Understanding the Production Process of Adipic Acid

Introduction

Adipic acid is a key intermediate in the global chemical industry, primarily used in the production of nylon-66, polyurethane foams, plasticizers, and performance coatings. Its high strength, chemical resistance, and thermal stability make it indispensable across automotive, textile, electronics, and consumer goods sectors. From engineered plastics to specialty resins, adipic acid contributes to the durability, flexibility, and safety of a wide range of industrial and consumer applications.

Understanding the production process of adipic acid is vital from both economic and environmental standpoints. It helps manufacturers optimize energy consumption, reduce operational costs, and manage emissions—particularly nitrous oxide (N2O), a potent greenhouse gas released during synthesis. With growing pressure to reduce carbon footprints, improve process yields, and adopt sustainable technologies, a detailed insight into adipic acid production helps identify key levers for cost optimization, regulatory compliance, and cleaner, more efficient alternatives.

Overview of the Production Process

The industrial production of adipic acid is primarily carried out through a continuous two-step oxidation process. It begins with the formation of KA oil—a mixture of cyclohexanol and cyclohexanone—produced via the catalytic oxidation of cyclohexane using air. This KA oil is then subjected to further oxidation using concentrated nitric acid in a controlled reactor environment, resulting in the formation of adipic acid along with minor by-products such as glutaric acid and succinic acid.

While both batch and continuous systems exist, continuous processing dominates large-scale commercial production due to its higher efficiency, scalability, and energy integration capabilities. The reaction is exothermic and occurs under moderate pressures (8–10 bar) and temperatures (50–70°C). Following oxidation, the adipic acid is crystallized, separated from the reaction mixture, and dried to form the final product.

Typical yield efficiencies range between 94% and 96%, depending on feedstock purity, catalyst performance, and process controls. A major environmental consideration is the generation of nitrous oxide (N2O), a potent greenhouse gas, which is mitigated through abatement technologies in modern plants. Despite its technical complexity and environmental footprint, this route remains the most widely used for adipic acid production due to its established reliability and integration with upstream nylon value chains.

Raw Materials and Input Requirements

The primary raw materials for adipic acid production include cyclohexane or KA oil and concentrated nitric acid (HNO3). Cyclohexane is usually derived from petroleum refining, while nitric acid is produced through ammonia oxidation. The purity of these inputs is crucial—impurities can lead to catalyst deactivation, reduce yield, and increase the formation of unwanted by-products.

Key Inputs:

•             Cyclohexane/Cyclohexanol-Cyclohexanone (KA oil): Primary feedstock for oxidation

•             Nitric Acid (HNO3): Oxidizing agent, typically 50–65% concentration

•             Catalysts (in newer processes): Metal-based or enzymatic, used in green alternatives.

•             Utilities: Steam, process water, compressed air, and cooling systems

Careful monitoring of input quality, especially regarding sulfur, chlorine, and heavy metal content, is essential for maintaining process stability and minimizing waste generation.

Major Production Routes

1.           Conventional Nitric Acid Oxidation:

This is the most established method, where KA oil is oxidized with nitric acid to yield adipic acid and N2O. The process is mature, scalable, and well-integrated into existing nylon value chains, especially in the US, Europe, and China.

2.           Hydrogen Peroxide or Oxygen-Based Oxidation:

These routes eliminate the use of nitric acid, thereby reducing N2O emissions. They are under pilot development and involve catalysts that enable selective oxidation of cyclohexanone or cyclohexane.

3.           Bio-Based Routes:

Emerging technologies use genetically engineered microbes to ferment glucose or other biomass feedstocks into adipic acid. While these processes are still evolving, they represent a lower-emission alternative and are gaining traction under sustainability targets.

4.           Electrochemical and Photocatalytic Oxidation:

Electrochemical and photocatalytic oxidation methods are still at early research or pilot stages. They often target renewable feedstocks such as furans or lignin derivatives rather than conventional KA oil, offering potential for near-zero emissions if powered by renewable electricity.

Regional preferences vary—developed economies are exploring green alternatives aggressively, while conventional oxidation remains dominant in cost-sensitive markets.

Figure 1 Simplified flow sheet of oxidation of KA oil to Adipic acid.

Equipment and Technology Used

The production of adipic acid relies on specialized chemical processing equipment designed to handle corrosive materials, manage exothermic reactions, and ensure consistent product quality under continuous operation. Given the involvement of nitric acid and nitrous oxide emissions, the technology used must also prioritize safety, emission control, and energy recovery.

Key Equipment Includes:

•             Oxidation Reactors – Stainless-steel continuous stirred-tank reactors (CSTRs) lined with corrosion-resistant alloys are used for the nitric acid oxidation of KA oil. These reactors are equipped with advanced temperature and pressure control systems to manage the exothermic nature of the reaction.

•             N2O Abatement Units – Secondary or tertiary catalytic systems (such as cobalt or zeolite-based beds) are installed to decompose nitrous oxide emissions thermally or catalytically before release into the atmosphere.

