Modeling and Simulation for FCC unit using Aspen Hysys

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CHEMCON 2013
66
th
Annual Session of Indian Institute of Chemical Engineers
Hosted at Institute of Chemical Technology, Mumbai 400 019
27-30 December 2013
1
Modeling and Simulation for FCC unit using Aspen Hysys
®
Ms. Sreshtha G. Bhende
1
, Dr. Kiran D. Patil
2*
1, 2
Department of Petroleum and Petrochemical Engineering,
Maharashtra Institute of Technology, Paud Road, Kothrud, Pune-411 038
Corresponding author e-mail: sresh11@gmail.com kiran.patil@mitune.edu.in
Abstract: Fluid catalytic cracking (FCC) is one of the most important processes in the petroleum refining industry for the
conversion of heavy gasoil to gasoline and diesel. Furthermore, valuable gases such as ethylene, propylene and isobutylene are
produced. The performance of the FCC units plays a major role on the overall economics of refinery plants. Any improvement
in operation or control of FCC units will result in dramatic economic benefits. This work describes development of a
mathematical model that can simulate the behavior of the FCC unit, which consists of feed and preheat system, riser, stripper,
reactor, regenerator, catalyst circulation lines, and the main fractionator. The model describes the seven main sections of the
entire FCC unit: (1) the feed and preheating system, (2) riser, (3) stripper, (4) reactor, and (5) main fractionator. The process is
simulated in Aspen Hysys v7.3 environment. In this work, a basic refinery process is designed and the vacuum gas oil from the
vacuum distillation column is used as feed in the FCC unit. The data used are the typical data of refinery, in which few changes
were made in order to achieve proper simulation. A parametric sensitivity study is also carried out to give a view of the effect of
these parameters on the production of Gasoline, Light cycle oil, Heavy cycle oil and the total conversion in the FCC unit.
Keywords: Fluidized Catalytic Cracking Unit, Aspen Hysys, parametric sensitivity studies, reactor, and riser.
1. Introduction:
An oil refinery or petroleum refinery is an industrial
process plant where crude oil is processed and refined into
more useful products such as petroleum
naphtha, gasoline, diesel fuel, asphalt base, heating
oil, kerosene, and liquefied petroleum gas. Oil refineries are
typically large, sprawling industrial complexes with
extensive piping running throughout, carrying streams
of fluids between large chemical processing units. An oil
refinery is considered an essential part of
the downstream side of the petroleum industry. Fluid
catalytic cracking (FCC) continues to play a key role in an
integrated refinery as the primary conversion process. For
many refiners, the cat cracker is the key to profitability in
that the successful operation of the unit determines whether
or not the refiner can remain competitive in today’s market.
Approximately 350 catalytic crackers are operating
worldwide, with a total processing capacity of over 12.7
million barrels per day. Most of the existing FCC units have
been designed or modified by six major technology licensers
out of which the most used is by UOP [4]. The FCC process
is very complex. The FCC unit uses a microspheroidal
catalyst, which behaves like a liquid when properly aerated
by gas. The common objective of these various FCC units is
to upgrade low-value feedstock to more valuable products.
The main purpose of the unit is to convert high-boiling
petroleum fractions called gas-oils to high value, high-
octane gasoline and heating oil. Since the start-up of the first
commercial FCC unit in 1942, many improvements have
been made. These improvements have enhanced the unit's
mechanical reliability and its ability to crack heavier, lower
value feedstocks [2]. The FCC has a remarkable history of
adapting to continual changes in market demands.
