Because of the reactivity of maleic anhydride, high yields are achieved in the production of polyesters, especially if the reaction time is prolonged. Small amounts of unreacted maleic anhydride can, however, remain behind. This can be washed out with water, but maleic acid quickly re-forms. To recover maleic anhydride from the aqueous solution one must make use of a dehydration process, which uses up relatively large amounts of energy.
Because of the risk of the maleic anhydride decomposing, care should be taken to ensure that the washing water is free from alkalis. Corrosion resistant equipment is likewise essential.
To recover the maleic anhydride, the aqueous maleic acid solution is passed to a distillation colums whose sump consists of a mixture of maleic anhydride and xylene or some other aromatic solvent. The enrichment of maleic anhydride in the sump can be as much as 40 %.
Solvent and water evaporate as an azeotropic mixture. Separation of maleic anhydride can be continuous or discontinuous. Thin—film vapourising technology can also be used for thermal dehydration, although only for small quantities. The brief residence time of maleic acid at temperatures of up to 200 °C is sufficient for water to separate without the risk of isomerisation (formation of fumaratic acid) taking place. Multi-stage plants of this kind will produce relatively pure maleic anhydride.