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1
Development of a Kinetic Model for Catalytic Reforming
Catarina Alexandra Semedo Barros Ferro
Abstract
The Catalytic Reforming is the main process in a refinery to produce gasoline with a high octane number. It is
equally an excellent producer of hydrogen reused for hydrotreatment processes and BTX production (benzene,
toluene and xylene) widely used in petrochemical industry.
The aims of this work were monitoring an industrial unit of catalytic reforming and study a kinetic model
developed by CEPSA Company for a year of cycle.
The monitoring was performed in order to understand the functioning of an industrial unit. The unit diagram was
studied in detail, the process variables were collected and subsequently represented in order to understand their
variations.
The kinetic selected for this model was Henningsen and Bundgaard – Nielson in which the reaction rates are
represented by simple first order and the reaction rate constants shown by Arrhenius Law.
Posteriorly simulations were realized to comprehend if the chosen kinetic would be the most appropriated. It was
also tested the calculus change of RON and it was verified that the initial correlation produces better results than
other correlations.
The model developed by CEPSA Company presents consistent data with the real ones, noting that the chosen
kinetic for the model was a good possibility.
Keywords: Catalytic Reforming, Monitoring, Kinetic Model, Simulations, Octane Number
1. Introduction The first Catalytic Reforming industrial unit was
developed in 1940 by UOP and is worldwide
recognized as Platforming process.
The goal of this process is to convert naphtha with
low octane number into high octane number gasoline
called reformate as a blending component of motor
fuels. It is also a primary source of hydrogen and
aromatics used in the petrochemical industry.
The octane number represents the ability of a
gasoline to resist knocking during combustion of the
air-‐gasoline mixture in the engine. This number is
defined as a volume percentage of isooctane in
blending with n-‐heptane that equals the knocking
performance of the gasoline. By definition the octane
number of n-‐heptane is zero and the octane number
of isooctane is 100.
Normally there are two ways to measure octane
number, the research octane number (RON) and the
motor octane number (MON).
2
2. Catalytic Reforming Process A typical feed to Catalytic Reforming is a mixture of
medium and heavy straight run naphtha obtained
directly from the atmospheric crude oil distillation
column. This naphtha normally contains 40-‐70 wt.%
paraffins, 20-‐50 wt.% naphthenes, 5-‐20 wt.%
aromatics and 0-‐2 wt.% olefins. [1]
The distribution of paraffins, olefins, naphthenes
and aromatics in the feed to catalytic reforming
determines the richness of the feedstock, which is
normally rated by its naphthenes + aromatics or
naphthenes + 2 aromatics value. A high concentration
of aromatics means that the octane number is quite
high. The naphthenes are transformed into aromatics
with a high selectivity and a high octane number is
easily achieved. However a paraffinic feedstock has a
low octane number and it is more difficult convert into
naphthenes.
Presently the standard operating conditions in this
process are elevated temperature (450-‐520ºC) and
moderate pressure (4-‐30 bar). [1]
A large number of reactions occur in catalytic
reforming such as dehydrogenation of naphthenes to
aromatics, isomerization of paraffins and naphthenes,
dehydrocyclization of paraffins and hydrocraking of
paraffins and naphthenes to lower hydrocarbons.
All reactions are desirable except hydrocraking,
which occurs at high temperature and consumed a
high amount of hydrogen.
2.1. Process Description
An usual Catalytic Reforming unit is presented in
Figure 1. The feed is initial mixed with recycled
hydrogen gas, and then the mixture passes through
the effluent-‐to-‐feed heat exchanger. The charge is
completely vaporized and it is transported to the
reactor section. Effluent from the last reactor is cooled
by the effluent-‐to-‐feed heat exchanger for maximum
heat recovery.
After the effluent is charged to the separation
section, where the liquid and gas products are
separated.
A fraction of the gas from the separator is
compressed and recycled back to the reactor section.
The separator liquid is pumped to the reformate
stabilizer for separate the desired product, reformate,
from the rest of hydrogen and light hydrocarbons.
Figure 1 – Catalytic Reforming flow diagram [2]
These processes are commonly classified according
to the frequency and mode of catalyst is regenerated,
into semiregenerative, cyclic regeneration and
continuous regeneration. The main difference
between the three types of processes is the need of
shutdown of the unit for catalyst regeneration in the
case of semiregenerative process, the use of an
additional swing reactor for catalyst regeneration for
the cyclic process and catalyst replacement during
normal operation for the continuous regeneration
type.
