Recovery of 1,3-butadiene by fractional crystallization from four-carbon mixtures

Freezing and melting techniques are described for separating 1,3-butadiene from a mixture of four-carbon compounds by fractional freezing of the liquid mixture and by fractional melting thereof. It has been discovered that when a frozen four-carbon mixture (BB) containing about 36% 1,3-butadiene (BD) is initially frozen, the first frozen fraction of up to about 30% of the mixture is enriched about 80% over the feed sample while the filtrate is decreased in BD about 36%. A second fractional crystallization of the melt and of the filtrate shows continued enrichment and impoverishment as to BD content, respectively. It has also been discovered that when a solidly frozen BB mixture is slowly thawed and the melt is progressively removed, the melt becomes markedly enriched in BD. A continuous fractional crystallization process that uses these discoveries for producing BD at purities greater than 99% is described. Utilizing liquefied natural gas (LNG) as the cryogen source for this process is also suggested, so that the refrigeration need for fractional crystallization of BB can be combined with the heating need for LNG vaporization.