2. Literature Review:
The basic process of FCC has got two major components
i.e. reactor and regenerator. The feed in the FCC riser are
the residue and the Atmospheric gas oil which comes out
from the distillation column. The feed needs to be preheated
before entering in the riser part. This is done by the feed
preheat system which heats both the fresh and recycled feed
through several heat exchangers and the temperature is
maintained at about 500-700 °F. The gas oil consists of
paraffinic, aromatics and naphthenic molecules and also
contains various amounts of contaminants such as Sulphur,
nitrogen which have detrimental effect on the catalyst
activity. Hence, in order to protect the catalyst feed pre-
treatment is essential which removes the contaminants and

CHEMCON 2013
66
th
Annual Session of Indian Institute of Chemical Engineers
Hosted at Institute of Chemical Technology, Mumbai 400 019
27-30 December 2013
2
have better cracking ability thus giving higher yields of
naphtha [1, 2]. The riser is the main reactor in which most of
the cracking reactions occur and all the reactions are
endothermic in nature. The residence time in the riser is
about 2–10 s. At the top of the riser, the gaseous products
flow into the fractionator, while the catalyst and some heavy
liquid hydrocarbon flow back in the disengaging zone.
Steam is injected into the stripper section, and the oil is
removed from the catalyst with the help of some baffles
installed in the stripper.
The earlier practice of carrying out the cracking reactions
in the reactor has now been completely replaced by carrying
out it in the riser part. This is done to utilize the maximum
catalyst activity and temperature inside the riser. Earlier, no
significant attempts were made for controlling the riser
operations. But after the usage of the reactive zeolites
catalyst the amount of cracking occurring in the riser has
been enhanced. Now the reactor is used for the separation
purpose of both the catalyst and the outlet products.
Reactions in the riser are optimized by increasing the
regenerated catalyst velocity to a desired value in the riser
reactor and injecting the feed into the riser through spray
nozzles. The main purpose of reactor is to separate the spent
catalyst from the cracked vapors and the spent catalyst flows
downward through a steam stripping section to the
regenerator. [3, 4]
The cracking reaction starts when the feed is in contact
with the hot catalyst in the riser and continues until oil
vapors are separated from the catalyst in the reactor
separator. The hydrocarbons are then sent to the fractionator
for the separation of liquid and the gaseous products. In the
reactor the catalyst to oil ratio has to be maintained properly
because it changes the selectivity of the product .The
catalyst’s sensible heat is not only used for the cracking
reaction but also for the vaporization of the feed. During
simulation the effect of the riser is presumed as plug flow
reactor where there is minimal back mixing, but practically
there are both downward and upward slip due to drag force
of vapour. The spent catalyst coming out from steam
stripping section goes in the regenerator. Regenerator
maintains the activity of the catalyst and also supplies heat
to the reactor. Depending upon the feed stock quality there
is deposition of coke on the catalyst surface. To reactivate
the catalyst, air is supplied to the regenerator by using large
air blowers. High speed of air is maintained in the
regenerator to keep the catalyst bed in the fluidized state.
Then through the distributor at the bottom air is sent to the
regenerator. Coke is burned off during the process in
significant amount. The regenerator operates at a
temperature of about 715 °C and a pressure of about 2.41
bars.
The hot catalyst (at about 715 °C) leaving the regenerator
flows into a catalyst where any flue gases are allowed to
escape and flow back into the upper part to the regenerator.
The flow of the regenerated catalyst is regulated by a slide
valve in the regenerated catalyst line. The hot flue gas exits
the regenerator after passing through multiple sets of two-
stage cyclones that removes entrained catalyst from the flue
gas. [4,5]The heat is produced due to the combustion of the
coke and this heat is utilized in the catalytic cracking
process. Heat is carried by the catalyst as sensible heat to the
reactor. Flue gas coming out of the regenerator is passed
through the cyclone separator and the residual catalyst is
recovered.
The specification of the catalyst will be discussed in
detail at literature review. The regenerator is designed and
modeled for burning the coke into carbon monoxide or
carbon dioxide. Earlier, conversion of carbon to carbon
monoxide was done which required lesser air supply hence
the capital cost was reduced. But now a day’s air is supplied
in such a scale that carbon is converted into carbon dioxide
in this case the capital cost is higher but the regenerated
catalyst has minimum coke content on it. The flue gases like
carbon monoxide are burned off in a carbon monoxide
furnace (waste heat boiler) to carbon dioxide and the
available energy is recovered.
The hot gases can be used to generate steam or to power
expansion turbines to compress the regeneration air and
generate power [4]. There are two stage cyclones which
remove any entrained catalyst from the flue gases.

