MOBIL OIL CORP. let a turnkey contract to Chas. T. Main Inc., Boston, for a 54,000 kw, combined cycle cogeneration plant at its Beaumont, Tex., refinery. The plant will burn refinery fuel gas and natural gas. The project is to be complete in summer 1993. UNOCAL CORP. ordered an eight processor C3880 supercomputer for seismic data processing from Convex Computer Corp., Richardson, Tex. Purchase price was not disclosed.