Cellular honeycomb ceramics have been used in a
variety of applications, including support materials for catalytic conversion,
adsorptive separations, membrane supports, cross-flow filtration and fluid
filtration. The honeycomb design offers a remarkably high geometric surface
area per unit volume and improves the chemical residence times for reactions to
reach completion in high-flow conditions. Extrusion is a well-understood and
highly scalable ceramic forming process, and extruded cellular honeycomb
monoliths provide unmatched structural
rigidity and robustness.
Figure 1. In the flow-through configuration,
cross-flow through the walls is extremely limited.
Existing Systems Face Limitations
One of the best-known uses of honeycomb materials is in
gasoline automobile catalytic converters. Since the late 1970s, every car on
the road in the U.S.
has been equipped with a honeycomb catalytic converter under the chassis. These
converters provide a complex chemical reactor to reduce emissions of regulated
gas species from gasoline engines. Ceramic flow-through honeycombs have been
used in converters because of their high mechanical and chemical durability
(see Figure 1). Typically, these ceramic honeycombs are made by extruding
powder-based materials like alumina, mullite and silicon carbide. The most
common material used in these applications is cordierite, which features a low
coefficient of thermal expansion (CTE).
The cellular geometry of the honeycomb provides a high
surface area per unit volume for deposition of catalysts or absorptive
materials. For example, in the case of a gasoline-powered car, a layer of
high-surface-area alumina washcoat is placed on the wall, and precious metal
catalysts are dispersed finely on the washcoat bed. At high temperatures, the
reactions occur quickly but are typically mass-flow limited in a laminar-flow
regime. In the flow-through configuration, cross-flow through the walls is
Figure 2. Alternate channels are blocked in the
wall-flow design, leading to turbulent flow and forced convection of fluid
through the permeable walls of the honeycomb.
While the flow-through configuration of honeycomb
ceramics works well for laminar-flow catalytic conversion, its utility is
limited in filtration. Unlike gasoline engines, diesel engines emit gas- and
condensed-phase toxic emissions. When these engines became popular for use in
the European light-duty market, the emission control industry recognized a need
for soot nano-particle filtration functionality. As a result, the wall-flow
design was added to increase filtration functionality while retaining the
benefits of the cellular honeycomb structure (see Figure 2).
Alternate channels are blocked in the wall-flow design,
leading to turbulent flow and forced convection of fluid through the permeable
walls of the honeycomb. As the fluid is forced through the wall, filtration of
suspended particulate matter takes place while fluid passes through
unobstructed. Depending on the size of the particles, diffusion, inertial and
interception mechanisms lead to the entrapment of suspended particles on the
In order to create a gas-permeable wall microstructure
out of the standard flow-through honeycomb material, the internal wall porosity
of powder-based chemistries was increased. Since then, however, the
microstructure has been deemed ill-suited for such applications. Unfortunately,
the obstruction to flow is relatively high in powder-based honeycomb filters,
which leads to backpressure and poor filtration efficiency.
Studies of this obstruction of flow affirm that
powder-based media, however porous, are not ideal for the filtration of
particles in the nanometer and micrometer size ranges. Advanced filters in
applications as diverse as high-efficiency particulate air (HEPA) filtration
and air-oil separation have typically relied on fiber-based filtration media.
The cross-linked microstructure formed by intersecting and overlapping fibers
improves filtration performance without causing significant backpressure.
However, until now, this desired microstructure was not available in a robust
and durable extruded honeycomb form. Pleated and rolled fiber-paper sheets do
not allow for robust filters, and they often exhibit a telescoping failure
mode, among other problems.
Figure 3. A top view of two parallel channels in
an extruded porous honeycomb with the patented cross-linked
Developing a New Alternative
New extruded ceramic honeycombs* have been developed with
a unique cross-linked pore microstructure in multiple chemistries. The
resulting honeycombs have a high cell density, which leads to high geometric
surface area. In addition, high permeability of the wall microstructure results
in low filtration backpressure, and the interconnected pore architecture enables
high filtration efficiency.
