Hydrothermal liquefaction processes for plastics recycling: A review
Georgina C. Laredo, Joel Reza, Edith Meneses-Ruiz
Abstract
• Hydrothermal liquefaction (HTL) has been studied for plastic recycling. • At sub- and supercritical conditions, HTL depolymerization may be carried out. • Subcritical conditions can depolymerize polymers with functional groups. • It is easier to depolymerize polyolefins with supercritical water(> 374 °C; > 22 MPa). • Low-pressure hydrothermal liquefaction (LP-HTL) can handle polyolefin hydrocarbons. If our expectations are to have a future with the resources provided by the earth, the recycling of plastics has become one of the most important topics that, as humans, we must deal with. Among the technologies developed for treating this issue is the hydrothermal liquefaction (HTL) method. In this review, subcritical and supercritical hydrothermal processes are presented. Experimental methods and product yields are disclosed and discussed. Subcritical conditions have previously been used to depolymerize synthetic polymers containing heteroatoms, such as bisphenol-A-based epoxy resin (Epoxy), polyamide 6 (PA6), polyamide 6/6 (PA66), polyethylene terephthalate (PET), polycarbonate (PC), and polyurethane (PU). This type of polymer can be broken down using this low-temperature, low-pressure method because it has heteroatoms that are easy to break down. To depolymerize polyolefins like polyethylene (PE) and polypropylene (PP), derivatives and mixtures, formed by long hydrocarbon chains, supercritical water conditions (> 374 °C; > 23 MPa) seem to be required. These requirements make the procedure quite expensive. The review showed that a new method that uses pressures between 2.5 and 30 MPa, temperatures above 400 °C, and residence times of 20 to 60 min, named low-pressure hydrothermal liquefaction (LP-HTL), can be used to handle this type of polyolefin hydrocarbon. This review describes the conditions needed to handle this problematic type of feedstock and, in a certain way, the possible utilization of such technology for treating more complex waste mixtures.