Chemical Engineering October 2016 - 7

Chementator
Purify p-xylene without the heat
C
urrent commercial technologies
for separating and purifying p-xylene
- an important precursor for
polyesters and plastics - from hydrocarbon
mixtures involve phase-change
techniques that require large amounts
of thermal energy. Recently, a research
team from Georgia Institute of Technology
(Ga. Tech; Atlanta.; www.gatech.edu)
and Exxon Mobil Corp. (Irving, Tex.; www.
exxonmobil.com) demonstrated the separation
of p-xylene at room temperature using
organic-solvent reverse-osmosis (OSRO;
diagram). Since it requires no thermal input,
the OSRO method has the potential to significantly
reduce the amount of energy required
for p-xylene purification.
Although reverse osmosis has been used
for decades in water desalination, this is said
to be its first application for the separation of
hydrocarbon mixtures. The keystone of the
OSRO technology is the complex structure
of the membrane. First, a hollow-fiber membrane
(HFM) is constructed from a commercially
available polymer. Then, the HFM is
chemically modified with crosslinking molecules,
which protect the membrane's mechanical
properties. The fiber is next carbonized
using a pyrolysis step, which converts
the structure into a carbon molecular-sieve
HFM. The molecular sieve has large pores,
Apply pressure
Exxon Mobil
Edited by:
Gerald Ondrey
Carbon molecular sieve membrane
Hydrocarbon mixture
Para-xylene
building blocks
MAKING CO FROM CO2
Professor
Rhosuke Suwhich
provide mechanical integrity without
impeding mass transport. These larger pores
eventually terminate into a 30-nm membrane
layer with extremely small (less than 1 nm)
micropores. It is here, at the micropore level,
that the individual isomers of xylene can be
isolated. At the laboratory scale, using just a
single HFM, researchers enriched a hydrocarbon
stream to over 80% p-xylene.
The membrane's carbon-based structure
imbues stability under the high pressure
(approximately 125 bars) required for
reverse osmosis. Furthermore, the carbon
fibers are inert in the presence of xylene
mixtures and also allow for the pore sizes
to be precisely tuned for molecular selectivity.
Going forward, the team will continue to
add more fibers to test OSRO, and will also
seek to separate hydrocarbon streams of
varying purity.
This ceramic membrane converts natural gas to
liquid hydrocarbons
C
onverting natural gas to liquid hydrocarbons
can theoretically be accomplished
at high temperatures
with the help of zeolite catalysts,
but the reaction is hindered by two major
factors. The conversion to products is thermodynamically
limited, and coke formation
on the zeolite surface rapidly decreases catalyst
activity.
Now, technology involving a ceramicmembrane
reactor offers a pathway around
these obstacles. Along with scientists from
the University of Oslo and the Institute of
Chemical Technology in Valencia, Spain,
engineered ceramics maker CoorsTek Inc.
(Golden, Colo.; www.coorstek.com) has
developed a reactor that integrates an ionconducting
membrane to shift the thermodynamic
equilibrium of the reaction and drive
the process toward increased product formation
without generating CO2.
" The membrane is a proton-conducting
ceramic material with electrodes similar to
a solid-oxide fuel cell, " explains Per Vestre,
managing director of CoorsTek Membrane
Sciences. " It provides a means of removing
hydrogen from the reaction and thus
shifting
the
thermodynamic
equilibrium
toward formation of aromatic rings as reaction
products. " The CoorsTek reactor
is also designed to allow oxygen to be
injected across the membrane surface to
remove carbon deposits, thus preventing
coke buildup from killing catalyst activity,
Vestre says.
Heated natural gas flows into the reactor,
where it encounters shape-selective
zeolite catalysts. As methane molecules are
activated at catalyst active sites and products
begin to form, hydrogen is transported
across the solid ceramic membrane. Protons
recombine as H2 on the other side.
Copper electrodes on the reaction side of
the membrane and nickel electrodes on the
hydrogen-permeate side aid the hydrogen
transport process.
Further details of the process are described
in the August 5 issue of Science.
Note: For more information, circle the 56-digit number on p. 78, or use the website designation.
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
OCTOBER 2016
7
zuki at Hokkaido University
(Sapporo City, Japan; www.
eng.hokudai.ac.jp) has
developed a molten-salt
electrolysis process that
can produce CO from high
temperature CO2 present
in the fluegas of industrial
furnaces. The process uses
a molten salt containing
CaCl2-CaO melt as a media
and a solid-state electrolyte,
containing 8 mol% YZrO2 in
Y2O3, as the anode. Operating
at temperatures of 800
to 1,000°C, the electrolyzer
decomposes CO2 into CO,
with a conversion efficiency
of 88.2% (at 1,000°C). The
CO forms by the decomposition
of carbonate ions,
which coexist with the CaO
as CO2 is bubbled into the
melt. Suzuki believes the
process provides a way to
utilize the CO2 generated
from industries having hot
fluegases, such as steel and
cement
production, nonferrous
metal smelting and
waste incineration.
ELECTROLYSIS
Toshiba Corp. (Tokyo,
Japan; www.toshiba.co.jp)
has recently started up Japan's
largest alkaline waterelectrolysis
system,
which
produces approximately
100 Nm3/h of hydrogen -
sufficient for fueling two fuelcell-powered
cars. Conventional
water electrolysis uses
an acidic electrolyte and
requires precious metals for
the electrodes. With alkaline
electrolysis, less expensive
metal oxides can be used,
making it more economical
for scaleup to large systems.
Toshiba
had
previously
demonstrated and tested a
smaller version of the system
as part of the " Regional Cooperation
and Low-Carbon
Hydrogen Technology Demonstration
Project, " funded
(Continues on p. 8)

http://eng.hokudai.ac.jp http://www.gatech.edu http://www.exxonmobil.com http://www.toshiba.co.jp http://www.coorstek.com http://WWW.CHEMENGONLINE.COM

Chemical Engineering October 2016

Table of Contents for the Digital Edition of Chemical Engineering October 2016