TECHNOLOGY FOCUS - CHEMICAL ENGINEERING - Preferred Process: The vapor side stream is passed to a first side column (SK I) giving a top stream containing (II), which is condensed in a top condenser, part of which is returned to the side column as reflux and the rest discharged, and a sump stream containing (IV), which is recycled to the first extractive distillation column (K I). A sump stream is discharged from K I, cooled by indirect heat exchange with (I), condensed in a condenser and recycled to K I as (IV) stream. The top stream from K I is condensed in a top condenser and partly recycled as reflux, whilst the rest is passed to a second extractive distillation column (K II), in which it is separated into raffinate 1 and crude buta-1,3-diene; or the top stream is partly condensed, the condensed fraction being used as reflux and the vapor passed to K II. The streams from K II are a top stream, which is condensed in a condenser, partly recycled to K II as reflux and partly discharged as raffinate 1; and a side stream, which is discharged below the feed point and preferably passed to a second side column (SK II). SK II gives a top stream, which is condensed, part being used as reflux and the rest discharged as crude buta-1,3-diene, and a sump stream of (IV), which is recycled to K II. One or more separation stages below the vapor side stream discharge, liquid or a partial stream of the liquid is discharged from K I, heated and/or vaporized by indirect heat exchange with the sump stream from K I and recycled to or above the same stage, which is selected to minimize the energy demand for K I. A (partial) stream of liquid taken one or more separation stages below the side discharge of K II is treated similarly. EXAMPLE - An extractive distillation column K I with 28 theoretical plates was charged at plate 15 (from the bottom) with 32 tonne/hour crude 4 carbon (C) stream (1) containing (wt.%) 5.75% n-butane, 2.45% iso-butane, 13.89% but-1-ene, 25.65% iso-butene, 4.44% trans-but-2-ene, 2.96% cis-but-2-ene, 43.84% buta-1,3-diene, 0.14% buta-1,2-diene, 0.13% ethylacetylene and 0.74% vinylacetylene. The column was operated at a top pressure of 4.5 bar absolute and top temperature of 58.8degreesC. A stream (2) of 120 tonne/hour N-methylpyrrolidone (NMP) with 8.3% water was fed to the top plate. The stream (3) discharged at plate 3 comprised 224 kg/hour stream of 2.45% buta-1,3-diene, 1.21% buta-1,2-diene, 1.67% ethylacetylene, 10.40% vinylacetylene, 70.10% water and 14.08% NMP. The solvent was scrubbed from this in side column SK I, which operated as pure concentration column. This gave a top stream (6) containing 3.24% buta-1,3-diene, 1.60% buta-1,2-diene, 2.20% ethylacetylene, 13.56% vinylacetylene, 0.16% 5 C hydrocarbons and 79.24% water. The costly NMP component of the extractive solvent was separated completely, down to 1 ppm in the top stream of the side column and recycled to column K I, so that the acetylenes could be removed from the process practically without loss of NMP.