Process Innovation

Specialists in process optimization: Dr. Thomas Frey (left), head of Process Development and Productivity at Corporate Engineering, and Jochen Groß, silane production manager in Burghausen.

Process Innovation

Jochen Groß knows “his” plant on the WACKER site in Burghausen like the back of his hand. The 3,000-square-meter complex has been the focus of his working life for over 15 years. “Up there,” says the production manager pointing to the walkways at a dizzying height, “you can feel the eddies inside the reactors. Standing there, I can sense whether the reactor is running properly.” Each year, the facility produces several hundred kilotons of methyl chlorosilanes. These serve as feedstock for producing silicones – fluids, rubbers and resins with applications in such industries as construction, automotive, plastics, electrical and textiles.

A Fluid Contact Mass

The principle behind the process is anything but new. It was discovered back in 1940 by Eugene Rochow, an American, and Richard Müller, a German. It starts from solid silicon and gaseous methyl chloride. Copper is also required as a catalyst, to increase the reaction rate and make the process economical.

A radically new insight into two key processes used in the chemical industry will boost the efficiency of silicone and hyperpure silicon production while reducing raw material consumption.

The silicon and copper, along with co-catalysts, are finely ground and then mixed to make the contact mass. In the Müller-Rochow reactors at WACKER, the methyl chloride flows up through a bed of contact mass particles at high speed. “This causes the particles in the contact mass to behave like a boiling liquid. In this fluidized bed, bubbles are created and the particles constantly rise and sink,” explains Groß.

There is another key production process, well-established at WACKER, that is performed in fluidized-bed reactors, namely the industrial-scale production of trichlorosilane – the precursor of hyperpure silicon in solar cells and semiconductors. Trichlorosilane (TCS) synthesis, too, is the first stage of a value-creation chain but distinct from that of the Müller-Rochow process. TCS synthesis is therefore the responsibility of a different WACKER business division. Yet the two processes are in fact very similar. In TCS synthesis, too, a gas is forced through solid metallurgical grade silicon, where it reacts to form the product.

WACKER performs both of these processes in fluidized-bed reactors because these systems offer optimum thermal and mass transfer. The yield is higher, i.e. more product. And local hotspots are avoided, which could otherwise compromise the product’s composition.

“Up until now, Müller-Rochow and TCS synthesis have been studied empirically,” says Dr. Thomas Frey, WACKER’s head of Process Development and Productivity at Corporate Engineering. This means that records are kept of how changes in the conditions within the reactor affect the quantity and composition of the product. The information and experience accumulated over the years have helped WACKER to make continuous improvements to its processes. “However, there’s limited scope left for making further empirical optimizations ,” admits Frey.

The problem is that there are too many variables. Hundreds of them, he says: starting with desirable and undesirable impurities in the silicon and catalytic additives, then the crystal structures of the solids, and their particle sizes, and finally process engineering variables such as pressure, temperature and flow rate. “Looked at on this scale, the Müller-Rochow and TCS syntheses constitute a black box – very complex systems that can only be examined in terms of their external behavior,” explains Frey. Consequently, attempts to elucidate the process in recent years were limited to the use of computers and ingenious algorithms to explain the interplay of the various factors.

The Reactor Is No Longer a Black Box

“Now, though, a new and highly promising approach has been developed that enables us to simulate the processes inside the reactor realistically using a computer,” adds Frey. Meanwhile, in production, Jochen Groß is also testing the new findings, which could prove to be a milestone in making production more versatile, cheaper and more economical on resources. What has made the two so optimistic is the outcome of some truly cutting edge research. A team led by Dr. Anne Alber from Corporate R&D has refined a special analytical technique to the point where it can now be used to view individual, fundamental processes occurring on the surface of the solid particles. The graduate chemist was honored with the Alexander Wacker Innovation Prize 2011 for her work.

The Particles Can Now Remain in the Reactor for Analysis

“What makes the technique so special is that we can use it to analyze the solids at the molecular level minute by minute in realistic conditions as the reaction is happening,” explains Alber. In the past, analysis entailed removing solid particles from the reactor for examination. The time delay allowed the particles to cool down and also to react with oxygen in the air. As a result, this analysis gave little information about what was really happening inside the reactor. Thanks to this new, innovative technique, WACKER researchers have already gained a very detailed idea of what Alber calls the reaction network of the Müller-Rochow process. The formation of methylchlorosilanes on the surface of the contact mass particles actually proceeds via a complicated process involving scores of intricately interlinked intermediate steps. The catalyst, too, plays a number of active roles, even though it ultimately emerges from the process virtually unchanged.

An innovative analytical method improves production processes.

As Anne Alber has only recently begun to apply her special analytical technique to trichlorosilane synthesis as well, concrete results are not yet available. “Much of what we learned from our pioneering work on the Müller-Rochow process is transferable to the trichlorosilane synthesis. So, progress will be swift,” assures Alber.

Armed with the fresh knowledge of the fundamental processes occurring in the Müller-Rochow direct synthesis, the WACKER process engineers have identified ways to further improve it. They are now assessing which of the possible recipes will be particularly effective at lowering silicon consumption in practice and at boosting productivity. For now, they are conducting model trials in fluidized-bed reactors the size of a glass beaker. Then, they will scale this up to reactors capable of converting several kilograms of raw materials. “Many of the new optimization methods have already gone to factory trials at the ton scale and will be phased into regular production,” adds Frey, visibly pleased.