thatia, Ehira

Accepted 9 January 2012Available online 9 February 2012

Keywords:DE optimizationOctane boostingThermally coupled reactor

ne

cessing and equipment pieces of the technology. One of the promising developments in the naphthareformers would be thermal coupling of the reactors in a heat exchanger conguration. In this study,

process in the renery would generate continuous evolution ofthe technology [1]. Primary attempts have been made to nd aproper kinetic model for the catalytic naphtha reforming. A simplemodel was presented by Smith [2] in which naphtha reformingwas considered as a combination of four main reactions. Thekinetic model of Krane et al. [3] was one of the most elaboratedmodels which considered all possible reactions in the process.

scholars such as Shanyinghu and Zhu [7], Mazzieri et al. [8],Salusinszky [9], Towghi et al. [10], Pieck et al. [11], Yan [12], Iran-shahi et al. [13,14] and Rahimpour et al. [15,16] investigatedvarious ways of improving the efciency of the reformers.

1.2. Process intensication (coupled reactors)

Process intensication (PI) is currently one of the most signi-cant trends in chemical engineering and process technology [17].It is the strategy of reducing environmental emissions, energyand materials consumption. Innovations in catalytic reactors,

Corresponding author at: Department of Chemical Engineering, School ofChemical and Petroleum Engineering, Shiraz University, Shiraz 71345, Iran. Tel.:

Fuel 97 (2012) 109118

Contents lists available at

ue

.e+98 711 2303071; fax: +98 711 6287294.Considering the worldwide high octane gasoline demand, thecatalytic naphtha reforming units attract increasing attention. Cat-alytic naphtha reforming process contains the reconstruction oflow-octane hydrocarbons in the naphtha and production of morevaluable high-octane gasoline components without changing theboiling point range. Naphtha and reformate are complex mixturesof parafns, naphthenes, and aromatics in the C5C12 range. Cata-lytic reforming is carried out at 450520 C and moderate pres-sures (430 bar). The importance of the naphtha reforming

deactivation of the catalyst due to coke formation. Also, a new ki-netic model developed by Weifeng et al. [5] involving 20 lumpedcomponents and 31 reactions for catalytic reforming process. Thesubdivision of 8-carbon aromatics and all the hydrocracking reac-tions of the parafn lumps were considered in their model.Stijepovic et al. [6] developed a semi-empirical kinetic model forcatalytic reforming. The most important reactions of the catalyticreforming process were taken into account in their kinetic model.One of the advantages of the presented kinetic model was highpredictability of hydrogen and light gases concentration. OtherCatalytic naphtha reformingHydrogenation of nitrobenzene

1. Introduction

1.1. Naphtha reforming0016-2361/$ - see front matter 2012 Elsevier Ltd. Adoi:10.1016/j.fuel.2012.01.015the DE optimization technique has been used as a powerful tool to nd the best operating parametersin the naphtha coupled reactors. Twenty-six decision variables have been considered in this optimizationproblem. The endothermic dehydrogenation of the catalytic naphtha reforming is coupled with an exo-thermic hydrogenation reaction (hydrogenation of nitrobenzene to aniline) through a shell and tube heatexchanger reactor. Catalyst mass distribution, exothermic inlet temperature, operating pressures, num-ber of tubes in the exothermic side and the total molar ow rates are the most important optimizedparameters. Maximizing the aromatics (octane number) in reformate is the best possible objective func-tion in this research. The results show a signicant increase in hydrogen and aromatics production rate ofthe optimized reactor compared with the non-optimized one. Hydrogen and aromatics have increased 72and 41 kmol/h, respectively. Besides, the exothermic tubes are considered for the third reactor in thiswork to maximize the aromatic and hydrogen production rates. These results are valuable for the proce-dure of designing a reactor in which both reactions are integrated.

