(608a) Optimal Energy Consumption in the Production of Bioethanol From Lignocellulosic Biomass

To reduce the use of fossil fuels, whose emissions are one of the main causes of global warming, the production of biofuels has been proposed as an alternative to diminish the harmful effects of greenhouse emission gases. Bioethanol already has a wide application in industry, i.e. in pharmaceuticals, food and chemical industries, and the main current producers are the USA, Brazil and China. The main raw materials for bioethanol production in these countries are sugar cane bagasse and corn, but there are many other sources which may also be suitable. In this work, the raw material considered is lignocellulosic biomass which is mainly composed of cellulose, hemicellulose and lignin, although the variety of lignocellulosic materials is vast, including solid urban waste and agricultural residues. This work aims to address the use of lignocellulosic biomass as potential feedstock for bioethanol production, in particular in terms of the energy consumption required and how this may potentially be reduced.

The bioethanol production process is mainly composed of four stages: pretreatment, enzymatic saccharification, fermentation and separation. The pretreatment considered in this work is based on dilute-acid hydrolysis in which the hemicellulose is degraded into xylose and furfural. The unreacted cellulose and other insoluble solids are filtered and stored for further enzymatic hydrolysis. The solution is then treated with Ca(OH)₂ to reduce the concentration of furfural to avoid inhibitory effects on the fermenting microorganism. This solution, rich in xylose, is then concentrated by evaporation. The unreacted solid fraction and the concentrated solution of xylose are mixed and added to a bioreactor in which enzymatic hydrolysis and fermentation of xylose and glucose take place simultaneously, also known as simultaneous saccharification and co-fermentation (SSCF). Further separation through distillation and pervaporation is required to purify the ethanol up to biofuel standard specifications (i.e. >99 % w/w). An energy evaluation of the process has identified that the evaporation in the pretreatment stage and the separation in the distillation columns are the biggest energy consumers.

A detailed dynamic mathematical model has been developed which accounts for all the four stages of the production, including both batch operated units and continuously operated units. The model includes mass and energy balances and has been verified against experimental data, mainly from literature. The model is solved using gPROMS with Multiflash for the thermodynamic properties. The model is used to determine the minimum energy requirements for the overall process through rigorous optimisation and to identify where the potentials for energy savings is greatest. The separation stage is the largest energy consumer and different alternatives for energy reduction, such as double effect distillation, heat pumping, and internally heat-integrated distillation, has been considered and has been found to reduce the energy requirement. The use of hybrid separation units combining distillation and pervaporation is also an option to reduce the energy consumption in this stage. Effective use of heat integration across units has been considered, and it has been found that it is possible to greatly reduce the energy requirements for the overall process by improving the energy efficiency of each stage combined with appropriate use of heat integration across units.