Method for ring hydrogenation of dialkyl terephthalates with low by-product formation

A process for ring hydrogenation of a dialkyl terephthalate having C3- to C16-alkyl groups, to give a 1,4-cyclohexanedicarboxylic ester having corresponding alkyl groups, the process comprising:

• contacting the dialkyl terephthalate with a hydrogen-containing gas, in a hydrogenation unit consisting of a first reaction unit and a second reaction unit connected in series,
• wherein the first reaction unit and the second reaction unit each consist of one reactor or two or more parallel reactors,
• wherein reactor(s) of the second reaction unit are operated in a straight pass,
• wherein reactor(s) of the first reaction unit and the reactor(s) of the second reaction unit each independently comprise a heterogeneous hydrogenation catalyst,
• wherein the dialkyl terephthalate is used as feed to the reactor(s) of the first reaction unit and the reactor(s) of the second reaction unit,
• wherein a reaction conversion in the first reaction unit is monitored, and
• wherein at least one parameter in relation to the first reaction unit, selected from the group consisting of an amount of feed supplied to the first reaction unit (total amount of fresh feed and recycle), an amount of dialkyl terephthalate to be hydrogenated, a reactor temperature in the reactor(s) of the first reaction unit, a temperature increase, a circulation volume flow, a feed/recycle ratio, a ratio of hydrogen to the dialkyl terephthalate, a reactor pressure, a feed composition, an amount of the heterogeneous hydrogenation catalyst, a catalyst composition, if cooling is present, an amount of coolant circulated or an amount of product in a discharge of the first reaction unit, and a combination thereof, is controlled such that a reaction conversion in the range of 85 to 93% is achieved.