Process for converting unsaturated C.sub.4 hydrocarbons into normal butane

In a process for the production of ethylene by the pyrolytic cracking of hydrocarbons in a cracking zone to produce a process stream of cracked gases, introducing said process stream of cracked gases into a rapid cooling zone to cool said gases, passing said cooled process stream of cracked gases into a quench prefractionation and compression zone to further cool and compress said gases and remove heavy hydrocarbons, passing said process stream of compressed cracked gases through a demethanization zone wherein methane is removed from said process stream and a stream of hydrogen is generated, thereafter, directing said process stream of compressed cracked gases through a recovery zone wherein ethylene is recovered and a stream of butene raffinate is formed, the improvement comprising the steps of:

• a. introducing said stream of butene raffinate into a hydrogenation zone wherein said butene raffinate is passed in contact with said stream of hydrogen generated in said demethanization zone at temperatures of about 200.degree. to about 450.degree. F. and a pressure of about 100 to about 500 psig to form a stream consisting essentially of normal butane and isobutane;
• b. discharging said stream of normal butane and isobutane from said hydrogenation zone and introducing said discharged normal butane and isobutane stream into a separation zone to separate said normal butane from said isobutane in a separation tower which is operated at overhead temperatures of from about 100.degree. to about 200.degree. F. and overhead pressures of from about 55 to about 240 psig;
• c. discharging said normal butane from said separation zone and introducing said normal butane from said separation zone into said cracking zone as a feedback for the production of ethylene;
• d. passing said isobutane from said separation zone into an isomerization zone wherein a portion of said isobutane is converted into normal butane in the presence of an isomerization catalyst to form a stream of normal butane and isobutane;
• and
• e. thereafter, returning said stream of normal butane and isobutane produced in said isomerization zone into said separation zone to separate the normal butane produced in said isomerization zone.