Effect of hydrogen combustion reaction on the dehydrogenation of ethane in a fixed-bed catalytic membrane reactor

Masoud Hasany*,Mohammad Malakootikhah,Vahid Rahmanian,Soheila Yaghmaei

Department of Chemical and Petroleum Engineering,Sharif University of Technology,Tehran,Iran

Keywords:Catalytic membrane reactor Mathematical modeling Ethane dehydrogenation Hydrogen combustion

ABSTRACT A two-dimensional non-isothermal mathematical model has been developed for the ethane dehydrogenation reaction in a fixed-bed catalytic membrane reactor.Since ethane dehydrogenation is an equilibrium reaction,removal of produced hydrogen by the membrane shifts the thermodynamic equilibrium to ethylene production.For further displacement of the dehydrogenation reaction,oxidative dehydrogenation method has been used.Since ethane dehydrogenation is an endothermic reaction,the energy produced by the oxidative dehydrogenation method is consumed by the dehydrogenation reaction.The results show that the oxidative dehydrogenation method generated a substantial improvement in the reactor performance in terms of high conversions and significant energy saving.It was also established that the sweep gas velocity in the shell side of the reactor is one of the most important factors in the effectiveness of the reactor.

1.Introduction

Unsaturated hydrocarbons,such as ethylene,propylene,butylene and isobutylene,are critical intermediates in the petrochemical industry[1].The presence of unsaturated bonds in the structure of these hydrocarbons makes them very reactive to variant intermediates and end products.Ethylene is one of the important types of these hydrocarbons[2].The major use of ethylene is in producing low and high density polyethylene,polyvinyl chloride(PVC),ethylene oxide,ethylene dichloride,ethyl alcohol,vinyl acetate and ethyl benzene[3].

Ethylene is commercially produced by thermal and steam cracking processes.In the thermal(steam)cracking process,hydrocarbons are thermally cracked at high temperatures(750–900 °C)[3,4].Economically,this process has problems such as demanding high temperature and lateral processes for separation,purification of products,etc.[5,6].Due to the problems associated with the thermal(steam)cracking process,ethylene production from the dehydrogenation of ethane in a fixed-bed catalytic reactor(FBCR)has been developed.The main challenge in the ethane dehydrogenation process is the presence of rigid thermodynamic constraints.The ethane dehydrogenation is an equilibrium reaction which continues until the reaction stops at a very low conversion in FBCRs[6].The endothermic nature of this reaction is another serious problem leading to high energy consumption.The use of FBCR for this reaction is highly energy intensive and thus currently not economically affordable[7].

In the last few years,some researchers attempted to overcome these problems by focusing on fixed-bed catalytic membrane reactors(FBCMR)for dehydrogenation processes.This type of membrane reactor is very suitable for the ethane dehydrogenation process[6,8–10].

The used FBCMRs for ethane dehydrogenation processes consist of three parts as shown in Fig.1:

·The tube side

·The ceramic substrate-membrane

·The shell side.

In the FBCMRs,the ethane dehydrogenation reaction,in the presence of catalytic pellets,occurs in the tube side.In this case,produced hydrogen permeates through the ceramic substrate membrane and then the dehydrogenation equilibrium reaction shifts to ethylene production.In these reactors,palladium based membrane was used for hydrogen separation[9–12].

Recently,some researchers have focused on increasing the efficiency of dehydrogenation processes in FBCMRs.Gobina et al.[10,12]reported that the conversion of ethane in FBCMRs can be increased up to seven times(in co-current mode)and eight times(in counter-current mode)higher than the achievable equilibrium value in conventional FBCRs.

Abashar et al.[6]used the exothermic reaction of benzene with hydrogen in the tube side of the reactor to decrease the hydrogen concentration and subsequently shift the ethane dehydrogenation equilibrium reaction to the production of ethylene.On the other hand,the generated heat in the hydrogenation of benzene is consumed in the ethane dehydrogenation process,so this approach is great for saving energy.In this process,separation and purification of the products are the main problems.

