NOVELTY - Processing at least one first crude 4C feed (1) mixture comprises: extractive distillation of 4C feed mixture in a first tower (3); thermally distilling the solvent/1,3-butadiene mixture in a second tower, to separate solvent (4) and produce a 1,3-butadiene concentrate; thermally distilling the 1,3-butadiene concentrate in a third tower to form a 1,3-butadiene product and a separate 4C bottoms stream; physically removing the cis-butene-2 rich bottoms stream; and replacing the removed cis-butene-2 rich bottoms stream with fresh crude 4C feed to first tower in addition to first crude 4C feed.    USE - The method is useful for processing the first crude C4 feed mixture containing 1,3-butadiene and cis-butene-2 to produce a 1,3-butadiene process product (claimed).    ADVANTAGE - The method provides the addition of fresh crude 4C feed to the 1,3-butadiene extraction process at a volumetric rate greater than the volumetric rate of the removed and separated finishing tower bottoms stream, without physical modification of the extractive distillation tower increased operating capacity.    DETAILED DESCRIPTION - Processing at least one first crude 4C feed (1) mixture containing at least 1,3-butadiene and cis-butene-2 to produce a 1,3-butadiene process product comprises: extractive distillation in a first tower (3) of the first crude 4C feed mixture being extractively distilled in the first tower with a solvent (4) to produce a solvent/1,3-butadiene mixture that is rich in 1,3-butadiene and containing cis-butene-2; thermally distilling the solvent/1,3-butadiene mixture containing cis-butene-2 to separate the solvent and produce a 1,3-butadiene concentrate containing cis-butene-2 in a second tower; thermally distilling in a third tower the 1,3-butadiene concentrate containing cis-butene-2 to form a 1,3-butadiene product as an overhead stream (7) and a separate 4C bottoms stream containing cis-butene-2, where the third tower has a finite vertical height; carrying out the thermal distillation of 1,3-butadiene concentrate containing cis-butene-2 in the third tower to produce a cis-butene-2 bottoms stream (13) that is rich in cis-butene-2; physically removing the cis-butene-2 rich bottoms stream from the process at a first volumetric rate; and replacing the removed cis-butene-2 rich bottoms stream with fresh crude 4C feed to the first tower in addition to the first crude 4C feed, where the addition of the fresh crude 4C feed being at a second volumetric rate that is at least 1.1 times the first volumetric rate of the removed cis-butene-2 rich bottoms stream.    DESCRIPTION OF DRAWING(S) - The figure shows the schematic representation of a modified butadiene extraction process.    Feed (1)    Extractive distillation tower (3)    Solvent (4)    1,3-Butadiene concentrate overhead stream (7)    Cis-butene-2 rich bottoms stream (13)