Page 235 - Fluid Catalytic Cracking Handbook

Products and Economics 203

* Reducing operating costs
* Meeting product specifications
» Reducing emissions

Product selectivity simply means producing more liquid products and
less coke and gas. Depending on the unit's objectives and constraints,
below are some of the steps that directionally improve product selectivity;
» Feed injection. An improved feed injection system provides
optimum atomization and distribution of the feed for rapid mixing
and complete vaporization. The benefits of improved feed injection
are reduced coke deposition, reduced dry gas yield, and improved
gasoline yield.
* Riser termination. Good riser termination devices, such as closed
cyclones, minimize the vapor and catalyst holdup time in the
reactor vessel. This reduces unnecessary thermal cracking and
nonselective catalytic re-cracking of the reactor product. The
benefits are a reduction in dry gas and a subsequent improvement
in conversion, gasoline octane, and flexibility for processing
marginal feeds.
* Reactor vapor quench. LCO, naphtha, or other quench streams can
be used to quench reactor vapors to minimize thermal cracking.
* Reactor stripper. Operational and hardware changes to the strip-
per improve its performance by removing the entrained and
adsorbed hydrocarbons. The benefits are lower delta coke and
more liquid products.
* Air and spent catalyst distribution. Modifications to the air and
spent catalyst distributors permit uniform dispersion of air and
spent catalyst into the regenerator. Improvements are lower carbon
on the catalyst and less catalyst sintering. The benefits are a
cleaner and higher-activity catalyst, which results in more liquid
products and less coke and gas.

Examples of increasing operating flexibility are:

* Processing residue or "purchased" feedstocks. Sometimes, the
option of processing supplemental feed or other components,
such as atmospheric residue, vacuum residue, and lube oil extract,
is a means of increasing the yields of higher-value products
and reducing the costs of raw material by purchasing less expen-
sive feedstocks.