Separation steps in polymer recycling

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. 

The Ticona materials are prepared by continuous polymerisation in solution using metallocene catalysts and a co-catalyst. The ethylene is dissolved in a solvent which may be the comonomer 2-norbomene itself or another hydrocarbon solvent. The comonomer ratio in the reactor is kept constant by continuous feeding of both monomers. After polymerisation the catalyst is deactivated and separated to give polymers of a low residual ash content and the filtration is followed by several degassing steps with monomers and solvents being recycled. 

One of the most successful petrochemical applications is illustrated in Figure 8.14 treatment of resin-degassing vent gas in a polyolefin plant [28]. In these plants, olefin monomer, catalyst, solvents, and other coreactants are fed at high pressure into a polymerization reactor. The polymer product (resin) is removed from the reactor and separated from excess monomer in a flash-separation step. The recovered monomer is recycled to the reactor. Residual monomer is removed from the resin powder by... 

The reactor-outlet stream contains a dispersion of hydrocarbons in sulfuric acid. The first separation step is therefore a liquid-liquid split The sulfuric-acid phase contains some amounts of sec-butyl acid sulfate, which decomposes at higher temperature (15 °C) to produce conjuct polymers dissolved in the acid and a mixture of C4-C1( isoparaffins with low octane number (pseudoalkylate) that separates as a second liquid phase. The hydrocarbon phase contains a small amount of di-isoalkyl sulfates. These need to be removed before entering the distillation units otherwise they will decompose and release sulfuric acid. The sulfates are removed by washing with either dilute caustic or sulfuric acid. In the first case, sulfates are converted to salts that are discarded. With sulfuric acid, sulfates are converted to isoalkyl acid sulfates that can be recycled to the alkylation reactor [15, 10]. 

Finally, the HDPE slurry from the second reactor is sent to the postreactor (3) to reduce dissolved monomer, and no monomer recycling is needed. In the decanter (4), the polymer is separated from the dispersing medium. The polymer containing the remaining hexane is dried in a fluidized bed dryer (5) and then pelletized in the extrusion section. The separated and collected dispersing medium of the fluid separation step (6) with the dissolved co-catalyst 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. 

However, these methods of plastic recycling are mainly limited to the degradation of polyolefinic plastics, because the presence in the feed of Cl or N-containing polymers may lead to a poisoning of the catalyst active sites. Likewise, the inorganic fillers and contaminants contained in the raw wastes tend to remain with the solid catalysts, which means that further separation steps are necessary. 

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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