simultaneous isobutane dehydrogenation and hydrogen production in a hydrogen-permselective membrane...

ISSN 0040�5795, Theoretical Foundations of Chemical Engineering, 2014, Vol. 48, No. 6, pp. 799–805. © Pleiades Publishing, Ltd., 2014.

799

1 INTRODUCTION

Isobutene as one of the unsaturated hydrocarbonsis used as a raw material to produce a variety of petro�chemical components such as polybutene, methyltert�butyl ether and ethyl tertiary butyl ether toincrease the fuel octane number. In addition, alkyla�tion of isobutene with butane produces isooctane asanother fuel additive. Although isobutene can be iso�lated from refinery streams, the industrial method toproduce isobutene is catalytic dehydrogenation ofisobutane.

Some of commercial technologies have been devel�oped for dehydrogenation of light alkanes such asisobutane that differ in catalyst, reactor type and theutilized regeneration system. The Catofin isobutanedehydrogenation technology is a cyclic process to pro�duce isobutene over chromia–alumina catalyst in thefixed�bed reactor [1]. The reactors operate adiabati�cally and to regenerate the deactivated catalyst, thereactors are changed periodically. In the UOP process,dehydrogenation occurs in the several adiabatic mov�ing�bed reactors over modified Pt�alumina catalyst[2]. The reactor section consists of three radial�flowreactors, charge and inter�stage heaters and catalystregeneration reactor.

1 The article is published in the original.

Currently, researchers have focused on then�butane and n�butene dehydrogenation and few arti�cles discuss about isobutane dehydrogenation processin the literature. Cortright et al. presented a rate equa�tion for isobutane dehydrogenation over Pt–Sn cata�lyst [3]. Gucuyener et al. studied isobutane dehydro�genation in a DD3R type zeolite membrane over theCr2O3 based Al2O3 catalyst. The experimental resultsshowed that hydrogen removal can increase isobutaneyield about 50% [4]. Bakhshi et al. modeled and simu�lated a bench scale fixed bed reactor for selective dehy�drogenation of isobutane over Pt–Sn/Al2O3 catalyst atsteady state condition [5]. Sahebdelfar et al. modeledthe isobutane dehydrogenation to isobutene in an adi�abatic radial�flow moving�bed reactor without consid�ering side reactions. They assumed that the productselectivity is 100% [6]. Vernikovskaya et al. investi�gated simultaneous dehydrogenation of isobutene andpropane in a pilot fluidized and in a lab fixed bed reac�tors over an industrial isobutene dehydrogenation cat�alyst [7]. The results showed that adding C3H8 to thereactor inlet increases C3–C4 mixture conversion andthe total process selectivity to olefins. Zangeneh et al.investigated applicability of a commercial Pt–Sn/Al2O3

isobutane dehydrogenation catalyst in dehydrogena�tion of propane experimentally [8]. They showed thatthe single carbon�carbon bond rupture is the main

Simultaneous Isobutane Dehydrogenation and Hydrogen Production in a Hydrogen–Permselective

Membrane Fixed Bed Reactor1

M. Farsi, A. Jahanmiri, and M. R. RahimpourDepartment of Chemical Engineering, School of Chemical and Petroleum Engineering, Shiraz University, Shiraz, Iran

e�mail: [email protected] 07.11.2012

Abstract—In this study, performance of hydrogen–permselective membrane fixed bed reactors to produceisobutene is studied at steady state condition. The proposed reactors have been modeled heterogeneouslybased on the mass and energy conservation laws. The considered reaction networks in the model areisobutene dehydrogenation as the main reaction, and hydrogenolysis, propane dehydrogenation as well ascoke formation as side reactions that all occur on the catalyst surface. The coke deposition on the catalystsurface results an activity profile along reactors. The reactions occur in the tube side and the hydrogen per�meates from the reaction zone to the sweep gas stream. Decreasing the hydrogen concentration over the cat�alyst pellets improves isobutane conversion and isobutene selectivity. To prove the performance of the pro�posed configuration, simulation results for membrane process are compared with the conventional process atthe same operating condition. In this configuration, the isobutene production rate is enhanced about 10.81%compared to the conventional process at the same catalyst loading.

