(177c) Recombination: A Complicating Issue in FCC Naphtha Desulfurization

The aim of
cracked naphtha selective hydrodesulfurization (SHDS) technologies, such as the
SelectFining? process licensed by UOP, is to reduce the sulfur content of the
naphtha while simultaneously maintaining its octane value.  Both of these
objectives are complicated by recombination, the term applied to the reaction
of H₂S and olefins to form mercaptans.    A solid understanding of
recombination reaction kinetics is critical in the design and operation of a
selective hydrodesulfurization unit as the lowest achievable sulfur level is
often dictated by the extent of recombination.  Furthermore, recombination
contributes to the conversion of the valuable high octane olefins. 
Recombination is a very rapid reaction which frequently, but not always,
approaches equilibrium.  The extent of reaction is governed either by the
equilibrium relationship or kinetics depending on the process conditions,
feedstock composition, and reaction rates.  If the rate of desulfurization is
slow relative to mercaptan formation, the extent of recombination will be
driven by the mercaptan-olefin equilibrium.  Predicting the extent of
recombination is a non-trivial challenge as it is impacted not only by the
total olefin and H₂S content, but also the carbon number, double
bond position, and branching of the olefins.  These issues will be discussed in
the context of the results of a series of numerical simulations and pilot plant
experiments.