International Journal of Chemical Sciences and Applications

ISSN 0976-2590, Online ISSN 2278 – 6015

Vol 5, Issue 2, 2014, pp 38-45
http://www.bipublication.com

MODELING AND SIMULATION FOR FCC UNIT FOR THE ESTIMATION

OF GASOLINE PRODUCTION

Sreshtha G. Bhende and Kiran D. Patil*

Department of Petroleum and Petrochemical Engineering,
Maharashtra Institute of Technology, Paud Road, Kothrud, Pune-411 038, India
* Author for correspondence: kiran.patil@mitpune.edu.in, Tel: +91-20-30273512, Fax: +91-2025442770

[Received-24/03/2014, Accepted-14/04/2014]

ABSTRACT
Fluid catalytic cracking (FCC) is one of the most important processes in the petroleum refining industry for the
conversion of heavy gas oil to gasoline and diesel. Furthermore, valuable gases such as ethylene, propylene and
isobutylene are produced. 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, 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, (5) regenerator, and (6) main fractionator. This model is able to predict and describe the
compositions of the final production rate, and the distribution of the main components in the final product. This allows
the estimation of economic factors, related to the operation of the FCCU. For the present study, a refinery process is
simulated 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). 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.

Keywords: FCC unit, Mathematical modeling, Aspen Hysys, Simulation, catalyst, riser.

I. INTRODUCTION:
An oil refinery or petroleum refinery is the cat cracker is the key to profitability in that
an industrial process plant where crude oil is the successful operation of the unit determines
processed and refined into more useful products whether or not the refiner can remain
such as petroleum naphtha, gasoline, diesel competitive in today’s market. Approximately
fuel, asphalt base, heating oil, kerosene, 350 catalytic crackers are operating worldwide,
and liquefied petroleum gas. Oil refineries are with a total processing capacity of over 12.7
typically large, sprawling industrial complexes million barrels per day. Most of the existing FCC
with extensive piping running throughout, units have been designed or modified by six
carrying streams of fluids between major technology licensers out of whom the
large chemical processing units.[1] An oil most used is by UOP. [1]
refinery is considered an essential part of The FCC process is very complex. The FCC unit
the downstream side of the petroleum industry. uses a microspheroidal catalyst, which behaves
Fluid catalytic cracking (FCC) continues to play like a liquid when properly aerated by gas. The
a key role in an integrated refinery as the common objective of these various FCC units is
primary conversion process. For many refiners, to upgrade low-value feedstock to more valuable
 MODELING AND SIMULATION FOR FCC UNIT FOR THE ESTIMATION OF GASOLINE PRODUCTION

