Feedstock recycling

Feedstock recycling processes recover the carbon in PVC, in the form of low-molecular-weight species (or low-molecular-weight organic molecules) that can be used as feedstock for chemical processes. Some procedures may also recover hydrogen chloride (HCl) or a neutralised salt.

Gasification

Gasification is a high-temperature reaction with restricted amounts of air, oxygen or steam. Part of the PVC waste is converted into carbon dioxide, the rest to syngas which is used to produce chemical feedstock such as methanol, ammonia, oxo-aldehydes, or for making fuels. An advantage of this process is that the chlorine is almost entirely liberated as water-soluble HCl, which is easily scrubbed out and reclaimed.

Gasification has been successfully applied commercially in large plants processing coal (Sasol in South Africa) or mixed plastic waste, such as the Ebara Ube Industries Processes (EUP) in Japan. The process involves high pressure and a high temperature and therefore requires significant investments and a large scale to operate profitably.

Sumitomo Metals: The Sumitomo process is a waste-gasification and secondary-ash melting system for plastic waste using iron-making and steel-making technologies. This process developed in Japan is able to treat both mixed plastics waste and pure PVC waste.

The gasifier consists of a packed ("fixed") bed at a temperature of 2,000°C and a fluidised bed at the top of the reactor at a temperature of 800 to 1,100°C. The reactor operates close to atmospheric pressure with a reduced atmosphere to avoid formation of dioxins or furans. The ash residue in the gasifier is melted in the smelting furnace and removed from the bottom of the gasifier. The furnace is equipped with top and sideway oxygen blow lances, ensuring a local temperature above 2,000°C. Plastic waste with low calorific value needs additional coke or wood as a carbon source for steady operation. PVC waste has a low calorific value and therefore needs more additional coke than other plastics waste (7-10% for PVC).

Ecoloop: The German plant produces syngas from waste with a capacity of 40,000 tonnes of incoming waste (plastics and biomass), and can also cope with chlorinated waste. It produces syngas, which is currently used to generate energy for lime production, and calcium chloride, but has the potential to be used for the production of chemicals. The plant started operating in 2012.

Ebara proces: Ebara's gasification process, known as 'Twin Internally revolving Fluidized bed Gasifier', is combined with its well-proven technology for ash melting – the 'Meltox' process. This process – known as EUP (Ebara-Ube Process) – uses a cyclonic combustion chamber to turn the solid residues into a stable granulated slag that can be recycled. The low-temperature gasification takes place at 600-800°C, and the secondary high-temperature gasification at 1,350°C. Both reactors are operated at about 10 bars.

The process is developed to treat mixed plastics waste, with a chlorine limit of 5%, although it could probably accept higher chlorine contents with some design adaptations. At the moment, pure PVC cannot be treated in this process.

Two commercial plants are currently operating in Japan. The syngas produced can also be used in other applications, including methanol, H2, fuel cells and energy production. The chlorine is recovered as NH4Cl(s), which is used as a fertiliser agent.

Pyrolysis

Pyrolysis is high-temperature decomposition normally in the absence of air or oxygen that yields a residue of carbon or heavy hydrocarbons. The process is used to convert non-halogenated plastics, although the presence of chlorine creates specific challenges.

Dow/BSL: This mixed waste German plant, fully operational and commercially viable has been in operation since 1999. Trials with PVC waste were successfully conducted from January to March 2000 and July 2002 to April 2003.

The process can handle mixed PVC waste, contaminated oil, bio-sludge and hazardous solids containing chlorinated substances. HCl and energy are recovered. The annual capacity is 45,000 tonnes of waste intake.

An average of 90% of the chlorine from the PVC input is recovered as 20% HCl (aqueous). The HCl quality is within the specification for use in the on-site chlor-alkali plant via membrane electrolysis. PVC waste has a lower calorific value than other plastic waste due to the high chlorine content.

To treat PVC waste in the rotary kiln, other waste with high calorific value is added to support the combustion. A total energy recovery of 50% is achieved for PVC.

Dehydrochlorination

This method covers mild degradation processes removing chlorine in a first step, which can then be followed by gasification or pyrolysis. Dehydrochlorination can take place under pressure in water, in ionic high-boiling liquids, or in dry processes, such as melting, or by hydrogenation.

Dehydrochlorination in water

REDOP process: The REDOP process targets the mixed plastic fraction from municipal waste, which usually contains around 1% chlorine, with ranges 0.5 to 5.0 wt.%.

The process is broken down in the following steps:

  • Post separation of plastic and paper from municipal solid waste;
  • Separation of the mixed plastics fraction from the paper fraction;
  • Dechlorination of the mixed plastics fraction;
  • Co-injection (together with coal) into a blast furnace for the production of pig iron.

Of special interest is the dechlorination step, using a novel process patented by chemical company DSM. Mixed plastics waste is heated batch-wise in a stirred reactor. Degradation products from the cellulose still present act as emulsifiers, helping to stabilise the slurry. The released HCl is neutralised by the addition of a diluted water- soluble base. The non-PVC plastics melt into droplets. When the reactor is cooled down, the plastic droplets solidify and yield granules that only need filtering, washing and drying.

Alzchem: The plant has a production capacity of 150,000 tonnes per year of calcium carbide and it strives to use as much plastic waste as possible. A pilot project is ongoing in Germany to eliminate as much chlorine as possible before entering the reactor, with an upstream extruder operating at temperatures which can degrade PVC. The resulting HCl could be sold as a water solution.

Dehydrochlorination in ionic liquids

KU Leuven: A team of this Belgium university is studying dehydrochlorination of PVC in ionic liquid media. These liquids are essentially non-volatile even at elevated temperatures (250°C and more). This allows for evacuation of the HCl by vacuum or a gas stream, thus avoiding salt formation by reaction of the HCl with caustic soda. The dehydrochlorinated PVC would precipitate.