05 | 2010

Ethanol
Evaporation
Steam
Furnace
Water
Raw
ethylene
Cruede
ethylene
Aqueous
NaOH
Chemical
grade
ethylene
Light
contaminants
Polymer
Grade
Ethylene
REACTION
Aqueous
effluent
QUENCH
Caustic
effluent
SCRUBBING
DRYING
Heavy
contaminants
DISTILLATION AND
STRIPPING
Representation of a generic process diagram of an ethanol-based ethylene plant.
way, starch is obtained from corn by dry milling, then slurried
with water, and hydrolyzed to glucose. The resulting solution
of fermentable sugars obtained by both ways is fermented
typically in batch or fed-batch by Saccharomyces cerevisiae
yeast to produce a broth with 6 to 8% by weight of ethanol. The
fermentation of the sugarcane juice is quite simple, because
it can be fermented directly, and faster, taking in general less
than 16 hours.
By distilling the broth containing ethanol hydrated ethanol,
about 93% by weight, is produced. The stillage, the bottom
by-product stream of the distillation, is rich in nitrogen and
potassium and is commonly recycled to the sugarcane crop
by a practice called ferti-irrigation.
Energy for the process
A large amount of lignocellulosic material is also produced
from the sugarcane feedstock. For an average yield of 80–
85 metric tons per hectare and 14% by weight of sugars, it
produces, and in addition, 28% by weight of dry lignocelluloses
fibers as bagasse and leaves. These fibers can be used to
supply renewable heat and electricity to the ethanol process.
Cosmetic Bottle (Courtesy Braskem)
Its surplus of about 20–40% is used normally to co-generate
renewable electricity to the grid and may also be used in other
processes when integrated with the ethanol manufacture. If
in the future the hydrolysis of hemicelluloses and celluloses
would be economically competitive these fibers may be used
as an additional source of sugars.
As a consequence of these many aspects the energy
balance of the sugarcane based ethanol is very favorable.
This number is obtained dividing the fossil fuel energy input
required by the entire manufacturing process, since the crop
plantation, by the energy content of the biofuel output. For
the Brazilian sugarcane ethanol the input/output energy
balance is 1:9, while for the US corn ethanol this relationship
is 1:1.5.
Ethanol to Ethylene
To generate ethylene from ethanol, you simple need to take
the water out (dehydration).
C 2
H 5
OH → C 2
H 4
+ H 2
O
Well, in real life, it is not that simple.
The dehydration of alcohols, mainly ethanol, has been
studied during the last centuries with different technologies
and using a large variety of catalysts such as alumina, silica,
silica-alumina, zeolites, clays, metal oxides, phosphoric acid,
and phosphates.
While older technologies were based on supported
phosphoric acid, later activated alumina became predominant
as a catalyst.
The dehydration reaction is endothermic which means
that energy has to be put into the process. The most
accepted mechanism for the ethanol dehydration considers
a simultaneous reaction:
2 CH 3
CH 2
OH → CH 3
CH 2
OCH 2
CH 3
+ H 2
O → 2 H 2
C=CH 2
+ H 2
O
Ethanol Ether Ethylene Water
2 CH 3
CH 2
OH ─────────────── → 2 H 2
C=CH 2
+ 2 H 2
O
Ethanol Ethylene Water
Diethyl ether is considered an intermediate and not a
byproduct. Its formation is favored mainly between 150°C
and 300°C, while ethylene formation is predominant between
320°C and 500°C.
54 bioplastics MAGAZINE [05/10] Vol. 5