Processes for venting olefin polymerization systems

A process for venting a gas phase polymerization reactor, comprising:

• (a) providing a gas phase polymerization system comprising a reactor, a reactor recycle loop in fluid communication with an outlet and an inlet of the reactor, a flare in fluid communication with the reactor recycle loop, a vent column in fluid communication with the reactor recycle loop and the flare, a product discharge system comprising first and second pairs of lock hoppers and a product purge bin, and a gas vent line in fluid communication with the reactor and the product purge bin, wherein each pair of lock hoppers comprises an upstream lock hopper connected by a valve to the reactor and a downstream lock hopper connected by a valve to the upstream lock hopper and connected by a valve to the product purge bin, wherein a first cross-tie is provided between the upstream lock hoppers of the first and second pairs of lock hoppers and a second cross-tie is provided between the downstream lock hoppers of the first and second pairs of lock hoppers, and wherein the first and second cross-ties each comprise a valve configured to selectively allow and to selectively prevent fluid communication between the upstream lock hoppers and the downstream lock hoppers, respectively;
• (b) supplying an olefin monomer, a carrier gas, and a particulate catalyst to the reactor under conditions effective to maintain the particulate catalyst in a fluidized state and to polymerize the olefin monomer in the presence of the particulate catalyst to produce a polymer product;
• (c) supplying a reactor overhead comprising a reactor gas from the outlet of the reactor to the reactor recycle loop, wherein the reactor gas comprises unreacted olefin monomer and carrier gas;
• (d) supplying a first portion of the reactor overhead in the reactor recycle loop to the vent column;
• (e) contacting the first portion of the reactor overhead with a stripping medium within the vent column to remove at least a portion of the olefin monomer to produce a vent column overhead rich in the carrier gas and lean in the olefin monomer and a vent column bottoms rich in the olefin monomer and lean in the carrier gas;
• (f) supplying the vent column overhead to the flare;
• (g) supplying a second portion of the reactor overhead in the reactor recycle loop and the vent column bottoms to the reactor;
• (h) removing the polymer product and an additional quantity of reactor gas from the reactor through the first and second pairs of lock hoppers in removal cycles,
• wherein each removal cycle (h) includes the steps of:
• (1) transferring polymer product and reactor gas from the reactor to (1a) the upstream lock hopper of the first pair of lock hoppers and equilibrating the pressure therebetween or (1b) the upstream lock hopper of the second pair of lock hoppers and equilibrating the pressure therebetween;
• (2) while performing step (1), transferring polymer product and reactor gas from (2a) the upstream lock hopper to the downstream lock hopper of the second pair of lock hoppers and equilibrating the pressure therebetween or (2b) the upstream lock hopper to the downstream lock hopper of the first pair of lock hoppers and equilibrating the pressure therebetween;
• (3) while performing steps (1) and (2), transferring polymer product using a conveying gas from (3a) the downstream lock hopper of the first pair of lock hoppers to the product purge bin or (3b) the downstream lock hopper of the second pair of lock hoppers to the product purge bin;
• (4) after performing steps (1), (2), and (3), passing reactor gas from (4a) the upstream lock hopper of the first pair of lock hoppers to the upstream lock hopper of the second pair of lock hoppers via the first cross-tie to equilibrate the pressure therebetween or (4b) the upstream lock hopper of the second pair of lock hoppers to the upstream lock hopper of the first pair of lock hoppers via the first cross-tie to equilibrate the pressure therebetween;
• and
• further wherein some or all of the removal cycles (h) further include the step of:
• (5) after performing steps (1), (2), and (3), passing reactor gas from (5a) the downstream lock hopper of the first pair of lock hoppers to the downstream lock hopper of the second pair of lock hoppers via the second cross-tie to equilibrate the pressure therebetween or (5b) the downstream lock hopper of the second pair of lock hoppers to the downstream lock hopper of the first pair of lock hoppers via the second cross-tie to equilibrate the pressure therebetween;
• wherein steps (1a), (2a), (3a), (4a), and (5a) or steps (1b), (2b), (3b), (4b), and (5b), respectively, are carried out in sequential order and in alternating removal cycles (h) with respect to one another;
• (i) determining an additional quantity of carrier gas needs to be removed from the gas phase polymerization system;
• (j) increasing an amount of the first portion of the reactor overhead in the reactor recycle loop supplied to the vent column until a maximum removal capacity of the vent column is reached;
• and
• (k) reducing the frequency of step (5) such that after performing steps (1), (2), and (3), the reactor gas is passed from one of the downstream lock hoppers to the other downstream lock hopper via the second cross-tie in some, but not all, of the removal cycles (h).