Japan\'s third-biggest oil refiner Idemitsu Kosan Co. and the leading trading firm Mitsui and Company Limited will set up a 50-50 joint venture in the United States to study on a petrochemical plant there, the two companies announced on Monday.  Idemitsu and Mitsui have signed a basic agreement to commence studies to produce linear alpha olefins with a newly built world-scale unit, they said in a joint statement, noting that the project is driven by the shale gas/oil revolution in the US, where sustainable cost advantage is expected.  The two firms have also reached initial agreement with US chemical giant Dow Chemical Company on the procurement of feedstocks for the production of alpha olefins and the supply of a part of its production.  Under the agreement with Dow, the joint venture will secure access to producer cost-based ethylene from Dow\'s Gulf of Mexico coast production grid, Idemitsu and Mitsui said.  The plant will have an annual output capacity of around 330,000 tons of alpha olefins and begin production in 2016. Final investment decision is expected to be made in 2014.  According to the statement, Alpha olefins are used as additives in polyethylene and other synthetic resins, and world demand is in excess of 3 million tons and is expected to remain on a steady growth trend.  They are also widely used in products ranging from detergents to high-performance lubricants and paper-making chemicals.