(315a) Bioethanol Dehydration in Thermally Integrated Extractive Distillation Columns

Bioethanol plants produce dilute mixture of around 10 wt-% ethanol in water. Ethanol is distilled to 96 %, and most commonly, sent to a molecular sieve, which absorbs water. The molecular sieve is regenerated by heating to remove water. As the capacity of ethanol production increases molecular sieve becomes thermally inefficient. Beside that the distillation of ethanol could consume up to 50% of the overall energy used in a typical grain alcohol plant. Significant energy saving, up to 30-40%, is possible in thermally coupled distillation sequences compared to the conventional direct and indirect distillation sequences [1,2]. At the same time, there may be problems involved in operating the Petlyuk columns because of the bidirectional flow of the vapor interconnecting streams [2]. Ethanol purification by distillation requires extractive distillation with an entrainer, such as pentane, benzene, diethyl ether, ethylene glycol, toluene, cyclohexane, methoxy-ethanol, ethyl tert-butyl ether, and bioglycerol. Entrainer breaks the azeotrope which forms between ethanol and water at 95.6 wt% ethanol (at atmospheric pressure). The selection of entrainer usually affects the column operating conditions [1,2]. This study analyzes the options of energy saving of Petlyuk columns for purification of ethanol with a particular entrainer. With various type of entrainers, the residue curve maps are constructed with the Aspen Plus simulator. Residue curve maps can help interpreting the behavior of these systems and identifying the regions between the distillation boundaries. Energy saving in ethanol purification using thermally integrated columns may be improved by the selection of an appropriate entrainer.

1. Hernandez, S., Analysis of energy-efficient complex distillation options to purify bioethanol, Chem. Eng. Technol. 31 (2008) 597-603. 2. Bruggemann, S., Marquardt, W., Rapid screening of design alternatives for nonideal multiproduct distillation processes, Comp. Chem. Eng. 29 (2004) 165-179.