Fig.1.Schematic of fixed-bed catalytic membrane reactor for ethane dehydrogenation.

As a new approach,Shelepova et al.[13,14]utilized the oxidative dehydrogenation method in the propane dehydrogenation process in FBCMRs.In the case,air is used as a sweep gas and oxidative dehydrogenation reaction uses oxygen to react with the released hydrogen from the hydrocarbon,in situ,so that the equilibrium limitation is removed.On the other hand,the generated heat in the combustion reaction is consumed by the propane dehydrogenation process and ascending temperature causes the higher conversion of ethane.Saving energy is the most important characteristic of this approach.

Mathematical modeling of this process allows one to theoretically investigate the dehydrogenation process and its advantages in FBCMRs.Developed mathematical models,in the literature,can be divided into one and two-dimensional,isothermal and non-isothermal models[12–15].Due to several simplifying assumptions,one-dimensional mathematical models are not suitable for investigation of the process effectiveness[15,16].

However,in most cases existing isothermal mathematical models cannot be used to investigate the effects of combustion reaction and describing the heat transfer phenomenon on the parameters of the dehydrogenation process.Also an isothermal process is not a realistic assumption because of high enthalpy of the reactions in the shell and tube sections.Because of high reaction rate and high enthalpy of reactions,in the actual process,there will be a significant temperature distribution that could affect the equilibrium and rate of reaction at every point in the reactor.These conflicting results point out the necessity of a more complex model in which the thermal effects,necessary to simulate the behavior of a FBMR,could be studied.

Therefore due to the fact that the temperature and concentration distribution have a great influence on the model behavior and parameters,two-dimensional non-isothermal mathematical models were developed[13,14,17].

In this investigation,oxidative dehydrogenation of ethane in a FBCMR was theoretically studied.The objective of this study was to explore the potential effects of a combustion reaction in the shell side of FBCMRs for the purpose of simultaneous production of ethylene at relatively low temperatures.A two-dimensional non-isothermal mathematical model was developed to study hydrogen combustion in the shell side of the FBCMR and its effects on the main characteristics of the dehydrogenation process.

2.Mathematical Model

A two-dimensional non-isothermal model was developed to evaluate the performance of the oxidative dehydrogenation of ethane in a co-current FBCMR.A schematic of this reactor was depicted in Fig.1.The following simplifying assumptions were considered in the derivation of mass and heat balance equations of the model:

·The reactor is operated in a steady-state condition.

·low pressure and high temperature condition,thus obeying the ideal gas law.

·According to Ergun's equation,pressure drops in the tube and shell side are negligible along the reactor.

·The Pa–Ag membrane was found to be 100%selective for hydrogen[9].

·The Pa–Ag membrane has very high heat conductivity and negligible thickness,which makes the thermal resistance of the membrane insignificant[13].

·Diffusion and heat transfer resistance at the surface of the catalytic pellets are negligible in both the tube and shell sides.

·Axial mass diffusion and convection along the radius are negligible.

The mathematical model was developed according to these assumptions.Note that in the following equations,i=1,2,3,4,5 and 6 are for C2H6,C2H4,H2,O2,N2and H2O respectively.

2.1.Mass transfer equations

2.1.1.Tube side:0＜r1＜R1

The ethane dehydrogenation equilibrium reaction that occurs in the tube side of the reactor is:

To describe the mass transfer equation in the tube side,the convective component along the radius and the diffusion component along the reactor are neglected;thus,the mass transfer differential equation will be:

The boundary conditions are:

The boundary condition for all species at the tube/ceramic substrate interface was given in Eq.(3c).

2.1.2.Ceramic substrate:R1＜r2＜R2

To describe the mass transfer equation in the ceramic substrate,the diffusion component along the radius is taken into account.Then,the mass transfer differential equation will be:

The boundary conditions at the ceramic substrate/shell interface are:

2.1.3.Shell side:R2＜r3＜R3

The hydrogen combustion reaction that occurs inside the shell side of the reactor is:

To describe the mass transfer equation in the shell side,the convective component along the radius and the diffusion component along the reactor are neglected;thus,the mass transfer differential equation will be:

The boundary conditions are:

The boundary condition for hydrogen at the ceramic substrate/shell interface is same as Eq.(5b).

