Separation of SO2/O2 using membrane technology
Hlabano-Moyo, Bongibethu Msekeli
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The Hybrid Sulphur process is one technology out of a multitude of known technologies responsible for hydrogen production. Within the latter hydrogen production cycle, it is pivotal to recover O2 as a by-product from a sulphuric acid decomposition reaction that produces SO2, H2O and O2. It is assumed that a simple phase separation stage carried out on the reaction products would liberate SO2 and O2 as a gaseous mixture leaving behind H2O in the liquid state. Several separation technologies are available to effect SO2/O2 separation, but membrane technology has proved to be dearer due to simplicity of the technology, low capital and energy costs. It is a pity though that insignificant work has been done that considers the SO2/O2 binary system in the membrane technology context. Of the insignificant work done, non – commercial membranes were employed. It is on the latter background that the present study was proposed. Six commercial membranes were selected from literature, two (Udel Polysulfone and Teflon AF 2400) of which are currently used in gas separation applications and the remainder (Hyflon M, Hyflon F, Halar and Nafion 117) not necessarily used as gas separation membranes but present a potential of separating SO2/O2. The inclusion of the latter four membranes sought to unearth unknown gas separation potentials of the membranes based on hypothetical 1 μm thick membranes. A screening technique was employed to eliminate poor performing membranes through pure component permeation of SO2, O2, N2 and CO2. The use of the additional gases (N2 and CO2) was meant to allow the generation of a pool of data that would be used as a yardstick to compare to literature and thus validate the authenticity of the designed set up. The single permeation experiments were carried out at 25°C and at absolute gas feed pressures of 1 bar, 2 bar and 3 bar, with the exception of Hyflon F experiments that were carried out at 3.85 bar, 2.85 bar and 1.85 bar also at 25°C. The effect of pressure on gas permeability and ideal selectivity of all gases against O2 was investigated. Udel Polysulfone and Nafion 117 presented clearly evident pressure dependant SO2 permeabilities whilst CO2, N2 and O2 permeabilities were sluggishly dependant on pressure in all membranes. Gas flux in general increased with increasing pressure as pressure is essentially the driving force for permeability. Membrane screening for further investigation was then performed based on a compromise between SO2/O2 ideal selectivity and SO2 flux in hypothetical 1 μm thick membranes. Membranes that presented the best SO2/O2 selectivity include, Udel Polysulfone with SO2/O2 selectivities of 46, 58 and 314 at 1 bar, 2 bar and 3 bar respectively, Nafion 117 with SO2/O2 selectivities of 30, 35 and 40 at 1 bar, 2 bar and 3 bar respectively and Halar with a SO2/O2 selectivity of 17 at 3 bar. The best SO2 flux through hypothetical 1 μm thick membranes was manifested in Teflon AF 2400 with SO2 fluxes of 3.6 m3.m-2.hr-1, 5.9 m3.m-2.hr-1 and 9.9 m3.m-2.hr-1 at trans-membrane pressures of 1 bar, 2 Bar and 3 Bar respectively, Udel Polysulfone with SO2 fluxes of 0.13 m3.m-2.hr-1, 0.32 m3.m-2.hr-1 and 2.56 m3.m-2.hr-1 at trans-membrane pressures of 1 bar, 2 bar and 3 bar respectively and Nafion 117 with SO2 fluxes of 0.48 m3.m-2.hr-1, 1.03 m3.m-2.hr-1 and 1.79 m3.m-2.hr-1 at 1 bar, 2 bar and 3 bar trans-membrane pressures respectively. Despite Teflon AF 2400 presenting the highest SO2 flux, the poor SO2/O2 ideal selectivity ≈ 1 rendered the membrane unfit for further investigation. The low SO2 flux (0.02 m3.m-2.hr-1) presented by Halar also rendered the membrane unfit for further investigation despite the relatively fair SO2/O2 ideal selectivity of 17. Binary permeation experiments were then performed on Udel Polysulfone and Nafion 117 after passing the single permeation screening test. Gas mixture compositions of (25 wt %:75 wt %, SO2:O2), (50 wt %:50 wt %, SO2:O2) and (75 wt %:25 wt %, SO2:O2) were employed. The binary permeation experiments were carried out at a temperature range of 15°C to 55°C and a SO2 feed partial pressure range of 1.1 ± 0.1 bar to 2.3 ± 0.1 bar. The SO2 permeate composition increased with pressure and decreased with temperature in both Udel Polysulfone and Nafion 117. Udel Polysulfone presented a superior SO2/O2 separation potential, concentrating a (25 wt %:75 wt %, SO2:O2) gas mixture to (94 wt %:6 wt %, SO2:O2) in a single step at 15°C and 2.2 ± 0.1 bar SO2 feed partial pressure. Nafion 117 concentrated the same gas mixture to (87 wt %:13 wt %, SO2:O2) also in a single step at 15 °C and 2.4 ± 0.1 bar SO2 feed partial pressure. Based on hypothetical 1 μm thick membranes, Nafion 117 presented generally high SO2 molar fluxes in mixture with O2 of about a magnitude higher than the SO2 molar fluxes presented in Udel Polysulfone. Also, Nafion 117 proved to be less prone to plasticisation within the pressure range considered. Despite Udel Polysulfone presenting generally lower SO2 molar fluxes, Udel Polysulfone was deemed to be the ideal membrane for the current SO2/O2 separation application as thicknesses of 1 μm of Nafion the perfluorosulfonic acid based membrane are currently unknown and also Udel Polysulfone presented the best SO2/O2 separation capability. The latter findings are envisaged to prompt further research on the production of ultra-thin perfluoro-sulfonic acid based membranes for the current application.
- Engineering