

      Source: Adapted from Baker and Tanaka et al. [16, 17].

      where v is the specific volume of the polymer (cm³ g⁻¹), i.e. the correlative of the polymer density, and v₀ is the volume occupied by the polymer molecules (cm³ g⁻¹). In principle, FFV is the sum of all the spaces between the polymer chains.

      Nowadays, polymer membranes for CO₂ capture are being intensively investigated, specifically regarding post‐combustion carbon capture (CO₂/N₂), hydrogen purification (CO₂/H₂), and natural gas sweetening (CO₂/CH₄) [19]. In terms of post‐combustion carbon capture, competing with chemical absorption, gas separation membranes appear as a more environmentally friendly solution. As an example, a simulation of two potential polymer membranes, Polyactive® and PVAm/polyvinyl alcohol membranes, was performed for a 600 MW (gross) reference power plant [20]. Other materials, such Pebax, were also investigated for similar purposes, regarding CO₂ capture in post‐combustion processes [21].

Regarding H₂ purification, the predominant technology is steam methane reforming (SMR), where H₂ is obtained together with, mainly, CO₂ (15–20%) among other gases [22]. Polymeric membranes not only show advantageous properties, such as ease of processing or low cost, but also would be more accessible for large‐scale applications over other types of membranes. Consequently, and regarding high‐temperature processes, different polyimides such as 6FDA–6FpDA and Matrimid, with outstanding H₂/CO₂ separation properties at temperatures up to 270 °C, have been recently reported [23, 24].

      With regard to natural gas sweetening, or in other words, CO₂ removal from natural gas, membrane systems show greater potential than conventional chemical absorption because they have main advantages such as low cost, environmentally friendly technologies, and process flexibility [25]. Several commercial membranes are available for CO₂ elimination from natural gas. The most common representative materials for CO₂ removal are cellulose acetate/triacetate and polyimide [25].

      In order to study the properties of different membrane materials, and to be able to compare different polymer materials, Robeson published several charts, the first time in 1991 and the latest in 2008, about membrane permeability as a function of selectivity. These charts are called upper bound correlation [26, 27]. These experimental data are obtained from single gas tests at 30 °C and 1 bar. This indicates that permeabilities were measured using pure gas tests, and selectivities were obtained from the ratio of the pure gas permeabilities. This gives ideal selectivities, despite the fact that industrial processes of gas separation membranes are commonly performed with gas mixtures. Nevertheless, it is possible to extrapolate to industrial applications, this is because, in principle, if gases do not interact with the membrane material, the difference between single gas selectivity and gas mixture selectivity will be small. Consequently, in gas mixtures, molecules that possess high‐solubility coefficients will be sorbed enough by the membrane material to affect the other gas permeabilities.

Hence, upper bound correlations are described for several gas pairs. Regarding CO₂ separation, two different gas pair (CO₂/N₂ and CO₂/CH₄) are studied, CO₂/CH₄ being the second most investigated gas pair for membrane separation. In Figure 3.3a,b, a large number of polymers have been shown, which are mostly placed between a selectivity of 2–100 and a permeability of 0.1–1000. In addition, two black lines are identifying the prior upper bound and the present upper bound. Any polymer that surpasses the present upper bound can be considered as the most promising polymer for CO₂ separation.

As an example, PIM‐1 with a permeability of 2300 Barrer and a selectivity of 18.4, or PIM‐7 with a permeability of 1100 Barrer and a selectivity of 17.7 [28], is considered as a ladder polymer for the gas pair CO₂/CH₄, which surpasses the upper bound [29]. Thermally rearranged polymers have been recently published, which exhibit outstanding improved separation characteristics, although the present upper bound is determined without these data [30], see Figure 3.3a.