Depending on the fabrication parameters, filters can
capitalize on diffusion flow to achieve extremely high nano-particle filtration
efficiencies. In addition, benefits such as structural rigidity, robustness and
low cost of production are also achieved. By altering the fiber and bonding
composition chemistry and processing conditions, high-porosity cellular
honeycombs with mullite, silicon carbide, cordierite, alumina and steel alloys
have been developed.
Figure 3 shows a top view of two parallel channels in an
extruded porous honeycomb with the patented cross-linked microstructure. The
filtration media in the cellular honeycomb closely resembles the kind of
surface texture and structure expected in the typical fiber-based membranes or
candle-type tube and bag filters-but in an extruded, monolithic honeycomb
geometry. The requirements and processing conditions for mixing, extruding,
drying and firing the materials into numerous desired honeycomb configurations
have been determined. Furthermore, based on application needs, similar internal
cross-linked wall microstructures can be created in several chemistries by
altering material and processing parameters.
*Developed and patented by GEO2
Figure 4. The size-specific filtration efficiency of a
wall-flow filter located downstream of a soot generator source.
The patented, cross-linked microstructure of the new
honeycomb substrates and processes provide several advantages over conventional
powder-based ceramics. The fiber-based structure exhibits a mechanical strength
similar to low-porosity, powder-based filters (~ 45% porous), but at a porosity
of greater than 65%. Depending on the pore size distribution, > 99%
filtration efficiencies for nano-particles (10-470 nm) have been observed.
For example, Figure 4 shows the size-specific filtration
efficiency of a wall-flow filter measured using a scanning mobility particle
sizer (SMPS, 135-second scans across the diameter range) downstream of a soot
generator source. The filter had an average pore size of 15 µm with a narrow
distribution curve. As can be seen, not only is the initial particle filtration
efficiency very high, but the filtration efficiency also continues to improve
as a fine layer of soot cake builds up on the filter.
Figure 5. In wall-flow honeycombs for use in turbulent
flow fixed-bed chemical conversion reactors, the catalytic material can be
dispersed throughout the internal porosity of the walls.
The internal pore microstructure of the honeycombs
provides additional "open" and "available" porosity for the
placement of reactive or catalytic species in the pore volume. For example, in
the case of wall-flow honeycombs for use in turbulent flow fixed-bed chemical
conversion reactors, the catalytic material can be dispersed throughout the
internal porosity of the walls (see Figure 5). This is achieved without causing
significant obstruction to the flow of fluids through the wall.
This scenario has been tested in the case of automotive
emissions control, where low backpressures were measured on a filter and
compared to a standard particle-based cordierite filter. Despite having five
times the typical washcoat loading, the filter exhibited lower backpressure and
higher catalyst efficiency than the standard filters.
In the case of a wall-flow configuration, fluids such as
gases from engine exhaust are forced through the wall structure where they come
into direct contact with the residing catalyst. Instead of the gas-phase
reactions being rate-limited by mass transfer in the channels, as is the case
in flow-through honeycombs, the reactions become pore-diffusion limited, which is
much more effective. The induced turbulence leads to an overall increase in
reaction rates and better utilization of catalyst loadings.
Figure 6. Honeycomb structures with a highly
permeable, cross-linked pore microstructure can be used as membrane support
structures for adsorptive reaction, catalytic conversion or cross-flow filtration-type applications.
Finally, honeycomb structures with a highly
permeable, cross-linked pore microstructure can also be used as membrane
support structures for adsorptive reaction, catalytic conversion or cross-flow
filtration-type applications (see Figure 6). Membranes, including zeolite-based
or polymeric types, placed as a layer on top of the honeycomb structures can
act as the active layers while the honeycomb provides a robust and permeable
structure for high-flow-rate and otherwise
harsh operating conditions.
Additional Applications on the Horizon
The new honeycomb structures in wall-flow and
flow-through configurations are already in use for several applications,
including diesel particulate filters, air-oil separators, selective absorption
of contaminants in gases and multi-functional catalytic conversion. Additional
applications are planned in a wide variety of areas, including gas separations,
alcohol-water separations, water filtration
For more information
regarding cellular honeycomb ceramics, contact GEO2 Technologies Inc.,
12-R Cabot Rd., Woburn, MA 01801; (781) 569-0559; fax (781) 721-6301; e-mail firstname.lastname@example.org; or visit www.geo2tech.com.