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| Publications/ArchivesPictures
of the Future Fall 2002 |
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INDUSTRY –
Miniaturization
Lilliputian
Factories—The Micro Revolution in the Chemical Industry
The chemical industry is under
pressure to become more flexible and to bring its products to
market more quickly. But with huge plants, that's simply not
possible—which is why companies are turning to microreaction
technology. Siemens is participating in a research project
designed to explore the industrial suitability of tiny
reactors.
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| Hardly larger than an ant, part of a miniaturized
pressure sensor from Siemens allows forces to be
transmitted to a membrane via the round surface
(center). The contact points for a temperature sensor
are located at T-shaped indentations |
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On Dr. Arno Steckenborn's
fingertip is a square, glittering object that resembles a
computer chip. The structures on its silicon surface are
hardly recognizable. "What we have here," says Steckenborn, a
physicist with Siemens Corporate Technology in Berlin, "is a
pressure and temperature sensor." The modest-looking device is
a key component of a research project that could signify a
radical change in chemical and pharmaceutical production.
In the past, calculations were made according to a
simple rule of thumb: the bigger the better. As a consequence,
even large investments in technology represented a relatively
small share of total costs. But this strategy is starting to
show signs of strain. If product demand weakens or dries up,
the manufacturer is left with an expensive, large-scale
installation that is prohibitively expensive to convert. One
solution is a miniplant that is easy to realize and can be
augmented as needed by any number of additional miniplants.
Here, investment costs fall, and there is no need to spend
time scaling up from laboratory to industrial production.
"Miniaturization would also have other advantages for
the chemical industry," says Inga Leipprand of Siemens Axiva,
a company that specializes in plant construction and process
engineering. Reactions on a miniature scale can be realized in
modular units—in other words, very flexibly. Moreover, they
take place continuously and without time-consuming overhauls
or boiler cleaning—features that represent another cost
benefit compared with today's installations.
Many
chemical reactions involve significant danger. However, it is
the quantity of chemicals used that ultimately determines how
much heat develops or whether a process could lead to an
explosion. In contrast, the heat given off by a process can
easily be dissipated in a reactor whose conduits are as narrow
as threads. That's because small volumes are associated with
relatively large surfaces. Furthermore, the yield is often
greater, and there are fewer by-products because conditions
can be adjusted far more precisely in miniature. Poisonous
materials can also be processed more safely on an extremely
small scale.
Large-Scale Use of Microreactors.
A change appears to be in the offing for the chemical
industry. According to a study carried out by the Institute
for Microtechnology in Mainz (IMM), Germany, all of the
industry's top 30 companies are interested in exploring
miniaturization technologies. In fact, many experts expect to
see the large-scale use of microreactors in production
processes from 2005 onward.
Probably the world's smallest
thermal conductivity detector. A gold wire in the middle
of a measuring cell is 200 times thinner than a
hair. Such sensors are particularly suitable for
analyzing gases
| Several companies have been investigating the new
technology for some time, and some have already presented
initial results in the area of industrial-scale use. Since
August 1998, for example, Merck KGaA has
been running several microreactors for the production of a
fine chemical. For the Darmstadt, Germany-based company,
flexible production is especially important. Merck sells more
than 10,000 different chemicals, of which more than two-thirds
are manufactured in quantities of less than ten kilograms per
year. Similarly, at BASF in Ludwigshafen, Germany syntheses
have been studied and the results used to optimize several
processes.
There is, however, a drawback. The
structures of the tiny reactors, whose conduits and supply
lines are measured in micrometers (a millionth of a meter), do
not allow reactions with solids, which would immediately block
the paths. Nevertheless, experts think many basic chemicals
and numerous fine chemicals, including pharmaceuticals, can be
manufactured in microreactors. Nor is output volume expected
to be a problem. In Karlsruhe, Germany, a research center has
developed a cube-shaped reactor measuring a mere three cubic
centimeters that can pump 7,000 l/h in continuous
operation. That's 60,000 t per year.
Nowadays,
materials like silicon, steel, glass and ceramic substances
are used to make many mixers and heat exchangers with conduits
measuring just three to 300 µm in diameter. But most are
independently developed designs that are not compatible with
one another. Another drawback is that they cannot be operated
fully automatically.
