NOVELTY - Propane is separated from alkylation reaction liquid (4) and transalkylation reaction liquid (11) obtained from isopropylbenzene production device by introducing benzene (3), first alkylation reaction liquid (4-1) and first transalkylation reaction liquid (11-1) into debenzenizer tower (1) to obtain first overhead stream (6), first benzene stream (8) and first bottom stream (5); introducing first overhead stream, second alkylation reaction liquid (4-2) and second transalkylation reaction liquid (11-2) into depropanizer tower (2); and returning second bottom stream (7) into debenzenizer tower.    USE - Method for separating propane from alkylation reaction liquid and transalkylation reaction liquid obtained from isopropylbenzene production device (claimed).    ADVANTAGE - The method employs debenzenizer tower to separate the first benzene stream, first bottom stream containing isopropylbenzene and diisopropylbenzene, and first overhead stream while enabling the second bottom stream from the depropanizer tower to be returned into the top of the debenzenizer tower, thus reducing the amount of material to be handled in the depropanizer tower, the size required for the depropanizer tower, the investment cost, and the energy consumption for cooling and heating processes; enabling effective separation of propane in the depropanizer tower; reducing the rate of benzene loss; solving the problems associated with the separation technique employed in the prior art (such as large depropanizer tower size and high investment cost, energy consumption and rate of benzene loss); and making the method suitable for industrial production.    DETAILED DESCRIPTION - Propane is separated from alkylation reaction liquid (4) and transalkylation reaction liquid (11) obtained from isopropylbenzene production device by (a) dividing the alkylation reaction liquid generated from the alkylation reactor into first alkylation reaction liquid (4-1) and second alkylation reaction liquid (4-2); dividing the transalkylation reaction liquid generated from the transalkylation reactor into first transalkylation reaction liquid (11-1) and second transalkylation reaction liquid (11-2); (b) introducing the raw material benzene (3), first alkylation reaction liquid and first transalkylation reaction liquid into the middle part of debenzenizer tower (1); distilling to obtain first overhead stream (6) from the top of the debenzenizer tower, first benzene stream (8) drawn from the middle side line of the debenzenizer tower, and first bottom stream (5) containing isopropylbenzene and diisopropylbenzene; sending the first bottom stream and first benzene stream into subsequent process; (c) introducing the first overhead stream into the tower kettle of depropanizer tower (2); introducing the second alkylation reaction liquid and second transalkylation reaction liquid into the middle part of the depropanizer tower; and distilling to obtain second overhead stream (9) containing propane from the top of the depropanizer tower, second benzene stream (10) containing water from the middle side line of the depropanizer tower, and second bottom stream (7) which is returned into the top of the debenzenizer tower.    DESCRIPTION OF DRAWING(S) - The drawing is a schematic flow diagram illustrating the method for separating propane from the alkylation reaction liquid and transalkylation reaction liquid obtained from isopropylbenzene production device.    Debenzenizer tower (1)    Depropanizer tower (2)    Raw material benzene (3)    Alkylation reaction liquid (4)    First alkylation reaction liquid (4-1)    Second alkylation reaction liquid (4-2)    First bottom stream (5)    First overhead stream (6)    Second bottom stream (7)    First benzene stream (8)    Second overhead stream (9)    Second benzene stream (10)    Transalkylation reaction liquid (11)    First transalkylation reaction liquid (11-1)    Second transalkylation reaction liquid (11-2)