O

Reject

Rear-illuminated falling particles

Product

Lamps

Background

Lamps

Background

Ejected particle (clear)

Reject o

Figure 14.24. Schematic showing the operating principles of a commercial flake color sorter [112].

Figure 14.25. Quality control laboratory supporting a plastics recycling plant. (Photo courtesy of MBA Polymers, Inc.).

Feedstock Recycling The term feedstock recycling broadly defined encompasses chemical recycling but is most often applied to the thermal cracking of polyolefins (polyethylenes and polypropylene) and substituted polyolefins into a variety of smaller hydrocarbon intermediates. In some processes, the yields of ethylene, propylene, and butylene can be significant [1]. In some cases, other addition polymers can be thermally depolymerized directly back into monomers in reasonably high yield. Examples are polystyrene to styrene [118] and polymethyl methacrylate to methyl methacrylate [119, 120]. A special class of feedstock recycling processes yields an important raw material called synthesis gas, a mixture of hydrogen and carbon monoxide [121, 122]. Hydrocarbons and synthesis gas can all be used as chemical feedstocks for further upgrading to commercially important products at oil refineries and chemical plants. The production of methanol can be mentioned as a specific example [122]. Feedstock recycling has been extensively applied to the recycling of mixed postconsumer packaging plastics, but feedstock recycling of plastics from end-of-life electrical and electronic equipment and end-of-life automobiles and appliances has also received attention [1, 123-126]. The U.S. Department of Energy has extensively investigated both the co-liquefaction and co-gasification of postuse plastics with coal to produce petrochemical feedstocks and transportation fuels [127, 128].

Chemical Recycling The term chemical recycling is most often applied to the depolymerization of certain condensation and addition polymers back to monomers using chemical reagents and/or solvents [1, 116]. Polyethylene

EFG - entrained flow gasifier SBR - spouted bed reactor MMB - molten metal bath <SCF - almost supercritical fluid FXB - fixed bed

Figure 14.26. Overview of range of feedstock recycling technologies that have been investigated for the recycling of postuse plastics. Source: American Plastics Council.

EFG - entrained flow gasifier SBR - spouted bed reactor MMB - molten metal bath <SCF - almost supercritical fluid FXB - fixed bed

Figure 14.26. Overview of range of feedstock recycling technologies that have been investigated for the recycling of postuse plastics. Source: American Plastics Council.

terephthalate (PET), certain polyamides (nylon 6 and 66), and polyurethanes (PURs) can be efficiently depolymerized. The resulting chemicals can then be used to make new plastics that can be indistinguishable from the initial virgin polymers.

Several chemical recycling processes for postconsumer PET have been scaled up [1, 116]. For example, reacting PET with ethylene glycol forms bis-hydroxyethyl terephthalate, a starting monomer for the synthesis of PET. Hydrolysis of PET yields terephthalic acid, another monomer. A third process uses methanol to depolymerize PET to the dimethyl ester of terephthalic, which can also serve as a monomer for the manufacture of PET.

Nylon 6 can be cracked back to caprolactam in the presence of phosphoric acid catalyst and superheated steam, and one nylon manufacturer has used the process to recycling the nylon face fiber on carpets [1, 116, 129]. Mixtures of nylon 6 and 66 can be depolymerized to useful monomers by ammonolysis at above 300°C by combining nylons and ammonia in the presence of catalysts [130].

Commercial polyurethanes depolymerization has been by glycolysis. By reacting polyurethanes with glycols, new polyols can be made. Following purification, the new polyols can be used for new polyurethane manufacture [1, 5, 6].

Was this article helpful?

0 0
Building Your Own Greenhouse

Building Your Own Greenhouse

You Might Just End Up Spending More Time In Planning Your Greenhouse Than Your Home Don’t Blame Us If Your Wife Gets Mad. Don't Be A Conventional Greenhouse Dreamer! Come Out Of The Mould, Build Your Own And Let Your Greenhouse Give A Better Yield Than Any Other In Town! Discover How You Can Start Your Own Greenhouse With Healthier Plants… Anytime Of The Year!

Get My Free Ebook


Post a comment