Cooling of reaction mixture obtained by high-pressure polymerization process of ethylenically unsaturated monomers

A process for preparing ethylene polymers comprising the step of:

• polymerizing ethylene or copolymerizing ethylene and one or more comonomers at a temperature from 180° C. to 350° C. and a pressure of from 110 MPa to 350 MPa in a continuously operated tubular polymerization reactor, thereby yielding a reaction mixture,
• wherein the tubular polymerization reactor has a reactor cooling jacket for removing the heat of the reaction and a pressure control valve,
• wherein the reaction mixture leaves the reactor through the pressure control valve,
• wherein the reaction mixture then passes a post-reactor cooler equipped with a post-reactor cooling jacket,
• wherein the reactor cooling jacket is provided with a reactor coolant having an inlet temperature and the post-reactor cooling jacket is provided with a post-reactor coolant having an inlet temperature,
• wherein the inlet temperature of the post-reactor coolant is independently controlled from the inlet temperature of the reactor coolant,
• wherein the post reactor cooler is equipped with an upstream cooling jacket and a downstream cooling jacket, wherein the upstream cooling jacket is provided with an upstream coolant, the downstream cooling jacket is provided with a downstream coolant, and the inlet temperature of the upstream coolant is independently controlled from the inlet temperature of the downstream coolant, and
• wherein the coolants for cooling the reactor cooling jackets of the tubular polymerization reactor and for cooling the post-reactor cooling jackets of the post-reactor cooler are circulated in a cooling system comprising a volume buffer system for accommodating volume expansion of the coolant, a coolant circulation pump, and a cooling arrangement for removing the absorbed heat of polymerization from the cooling system.