Besides, PTT Global Chemical PCL, Thailand's largest petrochemical maker, shut down for maintenance its olefins plant I-4/1 for 21 days from March 29, 2015, ahead of schedule. The shutdown, earlier planned for August, which involves cleaning up quench oil tower, will allow the plant to boost its efficiency faster and suit with the market situation. The olefins plant has annual production capacity of 515,000 tonnes of ethylene and 310,000 tonnes of propylene while the by-product unit has an annual capacity of 75,000 tonnes of butadiene and 25,000 tonnes of butene.