Keywords: Isobutane dehydrogenation, Pd/Ag membrane reactor, Heterogeneous model

DOI: 10.1134/S0040579514060049

800

JOURNAL OF THEORETICAL FOUNDATIONS OF CHEMICAL ENGINEERING Vol. 48 No. 6 2014

FARSI et al.

route for the formation of lower hydrocarbonbyproducts.

The integration of membrane separation and reac�tion has attracted much attention in the recent years[9]. A membrane reactor is a piece of chemical equip�ment that couples a reactor with membrane layer toadd reactants or remove products from the reactionzone. Removing product components from the reac�tion zone in a membrane reactor increases residencetime and shifts thermodynamic equilibrium limita�tions towards higher conversion. Also, the simulta�neous occurrence of reaction and separation leads tolower cost in the separation stage compared to con�ventional processes. Casanave et al. studied isobutanedehydrogenation over Pt–In catalyst in a packed�bedzeolite membrane reactor [10]. Higher dehydrogena�tion yield was observed in the membrane reactor dueto the hydrogen removal from the reaction zone. Cia�varella et al. investigated isobutane dehydrogenationover Pt–In catalyst in a MFI type membrane reactor[11]. The performance of membrane reactor was stud�ied as a function of the feed and sweep gas flow rates.Liang and Hughes studied isobutene synthesis fromisobutane in a membrane reactor over Pt/Al2O3 cata�lyst, experimentally [12]. In addition, they modeledthe considered Pd/Ag based membrane reactor atsteady state condition.

The object of this research is modeling and simula�tion of membrane fixed bed reactors to produceisobutene and hydrogen. The considered reaction net�works in the model are isobutene dehydrogenation asmain reaction, and hydrogenolysis, propane dehydro�genation as well as coke formation as side reactionsthat all occur on the catalyst surface. The performanceof the proposed membrane reactors is compared withconventional reactors at the same process condi�tion. The main advantages of this catalytic reactorare hydrogen production, improving isobuteneproductivity and selectivity, lower byproduct pro�duction.

KINETICS MODEL

The isobutane dehydrogenation is a reversible andan endothermic reaction. Although increasing tem�perature and decreasing pressure shift the reactiontoward completion, it promotes side reactions, cokeformation, and catalyst deactivation. The isobutanedehydrogenation over Pt–Sn/Al2O3 catalysts is as fol�lows:

i–C4H10 ↔ i–C4H8 + H2. (1)

As well as isobutane dehydrogenation, hydrogenolysis,propane dehydrogenation and coke formation reac�tions take place over the catalyst surface. High resi�dence time and temperature result isobutane crackingto methane and propane (hydrogenolysis reaction) asthe main side reaction.

i–C4H10 + H2 ↔ C3H8 + CH4. (2)

Other side reactions (propane dehydrogenationand coke formation from isobutene reactions) areas follows:

C3H8 ↔ H2 + C3H6, (3)

i–C4H8 ↔ 4C + 4H2. (4)

In this work, the rate expressions have been selectedfrom literature [13, 14]. To complete the simula�tion, rate equations are substituted in the govern�ing equations and generated differential equationsare solved.

PROCESS MODELING

Reaction side. The conventional dehydrogenationprocess consists of three series reactors that the feedstream is entered to the first reactor. The inter heatersare placed between rectors to increase temperature ofinlet streams. In this work, the conventional reac�tors have been substituted by Pd/Ag based mem�brane reactor at the same catalyst loading. Figure 1shows the schematic diagram of the consideredprocess.

In this work, a one�dimensional heterogeneousmathematical model has been developed to simulatethe hydrogen�permselective membrane reactors atsteady state condition. In this model, the followingassumptions are considered.

The gas is ideal condition at the considered operat�ing condition:

—Mass and energy radial diffusion is negligible.—Mass and heat axial diffusion is negligible.

—The system is well isolated.—The chemical reactions take place on the cata�

lyst surface.—The membrane is completely selective.

Feed Product

Sweep Gas Reactor 1Reactor 2 Reactor 3

Fig. 1. The schematic diagram of the considered process.

