Ashland Oil Inc. announced yesterday that it has developed a process to refine 25 percent more gasoline from a barrel of crude oil than is now possible.

The oil company, calling the new refining method a "breakthrough," said it will build a $70 million refinery addition in Ashland, Ky., that will use the "reduced crude conversion" process.

"We believe we've found a method that allows us to produce more gasoline from the heavy, leftover end of a barrel of crude oil," Oliver Zandona, Ashland vice president for manufacturing and engineering research, said at a press conference. "We think this is a real breakthrough in our industry."

In New York, Ashland Chairman Orin E. Atkins told the New York Society of Security Analysts that a patented Ashland process to be used by the new refinery will extract from a barrel of oil a product that is made up of more than 70 percent gasoline, rather than the 50 percent usually extracted.

"This promises to be one of the more revolutionary developments in the petroleum refining industry and can be a major step toward reducing our nation's dependence on imported crude oil," Atkins said. "It is important to realize" that the possible 25 percent reduction in crude oil requirements" is applicable for both domestic and imported crude oil."

In Washington, Zandona said that Ashland will build a 40,000 barrel-a-day addition to its 180,000 barrel-a-day refinery in Ashland and it will be ready to start up in early 1982. He said the addition will produce up to 25,000 barrels of gasoline on top of the 90,000 barrels of gasoline the plant now produces on an average day.

"Like most refineries, our Ashland facility gets from a barrel of oil a product that's 50 percent gasoline," Zandona said. "This new addition will be getting a product that's close to 75 percent gasoline."

In the refining process, crude oil is refined into its lighter and heavier components. The lighter fraction contains gasoline and heating oil, which are physically separated after the refining process. Jet fuel and kerosene are in the middle fraction and residual fuel is left over in the heavy fraction.

Zandona said the Ashland process works by converting most of the heavy bottom of the crude oil barrel into gasoline. Normally, the residual or leftover 25 percent of a barrel of crude is sold as lower-priced boiler fuel or made into asphalt.

The heaviest fraction of a barrel of oil, Zandona said, is contaminated nickel and vanadium that "poison" the refining process and prevent the conversion of the heavy oil fraction into gasoline.

The Ashland "reduced crude conversion" process changes the sulfur into hydrogen sulfide gas which can be removed from the oil and binds the nickel and vanadium to a catalyst which Zandona described as being made of microspheres of silica and alumnium.

Zandona said details of the catalyst are "proprietary" information. He also said the Ashland process depends on refining the oil at "elevated temperatures" that keep the carbon locked up in the hydrocarbon form that produces gasoline.

"We had to find a method of pacifying the metals and of decreasing the carbon content in the oil," Zandona said.

Zandona said that Ashland has been running a 200 barrel-a-day demonstration refinery since April 1978, and has produced greater gasoline runs from as many as a dozen difficult crude oils, including the heaviest from Mexico and Venezuela.

One penalty the new process appears to exact is that it cuts down drastically the amount of residual fuel refined from a barrel of oil, Zandona said.Low-sulfur residual fuel is still used by electric companies on the East and West coasts.