Cold box low-temperature separation method for PDH project

Provided is a cold box low-temperature separation method for a PDH project. According to the method, four plate-fin heat exchangers, six gas-liquid separators, a flash tank, two expansion units and a low-temperature pump are included; first feed is cooled through the first plate-fin heat exchanger and the second plate-fin heat exchanger and then enters the second gas-liquid separator; gas separated by the second gas-liquid separator enters the expansion end of the first expansion unit to be expanded, and then the second gas-liquid separator is connected with the third gas-liquid separator; gas separated by the third gas-liquid separator is divided into two sections, gas separated by the fourth gas-liquid separator enters the third plate-fin heat exchanger to be subjected to heat exchange with another dry gas product, and gas separated by the sixth gas-liquid separator and circulating gas both enter the second plate-fin heat exchanger. The cold box low-temperature separation method has the advantages of being simple in procedure, high in product recovery rate, easy to operate and good in economic performance, saving energy and the like.