2.2. Catalyst
Catalytic reforming reactions are conducted in the
presence of heterogeneous and bifunctional catalysts.
The double function is provided by the acid sites of the
support, usually alumina (Al2O3), and the metallic sites,
platinum (Pt) dispersed on the support.
3
The addition of components to the acid function,
such as chloride or fluoride, changes the strength and
amount of support acid sites.
A simplified schematic diagram of the alumina
functionality is given in Figure 2.
Figure 2 -‐ Alumina schematic [3]
The acid function catalyses the C-‐C bond reactions
such as isomerization, dehydrocyclization and
hydrocraking. For other hand the dehydrogenation
and hydrogenolysis reactions, C-‐H bond, are catalysed
by a metallic function.
Currently catalysts are bimetallic or multimetallic,
the platinum has remained the key component and
the second element is Re, Sn, Ge or Ir which interacts
with platinum and offers a better selectivity and
stability.
2.3. Kinetics Model
Naphtha is a very complex mixture of hydrocarbon
and there are more than 300 components present in
this mixture. [4] Different reactions occur through the
process such as dehydrogenation, hydrocraking,
isomerization and dehydrocyclization.
Due to the large number of components involved in
the reactions, the development of kinetic models
becomes a very complex process. To simplify this
problem were used groups of compounds, known as
lumps, which organise the species as a single entities
when they present affinity (chemical, structural, etc.).
The first effort to model a reforming system has
been made by Smith in 1959 and is described by
Figure 3. His model contained three classes of
hydrocarbons: paraffins, naphthenes and aromatics.
No distinction was made on the basis of the number of
carbons atoms within each class. Hydrogen and light
gases were also taken into account.
This model involves five pseudocomponents:
paraffins, naphthenes, aromatics, light gases and
hydrogen and four reactions. [5]
Figure 3 -‐ Reaction schemes of Smith model. [4]
Krane et al. also in 1959 proposed another model
for catalytic reforming reactions. In this model there
are 20 pseudocomponents, containing hydrocarbons
from C6 to C10. It is also recognized the difference
between paraffins, naphthenes and aromatics within
each carbon number group. All reactions are
represented by a pseudo-‐first order rate equation with
respect to hydrocarbons concentration.
The model chosen by CEPSA Company was
developed in 1970 by Henningsen e Bundgaard –
Nielson (Figure 4) and it was considered an
improvement to Krane’s model.
Figure 4 -‐ Reaction schemes of Henningsen and Bundgaard –
Nielson model. [6]
This model takes into account the differences in
the behaviour of cycloalkanes with five and six carbon
atoms in the ring. [7]
The reaction rates are normally represented by
simple first order with respect to partial pressures of
hydrocarbons and the pressure drop through the
reactores is despised. [7] The reaction rates constants
are expressed in the form of an Arrhenius law to
4
account for the influence of temperature and catalyst
deactivation were also included in the model. [5]
A heat balance was added into the system of
equations. This was a considerable improvement on
the previous models that treated catalytic reforming
as an isothermal system. [6]
The differential equations that describe the
reaction rates are equations (1) to (6). [6] [7]
𝑑C𝑑𝜏
= 𝑘!(𝑃!" + 𝑃!") (1)
𝑑𝑁𝑃𝑑𝜏
= −(𝑘! + 𝑘! + 𝑘! + 𝑘!)𝑃!" + 𝑘!𝑃!"#
+ 𝑘!𝑃!"# + 𝑘!𝑃!" (2)
𝑑𝐼𝑃𝑑𝜏
= −(𝑘! + 𝑘! + 𝑘! + 𝑘!")𝑃!" + 𝑘!𝑃!"#
+ 𝑘!𝑃!" + 𝑘!!𝑃!"# (3)
𝑑𝐴𝐶𝐻𝑑𝜏
= −(𝑘! + 𝑘! + 𝑘!" + 𝑘!")𝑃!"# + 𝑘!𝑃!"
+ 𝑘!𝑃!" + 𝑘!"𝑃!"# (4)
𝑑𝐴𝐶𝑃𝑑𝜏
= −(𝑘! + 𝑘!! + 𝑘!")𝑃!"# + 𝑘!𝑃!"
+ 𝑘!"𝑃!" + 𝑘!"𝑃!"# (5)
𝑑𝐴𝑅𝑑𝜏
= 𝑘!"𝑃!"# (6)
The differential equation that describes the heat
balance is equation (7). [6] [7]
!"!"= − !
