Acetic Acid is a largely used industrial product, with a world demand of about 6 million tons per year. Most of the production processes are based on the carbonylation of methanol promoted by an iodine compound and catalysed by Rhodium catalyst (Monsanto process) or Iridium catalyst (Cativa process). Monsanto method was used intensively until 1996 when BP Chemicals introduced the Cativa process, which is a more efficient technology that significantly reduces the cost and produces a high quality acetic acid with very low impurity content.
Iridium-based catalyst is responsible for a series of major improvements on the carbonylation of methanol process. Being more stable allows to extend the previously limited operating conditions. For instance a highly concentrated methanol feed can be used (0.5% water) instead of a 10% water content in Monsanto. This greatly reduces the impact of the side reaction between water and carbon monoxide and consequently improves the selectivity. The overall impact is a less expensive downstream purification process of the acetic acid compared to the technology used in Monsanto process. To be more specific, the new configuration uses a compact two distillation columns configuration.The major units of a commercial scale Cativa methanol carbonylation plant are shown in the following figure.