2012 Elsevier Ltd. All rights reserved.

Ramage et al. [4] developed a comprehensive kinetic model whichattempted to consider the reactivity differences between particularraw materials and modify the process kinetics by incorporatingReceived 29 October 2010Received in revised form 9 December 2011

highly interest. This importance would generate continuous evolution of the technology in both the pro-Boosting the gasoline octane number inheat exchanger reactor using de optimiz

Mohammad Reza Rahimpour a,b,, Davood IranshahiaDepartment of Chemical Engineering, School of Chemical and Petroleum Engineering, SbGas Center of Excellence, Shiraz University, Shiraz 71345, Iran

a r t i c l e i n f o

Article history:

a b s t r a c t

Since the reneries are be

F

journal homepage: wwwll rights reserved.ermally coupled naphtha reformingon technique

hsan Pourazadi a, Ali Mohammad Bahmanpour a

z University, Shiraz 71345, Iran

tted mostly from the catalytic naphtha reforming units, these units are of

SciVerse ScienceDirect

l

l sevier .com/locate / fuel

/ FueNomenclature

a catalyst activityA moles of aromatic formed, kmol h1

Ac cross-section area of reactor, m2

Cp specic heat capacity, kJ kmol1 K1

dp particle diameter, mEd activation energy of catalyst, J mol1

Ei activation energy for ith reaction, kJ kmol1

Fi molar ow rate of component ihf heat transfer coefcient, W m2 K1

keff effective thermal conductivity, W m1 s1

kf1 forward rate constant for reaction (1),kmol h1 kgcat1 MPa1

kf2 forward rate constant for reaction (2),kmol h1 kgcat1 MPa2

kf3 forward rate constant for reactions (3),kmol h1 kgcat1 MPa2

kf4 forward rate constant for reactions (4),kmol h1 kgcat1 MPa2

3

110 M.R. Rahimpour et al.which constitute the heart of the process technologies, are oftenthe preferred starting point. In this way, multifunctional auto-ther-mal reactor is an applicable idea in process intensication. Cur-rently, a promising eld of using multifunctional auto-thermalreactors is the coupling of endothermic and exothermic reactions.In this type of reactors, an exothermic reaction is used as the heatproducing source to drive the endothermic reaction(s) [18,19].During recent years, many researchers have focused on the auto-thermal reactors and their applications [20,21].

The goal of this study, is to enhance maximum possible aromat-ics production rate (octane number) in reformate and hydrogenproduction in a catalytic reformer. A one-dimensional homoge-neous mathematical model has been considered in this study andmass and energy balance equations have been obtained to predictthe coupled conguration behavior. Many different aspects havebeen considered in the optimization process in order to achievethe best possible operating conditions. The main issue while utiliz-ing the traditional optimization techniques is that the global opti-mum conditions may not be achieved due to the sensitivity ofthese techniques to the initial given values. Therefore, more ad-vanced systems based on natural phenomena (evolutionary com-putation) such as simulated annealing (SA) [22], evolutionstrategies (ESs) [23], genetic algorithms (GAs) [24,25] and differen-

Ke1 equilibrium constant, MPaKe2 equilibrium constant, MPa1

L length of reactor, mMi molecular weight of component i, kg kmol1

Mw average molecular weight of the feedstock, kg kmol1

n average carbon number for naphthaNA molar ow rate of aromatic, kmol h1

Ni molar ow rate of component i, kmol h1

p moles of parafn formed, kmol h1

Pi partial pressure of ith component, kPaP total pressure, kPaQ volumetric ow rate, m3 s1

ri rate of reaction for ith reaction, kmol kgcat1 h1

r rate of hydrogenation of nitrobenzene, mol kgcat1 s1

R gas constant, kJ kmol1 K1

sa specic surface area of catalyst pellet, m2 kg1

T temperature of gas phase, KWi catalyst weight fraction of ith reactorYFi fraction of exothermic feed to the ith reactoryi mole fraction for ith component in gas phaseGreek letterse void fraction of catalyst bedl viscosity of gas phase, kg m1 s1

q density of gas phase, kg m3

qb reactor bulk density, kg m3

us sphericityDH heat of reaction, kJ kmol1

z axial direction

Subscript and superscripta aromaticcal calculatedh hydrogeni number of componentsj number of reaction sides (1: endothermic, 2: exother-

mic)n naphtheneNB nitrobenzene

l 97 (2012) 109118tial evolution method (DE) [26] were invented. These methods aremore useful considering their essential advantages compared withtraditional techniques. In the present research, the operating con-ditions have been optimized using Differential Evolution (DE)method. By utilizing this method the hydrogen and aromatics pro-duction rate has been maximized and the decision variables havebeen assigned according to the dened objective function.

