Technique for recovering propylene from off gas discharged by polypropylene device

The invention relates to a technique for recovering propylene from off gas discharged by a polypropylene device. The polypropylene device has a plurality of off gas (average propylene content being about 85%) discharging points, and the pressure and composition of off gas discharged from different discharging points are different. According to the technique, no off gas compressor is arranged, off gas with different pressure grades discharged by the polypropylene device is condensed through a refrigerant generated by a refrigerating unit, and 95% of propylene in off gas by mass is condensed; meanwhile, equilibrium flash vaporization is conducted with a condensate tank with the minimum pressure to enable light components including hydrogen and nitrogen in condensate to be flashed out almost completely, and then recovery of propylene is realized in the true sense. The pressure of off gas is fully utilized, and condensation is conducted according to different compositions of off gas, so that optimal condensation efficiency is realized. By conducting low-pressure flashing on obtained condensate, light components including hydrogen and nitrogen in condensate can be flashed out almost completely, so that recovery of propylene is realized in the true sense. The technique is high in efficiency, low in energy consumption, simple, practical, safe and reliable and is realized by improving an existing polypropylene process.