Diesel fuel can be derived from **waste plastics** such as **polyethylene (PE)** and **polypropylene (PP)** through a process called **thermal depolymerization** or **pyrolysis**. This process involves breaking down long polymer chains of plastics into shorter hydrocarbon molecules, which can be further refined into fuels like diesel. Pyrolysis is typically performed at high temperatures (400–700°C) in the absence of oxygen, causing the plastics to decompose into various fractions, including gases, liquids (such as diesel), and solids (char). ![[Pasted image 20241017233133.png]] ### **Chemistry of Thermal Depolymerization of Plastics** **Polyethylene (PE)** and **polypropylene (PP)** are long-chain hydrocarbons consisting mainly of carbon and hydrogen. The general formula for polyethylene is \((C_2H_4)_n\), where \(n\) is the number of repeating ethylene units, and for polypropylene, it is \((C_3H_6)_n\). During **thermal depolymerization**, the polymer chains undergo **cracking** reactions, breaking down into smaller hydrocarbons, which can range from light gases to heavy oils. ![[Principle-of-Plastic-Pyrolysis-Process.webp]] #### **Key Steps of Thermal Depolymerization of PE and PP:** 1. **Initiation:** At temperatures of around 400–500°C, the heat energy causes the polymer bonds (C-C and C-H bonds) to break, generating free radicals and smaller fragments. 2. **Chain Scission:** The polymer chains break into smaller hydrocarbon fragments (alkanes and alkenes) through random scission, β-scission, or hydrogen transfer reactions. 3. **Decomposition to Hydrocarbons:** The broken fragments undergo further degradation, producing hydrocarbon compounds of various lengths. These can include gases like methane (CH₄), ethylene (C₂H₄), and propane (C₃H₈), as well as liquids like gasoline-range hydrocarbons (C₅-C₁₂) and diesel-range hydrocarbons (C₁₂-C₂₀). --- ### **Composition of Pyrolysis Products** The pyrolysis of polyethylene or polypropylene yields a mixture of gaseous, liquid, and solid fractions. The distribution of these fractions depends on the pyrolysis conditions (temperature, heating rate, and residence time). The typical composition by **molecular weight** for the pyrolysis of polyethylene (PE) is: - **Gaseous Fraction (~10–20% by weight):** - Contains small hydrocarbons like methane (CH₄), ethylene (C₂H₄), propane (C₃H₈), and butane (C₄H₁₀). - This fraction can be burned as fuel to provide heat for the pyrolysis process, reducing external energy inputs. - **Liquid Fraction (~60–80% by weight):** - This is the primary product and consists mainly of liquid hydrocarbons, including **gasoline-range (C₅-C₁₂)** and **diesel-range (C₁₂-C₂₀)** hydrocarbons. - The composition can vary, but approximately **60–70%** of the liquid fraction can be refined into **diesel**. - **Solid Residue/Char (~5–10% by weight):** - Composed mainly of carbon, non-degraded polymer residues, and sometimes ash if there are impurities in the plastic. #### **Typical Yields by Fraction:** - **Diesel:** 50–70% of the total product, by weight. - **Gasoline:** 15–25% of the total product, by weight. - **Gases:** 10–20% of the total product, by weight. - **Solid Residue/Char:** 5–10% of the total product, by weight. --- ### **Typical Diesel Yield from 1 kg of Plastic** Assuming **polyethylene (PE)** as the feedstock, the **diesel yield** from 1 kg of plastic is typically in the range of: - **Diesel Yield (C₁₂-C₂₀ hydrocarbons):** 60–70% by weight. Therefore, from **1 kg of polyethylene**, you can expect to obtain approximately **600–700 grams of diesel**. --- ![[distillation-of-crude-oil 1.gif]] ### **Heat Energy Input for Pyrolysis of 1 kg of Polyethylene** The energy required to break down 1 kg of plastic through pyrolysis depends on the pyrolysis system and the efficiency of heat transfer. The typical heat energy input for pyrolysis is calculated based on the **specific heat of the plastic**, the **temperature rise**, and the **energy required for bond breaking**. 1. **Heat Energy Input:** - To pyrolyze polyethylene, the process typically requires about **1.2 to 1.4 MJ/kg** of thermal energy. - This is the amount of heat needed to raise the temperature of the plastic to the required pyrolysis range (400–500°C) and break the chemical bonds. 2. **Energy Produced from Gaseous Fraction:** - The **gaseous fraction** produced during pyrolysis (10–20% of the total yield) can be burned to supply part of the thermal energy for the process, reducing the external energy input. - The energy content of the gases is typically in the range of **40 MJ/kg**. --- ### **Summary of Pyrolysis of Polyethylene (PE):** - **Process:** Thermal depolymerization (pyrolysis) at 400–500°C. - **Feedstock:** Polyethylene (PE). - **Diesel Yield:** 600–700 grams of diesel per 1 kg of plastic (60–70% yield). - **Energy Input Required:** 1.2–1.4 MJ/kg of plastic. - **Energy Recovery:** The gaseous fraction (10–20%) can be used to partially supply the required heat energy. ### **Concluding Remarks** The pyrolysis of polyethylene and polypropylene to produce diesel is a promising way to recycle plastic waste into valuable fuels. With an expected diesel yield of 60–70% from 1 kg of plastic and an energy input requirement of 1.2–1.4 MJ/kg, this process can provide a viable route for converting plastic waste into useful energy. The gaseous byproducts can also help offset energy costs by fueling the pyrolysis process itself.