Process and apparatus for the polymerization of ethylene

An apparatus for carrying out a process for polymerizing ethylene or ethylene with further 1-olefins, in which the ethylene is polymerized in presence of a catalyst in a gas-phase reactor to form polymer particles, the gas-phase reactor comprising a reaction gas comprising propane and unpolymerized ethylene, and the reaction gas being circulated to remove heat of polymerization;

• the process comprising:
• the polymer particles are discharged continuously or discontinuously from the reactor;
• the polymer particles are separated from a major part of concomitantly discharged gas, and the polymer particles are degassed;
• gas is freed of entrained fine particles;
• in a first separation stage, gas is separated from a low-boiling fraction comprising ethylene, or from a high boiling fraction comprising further 1-olefins or alkanes, the 1-olefins or alkanes comprising from 4 to 12 carbon atoms;
• in a second separation stage, a propane fraction is separated off and the propane fraction is used for degassing the polymer particles discharged from the reactor;
• wherein a partial pressure of ethylene in the propane fraction used for degassing is less than 6000 Pa, and a partial pressure of a sum of further 1-olefins and alkanes comprising from 4 to 12 carbon atoms in the propane fraction is less than 10000 Pa;
• the apparatus comprising:
• a gas-phase reactor comprising a bed of polymer particles;
• a recycle gas line connected to the reactor for carrying away and recirculating reaction gas, wherein the reaction gas comprises propane and unpolymerized ethylene;
• a product offtake line for continuously or discontinuously taking off polymer particles from the reactor, wherein the product offtake line is connected to a degassing vessel for separating polymer particles and gas;
• a particle precipitation unit connected to the degassing vessel for freeing gas of entrained fine particles;
• a first separation unit connected to the particle precipitation unit for separating off a low-boiling fraction comprising ethylene and ethane;
• a second separation unit connected to a bottom of the first separation unit for separating off a propane fraction;
• a propane line which is connected to a top of the second separation unit and in turn is connected to the degassing vessel to degas the polymer particles using the propane fraction;
• wherein the first separation unit is such that the partial pressure of ethylene in the propane fraction entering the degassing vessel is less than 6000 Pa, and the second separation unit is such that the partial pressure of 1-olefins and alkanes comprising from 4 to 12 carbon atoms in the propane fraction entering the degassing vessel is less than 10000 Pa.

An apparatus for carrying out a process for polymerizing ethylene or ethylene with further 1-olefins, in which the ethylene is polymerized in presence of a catalyst in a gas-phase reactor to form polymer particles, the gas-phase reactor comprising a reaction gas comprising propane and unpolymerized ethylene, and the reaction gas being circulated to remove heat of polymerization;

• the process comprising:
• the polymer particles are discharged continuously or discontinuously from the reactor;
• the polymer particles are separated from a major part of concomitantly discharged gas, and the polymer particles are degassed;
• gas is freed of entrained fine particles;
• in a first separation stage, gas is separated from a low-boiling fraction comprising ethylene, or from a high boiling fraction comprising further 1-olefins or alkanes, the 1-olefins or alkanes comprising from 4 to 12 carbon atoms;
• in a second separation stage, a propane fraction is separated off and the propane fraction is used for degassing the polymer particles discharged from the reactor;
• wherein a partial pressure of ethylene in the propane fraction used for degassing is less than 6000 Pa, and a partial pressure of a sum of further 1-olefins and alkanes comprising from 4 to 12 carbon atoms in the propane fraction is less than 10000 Pa;
• the apparatus comprising:
• a gas-phase reactor comprising a bed of polymer particles;
• a recycle gas line connected to the reactor for carrying away and recirculating reaction gas, wherein the reaction gas comprises propane and unpolymerized ethylene;
• a product offtake line for continuously or discontinuously taking off polymer particles from the reactor, wherein the product offtake line is connected to a degassing vessel for separating polymer particles and gas;
• a particle precipitation unit connected to the degassing vessel for freeing gas of entrained fine particles;
• a first separation unit connected to a particle precipitation unit for separating off a high-boiling fraction comprising further 1-olefins, alkanes comprising 4 to 10 carbon atoms, or mixtures thereof;
• a second separation unit connected to a top of the first separation unit for separating off a propane fraction;
• a propane line connected to a bottom of the second separation unit and in turn is connected to the degassing vessel to degas the polymer particles using the propane fraction after passing an evaporation unit;
• wherein the first separation unit is such that the partial pressure of ethylene in the propane fraction entering the degassing vessel is less than 6000 Pa, and the second separation unit is such that the partial pressure of 1-olefins and alkanes comprising from 4 to 12 carbon atoms in the propane fraction entering the degassing vessel is less than 10000 Pa.