Reaction system for preparing stress whitening resistant polypropylene and method for preparing polypropylene in system

The invention discloses a reaction system for preparing stress whitening resistant polypropylene and a method for preparing polypropylene by employing the system. A liquid-phase stirring reactor is sequentially connected with a gas-phase fluidized bed flashing reactor and at least two gas-phase fluidized bed reactors which are connected in series. According to the method, polypropylene granules are sequentially subjected to propylene liquid-phase homopolymerization, propylene gas-phase homopolymerization, propylene ethylene gas-phase copolymerization and ethylene gas-phase spontaneous polymerization through the reaction system and a Ziegler-Natta catalytic system, and pass a gas-solid drying and separating system and are discharged, so that the stress whitening resistant polypropylene is obtained. The system has the advantages that a copolymerization reactor is additionally arranged at the stage of gas-phase polymerization, a flow of propylene liquid-phase homopolymerization, propylene gas-phase homopolymerization, propylene ethylene gas-phase copolymerization and ethylene gas-phase spontaneous polymerization is formed, a notable characteristic is reflected in the stage of gas-phase polymerization, polymer granules are subjected to three polymerization stages including propylene homopolymerization, propylene ethylene copolymerization and ethylene spontaneous polymerization, and the interface integration of different structural molecules is properly improved, so that the stress whitening resistant polypropylene has relatively good mechanical property and processing property, and the reaction system further has outstanding characteristics that the occurrence probability of stress cracking is reduced, and the stress whitening phenomenon of products is obviously improved.