2.1.4.Parameters of mass transfer equations

The molecular diffusion coefficient is determined by Wilkes formula as follows:

In the FBCMR that includes the catalyst pebble,the effective diffusion coefficient is calculated from:

where ε is the porosity and τ is the curvature.

For the ceramic substrate,the effective coefficient of radial diffusion is described by[18]:

where ui(the average thermal velocity of the molecule)is[18]:

2.2.Heat transfer equations

2.2.1.Tube side:0＜r1＜R1

To describe the heat transfer equation in the tube side,the convective component along the radius and the thermal conductivity along the reactor are neglected;thus,the heat transfer differential equation will be:

The boundary conditions are:

2.2.2.Ceramic substrate:R1＜r2＜R2

To describe the heat transfer equation in the ceramic substrate,thermal conductivity along the radius is taken into account.So,the mass transfer differential equation will be:

The boundary condition at the tube/ceramic substrate interface is given by Eq.(14c).The boundary condition at the ceramic substrate/shell interface is:

2.2.3.Shell side:R2＜r3＜R3

To describe the heat transfer equation in the tube side,the convective component along the radius and the thermal conductivity along the reactor are neglected;thus,the heat transfer differential equation will be:

The boundary conditions are:

The boundary condition at the ceramic substrate/shell interface is given by Eq.(16).

2.2.4.Parameters of heat transfer equations

The density and heat capacity of the gas mixture are calculated as follows:

The effective coefficient of thermal conductivity along the radius is[19]:

3.Reaction Kinetics

3.1.Dehydrogenation reaction

The ethane dehydrogenation equilibrium reaction is same as Eq.(1).The rate expression for this reaction was given by Gobina et al.[12]as follows:

The reaction equilibrium constant(Keq)as a function of Temperature(T)and Gibbs free-energy(ΔG0)was given by[20]as follows:

where the Gibbs free-energy is:

3.2.Combustion reaction

The catalytic combustion reaction of Hydrogen is same as Eq.(6).The rate expression for this reaction based on a Rh/α-Al2O3catalyst was given by[13]as follows:

where k2and ?O2are:

In Eq.(26a),k873K=41.96 mol·g?1·s?1·MPa?1and Eatt/R=5000 K.

4.Numerical Solution

The model equations related to the tube,ceramic substrate and shell sides with the appropriate boundary conditions,physical and chemical properties,and reaction rate equations were solved using COMSOL Multiphysics Software,which uses the finite element method(FEM)for numerical solutions of differential equations.Distributed mesh with an extremely fine mesh size was used at all interfaces to solve this system of equations.

5.Result and Discussion

The used parameters for the mathematical simulation are presented in Table 1.Ethane and air(O2,21%and N2,79%)entered into the tube and shell side of the reactor respectively,at a temperature of 850 K with given flow rates;and finally,produced ethylene,the desired product left the reactor.The developed mathematical model allows one to analyze the concentrations,conversion,and temperature pro files in the reactor.

The ethane concentration distribution in the tube side of the reactor is presented in Fig.2.The ethane concentration reduced from 21.5 to 1.2 mol·m?3along the reactor.Due to the high rate of the dehydrogenation reaction at the initial section of the reactor,a high reduction of the ethane concentration was observed at this section.

Fig.3 depicts the hydrogen concentration distribution in the tube side,ceramic substrate,and shell side of the reactor.Hydrogen concentration increased very quickly within the first 30%of reactorlength.Then the hydrogen permeated through the ceramic substrate membrane into the shell side which shifted the dehydrogenation equilibrium reaction to further ethylene production.Subsequently,in the shell side,the hydrogen combustion reaction led to a decrease in hydrogen concentration at the ceramic substrate/shell interface.The reduction of hydrogen concentration at this interface caused a high gradient in the hydrogen concentration on both sides of the ceramic substrate-membrane.Thus,hydrogen flux through the membrane increased and led to further production of ethylene.

