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Monomer Recovery Unit (MRU)

The reactor waste gases, from polyethylene reactor stream and/or product purge tank are optionally compressed before being cooled in a waste gas cooler.  Typically, the cooling is staged using ambient air or cooling water followed by cross-exchange with the light components stream from the overhead of the absorber.  The final cooling may be provided by an appropriate level refrigerant.  Depending upon the temperature, heavy co-monomers and other hydrocarbons which may have condensed are separated and returned to the reactor as a recovered co-monomer product.

The separated gas stream is conveyed into the absorber column and flows up the column.  A chilled lean solvent stream enters the top of the absorber column and flows down the column counter-currently contacting the rising gas stream over a conventional mass transfer medium such as trays or packing.  As a result of this counter-current contact, ethylene, higher alpha olefin co-monomers and heavier hydrocarbons are absorbed into the lean solvent.

The absorber column overhead gas stream is comprised of the light components, primarily nitrogen, hydrogen and other light gases that come into the absorber column with the reactor waste gases.  The high pressure absorber column overhead gas stream is rich in the light components of the reactor fluidization system and therefore are returned to the polymerization system.  This inert gas stream may also be used in the product purge tank for desorbing monomers from the polymer product or as the product transfer gas stream to convey product powder from the product discharge tank to the product purge tank.

Even though the solubility of the light components of the waste gases in the lean solvent is very low, the chilled solvent descending down the column unavoidably absorbs some of the nitrogen from the feed.  These light components are stripped out of the rich solvent containing ethylene monomer by reboiling the bottoms of the absorber column.

The heavier components of the waste gases form the preferred lean solvent, whereby no external solvent is required.  The operating conditions within the MRU facility are selected so as to balance the distribution of heavier components between feed and product streams.

The absorber column bottoms stream, which comprises absorption solvent containing absorbed ethylene, co-monomers and heavy hydrocarbons, is conveyed to a regenerator column which fractionates the rich solvent stream into a raw monomer overhead stream containing ethylene and saturated reactor byproducts such as ethane, and a bottoms stream comprised of co-monomer and heavier hydrocarbons present in the waste gas stream.

The regenerator column bottoms stream is the absorption solvent that is fed into the top of the absorber column.  This stream is comprised of heavy hydrocarbons from the reactor waste gas stream which includes co-catalyst solvent, inert condensable added to the reactor to increase heat removal capacity in the reactor, and possibly butene 1, hexene 1, or other olefin co-monomers.  Excess amount of the bottoms stream is returned to the polymerization reactor system as recovered co-monomer and heavies product.

The regenerator column overhead stream is partially condensed to reflux the column.  Doing so maximizes the recovery of co-monomers present in the waste gas stream.  The vapor stream leaving the reflux separator forms the raw monomer product.  If a nearby fractionation facility is available, such as a nearby ethylene plant equipped with a C2 splitter column, the raw monomer stream may be fractionated off-site to reject the saturated reactor byproduct ethane.  Else, the raw monomer product may be further fractionated onsite to reject the desired amount of reactor byproduct.

The raw monomer product is fractionated into an overhead recovered monomer product and a bottoms reject saturated byproduct hydrocarbons stream.  Accordingly, the byproduct column separates ethane from ethylene.  Unlike conventional C2 splitters, the byproduct column is operated with the objective to reject byproduct ethane at a rate which equals its rate of formation in the polymerization reactor.  In this manner the amount of ethane recycled is controlled, thereby establishing partial pressures in the reactor, while eliminating nearly all loss of ethylene from the system.

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