Usage of Cfd Modelling for Improving an Fcc Riser Operation
Oil & Gas Science and Technology - Rev. IFP, Vol.
Usage of CFD Modelling for Improving an FCC Riser Operation
T. Patureaux 0
D. Barthod 0
0 TotalFina Raffinage & Marketing, CdR, département Procédé et Raffinage , BP 27, 76700 Harfleur - France
fluid catalytic cracking; riser; catalyst distribution; CFD; computational fluid dynamics; conveying steam
Total (now TotalFina) has been deeply involved in FCC for a
long time, both as an operator (its first unit in France was put
on stream in 1954) and as a licensor since the early 80’s,
when was developed the original two-stage regenerator
process (R2R), as the result of an intensive research in
association with IFP.
This process is now licensed by IFP and Stone & Webster
Engineering Corp., with the continuous support of a heavy
R&D program from TotalFina and IFP.
This research effort, aimed at improving the design, operation and reliability of the R2R process, led to a better identification and comprehension of what are the key points of an industrial FCC unit.
A lot of knowledge and experience being then accu
mulated through research and operation, every opportunity
to improve the existing units of TotalFina was seized,
especially during major revamps and turnarounds.
This article describes the work that has been done to
improve the feed/catalyst initial contacting at the FCC unit in
TotalFina refinery near Dunkerque (France), using CFD as a tool for analysing an existing situation and implementing a solution to tackle the problem .
1 BACKGROUND OF THE UNIT
The FCC unit at TotalFina Dunkerque refinery is an Exxon
Flexicracker and was put on stream in 1982. Over the years,
it has been subjected to several modifications, such as the
change from a dense bed reactor to riser operation, a better
catalyst entry in the regenerator, the implementation of
highefficiency feed injectors and acceleration steam nozzles, etc.,
while keeping the general layout of the transfer lines.
In 1994, a series of three major modifications was done on
the unit (Fig. 1):
– installation of a new feed injection system with injectors
pointing downwards instead of upwards in conventional
– a new riser termination device aimed at separating quickly
reaction products and catalyst so as to minimise the
importance of non-selective post riser cracking reactions;
– a new set of internals in the stripper consisting mainly of a
structured packing specially adapted to countercurrent
flow of catalyst and stripping gas.
This revamp was very successful in terms of increased products yields and qualities and general profitability of the unit.
As the changes were really a technological breakthrough, a
lot of attention was paid to the post-revamp performance
evaluation and every part of the unit which had been subjected
to a modification was scrutinised and evaluated; within this
prospect, attention focused on the riser which, in addition to
Riser termination device
the existing instrumentation, had been equipped during the
revamp with sampling ports at various levels above the feed
Some doubts arose concerning the homogeneity of the
mixture of feed and catalyst above the injection level, mainly
from the indications of the two rows of two thermocouples
located at two different elevations above the injector nozzles.
It triggered a series of investigations to analyse the situation.
2 INVESTIGATION ON CATALYST DISTRIBUTION
The primary hint came from the thermocouples labelled
TJ 2056/2057 and TJ 2058/2059 located respectively 5.5 and
8.5 m above the injectors level, as shown in Figures 2 and 3.
It appeared clearly that, in spite of a good balance of the
feed and dispersion steam through the eight injectors, there
was a permanent difference in the temperatures at the two
points at the same level, which could be as high as 30-35°C,
when they should have been very close if the mixture catalyst
feed had been homogeneous (Table 1).
TJ 2058, TJ 2059
TJ 2056, TJ 2057
it was clearly higher between and above injectors Nos. 4
to 6, which was coherent with the hypothesis of a
preferential flow path of the catalyst;
– a gamma scan at various levels in the injection zone
showed again a tendancy for the catalyst to flow in the
extrados region, but nothing quantitative nor definite
could be obtained by this technique;
– a new test was then designed to confirm the link between
the geometry of the J-bend and the catalyst flow pattern.
The idea was that a well balanced and symmetrical injection system could not obviously be able to correct a dissymmetry in the catalyst flow.
