Choosing the Ingredients -Feedstock
The journey of creating ethylene begins with selecting the right ingredients. These ingredients are special hydrocarbons like ethane, propane, butane, naphtha, etc. that are present in the oil and gas extracted from the ground. The plant operators carefully choose the best mix based on what is available and cost-effective. These feed hydrocarbons are preheated using heat recovered from the process.
Steam Cracking Furnace
Next, the warmed-up ingredients are mixed with steam and sent into the heart of the operation: the cracking furnace. This furnace heats the steam-hydrocarbon mixture to desired cracking temperatures between 750°C and 900°C (1380 to 1650°F). The steam helps keep things moving and minimizes unwanted gunk from sticking to the furnace walls. The heat breaks the big hydrocarbon molecules inside the furnace into smaller, more useful pieces, including ethylene, propylene, etc.
A Quick Cool Down
As soon as the molecules are cracked, they must be cooled down quickly to stop further reactions. This is done in a special transfer line exchanger (TLE), where the hot gases are rapidly cooled to about 350°C to 450°C (660 to 840°F). The heat is used to produce steam requirements for other parts of the plant, maximizing efficiency.
Sorting Out the Good Stuff
Now, the mixture of cracked gases enters a primary fractionator, a big sorting tower. Here, the heavier, less useful stuff like fuel oil and tar is separated. The lighter, more valuable gases are cooled further, condensing the heavier components, and leaving behind a cleaner gas mixture.
Compressing and Cleaning Up
The cooled gas mixture is then compressed in several stages, making it easier to handle and purify. In between these stages, the acid gases like hydrogen sulfide and carbon dioxide are removed using a special acid gas scrubber, ensuring the gas is clean and ready for the next steps. The scrubbed gases are dried before chilling these for further processing.
The Basic Separation
In a series of separation towers, the gas mixture is carefully separated.
The basic separation approach can vary depending on the arrangement of the first separation tower – Demethanizer, Deethanizer, or Depropanizer.
Deethanizer: This tower separates ethane and lighter gases from heavier ones.
Demethanizer: Here, methane and hydrogen are separated from ethylene and ethane (including any heavies).
Depropanizer and Debutanizer: These towers further sort out propane/propylene, C4s (butane, butadiene, butenes), and aromatics stream (pyrolysis gasoline).
Each step ensures that ethylene and other valuable byproducts are collected efficiently and purified to the desired levels. Separation of ethane and lighter components requires low-temperature steps. Refrigeration systems (e.g. propylene, ethylene, methane, etc.) provide low temperatures to cool and separate the light gases.
Selective Hydrogenation of Acetylene and MAPD
In the ethylene production process, acetylene and methylacetylene-propadiene (MAPD) are unwanted byproducts that need to be carefully managed. The plant employs selective hydrogenation techniques to convert these impurities into valuable products without compromising the purity and recovery of ethylene and propylene. The acetylene hydrogenation can be located either in a front-end (associated mostly with Deethanizer or Depropanizer first schemes) or a back-end location (associated mostly with Demethanizer first schemes).
Purifying Ethylene
The mixture of ethylene and ethane goes to an ethylene/ethane splitter (super fractionator), where ethylene is separated as a pure product, and ethane is sent back to the furnace to be cracked again. Any remaining impurities in the ethylene are removed, ensuring it is of the highest quality to meet the stringent requirements of downstream derivative units.
Purifying Propylene
The mixture of propylene and propane goes to a propylene/propane splitter (super fractionator), where propylene is separated as a pure product, and propane is sent back to the furnace to be cracked again. Any remaining impurities in the propylene are removed, ensuring it is of the highest quality to meet the stringent requirements of downstream derivative units.
Byproducts and Bonus Materials
The plant also recovers valuable byproducts like hydrogen, which can be used in other processes or as fuel. Other chemicals, like butadiene and benzene, are also recovered and sold. Any light gases like methane are used to fuel the cracking furnaces, making sure every part of the process is efficient.
The Future of Ethylene Production: A Journey Toward Decarbonization and Energy Transition
A New Beginning
As the world began to recognize the urgent need to combat climate change, petrochemical plants have embarked on a new chapter in their story: the journey towards decarbonization and energy transition. The plant operators understood that producing ethylene and other chemicals is needed to become more sustainable and environmentally friendly.
Embracing Green Feedstocks
Industry is exploring the use of renewable and bio-based feedstocks. Instead of relying solely on traditional hydrocarbons, naphtha and other feeds derived from renewable sources like biomass are being incorporated in the mix. This shift helps reduce the carbon footprint of the ethylene production process.
Advancing Cracking Technology
To further reduce emissions, the engineers are focusing on improving the cracking technology by integrating advanced technologies like electric reactors powered by renewable energy. This innovation will significantly reduce greenhouse gas emissions, making the production process cleaner and more efficient.
Capturing Carbon
In its quest for decarbonization, the industry is implementing state-of-the-art carbon capture and storage (CCS) systems utilizing pre- or post-combustion approaches. These systems capture CO₂ emissions from the cracking process and store them underground or use them in other industrial applications. By doing so, the plant will be able to reduce its overall carbon emissions drastically.
Enhancing Energy Efficiency
Energy efficiency has become a top priority. The plants adopt advanced heat recovery systems and optimize their energy use at every step of the process. By using less energy to produce the same amount of ethylene, the plants not only cut costs but also minimize their environmental impact.
Integrating Renewable and Carbon-Free Energy
The plant’s power needs can be increasingly met by carbon-free energy sources. Solar panels, wind turbines, and/or nuclear reactors (Small Modular Reactors) can be installed on-site or nearby, providing clean electricity for the plant’s operations. This shift will further reduce reliance on fossil fuels and help the plant move towards a more sustainable energy mix.
Circular Economy and Recycling
In line with the principles of a circular economy, the plants have begun recycling plastic waste back into their feedstock. Advanced chemical recycling technologies allow the plant to convert waste plastics into valuable hydrocarbons, which are then used to produce new ethylene. This closed-loop system minimizes waste and reduces the demand for virgin hydrocarbon feedstocks.
Continuous Innovation and Collaboration
The owners know that staying at the forefront of the industry requires continuous innovation and collaboration. They partner with research institutions, technology providers, and other industry players to develop and implement cutting-edge technologies. These collaborations ensure that the plants can continuously improve production processes and remain a leader in sustainable ethylene production.
Educating and Training for the Future
Understanding the importance of knowledge transfer, plants are investing in training programs focused on reliability, sustainability, and green technologies. Engineers, operators, and business leaders need to be educated and trained on the latest advancements and best practices in operation/maintenance, decarbonization, and energy transition. This will empower the workforce to drive the plant’s sustainability initiatives forward.
Shaping our Destiny
The choices of actions taken today will determine the world of tomorrow. With these transformative steps, the plants will not only continue to produce high-quality ethylene but will do so in a way that is kinder to the planet. The journey towards decarbonization and energy transition is challenging but necessary as the demand for materials will grow to serve the needs of the growing middle class and world population. By embracing innovation and sustainability, the owners will pave the way for a greener future in the petrochemical industry. This new chapter in the story of ethylene production will be a testament to the industry’s commitment to excellence, resilience, and environmental stewardship.
Thanks to this meticulous and clever process, the plants will continue to produce high-quality ethylene and other valuable chemicals efficiently and sustainably. The story of ethylene production is a tale of innovation, efficiency, and smart engineering, ensuring that the plants can continue to thrive and support countless industries with their valuable products. And so, the adventure of steam cracking continues, always striving for better ways to create the essential building blocks of our modern world.

