Saudi Aramco said on Friday it had completed the purchase of a 10% stake in a Chinese petrochemical firm for the equivalent of $3.4 billion as the Saudi oil giant continues to expand its downstream footprint in one of its key export markets. Aramco successfully closed the acquisition of a 10% interest in Rongsheng Petrochemical Co Ltd, which followed the signing of definitive strategic agreements in March this year. “Our strategic partnership with Rongsheng advances Aramco’s liquids to chemicals strategy while growing our presence…