Challenge
ATOFINA Italia's methyl methacrylate production facility in Rho, Italy, needed to treat blow down from an ammonium sulfate crystallizer.
Methyl methacrylate (MMA) is the most important ester of methacrylic acid. It can be homo- and copolymerised to produce acrylic resins with good strength, transparency and with excellent weather resistance. Acetone cyanohydrin was the first commercial process for making MMA, and is the predominant process used today.
However, acetone cyanohydrin generates by-products due to the strength of the acid and high temperature steps in the process. Ammonium hydrogensulphate is an unavoidable by-product of the reaction.
The most common way to treat the hydrogensulphate by-product is to pyrolyze it (recovers no ammonia). Alternatively, it can be neutralized with dry ammonia through crystallization to produce ammonium sulphate, which is used as a fertilizer. If crystallization is used, crystallizer blowdown is created and needs to be treated or disposed of.
Solution
After a thorough investigation of available treatments, wet air oxidation was chosen to treat the crystallizer blowdown stream.
Wet air oxidation was chosen because:
- the agricultural use of industrial by-products was becoming increasingly restricted,
- incineration was expensive and would have adverse environmental impacts, making it difficult to obtain local authorization for gas emissions (NOx, SO2),
- biological treatment was expensive due to the high ammonium salt contained in the purge,
- the use of wet oxidation allows energy recovery as well as a ammonium sulfate recovery while maintaining the lowest environmental impact.
Wet air oxidation is the process of oxidizing organic matter in the presence of liquid water, and is an ideal process for pretreatment of wastes that are toxic to conventional biological facilities.
Result
The use of wet air oxidation at ATOFINA has eliminated the need for off-site disposal of the crystallizer blowdown stream, and has increased ammonium sulfate recovery.
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