Ethylene recovery from a polymerization process
View Patent ↗A method for solution polymerizing ethylene wherein an ethane purge stream is separated, subjected to high efficiency cryogenic distillation, and a side cut from that distillation recycled to the polymerization process while the overhead and bottoms from that distillation are removed from the polymerization process.
1. In a process for the solution polymerization of ethylene wherein a single phase solution containing at least ethylene and catalyst dissolved in a solvent is employed to form polyethylene dissolved in said single phase solution, said ethylene polymerization being terminated with at least one deactivator that leaves a decomposition residue, said single phase solution being subjected to a series of process steps to produce an ethylene rich stream containing ethane, hydrogen, CO 2 , C4 and heavier hydrocarbons, and de-activator residue, said ethylene rich stream being suitable for recycling as feed to said ethylene polymerization process, the improvement comprising removing an ethane purge stream downstream of said series of steps from said ethylene rich stream, wherein no purge stream is removed during the series of steps containing ethylene, ethane, hydrogen, CO 2 , C4 and heavier hydrocarbons, and deactivator residue, said ethane purge stream being removed in an amount sufficient to maintain the quantity of ethane in said polymerization process below the level at which polyethylene separates from said single phase solution upstream of said series of steps, cooling said removed ethane purge stream to essentially liquefy it, subjecting said liquefied ethane purge stream to distillation in a high efficiency distillation column that contains the equivalent of at least about 45 trays and produces an overhead stream and a bottoms stream, removing from said high efficiency distillation column a side cut stream that contains a major amount of ethylene, a minor amount of ethane, and essentially no hydrogen, CO 2 , C4 and heavier hydrocarbons, and deactivator residue, recycling said side cut stream as ethylene feed to said polymerization process, and removing said overhead and bottoms streams from said polymerization process, whereby the quantity of said removed overhead and bottoms streams combined is substantially less than if an ethane purge stream was removed from said process during said series of steps in an amount sufficient to maintain the quantity of ethane in said polymerization process below the level at which polyethylene will separate from said single phase solution.
2. The method of claim 1 wherein said series of steps comprises 1) formation of a polymer rich phase and a solvent rich phase, 2) separation of said polymer rich phase and solvent rich phases from one another, said solvent rich phase containing solvent, ethylene, and impurities including ethane, waxes, hydrogen, CO 2 , water, C4 and heavier hydrocarbons, and deactivator residue, 3) removal from said solvent rich phase of both solvent and waxes, and 4) removal from the remainder of said solvent rich phase part of said hydrogen, CO 2 , and deactivator residue and essentially all of said water to produce said ethylene rich stream ( 30 ).
3. The method of claim 1 wherein said high efficiency distillation column overhead and bottoms streams combined constitute the sole ethane purge stream removed from said polymerization process.
4. The method of claim 1 wherein said deactivator residue is at least one oxygenate.
5. The method of claim 1 wherein said solvent is hexane.
6. The method of claim 1 wherein at least one co-monomer is employed in said polymerization process.
7. The method of claim 1 wherein said side cut stream contains at least about 95 wt. % ethylene, and less than about 5 wt. % ethane.
8. The method of claim 1 wherein the quantity of said removed overhead and bottoms streams combined is at least about 75% less than if during said series of steps an ethane purge stream was removed from said process in an amount sufficient to maintain the quantity of ethane in said polymerization process below the level at which polyethylene will separate from said single phase solution upstream of said series of steps.
9. The method of claim 1 wherein said series of steps comprises separation of said single phase solution into a polymer rich phase and a solution rich phase, distillation of said solvent rich phase to remove said solvent and said waxes, scrubbing of the remainder of said solvent rich phase to remove at least part of said CO 2 , dehydrogenation of said remainder, and water removal from said remainder.