Insights in Innovative Technology Homecoming of a Wonder Material: Gas–Phase Phosgenation Brings Back TDI!

Author / Editor: Dominik Stephan* / Peter Steinmüller

Rejuvenation treatment for a chemical senior–Bayer has shaken up TDI production with a gas phase phosgenation process. Now the chemicals giant puts its second large plant on-stream – in Europe, of all places.

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The 50 m high TDI plant contains around 4000 tonnes of steel and a total of 10,000 individual parts (including 60 km of tubing).
The 50 m high TDI plant contains around 4000 tonnes of steel and a total of 10,000 individual parts (including 60 km of tubing).
(Picture: Bayer Material Science)

Festive fanfares for a chemical plant: Bayer celebrated the opening of one of the world’s most modern TDI production sites in great style. Guests of honour included Bayer CEO Dr. Marijn Dekkers alongside Patrick W. Thomas, who, as chairman of the board at Bayer Material Science (BMS), intends to lead the company’s materials division into independence, as well as the state's Minister President Hannelore Kraft. In between: Some snacks, big speeces and the artists of dance theatre Mobilé.

Bayer has spent 250 million Euros just on constructing the plant. Together with the associated projects and investments by suppliers and partners, the total sum reaches a staggering 400 million Euros. 300,000 tons of TDI per year are to be produced in Dormagen in the future – almost a fifth of the current global production capacity. Setting up a world-scale project like this in Germany is not an obvious step – which is just one reason for the huge crowd at the opening. Bayer in fact aims to score points on the home front with efficiency and catch up ground on the international competition.

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Between dancing artists and notable politicians, one could almost forget that it was all about chemicals. More precisely, about a revolutionary process intended to breathe new life into a product which had been gathering some dust. About 80 years have passed since the development of the wonder material 2.4.-toluene diisocyanate, TDI for short, the suitability of which as a pre-product for polyaddition reactions is rivalled by few other isocyanates. Since its discovery by Otto Bayer in the 1930s, TDI has been one of the most important plastic pre-products: Sector experts estimate the annual production capacity worldwide at over 1.5 million tons. Now the Saudis want a piece of the action: in Sadara, Arabia, the largest TDI production plant in the world is due to be completed next year. Under these conditions one cannot rest on one's laurels – even as the discoverer of TDI.

Usually, TDI is based on 2.4-toluene diamine, TDA for short, which is formed catalytically from nitrated toluene and hydrogen. This is then converted with phosgene, via intermediate steps, to isocyanate. This so-called basic phosgenation was developed at Bayer during the 1950s and has been of global significance ever since – but it brings certain problems too: For example, highly toxic phosgene passes through the plant at a slight overpressure as a reaction educt. Critics therefore fear that even small leakages could have grave consequences. The solvent requirements, and thus also the energy consumption for regeneration, are also considerable.

In Germany in particular, where energy is expensive and chemical works generally grew up next to rivers and towns, basic phosgenation increasingly reached its limits. For this reason, the TDI pioneers in Dormagen tried out a completely new process on a pilot scale in 2004: In a so-called gas phase phosgenation, the decisive final reaction takes place not under pressure in liquid solvents, but at temperatures of over 300° C in a tube reactor.

Bayer’s new TDI process moves the amine phosgenation into the gas phase
Bayer’s new TDI process moves the amine phosgenation into the gas phase
(Picture: Bayer Material Science)

It was therefor possible to induce both diamine TDA and phosgene to react in the gas phase. This enables solvent consumption to be reduced by 80 %, project leaders explain. Because the reaction does not take place under pressure, the risk of phosgene leakages is significantly reduced. Less solvents, raised efficiency and improved safety – with these benefits, Bayer is convinced, the gas phase technology can score points in international competition – Even against competitors who have access to cheap energy from crude or natural gas or who, in the unpopulated desert, have less worries about environment and people.

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