2. Process description

2.1. Conventional tubular naphtha reactor (CTR)

A simplied process ow diagram for conventional catalyticnaphtha reforming is presented in Fig. 1. CTR conguration hasbeen described extensively in the previous research [27].

2.2. Thermally coupled naphtha reforming reactor (TCR)

A conceptual schematic for the coupled conguration is de-picted in Fig. 2. As it is illustrated, the tubes which contain the exo-thermic reaction are placed in a way to provide the necessary heatof the endothermic reaction in the shell side. The exothermic reac-tion occurs on the spherical commercial PdAl2O3 (1.1 wt.%)

out outletp parafnTube tube sideShell shell side

AbbreviationCTR Conventional tubular naphtha reactorFBP nal boiling pint, CIBP initial boiling pint, CLHSV liquid hourly space velocity, h1

LOD length over diameterOTCR optimized thermally coupled reactorPt platinumRe rheniumRON research octane numberTBP true boiling point, KTCR thermally coupled reactorWHSV weight hourly space velocity, h1

Condenser

/ FueR-2

M.R. Rahimpour et al.catalyst [28] while the catalyst of the naphtha reforming process iscomposed of 0.3% Platinum (Pt) and 0.3% Rhenium (Re) on analumina basis (Pt/Re/Al2O3). Moreover, unlike the previous re-search, the third reactor has been substituted by the mentionedthermally coupled reactor. The specication and details of bothexothermic and endothermic sides have been presented in previ-ous study [29]. Note that the molar ow rates of nitrobenzeneand steam are considered as those reported by Abo-Ghanderet al. [21]. Adding steam to the feed line prevents the cokeformation on the catalysts. It may reduce the temperature along

H-1 H-3H-2

Pump

Naphtha Feed

R-1

Fig. 1. Schematic for conventional t

Fig. 2. A conceptual schematic for thermR-3 Reflux DrumOff GasCompressor

Hydrogen

l 97 (2012) 109118 111the hydrogenation side and also reduce the exothermic reactionrate by diluting the reacting mixture.

3. Reaction scheme and kinetic expressions

3.1. Endothermic side (shell side)

A kinetic model based on the Smiths model is taken into con-sideration [2]. Four dominant reactions are considered as follows:

E-1

Cooler

V-1

Sep-1

Stabilizer

Reboiler

Vapor

Reformate

Reformate

ubular naphtha process (CTR).

ally coupled naphtha reactor (TCR).

6. Differential evolution (DE) method

Differential evolution (DE) is a heuristic approach based onoptimization evolutionary algorithms and random search methods.This technique was presented by Storn and Price [33] for the rsttime and soon it has been paid intensive attention as a fast, easyto use, and efcient method of optimization. Consequently, thismethod has been employed for several kinds of problems withcomputation intensive cost functions. DE is used for minimizationof nonlinear, non-differentiable, multimodal and continuous spacefunctions and is able to handle optimization problems involvingcomplex mathematical models. It is a self-organizing and stochas-tic direct search approach which is compatible for numerical opti-mization and is more likely to nd the global optimum of thefunction. These advantages make DE a useful and an applicable

/ Fue Dehydrogenation of naphthenes to aromatics

NaphthenesCnH2n $ AromaticsCnH2n6 3H2 1 Dehydrocyclization of parafns to naphthenes

NaphthenesCnH2n H2 $ ParaffinsCnH2n2 2 Hydrocracking of naphthenes to lower hydrocarbons

NaphthenesCnH2n n=3H2 ! Lighter endsC1C5 3 Hydrocracking of parafns to lower hydrocarbons

ParaffinsCnH2n2 n 3=3H2 ! Lighter endsC1C5 4The rate equations of these reactions are expressed in the

previous studies [29].