Microreformers for fuel cells.
The Institute for Microtechnology in Mainz is using
these devices to convert methanol into hydrogen, which
could in turn be used to power fuel cells
| If microreactors are to succeed on an industrial scale,
processes must become fully automated—which is where Siemens
Automation and Drives (A&D) comes in. Together with Axiva,
Merck and the Fraunhofer Institute for Chemical Technology
(ICT) in Pfinztal near Karlsruhe, A&D is participating in
a project sponsored by Germany's Federal Ministry of Education
and Research (BMBF) designed to develop a microreaction system
for industrial use. In addition to containing modular
microfluidic components to supply it with starting materials
and process the product, the system will also be equipped with
sensors, analytical elements and process control technology.
The dimensions of the components are being chosen in
accordance with the dictates of process development and the
goal of continuous production on a kilogram scale. The
project's partners want to investigate a specific nitration.
Nitration is one of the most important transformations in
chemistry because the nitro (NO2) groups attached
to molecules can easily be transformed into other functional
groups. As nitrations usually generate a great deal of heat
and often result in many by-products, they are very suitable
for testing microtechnology in an industrial context.
"The special thing is the integration of a fluidic bus
system for chemicals and an electrical bus system for
communication," says Axiva's Inga Leipprand. During the BMBF
project, A&D will be responsible for the system, including
the control system. Arno Steckenborn and his colleagues at
Siemens' Micromechanics & Coating center will supply
sensors for it. These are essential because the exact
regulation of a chemical process requires detailed knowledge
of the mixture's pressure, temperature, mass flow and
density—at every stage. "The main drawback of older pressure
sensors for microreaction systems is that chemical residues
get left over in their openings," explains Steckenborn.
Refining Microarchitectures. A membrane in the new
pressure sensor, which is made of silicon, imparts the
mixture's pressure to a conductive structure by way of a
stamp. The resistance of the structure changes and provides a
signal proportional to the difference in pressure. A
temperature sensor is located behind the membrane. The
pressure sensor itself consists of two silicon elements that
are bonded together. The "direct bonding" involved here relies
on a type of cementing in which the silicon is pretreated with
chemicals. As a result, hydroxide groups are deposited on the
surface. When pressure is applied, the parts tightly adhere to
one another via hydrogen bonds. And when they are heated to
1,000 °C, a seamless and inseparable connection is
created. Temperatures are particularly critical when metals
are involved— too high, and metal atoms will migrate into the
silicon layer, which reduces the sensitivity of the sensor.
But Steckenborn and his team have now refined the technique to
the point that only 250 °C is needed, which allows for
even more complexcomponents.
Steckenborn has also
succeeded in building what is probably the world's smallest
thermal conductivity detector. Its measuring chamber is one
millimeter in length and contains a 0.3-µm-wide gold wire that
is around 200 times thinner than a hair. The sensor can
be used for the analysis of gases. Here, the sensor is heated
up to the point where the wire is about 100 K hotter than
surrounding gases. When their composition changes, the
differences in the thermal conductivity of the gases results
in a change of temperature at the wire and hence a change in
its resistance, which is converted into a signal. This is
nothing new. What is revolutionary, though, is the extreme
miniaturization of the process.
Thinking Small.
Miniaturization is also the centerpiece of another one of
Steckenborn's components—a flow sensor that looks like a tiny
antenna. The sensor is based on the principle of the Coriolis
force, which appears in the context of rotating bodies and,
for example, makes clouds in the northern hemisphere drift
eastward. In the sensor, the chemicals flow through a ring
that is designed to vibrate. The Coriolis force causes
extremely small displacements in the plane of vibration, from
which the mass flow rate and therefore also the density can be
calculated. Sensors based on this principle have been used for
years in the chemical industry, but are up to 100 times
larger. Steckenborn looks at the tiny antenna in his hand and
says: "Many people at Axiva and Siemens are used to thinking
in the dimensions of conventional plant engineering.
Naturally, for them, the trend toward miniaturization means a
huge adjustment."
Norbert
Aschenbrenner
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