Table 1 Model parameter values

Fig.2.Ethane concentration distribution in the tube side.

Temperature distribution in the tube,ceramic substrate and shell sides of the reactor is presented in Fig.4.Due to the fact that the ethane dehydrogenation reaction is an endothermic one,the temperature decreased immediately from 850 K to 834 K at the entrance of the reactor.Then,the generated heat from hydrogen combustion reaction,conducted in the tube side,increased the temperature to 1048 K at the output of the tube side.

5.1.Effect of combustion reaction

In this section,the effect of the hydrogen combustion reaction on the ethane dehydrogenation process in a FBCMR was studied.Two cases were compared with each other so that in the first case,hydrogen combustion reaction was considered in the shell side of the reactor and in the second,pure nitrogen was considered as a sweep gas and the hydrogen combustion reaction didn't occur.In this investigation,two effects were observed:

Fig.3.Hydrogen concentration distribution in the reactor.

Fig.4.Temperature distribution in the reactor.

·Hydrogen concentration reduction

·Heat generation.

5.1.1.Hydrogen concentration reduction effect

The hydrogen concentration at the tube/ceramic substrate interface with and without the combustion reaction was depicted in Fig.5.Without the combustion reaction,hydrogen concentration decreased uniformly from4.5 to 2.7 mol·m?3.In the other case,hydrogen concentration increased from 4.5 to 5.5 mol·m?3at the initial section of the reactor and then decreased to 1.6 mol·m?3at the output.The increase in hydrogen concentration at the initial section of the reactor was due to the bigger rate of produced hydrogen by the dehydrogenation reaction in the tube side of the reactor than the hydrogen flux through the membrane at this section of the reactor.

The hydrogen concentration at ceramic substrate/shell interface with and without the combustion reaction,depicted in Fig.6,shows that in the case of the absence of the combustion reaction,hydrogen concentration increased uniformly from 1.8 to 2.2 mol·m?3.On the contrary,with a combustion reaction,hydrogen concentration decreased dramatically from 4.5 to 0.3 mol·m?3at the initial section of the reactor.

Fig.5.Hydrogen concentration at the tube/ceramic substrate interface in cases with and without combustion reaction.

Fig.6.Hydrogen concentration at the ceramic substrate/shell interface in cases with and without combustion reaction.

In Fig.7,the hydrogen concentration gradient on both sides of the ceramic substrate-membrane,with and without a combustion reaction was compared.The hydrogen concentration gradient along the reactor in the case of the combustion reaction was greater than in the case without it.Therefore,the combustion reaction led to higher permeation of hydrogen through the membrane and subsequently an increment of ethylene production.

5.1.2.Heat generation effect

Temperature of the tube center,with and without the combustion reaction was compared in Fig.8.Without the combustion reaction the temperature decreased uniformly from 850 to 830 K;although,with the combustion reaction it increased to 1050 K.Since the ethane dehydrogenation reaction is an endothermic reaction,the temperature increment in the case with combustion reaction led to higher ethylene production.

Comparison of ethane conversion in cases with and without the combustion reaction is shown in Fig.9.With the combustion reaction,conversion increased to 0.97 at the outlet of the reactor.Without a combustion reaction,due to the decrease in temperature,the dehydrogenation reaction shifted to production of ethane so conversion decreased uniformly from 0.3 to 0.25.

Fig.7.Hydrogen concentration at both sides of the ceramic substrate-membrane part in cases with and without combustion reaction.

Fig.8.Temperature at the tube center along the reactor in cases with and without combustion reaction.