Propylene (CH3-CH=CH2) is

• The second largest volume building block for many petrochemicals
• End products such as plastics, resins, fibers, solvents, polyurethanes, etc.
• Colorless, flammable gas and practically odorless
• A very reactive intermediate and, therefore, is involved in many chemical reactions
  • Polymerization for the production of polypropylene
  • Cumene through benzene alkylation
  • Acrylonitrile by ammoxidation
  • Propylene oxide via chlorohydrin chemistry or peroxidation
  • Alcohols via hydration
  • Acrylic acid via oxidation
• Acts as a mild anesthetic
• Plants emit a small amount of propylene naturally

Propylene manufacturing

• By-product (also referred to as co-product) of the thermal cracking process for ethylene production
• By-product from the refinery Fluid Catalytic Cracking (FCC), including high-severity, units
• On-purpose propylene production via
  • Propane dehydrogenation
  • Methanol to propylene and/or ethylene
  • Metathesis process
  • Olefins interconversion processes
• Propane dehydrogenation processes
  • Reactor section – fixed bed or continuous catalyst regeneration system
    • Once through conversion of propane is limited
    • Platinum or Chromium-based
  • Recovery Section
    • Including reactor effluent compressor, lights recovery, propane/propylene separation

Alternate Routes for Propylene Production
These technologies are potential options for propylene production.

