reactor temperatures. More recently, changes in both gasoline versus distillate demand
and gasoline specifications have reduced severity in many reformers and FCCUs, which
shifts the fuel gas balance back toward shortness. However, lower severity reformer
operation also reduces the availability of hydrogen for feedstock and product
desulfurization, whereas product sulfur specifications have been tightened and the
quality of some feedstocks has declined. Many refiners have found it necessary to
improve hydrogen recovery from refinery fuel gas and even to supplement hydrogen
supply through purchase or on-purpose production.
In addition to the higher severity operations of the FCCU, simple increases in FCCU
throughput can also impact the refinery’s fuel balance. Hot gas from the FCCU catalyst
regenerator is routed to a CO boiler, where its heat is used to generate steam. At higher
throughput, even if conversion remains the same, increased amounts of steam are
generated from waste heat in the CO boiler. Since FCCUs often generate steam at the
highest level in the refinery, this causes the refinery’s on-purpose boilers to reduce
production, thus reducing the demand for fuel gas.
In addition to conversion unit fundamentals, inefficient operations can send light end
molecules into the fuel system. For example, poor use of hydrogen can lead to hydrogen
letdown into the fuel system. Furthermore, refineries often have a greater fuel gas
containment problem in the hot summer months when higher volumes of C3+material
make their way to the fuel system. This can be due to poor cooling tower operations
where summertime cooling water supply temperatures are as high as 90°F