Hydrocarbon Processing - August 2021 - 15

Special Focus Valves, Pumps and Turbomachinery
G. RUIBO, PetroChina Jinzhou Petrochemical Co.,
Jinzhou, Liaoning, China; and P. WEI and L. YANSHENG,
China University of Petroleum, Beijing, China
Online treatment of a slide valve fault in an FCCU
The regeneration slide valve is a key
piece of equipment that controls the ratio
of catalyst to oil and reaction temperature
in FCC units (FCCUs). Unplanned shutdown
caused by slide valve stem fracture is
a challenging problem that limits the operational
period of FCCUs.
In this article, the reaction-regeneration
system in a commercial, 1-MMtpy FCCU
was studied to determine the influence of
a regeneration slide valve fracture on the
key parameters in the regenerated and
spent catalyst routes. Double-valve control
technology was introduced to manage the
slide valve stem fracture without requiring
a shutdown. Meanwhile, the axial pressure
distribution in the regeneration standpipe
and the variation of the reaction temperature
in the riser were recorded by adjusting
the sensitive aeration nozzles.
Based on the stability of the reaction
temperature, the catalyst flow patterns in
the vicinity of the regeneration slide valve
were divided into packed-bed flow, bubbling
fluidized flow and slug flow, respectively.
In addition, the operating conditions
corresponding to the different catalyst flow
patterns were outlined to help guide the
operation and adjustment of the FCCU.
Fluid catalytic cracking in China. In
2019, more than 200 FCCUs produced 70
wt% of gasoline, 30 wt% of diesel oil and 10
wt% of propylene in China.1
The FCCU
comprises two main sections: a reactor
and a regenerator. The catalyst circulation
between the two vessels is carried out in
a vertical pipe or inclined pipe, which are
collectively referred to here as a standpipe.
Electro-hydraulic slide valves are typically
installed at the bottom of the standpipe to
control the catalyst circulation rate.
The equipment pinpointed for this
study are the FCCU riser and combustor.2,3
Since
the standpipe valve system is
not involved in the oil and gas cracking reaction
or the catalyst regeneration process,
few research reports are available on the
standpipe valve system. However, this system
is prone to failure in commercial FCCUs,
which can cause temperature fluctuation
and even unplanned shutdown.4
The FCC slide valve comprises a stem,
disc and guide rail, with the stem controlling
the opening of the disc. If the stem is
broken, then the opening of the slide valve
and catalyst circulation rate cannot be controlled,
leading to FCCU shutdown.
In November 1994, a 1-MMtpy FCCU
suffered a fracture in the regeneration slide
valve stem, leading to an unplanned, 5-d
shutdown.5
This article examines a proposed,
online treatment technology for
the slide valve stem fracture and a method
of stability control for reaction temperature.
Both measures can be adopted to
guide the operation of the unit.
FCCU case study introduction. As
shown in FIG. 1, the commercial setup for
the 1-MMtpy FCCU comprises a reactor
and a regenerator. The regenerator encompasses
the combustion process and the premixing
pipe technology. The riser outlet
includes the direct connection technology
to shorten the oil-gas reaction time and
reduce the secondary cracking reaction.
The feed to the FCCU includes vacuum
gasoil (33 wt%), coking gasoil (17 wt%)
and atmospheric residual oil (50 wt%).
To improve gasoline yield, the FCCU
TABLE 1. Operating parameters of the FCCU
Fresh feed fl owrate, t × hr-1
Reaction pressure, kPa
130
197
Regeneration pressure, kPa
Reaction temperature, °C
Combustion temperature, °C
200
530
685
FIG. 1. Schematic diagram of the reaction-
regeneration system.
utilizes low regeneration temperature and
high reaction temperature, and the ratio of
catalyst to oil is approximately 8, which is
calculated according to the heat balance of
the reaction-regeneration system. The key
process parameters are shown in TABLE 1.
FIG. 2 shows a schematic diagram of the
regeneration standpipe structure. An overTemperature
of the dense phase bed, °C
Combustor density, kg × m-3
Density of the dense phase bed, kg × m-3
Oxygen content of fl ue gas, vol%
Ratio of catalyst to oil
670
170
550
3
8
Hydrocarbon Processing | AUGUST 2021 15
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