The dozen or so cranes visible from Interstate 10 in Baytown will only remain for a few more months as Chevron Phillips Chemical's $6 billion petrochemical expansion moves toward completion. Chevron Phillips' complex is more than 80 percent complete, although it won't be fully operational for almost another year. The project involves a massive ethane cracker - on a plot the size of 44 football fields - that will separate a component of natural gas called ethane, which will provide the feedstock for some 1.5 million metric tons a year of ethylene, a common building block of plastics.