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Supercritical water to recycle plastics

The importance of the need to chemically revamp the recycling of waste plastics was appropriately featured in C&EN’s June 18 issue (page 24). Strangely, the successful use of water alone to cleave the bonds in these and other giant molecules in a matter of seconds was not mentioned.

Of course, the water must be in a supercritical state, when it has remarkable solvent and heat transport properties, as collected by Yizhak Marcus in “Supercritical Water: A Green Solvent; Properties and Uses.” A more recent book is “Hydrothermal and Supercritical Water Processes” by Gerd Brunner.

In addition to the extreme rapidity of the supercritical water reactions, simultaneously hydrogen atoms are provided from the water that stabilize the free radicals in the small molecules created by bond cleavage in the substrate and thereby reduce or eliminate the objectionable char formation of conventional, purely thermal pyrolysis processes.

To take advantage of these unique characteristics, a continuous reactor system is desirable, and a demonstration-sized unit has been constructed and patented (US Patent No. 9932285).

Several industrial companies (Licella, Renmatix, and Xtrudx) are actively involved with high-temperature water, and Massachusetts Institute of Technology has a substantial program under the leadership of professor William H. Green of the chemical engineering department.

Graham Allan
Seattle

Editor’s note: Allan is an inventor on the patent mentioned in the letter and is involved in the company Xtrudx.