products. The main purpose of the unit is to [3] But after the usage of the reactive zeolite
convert high-boiling petroleum fractions called catalyst the amount of cracking occurring in the
gas-oils to high value, high-octane gasoline and riser has been enhanced. Now the reactor is used
heating oil. Since the start-up of the first for the separation purpose of both the catalyst
commercial FCC unit in 1942, many and the outlet products. Reactions in the riser are
improvements have been made. These optimized by increasing the regenerated catalyst
improvements have enhanced the unit's velocity to a desired value in the riser reactor and
mechanical reliability and its ability to crack injecting the feed into the riser through spray
heavier, lower value feedstock. The FCC has a nozzles.
remarkable history of adapting to continual The main purpose of reactor is to separate the
changes in market demands. spent catalyst from the cracked vapors and the
spent catalyst flows downward through a steam
[II] LITERATURE REVIEW:
stripping section to the regenerator. The cracking
The basic process of FCC has got two major
reaction starts when the feed is in contact with
components i.e. reactor and regenerator. The
the hot catalyst in the riser and continues until oil
feed in the FCC riser are the residue and the
vapors are separated from the catalyst in the
Atmospheric gas oil which comes out from the
reactor separator. The hydrocarbons are then sent
distillation column. The feed needs to be
to the fractionator for the separation of liquid
preheated before entering in the riser part. This
and the gaseous products. In the reactor the
is done by the feed preheat system which heats
catalyst to oil ratio has to be maintained properly
both the fresh and recycled feed through several
because it changes the selectivity of the product.
heat exchangers and the temperature is
The catalyst’s sensible heat is not only used for
maintained at about 500-700°F. The gas
the cracking reaction but also for the
oil consists of paraffinic, aromatics and
vaporization of the feed. During simulation the
naphthenic molecules and also contains various
effect of the riser is presumed as plug flow
amounts of contaminants such as Sulphur,
reactor where there is minimal back mixing, but
nitrogen which have detrimental effect on the
practically there are both downward and upward
catalyst activity. Hence, in order to protect the
slip due to drag force of vapour.[4] The spent
catalyst feed pre-treatment is essential which
catalyst coming out from steam stripping section
removes the contaminants and have better
goes in the regenerator.
cracking ability thus giving higher yields of
Regenerator maintains the activity of the catalyst
naphtha. The riser is the main reactor in which
and also supplies heat to the reactor. Depending
most of the cracking reactions occur and all the
upon the feed stock quality there is deposition of
reactions are endothermic in nature.
coke on the catalyst surface. To reactivate the
The residence time in the riser is about 2–10 s.
catalyst, air is supplied to the regenerator by
At the top of the riser, the gaseous products flow
using large air blowers. High speed of air is
into the fractionator, while the catalyst and some
maintained in the regenerator to keep the catalyst
heavy liquid hydrocarbon flow back in the
bed in the fluidized state. Then through the
disengaging zone. Steam is injected into the
distributor at the bottom air is sent to the
stripper section, and the oil is removed from the
regenerator. Coke is burned off during the
catalyst with the help of some baffles installed in
process in significant amount. The regenerator
the stripper. The earlier practice of carrying out
operates at a temperature of about 715 °C and a
the cracking reactions in the reactor has now
pressure of about 2.41 bars. The hot catalyst (at
been completely replaced by carrying out it in
about 715 °C) leaving the regenerator flows into
the riser part. This is done to utilize the
a catalyst where any flue gases are allowed to
maximum catalyst activity and temperature
escape and flow back into the upper part to the
inside the riser. Earlier, no significant attempts
regenerator.
were made for controlling the riser operations.

Sreshtha G. Bhende and Kiran D. Patil 39

 MODELING AND SIMULATION FOR FCC UNIT FOR THE ESTIMATION OF GASOLINE PRODUCTION

The flow of the regenerated catalyst is regulated remove any entrained catalyst from the flue
by a slide valve in the regenerated catalyst line. gases.
The hot flue gas exits the regenerator after [II] ANALYTICAL MATHEMATICAL
passing through multiple sets of two-stage MODELING:
cyclones that removes entrained catalyst from In current refineries, the FCC unit plays a
the flue gas. The heat is produced due to the prominent role, producing gasoline and diesel, as
combustion of the coke and this heat is utilized well as valuable gases, such as ethylene,
in the catalytic cracking process. Heat is carried propylene and isobutylene, from feedstocks that
by the catalyst as sensible heat to the reactor. comprise atmospheric gas oils, vacuum gas oils
Flue gas coming out of the regenerator is passed and hydrocracker bottoms.[2] The significant
through the cyclone separator and the residual economic role of the FCC unit in modern-day
catalyst is recovered. 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. Since maximum conversion takes place in
riser it was assumed to be a conversion reactor
where in mass transfer takes place.[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
Figure 1: Typical UOP type FCC unit. constant-mass-transfer coefficient: t k, (with
units of length per time) is used to give the flux
The specification of the catalyst will be
of A into the liquid through the liquid film as a
discussed in detail at literature review. The
regenerator is designed and modeled for burning function of the driving force.
the coke into carbon monoxide or carbon
NA=kL (CA*-C1) (1)
dioxide. Earlier, conversion of carbon to carbon
monoxide was done which required lesser air Mass transfer is usually limited by diffusion
supply hence the capital cost was reduced.[4] through the stagnant liquid film because of the
But now a day’s air is supplied in such a scale low liquid diffusivities.
that carbon is converted into carbon dioxide in We will assume the vapour-phase dynamics are
this case the capital cost is higher but the very fast and that any unreacted gas is vented off
regenerated catalyst has minimum coke content the top of the reactor. So we can write,
on it.
The flue gases like carbon monoxide are burned FV=FA-(AMTNAMA)/PA (2)
off in a carbon monoxide furnace (waste heat Component continuity for A :
boiler) to carbon dioxide and the available
V (d CA/dt) = AMTNA-FLCA-VkCACB (3)
energy is recovered. The hot gases can be used
to generate steam or to power expansion turbines Component continuity for B:
to compress the regeneration air and generate
V(d CB/dt) = FBCBO-FCCB- VkCACB (4)
power. There are two stage cyclones which

