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Different Processes for Waste Plastics Treatment

Gasification process consists of two parts, a liquefaction step and an entrained bed gasifier. In the liquefaction step the plastic waste is mildly thermally cracked (Depolymerization) into a synthetic heavy oil and some condensable and non-condensable gas fractions. The non-condensable gases are reused in the liquefaction as fuel (together with natural gas). This liquefaction process is comparable to visbreaking of vacuum residue from oil refining.

The heavy oil is filtered to remove large inorganic particles. The oil and condensed gas are then injected to the entrained gasifier. Also, chlorine containing gases from the plastic waste are fed to the gasifier. The gasification is carried out with oxygen and steam at a temperature of 1200 — 1500 °C. The gasification pressure is normally adjusted to the pressure of the process which will consume the resulting synthesis gas. After a number of cleaning processes (amongst others, HC1 and HF removal), a clean and dry synthesis gas is produced, consisting predominantly of CO and H with smaller amounts of CH CU H and some inert gases.

Virtually all chlorine present in MPW is captured by washing the raw syngas under addition of NH and converted into saleable NH Sulphur from MPW is won back in a pure, saleable form. One may assume that any metals present in a PVC-formulation end up mainly in the solid residues. Filtrated waste water from the scrubber and quench is distilled, yielding reusable water, crystallized NH and a brine purge that is recirculated to the gasifier.

THE POLYMER CRACKING PROCESS

The “Cracking Process”, is a fluid bed cracking process. In this process some elementary preparation of the waste plastics feed is required, including size reduction and removal of most non-plastics. This prepared feed is fed directly into the heated fluidized bed reactor which forms the heart of the Polymer Cracking process. The reactor operates at approximately 500°C in the absence of air. The plastics crack thermally under these conditions to hydrocarbons which vaporize and leave the bed with the fluidising gas. Solid impurities, including metals for e.g. PVC stabilisers and some coke, are either accumulated in the bed or carried out in the hot gas as fine particles for capture by cyclone. The decomposition of PVC leads to the formation of HC1. Which is neutralised by bringing the hot gas into contact with a solid lime absorbent? This results in a CaCl fraction that has to be land filled. The purified gas is cooled, to condense most of the hydrocarbon as valuable distillate feedstock. This is then stored and tested against agreed specifications before transfer to the downstream user plant. The remaining light hydrocarbon gas is compressed, reheated and returned to the reactor as fluidizing gas. Part of the stream could be used as fuel gas for heating the cracking reactor, but as it is olefin-rich, recovery options are being considered.

THE BASF CONVERSION PROCESS

The BASF feedstock recycling process was designed to handle the recycling of mixed plastic waste. Before the waste plastics can be fed to the process, a pretreatment is necessary. In this pretreatment the plastics are ground, separated from other materials like metals and agglomerated. The conversion of the pretreated mixed plastic into petrochemical raw materials takes place in a multi-stage melting and reduction process. In the first stage the plastic is melted and dehalogenised to preserve the subsequent plant segments from corrosion. The hydrogen chloride separated out in this process is absorbed and processed in the hydrochloric acid production plant. Hence, the major part of the chlorine present in the input (e.g. from PVC) is converted into saleable HC1. Minor amounts come available as NaC1 or CaC1 effluent. Gaseous organic products are compressed and can be used as feedstock in a cracker. In the subsequent stages the liquefied plastic waste is heated to over 400 °C and cracked into components of different chain lengths. About 20-30% of gases and 60-70% of oils are produced and subsequently separated in a distillation column.

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