Abstract:
LanzaTech NZ Ltd is a biofuel company which produces Bioethanol through the conversion of waste carbon gases via the mechanism of fermentation. Currently, Bioethanol is recovered from fermentation broths through the process of distillation. However the energy demand for this system is large, and results in ever-increasing operating costs. Membrane technology such as pervaporation, has recently been proposed as an alternative technology to distillation which can be incorporated in-situ with LanzaTech’s fermenter to increase the ethanol production yield, or coupled with the existing distillation process to lower the energy requirement. This is achieved by concentrating ethanol solutions above its azeotropic concentration, which cannot be attained through distillation alone. This project was established to investigate the potential benefits of pervaporation in LanzaTech's process. The first objective was to study the separation mechanism by developing a lab-scale vacuum driven rig to recover low concentration ethanol from aqueous solution. The second objective was to determine the membrane requirements to achieve the optimum pervaporation performance, as well as the effect of fermentation by-products on the separation process. Three different hydrophobic polymer membranes were used in this study to identify the optimal performance in terms of flux, permeability and selectivity of ethanol using different feed solutions. It was found that ethanol can be concentrated up to 25 wt%, with a flux of 530 gm-2h-1, from a 5 wt% ethanol and water mixture. Ethanol could be concentrated up to 18wt%, with a flux of 350 gm-2h-1, directly from LanzaTech’s fermentation broth using pervaporation. It was also found that mechanism of pervaporation is not only governed by solution diffusion, but also by pore flow mechanism. To optimise the mass transfer, the membrane material must have high ethanol solubility, allow diffusion, minimal coupling effect and low concentration polarization on the surface. Fermentation by-products played an important role in pervaporation, where the effect can be negative, positive or have no influence on the separation, depending on the physiochemical properties of the feed components, their interactions and the kinetic aspects on the membrane.