Systems and techniques for polymer product withdrawal

A method of operating a polymerization system including a polymerization reactor and a downflow polymer product withdrawal system coupled to a reactor outlet of the polymerization reactor, the method comprising:

• opening a fill valve of the downflow polymer product withdrawal system to fluidically couple a lock hopper to the polymerization reactor through a withdrawal line, the withdrawal line comprising at least one downward sloped descending section, the withdrawal line being absent of any non-self draining regions;
• after the opening the fill valve, receiving a plug comprising powder and gas from the withdrawal line into the lock hopper;
• after receiving the plug, allowing the lock hopper to be pressurized with gas received from the reactor through the withdrawal line to a reactor pressure;
• after the pressurization, allowing powder from the withdrawal line to continue to settle into the lock hopper for a predetermined settling time;
• after the settling, closing the fill valve to isolate the lock hopper from the withdrawal line and from the reactor;
• and
• after the closing the fill valve, opening the discharge valve to discharge the contents of the lock hopper.