THEORETICAL FOUNDATIONS OF CHEMICAL ENGINEERING Vol. 48 No. 6 2014

SIMULTANEOUS ISOBUTANE DEHYDROGENATION AND HYDROGEN PRODUCTION 801

Subject to these assumptions, mass and energy bal�ances for the gas phase in the reaction zone areexpressed by:

(3)

(4)

where QH presents simultaneous heat and mass trans�fer through the membrane layer. Mass and energy bal�ances for the solid phase are expressed by:

(5)

(6)

In the above equations, the internal mass transferresistance has been inserted in the model consideringeffectiveness factor (η) [15]. The pressure dropthrough the catalytic packed bed is calculated basedon the Tallmadge equation that is usable for laminarand turbulent flow regimes [16].

(7)

(8)

Feed specifications, reactor and catalyst characteris�tics in the commercial dehydrogenation plant areshown in Table 1.

Membrane side. Hydrogen permeation from thereaction zone to the sweep gas stream results increas�ing the hydrogen content in the sweep gas stream. Inaddition, the heat transfer between endothermic sideand sweep gas stream results decreasing temperatureof the sweep gas along the reactor. Mass and energybalance equations are written for hydrogen permselec�tive side as follows:

(9)

(10)

( )− + −

α− − =

v

,

1 ( )

( ) 0,

gi s g

t gi i ic

ii i m

c

d Fya c k y y

A dz

P PA

− + −

π+ − − =∫ H

v

0

( )

1( ) 0,

g gs gp

fc

T

g gm p

c cT

C dTF a h T TA dz

DU T T Q C dTA A

( ) 0,g st gi i i i ba c k y y r− + η ρ =∑v

,

1

( ) ( ) 0.N

g sf b i f i

i

a h T T r H−

− + ρ η −Δ =∑v

1.166

1 6,f

− ε − ε= +

ε ε

2

3 3

(1 ) (1 )150 4.2Re Re

2

.p

uP fL D

ρΔ=

( ) ( ),

, 0,

gm i m

i i i m

d F yP P

dz− + α − =

( )

0

( ) 0,mT

gg g

m p m p

T

dTF C D U T T Q C dTdz

− − π − + =∫ H

where the hydrogen permeation constant is calculatedfrom [17]:

(11)

Auxiliary equations. There are some parameters inthe heterogeneous models such as heat and masstransfer coefficients between gas and solid phases thatconnect gas and solid phases governing equations. Inaddition, suitable temperature, pressure and compo�nent dependent correlations should be chosen to esti�mate physical properties of components and mixturesuch as viscosity, specific heat capacity, heat conduc�

.

p

o

i

EL P

RTD

D

−⎛ ⎞π ⎜ ⎟

⎝ ⎠α =⎛ ⎞⎜ ⎟⎝ ⎠

2

0

H

2 exp

ln

Table 1. Feed and product specifications of the commer�cial dehydrogenation reactors

Parameter Reactor 1 Reactor 2 Reactor 3

Feed

Temperature (K) 600 610 605

Flow rate (ton hr–1) 106 106 106

Pressure (barg) 1.4 0.9 0.4

Catalyst

Catalyst loading (ton) 11.2 12 13.8

Catalyst density (kg m–3) 800

Catalyst diameter (m) 6 × 10–4

Table 2. The used correlations for physical properties, massand heat transfer coefficient

Parameter Equation

Gas conductivity Lindsay and Bromley [18]

Mixture heat capacity [19]

Viscosity of reaction mixtures Lucas [19]

Mass transfer coefficient Cusler [20]

Binary diffusion coefficient Hirschfelder et al. [21]

Effective diffusion coefficient in pellet

[22]

Permeation–exothermic side heat transfer coefficient

[23]

Gas–catalyst heat transfer coef�ficient

[23]

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FARSI et al.

tivity and diffusion coefficients along the reactor. Thesource of considered correlations to calculate compo�nents and mixture physical properties, mass and heattransfer coefficient between phases are summarized inTable 2.

NUMERICAL SOLUTION

The considered mathematical model consists ofsome ordinary differential equations as an initial valueproblem, which is not solved analytically. This set ofdifferential equations is solved numerically with 4th

order Runge�Kutta method. Fourth order Runge�Kutta method as a simple and robust numericalmethod is a good candidate to solve this set of differ�ential equations. At the end of solution, it is possible toplot the concentration of components and tempera�ture versus reactor length.