1. Catalytic pyrolysis process (based on a combination of carbonium and free radical mechanisms) for production of propylene (and light olefins) using heavy hydrocarbon feeds. This process has been used in commercial applications in China.
2. Propane dehydrogenation process based on chemical looping concept – currently under development
3. Fluid solids cracking processes – currently under development
4. Bio-propylene routes – currently under development

Ethylene: Ethylene (H2C=CH2) is

• The largest volume building block for many petrochemicals
• End products such as plastics, resins, fibers, etc.
• Colorless, flammable gas with a slight odor
• A very reactive intermediate and, therefore, is involved in many chemical reactions
  • Polymerization – one of the main reactions for the production of polyethylene (including Low-density Polyethylene – LDPE, High-density Polyethylene – HDPE, and Linear Low-density Polyethylene – LLDPE)
  • Oxidation – for production of ethylene oxide and ethylene glycols (used in PET and fibers)
  • Addition – many addition reactions are important.
    • Halogenation-hydrohalogenation reaction chemistry is used for forming Ethylene Dichloride (EDC) which is cracked to produce Vinyl Chloride Monomer (VCM) used in the production of Polyvinyl Chloride (PVC).
    • Similarly, Ethylbenzene (EB) is produced from benzene and ethylene. Ethylbenzene is dehydrogenated for styrene manufacture and used for producing Polystyrene (PS).
    • Oligomerization for the production of alpha-olefins and linear primary alcohols
    • Ethanol is manufactured from ethylene by hydration
• Slightly more potent anesthetic than nitrous oxide; smell can cause choking, so it is no longer used as an anesthetic.
• Used in controlled ripening of fruits and vegetables

Ethylene manufacturing

• Thermal cracking of hydrocarbons (ethane and heavier) is the major route for industrial ethylene production
  • Also produces valuable by-products – like propylene, butadiene, butenes (Butene-1, Butene-2, Isobutene), Benzene, Toluene, and hydrogen. Less valuable by-products include methane and fuel oil.
  • Thermal cracking is accomplished in tubular reactors commonly known as cracking furnaces. These are direct-fired reactors for providing high-temperature heat of reaction and sensible heat. Most plants have 6 to 8 cracking furnaces for achieving the desired plant capacity.
    • Cracking furnaces experience coke laydown inside the tubes and need decoking at frequent intervals to stay within the temperature limits of tube metallurgy
    • Coke mitigation technologies are used to minimize the coke laydown
    • Cracking furnaces operate with the lowest optimum pressure to maximize the reaction selectivity for light olefins. Dilution steam is used for minimizing the secondary reactions that eventually lead to coke formation
    • Conversion of feed or severity of operation is selected based on the desired product slate and overall plant optimization
  • Feedstocks include – ethane, propane, butanes, naphtha (light to heavy), gas oils (light to vacuum, including hydrotreated and/or hydrocracked), condensates, light crudes, refinery off-gases, etc.
  • Cracking furnace effluents are cooled and compressed for recovering ethylene and by-products. The separation of lighter products requires very low temperatures that are achieved by application of the refrigeration systems.
  • Ethylene is a highly energy-intensive process; therefore, energy recovery and conservation are important to minimize the cost of production and improve environmental performance. Sustainability, environmental performance, and emission reduction have been a constant theme over the years and are now at the forefront during the process of energy transition and net-zero targets.
• Methanol to Olefins (methanol is mostly produced either starting from coal or methane) utilizes catalytic processes for converting methanol to ethylene and/or propylene.
• A small amount of ethylene is recovered from refinery conversion processes (like Fluid Catalytic Cracking) and the Fischer-Tropsch process.
• A small amount of ethylene is produced by dehydrating ethanol (mostly produced from biomass).

Alternate Routes for Ethylene Production: These technologies are potential options for ethylene production.

1. Oxidative coupling of methane – was developed in the late 1970s and 1980s. This process concept has been demonstrated and so far has no commercial application.
2. Catalytic oxy-dehydrogenation of ethane – this process is available for commercial applications (currently no plant in operation) and produces a large amount of acetic acid as a by-product.
3. Catalytic pyrolysis process (based on the free radical mechanism – like thermal cracking) for production of ethylene (and light olefins) using heavy hydrocarbon feeds. This process has been used in commercial applications in China.
4. Ethane dehydrogenation process based on the chemical looping concept is currently under development, as a potential option for decarbonizing ethylene manufacturing.
5. Shockwave reactor concept using supersonic speeds (Roto Dynamic Reactor), currently under development, is a potential option for using carbon-free electric power for ethylene production.