Page 40 - Plastics News June 2019

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Chemical recycling Pyrolysis and energy recovery
There are different methods of chemical recycling. This has two routes: pyrolysis into liquid fuels or gaseous
One method is to depolymerize the waste polymer into fuels. The plastic waste is heated in presence of catalyst
monomer(s) and repolymerize to the desired level. at temperatures in the range of 300 to 450 C. The gasses
This works very well and is truly sustainable if clean produced due to heating, are condensed. The non
waste polymer is available without any contamination. condensed gasses are used as fuel during heating of plastic
The scale of operation decides the economics of such waste. The condensed liquid is similar to hydrocarbon
chemical recycling. Polystyrene, poly acrylates, nylon 6 oil or similar to diesel. The yields can be 60 to 75 %
and polyesters can be treated like this. As a matter of fact depending upon the type of waste. The technology needs
the condensation or ring opening polymers are suitable to be upgraded for consistent yields. The yields vary if the
for chemical recycling by depolymerization. composition of waste changes. The resultant fuel may have
oxygenated and cyclic unsaturated compound. During the
Polyester can be depolymerized in a controlled manner
and the new grade can be used effectively. Thus bottle storage, such fuels can become viscous.
waste has high molecular weight (high Intrinsic viscosity, If the waste plastic is heated at higher temperatures,
IV)and it is depolymerized to lower IV to suit fiber gaseous fuels can be produced. The waste itself can be
spinning. The waste polyester can be upgraded by solid considered as fuel. The calorific value of plastic waste
state polymerization to make bottle grade polyester of is much more ( up to 5000 to 7500 kcal/kg) than coal.
high IV. The additives, fillers etc can pose challenges for Plastic waste can be burnt and power of about 1kW per
this process. 1 kg of plastic. In Japan, more than 65 % of plastic waste
Feedstock recycling is incinerated to produce power.
Incineration without power generation
The other type of chemical recycling is to heat the
polymers to a high temperature in the presence of catalyst When plastic waste is in small quantity or it contains
and in the controlled amount of air, to produce a mixture hospital waste or it is available at remote places, plastic
of carbon monoxide, CO, and hydrogen, H2. This mixture waste can be incinerated in the absence of air to produce
is known as synthesis gas or syn gas. By introducing char or carbon. This is known as plasma technology. The
appropriate of amount of additional air and hydrogen, waste is heated at about 1200 C in a closed furnace or
different chemicals can be manufactured. This is known as chamber. The hydrogen released during heating is burnt
Fischer – Tropsch synthesis. This is commercially practiced outside the furnace. It is claimed that the char is safe to
to produce automotive fuel and industrial polyethylene be mixed with soil or burnt in air.
waxes in South Africa. The major reactions are given Sustainable solution:
below:
The waste which is not recycled and has no useful end
•nCO + (2n + 1)H2 CnH2n+2nH2O
use, can be used for end of life solution. This can be use
•nCO + 2nH2 CnH2n + nH2O in cement kilns as carbon source (limited utility). It can
be used in road making which has also limitations. The
•2nCO + nH2 CnH2n + nCO2
incineration of waste to power generation or chemical
The products of Fischer-Tropsch process depend upon: recycling is more sustainable as it can tackle any waste:
- a) CO: H2 ratio, b) catalyst type, shape and size, c) mixed and comingled plastic waste and also domestic
temperature and pressure, and d) space velocity of organic waste.
reactants.
Conclusion
Low pressures and temperatures yield methane and lower
alkanes. At high temperatures and pressures, branched Plastic waste is a resource material. It should be looked
paraffin and alicyclic compounds are produced. Catalyst at as an opportunity rather than waste. It offers excellent
selectivity and activity are important. Modern processes opportunity for circular economy.
use bimetallic or zeolites catalysts.

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