3.2. Exothermic side (tube side)

Identication of coupled reactions is a controversial and a num-ber of requisites should be considered in order to nd appropriatereactions which can be thermally coupled. Our strategy here can besummarized as below:

First and foremost, the exothermic reactions should be tookplace in higher temperature in order to have a desirable heat trans-fer from the exothermic side to the endothermic one while it worthto mention that the inlet temperature should also be nearly thesame. Secondly, both sides should have approximately the sameamount of heat generation and consumption rates (ri DHi). Be-sides, as long as the reactions are selected appropriately, the masstransfer as well as heat transfer can be carried out between shelland tube side by aid of membrane technology. This factor has beenconsidered here in order to use the produced hydrogen fromreforming and manage the exothermic reaction. This feature needsa consideration of pressures in both sides owing to having a desir-able mass transfer gradient (Shell side pressure is higher than thetubes). From commercial point of view, aniline is an important rawmaterial for synthetic of more than 300 different end productssuch as dyes, rubber chemicals, amino resins, and polyurethane(MDI). On the other hand, at the present time this product is im-ported and does not produced domestically. Hence, the hydrogena-tion of nitrobenzene to aniline has been considered as a part of ourstudy. The hydrogenation reaction of nitrobenzene to aniline isgiven by:

C6H5NO2 3H2 ! C6H5NH2 2H2O 9The rate expression provided by Klemm et al. [30] is as follows:

r k0KNBKH2PNBP

:5H2

1 KNBPNB KH2P:5H2 2 10

The reaction rate constant (k0), equilibrium constants (Ki), theactivation energy (E) and standard heat of reaction (DH298K) areall discussed in the previous studies [29].

4. Mathematical modeling

In this study, a homogeneous one-dimensional model has beenconsidered. The basic structure of this model is composed of heatand mass balance conservation equations obtained by using ther-modynamic and kinetic relations, as well as auxiliary correlationsfor predicting physical properties. A differential element (Fig. 3)is considered along the axial direction inside the reactor congura-

112 M.R. Rahimpour et al.tions to derive the mentioned equations. The mass and energybalance equations are written for uid phase in each side. The nalresults are summarized in Table 1. The positive sign in Eq. (14) isused for the endothermic side and the negative sign for the exo-thermic side.

The following assumptions have been considered in thisresearch:

(i) Steady state conditions.(ii) Plug ow pattern is considered.(iii) Adiabatic reactors have been used.(iv) Catalyst deactivation is not considered (since there is not a

reliable deactivation rate for the hydrogenation of nitroben-zene through the literature under the industrial condition).

(v) Ideal gas behavior on both sides.(vi) Negligible axial heat dispersion.(vii) Porosity is constant along the shell and tube sides.(viii) Pressures along both compartments are calculated based on

Erguns equation [31].

5. Steady state model validation for CTR

The model validation has been investigated considering ob-served plant data for CTR at steady state condition. Table 2 pre-sents the plant data and predicted mole fractions of thecomponents at the output of the process. Model results showacceptable agreement with the plant data (also the model has beenvalidated for dynamic condition during 800 days of operation[27]). By utilizing the PONA Test in Stan Hop Seta apparatus, eachcomponent (parafn, naphthene and aromatic) has been analyzed.The aromatic is tested especially by ASTM 2159 equivalent to UOP273 method [32].Heat transfer

FshellIz+dz FtubeIz+dz

FtubeIzFshellIz

dzEndothermic side

(Naphtha reforming)

Exothermic side(nitrobenzene hydrogenation)

Fig. 3. A differential element along the axial length of TCR.

l 97 (2012) 109118technique especially for chemical engineering relevant problems.DE has been successfully applied for solving several complex prob-lems and is now being identied as a potential source for more

Several decision variables are considered to be optimized in thisresearch. A complete list of optimized variables with their varia-tion range and optimized values has been presented in Table 3.

The total mass of catalyst in both exothermic and endothermicsides has been assumed to be constant and the related optimuminlet molar ow rates have been calculated. As it can be seen, byproper design and catalyst mass distribution in the three reactorsof naphtha reforming, the inlet fresh naphtha feed can be increasedas high as 33.46 % compared with the CTR. In the naphtha reform-ing process, the hydrogen amount is very effective. The recyclestream shown in Fig. 1 mainly consists of hydrogen and light-end

Table 1Mass and energy balance equations.

Denitions Equations Nos.

