Process for the production of polyethylene and ethylene copolymers
The invention provides a process and an apparatus for the high pressure polymerization of ethylene in which a jet pump is used to pump a fluid into a polymer/monomer product mixture leaving a bumped tubular reactor. The fluid may be recycled monomer or another fluid such as a modifier.
1. A process for producing polyethylene or ethylene copolymers comprising
polymerizing ethylene, optionally in the presence of one or more comonomers, in a tubular reactor while bumping the reactor pressure to produce a product mixture comprising polymer and unreacted monomer,
passing the product mixture through a high pressure let down valve which changes by no more than 15% of its opening range to effect a pressure bump,
passing recycled monomer to a jet pump to pump said recycled monomer into the product mixture exiting said let down valve,
passing the product mixture including the recycled monomer into a separation system for the separation of polymer from unreacted monomer, and
recycling said unreacted monomer through a recycle system,
wherein at least a portion of the recycled monomer is taken from a location downstream of a waste heat boiler and upstream of any recycle system coolers and passed into said jet pump.
2. The process of claim 1 , wherein the recycled monomer comprises monomer taken as off gas from a separation vessel in the separation system.
3. The process of claim 1 , wherein the bumping is effected by moving the high pressure let down valve to a more open position for a predetermined period of time.
4. A process as claimed in claim 1 in which during a bump the high pressure let down valve opens by at least 2% of its range.
5. The process of claim 1 , wherein the bumping frequency is at least once every ten minutes.
6. The process of claim 1 , wherein the pressure at a location upstream of the tubular reactor decreases by at least 40 bar during a bump.
7. The process of claim 1 , wherein the flow rate of product mixture into the jet pump increases by at least 100% during a bump.
8. An apparatus for the high pressure polymerization of ethylene under bumped conditions, optionally with one or more comonomers, comprising
a tubular reactor for the polymerization of monomer to produce a product mixture comprising polymer and unreacted monomer,
a high pressure let down valve downstream of the reactor for controlling the flow of product mixture from the reactor,
a jet pump downstream of the high pressure let down valve for pumping a recycled monomer into the product mixture, and
a separation system for separating the polymer from unreacted monomer, and
an unreacted monomer recycle system comprising at least one conduit located downstream of a waste heat boiler and upstream of any recycle system coolers, for conducting at least a portion of said unreacted monomer from said recycle system to said jet pump.
9. The apparatus of claim 8 , wherein the suction of the jet pump communicates with the recycle system via said conduit for the flow of recycle monomer from the recycle system to the jet pump.
10. The apparatus of claim 9 , wherein the suction of the jet pump communicates with a separation vessel of the separation system via said conduit for the flow of off gas from the separation vessel to the jet pump.