This SBIR Phase I project will ascertain the parameters for three of the most common waste plastics (namely, polystyrene (PS), polyethylene (PE), and polypropylene (PP)), and determine the solubility and stability characteristics of the resulting reaction products in commercial grade liquid diesel fuel. Waste plastics are a perceived societal problem; however, waste plastics comprise various hydrocarbon chains the fundamental components of which are valuable chemicals suitable for diesel engine combustion. Supercritical water (SCW) has recently been shown to be an effective solvent and reagent for the depolymerization of various polymers.
The broader/commercial impact of the proposed project will be to remove waste plastics from landfills and utilize them to produce a consumable fuel.
As way of background, our Phase I project proposed a novel approach to solve the chemical engineering problem of continuous and energetically favorable depolymerization of common waste plastics into diesel-like liquid hydrocarbons. Waste plastics are a perceived societal problem; however, waste plastics comprise various hydrocarbon chains the fundamental components of which are suitable for use in internal combustion engines. Supercritical water (SCW) has recently been shown to be an effective solvent and reagent for the depolymerization of various polymers. What was not known prior to our Phase I project (or at least has not been reported in the literature) was the minimum amount of water needed (together with required minimum reaction times, temperatures, and pressures) to achieve complete waste plastic depolymerization without the formation of unwanted char or degradation products. In view of these objectives, our Phase I project demonstrated (in a custom-built "batch" reactor) that under supercritical reaction conditions of less than 10 seconds, each of the three plastics studied required at least about an equal part of water in order to obtain complete liquification to small chain hydrocarbons of up to 100% yields without any significant char formation. Of the three plastics studied, polystyrene (PS) was found to be the most attractive starting material because of the conversion in near 100% yields into a water-insoluble oil within only a few seconds. This oil product was found to be completely miscible in a commercial diesel fuel at various levels up to the tested limit of 40%. The mixtures were found to be stable for at least 100 days and acceptable for use as a diesel engine fuel substitute. Polypropylene (PP) and polyethylene (PE) were also converted to liquid hydrocarbons by contact and reaction with supercritical water. However, and unlike polystyrene, some modest (<5%) gasification occurred with each that makes their processing transformation slightly more complex (with respect to future scale-up to a continuous feed extruder-based system as now being proposed under Phase II). During Phase I of our SBIR project, supercritical water (SCW) was shown to break down polyethylene (PE) by cleaving the carbons at various locations along the backbone and simultaneously capturing hydrogen atoms from the water. As a consequence of these reactions, a whole range of liquefied linear hydrocarbons of various lengths were formed. In the case of polypropylene (PP) the breakdown fragments were not found to be linear, but branched (because of the side chain methyl groups on every other carbon atom in the polymer backbone). These branched structures were found to be even more soluble in gasoline than their counterparts from polyethylene. When the starting renewable plastic was polystyrene (PS), the breakdown fragments were somewhat different because supercritical water was found not to readily cleave aromatic rings. The aliphatic backbone chain of polystyrene was instead cut by the supercritical water as were polyethylene and polypropylene backbones, but the polystyrene-derived fragments contained aromatic rings that originated from the phenyl rings of the polystyrene. Thus, the resulting hydrocarbon product was found to be more like the aromatic components of kerosene with its alkylbenzenes currently used in commercial grade diesel fuel. Consequently, these aromatic higher boiling substances were found to be more compatible as a diesel fuel as opposed to as a gasoline-type fuel. The Xtrudx team has shown that through its proprietary prototype "batch" reactor various waste plastics along with water can be heated to SCW conditions and cooled, within a matter of seconds, to achieve near 100% yields of a crude oil equivalent (called "neodiesel"). In view of these favorable results, the Xtrudx team has designed a "continuous" reactor proposed for Phase II.
