A lot of daily life products are made from polymers based on toluene diisocyanate (TDI) like glue, soft foams for padding and mattresses, shoe soles, spray paintings for car industries, air crafts and trains. About 90 % of the world´s TDI production is used for polyurethane, a plastic which can be foamed and therefore is used in several high-volume applications.
The TDI production process yields substantial quantities of distillation residue which contains anything from 30 to 70% by weight of free TDI. The recovery of the TDI from this distillation residue is one of the process steps governing the overall economy of the TDI production process. Since the recovery is a difficult undertaking, several recovery methods were developed based on
- chemical/physical separation using liquid-liquid-extraction
- chemical transformation with conversion to Toluene diamine (TDA) and
- thermal separation with evaporation and/or drying stage.
The thermal separation incl. a drying stage allows 100% recovery of the TDI from the residue and therefore is accepted as an up-to-date technology in TDI recovery.
However, even if in theory the recovery of 100% of TDI is possible the realisation of the recovery process is a very challenging task. As soon as the free TDI content of the distillation residue decreases to about 17%, the concentrated residue becomes highly viscous, sticky and suddenly converts into a solid state. Therefore actual process and equipment designs aim on avoiding the blocking by solid material. Convective dryers act as fluidized bed dryers under atmospheric pressure using nitrogen for fluidization. The pre-concentrated residue is distributed finely into the hot fluidized bed, so that the conversion to the solid phase is very quick. Thus, the formation of the highly viscous intermediate phase is disabled and the distillation residue is transformed quickly into fine solid particles.
Contact dryers in TDI recovery use one or two stirrers and are characterized by the large volume, the rugged, high torque and fully double jacket design, the containment of vapours and in some processes the use of additional high-boiling inert carbons to prevent caking of the solid residual at the walls and the stirrer. The contact dryers operate continuously and usually under vacuum.
Both implementations of the thermal separation TDI recovery process in convective and very special contact dryers have disadvantages resulting in high operation and/or investment costs. Therefore SMS investigated a new process, the TDI recovery in a Combined Fluidization Technology (CFT) dryer, which clearly improves the economy of the TDI recovery process. Because of the working principle of CFT dryers, this type of dryer is the ideal equipment to simplify the TDI recovery process. The final dry residue is discharged continuously by gravity from the CFT dryer into a product cooler. The overall material balance proves 100% TDI recovery.