Gas-phase process for preparing ethylene polymers

A process for the preparation of a multimodal ethylene polymer, comprising the step of

• forming an ethylene polymer in particle form in a gas-phase polymerization unit comprising a reactor cascade of two or more gas-phase polymerization reactors which comprises a first gas-phase polymerization reactor, into which a polymerization catalyst is fed, and one or more subsequent gas-phase polymerization reactors, which receive a polymerization catalyst fed to the first gas-phase polymerization reactor in active form incorporated into ethylene polymer particles formed in a preceding gas-phase polymerization reactor(s) by homopolymerizing ethylene or copolymerizing ethylene and one or more C4-C12-1-alkenes at temperatures from 20 to 200° C. and pressures of from 0.5 to 10 MPa in a reaction gas comprising propane as polymerization diluent in the presence of a polymerization catalyst,
• wherein a purified propane feed stream comprising at least 99% mol propane and not more than 100 ppm-mol propylene is fed into the gas-phase polymerization unit and purification of the purified propane feed stream has been carried out by passing a hydrocarbon stream containing propane, propylene, acetylene, carbon monoxide, carbon dioxide, oxygen and water through hydrocarbon purification units which reduce at least the concentration of propylene, acetylene, carbon monoxide, carbon dioxide, oxygen and water contained in the hydrocarbon stream to produce the purified propane feed stream, and
• wherein the purified propane feed stream fed to the gas-phase polymerization unit comprises not more than 0.03 ppm-mol carbon monoxide, not more than 0.4 ppm-mol carbon dioxide, not more than 2.0 ppm-mol oxygen, not more than 3.0 ppm-mol acetylene, and not more than 2.0 ppm-mol water.