Method for preparing ester-based composition

A method for preparing an ester-based composition, comprising:

• putting a polycarboxylic acid and a mono-alcohol having 3 to 12 alkyl carbon atoms into a mixer to form a reaction mixture (step S1);
• sequentially putting the reaction mixture into each of N batch reactors connected in parallel to perform a reaction, and sequentially completing the reaction in the N batch reactors to semi-continuously produce reaction products (step S2);
• and
• semi-continuously moving the reaction products to a separation unit to remove unreacted alcohol (step S3),
• wherein N is an integer of 3 or more,
• pressures of the N batch reactors are each independently configured such that pressure at an early stage is 0.3 barg to 1.0 barg and pressure at a latter stage is 0 barg to 0.5 barg, the pressure at the early stage is greater than the pressure at the latter stage, and
• the early and latter stages are divided based on any one of the time points when reaction conversion rate is 30% to 90%.