Design methodology for integrated downstream separation systems in an ethanol biorefinery.

摘要

Energy security and environmental concerns have been the main drivers for a historic shift to biofuel production in transportation fuel industry. Biofuels should not only offer environmental advantages over the petroleum fuels they replace but also should be economically sustainable and viable. The so-called second generation biofuels such as ethanol which is the most produced biofuel are mostly derived from lignocellulosic biomasses. These biofuels are more difficult to produce than the first generation ones mainly due to recalcitrance of the feedstocks in extracting their sugar contents. Costly pre-treatment and fractionation stages are required to break down lignocellulosic feedstocks into their constituent elements. On the other hand the mixture produced in fermentation step in a biorefinery contains very low amount of product which makes the subsequent separation step more difficult and more energy consuming.;In an ethanol biorefinery, the dilute fermentation broth requires huge operating cost in downstream separation for recovery of the product in a conventional distillation technique. Moreover, the non-ideal nature of ethanol-water mixture which forms an iseotrope at almost 95 wt%, hinders the attainment of the fuel grade ethanol (99.5 wt%). Therefore, an additional dehydration stage is necessary to purify the ethanol from its azeotropic composition to fuel-grade purity.;In order to overcome the constraint pertaining to vapor-liquid equilibrium of ethanol-water separation, several techniques have been investigated and proposed in the industry. These techniques such as membrane-based technologies, extraction and etc. have not only sought to produce a pure fuel-grade ethanol but have also aimed at decreasing the energy consumption of this energy-intensive separation. Decreasing the energy consumption of an ethanol biorefinery is of paramount importance in improving its overall economics and in facilitating the way to displacing petroleum transportation fuel and obtaining energy security.;On the other hand, Process Integration (PI) as defined by Natural Resource Canada as the combination of activities which aim at improving process systems, their unit operations and their interactions in order to maximize the efficiency of using water, energy and raw materials can also help biorefineries lower their energy consumptions and improve their economics. Energy integration techniques such as pinch analysis adopted by different industries over the years have ensured using heat sources within a plant to supply the demand internally and decrease the external utility consumption.;Therefore, adopting energy integration can be one of the ways biorefinery technology owners can consider in their process development as well as their business model in order to improve their overall economics.;The objective of this thesis is to propose a methodology for designing integrated downstream separation in a biorefinery. This methodology is tested in an ethanol biorefinery case study. Several alternative separation techniques are evaluated in their energy consumption and economics in three different scenarios; stand-alone without energy integration, stand-alone with internal energy integration and integrated-with Kraft. The energy consumptions and capital costs of separation techniques are assessed in each scenario and the cost and benefit of integration are determined and finally the best alternative is found through techno-economic metrics. Another advantage of this methodology is the use of a graphical tool which provides insights on decreasing energy consumption by modifying the process condition.;The pivot point of this work is the use of a novel energy integration method called Bridge analysis. This systematic method which originally is intended for retrofit situation is used here for integration with Kraft process. Integration potentials are identified through this method and savings are presented for each design. In stand-alone with internal integration scenario, the conventional pinch method is used for energy analysis.;The results reveal the importance of energy integration in reducing energy consumption. They also show that in an ethanol biorefinery, by adopting energy integration in the conventional distillation separation, we can achieve greater energy saving compared to other alternative techniques. This in turn suggests that new alternative technologies which imply big risks for the company might not be an option for reducing the energy consumption as long as an internal and external integration is incorporated in the business model of an ethanol biorefinery. It is also noteworthy that the methodology developed in this work can be extended as a future work to include a whole biorefinery system. (Abstract shortened by UMI.).