!! !!!(𝑘!𝑃!"∆𝐻!"→! +
𝑘!𝑃!"∆𝐻!"→! + 𝑘!𝑃!"#∆𝐻!"#→!" + 𝑘!𝑃!"∆𝐻!"→!"# +
𝑘!𝑃!"#∆𝐻!"#→!" + 𝑘!𝑃!"∆𝐻!"→!"# + 𝑘!𝑃!"∆𝐻!"→!"# +
𝑘!𝑃!"#∆𝐻!"#→!" + 𝑘!𝑃!"∆𝐻!"→!" + 𝑘!𝑃!"∆𝐻!"→!" +
𝑘!"𝑃!"∆𝐻!"→!"# + 𝑘!!𝑃!"!∆𝐻!"#→!" +
𝑘!"𝑃!"#∆𝐻!"#→!"# + 𝑘!"𝑃!"#∆𝐻!"#→!"# +
𝑘!"𝑃!"#∆𝐻!"#→!")
3. Analysis of Kinetic Model
For a better understanding of the model
developed by CEPSA Company was made a detailed
study and in Figure 5 is shown a flowchart to explain
the functioning of the simulator.
The reactors used in this type of processes are
heterogeneous because there is a presence of two
phases, one solid and other vapour. Reactors are
tubular fixed-‐bed with axial or radial flow and work in
the adiabatic regime.
Figure 5 – Model calculation flowchart
Since the differential equations described by
Henningsen and Bundgaard – Nielson kinetic are
complex equations is necessary use a numerical
method to solve these equations. The method chosen
was the Euler method described by the equation (8)
with unit step size.
𝑦 𝑡 + ∆𝑡 = 𝑦 𝑡 +𝑑𝑦𝑑𝑡 ∆𝑡 (8)
(7)
5
4. Experimental section In this chapter is described whole the work process
to obtain the results.
4.1. Monitoring of Industrial Unit
The data used in this study were obtained from
Excel Tool, called Pl@nt@. This tool gets all the tags
related to the industrial unit as well as all the
laboratory analyses.
Figure 6 shows the process diagram of Catalytic
Reforming unit designed from the available plans.
Figure 6 – Process Diagram of Catalytic Reforming unit.
Initially data were extracted for six months of
operation just to get an idea of how the refinery
operated and how the process variables were changed
through the working days.
After this first approach it was verified that for a
better utilization of the simulator it would be better to
use more data to have a greater range of values for
comparison. To accomplish this goal it were extracted
further data, in this case 18 months of operation.
Figure 7 – Recycle Gas Pressure for 18 months of operation
In Figure 7 it is obvious that this variable as other
shows discrepant values for a few days of operation.
This fact is justified with stops in the unit for catalyst
regeneration or operational problems.
Due to the presence of these shutdowns and
consequently missing of laboratory analysis data, it
was only considered the days of operation
represented by the circle.
4.2. Real Data Simulation of Industrial Unit
Before using the simulator it was important to
conduct a study of the optimal number of iterations in
order to minimize adjustment function selected. For
this study several iterations were performed as is
noticeable in Figure 8.
Figure 8 – Adjustment Function and Iteration Time as
function of number of iterations
As show in Figure 8 the adjustment function
decreases steeply until 3000 iteration, and this value is
relatively lower. It is also perceptible that from 5000
0
5
10
15
20
25
30
35
0 50 100 150 200 250 300 350 400 450 500 550 Recycle Gas Pressure (atm
)
Days of OperaJon
0
20
40
60
80
100
120
140
160
180
200
500
1000
1500
2000
2500
3000
3500
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
IteraJo
n Time
Adjustmen
t Fun
cJon
Number of IteraJons
Adjustment Funcgon Iteragon Time (mim)
6
iterations the value of adjustment function does not
change too much and since the simulation time begins
to be excessive, the best choice is 5000 iterations to
have one minimization of adjustment function and a
simulation time reasonable.
Moreover to prove the choice of 5000 iterations
five trials were conducted where it was verified that
results were consistent.
5. Results This chapter presents some results of the study.
5.1. Simulation of the real data
The results of this subchapter are associated to an
interval of 425 days of operation and in all figures
there are points which don´t converge represented by
the yellow colour.
Figure 9 -‐ Comparison between Real and calculated RON
The expression used to calculate the model RON is
indicated by the equation (9). This RON depends
exclusively on the molar percentage of aromatics
present in reformate.
𝑅𝑂𝑁 = 𝑚 𝑥!" + 𝑏 (9)
In Figure 9 is visible that RON calculated by the
simulator follows the same trend as the real RON, but
their values are always higher. Due to this fact it was
tested a new correlation for the calculated RON.