RESULTS AND DISCUSSION

To demonstrate the accuracy of the consideredmodel and assumptions, the model of isobutene syn�thesis side has been validated against the conventionaladiabatic reactor at design specifications [24]. Thecomparison between simulation results and plant datahas been shown in Table 3. It is observed that the sim�

ulation results of the conventional process have a goodagreement with the observed plant data.

In this research, the process performance to pro�duce isobutene has been investigated while the mem�brane layers are inserted in conventional reactors toremove hydrogen from the reaction zone. Figure 2shows the comparison of isobutene mole fractionalong the membrane and conventional reactors atsteady state condition. According to this figure, isobu�tane concentration in the first, the second and thethird membrane reactors is increased by 3.34%,11.84% and 26.41% compared to the conventionalprocess. The overall isobutane enhancement in themembrane reactors is 10.81%. These results show thateffect of hydrogen permeation on the reaction rate andisobutene production in the third reactor is more sig�nificant and in the first reactor is small. This configu�ration leads to delay in the thermodynamic equilib�rium, while conventional reactors approach to theequilibrium condition, particularly in the second andthird reactors. According to Le Châtelier’s principle,when an independent variable of an equilibrium sys�tem is changed, the system shifts to decrease effect ofthe change and approaches to a new equilibrium con�dition. Hydrogen permeation from the reaction zoneto the sweep gas stream reduces the hydrogen concen�tration over the catalyst pellets and shifts dehydroge�nation reaction to the right side and higher isobutaneis converted to isobutene.

Figure 3 shows the comparison of methane molefraction along the membrane and conventional reac�tors at steady state condition. According to this figure,methane production rate as a undesired byproduct inthe membrane process is decreased about 1.7% com�pared to the conventional process. These results show

Table 3. Comparison of simulation results and plant datafor an industrial process

Parameter Plant data Model Relative error

Total conversion 38.2 38.8 1.6%

Selectivity 90.4 90.8 0.4%

0.16

0.12

0.08

0.04

3.02.01.00 1.50.5 2.5

MFR

CFR

C4H

8 M

ole

Fra

ctio

n

Dimensionless Catalyst Loading

Fig. 2. Isobutene mole fraction along the membrane andconventional reactors.

0.018

0.012

0.006

3.02.01.00 1.50.5 2.5

MFRCFR

CH

4 M

ole

Fra

ctio

n


Fig. 3. Methane mole fraction along the membrane andconventional reactors.



that the hydrogen removal from the reaction zonedecreases isobutane dissociation and improvesisobutene selectivity. Selectivity as main parameters toinvestigate side reactions is defined as the ratio of pro�duced isobutene per consumed isobutane. Accordingto Le Châtelier’s principle, decreasing hydrogen con�centration as a product over the catalyst surface shiftsthe system toward a new equilibrium condition andresults decreasing isobutane conversion to methaneand propane. The results show that the isobuteneselectivity in the membrane process approaches to91.66% and it has been enhanced about 0.98% com�pared to the conventional process at the same catalystloading.

Figure 4 shows the temperature profile in the gasphase of membrane and conventional reactors,respectively. In this process, since the reaction isendothermic, conversion is increased by supplyingheat using interstage heaters that placed between reac�tors. There is not a considerable difference betweenthe thermal behavior of the membrane and conven�tional process under steady state conditions. In theconventional process, the temperature decreases alongthe reactor due to the endothermic behavior of dehy�drogenation reaction. In the membrane configura�tion, the reaction side is surrounded by hydrogenpermselective tubes. Thus, according to Le Châtelier’sprinciple, heat transfer from sweep gas stream to theendothermic side leads to shift the dehydrogenationreaction in the endothermic direction. At the outlet ofreactors, the sweep gas temperature decreases andapproaches toward reactor temperature.

The profile of hydrogen mole fraction in the reac�tion zone of membrane and conventional reactors isshown in Fig. 5. Hydrogen permeation has a positive

effect on the isobutane dehydrogenation and a nega�tive effect on the isobutane dissociation. Decreasingthe hydrogen content over the catalyst using mem�brane layer results the higher isobutane production. Inaddition, it shifts dissociation reaction to the left sideand decreases isobutane conversion to the methaneand propane and increases isobutene selectivity. At themembrane reactor entrance, the isobutane dehydro�genation reaction is fast, and increases rapidly which isdue to high temperature and reactant composition inthe feed stream. Then, decreasing the temperatureleads to decreasing the rate of isobutane dehydrogena�tion. After a certain position along the reactor, the rateof isobutane dehydrogenation reaction decreases andhydrogen production rate could be lower compared tothe hydrogen permeation rate from reaction zone, sohydrogen mole fraction can decrease along the reac�tor. While there is a difference between the hydrogenpartial pressure in the reaction side and permeationside, hydrogen can continuously pass through themembrane layer into the sweep gas side.