5.2.Sweep gas flow rate effect

One of the most important parameters that can change FBCMRs performance is the sweep gas(air) flow rate.Fig.10 shows the temperature at the center of the reactor in four different sweep gas flow rates.When the sweep gas flow rate increased,the temperature also increased which affects the ethylene production.The sweep gas flow rate influenced ethane conversion by changing oxygen concentration,hydrogen concentration,heat generation,etc.Fig.11 shows the ethane conversion along the reactor in the four sweep gas flow rates.At low flow rates,an increase in flow rate had a significant influence on the outlet ethane conversion but at high flow rates,the influence of flow rate increment was insignificant.So,a sweep gas flow rate of 5 × 10?6m3·s?1was an optimum flow rate at which the ethane conversion is about 0.97.

6.Conclusions

Fig.9.Ethane conversion along the reactor in cases with and without combustion reaction.

Fig.10.Temperature along the reactor center for various sweep gas flow rates.

The ethane dehydrogenation reaction in a fixed-bed catalytic membrane reactor was studied.It was demonstrated that using the membrane reactor can increase the yield of ethylene by shifting the ethane equilibrium reaction.A two-dimensional non-isothermal mathematical model was developed to evaluate the performance of the oxidative dehydrogenation of ethane in a FBCMR.In the co-current mode with the hydrogen combustion reaction,the results presented in this paper showed that an oxidative dehydrogenation reaction caused appreciable improvement in the reactor performance in terms of high conversion and high energy saving.Due to the fact that the ethane dehydrogenation reaction is endothermic,the heat generation in the shell side was conducted to the tube side and led to an increase in ethylene production.It was demonstrated that the hydrogen combustion reaction in the shell side led to an increment of ethane conversion up to 0.97.The sweep gas velocity was found to be an important parameter regarding the reactor performance.Further research should be focused on optimization of the ethane and sweep gas flow rates to achieve higher conversion.More improvements are still to be expected both in scientific knowledge and industrial practice in ethylene industry.

Nomenclature

Ciconcentration of the i th component,kmol·m?3

Cpspecific heat,kJ·g?1·K?1

Deieffective radial diffusion coefficient of the i th component,m2·s?1

Dijmolecular diffusion coefficient of the i th component in the j th component,m2·s?1

Dmimolecular diffusion coefficient,m2·s?1

Dkneffective Knudsen diffusion coefficient,m2·s?1

Fig.11.Ethane conversion along the reactor for various sweep gas flow rates.

deequivalent diameter of the pore channel,m

dcatdiameter of the catalyst,m

Eattactivation energy,J·mol?1

Keqequilibrium constant

k reaction rate constant,mol·g?1·s?1·Pa?1

L length of the reactor,m

Mimolecular mass of i th component,g·mol?1

N number of components in the mixture

Pipartial pressures of i th component,Pa

Q0constant permeability,kmol·m?1·s?1·Pa?1/2

QH2rate of hydrogen penetration through the membrane,kmol·s?1

R universal gas constant,J·mol?1·K?1

R1distance from the center of the tube to the substrate,m

R2distance from the center of the tube to the membrane,m

R3distance from the center of the tube to the wall of the reactor,m

Rjrate of j th reaction

r1radial coordinate in the tube side,m

r2radial coordinate in the ceramic substrate,m

r3radial coordinate in the shell side,m

rcapcapillary radius,m

T temperatures,K

T0temperature under normal conditions

Uxmean gas velocity,K

uiaverage thermal velocity of the molecules,m·s?1

X degree of conversion

x x direction along to length of the reactor,m

yimole fraction of i th component

δ membrane thickness,m

γ stoichiometric coefficient of the ith component

ε porosities of the catalyst bed

εcporosities of the catalyst bed

λeffeffective radial thermal conductivity,J·m?1·s?1·K?1

λcthermal conductivity of the ceramic substrate,J·m?1·s?1·K?1

λHethermal conductivity of helium,reference value,J·m?1·s?1·K?1

ρggas density,kg·m?3

ρcat1catalyst density in the tube side,kg·m?3

ρcat2catalyst density in the shell side,kg·m?3

τ curvature

Subscript

in inlet

Superscript

t tube side

s shell side

c ceramic substrate