To produce high-purity butadiene (BD) from a mixed C4 stream, typically a byproduct stream from an ethylene plant using liquid feeds (liquids cracker). The BASF process uses n-methylpyrrolidone (NMP) as the solvent.
The mixed C4 feed stream is fed into the main washer, the first extractive distillation column (1), which produces an overhead butanes/butenes stream (raffinate-1) that is essentially free of butadiene and acetylenes.
The bottoms stream from this column is stripped free of butenes in the top half of the rectifier (2). A side stream containing butadiene and a small amount of acetylenic compounds (C5 and C4-acetylenes) is withdrawn from the rectifier and fed into the after-washer [the second extractive distillation column (3)1. In recent designs, the rectifier (2) and after-washer are combined using a divided wall column.
The C4 acetylenes, which have higher solubilities in NMP than 1,3-butadiene, are removed by the solvent in the bottoms and returned to the rectifier. A crude butadiene (BD) stream from the overhead of the after-washer is fed into the BD purification train. Both extractive distillation columns have a number of trays above the solvent addition point to allow for the removal of solvent traces from the overheads.
The bottoms of the rectifier, containing BD, C4 acetylenes and C5 hydrocarbons in NMP, is preheated and fed into the degasser [the solvent stripping column (4)]. In this column, solvent vapors are used as the stripping medium to remove all light hydrocarbons from NMP. The hot, stripped solvent from the bottom of the degasser passes through the heat economizers (a train of heat exchangers) and is fed to the extractive distillation columns.
The hydrocarbons leaving the top of the degasser are cooled in a column by direct contact with solvent (NMP) and fed to the bottom of the rectifier.
Hydrocarbons having higher solubilities in the solvent than 1,3-butadiene accumulate in the middle zone of the degasser and are drawn off as a side stream. This side stream, after dilution with raffinate-1, is fed to a water scrubber to remove a small amount of NMP from the exiting gases. The scrubbed gases, containing the C4 acetylenes, are purged to disposal.
In the propyne column (5), the propyne (C5 acetylene) is removed as overhead and sent to disposal. The bottoms are fed to the second distillation column [the 1,3-butadiene column (6)1, which produces pure
BD as overhead and a small stream containing 1,2-butadiene and C5 hydrocarbons as bottoms.
Yield: Typically, more than 98% of the 1,3-butadiene contained in the mixed C4 feed is recovered as product.
