Motiva Enterprises, a joint venture owned by affiliates of Shell and Saudi Aramco, has commenced its 325,000 barrels per day refinery expansion in Port Arthur, Texas.

The expansion is expected to increase the refinery’s crude oil throughput capacity to 600,000 barrels per day, making it the largest refinery in the US and one of the largest in the world, the company said.

Motiva Enterprises said that the upgrades and modernization will lead to new supplies of transportation fuels for US markets, add jobs to the local economy and decrease the refinery’s ozone precursor emissions from present levels.

Motiva claims that the expansion will bolster energy security in the US by strengthening the supply of gasoline, diesel, aviation fuels and high-quality base oils.

The new production capacity is expected to come online in 2010 and will increase Motiva’s supply of Shell-branded fuels to the company’s wholesale and direct supply markets. The refinery is expected to produce about 23 million gallons of transportation fuels per day.

William Welte, president and CEO of Motiva, said: This is a major milestone in the journey to meet our nation’s growing energy demand. By adding 325,000 barrels per day capacity, the expansion is equivalent to building a new refinery.