•             Crystallization and Separation Units – Multi-effect crystallizers and centrifuges are used to isolate and purify adipic acid crystals from the reaction mixture. Downstream drying systems convert the wet cake into solid flakes or granules.

•             Heat Recovery Systems – Heat exchangers and steam generation units capture thermal energy from the reaction process, improving overall plant efficiency and lowering utility costs.

•             Emission Control Systems – Scrubbers and NO? reduction units handle gaseous by-products, ensuring compliance with environmental regulations.

•             Distributed Control Systems (DCS) – Modern adipic acid plants using nitric acid oxidation are automated with real-time monitoring and advanced control logic to ensure process stability, safety, and yield optimization.

Technological advancements such as digital twins, predictive analytics, and AI-assisted control are increasingly being adopted to improve reliability, reduce energy intensity, and extend equipment life.

For emerging processes like bio-fermentation or electrochemical oxidation, control systems are tailored to biological or redox environments and may include integrated monitoring of pH, microbial activity, voltage, or electrode performance. While these technologies are still evolving, automation and digital tools remain essential across all production platforms.

Environmental and Safety Considerations

The industrial production of adipic acid presents notable environmental and safety challenges, primarily due to the use of concentrated nitric acid and the generation of nitrous oxide (N2O), a highly potent greenhouse gas. As sustainability and regulatory pressures intensify, producers are prioritizing emission control, waste minimization, and plant safety improvements.

Emission Profile:

•             Nitrous Oxide (N2O): The oxidation of KA oil using nitric acid results in significant N2O emissions, which can exceed 300 kg per tonne of adipic acid if untreated.

•             NO? and VOCs: Additional gaseous emissions include nitrogen oxides and volatile organic compounds, especially during crystallization and drying stages.

•             Liquid Effluents: Spent nitric acid and acidic wash streams require neutralization and treatment before discharge.

Mitigation Measures:

•             N2O Abatement Technologies: Most modern plants are equipped with catalytic or thermal decomposition units that reduce N2O emissions by over 90%.

•             Scrubbers and Flue-Gas Systems: Wet and dry scrubbers’ control NO? and VOC emissions, often coupled with selective catalytic reduction (SCR) for compliance with air quality standards.

•             Effluent Treatment Plants (ETPs): These handle acidic wastewater streams and prevent harmful discharge into water bodies.

Regulatory Compliance:

•             Europe: Plants operate under REACH and the EU Emission Trading System (EU ETS), with strict limits on greenhouse gas and NO? output.

•             USA: Facilities comply with EPA Clean Air Act requirements and are subject to OSHA’s Process Safety Management (PSM) standards.

•             Asia-Pacific: While regulations vary, increasing alignment with international standards is driving investment in emission control systems.

Ongoing efforts include improved catalyst design, closed-loop acid handling systems, and digital monitoring to reduce risks while ensuring operational continuity. Environmental stewardship and safety compliance are now key differentiators in adipic acid production worldwide.

Conclusion and Future Innovations

Adipic acid remains a critical building block for numerous industrial applications, but its conventional production route poses sustainability challenges. Going forward, the industry is witnessing a shift toward low-emission technologies, including bio-fermentation and electrochemical processes.

Several companies are piloting bio-based adipic acid production using engineered microbes, while others are exploring catalysts that enable nitric acid-free oxidation. Carbon capture integration, modular plant designs, and digital process controls are also being adopted to enhance efficiency and reduce emissions.

As demand for green polymers like bio-nylon increases and regulatory pressure intensifies, adipic acid manufacturers will need to balance cost-efficiency with sustainable innovation. Future competitiveness will hinge on the ability to reduce environmental impact while maintaining product performance and scalability.

FAQs

Q1. Why is nitrous oxide (N2O) a major concern in adipic acid manufacturing?

Nitrous oxide is a by-product of the nitric acid oxidation process used in conventional adipic acid production. It is a potent greenhouse gas with a global warming potential nearly 300 times that of carbon dioxide. Due to increasing climate regulations and emission trading schemes, controlling N2O emissions has become a key operational and reputational priority for adipic acid producers.

Q2. What role does adipic acid play in the global chemical value chain?

Adipic acid is a foundational intermediate used in the production of nylon-66, polyurethanes, and plasticizers. These downstream materials are critical for automotive components, electrical insulation, footwear, textiles, and industrial machinery. Its durability, flexibility, and chemical resistance make adipic acid an essential feedstock for high-performance polymers used across various sectors.

Q3. How is the industry transitioning toward greener adipic acid production?

To meet sustainability goals, chemical companies are investing in low-emission and bio-based production methods. These include microbial fermentation using renewable feedstocks, electrochemical oxidation of biomass derivatives, and peroxide-based catalytic processes. While still in early adoption phases, these innovations aim to reduce reliance on nitric acid and eliminate N2O emissions, paving the way for a more sustainable adipic acid supply chain.