The chosen correlation is described by equation
(10), and it takes into account the contributions of
each hydrocarbon family presents in reformate. [8]
𝑅𝑂𝑁 = 𝑥!" 𝑅𝑂𝑁 !" + 𝑥!" 𝑅𝑂𝑁 !" +
𝑥! 𝑅𝑂𝑁 ! + 𝑥!" 𝑅𝑂𝑁 !"
(10)
Where xNP, xIP,xN, xAR are molar fraction of n-‐
paraffins, iso-‐paraffins, naphthenes and aromatics
groups, respectively.
For each hydrocarbon family the value of RON is
described by equation (11).
𝑅𝑂𝑁 = 𝑎 + 𝑏 𝑇 + 𝑐 𝑇 ! + 𝑑 𝑇 ! + 𝑒 𝑇 ! (11)
Where T=Tb/100 in which Tb is the normal boiling
point given by an average value between initial boiling
point and final boiling point in the ASTM D86.
Coefficients a-‐e are given in table 1.
Table 1 – Coefficients for equation (11) for Estimation of RON
In Figure 10 is visible that the correlation described
by equation (10) despite follows the same tendency as
the real RON is not appropriate because its values
show a percentage error of 16 % while the values of
85
90
95
100
105
110
91 141 191 241 291 341 391 441 491 541
RON
Days of Operation
Real RON Model RON
HC Family a b c d e
N-‐Paraffins 92,81 -‐70,97 -‐53 20 10
Iso-‐ Paraffins 2-‐
Methylpentanes
95,93 -‐157,53 561 -‐600 200
3-‐ Methylpenta
nes 92,07 57,63 -‐65 0 0
2,2-‐ Dimethylpen
tanes 109,38 -‐38,83 -‐26 0 0
2,3-‐ Dimethylpen
tanes 97,65 -‐20,8 58 -‐200 100
Naphthenes -‐77,53 471,59 -‐418 100 0
Aromatics 145,66 -‐54,33 16,27 0 0
7
RON calculated by equation (9) show an error of 5%
compared to the real values.
Figure 10 -‐ Comparison between Real RON and calculated
RON by equation (9) and (10)
Due to these results, the equation (10) was
modified to take into account only the compounds
with a high value of octane number, such like iso-‐
paraffins and aromatics. This new correlation is
described by equation (12) and the calculation method
is the same described previously.
𝑅𝑂𝑁 = 𝑥!" 𝑅𝑂𝑁 !" + 𝑥!" 𝑅𝑂𝑁 !" (12)
Likewise equation (12) is not appropriated for
adjustment the real data. Therefore the correlation
described by equation (9) remains the best option, as
confirmed in Figure 11.
Figure 11 -‐ Comparison between Real RON and calculated
RON by equation (9), (10) and (12)
From Figure 12 it is observed that production of
aromatics present consistent values with experimental
results with percentage error of 1%
Figure 12 – Comparison between the percentage real and
calculated of aromatics in reformate
The process variable most frequently used by
refiners to control reformer operation is weighted
average inlet temperature (WAIT). This variable is the
sum of the inlet temperature to each reactor
multiplied by the weight percent of total catalyst in
each reactor.
Figure 13 – WAIT
It is observed that the value of WAIT in Figure 13
increases due to the loss activity and stability of
catalyst. This loss of activity happens because coke
deposition in both acid and metal sites.
This increase in temperature happens to allow
octane number keeps constant and produces the
desired reformate.
75
80
85
90
95
100
105
110
186 196 206 216 226 236
RON
Days of OperaJon
Real RON RON Equagon 9 RON Equagon 10
75
80
85
90
95
100
105
110
186 196 206 216 226 236
RON
Days of OperaJon
Real RON RON Equagon 9
RON Equagon 10 RON Equagon 12
50
55
60
65
70
75
80
85
90
91 141 191 241 291 341 391 441 491
Mol% of A
romaJ
cs
Days of OperaJon
Mol % Real AR in Reformate
Mol % Model AR in Reformate
502 504 506 508 510 512 514 516 518 520 522
91 141 191 241 291 341 391 441 491 541
WAIT (ºC)
Days of Operation
8
Figure 14 – Evolution of compounds along reforming
catalytic
Figure 14 shows the evolution of compounds
present in naphtha along the reaction system. It is
observed that molar percentage of aromatics through
the reaction system increases while molar percentage
of naphthenes and paraffins decreases, as expected,
because aromatics formation by dehydrogenation
reaction requires a lower amount of naphthenes and
consequently the formation of naphthenes leads to a
reduction of the amount of paraffins.
