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Chemical recycling

Foreseeing the future with chemical recycling

Our environment has been dealing with the consequences of plastic pollution. Persistent usage of plastic products has led to an increase in plastic production by 4 to 5% annually. As of 2019, global plastic production reached over 350 million metric tons, and still, we are able to recycle only 20% of the annual production. With the growing amount of plastic trash, companies like Coco-Cola, Nestlé, HUL, and many others are transitioning towards recycled plastic products.

Where not every type of plastic can be mechanically recycled, this again highlighted the need to introduce a shift from the conventional recycling methodologies. An underdeveloped technique known as Chemical recycling was brought up into functioning to effectively deal with the increasing plastic waste.

Chemical recycling or advanced recycling is basically the categorization of technologies that easily target those plastics that have been left untreated with the usual mechanical recycling process. The chemical methodology also ensures to extract the invaluable properties from the discarded polymers while delivering a virgin-quality like raw material.

How does chemical recycling work?

Plastic is chemically structured as a long chain of repeating polymer units. The arrangements of these units decide the physical properties of the resultant material. These hard-to-break chemical structures make plastic difficult to biodegrade and chemical recycling targets to break these bonds.

In chemical recycling, external conditions like heat, pressure, and catalysts are targeted upon these polymer chains so that they can be broken down into their original monomers. These monomers are then extracted and used as virgin raw material for further processing.

Chemical recycling is an umbrella term that includes different techniques to work with plastic recycling. These techniques are Purification, Depolymerization, and Feedstock Recycling. Let’s understand each one of these:

1. Purification or Dissolution-

The plastic is immersed in a solvent so that additives can be removed and the basic structure of the polymer is not altered. Targeted polymers are clustered together based on the solvent used.

Dissolution differs from depolymerization and feedstock recycling as it just focuses on removing the impurities from the plastic. Also, thermal degradation is involved in this process which fails to deliver virgin-quality like raw material.

2. Depolymerization-

This process, also known as chemolysis or solvolysis uses chemicals to target the plastic to extract its monomer. By further purification, the end product acts like a virgin monomer and hence is used in the preparation of polymers. Pre-sorting and cleaning of material are required for this process to deliver the desired output.

The major setback this process has is that it uses condensate polymers (such as PET & PA) only.

3. Feedstock recycling or Thermal Conversion-

Like depolymerization, thermal conversion also breaks down the polymers into monomers, giving virgin quality raw material. To begin with the process, mixed plastic waste is screened and with the help of heat and chemicals, plastic waste is transformed into liquid, oil-like (pyrolysis) or gaseous (gasification) forms depending upon the chemistry used. Feedstock recycling is used to produce basic chemicals like hydrocarbons to extract fuel and petrochemicals.

Based upon the chemistry and extracted end product, feedstock recycling has 3 major processes, namely Gasification, Pyrolysis & Hydrothermal Treatment.

• Pyrolysis:

This technique is used to transform plastic waste into energy. It is a thermal degradation process where plastic waste is heated in a temperature range of 450-900oC, in an oxygen-free environment which results in the breaking down of polymers into simple compounds. This process leads to the conversion of plastic trash into synthetic crude oil which is purified to be further used in refineries to extract diesel fuel, wax & heating oils.

In pyrolysis catalysts like zeolite is used to increase the efficiency of the process and also to reduce the process time and temperature.

• Gasification:

In this process, plastic waste is pre-treated to remove moisture, and then the resultants are fed into high-temperature process units usually at 1000-1500oC. Unlike pyrolysis, gasification occurs in the presence of oxygen which reduces the plastic to syngas (or synthetic gas). Syngas if further cleaned at high temperatures to be used for further chemical processes.

The products then obtained are further used in electric power generation or conversion into fuel, ethanol, and also for the production of hydrocarbons. The reason why gasification is widely used is its ability to target mixed waste for recycling and delivering the product which could replicate virgin feedstock.

• Hydrogenation:

Hydrothermal treatment or hydrogenation uses hydrogen and heat (160-240OC) to break down the polymeric structure of plastic waste. At high temperatures, high corresponding pressure is applied, allowing the water to stay in the liquid phase. Different additives are also used to enhance hydrolysis. The end product produced after this chemical reaction is highly saturated in nature.

The targeted materials for the hydrolysis process are polyesters, polycarbonate, nylons. Hydrogenation results in the production of synthetic crude oil which is further used for refinery operations.

Where these technologies are still under development, they aim to eradicate the problem of non-biodegradable waste. With initiatives from several companies and startups, the production process could be paced up. Where the promise is to effectively deal with the plastic problem, there have still been several questionings & doubts if chemical recycling is worth the hype? What is Plastic Recycling Code and why is it important?

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