Environmental Engineering Reference
In-Depth Information
For this reason, simulation of conventional distillation process was carried out in Aspen Plus
®
considering nonequilibrium stage model. In this model, conservation equations are written for each
phase independently and solved together with transport equations that describe mass and energy
transfers in multicomponent mixtures; also it is assumed that equilibrium occurs only in the vapor-
liquid interface. Besides, in this way, empirical correcting factors, such as efficiencies used in the
equilibrium model, are no longer necessary (Pescarini et al. 1996).
Results of the simulation considering nonequilibrium stage model were compared to those
obtained using equilibrium stage model with constant plate efficiencies of 55, 70, 85, and 100%.
It was observed that, to obtain hydrous ethanol (93%), the idealized equilibrium stage model (effi-
ciency of 100%) predicts an energy requirement that corresponds to 80% of that given by non-
equilibrium stage model (~7,000 kJ/kg of hydrous ethanol). In addition, simulations showed that
equilibrium stage model with an efficiency of 70% provide results quite similar to nonequilibrium
stage model for the conventional distillation process.
21.9.4.2 multiple effect distillation
An alternative to the conventional distillation process is the multiple effect operation of the distilla-
tion columns. The operation in different pressure levels gives rise to different temperature levels on
condensers and reboilers of the different columns, thus it is possible to integrate the equipment and
reduce steam consumption on reboilers.
To optimize bioethanol production, simulations using Aspen Plus were carried out with five
different configurations: conventional fermentation and distillation (CFCD), vacuum extractive fer-
mentation and conventional distillation (VFCD), vacuum extractive fermentation and conventional
distillation (VFCD), vacuum extractive fermentation and double effect distillation (VFDD), and
vacuum extractive fermentation and triple effect distillation (VFTD).
Vacuum extractive fermentation consists of a fermentation reactor coupled to a vacuum flash
evaporator, which allows ethanol produced to be simultaneously removed from the reactor. In this
study, ethanol concentration in the reactor was kept at low levels (around 8 wt % ethanol) whereas
in the flash chamber, wine with 36 wt % ethanol was obtained.
The double effect configuration was similar to the conventional configuration, however, the dis-
tillation columns operate under vacuum (19-25 kPa), while rectification columns operate under
atmospheric pressure (101-135 kPa). In this way, different temperature levels are observed between
columns “A” reboiler and “B” condenser (65 and 78ºC, respectively), allowing thermal integration
of these equipments and consequently reducing energy consumption on the distillation stage.
In the triple effect configuration, the distillation columns operate under vacuum (19 - 25 kPa),
and the liquid phlegm stream produced on column D is split in two: one of them is fed to a recti-
fication column operating under nearly atmospheric pressure (column “B,” 70 - 80 kPa) and the
other is fed to a rectification column which operates under relatively high pressure (column “B-P,”
240 - 250 kPa).
Regarding thermal energy, results showed that the configuration that presents the lowest energy
demand is the triple effect configuration (VFTD), providing a reduction in energy consumption of
44 % when compared to the VFCD process and 77 %, when compared to the CFCD process, which
is the configuration most commonly used in Brazilian biorefineries.
21.9.5 S
imulationS
for
i
incrEaSing
E
fficiEncy
of
a
nhydrouS
E
thanol
p
roduction
Anhydrous bioethanol, suitable to be used as a gasoline additive or as raw material for produc-
tion of different renewable materials, must contain at least 99.3 wt % ethanol. Because water and
ethanol form an azeotrope with 95.6 wt % ethanol at 1 atm, alternative separation processes are
necessary to produce anhydrous bioethanol. The most used processes in Brazilian biorefineries
are azeotropic distillation with cyclohexane and extractive distillation with monoethyleneglycol
(MEG).
