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Can we reverse the process of making plastic that will break it into its original elements?
Can we reverse the process of making plastic or develop a new process that will break plastic back down into its original elements? Can we at least break it down into something that is less permanent?
I'm looking for someone who has experience in plastic engineering or who is up to date in this field to produce a half-page document answering these questions.
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Plastic can indeed be broken down into its original form (light crude oil) by a process known as “thermal depolymerisation”, which is effectively a form of industrial pressure cooking utilising hydrous pyrolysis. This involves mixing waste plastic with water and heating to a temperature between 220°C and 360°C (depending on the plastics to be broken down) and a pressure between 7Mpa and 18Mpa. The process generally takes 30 to 90 minutes depending on batch size and contents. The products of this breakdown process are a light crude oil, which can then be fractionally distilled into constituent hydrocarbons to be used as fuel or to create new plastics. Other waste materials, such as rubber tires, domestic organic waste and certain agricultural wastes can also be broken down via this process. The obvious benefits of this process are that it eliminates waste that would otherwise end up in a landfill, and produces a useful commodity from an essentially free waste stock (used plastics). However the downsides are that this process can use a significant amount of energy and water. However with the continual development of renewable energy sources, thermal depolymerisation is on the brink of becoming economically viable, with several test plants in operation in the United States (see this website for more information on a test plant http://www.changingworldtech.com/where/philadelphia.asp).
Plastics can also be broken down using anhydrous pyrolysis to create a bio-oil similar to the product of thermal depolymerisation. This process uses heat and pressure in the absence of oxygen to break down plastics and organic wastes. The disadvantage pyrolysis is that it requires temperatures around 430°C and the absence of oxygen to avoid combustion. As such this process may consume more energy per unit of oil produced; however this process consumes no water.
These processes are of course in addition to existing plastic recycling technologies, which can quite effectively recycle PET and HDPE. However the quality of the plastic reduces with each recycle, eventually leading to recycled products being discarded to landfills. This means that the breakdown of plastics into their constituent parts may become an important industry in the future as oil prices continue to rise.
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