Fluid phase 1Acj

dFi;jdz ri;jqb (11)

dFtj P

idFi;j (12)

dMave;j dFtjF2tj

_mj(13)

1Acj

CgpjdFtj Tj

dz pDjAcj

UTTube TShell qbP

iri;jDHf ;i (14)

dPdz 150l/2s d2p

1e2e3

QAc 1:75q/sdp

1ee3

Q2

A2c

(15)

Boundary z 0; Fi Fi0; T T0 (16)

M.R. Rahimpour et al. / Fuel 97 (2012) 109118 113accurate and faster optimization. Complementary descriptionregarding the DE approach can be found in previous publications[3437].

7. Optimization

In this study, DE method is used to maximize the aromatic,hydrogen and aniline molar ow rates as well as the aromatic inreformate mole fractions:

Sum YOutAromatic= YOutParaffin YOutNaphthene YOutAromatic

17

and the sum of aromatic and hydrogen yields:

Sum YOutH2

= YInH2

YOutAromatic

= YInAromatic

18

where YOut and YIn are the mole fractions in the outlet of the thirdreactor and the inlet of the rst reactor, respectively. The con-straints of optimization step are determined as follows:

X3

i1WExothermici 1;

X3

i1WEndothermici 1 19

Max TExoi ; TEndi

< 810 K 20

X3

i1YExoFi 1 21

H2=HC > 4:73 22where i represents the number of the reactors,Wi is the mass distri-bution of the catalysts in each reactor, Ti is the temperature in bothexothermic and endothermic sides and YFi is the fraction of exother-

conditionsmic feedstock ow which enters to each reactor. The difference be-tween the inlet and outlet pressure in the shell side of optimizedreactor ought to be lower than that of in the CTR.

Table 2Comparison between model prediction and plant data for fresh catalyst.

Reactor no. Inlet temperature (K) Inlet pressure (kPa)

1 777 37032 777 35373 775 3401

Reactor no. Outlet temperature (K)

Plant CTR TCR

Non-optimized Optimiz

1 722 726.13 760.77 751.952 753 751.75 801.19 796.493 770 770.83 799.59 808.52components. As a result, the amount of hydrogen entering thereactor is an effective parameter which determines the H2/HC ratioand can be adjusted in the process. Considering the increase in thenaphtha molar ow rate, in order to have a xed value of H2/HC ra-tio, the hydrogen mole fraction in the recycle gas should also in-crease. H2/HC molar ratio is one of the main parameters in thereforming units which should be controlled frequently and prop-erly. Low values accelerate the polymerization of the coke on thecatalyst surface. The outlet pressure of the compressor enteringthe rst reactor is also optimized. In order to reduce the coke for-mation on the surface of catalysts, it is generally attempted to in-crease the pressure while lower pressure increases the aromaticproduction [1]. The length to diameter ratio (LOD) is another effec-tive parameter in performance of the reactor that should be opti-mized. If this ratio increases, the pressure drop inside the reactoralso increases. However, at lower values of this ratio channelingoccurs. On the other hand, decreasing the length to diameter ratiohas the merit of processing excessive feed ow rate entering thereactor. The achieved result conrms that the total molar ow rateof the exothermic side has increased about 25.25% in comparisonwith the non-optimized conguration. In a short term, more ani-line is produced in the optimization process. The exothermic cata-lyst mass distribution of the third reactor is the least one owing toa small temperature drop in the shell side of the conventional reac-tor (naphtha side). As it is evident, the optimum inlet temperatureof rst reactor in the exothermic side is exactly the same as the in-let temperature of endothermic side which reduces the thermalloss in the coupled conguration.

8. Results and discussion

The results of the optimization process are categorized in twosections. First, the results of the shell side (naphtha reforming)are analyzed and then the exothermic side is considered.

8.1. Endothermic side

Aromatics have high octane numbers which is always above100 (except for benzene) and can be used to compensate the gas-oline octane loss due to elimination of MTBE [1,38]. Fig. 4 indicates

Catalyst distribution (wt.%) Input feedstock (mol%)

20 Parafn 49.330 Naphthene 3650 Aromatic 14.7

Aromatic in reformate (mol%)

Plant CTR TCR

ed Non-optimized Optimized 35.1422 44.0261 42.4436 47.4499 62.1032 58.371957.7 56.0612 85.7659 88.3592

Table 3Optimized parameters and their values.

