Chemical Engineering November 2013 - 26

Ethanol from Woodchips
By Intratec Solutions
E
thanol is widely produced by fermentation
of sugars from agriculture feedstocks,
such as corn and sugarcane. In order to
meet growing energy requirements, several
lines of research have focused on cellulosic
ethanol production, where some type of lignocellulosic
biomass, such as woodchips,
sugarcane bagasse, corn stover and others,
serve as the feedstock for the process.
Use of these raw materials is justified by their
low cost, ready availability and status as generally
unused residues. Also, such waste biomass
feedstocks do not interfere or compete with the
production of food from crop feedstocks.
The process
The production process for ethanol from woodchips
shown in Figure 1 is similar to the Green
Power+ process described in the U.S. patent
published by American Process Inc. (Atlanta,
Ga.; www.americanprocess.com; U.S. Patent
195,468; 2011). The major process steps are
described in the diagram below.
Pretreatment. Wood chips are pumped with
hot water into the pretreatment reactor, where
hemicelluloses are separated from lignin and
cellulose. The treated mixture is washed with
recycled filtrate. The hemicellulose extract is
sent to the first evaporation step, where water
is evaporated. Lignin and cellulose are burned
to generate steam and electricity to supply
plant demands.
Hydrolysis. The hemicelluloses undergo acid
hydrolysis and are converted to monomer sugars,
such as xylose, using dilute sulfuric acid.
Then, lime is used to increase pH. The diluted
sugar mixture passes through the second
evaporation step.
Potassium acetate production. Acetic acid is
released with water in both evaporation steps.
Potassium hydroxide is added to the evaporation
condensate to form potassium acetate.
The salt is separated in a membrane unit and
concentrated to 50% to be sold as a de-icer.
Fermentation. The monomer sugars are
fermented to ethanol, and gases released from
fermentation are washed with recycle water in
1
Wood
Hot water
Key research features
* According to American
Process, the approach of
selectively converting hemicelluloses
to monomer sugars leads to a greater
effective ethanol yield when compared to
processes converting both cellulose and
hemicelluloses
* It is possible to obtain valuable co-products,
including potassium acetate de-icer, from
acetic acid released from hemicelluloses during
the pretreatment and hydrolysis steps
Economic performance
An economic evaluation of the process was
conducted based on data from the fourth quarter
of 2012. The following assumptions were
taken into consideration:
* A wood processing unit producing 40 million
gal of anhydrous ethanol and 40 million gal
of potassium acetate de-icer per year built
on the U.S. Gulf Coast (process equipment
are represented in the simplified flowsheet in
Figure 1)
* The unit operates 300 days per year
* Storage of products is equal to 30 days
of operation, and there is no storage for
feedstock
* Surplus electricity generated in the plant is
sold to the electric power grid
The estimated total fixed investment for the
ST
Lignin
and
cellulose
14
Electricity
Vent
gases
7
10
2
Lime
Sulfuric
acid
3
4
Gypsum
to waste
treatment
Potassium
hydroxide
8
9
FigurE 1. Cellulosic ethanol production process similar to American Process' Green Power+
6
5
CW
Potassium
acetate
deicer
CW
13
CW Cooling water
ST Steam
To waste
treatment
ST
To waste
treatment
ST
11
Ethanol
CW
12
CW
construction of this plant would be between
$350 and $450 million.
Research perspective
Currently, there is great interest in developing
cellulosic ethanol production processes.
Several configurations are being studied.
The main differences among them are the
pretreatment and hydrolysis steps. The aim is
to maximize fermentable sugar yield in order
to enhance process economic competitiveness.
Figure 2 shows the distribution of some cellulosic
ethanol plants in U.S. that use diverse
ethanol production technologies.
Other signs of the attention directed to cellulosic
ethanol are the loan guarantees recently
provided by the U.S. government, for the
construction of commercial-scale facilities. ■
Editor's Note: The content for this column is supplied
by Intratec Solutions LLC (Houston; www.intratec.us)
and edited by Chemical Engineering. The analyses
and models presented herein are prepared on the
basis of publicly available and non-confidential information.
The information and analysis are the opinions
of Intratec and do not represent the point of view of
any third parties. More information about the methodology
for preparing this type of analysis can be found,
along with terms of use, at www.intratec.us/che.
1. Pretreatment reactor
2. First evaporation step
3. Hydrolysis reactor
4. Mixing tank
5. Second evaporation step
6. Fermenters
7. Gas washing column
8. Membrane unit
9. Evaporator
10. Concentration column
11. Rectifying column
12. Molecular sieve unit
13. Cooling tower
14. Energy cogeneration
unit
a gas washing column.
Distillation. In the concentration
column, the broth is concentrated
to about 50% ethanol in
a vapor sidestream, and gases
are released in the overhead
to the gas-washing column. In
the rectifying column, a 90%
ethanol vapor stream is generated
and the bottoms' recycle
water stream is used in the gaswashing
column. The ethanol
is dehydrated in a molecular
sieve unit, leading to an ethanol
product with 99.5% purity.
Up to 10 million gal/yr
Existing plants
Proposed plants
Up to 10 million gal/yr
From 11 to 20 million gal/yr
At least 21 million gal/yr
FigurE 2. Each mark in
the map corresponds to
an existing or proposed
cellulosic ethanol plant.
The nominal capacity of
each plant follows the
legend to the left:
http://www.americanprocess.com http://www.intratec.us http://www.intratec.us/che
Chemical Engineering November 2013

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