Process for start-up of a multizone circulating reactor

A process for starting a gas-phase olefin polymerization reaction comprising producing a first polyolefin by polymerizing one or more olefins in a multizone circulating reactor,

• wherein the multizone circulating reactor comprises:
• a vertical first reactor zone comprising a upper region and a lower region,
• a vertical second reactor zone comprising a upper region and a lower region, and wherein the vertical second reactor zone is equipped with a throttling valve at the bottom, and a line for discharging the polyolefin,
• wherein the upper region of the first reactor zone is connected to the upper region of the second reactor zone by a first connecting part, and
• wherein the lower region of the second reactor zone is connected to the lower region of the first reactor zone by a second connecting part;
• and
• a gas recycle line, which is equipped with a compressor and a heat exchanger for removing the heat of polymerization, wherein the gas recycle line connects the top of the first reactor zone, the top of the second reactor zone, or the first connecting part with at least
• a riser gas introduction point for feeding a riser gas at the bottom of the first reactor zone,
• a dosing gas introduction point for feeding a dosing gas into the second reactor zone above the throttling valve;
• and
• optionally, a transport gas introduction point for feeding a transport gas to the second connecting part at the end of the connecting part, which is connected with the lower region of the second reactor zone,
• i) filling the multizone circulating reactor with a reactor gas at a pressure from 0.1 to 20 MPa and a temperature from 20 to 200° C., wherein the multizone circulating reactor contains no polyolefin particles,
• j) conveying the reactor gas from the top of the first reactor zone, the top of the second reactor zone, or the first connecting part through the gas recycle line to the riser gas introduction point, to the dosing gas introduction point, and optionally to the transport gas introduction point;
• k) feeding a polymerization catalyst or a polymerization catalyst and a second polyolefin into the multizone circulating reactor;
• l) controlling the gas flow in the second reactor zone by adjusting the opening of the throttling valve and the flow rates of the gas fed to the riser gas introduction point, to the dosing gas introduction point, and optionally to the transport gas introduction point, wherein an upwards gas velocity in the bottom part of the second reaction zone is lower than the terminal free-fall velocity of the polymerization catalyst or a combination of the polymerization catalyst and the second polyolefin that was fed into the multizone circulating reactor;
• m) optionally feeding one or more olefins to the multizone circulating reactor and reacting the one or more olefins in the presence of the polymerization catalyst to produce the first polyolefin;
• n) feeding, with a closed polyolefin discharged line,
• one or more olefins,
• the polymerization catalyst or the combination of the polymerization catalyst and the second polyolefin, or
• one or more olefins, the polymerization catalyst or the combination of the polymerization catalyst and the second polyolefin,
• into the multizone circulating reactor until the weight of the first polyolefin in the second reactor zone is higher than the drag force of the upward moving gas and a settled bed of first polyolefin forms;
• o) thereafter, controlling the circulation rate of the first polyolefin within the multizone circulating reactor by adjusting the opening of the throttling valve and adjusting the flow rate of the dosing gas, optionally discharging the first polyolefin and controlling the feeding rates of:
• the one or more olefins, and the polymerization catalyst, or the combination of the polymerization catalyst and the second polyolefin,
• into the multizone circulating reactor and the rate for the optional discharge of the first polyolefin wherein the combined amounts of introduced second polyolefin and olefin(s) are higher than the amount of discharged first polyolefin until the level of the settled bed of first polyolefin reaches the upper part of the second reactor zone;
• p) thereafter, controlling the circulation rate of the first polyolefin within the multizone circulating reactor by adjusting the opening of the throttling valve and adjusting the flow rate of the dosing gas, discharging the first polyolefin and controlling the feeding rates of the one or more olefins, the polymerization catalyst, and the second polyolefin into the multizone circulating reactor and the discharge rate of the first polyolefin wherein the combined amounts of introduced second polyolefin and olefin(s) are balanced with the amount of discharged first polyolefin.