No. Decision variable Min. Max. Optimized value

Endothermic side (naphtha reforming)1 Total fresh naphtha feed to the 1st reactor (kmol/h) 266 360.00 3552 Hydrogen mole fraction in the recycle gas 0.65 0.81 0.763 Catalyst mass distribution for the 1st reactor (weight fraction) 0.1 0.5 0.224 Catalyst mass distribution for the 2nd reactor (weight fraction) 0.1 0.5 0.295 Catalyst mass distribution for the 3rd reactor (weight fraction) 0.1 0.5 0.496 Naphtha compressor outlet pressure (kPa) 2000 3703 35267 LOD of the 1st reactor 1.5 2.07 2.018 LOD of the 2nd reactor 1.5 1.8 1.629 LOD of the 3rd reactor 1.5 2.8 2.66

10 Inlet naphtha feed temperature to the 1st reactor 677 777 777

Exothermic side (nitrobenzene hydrogenation)11 Total molar ow rate for the exothermic side 100 1000 99212 Exothermic side molar ow rate to the 1st reactor (% of total molar ow rate) 0.1 0.5 0.4113 Exothermic side molar ow rate to the 2nd reactor (% of total molar ow rate) 0.1 0.5 0.4814 Exothermic side molar ow rate to the 3rd reactor (% of total molar ow rate) 0.1 0.5 0.1115 Catalyst mass distribution for the 1st reactor (weight fraction) 0.1 0.5 0.3316 Catalyst mass distribution for the 2nd reactor (weight fraction) 0.1 0.5 0.38

114 M.R. Rahimpour et al. / Fuel 97 (2012) 109118successful aromatic production in the OTCR. The aromatic molarow rate increases 41 and 65 kmol/h compared with the TCR andCTR, respectively. The aromatic yield increases about 26.30% incomparison with the CTR. The related yield denition is as follows:

FOut

17 Catalyst mass distribution for the 3rd reactor (weight fraction)18 Exothermic side temperature to the 1st reactor (K)19 Exothermic side temperature to the 2nd reactor (K)20 Exothermic side temperature to the 3rd reactor (K)21 Inlet pressure of the exothermic side for the 1st reactor (kPa)22 Inlet pressure of the exothermic side for the 2nd reactor (kPa)23 Inlet pressure of the exothermic side for the 3rd reactor (kPa)24 Number of tubes for the 1st reactor25 Number of tubes for the 2nd reactor26 Number of tubes for the 3rd reactorYieldAromatic AromaticFFresh naphtha feed 23

The related rate for aromatic production (reaction No. 1) is depictedin Fig. 5. The dimensions are considered in a way that the area undereach graph shows the aromatic production in the reactor. The totalareas for CTR, TCR and OTCR are 94.34, 118.08 and 158.93 kmol/h,respectively.

The superiority of the OTCR for hydrogen production is demon-strated in Fig. 6. Hydrogen demand has increased recently in thereneries mainly due to: (a) Stricter legislation on sulfur content

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

20

40

60

80

100

120

140

160

Length of reactor (Dimensionless)

Aro

mat

ic p

rodu

ctio

n (k

mole/

hr)

CTRTCROTCR

Fig. 4. Total molar ow rate of produced aromatic compounds in the CTR,optimized and non-optimized TCR.in fuels increases the need for hydrotreating and (b) Processingof heavier crude oils and reduced market for heavy fuels is forcingto greater use of hydrocracking and the catalytic reformers are un-der pressure to response such an urgent need. The outlet hydrogenproduction in TCR is even less compared with the CTR case. Most of

0.1 0.5 0.29777 800 777777 800 782777 800 79070 300 29370 300 28170 300 1091 1000 8821 1000 8811 1000 860the hydrogen is produced in the rst reactor which is due to thedehydrogenation reaction (No. 1).

