Analysis of energy saving by combination of distillation and pervaporation for biofuel production

• Upgrading the ethanol content of the fermentation broth is an energy intensive process.
• Energy demand of distillation reduces significantly with the feed ethanol concentration.
• Combined process of distillation and pervaporation can decrease the energy demand.
• At present, commercially available membranes are not real alternative to distillation.

Energy consumption of distillation needs high portion of the total energy demand of the fuel-grade bioethanol production. Pervaporation, being a membrane separation process, is a promising alternative process to distillation. A detailed analysis was performed considering the energy demand of concentration of ethanol solution obtained during fermentation up to the fuel-grade quality applying both hydrophilic and hydrophobic membranes. It was also discussed how the specific energy demand of the process varies during the pervaporation process and how energy consumption can be reduced applying pervaporation process with different operating modes. It was stated that standalone pervaporation process can provide fuel-grade quality biofuel at very high separation coefficient, only, using consecutively switched hydrophobic and hydrophilic membranes. Three-stage pervaporation process is needed at separation coefficient of 50, and two-stage one at separation coefficient of 100 to achieve the desired quality. Application of a hybrid process with lower membrane separation coefficient can give the desired quality by lower energy than that the distillation alone. Thus, the hybrid process can provide saving of essential amount of energy comparing it with the energy demand of distillation. However, the commercially available membrane, at present, has not the desired selectivity for replacing, at least partly, the distillation.