Hydrocarbon Processing - August 2021 - 16

Valves, Pumps and Turbomachinery
flow hopper at the entrance of the regeneration
standpipe is used to remove large
bubbles carried by the descending catalyst.
The standpipe consists of the top inclined
pipe, the middle vertical pipe and the bottom
inclined pipe. To control the stable
flow of catalyst, eight aeration nozzles
(labeled C1-C8) are installed along the
height of the regeneration standpipe. The
angle between the eight nozzles and the
pipe wall is 45°. The aeration medium of
the C1-C7 nozzles is 0.55-MPa plant air,
and the aeration medium of the C8 nozzle
is 1 MPa plant air and 270°C steam. The
aeration rate of each nozzle is controlled
by the flow limiting the orifice plate, which
has a diameter of 2 mm-3 mm. The pressure
at the standpipe entry (in kPa) is pi.
The pressure above the regeneration slide
valve (in kPa) is ps. The pressure at the
standpipe outlet (in kPa) is po.
FIG. 2. Schematic diagram of regeneration
standpipe.
Analysis of slide valve fault. In December
2019, the operator found that
the regeneration slide valve could not be
adjusted through the distributed control
system (DCS), which caused the reaction
temperature to fluctuate sharply. The hand
wheel control was used to open the regeneration
slide valve, and although the stem
of the regeneration slide valve followed the
shaking action of the hand wheel, the reaction
temperature failed to change. Meanwhile,
when the hand wheel was used to
close the slide valve, the shaking action
became more difficult and the opening of
the regeneration slide valve would close.
Based on the described phenomena, a
plant engineer determined that the stem
of the slide valve had been broken. In
September 2020, the FCCU was shut
for maintenance. When the cover of the
regeneration slide valve was opened, the
stem of the regeneration slide valve was
found to be broken, as shown in FIG. 3.
It could be seen that the back side of the
slide valve plate was seriously scoured,
forming two obvious grooves as shown in
FIG. 3A. The stem of the regeneration slide
valve was worn and fractured as shown in
FIG. 3B, which was caused by the scouring
of the eddy flow field formed by the scavenging
steam of the disc guide rail and the
catalyst carried by the aeration.4,5
Variation in regeneration catalyst
route. The regeneration standpipe has
two functions in the FCCU: (1) provide a
steady supply of high-temperature catalyst
for the riser and (2) establish a material
seal above the regeneration slide valve to
prevent the mixture of gas and oil. However,
due to the stem fracture, the valve plate
of the regeneration slide valve could only
be closed rather than opened, and the control
accuracy of the catalyst circulation rate
worsened, which affected the standpipe
pressure buildup and reaction temperature
(FIG. 4). The axial pressure gradually
increased along the height of the standpipe,
as shown in FIG. 4A, which indicated
that the catalyst was well fluidized and the
standpipe had strong pressure storage.
However, the reaction temperature
FIG. 3. Disc and stem system showing (a) the scoured valve plate and (b) the orifice of the
slide valve.
A
32
28
24
20
16
12
8
4
200
pi
C1-C2
515
C3
C4
C5-C7
Slide valve
C8
250
300
p, kPa
350
po
505
400
450
10
20
30
Time, min
FIG. 4. Regeneration catalyst route parameter profiles: (a) pressure profile of regeneration
standpipe and (b) reaction temperature curve.
16 AUGUST 2021 | HydrocarbonProcessing.com
40
50
60
ps
510
Variation in spent catalyst route. The
flow pattern of a stripper is a low-velocity
bubbling fluidized bed, and the range of
the apparent catalyst density is 700 kg/
m3
-800 kg/m3
. When the reaction temperature
fluctuates greatly, the unvaporized
heavy oil droplets enter the stripper
with the spent catalyst, leading to coke formation
at high temperature that block the
520
Reaction temperature
B
fluctuated sharply between 507°C and
518°C, as shown in FIG. 4B. The fluctuation
frequency was high-more than four
times per hour. When reaction temperature
fluctuates greatly, it not only affects
the product distribution but also reduces
the gasoline yield. Moreover, it reduces
the atomization effect of heavy oil. The
amount of unvaporized oil in the riser increases,
leading to an increase of coke.
Elevation, m
Temperature, °C

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Hydrocarbon Processing - August 2021

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