Today’s S&W design employs front-end vapor phase hydrogenation catalysis (higher selectivity,
eliminates regeneration) with optional back-end liquid phase hydrogenation of residual methyl
acetylene/propadiene. If appropriate, partial or complete saturation of C4/C5 fractions is also
included. Most ethylene units are linked to an associated pyrolysis gasoline hydrogenation unit. Such
mixed-phase systems reduce gum precursors by conjugated diene saturation (mixed phase, first stage)
to meet gasoline blendstock specifications and optionally produce a sulfur-free, olefin-free aromatic
concentrate (vapor phase, second stage) for BTX extraction.
Another emerging aspect of ethylene plant recovery design is the integration of the ethylene unit
with its adjacent refinery. As the degree of refinery conversion processing increases, recovery of
light olefins from FCC-type units becomes economically viable. Accordingly, it is frequently
attractive to separate these refinery offgas olefins within the adjacent ethylene unit. This requires a
series of contaminant removal systems to avoid safety risks and product specification deviations but
provides a low-cost supplementary feedstock source. Other areas of refinery-petrochemical
integration also have been devised to further improve combined operating economics, e.g.,
hydrocracker residue cracking, pyrolysis gasoline interchange, hydrogen management, recycle
cracking of various raffi-nate streams, metathesis-type conversions, etc.