To have the indications of the wall thermocouples at the
same level coming closer together, one had to shut off the
feed injector No. 8 (while maintaining the dispersion steam
in service); the gap between temperatures shrank then very
significantly (Table 2).
TJ 2057-TJ 2056
TJ 2059-TJ 2058
Temperatures close to the wall (°C)
It clearly indicated that with a perfectly balanced and close
to symmetry system, the catalyst flows preferentially on the
opposite lane of the J-bend and even on the right side.
More complete profiles inside the riser were elaborated at
15 m above the injectors by introducing a movable probe
equipped with a thermocouple; the best result concerning the
temperature profile flatness was obtained with injectors
Nos. 7 and 8 closed (see locations of injectors in Figure 3).
In the mean time, samples were taken in the vapour line to evaluate the consequences on the yield structure at the very outlet of the reaction section.
It was clear enough that with a homogeneous feed/catalyst
mixture, even at the expense of a misbalanced feed injection,
one was really able to modify the yield structure towards less
dry gas (0.32% absolute is really significant), with improved
selectivities (Table 3).
3 USAGE OF CFD
All this experimental investigation on the unit itself, which
was not always easy to carry out as it interfered in many
circumstances with operation, was sufficient in itself to
establish a diagnosis and imagine a permanent solution based
on technology. It was also a good opportunity to put into
practice all the knowledge and know-how gained in the field
of CFD applied to the flow of gas-solid fluidised systems,
through the research efforts accomplished jointly by
TotalFina and IFP.
The resulting modified code was used, at a 1/3 scale of the
exact J-bend configuration upstream of the feed injectors.
The main customisation of the code, necessary for predicting correctly gas/solid flows with FCC catalyst, was done by fitting CFD results to experimental measurements on bubbling fluidised beds (Appendix).
The previsions of the simulation were as expected
coherent with the observations, showing clearly that the
segregation occurring in the slanted part of the transfer line
was indeed the cause of the homogeneity of catalyst flow
immediately upstream of the injectors (Fig. 4).
4 ELABORATION AND IMPLEMENTATION
OF A CORRECTIVE MEASURE
As everything converged to demonstrate that the upflow of
regenerated catalyst was not homogeneous on the riser
section, and that this heterogeneity could not be corrected by
the existing steam system which was almost symmetric with
respect to the riser/J-bend plane, this steam system was
completed by other nozzles; several solutions were tested
initially by CFD to find the optimum position and steam rates
for these new nozzles.
In this process it was of course necessary to comply with the room available at the bottom of the riser. These new nozzles were indeed installed during the last revamp in May 1999.
Just to complete the information on the homogeneity of
flow in the riser, two additional rows of four thermocouples
each were installed respectively at 8.3 m (TJ 2115 to 2118)
and 11.5 m (TJ 2119 to 2122) according to the disposition of
With all the feed injectors in operation, the maximum
difference in temperatures (∆ T) at the same level has dropped
typically to 5°C at elevation 8.3 m and to 7°C at 11.5 m.
As shown in Table 4, the temperatures in the outside lane
which used to be the highest in the previous situation are now
the lowest. It is the demonstration that the corrective effect of
the new steam nozzles is effective, but also that a further
tuning is necessary.
to the feed nozzles (m)
The ∆ T on two opposite vertical lines TJ 2116-TJ 2119
and TJ 2117-TJ 2122 are the same, which means that the
reactions progress are the same pace on the two lanes; C/O
(cat to oil ratio) must be very close in these two regions.
It is probable that a fine tuning of the steam rates on the different nozzles would really level the temperature profiles.
5 INFLUENCE OF CONVEYING
AND REDISTRIBUTION STEAM RATES
The validity of the CFD modelling of the bottom zone of the riser having been established, it has been used to predict the effect of a variation of the conveying steam in the slanted portion and of the redistribution steam.
