Process Intensification

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Honey, I shrunk the hardware
Microchannel reactors for Fischer-Tropsch processes can lead to mega
benefits for biofuels production, say Derek Atkinson and Jeff McDaniel
OIL and liquid hydrocarbon fuels still also be used to produce liquid fuel from conventional BTL plants would need to
provide the mainstay of today’s energy biomass (via biomass to liquid, BTL). process biomass feedstocks of
economy. But change is in the air. World 10,000 t/d in order to produce 10,000 bbl/
demand for petroleum is continually
sustainable biofuels d of fuel, small microchannel FT reactors
increasing, while world oil production Biofuels have attracted much interest as can operate economically when processing
has plateaued. Along with factors such as an environmentally-friendly substitute for just 500–2000 t/d of waste.
high oil prices, fears about fuel security petroleum-based transport fuels. Biofuels
The secret of their success is down to
and concerns about global climate change based on vegetable oils will probably be
the fact that they offer a way to reduce the
are encouraging the development of the major technology for meeting the EU
size and cost of the chemical processing
alternative fuels. This, in turn, is sparking goal of having 5.75% in fuels by 2012.
hardware, while still enabling efficient
a revival of interest in the Fischer-Tropsch But to meet the goal of 10% biofuels
and precise temperature control, leading
(FT) reaction, a process first developed by by 2020 it will be necessary to look for
to higher throughput and conversion.
Franz Fischer and Hans Tropsch in Germany alternative methods for producing biofuels
Like the microelectronics technology that
in the 1920s and 1930s to produce liquid from different types of biomass – not
revolutionised the computer industry,
fuel from coal. least because existing first-generation
microchannel technology shrinks
In the FT process synthesis gas, biodiesel uses food crops as a feedstock.
processing hardware, while at the same
composed of a mixture of carbon There are concerns that this could lead to
time improving its performance.
monoxide (CO) and hydrogen (H2), is food shortages and promote destruction
For a start, plant size is small:
converted into various forms of liquid of the rain forests. In contrast, second-
microchannel reactor assemblies have
hydrocarbons using a catalyst at elevated generation biodiesel produced via FT will
diameters of around just 1.5 m. Capital
temperatures and pressures. Because rely on waste feedstocks, such as potato
costs are relatively low compared to
they don’t contain aromatics or sulphur- waste, animal waste, corn stover and
conventional reactor systems such as slurry
containing contaminants, the liquid fuels ligno-cellulose waste from trees.
beds, and the reactors can be operated
produced are typically of higher quality Although the BTL technology needed
with a minimum of staff. The microchannel
and burn cleaner than petroleum-based to produce second-generation biofuels
FT reactor design is also very flexible.
diesel and jet fuels, resulting in lower exists, the FT processes used to produce
The basic building blocks consist of
emissions of NOx and harmful particulates. it need to be optimised to make them
components with parallel microchannels,
In theory, any source of carbon can be economic. Because biomass isn’t very
which are arrays of channels with
used to generate the synthesis gas. The dense – as a rule of thumb it takes one
diameters in the 0.1–5.0 mm range (see
FT reaction is already used to produce ton of biomass to produce one barrel of
Figure 1). Their modular structure means
liquid fuels from natural gas (via gas-to- liquid fuel – it can’t be transported over
that maintenance and catalyst replacement
liquid, GTL) on a large scale in Qatar, and long distances to production facilities.
can be carried out by replacing individual
is widely used to generate liquid fuels This means that the facilities must be
modules, rather than requiring the
from coal (via coal to liquid, CTL). It can relatively small, producing around
prolonged shutdown of the entire system.
500–2000 bbl/d, compared to 30,000–
A great advantage of microchannel
140,000 bbl/d for a GTL plant. This
reactors is their capability to handle huge
depends on developing better catalysts,
volumes of feedstock and their ability to
intensifying the BTL process, and
produce high quality, energy-dense fuel
developing small-scale FT reactors. We
from a wide variety of resources, including
believe new reactor designs combined
waste wood, energy crops and municipal
with more efficient, optimsed FT catalysts
solid waste.
are the key to successful BTL process
intensification. To achieve this, reactor In terms of productivity – defined as
designers and catalysts developers must bbl/d of FT product per ton of reactor mass
work closely together. (bbl/d/t) – microchannel FT reactors far
Figure 1: In a microchannel reactor, a single reactor module outstrip their conventional cousins. For
consists of many hundreds of rows of microchannels each microchannel reactors: example, Velocys’ microchannel FT reactor
containing large numbers of parallel microchannels. The small and perfectly formed assembly, which has an output of
orientation and size of the channels within each row is Microchannel reactors are potentially 360 bbl/d, exhibits reactor productivities
determined by the application, adjacent rows of channels the best candidates for producing in the range of 12 bbl/d/t. In contrast,
potentially having very different duties. (Courtesy of Velocys) second-generation biofuels. While large Shell’s Bintulu and Pearl GTL FT reactors
42 november 2008 www.tcetoday.com
microreactors_809_to author.indd34 34 17/11/08 16:33:06

2. process intensification
with outputs ranging from 3500–6000 (see Figure 2). This stability means that the distributed production of second-
bbl/d have reactor productivities that that both microchannel reactors and generation biofuels becomes both a viable
range from around 3–5 bbl/d/t, and conventional systems will be able to economic reality and a practical way to
Sasol/QP’s Oryx FT plant has a reactor operate for longer without resorting to reduce carbon emissions, much sooner.