In addition to the temperatures of the reactors, the inlet H2/HCratio of the rst reactor should be controlled continuously. Thisratio has a dual role in the unit. First, the proper ratio will reducethe probability of coke formation on the catalyst surface and onthe other hand, lower ratios shift the reactions toward hydrogenand aromatic production. Accordingly, the superiority of usingmembrane reactors becomes noticeable. Membrane layers may ex-tract the extra hydrogen from the reaction side which reduces the

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.02

0.04

0.06

0.08

0.1

0.12

0.14

Length of reactor (Dimensionless)

Firs

t rea

ctio

n ra

te (k

mole/

kgca

t.hr)

CTRTCROTCR

Fig. 5. Change of dehydrogenation reaction rate (reaction No. 1) along the threereactors of CTR, optimized and non-optimized TCR.

/ FueM.R. Rahimpour et al.H2/HC ratio and increases the aromatic production, simulta-neously. H2/HC ratio along the reactors is shown in Fig. 7a. Thelight ends are other promising products in the reneries. These

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

50

100

150

200

250

300

350

Length of reactor (Dimensionless)

CTRTCROTCR

0.998 1234.8

235235.2235.4235.6

Hyd

roge

n pr

odu

ctio

n (k

mole

/hr)

Fig. 6. Total molar ow rate of produced hydrogen.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 14.5

5

5.5

6

6.5

7

7.5

8

8.5

Length of reactor (Dimensionless)

H2/

HC

mol

ar ra

tio

CTRTCROTCR

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

50

100

150

200

250

300

350

Length of reactor (Dimensionless)

Ligh

t end

s pro

duct

ion

(kmo

le/hr

)

CTRTCROTCR

a

b

Fig. 7. (a) H2/HC molar ratio and (b) the light ends production rate along the CTR,optimized and non-optimized TCR.l 97 (2012) 109118 115gases are used as LPG feed stock. It is claimed that more LPG canbe produced by using the OTCR (Fig. 7b).

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1720

730

740

750

760

770

780

790

800

810

Length of reactor (Dimensionless)

Tem

pera

ture

(K)

CTRTCROTCR

Fig. 8. Temperature prole of shell side (endothermic side) for three possible casestudies.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1750

760

770

780

790

800

810

820

Length of reactor (Dimensionless)

Tem

pera

ture

(K)

Endothermic sideExothermic side

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

1

2

3

4

5

6

7

8

9x 107

Length of reactor (Dimensionless)

react

ion

rate

(kmo

le/kg

cat.h

r)

Endothermic heat of reactionTransfered heat

a

b

Fig. 9. A comparison between (a) thermal proles of exothermic and endothermicsides and (b) the endothermic heat of reaction and transferred heat for optimizedTCR.

0.35 0.4 0.45 0.5 0.55 0.60

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5x 107

Length of reactor (Dimensionless)

Hea

t flu

x (k

J/kg c

at.h

r)

Transfered heatExothermic heat of reaction

0.6227 0.6228

1.24

1.25

1.26x 106

b

/ Fue0 0.05 0.1 0.15 0.2 0.25 0.30

Length of reactor (Dimensionless)

x 1060.5

1

1.5

2

2.5

3

3.5

4

4.5x 107

Hea

t flu

x (k

J/kg c

at.h

r)

0.3255 0.326 0.32651

1.05

1.1

1.15

1.2

x 106

Transfered heatExothermic heat of reaction

a

116 M.R. Rahimpour et al.The thermal behavior of endothermic side is investigated inFig. 8. A rule of thumb for estimating the effect of temperature isthat the octane number increases by one unit for an increase inthe reaction temperature of catalyst 2.8 C [39].

8.2. Exothermic side

Fig. 9a compares the temperature proles in both exothermicand endothermic sides of OTCR and Fig. 9b clearly justies thebehavior of Fig. 9a for the endothermic side. At the rst reactor,the transferred heat from exothermic side to the endothermicone is lower compared with the consumed heat in the reformingprocess. Consequently, the temperature of shell side falls gradually.

Fig. 10ac explains the generated and transferred heat from theexothermic side. Since the generated heat is more compared withthe transferred one at the entrance of the rst reactor, a peak isgenerated in the thermal prole of exothermic side (see Fig. 9a).Fig. 10bc magnies the thermal behavior in the other two reac-tors (the second one and the third one).

