Hydrocarbon Processing - July 2021 - 44

Process Optimization
pipe and the vertical line is 10°. The upper
inclined pipe encompasses two parts
with inner diameters of 760 mm and 630
mm, respectively, and a total length of
18.2 m. The top wider part is used to degas
bubbles. The angle between the lower
inclined pipe and the vertical line is 45°,
with a length of 2.3 m and an inner diameter
of 630 mm.
C1-C11 are the 11 axial cross-sections
of the standpipe. One aeration nozzle is installed
on each cross-section, and the angle
between the nozzles and the wall surface is
30°. Nozzles on C2, C4, C6 and C8 are located
at the bottom inclined pipe. The rest
of the nozzles are installed on both sides,
and the angle with the central busbar is
60°. The aeration gas above the regenerated
slide valve is nitrogen with a pressure
of 1.4 MPa and a normal temperature. The
aeration gas below the regenerated slide
valve is steam with a pressure of 1 MPa
and a temperature of 265°C. The aeration
flowrate is controlled by the flow limiting
orifice with a diameter of 2 mm-5 mm.
Standpipe
Riser
TABLE 2 shows the analysis of material
properties of the equilibrium catalyst at
20°C. Pressure measurement was carried
out with the selected aeration nozzles
(C1, C2, C3, C6, C9 and C10) at the
same time under different operating conditions.
Measuring frequency and time
were 1 Hz and 60 sec, respectively.
Results and discussion. In looking at
the pressure profiles of the standpipe, FIG. 3
shows the axial pressure profiles of the regenerated
standpipe from Case 1 to Case
4. The pi
depended on the regenerator
pressure and the static pressure of the
dense-phase bed in the regenerator, with
little change under the different operating
conditions. The axial pressure increased
gradually along the standpipe, but the
pressure between the C6 and C9 crosssections
was greatly affected with Gs
When Gs was 857.4 kg × m-2 × s-1
.
, the
pressure at the bottom standpipe increased
and had a maximum pressure of 255.6 kPa
at the C9 cross-section. In the other three
operating conditions, the pressure below
the C6 cross-section decreased. When Gs
was 435.4 kg × m-2
× s-1
Expansion joint
Nitrogen
Steam
Nozzle
FIG. 2. Schematic diagram of the regenerator
standpipe. EL is elevation; C1-C11 are crosssections
of the standpipe.
TABLE 1. Operating parameters of the FCCU
Gs/
Case
1
2
3
4
Feedstock,
t/hr-1
120
105
95
75
(kg × m-2
857.4
703.4
593.9
435.4
× s-1
)
26
24
22
20
18
16
14
12
10
8
6
4
6
8
10
12
14
16
18
20
22
24
26
C1 (pi
)
, pressure at the
C9 cross-section had a minimum pressure
C1 (pi)
Gs / (kgm ) :
-2
C2
C2
C3
C3
Expansion point
Slide valve
170
180
Slide valve
C9
C10
190
200
210
220
p, kPa
p / kPa
FIG. 3. Axial pressure profiles in the standpipe.
170 180 190 200 210 220 230 240 250 260
4
C10 (po(po)
230
)
240
250
260
C9 (ps(ps)
)
Expansion point
C6
C6
857.4
703.4
593.9
435.4
857.4
703.4
593.9
435.4
Gs / (kg × m-2 × s-1
s ):
-1
of around 210 kPa. The po
below the slide
valve ranged in pressure from 180 kPa-
223 kPa, which was mainly affected by the
pressure drop of the riser, and increased
with the increase of Gs
.
FIG. 4 shows the dynamic pressure at
the C6 and C9 cross-sections under different
operating conditions. When Gs
was
857.4 kg × m-2 × s-1
, the mean pressure at
the C6 cross-section was less than that at
the C9 cross-section, as shown in FIG. 4A.
In the other three operating conditions,
the pressure at the C6 cross-section was
greater than that at the C9 cross-section.
Moreover, the pressure difference between
the C6 and C9 cross-sections increased
gradually with the decrease of Gs
Gs was 435.4 kg × m-2 × s-1
. When
, the mean pressure
difference increased to 30 kPa.
In analyzing the flow patterns in the
standpipe, FIG. 5 shows the single and
double flow pattern modes in the FCC
standpipe proposed by Leung et al.7
Generally
speaking, the sliding speed of the
gas solids at the top standpipe is large,
and the catalyst flow pattens are densephase
fluidized solids flow. The fluidized
gas volume in the bottom standpipe is
reduced because it is compressed. If the
aeration flowrate is approximately equal
to the decreased gas volume, then the axial
pressure gradually increases along the
standpipe, as shown in FIG. 5A.
If the aeration flowrate is insufficient,
then the pressure gradient along the
standpipe decreases. The catalyst flow
pattern on the top standpipe is in a fluidized
state. Catalyst voidage, ε, decreases
gradually, and the axial pressure increases
continuously. However, in the bottom
standpipe, when ε is less than the incipient
fluidization voidage, εmf
, the catalyst
flow patterns change to transitional
Reactor
pressure, kPa
173
173
165
165
TABLE 2. Properties of the equilibrium catalysts
Apparent packing
density, kg × m-3
840
Skeletal density
(ρs
), kg × m-3
2,400
φ%
ρmf, kg × m-3
760
APS, μm
76
0-20, μm 20-40, μm 40-60, μm 60-80, μm 80-110, μm > 110, μm
7
Notes: φ = Particle size volume distribution; APS = Average particle size; ρmf = Incipient fluidization density
44 JULY 2021 | HydrocarbonProcessing.com
29
27
27
10
Regenerator
pressure, kPa
165
165
160
160
Reactor
temperature, °C
530
520
510
500
Regenerator dense
phase temperature, °C
705
705
705
705
Catalyst-tooil
ratio
8
7.5
7
6.5
Opening of
slide valve
65
60
55
45
ElevatElevation, mio n / m

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Hydrocarbon Processing - July 2021

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