A review of advanced recycling technology

This paper is intended as a review of the various processes put forth to convert mixed and unwashed waste plastic to a product with some economic value. Conventional mechanical or melt recycling of plastics works well when the processes can acquire large quantities of reasonably clean, single polymer articles such as PET soda bottles or natural HDPE milk bottles. The remaining rigid or flexible plastics in the waste streams are often heavily contaminated, multilayered, heavily pigmented, and difficult to sort into single polymer streams. This portion of the waste plastic stream, nicknamed the third bale, is best recycled using a thermal process to convert the mix to a liquid product which is suitable as a refinery feedstock. The three major process subgroups used are 1) pyrolysis, 2) gasification, and 3) hydrogenation. Pyrolysis converts the material to a liquid in the absence of oxygen while gasification converts the plastic to a mix of carbon monoxide and hydrogen in a limited oxygen atmosphere. Hydrogenation is a variant in the gasification process whereby hydrogen is added during the polymer cracking phase. An overview of the advantages and disadvantages of each process is described in this paper.