The profile of hydrogen mole fraction in the sweepgas stream is shown in Fig. 6. Hydrogen permeationincreases hydrogen content in the sweep gas stream. Inthis study, the hydrogen content in the inlet sweep gasstream to the each permeation side has been chosen sothat the hydrogen mole fraction in the reaction zonedoesn’t approach to the lower value compared to thefresh feed stream. At the last part of each reactor, theslope of the hydrogen mole fraction profile decreasesbecause of approaching the isobutane dehydrogena�tion to an equilibrium value and decreasing hydrogenpartial pressure difference between the reaction andpermeation sided. In the second reactor due to hydro�gen production through the hydrogenation reaction

595

575

555

535

3.02.01.00 1.50.5 2.5

MFRCFRT

emp

erat

ure

, K


Sweep Gas

515

615

Fig. 4. The axial temperature profiles in the membrane andconventional reactors.

0.56

0.54

0.57

0.52

3.02.01.00 1.50.5 2.5

MFR

CFR

H2

Mo

le F

ract

ion


0.50

Fig. 5. Hydrogen mole fraction profile in the membraneand conventional reactors.

804


FARSI et al.

and high hydrogen concentration in the inlet feedstream, hydrogen permeation is more significant com�pared to the first and third reactors. The lower hydro�gen content in the feed of the first reactor results lowerhydrogen permeation.

CONCLUSIONS

In this study, isobutane dehydrogenation in thePd/Ag based hydrogen permselective membrane fixedbed reactor was modeled based on the mass and energyconservation laws considering isobutane dehydroge�nation and hydrogenolysis reactions. This configura�tion represents some important improvement com�pared to the conventional adiabatic process such as thehigher isobutene production rate and its selectivity,lower purification cost and hydrogen production. Itwas shown that the isobutene production in the mem�brane reactor is enhanced about 10.81% compared tothe conventional process at the steady state condition.In addition, isobutene selectivity in the membraneprocess is increased about 0.98%. In general, decreas�ing hydrogen concentration over the catalyst pelletsusing membrane tubes improves the performance ofdehydrogenation process.

NOMENCLATURE

—Specific surface area of catalyst pellet (m2 m–3)Ac—Cross section area of each tube (m2)Ci —Molar concentration of component i (mol m–3)D—Tube diameter (m)Ep—activation energy of permeability, (kJ mol–1)F—Total molar flow rate (mol s–1)hf—Gas�solid heat transfer coefficient (W m–2 K–1)

av

L—Reactor length (m)P—Total pressure (bar)Pi—Partial pressure of component i (bar)P0—Pre�exponential factor of hydrogen perme�

ability, (mol m–2 s–1 Pa–1/2)ΔP—Pressure difference (Pa)Q—Volumetric flow rate (m3 s–1)QH—Hydrogen permeation rate (mol m–1 s–1)r—Rate of reaction for dehydrogenation (mol kg–1 s–1)Re—Reynolds numberT—Temperature (K)u—Superficial velocity of fluid phase (m s–1)U—Overall heat transfer coefficient (W m–2 K–1)yi—Mole fraction of component i (mol mol–1)z—Axial reactor coordinate (m)αH—Hydrogen permeation rate constant,

(mol m⎯ 1 s–1 Pa–1/2)ε—Bed void fractionη—Catalyst effectiveness factorρ—Density of fluid phase (kg m–3)

SUPERSCRIPTS

g—In bulk gas phasem—In the sweep gas sidep—particles—At surface catalyst

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0.525

0.515

0.535

0.505

3.02.01.00 1.50.5 2.5

H2

Mo

le F

ract

ion


0.495

Fig. 6. Hydrogen mole fraction profile in the sweep gasstream versus reactors length.



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