Chemical Engineering September 2010 - 11

Edited by Gerald Ondrey
September 2010
Enzyme-based method could make
CO2 capture economically viable
C
odexis (Redwood City, Calif.; www.
codexis.com) and CO2 Solution (Quebec
City, Canada; www.co2solution.com)
have jointly developed a cost-effective
method for capturing carbon dioxide
from coal-fired power plants.
Solvent-based systems for capturing
CO2 are relatively well-understood,
but they have not been widely deployed
because they are energy-intensive and
inefficient. Solvent-based approaches
involve a trade-off between the kinetics
of CO2 absorption by the solvent and
the energy required to regenerate the
greenhouse gas for sequestration or further
use, explains Jim Lalonde, Codexis
vice president for biochemistry and engineering
R&D.
" You want something that binds CO2,
but not too tightly, " he says. For example,
the solvent monoethanolamine (MEA)
offers efficient CO2 capture, but requires
high energy to regenerate the gas.
The new process depends on two technologies:
new forms of the enzyme carbonic
anhydrase (CA) that are tolerant
of elevated temperatures; and carboncapture
solvents that require less energy
to regenerate CO2.
Codexis is using a proprietary genetic-screening
and directed-evolution
approach in developing an early protoCO2
absorber
column
Enzyme
Rich
solvent
Steam
Fluegas
with CO2
40-60°C
type of CA that is stable for 30 min at
temperatures of 85°C (natural, human
CA is unstable above 55°C). The enzyme
catalyzes CO2 absorption into solvents
whose properties allow CO2 regeneration
at lower energies. Without the enzyme,
these solvents, such as monomethyl-diethanolamine
(MDEA), would
have unfavorably slow reaction kinetics
for CO2 capture.
In modest quantities, the optimized
enzyme accelerates the rate of CO2
capture in MDEA by about 50 times
compared to the rate without enzyme.
Once the CO2 is absorbed, the energy
required to drive off the CO2 is 30% less
than the conventional solvent, because
An attractive way to remove arsenic from water
A
magnetic composite based on reduced graphene
oxide (RGO) has been developed by
a Korean team with an exceptional capacity
to remove arsenic from drinking water. The
team, headed by professor Kwang S. Kim
from the Center for Superfunctional Materials,
Dept. of Chemistry, Pohang University
of Science and Technology (www.postech.
ac.kr), South Korea, claims the composite
can achieve nearly complete removal (more
than 99.9%) of arsenic - within about 1 ppb
- from water. The magnetite-RGO (M-RGO)
composite is superparamagnetic at room
temperature. It soaks up arsenic when dispersed
in water and is then easily removed
from the water with a permanent magnet.
Several methods have been developed to
remove arsenic from drinking water. For example,
arsenic can be removed from drinking
Note: For more information, circle the 3-digit number
on p. 62, or use the website designation.
Higher T
of MDEA's sterically hindered molecular
structure. The Codexis/CO2 Solution
process is designed to reuse solvent,
and is compatible with multiple
carbon sequestration approaches.
Codexis is applying its directed-evolution
technology to further increase
the stability of CA by orders of magnitude
and, if successful, will scale up
the process to eventually capture tons
per day of CO2 by the end of the current
project, Lalonde says. The project is partially
supported by a $4.7 million grant
from the U.S. Dept. of Energy's (DOE;
Washington, D.C.; www.energy.gov) Advanced
Research Projects Agency-Energy
(ARPA-E).
water through coprecipitation of iron minerals
such as magnetite (Fe3O4) nanocrystals.
However, those adsorbents are difficult
to use in continuous flow systems, such as
rivers, due to small particle size and instability
- magnetite is highly susceptible to
oxidation when exposed to the atmosphere.
Therefore, the team decided to synthesize
M-RGO due to its large surface area and the
stability of the RGO.
The team synthesized graphene oxide via
Hummer's method. The graphene oxide was
exfoliated in water to produce a suspension
of graphene oxide sheets. An aqueous mixture
of FeCl3 and FeCl2 was added slowly
to the solution, and ammonia solution was
added quickly to precipitate Fe+2 and Fe+3
ions for synthesis of magnetite nanopar(Continues
on p. 12)
World's largest PDH unit
lummus technology (bloomield,
n.J.), a Cb&i company
(the woodlands, tex.; www.
cbi.com), has been awarded a
contract by tianjin bohua petrochemical
Co. for the license and
engineering design of a grassroots
propane dehydrogenation
(pDh) unit to be built in tianjin,
China. it will be the irst pDh
plant in China, and upon startup
in 2012, will be the largest pDh
plant in the world, says Cb&i.
the unit will use the Catoin
dehydrogenation process to
produce 600,000 metric tons
per year of propylene. (for more
on Catoin and pDh, see pushing
propylene production, CE,
march 2004, pp. 20-24)
ChemiCal engineering www.Che.Com September 2010 11
CO2
stripper
column
Enzyme
Scrubbed
fluegas
Lean solvent
Pure CO2
(> 90%) for
compression
and storage
http://www.codexis.com http://www.co2solution.com http://www.energy.gov http://www.cbi.com http://www.postech http://www.ac.kr http://www.Che.Com
Chemical Engineering September 2010

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