Sreshtha G. Bhende and Kiran D. Patil 40

 MODELING AND SIMULATION FOR FCC UNIT FOR THE ESTIMATION OF GASOLINE PRODUCTION

Total continuity: the departure from equilibrium. [7]

d(ρV)/ dt= 0 = FBρB + MANAAMT - FLρ (5) Enj = (ynj - yn-1,jT) / (ynj* - yn-1,jT) (9)

The reactor of the FCC unit is assumed to be a Where ynj*= composition of vapour in phase
constantly-stirred tank reactor (CSTR) which equilibrium with liquid on nth tray with
operates under pressure. The outflow will vary composition xnj, ynj= actual composition vapour
with the pressure and the composition of the leaving nth tray, yn-1,jT= actual composition of
reactor. Density varies with pressure and vapour entering nth tray, Enj= Murphree vapour
composition.[6] efficiency for jth component on nth tray.
Total continuity (one per tray):

(6) d(Mn)/dt = Ln+1+FnL+Fvn-1+Vn-1-Vn-Ln-SnL-Snv

… (10)
ñ = MP/RT = {[yMA + (1-y) MB]P}/RT
[IV] SIMULATION:
(7)
The above models were developed in Aspen
Where,CV=valve sizing coefficient,
Hysys v7.3 environment. Simulation Basic
M=average molecular weight,
Manager, a fluid package is selected along with
MA = molecular weight of the reactants,
the components which are to be in the input
MB = molecular weight of the products.
stream. In the process, Peng-Robinson was
Total continuity: selected as the fluid package as it is able to
handle hypothetical components (pseudo-
V (dρ/dt) = ρoFO – Ρρ (8)
components).
The fractionator of the FCC unit is modeled as a The non-oil components used for the process
multi-component non-ideal distillation column. were H20, CH4, C2, C3, n-C4, i-C5, n-C5 and n-
The assumptions that we will make are, liquid C10. The pseudo-components were created by
on the tray is perfectly mixed and supplying the data to define the assay. The fluid
incompressible, tray vapour holdups are package contains 25 components (NC: 25). In
negligible, dynamics of the condenser and the order to go to the PFD screen of the process the
reboiler will be neglected and vapour and liquid option “Enter to simulation Environment” was
are in thermal equilibrium (same temperature) clicked on. An object palette appeared at right
but not in phase equilibrium. A Murphree hand side of the screen displaying various
vapour-phase efficiency will be used to describe operations and units.

Figure 2: Model of FCC unit in Aspen Hysys

Sreshtha G. Bhende and Kiran D. Patil 41

 MODELING AND SIMULATION FOR FCC UNIT FOR THE ESTIMATION OF GASOLINE PRODUCTION

Here the heater’s icon was changed and assumed (2) Pressure v/s Tray Position:
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.

[V] RESULT AND DISCUSSION:

The properties discussed in this section are
temperature, pressure, flow rates, light liquid, K-
value and transport properties.
(1) Temperature v/s Tray Position:

Figure 4: Plot of Pressure v/s Tray position

Usually during the model development we used

the unit of pressure as kg/cm2, but for this plot,
the unit of pressure is in kPa. From the plot it is
clear that the pressure increases from top to
bottom of the fractionator column. It has same
reason as that for the rise in the temperature from
top to bottom of the fractionator column.

(3) Flow v/s Tray Position:

From this plot as shown in Figure 5, we can see
that on the top section of the column the flow of
vapours is more than that of liquids. This means
Figure 3: Plot of Temperature v/s Tray Position
that more of light components are present in
Temperature increases from the top to bottom as
vapour phase and hence they are separated from
the position of the of the tray increases from top
top of the column, from which we can conclude
to bottom in the fractionator column. This is
that more of gasoline and LPG can be obtained
happening due to the true boiling point (TBP) of
from the gases separated from the top of the
the cuts. From this plot we can conclude that the
column?
light gaseous cut is obtained from the top of the
fractionator column this contains methane,
ethane, propane, butane and pentane. From the
composition, all the components that have TBP
are till 72oC are collected as gas from the top of
the fractionator column.
As we proceed downward the liquids become
heavy and have tendency to absorb more heat,
hence their TBP is higher and hence curve
moves steeply upward. Since the TBP of LCO is
115oC, hence at the cut of LCO the temperature
of its draw is 92 oC. And hence for HCO the
draw temperature is at 145 oC. The residue is
collected from the bottom of the fractionator Figure 5: Plot of Flow v/s Tray position
column at 245 oC.

