1,3 Butadiene is recovered from a crude C4 stream from olefins plants by extractive distillation. N-methylpyrrolidone (NMP) as the selective solvent substantially improves the volatilities of the components. Different process configurations are available.
The C4 cut enters the pre-distillation tower, in which methyl acetylene, propadiene and other light components are separated as gaseous overhead product. Its bottom product enters the bottom section of the main washer column while NMP solvent enters at the column top as overhead product C4 raffinate consisting of butanes and butenes is drawn off. The loaded solvent is sent to the rectifier, which comprises a vertical plate in its upper section. In its first compartment, the less soluble butenes are stripped and fed back into the main washer. In its second compartment, the C4 acetylenes are separated from crude butadiene (BD) due to their higher solubility in NMP.
The solvent from the rectifier bottoms is sent to the degassing tower, where it is completely stripped from hydrocarbons. The stripped hydrocarbons are recycled to the rectifier bottoms via a compressor. The side stream of the degassing tower containing diluted C4 acetylenes is fed into a scrubber to recover NMP solvent. After further dilution with raffinate or other suitable materials, the C4 acetylene stream is discharged to battery limits for further processing. The crude butadiene withdrawn as overhead product from the rectifier is sent to the butadiene column. In its top section, mainly water and some remaining light components are separated, while heavy ends are drawn off as bottom product. The butadiene product is withdrawn as liquid side product.
Due to the excellent properties of NMP the process has a better ecological fingerprint than competing BD extraction technologies.
Recovery rate: Typically more than 98% of 1,3-butadiene.
