Evonik Industries AG´s Steinau plant feels it has a particular obligation to protect the environment over and above the statutory requirements, as it is located in an idyllic setting surrounded by the Rhön, Spessart and Vogelsberg Mountains.

Since the plant’s cooling requirements are met by the regenerative use of waste cold, it seemed suitable here, both economically and environmentally, to use the DuoCondex process - developed for the recovery of pure substances and mixture for further recycling of residues. In other words, the gaseous nitrogen that is produced by the vapourisation of liquid nitrogen is required for production processes anyway.

The process, developed by Messer, makes use of the extremely low temperature of liquid nitrogen to separate all the waste gas components apart from air and nitrogen in multiple stages through freezing and condensing. Moisture is separated in a pre-condenser which is cooled with a refrigerating system. The cold supply for the main condenser begins with the vapourisation of the liquid nitrogen at -196ºC. Cooling for separation of the hydrocarbons takes place in a multi-stage, temperature-controlled way via circuits and the interconnection of regenerative heat exchanger. This reduces the temperature difference between the waste gas to be cleaned and the cooling, gaseous nitrogen on the walls of the condenser.

This, in turn, improves the cleaning performance, and prevents aerosol formation, localised freezing-out of concentrations and the blockage of the tube bundles, while also extending the running time before defrosting is necessary.

Adapting and designing the process required intensive cooperation between the plant engineers and the operator. It makes sense to run a pilot plant with appropriate instrumentation first, in particular to take account of the varying amounts of waste gases produced with very different compositions. This can be supplied by Messer.