The commercial Sohio process uses a fluid bed reactor. The reactor is fed with the reactants with are propylene, ammonia and air. These reactants come into contact with each other over a solid catalyst at a temperature of around 400-510 degrees Celsius and with a pressure of 49-196 kPa. The whole process is a single-pass process with around a 98% conversion rate of the propylene. The reaction utilizes 1.1 kg of propylene per kg of acrylonitrile produced. The reaction produces a variety of by-products which are both useful and important in industrial processes and ones which are not. The list of by-products include:
Useful
• Hydrogen Cyanide: Throughout the process, HCN is produced at a rate of 0.1kg per kg of acrylonitrile produced and is most commonly used in the manufacturing process of methyl methacrylate.
• Acetonitrile: During the process, acetonitrile is produced at a rate of 0.03kg per kg of acrylonitrile and is a very common industrial solvent.
• Ammonium sulphate: As the process progresses any unreacted ammonia from the reaction is neutralized with sulphuric acid to produce ammonium sulphate. Ammonium sulphate can then be recovered and is commonly used as a fertilizer.
Non-Useful
• Nitrogen: Vented into the atmosphere
• Carbon Monoxide: Directly vented into the atmosphere or passes through an incinerator to be combusted and produce carbon dioxide
• Carbon Dioxide: vented into the atmosphere
• Unreacted Hydrocarbon: directly vented into the atmosphere or is made to pass through an incinerator to be combusted in excess oxygen to produce carbon dioxide and water.
In common practise hot reactor effluent is quenched with cool water in a counter-current absorber.
The solution containing the acrylonitrile from the absorber is directed into the acetonitrile recovery column. Here an overheated crude acrylonitrile stream that contains hydrogen cyanide is produced. The bottom stream is fed into the second acetonitrile recovery column to remove water and produce a crude acetonitrile mixture. This is a by-product and is either incinerated or further processed, to remove the impurities and produce solvent quality acetonitrile. The desired acrylonitrile quality is obtained via fractionation of the top product to remove HCN, H2O, light ends and high boiling pint impurities. (Kirk-Othmer, 2010)
Safety
The Sohio process is considered to be a very safe process with rarely any major incidents. However, due to the nature of the reactants caution is advised when using these materials:
• Propylene: Propylene is not known to be very toxic. However, undiluted exposure to the eye can cause mild irritation. Similarly, if inhaled propylene can cause irritation of the respiratory track. Therefore, propylene is not recommended in the use of applications where it can be inhaled or exposed to the eye. (SEPA, n.d)
• Ammonia: Ammonia is not considered to be a toxic or harmful substance in low concentration (SEPA1, n.d)
• HCN: Hydrogen cyanide, in high enough concentrations, can be lethal to a human being. Safety precautions must be taken when working with cyanide over the safe concentrations (SEPA2, n.d)
• Acetonitrile: It is not considered to be very hazardous.
• Heavy metals (Catalyst): Heavy metals used as catalyst can be very unsafe to human beings and causes serious illness depending on the metal an individual is exposed to.
Environmental Emissions
In the unlikely event of an accident or an uncontrolled release of compounds into the atmosphere or water sources, these products can have various impact on the environment, such as:
• Heavy metals (Catalyst): heavy metals depending on the type of metal can be very toxic to wildlife and aquatic life. These metals can also accumulate in the food cycle, causing serious problems to health and wellbeing.
• Acetonitrile: has limited effects in low concentrations
• CO2 emissions: Carbon dioxide is one of the main contributors to global warming and climate change. Steps must be put in place to reduce the amount of carbon dioxide vented. A possible solution is to store it underground.
• Burning fossil fuels: fossils fuels are used to provide energy for the exothermic reaction and other various processes. Digging up fossil fuels can damage environment and the combustion of fossil fuel produce carbon dioxide and other greenhouse gasses.
• HCN: Hydrogen cyanide in low levels are not consider to be harmful to the environment but in high enough levels can disturb ground and aquatic life (SEPA2, n.d)
• Ammonia: Even in dilute concentration ammonia can be toxic to marine life. Steps must be taken to ensure that ammonia is not leaked into water sources. (SEPA1, n.d)
• Propylene: Propylene is known to have high levels of biochemical oxygen demand during degradation on water surfaces. This can affect aquatic creatures by reducing the amount of oxygen available to them. (SEPA, n.d)