Polymerization of ethylene optionally with 1-olefins involves polymerizing ethylene in presence of catalyst to form polymer particles and circulating unpolymerized ethylene to remove heat of polymerization

摘要

Polymerization of ethylene optionally with 1-olefins involves polymerizing ethylene in presence of a catalyst in gas-phase reactor (1) to form polymer particles and reaction gas containing propane, circulating unpolymerized ethylene to remove the heat of polymerization, and separating propane fraction in second separation stage and using for degassing polymer particles discharged from reactor such that the partial pressure of ethylene in propane fraction used for degassing is less than 6000 Pa and that of 1-olefins and 4-12C alkanes in propane fraction is less than 10000 Pa. Polymerization (p1) of ethylene optionally with further 1-olefins involves polymerizing ethylene in the presence of a catalyst in a gas-phase reactor (1) to form polymer particles and reaction gas containing propane, circulating unpolymerized ethylene to remove the heat of polymerization, discharging the polymer particles (dis)continuously from the reactor, separating the polymer particles from the major part of the concomitantly discharged gas and degassing them, freeing the gas of entrained fine particles, separating the gas from a low-boiling fraction containing ethylene or from a high boiling fraction containing further 1-olefins or 4-12C alkanes in a first separation stage, and separating a propane fraction in a second separation stage and using it for degassing the polymer particles discharged from the reactor. The partial pressure of ethylene in the propane fraction used for degassing is less than 6000 Pa and the partial pressure of the sum of further 1-olefins and 4-12C alkanes in the propane fraction is less than 10000 Pa. Independent claims are included for the following: (1) an apparatus for carrying out the process (p1) comprising: a gas-phase reactor (1) containing a bed of polymer particles, a recycle gas line (3) connected to the reactor for carrying away and recirculating the reaction gas comprising propane and unpolymerized ethylene, a product offtake line (2) for (dis)continuously taking off polymer particles from the reactor, which line is connected to a degassing vessel (5) for the separation of polymer particles and gas, a particle precipitation unit (9) for freeing the gas of entrained fine particles connected to the degassing vessel, a first separation unit (12) for separating off a low-boiling fraction containing ethylene and ethane connected to the particle precipitation unit, a second separation unit (15) for separating off a propane fraction connected to the bottom of the first separation unit, a propane line which is connected to the top of the second separation unit and is in turn connected to the degassing vessel to make it possible to degas the polymer particles by the propane fraction, where the first separation unit is designed so 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 designed so that the proportion of 1-olefins and 4-12C alkanes in the propane fraction entering the degassing vessel is less than 10000 Pa; (2) use of propane containing less than 1 mol% ethylene, and less than 4 mol% of 1-olefins and 4-12C alkanes based on the total amount of gas, obtained from the gas discharged from a gas-phase polymerization reactor together with the polymer particles for degassing ethylene homopolymers or ethylene copolymers.