In a reaction vessel for the purification of crude aromatic carboxylic acid, a bed of hydrogenation catalyst having a vapor space is located above the hydrogenation catalyst bed. A packed bed and a holdup section are located within the vapor space and spaced above the catalyst bed. The reaction vessel has means for introducing crude aromatic carboxylic acid and means for introducing hydrogen to the reaction vessel at respective locations such that in use the crude carboxylic acid contacts the hydrogen in the packed bed. The reaction vessel also has means for removing purified aromatic carboxylic acid.