Pretreatment is crucial for removing contaminants and non-plastic materials, especially polyvinyl chloride (PVC) and polyethylene terephthalate (PET). This is a crucial step that directly determines the entire pyrolysis of plastic to diesel process. Sorting plastic is crucial. Plastics unsuitable for pyrolysis of plastic to diesel, such as PVC and PET, must be removed.
The pyrolysis of PVC produces hydrogen chloride (HCl) gas, which corrodes plastic pyrolysis equipment and contaminates oil products, potentially generating highly toxic dioxins. PET contains oxygen, which can produce oxygenated compounds (such as benzoic acid), leading to high acidity, instability, and corrosiveness in the oil.
The ideal raw materials for pyrolysis plastic to diesel are primarily polyolefins, such as PE (polyethylene) and PP (polypropylene). They have high oil yields and relatively good oil quality.
Plastic Pyrolysis to Diesel Process
Feeding
The plastic is fed into the high-temperature plastic pyrolysis equipment via a closed feeding system (screw feeder). Air ingress is minimized throughout the entire feeding process.
Pyrolysis Reaction
This is the core conversion step in the plastic pyrolysis to diesel process. The reactor is heated to a specific temperature between 350°C and 500°C (temperature control is crucial).
In the absence of oxygen, the plastic's molecular chains undergo random cleavage (pyrolysis). Long-chain polymers are broken down into smaller molecules, transforming from a solid to a gaseous state. The reactor is typically equipped with an agitator to ensure uniform heating.
Condensation
The resulting high-temperature oil-gas mixture enters a condensation system (typically a multi-stage condenser) from the reactor. Condensable gases are cooled by a coolant (usually water) and condensed into liquid crude oil. Non-condensable gases (such as methane, ethane, propane, and ethylene) are then processed further.
The syngas is typically recovered to provide fuel for pyrolysis system heating. This makes the system energy self-sustaining and reduces external energy consumption.
The solid residue remaining after the reaction is primarily carbon black, along with some inorganic impurities. This can be burned as solid fuel or processed for industrial use as carbon black.
Plastic Pyrolysis Oil to Diesel Process
The crude oil obtained upon condensation is a dark brown mixture with a pungent odor. Its composition is complex and its stability is poor, making it unsuitable for direct use as diesel. It requires further distillation refining to separate its components into light naphtha, diesel fraction, and heavy oil based on their boiling points.
Pyrolysis oil is a complex mixture containing a variety of components, ranging from gasoline to diesel and heavy oil. Its separation and purification are key steps in plastic pyrolysis to diesel process.
First, pyrolysis oil is initially separated into light fractions (such as gasoline), middle fractions (diesel), and heavy fractions (heavy oil) through pyrolysis oil distillation equipment, based on the boiling point differences of these fractions. Precise control of the distillation temperature and reflux ratio is crucial during the distillation process.
Generally, the boiling point of the diesel fraction ranges from 180°C to 360°C. By adjusting the temperature of the distillation column, the diesel fraction vaporizes and condenses within this temperature range for collection.
The diesel fraction obtained after initial separation still contains impurities, such as heteroatom compounds like sulfur, nitrogen, and oxygen, as well as incompletely cracked large hydrocarbons. These impurities can affect the quality and performance of the diesel, necessitating further purification.
Common purification methods include acid washing, alkaline washing, and adsorption refining.
Acid washing, typically using dilute sulfuric acid or phosphoric acid solutions, removes basic nitrogen compounds and some sulfides from diesel fuel. Alkaline washing, using sodium hydroxide solution, neutralizes acidic impurities. Adsorption refining utilizes adsorbents, such as activated clay, to remove pigments, colloids, and residual impurities from diesel fuel, improving its purity and stability.
After these purification steps, pyrolysis of plastic to diesel quality can be significantly improved, reaching or approaching the quality standards of conventional diesel fuel.