with an output of more than 12,000 elevated temperatures – which accelerate
bbl/d, with a productivity of around 8 the decay in catalyst activity – in order
bbl/d/t. to maintain productivity. The key to the
improved performance of Oxford Catalysts’
heat handling FT catalyst lies in a new patented catalyst
This sterling performance is largely down preparation method, known as organic
to the process-intensified design, which matrix preparation, OMX (see box).
results in massively enhanced heat and
mass transfer capabilities (see Table 1). closer than you think
Conventional reactor systems rely on the Although the FT reaction has been around
use of massive hardware to manage the for many years, there are just seven
heat in FT reactions and have relatively FT plants in operation worldwide, and
small heat transfer areas per volume of these are used for producing liquid fuels,
catalyst. In contrast, in microchannel lubricant feedstocks and industrial waxes
reactors, each reactor block has from coal or gas on a large scale. Use of
thousands of thin process channels filled FT to produce second-generation biofuels,
with FT catalyst which are interleaved economically, and on a small-distributed Figure 2: Comparison of deactivation rates between
with water-filled coolant channels. As a scale, presents new challenges. Some Oxford Catalysts new FT catalyst and a catalyst used in the
result they are able to dissipate the heat experts believe that we may have to wait Syntroleum FT process (data on this catalyst supplied by the FT
produced from the exothermic FT reaction as long as 5–10 years before commercial process licensor, Syntroleum)
much more quickly than conventional production of second-generation biofuels
systems. becomes viable. Microchannel Conventional
This makes them ideally suited for From both the environmental and Heat transfer (W/cm2)
carrying out both highly exothermic commercial perspective, existing BTL
Convective 1–20 <1
catalytic reactions, such as FT synthesis, processes are not suitable for the
and highly endothermic reactions, such as production of second-generation biofuels. Boiling 1–20 <1
methane reforming, in which heat must However, we believe that by working Mass transfer 0.001–0.3 1–10
be efficiently transferred across reactor closely together to optimise and intensify (contact time in seconds)
walls in order to maintain an optimal the FT process, catalyst developers and
and uniform temperature to maximise microreactor designers could ensure Table1: Comparison of heat and mass transfer capabilities
the catalyst activity and prolong catalyst
life. This allows microchannel reactors Transmission electron microscope picture
to operate at much higher conversions. of the FT catalyst produced using the OMX
Microchannel reactors exhibit conversion method (photo courtesy of Oxford Catalysts)
efficiencies in the range of 70% per pass.
By comparison, conventional reactor
systems typically operate at conversion The OMX method
efficiencies of only 50% per pass. The level of catalyst activity is related to its
surface area, which is related to crystal size, so
catalysts:
producing catalysts with the optimal crystal size
improved and optimised
for a given application is a key goal for catalyst
Taking advantage of these high
developers. The big challenge lies in achieving the
conversion efficiencies requires the
right balance between catalyst activity and stability. If the crystal size is too
right FT catalyst for the job. In order to
large, the catalyst activity – and hence, conversion rates – will be reduced. If
boost conversion rates to an economic
too small, the catalyst becomes unstable. The aim is always to produce a catalyst
level, microchannel reactors require an crystal size that is just right.
FT catalyst with an exceptional level of
The OMX method combines the metal salt and an organic component to make a Derek Atkinson
activity. A new FT catalyst developed for
complex that effectively stabilises the metal. On calcination, combustion occurs (Derek.Atkinson@
this purpose by Oxford Catalysts allows
that fixes the crystallites at this very small size. Since the calcination is quick, oxfordcatalysts.
operators of microchannel reactors
the metal crystallites do not have time to grow, and hence remain at the ideal com) is business
to achieve productivities (defined as
size. This is important because the improvements in catalyst performance are development
kg/m3/h) that are orders of magnitude
down to the fact that the OMX method produces crystallites in the director responsible
higher than for conventional systems. For
8–15 nm range that exhibit a terraced surface (pictured). These are both features for the petroleum
comparison, fixed-bed reactors typically
that enhance catalyst activity. OMX also produces fewer very small crystallites and petrochemical
operate at catalyst productivities of
that could sinter at an early stage of operation. This results in greater catalyst markets at Oxford
100 kg/m3/h, while slurry-bed reactors stability. Less stable crystallites tend to deactivate quickly, reducing the activity Catalysts (www.
operate at productivities of around of the catalysts. oxfordcatalysts.
200 kg/m3/h. In contrast, the use of the
Aside from their higher activity, the FT catalysts produced using OMX have a com); Jeff McDaniel
new catalyst makes it possible to achieve
longer life, and the need for precious metal promoters on the catalysts can be (mcdaniel@velocys.
productivities of over 1500 kg/m3/h.
reduced, or in some cases, eliminated (see for example, tce March 2008, pp46– com) is business
The same catalyst will also be of 47), while still retaining or even exceeding the benefits of traditional catalysts. development
great benefit for use in conventional Oxford Catalysts are now working to scale up the OMX process to make it possible director for Velocys
FT systems, since another key feature to supply formed catalysts in commercial quantities. (www.velocys.com)
of the catalyst is exceptional stability
www.tcetoday.com november 2008 43
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