Environmental Engineering Reference
In-Depth Information
21.9.5.1 azeotropic distillation
In azeotropic distillation for bioethanol production, the entrainer (cyclohexane) is added to the
binary mixture, producing a heterogeneous azeotrope with water and ethanol. In this process, two
distillation columns are used: azeotropic and recovery.
Simulation of the azeotropic distillation process is often complex and extremely sensitive to proj-
ect parameters and specifications, mainly because of the formation of a second liquid phase inside
the azeotropic column. Higler et al. (2004) reported that column efficiencies between 25 and 50%
are not uncommon when a second liquid phase is present.
To study the formation of the second liquid phase inside the column, simulations of the azeo-
tropic distillation process with cyclohexane for anhydrous bioethanol production were carried out
using the software Aspen Plus with three configurations.
In the configuration that presented the best results, hydrous bioethanol is mixed with the recycle
stream of the recovery column and fed to the azeotropic column. Entrainer is comprised of the
organic phase obtained in the decanter and a solvent make-up stream. Anhydrous bioethanol is pro-
duced on the bottom of the azeotropic column, and the ternary azeotrope in the top. After cooling,
the ternary azeotrope splits into two liquid phases in the decanter. The aqueous phase is fed to the
recovery column, producing pure water on the bottom.
For all configurations studied, it was observed that the mixture inlet stage in the azeotropic
column determines the beginning of a second liquid phase, because this stage is the one that first
presents two liquid phases. For this reason, it is important to define the inlet stage near to the bot-
tom of the column; therefore the best feed inlet stage is the last one that allows column convergence.
21.9.5.2 extractive distillation
In extractive distillation, also known as homogeneous azeotropic distillation, a separating agent,
called solvent or entrainer, is added to the azeotropic mixture to alter the relative volatility of the
components in the original mixture.
In the conventional extractive distillation process, solvent is fed to the first column (extractive
column), above the azeotropic feed. Anhydrous ethanol is produced on the top of the extractive
column, while in the bottom a mixture containing solvent and water is obtained. The solvent is
recovered in a second column (recovery column), cooled and recycled to the extractive column
(Higler et al. 2004).
Conventional extractive distillation processes used in the industry for the separation of ethanol-
water mixtures use monoethyleneglycol (MEG) as a solvent in the distillation column, which is a
toxic component.
Energy consumption on column reboilers for the optimized configuration is 1057 kJ/kg of anhy-
drous ethanol, which is a reasonable value when compared to other distillation-based dehydra-
tion processes. Bioglycerol is not harmful to humans or the environment, and its availability has
increased in the last years because it is a byproduct of the biodiesel production process.
21.9.6 S
imulationS
for
i
incrEaSing
E
fficiEncy
of
E
xtraction
p
rocESSES
The need to decrease residues generation and the pursuit of cost reduction in bioethanol pro-
duction has motivated the investigation of more efficient processes to produce this biofuel. One
of the proposed options is the use of reaction-separation systems, such as the continuous flash
fermentation.
Cardona and Sanchez (2007) point out the reaction-separation integration is a particularly inter-
esting choice for the intensification of ethanol production. The removal of ethanol from the culture
broth diminishes the inhibition effect on the growth rate. The continuous extractive fermentation
has shown several advantages, such as low vinasse generation and fresh water consumption because
of the possibility of feeding molasses at higher concentrations, which reduce costs in waste treat-
ment, and the potentiality of eliminating one distillation column from the process. It is worthwhile
