Advanced Chemical Recycling for Plastics

The term chemical recycling covers a variety of processes for breaking down plastic waste so it can be remanufactured, much as it was originally, or made into something else entirely. It uses several techniques that reflect the extent to which the waste is broken down. 

Dissolution mixes waste with a suitable solvent to liquify it. While in this state, it is possible to remove incompatible waste and pigments. Once cleaned, application of an anti-solvent causes the polymer to resolidify in its pure form, ready to be used again. The solvent and anti-solvent can be separated via distillation for reuse. 

Depolymerization uses a chemical reaction to break polymers back into their monomer form, allowing contaminants to be removed. A second reaction repolymerizes the monomer, turning it back into solid plastic again, in its pure form. This process operates under high pressure and temperature. Both of these require sorting prior to processing.

Other processes use pyrolysis, sometimes combined with steam, to break down waste, with the amount of heat determining the extent of decomposition. Mild approaches recover hydrocarbons suitable for use as fuels or feedstocks for manufacturing new plastic. More thorough pyrolysis can create syngas for a variety of uses. Pyrolysis processes are not as picky about sorting and can accept mixed waste but can be energy intensive. 
 

Companies moving into the chemical recycling space start from scratch. Some are startups, others are new divisions of larger companies, but the process units are brand new. For example, PureCycle can create near-virgin polypropylene from waste plastic feedstock using a dissolution process. The company has very ambitious plans to build 50 new plants in the next 15 years using this approach. To make that goal a reality, PureCycle is partnering with Emerson to equip these new facilities with intelligent sensors and control valves, advanced operations software, and cloud data management systems. Adopting advanced digital automation technologies, it enables for faster project completion, fully integrated systems, and world-class operating performance.

It’s not difficult to see that the world is drowning in plastic trash, so recycling is enormously important, and we must be doing everything we can to encourage it. But it is only one step in the development of a true circular economy. Recycling acknowledges and facilitates our “single use” mentality where we buy something, use it, and throw it away. We must replace this linear mindset and move to a circular economy where products last several lifecycles. They’re designed to be reused, repaired, and remanufactured. When they reach the end of multiple uses, they are recycled easily to begin a new life. Obviously, making this happen requires designing this capability into a product from its conception. If practiced, this reduces environmental impact enormously, and reduces the very need for recycling.

Yes, this technique is called mechanical recycling and is practical within specific limits. Carefully sorted plastic trash, including sorting by color as well as material, can be shredded and cleaned, and then remelted. This is used primarily for polyethylene terephthalate (PET) material, which is used extensively for plastic beverage bottles. PET can withstand such treatment with minimal loss of quality, however most other plastic types are degraded in the remelting process.

The degree of sorting necessary depends on the nature of the process. For those processes producing recycled plastic equal to virgin material, such as polypropylene, the sorting must be very thorough, although most depolymerization and dissolution processes can remove colors. Pyrolysis processes are more tolerant of mixed feedstocks, but they reduce the final product to a much lower chemical state.