Separation steps in polymer recycling process

RODRIGUEZ ET AL. Separation Steps in Polymer Recycling Processes... 

Lewis acids, such as the haUde salts of the alkaline-earth metals, Cu(I), Cu(II), 2inc, Fe(III), aluminum, etc, are effective catalysts for this reaction (63). The ammonolysis of polyamides obtained from post-consumer waste has been used to cleave the polymer chain as the first step in a recycle process in which mixtures of nylon-6,6 and nylon-6 can be reconverted to diamine (64). The advantage of this approach Hes in the fact that both the adipamide [628-94-4] and 6-aminohexanoamide can be converted to hexarnethylenediarnine via their respective nitriles in a conventional two-step process in the presence of the diamine formed in the original ammonolysis reaction, thus avoiding a difficult and cosdy separation process. In addition, the mixture of nylon-6,6 and nylon-6 appears to react faster than does either polyamide alone. 

Finally, the HDPE-slurry from the second reactor is sent to the post reactor (3) to reduce dissolved monomer. The process total conversion is up to 99.5%. In the decanter (4), the polymer is separated from dispersing medium. The polymer containing the remaining hexane is dried in the fluid bed dryer (5) and then pelletized in the granulation section. The separated and collected dispersing medium of the fluid separation step (6) with the dissolved cocatalyst and comonomer is recycled to the polymerization reactors. A small part of the dispersing medium is distilled to maintain the composition of the diluent. 

Depending on the silicon-bonded substituents R in the starting compound, the viscosity of the solution increased considerably within 30 to 60 min (R = H, Cl) or within 24 h (R = CH3). The obtained gels aged readily and excess Bis as well as the couple product, Me3SiCl, which separated from the polymer, were distilled off under reduced pressure. The whole process is inexpensive, since excess bis and Me3SiCl can be recycled. Time-intensive processing steps are not required. 

The MD/PEG system offers the combined advantages of low-cost, reduced lower phase viscosities and high density differences for inexpensive polymer-polymer affinity partitioning. When coupled with low-cost affinity ligands i.e. triazine dyes, two-phase aqueous affinity partitioning could be used as the first step in a separation train for the recovery of industrially important enzymes. The bottom phase, which is generally considered to be a waste stream and non-recyclable if dextran or salt is used could be used as a substrate for additional fermentations if maltodextrin is used, thereby aiding the overall economics of the process. 

In the Hoechst process, for example, hexane is used as the diluent (108,109). Hexane, ethylene, alpha-olefin, catalyst components, and hydrogen are continuously fed into a stirred reactor for polymerization. The slurry is then transferred into a smaller reactor for post-polymerization, after which the total charge is separated by a centrifuge into a liquid stream (which is returned to the initial reactor) and solid polymer. The wet polymer is steam-stripped from the solvent, dried, and pelletized. The stripped hexane is purified and recycled. Although stirred tanks are most common, loops can also be used in this fashion. In some schemes, a portion of the recycle diluent from the centrifuge is returned to the reactor, and a portion is fed to recycle purification for wax removal. This step removes some of the lowest molecular weight pol5mier, which dissolves in the diluent. 

A disadvantage with the slurry processes is that the diluent contained in the slurry from the reactor has to be separated Ifom the polymer powder and purified before it is recycled back into the reactor. This process step is more complicated and more expensive than the corresponding recycling system for the gas phase process. Using a light diluent (isobutane, propane) makes it possible to separate most of the diluent by a direct flash of the slurry Ifom the reactor, which is not feasible with heavier diluents due to the higher boiling point. 

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