Industry – Miniaturization

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.

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.

"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