Generally, Nitrobenzene has been used as the raw material inorder to produce aniline by utilizing xed-bed or uidized bedvapor phase reactors [40,41]. Over the last 145 years, aniline hasbecome one of the hundred most important building blocks in

0.65 0.7 0.750

2

4

6

8

10

12

14

16

18

Length of rea

Hea

t flu

x (k

J/kg c

at.h

r)

TransfExoth

c

Fig. 10. A comparison between the generated heat in the exothermic side and transferredreactor of optimized TCR.l 97 (2012) 109118chemistry. Aniline is known to be used in the production procedureof more than 300 different end products such as, isocyanates,

0.8 0.85 0.9 0.95 1ctor (Dimensionless)

0.9999 1

1.67

1.68

1.69

1.7

1.71x 105

ered heatermic heat of reaction

heat from exothermic section in the (a) rst reactor, (b) second reactor and (c) third

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

10

20

30

40

50

60

70

80

90

100

Length of reactor (Dimensionless)

Nitr

oben

zene

con

vers

ion

TCROTCR

Fig. 11. (a) Nitrobenzene molar ow rate and (b) nitrobenzene conversion in theexothermic side of optimized TCR.

Finally, Table 4 is prepared to show the best possible resultsaccording to the optimization process and compares the conven-

naph

oduc

/ Fuetional, thermally coupled and optimized thermally coupledreactors.

9. Conclusion

Considering the higher thermal efciency as well as the loweroperational costs, utilizing thermally coupled conguration hasbeen given priority in this study. Differential Evolution (DE) meth-od has been used in order to obtain the best possible operatingconditions. Twenty-six decision variables have been assigned tocontrol every aspect of the process. Catalyst mass distribution, exo-thermic inlet temperature, operating pressures, number of tubes inthe exothermic side and the total molar ow rates are the mostimportant optimized parameters. By controlling these decisionvariables, the hydrogen and aromatic production rates have beenimproved compared with the non-optimized situation. Keepingthe H2/HC molar ratio in the acceptable range and consideringthe fact that the sum of the catalyst weight fraction for the threereactors should be unity are among the main constraints in theoptimization process. The obtained results imply that using theoptimized thermally coupled reactor conguration can be a com-pelling way to boost the hydrogen and aromatic production rate.rubber processing chemicals, dyes and pigments, agriculturalchemicals and pharmaceuticals [40]. Fig. 11 presents the nitroben-zene conversion vs. the dimensionless length of reactors. Note thatthe length of the reactors is not the same in TCR and OTCR. Itshould be mentioned that the mass of catalyst is the same for bothTCR and OTCR. However, the total mass of catalyst is distributed inthree reactor of OTCR while it is situated in the rst and secondreactors of TCR. The optimization allows us to increase the totalaniline production as high as 39.21% compared with non-opti-mized TCR conguration.

The pressure drop in the CTR is about 400 kPa. Temperature in-creases along the TCR and consequently the viscosity and relatedlosses are increased. Since the length to diameter ratio of the opti-mized reactor is lower compared with the non-optimized reactor,the pressure drop in the OTCR decreases signicantly. This is favor-able and reduces the operational costs of recompression. The opti-mization program dened the value of 3526 kPa as the best outletpressure of the compressor for naphtha feed. Depletion in the pres-sure is just about 120 kPa for OTCR.Table 4Comparison between Conventional, optimized and non-optimized thermally coupled

No. Reformate production (kmol/h) Aniline pr

CTR TCR OTCR

1 263.7 259.0 346.3 2 256.6 224.6 309.4 3 237.9 183.3 239.0

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118 M.R. Rahimpour et al. / Fuel 97 (2012) 109118

Boosting the gasoline octane number in thermally coupled naphtha reforming heat exchanger reactor using de optimization technique1 Introduction1.1 Naphtha reforming1.2 Process intensification (coupled reactors)

2 Process description2.1 Conventional tubular naphtha reactor (CTR)2.2 Thermally coupled naphtha reforming reactor (TCR)

3 Reaction scheme and kinetic expressions3.1 Endothermic side (shell side)3.2 Exothermic side (tube side)

4 Mathematical modeling5 Steady state model validation for CTR6 Differential evolution (DE) method7 Optimization8 Results and discussion8.1 Endothermic side8.2 Exothermic side

9 ConclusionReferences