Sreshtha G. Bhende and Kiran D. Patil 42

 MODELING AND SIMULATION FOR FCC UNIT FOR THE ESTIMATION OF GASOLINE PRODUCTION

The flow rates within the phases changes almost respectively. The values of the ratio Ki are
from where the heavy liquids are drawn off from correlated empirically or theoretically in terms of
the fractionator column. But from the plot we temperature, pressure and phase compositions.
see that the flow of vapours almost diminishes to In large-scale industrial distillation it is seen that
the lower trays. This means the cuts drawn off K values are widely used in the design
from the column and the residue obtained are calculations of continuous distillation columns
major in liquid phase but also consists some of for distilling multicomponent mixtures. It is seen
vapors, hence care needs to be taken during the from the plot, that the K-value of only light
transportation of these liquids. First the vapour hydrocarbons are considered.
needs to be separated and then the liquid should We can observe that the vapour-liquid
be transported to the storage tanks. distribution ratio is high in the upper section of
the fractionator column and then the values
(4) K-Value v/s Tray Position: decline down the column and negligible for the
last few trays of the fractionator column.
This plot is more important for the production of
LPG. Since we are dealing with the estimation of
gasoline production, we will neglect that plot.
(5) Light Liquid v/s Tray Position:

Figure 6: Plot of K-value v/s Tray position

Vapour–liquid equilibrium (VLE) is a condition

where a liquid and its vapour (gas phase) are
in equilibrium with each other, a condition or
state where the rate of evaporation equals the Figure 7: Plot of Light Liquids v/s Tray position
rate of condensation on a molecular level such
The primary end-products produced in petroleum
that there is no net or overall vapour–liquid
refining may be grouped into four categories:
inter-conversion. A vapour-liquid equilibrium
light distillates, middle distillates, heavy
ratio (K-value) is defined for a component as the
distillates and others. The light distillates consist
ratio of mole fraction in the vapour to mole
of Liquid petroleum gas (LPG), Gasoline (also
fraction in the liquid for that component.
known as petrol), kerosene, jet fuel and other
The tendency of a given chemical species to
aircraft fuel. Naphtha normally refers to a
partition itself preferentially between liquid and
number of flammable liquid mixtures
vapour phases is the equilibrium ratio Ki. For a
of hydrocarbons, i.e. a component of natural gas
multi-component mixture, the vapour–liquid
condensate or a distillation product
equilibrium data are represented in terms of K-
from petroleum, coal tar, or peat boiling in a
values (vapour–liquid distribution
certain range and containing certain
ratios) defined by:
hydrocarbons. It is a broad term covering among
Ki = yi / xi. (11) the lightest and most volatile fractions of the
Where yi and xi are the mole fractions of liquid hydrocarbons in petroleum. In petroleum
component ‘i’ in the vapour and liquid phases engineering, full range naphtha is defined as the

Sreshtha G. Bhende and Kiran D. Patil 43

 MODELING AND SIMULATION FOR FCC UNIT FOR THE ESTIMATION OF GASOLINE PRODUCTION

fraction of hydrocarbons in petroleum boiling scientific importance. They are diffusion