It is also noted that the largest decrease in the
naphthenes percentage occurs in the first reactor,
because hydrogenation reaction is the fastest and the
most endothermic reaction happens mainly in the first
reactor.
Furthermore it is observed that the molar
percentage of cracked products undergoes a
significant increase in the second and third reactor,
because these reactions are considered the slowest
reactions.
6. Conclusions The kinetic model developed by Cepsa Company
was applied to real data of an industrial unit of
reforming catalytic.
In relation to the monitoring carried out it was
observed the behaviour of an industrial unit compared
to a pilot unit and how to change any variable affects
the whole production of reformate.
A study of the optimal number of iterations was
done to ensure that the adjustment function would be
minimized within a reasonable simulation time. We
conclude the more appropriate number of iterations is
5000.
The correlation used initially by kinetic model for
estimate of RON is the best option despite having
higher values than the real.
The real values were compared with those
calculated by the model. The values for the aromatics,
n-‐paraffins and iso-‐paraffins are consistent with
percentage error of 2%.
However in relation to naphthenes there is a
greater discrepancy in the values. This difference can
be explained by the fact that their values are lower
and the possible integration errors made in
chromatography equipment (GC-‐Reformulizer).
On the other hand one of the assumption of
chosen kinetic indicates there is a distinction between
the behaviour of alkyl-‐cyclopentanes and alkyl-‐
cyclohexanes. However this assumption in the model
was not considered, feedstock presents only alkyl-‐
cyclohexanes and reactions where ACP appears were
not taken into consideration. This fact can also be a
possible reason for the disparity of values.
The difference between model yield and real one is
5% and it is considered acceptable.
In a following work the model used should be
complemented using more reactions to describe the
process as well as improve the way of calculating the
RON.
0 5
10 15 20 25 30 35 40 45 50
Mol% of C
ompo
unds
EvoluJon over of reacJon system
mol% nP mol% IP mol% N
mol% AR mol% C
9
7. Nomenclature ∆𝐻!→! -‐ Heat of reaction
𝐶! -‐ Heat capacity
𝑘! – Reaction rate constant
𝑃! -‐ Partial pressure of the component i
ACH – Alkyl-‐cyclohexanes
ACP – Alkyl – cyclopentanes
AR – Aromatics
C – Hydrocraking products
HC -‐ Hydrocarbon
IP – iso-‐Paraffins
𝑛 -‐ Hydrogen/Hydrocarbon ratio
N-‐Naphthenes
NP-‐ normal-‐Paraffins
WAIT -‐ Weighted Average Inlet Temperature
τ – Reaction time
8. Bibliography
[1] Antos, George J; Aitani, Abdullah M;, Catalytic
Naphtha Reforming, 2nd ed. New York: Marcel
Dekker, Inc., 2004, Revised and Expanded.
[2] Lapinski, Mark; Baird, Lance; James, Robert;,
"Chapter 4.1 UOP Platforming Process," in
Handbook of Petroleum Refining Processes, 3rd
ed.: McGraw-‐Hill, pp. 4.3-‐4.31.
[3] Mark Moser and Peter R. Pujadó, "Chapter 5 -‐
Catalytic Reforming," in Handbook of Petroleum
Processing. The Netherlands: Springer, 2006, pp.
217-‐237.
[4] Rahimpour, Mohammad Reza; Jafari, Mitra;
Iranshahi, Davood;, "Progress in catalytic naphtha
reforming process: A review," Elsevier, no. 109, pp.
79-‐93, 2013, www.elsevier.com/locate/apenergy.
[5] Jorge Ancheyta, "Chapter 4 -‐ Modeling of Catalytic
Reforming," in Modeling and Simulation of
Catalytic Reactors for Petroleum Refining. New
Jersey: John Wiley & Sons, Inc, 2011.
[6] Raseev, Serge;, "Chapter 13 -‐ Catalytic Reforming,"
in Thermal and Catalytic Processes in Petroleum
Refining. New York: Marcel Dekker, Inc., 2003, pp.
771-‐786.
[7] Henningsen, J; Bundgaard-‐Nielson, M;, "Catalytic
Reforming," Bristish Chemical Engeneering, vol. 15,
no. 11, pp. 1433-‐1436, November 1970.
[8] M. R. Riazi, Characterization and Properties of
Petroleum Fractions, 1st ed., AMERICAN SOCIETY
FOR TESTING AND MATERIALS, Ed. Philadelphia,
PA, 2005.