The main commercial route for the manufacture of TDI starts with the nitration of toluene using nitric acid to produce dinitrotoluene followed by catalytic hydrogenation to toluene diamine. The toluene diamine is dissolved in an inert solvent and reacted with phosgene to produce a crude TDI solution.
The slurry is digested in stages and the temperature progressively raised until the phosgenation is complete. Hydrogen chloride gas is blown into the mixture to reduce the activity of the free amine and removed by blowing an inert gas through the solution. The crude solution of TDI is fractionated to remove any unreacted phosgene, solvent and hydrogen chloride before being distilled under vacuum.
A new development by Bayer involves the phosgenation being carried out in the gas phase instead of the liquid phase. The toluene diamine and phosgene are heated to more than 300oC and then transferred in gaseous form to the reaction via a specially designed nozzle. The liquid TDI is condensed out and purified by distillation during which solvent and excess phosgene are recovered.
Bayer says its gas phase route reduces the amount of solvents by 80% with some 40% less energy used, particularly in the subsequent distillation step. The residence time in the reactor is substantially reduced, also reducing the size of the plant equipment. Investment costs are claimed to be 20% lower.
TDI can be produced directly from dinitrotoluene by liquid phase carbonylation with o-dichlorobenzene. This route avoids the use of phosgene and waste recovery problems associated with HCl. One-step processes based on the reaction between dinitrotoulene and carbon monoxide have been developed but not commercialised.
Toluene diisocyanate
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Toluene diisocyanate
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The main commercial route for the manufacture of TDI starts with the nitration of toluene using nitric acid to produce dinitrotoluene followed by catalytic hydrogenation to toluene diamine. The toluene diamine is dissolved in an inert solvent and reacted with phosgene to produce a crude TDI solution.
More about Toluene diisocyanate Process Technologies
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