between 30 °C and 200 °C. coefficient, thermal conductivity and the
It consists of a complex mixture of hydrocarbon viscosity and are associated with the transport of
molecules generally having between 5 and 12 mass, energy and momentum respectively.
carbon atoms. It typically constitutes 15–30% of
crude oil, by weight. Light naphtha is the
fraction boiling between 30 °C and 90 °C and
consists of molecules with 5–6 carbon
atoms. Heavy naphtha boils between 90 °C and
200 °C and consists of molecules with 6–12
carbons.
As it is seen in the plot, in Figure 7, most of the
light liquids can be obtained from the initial
trays of the fractionator column. In these initial
trays, the light gases are obtained and with the
help of further separation processes we hence
Figure 8: Plot of Transport Properties v/s Tray
obtain LPG and gasoline. Therefore it is
position (Part 1)
beneficial that light liquids should be more at the
top of the column as the light components are
separated from the top of the fractionator
column. In the middle section of the fractionator
column, still consists of some light liquids.
It is from one of these trays only from where we
draw off the light liquid and name it as light
cycle oil (LCO). After tray number 18, we see
that there is a sudden decline in the amount of
light liquid in the bottom trays of the
fractionator column. We find very negligible
amount of light liquids as we proceed
downwards in the column. That is because in the
lower section of the column we draw off the Figure 9: Plot of Transport Properties v/s Tray
heavy cuts and receive the residue at the bottom position (Part 2).
of the fractionator column. The transport properties discussed for the plots
are density, mole weight, surface tension, heat
(6) Transport Properties v/s Tray Position: capacity, viscosity and thermal conductivity.
The transport properties (viscosity, thermal Properties like thermal conductivity and surface
conductivity, etc) of fluids are important for the tension show no changes throughout the
most efficient engineering design of many fractionator column, from top to bottom. On the
processes in the oil and chemical industries. other hand, the rest of the properties show a
They characterize the response of a fluid to peculiar pattern in their behavior in the
changes in its temperature, speed of flow and/ or fractionator column. The other properties are
composition. Transport processes are the process initially steady at the top section of the column
whereby mass, energy or momentum is and then they change for the bottom section of
transported from one region of a material to the column. These changes occur at the tray
another, under the influence of composition, number 18, exactly from the same tray number
temperature or velocity gradients. There are a after which the light liquid rate reduce and only
large number of transport properties in principle, heavy liquid is obtained thereafter. Though the
but three have, by far, the greatest practical and changes in these properties are not so drastic that

Sreshtha G. Bhende and Kiran D. Patil 44

 MODELING AND SIMULATION FOR FCC UNIT FOR THE ESTIMATION OF GASOLINE PRODUCTION

they could not affect the behavior of the fluid [2] Rohit Ramachandran, G.P.Rangaiah, S.
within the fractionator column, which hence Lakshminarayanan,(2006), “Data Analysis,
could not affect the outlet streams. Modeling and Control Performance Enhancement
The change in most of the plots is that the
of an Industrial Fluid Catalytic Cracking Unit”,
changes in the plotting are almost after the 18th
tray, the tray after which the heavy liquids start Chemical Engineering Science, Vol. 62,
to rule the fractionator column. Above 18th tray, pp 1958-1973
mostly we find light liquid and light gases. From [3] A.R. Secchi, M.G. Santos, G.A.Neuman, J.D.
the light gases, we obtain LPG and gasoline with Trierweiler,(2001), “A Dynamic Model for A
the help of further processes.
FCC UOP Stacked Converter Unit”, Computers
Since we are more concerned with the gasoline,
and Chemical Engineering, Vol. 25, pp 851-858
hence for maximizing the production of it, we
must have more flow rate of light liquids and [4] Reza Sadeghbeigi,(2000), “Fluid Catalytic
vapours along with less of methane, ethane, Cracking Handbook”, Second Edition, Gulf
propane and butane altogether in the upper Professional Publishing
section of the fractionator column. [5] William L. Luyben,(1996) “Process Modeling,
Simulation and Control for Chemical Engineers”,
[VI] CONCLUSION:
A model for preheater, riser, stripper, reactor Second Edition, Mc Graw Hill International
and fractionator of modern UOP type FCC unit Publication
was developed. The proposed model is capable [6] Warren D. Seider, J. D. Seader, Daniel R.
of predicting overall conversion, product yields, Lewin,(2003) “Process Design Principles”,
temperature and pressure. The model results are
Second Edition, Wiley International Publications,
in close agreement with the industrial data and
[7] Lenvenspiel, Octave (2007), “Chemical Reaction
the data predicted by the simulator. The
predictions of the FCC model are dependent on engineering”, Third Edition, Wiley- India
the value of cracking reactions rate constants, Edition.
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 the financial support for this
project.

REFERENCES:

[1] Ajay Gupta, D. Subba Rao,(2001), “Model for

the Performance of a Fluid Catalytic Cracking
(FCC) Riser Reactor”, Elsevier.

Sreshtha G. Bhende and Kiran D. Patil 45