Figure 7 shows the variation of the local C/O ratio at the
feed injector levels on the intrados and the extrados following
the bend in different cases:
– conveying steam 2 t/h, no redistribution steam: it
corresponds to an overaerated transfer line. The segregation is
at its worst and the difference in C/O is formidable but
without too high fluctuations with time;
– conveying steam 1 t/h, no redistribution: the segregation is
less, but fluctuations are really high;
– conveying steam 1 t/h, redistribution 1 t/h: the C/O in the
opposite regions are very close to the mean values and
Figure 5 shows the CFD-predicted catalyst concentration
map upstream of the feed injectors with the new redistribution
steam system. It is clear that the time-averaged catalyst flow
is more uniform.
C/O evolution at feed injectors level
Influence of conveying and redistribution steam rate
Conveying 2 t/h
Redistribution 0 t/h
Conveying 1 t/h
Redistribution 0 t/h
These results confirm the effectiveness of the new
CFD was eventually extremely useful for optimising the
redistribution steam system, and remind that a slanted
uptechnological solution selected and finally implemented.
flowing transfer line is very sensitive to an overaeration.
Overaeration can cause a segregation between catalyst and
steam upstream of the feed injection level; if not corrected it
will be detrimental to the selectivity in the yield structure.
This study confirmed that the radial mixing in a straight riser or fast fluidised bed is rather poor [2-4] and will not be able to correct any default in the initial feed/catalyst contacting zone.
This observation emphasises the major role played by the
design of this part of the unit. Once again, CFD along with a
good mastery of mass transfer and catalytic aspect is a
preferential tool for imagining and implementing an efficient
Several tools were used in this case history to analyse the
situation, starting from basic
observations such as
temperatures to more sophisticated approaches like CFD
Each of them has its value. Interpretation of temperature
maps and surveys was sufficient by itself for conceiving
a solution. The coherence between CFD results and
observations was in the mean time a confirmation of the
validity of the interaction laws introduced in the model and
soundness of the analysis of what occurred in the transfer line
and the injection zone.
FCC GAS/SOLID FLOW MODEL
In a bubbling fluidised bed, the flow is made up of coexisting
very dilute (bubbles) and dense (emulsion) regions.
Therefore, the hydrodynamic model must take into account
simultaneously the gas-particle and particle-particle
interactions in the mean and fluctuating motions.
To simulate such a flow, the approach used is that
developed by Simonin [
]. This approach, based on the
two-fluid model, uses the kinetic theory to account for
particle-particle and gas-particle interactions. A 2D version
of this model was first implemented into PHOENICS
package in order to simulate both dilute and dense FCC
gassolid flows [
]. The results showed that it was necessary to
modify this first version of the model to properly simulate
these flows. Then it was decided to correct empirically the
drag forces by fitting predicted CFD results to experimental
measurements on bubbling fluidised beds.
For this purpose, a simple stationary equilibrium 1D
model was developed and used. To build this model, one can
write both momentum balance and momentum transfer
∂P1 + α1ρ1g + I1 = 0
−α2 ∂∂Px1 + α2ρ2g + I2 = 0
I2 = – I2 = – α2ρ1FDVr
The particularity of the FCC flows is that ρ2 >> ρ1 and, in
this case, the forces due to the gas on a single particle are
reduced, as we see in these equations, to the mean pressure
gradient and to the drag force.
• For dilute gas/solid flows, following the approach of
Wen and Yu , we modified the standard law of
Richardson and Zaki  in order to account for Reynolds
number influence and the concentration effects:
α2 < 0.2:
FD = 3 C
⋅ D ⋅ Vr ⋅ α1−1.65 ⋅ f (Re, d)
Re is the particle Reynolds number defined by:
d is the mean particle diameter and f(Re, d) is a correcting
factor which depends on Re and d. One can notice that
when f(Re, d) = 1, we find the Wen and Yu drag force.
• For dense gas/solid flows, a correlation based on Ergun’s
relation is used in the same way:
α2 ≥ 0.2:
Then, by substituting the pressure gradient in Equations (1) to (3), one obtains the stationary 1D model used:
α1 ρ2 − ρ1 g − FDVr = 0
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