PROCESS FOR PREPARING 4-AMINODIPHENYLAMINE

1 . A falling film evaporator comprising

a shell pass,

a tube pass,

a steam inlet on top of the shell pass,

a condensed water outlet at bottom of the shell pass,

a lower concentration aqueous phase inlet on top of the tube pass,

a higher concentration aqueous phase outlet at bottom of the tube pass,

a flow-aiding steam inlet on top of the tube pass, and

a distributing tray below the lower concentration aqueous phase inlet.

2 . The falling film evaporator according to claim 1 , wherein the evaporator is a one-stage or multi-stage gas-aid falling film evaporator(s).

3 . The falling film evaporator according to claim 1 , wherein an aqueous phase enters the evaporator through the lower concentration aqueous phase inlet and forms a film in the tube pass through the distributing tray during evaporation.

4 . The falling film evaporator according to claim 3 , wherein residence time for the aqueous phase inside the tube pass ranges from 2 to 60 seconds.

5 . The falling film evaporator according to claim 1 , wherein inside the falling film evaporator, a system pressure ranges from 0.005 to 0.1 MPa (absolute pressure).

6 . The falling film evaporator according to claim 1 , wherein a flow-aiding steam enters the tube pass through the flow-aiding steam inlet.

7 . The falling film evaporator according to claim 1 , wherein the evaporator contains two or more flow-aiding steam inlets.

8 . The falling film evaporator according to claim 1 , wherein the shell pass is heated by a steam which is maintained at a pressure in a range of from 0.005 to 0.1 MPa (absolute pressure).

9 . The falling film evaporator according to claim 1 , wherein heat medium in the evaporator is water, steam, or secondary steam.