Abstract:
As the problem of plastic waste management increases in Pakistan, the need for
better and more efficient chemical recycling methods also arises. Our team worked
on the utilization of waste PET (Polyethylene Terephthalate) bottles in order to obtain
useful monomers to close the loop on chemical recycling of plastics. This includes the
process we have used to de-polymerize the PET, “supercritical methanolysis” as well
as the challenges associated with separation of useful products from the
depolymerization.
The PET flakes must be reduced in size through some pre-treatment like crushing, or
even dissolution is methanol/ethylene glycol. The reactant methanol must be
vaporized for the reaction to occur in the supercritical phase of methanol through
heating whether in the reactor itself or in a heat exchanger. The solid-vapor mixture
is to be fed to a reactor where along with high RPM agitation, the mixture is allowed
to react for 30 minutes under 300 oC temperature and 80 bar pressure. The reaction
mixture now contains the desired products, ethylene glycol (EG) and dimethyl
terephthalate (DMT).
In order to obtain a high purity of EG, DMT must be separated from the reactor
product stream using crystallization, after which it is filtered out and dried. The rest
of the mixture is sent to two distillation columns, one removing the large quantity of
methanol from the mixture and the next distilling out our main product, EG at a very
high purity of 99.95%.
The objective of this proposed process is to provide two valuable monomer products
needed to make PET plastic which in turn can be used in the polymerization process
to replace some requirement of fresh monomers needed to produce it. Furthermore,
these chemicals are also valuable in other applications such as being used in making
solvents, pharmaceutical products, plasticizers, dyes, and pigments etc. This in turn
completes the loop on recycling plastics chemically.
