Extractive Distillation Using High Boiling Solvents
This extractive distillation method uses a relatively non-volatile liquid solvent which is fed into a distillation column some trays above the ethanol feed tray. The presence of the high boiling new component at relatively high concentration in the liquid on the trays alters the volatility of one of the feed components more than the other, so a separation of the feed components can be made in the column. Such a solvent is referred to as an extractive distillation solvent. A number of readily available relatively stable organic liquids have been used to produce dry ethanol by this method. Lee and Phal screened a series of solvents for extractive distillation and reported that promising solvents are glycerin, ethylene glycol, tetraethylene glycol, and triethylene glycol [25]. Furthermore, they reported that with a suitable solvent, such as ethylene glycol, anhydrous ethanol could be produced with only 18 theoretical trays. A low reflux ratio of 1.5 and a low solvent-to-feed ratio of 0.27 would be needed for
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Figure 15.2 Dehydration of ethanol water mixture by extractive distillation using ethylene glycol. (Reprinted with permission from reference [2]; copyright 2010 Elsevier). |
the separation [25]. In addition to this, toluene, and furfural have also been used in this technique [2].
In the extractive distillation, added solvent might be the one which enhances the volatility of ethanol more than that of water. In this case, the ethanol would appear in the overhead product. Conversely, the solvent selected might enhance the volatility of water more than that of ethanol. In this situation, water would appear in the overhead product. Figure 15.2 shows a flow diagram of extractive distillation process for separating water from ethanol using ethylene glycol as the solvent [2].
In this type of extractive distillation process, a water-rich mixture from the bottom of the extractive distillation column is fed into a solvent recovery column, where high boiling ethylene glycol and water are separated. Water is removed from the bottom of this column, and ethylene glycol with some ethanol is fed back to the extractive distillation column. Extractive distillation with ethylene glycol as solvent has features of high quality of product and suitability for large-scale production, relatively less volatilization amount and lower consumption of solvent. However, there are a few weaknesses in high boiling solvent extraction technology such as the need to recycle a large amount of the high boiling solvent, which requires a large amount of energy. Brito and coworkers have used process simulation methods for optimization of extractive distillation using ethylene glycol as the added solvent [26, 27]. As expected, the increase in the number of stages causes a reduction in energy consumption, even though it is a minor decrease. However, unlike the conventional distillation column, this is not always in line with the reduced reflux ratio, which shows once again the strong influence of solvent flow rate on the separation in extractive distillation [26].