Modification of the catalytic system in an industrial process for making acetic and/or methyl acetate acid

In a process for continuous manufacture of acetic acid and/or methyl acetate by carbonylation of methanol or a carbonylatable derivative of methanol into acetic acid and/or methyl acetate in a homogeneous liquid phase and under carbon monoxide pressure, in the presence of a homogeneous catalyst system comprising a rhodium-based homogeneous catalyst and a halogenated promoter, and in the presence of at least 14% by weight water,

• said continuous process being carried out in a plant comprising a reaction zone I, with a temperature maintained at a value between 150 and 250° C. a total pressure between 5.105 Pa and 200.105 Pa, and a flash zone II, where the product from the reaction zone I is partially vaporized at a pressure below the pressure in the reaction zone I, the non-vaporized fraction from this flash zone II being recycled into the reaction zone I and the vaporized fraction being purified in a zone III by separation/distillation/purification of the acetic acid and/or methyl acetate formed,
• the improvement comprising:
• adding an iridium compound to the catalyst system gradually over time, in order to gradually modify the composition of the homogeneous catalyst system, to obtain a catalyst system containing a predetermined atomic ratio of rhodium to iridium, thereby modifying the catalyst composition without stopping the process, and
• continuing the continuous manufacture of acetic acid and/or methyl acetate, while maintaining the catalyst system containing said predetermined atomic ratio by adding a rhodium compound, an iridium compound or both a rhodium and an iridium compound.