The present invention relates to a two-stage process for producing pure terephthalic acid according to a first oxidation stage and a second purification stage, and, more particularly, to a method for recovering methyl acetate and residual acetic acid in such two-stage process.
Terephthalic acid is produced on a commercial scale by oxidation of paraxylene in the presence of a metal bromide catalyst system in acetic acid solvent. A crude, i.e., impure, terephthalic acid product is isolated from a slurry in the oxidation stage, usually as a dry crystalline powder. The crude terephthalic acid in the form of a wet cake is washed as necessary with acetic acid or water and is then sent to a dryer where any adherent solvent is removed to form crude terephthalic acid. Water is produced in a reaction off-gas stream as a significant by-product of the oxidation reaction. The off-gas stream also includes acetic acid and low levels of methyl acetate. A preferred means for separating and recovering the acetic acid from the off-gas stream is through azeotropic distillation of the off-gas stream condensate using an organic entrainer selected from, for example, n-butyl acetate, n-propyl acetate and isobutyl acetate. In such cases where the main feed to the azeotropic distillation process is derived from the oxidation reaction overheads, the presence of methyl acetate in the feed stream can adversely affect the amount of water which can be removed azeotropically because methyl acetate's water azeotrope lies in the single phase region, i.e. its water azeotrope will have a low water content.
In the second, i.e., purification, stage of the process, crude terephthalic acid crystals are dissolved in water at elevated pressure and temperature and the solution is subjected to hydrogenation in the presence of a Group VIII noble metal hydrogenation catalyst. The purified acid is recovered by crystallizing the acid from the hydrogen treated aqueous solution. A majority of the principal impurities, which are p-toluic acid derived from the compound 4-carboxybenzaldehyde and unidentified color bodies, along with some other organic components, such as benzoic acid and residual terephthalic acid, remain dissolved in the aqueous solution. This remaining aqueous solution is referred to hereinafter as "pure plant mother liquor", i.e., PPML. More recent commercial two-stage processes, however, have sought to eliminate the need to recover the crude terephthalic acid as a dry product with a separate drying step. Instead, the terephthalic acid crystals can be separated from the slurry formed in the oxidation stage by depositing the slurry on a moving band of filter material to form a wet cake and then washing the wet cake with water or other solvent according to a predetermined series of washing steps. The resulting wet cake can then be dissolved in water for purification without the need for a separate drying step.
For improved economy, it is desirable to recover and recycle the resulting pure plant mother liquor. However, under some process operating conditions, a residual amount of acetic acid can "slip" through the filtration/solvent exchange process, i.e., residual amounts retained within the recovered crude acid, and find its way into the aqueous mother liquor solution. The presence of acetic acid in the pure plant mother liquor can be problematic in attempting to recycle it for use elsewhere in the process. Hence, a method is needed to recover or otherwise account for the presence of a residual amount of acetic acid in the pure plant mother liquor.