Hydrocarbon Processing - January 2021 - 36

Sustainability
(end) impurities are removed by distillation columns C2, C5 and C7.
The main goals in the purification of
ethanol are to obtain a product with a min-

imum impurities content and to maximize
product yield. In addition, energy savings
can be achieved by operating the columns
at different pressures; for example, the va-

FIG. 6. Flowsheet of an ethanol production plant.

FIG. 7. XY diagrams for ethanol-water separation for classic (left) and cyclic (right) distillation.

FIG. 8. Ethanol-water separation efficiency in a cyclic distillation column (perfect displacement
mode) vs. classic distillation (perfect mixing mode).

36 JANUARY 2021 | HydrocarbonProcessing.com

por from the beer column is used to heat
the hydro-selection and the methanol columns. All these distillation columns can
be replaced with cyclic distillation units.
However, for the production of food-grade
ethanol, column C3 remains due to insufficient knowledge about the actual distribution of impurities in this multi-feed, multiproduct column in cyclic operation mode.
FIG. 7 illustrates the difference in the
shape of the XY diagrams for the ethanolwater mixture for classic distillation and
cyclic distillation. The operating lines
for cyclic distillation allow a much lower
number of theoretical stages due to the
larger driving force of the process.
FIG. 8 (left) illustrates the separation
efficiency in a cyclic distillation column
(perfect displacement model) vs. classic
distillation (perfect mixing mode) for
an ethanol-water mixture.14 At minimum
reflux ratio operation, fewer trays are required for cyclic distillation as compared
to classic distillation. Similarly, a lower
reflux ratio is needed for a cyclic distillation operation as compared to classic
distillation, for the same number of trays
used. FIG. 8 (right) shows the difference
between the cyclic operation in a pilot
column working in parallel to an existing
industrial column, thereby performing
the same separation task.14 Note the large
steam usage (per liter of ethanol product),
which translates into 30% energy savings
in the case of cyclic distillation.
FIG. 9 illustrates the quantitative effect
of applying the cyclic operation mode to
ethanol-water mixtures. The optimal reflux ratio (corresponding to the minimum
annualized cost) shifts to the left (lower
values), thereby allowing 25% lower energy use as compared to classic distillation.
The reflux ratio and the number of trays
can be used to achieve various targets-
e.g., at reflux ratio 4, the energy savings
are 0% but 2.5 times fewer trays are required; at reflux ratio 3.5, the energy savings are 10% and 2.3 times fewer trays are
required; and at reflux ratio 2.6, the steam
savings are 30% and require 1.5 times fewer trays as compared to classic distillation.
Other industrial applications. Cyclic distillation also offers new opportunities by applying the same principles
of the cyclic operation mode to other
intensified processes, such as catalytic
distillation (CD)16 or even dividing-wall
column (DWC). FIG. 10 illustrates cyclic
distillation DWCs with a coaxial parti-

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Hydrocarbon Processing - January 2021

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