MAXOFIN FCC
The proprietary MAXOFIN FCC process, licensed by KBR, is designed to maximize the production of propylene from traditional FCC feedstocks and selected naphthas (Fig. 3.1.6).In addition to processing recycled light naphtha and C4 LPG, the riser can accept naphtha
from elsewhere in the refinery complex, such as coker naphtha streams, and upgrades
these streams into additional light olefins. Olefinic streams, such as coker naphtha, convert
most readily to light olefins with the MAXOFIN FCC process. Paraffinic naphthas, such
as light straight-run naphtha, also can be upgraded in the MAXOFIN FCC unit, but to a
lesser extent than olefinic feedstocks.
A MAXOFIN FCC unit can also produce an economic volume of ethylene for petrochemical
consumption if there is ready access to a petrochemical plant or ethylene
pipeline. For instance, while traditional FCC operations have produced less than about 2
wt % ethylene, the MAXOFIN FCC process can produce as much as 8 wt % ethylene.
Spent Catalyst Stripping
Catalyst separated in the cyclones flows through the respective diplegs and discharges into
the stripper bed. In the stripper, hydrocarbon vapors from within and around the catalyst
particles are displaced by steam into the disengager dilute phase, minimizing hydrocarbon
carry-under with the spent catalyst to the regenerator. Stripping is a very important function
because it minimizes regenerator bed temperature and regenerator air requirements,
resulting in increased conversion in regenerator temperature or air-limited operations. See
Fig. 3.1.7.
The catalyst entering the stripper is contacted by upflowing steam introduced through
two steam distributors. The majority of the hydrocarbon vapors entrained with the catalyst
are displaced in the upper stripper bed. The catalyst then flows down through a set of hat
and doughnut baffles. In the baffled section, a combination of residence time and steam
partial pressure is used to allow the hydrocarbons to diffuse out of the catalyst pores into
the steam introduced via the lower distributor.
Stripped catalyst, with essentially all strippable hydrocarbons removed, passes into a
standpipe, which is aerated with steam to maintain smooth flow. At the base of the standpipe,
a plug valve regulates the flow of catalyst to maintain the spent catalyst level in the
stripper. The catalyst then flows into the spent catalyst distributor and into the regenerator.
Regeneration
In the regenerator, coke is burned off the catalyst with air in a fluid bed to supply the heat
requirements of the process and restore the catalyst’s activity. The regenerator is operated
in either complete CO combustion or partial CO combustion modes. In the regenerator
cyclones, the flue gas is separated from the catalyst.
Regeneration is a key part of the FCC process and must be executed in an environment
that preserves catalyst activity and selectivity so that the reaction system can deliver the
desired product yields.
The KBR Orthoflow converter uses a countercurrent regeneration system to accomplish
this. The concept is illustrated in Fig. 3.1.8. The spent catalyst is introduced and distributed
uniformly near the top of the dense bed. This is made possible by the spent catalyst
distributor. Air is introduced near the bottom of the bed.
The design allows coke burning to begin in a low-oxygen partial pressure environment
which controls the initial burning rate. Controlling the burning rate prevents excessive particle
temperatures which would damage the catalyst. The hydrogen in the coke combusts more
quickly than the carbon, and most of the water formed is released near the top of the bed.
These features together minimize catalyst deactivation during the regeneration process.
With this unique approach, the KBR countercurrent regenerator achieves the advantages
of multiple regeneration stages, yet does so with the simplicity, cost efficiency, and
reliability of a single regenerator vessel.
Catalyst Cooler
A regenerator heat removal system may be included to keep the regenerator temperature
and catalyst circulation rate at the optimum values for economic processing of the feedstock.
The requirement for a catalyst cooler usually occurs when processing residual feedstocks
which produce more coke, especially at high conversion.
The KBR regenerator heat removal system is shown in Figure 3.1.9. It consists of an
external catalyst cooler which generates high-pressure steam from heat transferred from
the regenerated catalyst.
Catalyst is drawn off the side of the regenerator and flows downward as a dense bed
through an exchanger containing bayonet tubes. The catalyst surrounding the bayonet tubes
is cooled and then transported back to the regenerator. Air is introduced at the bottom of the
cooler to fluidize the catalyst. A slide valve is used to control the catalyst circulation rate and
thus the heat removed. Varying the catalyst circulation gives control over regenerator temperature
for a broad range of feedstocks, catalysts, and operating conditions.
Gravity circulated boiler feedwater flows downward through the inner bayonet tubes
while the steam generated flows upward through the annulus between the tubes.
