BASF Corp.'s top management Thursday announced a number of new capital projects that will strengthen the company's position as a major U.S. manufacturer of urethane-based products.

The most significant of the projects is an $83.5 million aniline and nitrobenzene plant to be built at the company's industrial site in Geismar, La. The plant, which is scheduled to come on stream by the end of 1991, will have a combined total capacity of 80,000 tons a year.Aniline produced at the plant will be used to make diphenyl methane diisocyanate (MDI), a primary raw material used to manufacture urethane foams.

"Sales of MDI have grown at a 6 percent compounded annual rate over the last decade," according to Manfred Buller, president of BASF Corp.'s polymers division. "Continued annual growth of 4 percent is anticipated over the next decade."

The demand for MDI has been driven by healthy sales of urethane foam products used in insulation, Mr. Buller said.

The company also said that ground will be broken this May for a $52.5 million paper chemicals plant sited in Freeport, Texas. The facility, scheduled for completion by late 1991, will produce "Polymin" polyethylene imine, used to improve yields in the paper-making process.

BASF also announced it will build a $17 million plant at Wyandotte, Mich., to produce polyurethane elastomers, as well as a $30 million expansion at its polyacrylonitrile precursor fiber plant in Rock Hill, S.C.

BASF Corp. is the North American member of the German BASF Group, which had net sales of $25.3 billion in 1989.