Bioethanol Dehydration Assessment with Environmental Issues

Environmental effects and health hazards posed by fossil-fuel based technologies complemented by changes in the global economy have further demanded the need for developing “cleaner” and more efficient technologies that rely on renewable or synthetic resources. An alternative, commonly referred to as bio-fuels, has significantly matured and today’s economy recognizes the significance of being able to produce ethanol from renewable resources such as biomass. Moreover, the potential of ethanol to be further converted to hydrogen makes it a very attractive alternative to replace or complement fossil fuels as sources of energy.

Argentina has recently enacted legislation to promote the use of bio-fuels to ameliorate its ever increasing annual gasoline demand, currently bordering 4 billion liters. This new legislation (Law 26.093 of Biofuels Promotion) would result in a demand of over 200 million liters of dehydrated alcohol. This quota would be equivalent to produce ethanol out of all the sugar that is currently exported. With fifteen (15) active Sugar Cane processing plants, a mill capacity of 15 million tons/year, and thirteen (13) distilleries with 1.5 million liters/year of hydrated alcohol, Tucumán (located at North West of Argentina) could meet more than 50% of the ethanol demands for Argentina. 

Process simulation allows a systematic analysis that leads to an advantageous combination of all the values of process variables. In particular, since they demand significant energy influx, an important subsystem for analyzing is the separation of alcohol-water mixtures. Though many techniques for ethanol dehydration are known; adsorption, distillation, hybrid processes, and pervaporation, are the most common technologies in practice. Two alternatives for ethanol dehydration technologies are considered in this work. The first is based on the combination of distillation and azeotropic distillation, while the second relies on hybrid distillation and pervaporation processes, so it was necessary to develop a simulation module for calculating the membranes performance, typically not included in commercial process simulators. Both alternatives are simulated and their optimal design and operating parameters are identified by means of rigorous simulation using Aspen Plus^®.

To achieve sustainable designs process integration is a critical task. For this, we consider the dehydration step as an integral part of the overall scheme of alcohol / water separation, taking into account potential energy integration scenarios considering different concentrations of the products obtained in the different units, to analyze its impact on global energy consumption and other environmental indicators. Particular attention is given to energetic integration by performing a pinch-analysis to each of the fundamental stages to find minimum energy consumptions.

This paper uses an ad-hoc pervaporation module to assess the production of fuel-grade bio-ethanol in distillery installations complementing a sugar mill. A baseline condition is defined as a case-study to illustrate the effect of structural and parametric changes on membrane separation units.

The user-defined module is integrated in commercial process simulators to study the environmental and energetic impact of different process configurations. Different schemes meeting equivalent performance standards in terms of purity and recovery are examined in this paper. Particular attention is given to energetic demands as comparison metrics.

This paper formulates an approach that accounts for environmental issues in explicit form. The approach makes use of Life Cycle Assessment (LCA) as described by the ISO 14000 series. Unlike most common approaches that consider environmental impact by focusing on reducing effluents, this methodology also considers the impact associated with all the involved processes in the FPD.  The software used to evaluate the environmental impact is SimaPro®.

Process alternatives that meets same specifications of purity and recovery in exit streams, are examined in this paper. Of particular relevance is the assessment of costs and environmental impact of these processes. A baseline case is defined in our case-study, and structural and parametric changes are made. Different scenarios with respect to economic and environmental issues taking into account the specifications of the final product are generated. The impact of different operating policies for the production of anhydrous alcohol on the environment is examined in detail.

The membrane area, the number of plates in the distillation units, the heat exchange area, and the pressure manipulators are considered as structural variables. Among the most relevant operational variables considered one can mention reflux ratio, pressure ratio, as well as compositions and flow rates of streams.

One could consider the addition of environmental aspects within energy systems optimization as a promising contribution to the energetic optimization and LCA. 

Keywords: Bio-fuels, Ethanol Dehydration, Life-cycle Assessment, Process Simulation