Abstract: In order to increase the capacity of a toluene diisocyanate (TDI) plant from 20kt/a to 75kt/a, we analyzed and made simulation of the TDI refining column, which was used to separate TDI from DEIP, an inert solvent used during reaction process. Simulation results showed that the tray number of the original column was too small that needed a larger reflux rate to meet the separation requirement, and the column load had a big difference between top section and lower section. Based on the simulation results, we proposed a two-column flow sheet and found the optimal equipment and operation parameters by simulation and optimization on the new flow sheet. The new flow sheet needs a new column with a diameter of 3 m, tray number of 25 to be combined with the original column to meet 75 kt/a TDI capacity. When the recycle flow rate between the two columns is about 14 000 kg/h, the new flow sheet could not only use the old column, but also ensure that the total energy consumption is not too high. By this way, the proposed flowsheet can achieve the objects of process-optimized and invest-saving.