CHEMCON 2013
66
th
Annual Session of Indian Institute of Chemical Engineers
Hosted at Institute of Chemical Technology, Mumbai 400 019
27-30 December 2013
3
Figure 1: Typical UOP type FCC unit
3. Mathematical Analytical Modeling and Simulation:
In current refineries, the FCC unit plays a prominent role,
producing gasoline and diesel, as well as valuable gases,
such as ethylene, propylene and isobutylene, from
feedstocks that comprise atmospheric gas oils, vacuum gas
oils and hydrocracker bottoms. The significant economic
role of the FCC unit in modern-day petroleum refining has
attracted great interest in academia and industry in terms of
developing and modeling control algorithms for efficient
FCC application. The main parts of the FCC unit that have
been modeled are riser, reactor and fractionator. The riser
of the FCC unit is assumed to be a reactor in which all the
complex reactions take place [5,7]. Since maximum
conversion takes place in riser it was summed to be a
conversion reactor where in mass transfer takes place [1,
2]. For simplicity, we will assume isothermal, constant-
holdup, constant-pressure, and constant density conditions
and a perfectly mixed liquid phase. The total mass-transfer
area of the bubbles is A and it could depend on the gas feed
rate FA. A constant-mass-transfer coefficient: t k, (with
units of length per time) is used to give the flux of A into
the liquid through the liquid film as a function of the
driving force.
N
A
=k
L
(C
A
*
-C
1
) [1]
Mass transfer is usually limited by diffusion through the
stagnant liquid film because of the low liquid diffusivities
[6,7] We will assume the vapour-phase dynamics are very
fast and that any unreacted gas is vented off the top of the
reactor.
F
V
=F
A
-(A
MT
N
A
M
A
)/P
A
[2]
Component continuity for A :
V (d C
A
/dt) = A
MT
N
A
-F
L
C
A
-VkC
A
C
B
[3]
Component continuity for B:
V(d C
B
/dt) = F
B
C
BO
-F
C
C
B
- VkC
A
C
B
[4]
Total continuity:
d(ρV)/ dt= 0 = F
B
ρ
B
+ M
A
N
A
A
MT
- F
L
ρ [5]
The reactor of the FCC unit is assumed to be a constantly-
stirred tank reactor (CSTR) which operates under pressure.
The outflow will vary with the pressure and the
composition of the reactor. Density varies with pressure
and composition.
[6]
ρ = MP/RT = {[yM
A
+ (1-y) M
B
]P}/RT [7]
Where C
V
= valve sizing coefficient, M = average
molecular weight, M
A
= molecular weight of the reactants,
M
B
= molecular weight of the products. Total continuity:
V (dρ/dt) = ρ
o
F
O
– ρ
.
F [8]
The fractionator of the FCC unit is modeled as a multi-
component non-ideal distillation column. The assumptions
that we will make are, liquid on the tray is perfectly mixed
and incompressible, tray vapour holdups are negligible,
dynamics of the condenser and the reboiler will be
neglected and vapour and liquid are in thermal equilibrium
(same temperature) but not in phase equilibrium. Murphree
vapour-phase efficiency will be used to describe the
departure from equilibrium.
E
nj
= (y
nj
- y
n-1,j
T
) / (y
nj
*
- y
n-1,j
T
) [9]
Where y
nj
*
= composition of vapour in phase equilibrium
with liquid on n
th
tray with composition x
nj
, y
nj
= actual
composition vapour leaving n
th
tray, y
n-1,j
T
= actual
composition of vapour entering n
th
tray, E
nj
= Murphree
vapour efficiency for j
th
component on n
th
tray.

CHEMCON 2013
66
th
Annual Session of Indian Institute of Chemical Engineers
Hosted at Institute of Chemical Technology, Mumbai 400 019
27-30 December 2013
4
Total continuity (one per tray):
d(M
n
)/dt = L
n+1
+F
n
L
+F
v
n-1
+V
n-1
-V
n
-L
n
-S
n
L
-S
n
v
[10]
The above models were developed in Aspen Hysys v7.3
environment. Simulation Basic Manager, a fluid package is
selected along with the components which are to be in the
input stream. In the process, Peng-Robinson was selected
as the fluid package as it is able to handle hypothetical
components (pseudo-components).
