Separation of Ethanol–Water Azeotrope Using Waste Carrageenan Filter-Cake Adsorbent

Abstract

The final step in the downstream processing of bioethanol is typically the removal of water from the ethanol–water azeotrope (95.6 wt.% ethanol) to obtain fuel-grade ethanol (> 99 wt.%). This can be done by various techniques including distillation (azeotropic or extractive), membrane-based separation, and adsorption using molecular sieves or bio-based adsorbents. In this work, a waste by-product of carrageenan production is studied for its efficacy as water adsorbent. This waste carrageenan filter-cake (CFC) material is primarily composed of organics (22 wt.% cellulose and carrageenan) and ash (78 wt.% perlite) as inferred from the proximate analysis. The functional groups present in the material were identified by FTIR analyses, and the surface morphology was checked by FESEM. Liquid phase water adsorption isotherms at 30°C, 40°C, and 50°C were established and were found to be adequately described by both Langmuir (R² = 0.93 to 0.97) and Brouers-Sotolongo (R² = 0.96 to 0.97) models. An enthalpy change of adsorption of about −17 kJ/mol was computed through the van't Hoff equation. An azeotropic mixture (95.6 wt.% ethanol) was successfully purified to above 99.2 wt.% ethanol in a CFC-packed column. The CFC adsorbent was used in three adsorption–desorption cycles without much loss in capacity. Moreover, the Thomas model adequately described the breakthrough curves.