Acrylic acid is a critical industrial chemical used in the manufacture of superabsorbent polymers, adhesives, coatings, and textiles. Its growing demand across diverse sectors makes it an essential commodity in the global chemical market. One of the most efficient and widely adopted methods of producing acrylic acid is through the oxidation of propylene. This article provides a comprehensive guide to the entire production process of acrylic acid from propylene, focusing on raw materials, production technology, catalysts, reactor configurations, separation techniques, and environmental management.
Overview of Acrylic Acid Manufacturing Process
The process of acrylic acid production from propylene involves complex chemical reactions and multiple processing units. It is typically performed in two sequential oxidation stages. In the first stage, propylene is oxidized to acrolein, and in the second stage, acrolein is further oxidized to acrylic acid. These reactions are exothermic and require precise temperature and pressure control to ensure high yield and selectivity.
Raw Materials and Feed Preparation
Propylene is the primary raw material for the process. It is a by-product of petroleum refining and is available in large quantities. The feedstock must be of high purity to prevent catalyst poisoning. Air or molecular oxygen is used as the oxidizing agent, and in some cases, steam is introduced to control the reaction temperature and reduce hot spots in the reactor.
Feed preparation also includes filtering out impurities and adjusting the molar ratios of the reactants. The proper balance of propylene, oxygen, and steam is crucial to achieving high conversion rates and minimizing the formation of unwanted by-products such as carbon dioxide and acetic acid.
First Oxidation Stage – Propylene to Acrolein
The first reaction in the production of acrylic acid is the partial oxidation of propylene to acrolein. This is typically carried out in a fixed-bed tubular reactor filled with a catalyst containing bismuth molybdate. The reaction takes place at a temperature of around 300–350°C and a pressure of 1–3 atm.
The reaction is as follows:
C?H? + O? ? C?H?O (Acrolein) + H?O
This step is highly exothermic and requires efficient heat removal systems. The catalyst selection is crucial for maintaining high selectivity toward acrolein while minimizing unwanted side products.
Second Oxidation Stage – Acrolein to Acrylic Acid
In the second stage, acrolein is oxidized further to form acrylic acid. This step also takes place in a fixed-bed reactor with a catalyst based on vanadium and molybdenum oxides. The operating temperature is slightly lower, around 250–300°C.
The reaction proceeds as follows:
C?H?O (Acrolein) + ½ O? ? C?H?O? (Acrylic Acid)
This stage must also be carefully controlled to maximize yield and avoid over-oxidation to carbon oxides. Reactor cooling and optimal catalyst performance are key to achieving efficient production.
Product Separation and Purification
After the reaction, the product stream contains acrylic acid, unreacted acrolein, water, carbon oxides, and traces of other organics. The stream is first cooled and condensed to separate the liquid and gas phases.
The liquid is then subjected to a series of distillation and extraction steps. Water and low-boiling point components are separated out, and acrylic acid is isolated typically using azeotropic distillation or solvent extraction. High-purity acrylic acid (up to 99.5%) is required for polymer-grade applications.
The unreacted acrolein and propylene are often recycled back into the system to improve overall efficiency and reduce raw material consumption.
Catalysts Used in Acrylic Acid Production
Catalysts are central to the efficiency of acrylic acid production. In the first stage, bismuth molybdate catalysts are widely used due to their high selectivity toward acrolein. In the second stage, mixed metal oxide catalysts containing vanadium and molybdenum are employed to promote the conversion of acrolein to acrylic acid.
These catalysts must be regenerated periodically to maintain activity. Poisoning by impurities in the feed or side reactions can lead to catalyst deactivation. The catalyst’s physical structure and surface area also influence the reaction performance.
Process Equipment and Reactor Design
Acrylic acid plants typically use multitubular fixed-bed reactors because they offer efficient heat transfer and temperature control, which are critical in exothermic reactions. The reactors are made of high-alloy materials resistant to corrosion by acidic compounds.
Heat generated from the reaction is often recovered in waste heat boilers and used to produce steam, improving the plant’s energy efficiency. Downstream, the plant includes absorbers, separators, distillation columns, and purification units.
Process automation and real-time monitoring systems are employed to ensure the entire production line operates within optimal parameters, thus maximizing safety, yield, and product quality.
Environmental Considerations
Producing acrylic acid involves handling volatile organic compounds and acidic by-products, which pose environmental challenges. Emission control systems such as scrubbers, catalytic oxidizers, and activated carbon beds are used to treat gaseous emissions before discharge.
Wastewater treatment is another critical component. Water used in the process may contain dissolved organics and must be treated through biological or chemical means before release or reuse. Spent catalysts and process residues must be disposed of or recycled according to environmental regulations.
Economic Aspects and Market Trends
The global market for acrylic acid continues to grow, driven by demand from the hygiene products industry (superabsorbent polymers), adhesives, textiles, and paints. Setting up a production facility involves significant capital investment in equipment, infrastructure, and technology.
Operational costs depend on feedstock prices (primarily propylene), energy consumption, catalyst life, and maintenance. Innovations such as integrating renewable propylene sources and improving catalyst life cycles are actively being researched to reduce environmental impact and improve profitability.
Countries like China, the USA, Germany, and South Korea dominate the acrylic acid manufacturing landscape. However, emerging markets in India and Southeast Asia offer promising opportunities for capacity expansion, driven by the growing domestic demand for end-use products.
Safety and Risk Management
Handling flammable gases like propylene and oxygen, along with high-temperature exothermic reactions, poses serious risks. Safety measures include automated shutdown systems, explosion-proof equipment, rigorous maintenance protocols, and regular employee training.
Proper storage and handling of acrylic acid, which is both corrosive and volatile, are also essential. It must be stored in stainless steel tanks equipped with inert gas blanketing to prevent polymerization and fire hazards.
Future Technologies in Acrylic Acid Production
With increasing environmental concerns, research is underway to develop bio-based routes for acrylic acid using renewable feedstocks like glycerol and lactic acid. Though not yet commercially viable at scale, these technologies could transform the industry by reducing dependency on fossil fuels.
Process intensification strategies, including membrane separation, microreactors, and energy integration, are also being explored to enhance efficiency, reduce costs, and lower carbon footprints.
Conclusion
In conclusion, acrylic acid production from propylene is a mature but technologically complex process. It requires careful control of multiple reaction and separation stages, supported by robust catalyst systems and environmental safeguards. As the demand for acrylic acid continues to grow, especially in emerging markets, adopting advanced technologies and sustainable practices will be key to future success. Whether for new entrants or established producers, understanding the entire value chain from feedstock to final product is essential for operational efficiency, safety, and profitability.