The non-oil components used for the process were H20,
CH
4
, C
2
, C
3
, n-C
4
, i-C
5
, n-C
5
and n-C
10
. The pseudo-
components were created by supplying the data to define
the assay. The fluid package contains 25 components (NC:
25). In order to go to the PFD screen of the process the
option “Enter to simulation Environment” was clicked on.
An object palette appeared at right hand side of the screen
displaying various operations and units.
Figure 2: Model of FCC unit in Aspen Hysys.
Here the heater’s icon was changed and assumed to be pre-
heater. The conversion reactor is assumed to be the riser
and the constantly-stirred tank reactor as the reactor. The
operating data for these equipments inserted is equal to the
data on which a normal FCC unit runs in any refinery.
4. Result and Discussion:
With the help of model developed in the simulator data was
obtained and using that data various plots were obtained.
Figure 3: plot of temperature v/s tray position.
Figure 4: plot of pressure v/s tray position.
Figure 5: plot of flow v/s tray position.
Figure 6: plot of component properties v/s tray position.

CHEMCON 2013
66
th
Annual Session of Indian Institute of Chemical Engineers
Hosted at Institute of Chemical Technology, Mumbai 400 019
27-30 December 2013
5
The above figures show the variations in various
parameters versus the tray position of the fractionator from
top to bottom. The temperature and pressure increases from
top to bottom in the fractionator. Whereas when flow is
considered the vapour flow decreases down the fractionator
and the liquid flow increases on the other hand. The
component properties such as molecular weight and density
remain more or less constant but show a sharp increase
during the last few tray positions.
5. Conclusion:
A model for preheater, riser, stripper, reactor and
fractionator of modern UOP type FCC unit was developed.
The proposed model is capable of predicting overall
conversion, product yields, temperature and pressure. The
model results are in close agreement with the industrial
data and the data predicted by the simulator.
The predictions of the FCC model are dependent on the
value of cracking reactions rate constants, which can easily
be obtained with the help of proposed model for different
characteristics of the feed stocks, type of catalyst, activity
of catalyst and operating parameters. Therefore, it seems to
be more appropriate to use these rate constant parameters
obtained for a pair of feedstock and catalyst.
Acknowledgements:
We were grateful to MAEER’s MIT, Pune for providing
necessary infrastructure and facilities and ONGC Chair
program in Petroleum Engineering for financial support for
this project
References
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of a Fluid Catalytic Cracking (FCC) Riser Reactor”,
Elsevier, March 2001.
[2] Rohit Ramachandran, G.P.Rangaiah, S.
Lakshminarayanan, “Data Analysis, Modeling and
Control Performance Enhancement Of An Industrial
Fluid Catalytic Cracking Unit”, Chemical Engineering
Science, Volume 62, Pgs 1958-1973, Elsevier,
December 2006.
[3] A.R. Secchi, M.G. Santos, G.A.Neuman, J.D.
Trierweiler, “A Dynamic Model for A FCC UOP
Stacked Converter Unit”, Computers and Chemical
Engineering, Volume 25, Pgs 851-858, Elsevier, January
2001.
[4]
Reza Sadeghbeigi, “Fluid Catalytic Cracking
Handbook”, Second Edition, Gulf Professional
Publishing, 2000.
[5]
William L. Luyben, “Process Modeling, Simulation and
Control for Chemical Engineers”, Second Edition, Mc
Graw Hill International Publication, 1996.
[6]
Warren D. Seider, J. D. Seader, Daniel R. Lewin,
“Process Design Principles”, Second Edition, Wiley
International Publications, 2003.
[7]
Lenvenspiel, Octave, “ Chemical Reaction engineering”,
Third Edition, Wiley- India Edition, 2007
[8]
www. aspentech.com, Accessed on October 03, 2013