Bioethanol is one of the most important biofuels contributing to the mitigation of negative effects on the environment and quality of life due to the exploitation of fossil fuels. An advantage of bioethanol over other energy alternatives, such as hydrogen, is that it can be mixed with gasoline without any modification on current engines (Costa & Sodré 2010). However, to reach the bioethanol purity for biofuel applications two major processing steps must to be carried out, this due to the ethanol-water azeotrope. The first step is typically a conventional distillation tower in where near-azeotrope concentrations are achieved up to the level of 92-94 wt.%. The second step is much more complex and of greater interest. Several alternatives are available to achieve high purities (>99 wt.%), among them extractive distillation, which has being commonly used for large-scale production. Conventional extractive distillation process is performed in a sequence of two columns, where the first one (extractive column) purifies the ethanol and the other one (recovery column) splits water from the solvent, which is recycled back to the first column.
Dividing wall column (DWC) technology has demonstrated to be a feasible option to separate not only multi-component mixtures using a single shell in distillation tower, but also to be used in processes involving azeotropic, extractive and reactive distillation. As a consequence, alternative configurations to the conventional extractive system have been proposed for bioethanol purification. Hernández (Hernández 2008) and Kiss & Suszwalak (Kiss & Suszwalak 2012) proposed the use of thermally coupled distillation columns using complex extractive distillation schemes, which have demonstrated to have good dynamic control properties (Tututi-Avila et al. 2014).
Modified alternative schemes to thermally coupled systems have been considered for zeotropic purification such as those proposed by Agrawal (Agrawal 2000) and analyzed by Ramírez and Jiménez (Ramirez & Jimenez 2004) at steady state conditions and by Segovia-Hernández et al. (Segovia-Hernández et al. 2005) under closed-loop operation. Such systems consist of an alteration to the thermally coupled systems with side columns by removing the vapor-liquid coupling and adjusting the system structure to generate more operable options. In this study we consider such systems and extent them towards azeotropic systems, particularly an alternative extractive distillation system for bioethanol purification is studied. The proposed extractive distillation system consist of an extractive side-stream column from which bioethanol is removed as the distillate product. Water is removed as part of a liquid side stream and fed to a second column, which produces water as distillate product. The bottom stream from the first and second column is the solvent recycled back to the first column.
In this work, the design and control of such novel distillation system and a comparison of economic and dynamic performance to those of a conventional extractive distillation sequence and an extractive divided wall system is addressed. Steady state optimal configurations for conventional and alternative systems is obtained by simulation in Aspen Plus. Sensitivity analysis is carried out to design temperature control schemes. Responses are analyzed based upon a multi-loop framework under feed disturbances in Aspen Dynamics. The results show that the novel extractive distillation system is able of maintain the bioethanol purity under large feed disturbances, being its dynamic behavior comparable to the conventional and thermally coupled distillation systems.
Agrawal, R., 2000. Thermally coupled distillation with reduced number of intercolumn vapor transfers. AIChE Journal, 46(11), pp.2198–2210.
Costa, R.C. & Sodré, J.R., 2010. Hydrous ethanol vs. gasoline-ethanol blend: Engine performance and emissions. Fuel, 89(2), pp.287–293.
Hernández, S., 2008. Analysis of Energy-Efficient Complex Distillation Options to Purify Bioethanol. Chemical Engineering & Technology, 31(4), pp.597–603.
Kiss, A.A. & Suszwalak, D.J.P.C., 2012. Enhanced bioethanol dehydration by extractive and azeotropic distillation in dividing-wall columns. Separation and Purification Technology, 86(0), pp.70–78.
Ramirez, N. & Jimenez, A., 2004. Two alternatives to thermally coupled distillation systems with side columns. AIChE Journal, 50(11), pp.2971–2975.
Segovia-Hernández, J.G. et al., 2005. Control properties and thermodynamic analysis of two alternatives to thermally coupled distillation systems with side columns. Chemical and Biochemical Engineering Quarterly, 19(4), pp.325–332.
Tututi-Avila, S., Jiménez-Gutiérrez, A. & Hahn, J., 2014. Control analysis of an extractive dividing-wall column used for ethanol dehydration. Chemical Engineering and Processing: Process Intensification, 82, pp.88–100.