A substantial part of production in Marl, Evonik’s largest location, is based on the formula C4. It stands for liquid gases, the molecules of which contain four carbon atoms. These liquid gases are refined more and more, via a series of steps, at the C4 facilities in Marl and similarly at the Antwerp location. The products of these processes include MTBE (methyl tert-butyl ether), which is used as an antiknock additive to increase the octane value of gasoline and thus improve combustion, DINP (diisononyl phthalate), a plasticizer that turns brittle PVC into a flexible material, and butadiene, which is processed into synthetic rubber for car tires. All of the C4 facilities are connected closely together by material and energy flows. Even the byproducts of a process can be used as raw materials in other facilities, and the waste heat from one process step can supply the energy needed for another.

For decades, the production network in Marl has been using crude C4—an unprocessed stream of C4 from petrochemical plants that thermally decompose (“crack”) crude oil fractions in steam or naphta crackers. The petrochemical industry uses these methods to obtain chemicals such as ethylene and propylene, as well as the crude C4 stream.

But the supply of crude C4 is not infinite. Thanks to the growth of the market for the secondary products, more and more competitors are taking an interest in this raw material flow. At the same time, the number of steam crackers in Europe has stagnated—as has the supply of crude C4. Oil and gas extraction from non-conventional sources by means of fracking processes is booming in the USA, but the material extracted has a different composition from that of traditional crude oil. As a result, if the fracking material is then processed in crackers, the result is smaller quantities of lower-quality crude C4.

Increasing demand for C4-based industrial chemicals but stagnating availability of the raw material stream—Performance Intermediates recognized this dilemma ten years ago and started watching out for alternative C4 raw materials. During this time, the Business Line came across fluid catalytic cracking (FCC), a process used by refineries to catalytically crack high-boiling petroleum crude fractions into fuel components. As in a steam cracker, the resulting material stream is mainly made up of compounds containing four carbon atoms.

But there is one substantial difference in the composition: Crude C4 contains a maximum of five percent n-butane and isobutane, while FCC-C4 contains more than 25 percent of these compounds. Because these two alkanes contain exclusively single bonds, they are relatively unreactive and are difficult to process into valuable chemicals. “That would be a clear disadvantage of FCC-C4 compared to the established crude C4 as a feedstock for the Marl production network,” explains Dr. Markus Winterberg, who was the project leader at Process Development and later took on responsibility for its implementation.

A further negative aspect of the FCC-C4 stream quickly became apparent in the laboratory. It poisons the catalysts used in the plants of the Marl network. That means the catalysts, which are essential for the highly efficient and economic functioning of the production processes, would become less effective within a matter of hours.

The analytical experts from Evonik had tested new detectors and developed appropriate methods of gas chromatography that use a different operating principle than the standard methods. After that approach was used, the reason for poor catalyst performance became clear. The FCC-C4 mixture contained more than 50 unwanted nitrogen and sulfur components. The new detector for nitrogen compounds is so sensitive that it can measure concentrations as low as 50 ppb (parts per billion)—that corresponds to 50 grams of nitrogen compounds in 1,000 metric tons of FCC-C4 mixture. That laid the foundation for developing a concept to separate these components. Firstly, the boiling points and other thermodynamic properties of all of the components were determined. The data obtained about the material properties was fed into computer simulations. That made it possible to investigate which unwanted components could be relatively easily separated from the FCC-C4 flow by means of distillation.