Tianpeng LiZhou,Jiajia Luo,Tiefeng Wang

Beijing Key Laboratory of Green Reaction Engineering and Technology,Department of Chemical Engineering,Tsinghua University,Beijing 100084,China

Keywords:Partially decoupled process (PDP)Ethane Computational fluid dynamics (CFD)Steam addition

ABSTRACT In our previous work,a partially decoupled process(PDP)was proposed for efficient conversion of ethane to increase the ethylene yield and a new structural reactor called forward-impinging-back reactor (FIB)was proposed for scale-up.In this work,the influence of changing the composition and temperature of the heat carrier was investigated by simulations with detailed chemistry to further increase of the C2(C2H2 +C2H4)yield in the PDP of ethane.At ideal mixing conditions,the C2 yield is 75.3%without steam addition and it is 82.9%at steam addition ratio of β=1.4.In comparison,the C2 yield in an FIB reactor is 62.4%without steam addition and it increases to 78.5%with steam addition(β=1.4).The requirement of high mixing efficiency is diminished by steam addition,which is favorable for reactor scale-up.

1.Introduction

As the problems of climate change and global warming become severe,extensive research has focused on the use of natural gas as energy resource.Compared to coal and oil,natural gas can reduce CO2emission[1].It is anticipated that a large portion of natural gas will be supplied by shale gas,tight gas,and coal bed methane [1].With the advances in the technologies of drilling horizontal wells and multistage hydraulic fracturing [2,3],the production of shale gas has increased greatly in recent years[4].Also,the dramatic rising of shale gas has made it a dominant source of energy in some places,and has a great influence on the world gas supply chains[5-10].Shale gas contains a considerable amount of ethane [4],which is used as feedstock for production of chemicals.

As one of the most important raw materials,olefins are produced by catalytic or thermal cracking of hydrocarbons,such as ethane,propane,and naphtha [11].At present,ethane is mainly used as the feedstock of steam cracking process for producing C2H4with～52% C2H4yield at 65% C2H6conversion [12].Although the steam cracking process has a high selectivity to C2H4,it is energy consuming due to its low one-pass conversion[13].The catalytic oxidative dehydrogenation is another potential approach to convert ethane to ethylene,with an ethylene yield of 40%-75%[14-19].However,severe catalyst deactivation limits the industrial applications of the oxidative dehydrogenation process.

In our previous work,based on in-depth analysis of the methane partial oxidation (POX) process,a new process named partially decoupled process (PDP) was proposed,which separates the oxidation and pyrolysis reactions by a special reactor design[20,21].In the PDP,the proportion of methane and ethane consumed by combustion for heat supply can be replaced by other cheap fuel gases such as coke oven gas.The fuel gas combusts with stoichiometric oxygen to generate heat carrier,and the ethane or methane feedstock is mixed with the heat carrier and undergoes pyrolysis at a high temperature.Simulations with detailed chemistry show that the maximum C2(C2H2+C2H4) yield is 65% at 97% ethane conversion in the ethane PDP.In the validation experiments,the C2yield reached 52% at 70% ethane conversion [22],confirming that the PDP is at least comparable to the ethane steam cracking process in ethylene yield.In order to solve the scale-up problem of the jet-in-cross-flow (JICF) reactor,a new reactor named forward-impinging-back (FIB) reactor was proposed for scaling up the PDP[23].The C2yield of ethane PDP only decreased from 64.4% to 63.8% in a FIB reactor when reactor diameter is scaled up from 30 mm to 390 mm,which is much better than the JICF reactor.

The heat carrier of the ethane PDP in our previous work was set as coke oven gas combusted with stoichiometric oxygen.Since the high calorific value of coke oven gas,the temperature of heat carrier could reach 3100 K.The excessively high temperature of heat carrier promotes the homolysis of ethane and formation of CH4.In addition,H2O molecules split into H and OH radicals,and CO2molecules split into CO and O radicals.The target products,C2H2and C2H4,will be oxidized by the oxidative radicals,causing a decrease in C2yield.Therefore,the C2yield is expected to further increase by decreasing the temperature of heat carrier.The stability of CO2and H2O molecules is different at high temperatures,therefore the ratio of CO2to H2O(C/H)in the heat carrier will also affect the C2yield.

In this work,the influence of the heat carrier composition on C2yield and selectivity were investigated in detail by both 0-dimensional (0-D) and three-dimensional (3-D) CFD simulations coupled with a detailed reaction mechanism.The heat carrier was changed by adding steam and adjusting the C/H ratio of the fuel gas,and their influences on the C2yield and selectivity were investigated by CFD simulations in the FIB reactor.

2.Computational Methods

The composition of heat carrier under stoichiometric combustion and with addition of steam were calculated by 0-D simulations under ideal mixing conditions.The 0-D simulations were also used to preliminarily optimize the reaction conditions for 3-D CFD simulations at a much lower computational cost.The 3-D CFD simulations were conducted to predict the C2yield and selectivity in a real reactor and were used to further optimize the operating conditions.

2.1.0-D simulations

The simplified flow chart of the PDP is shown in Fig.1.The 0-D simulations were carried out using CHEMKIN 18.0.The combustion of coke oven gas was simulated by the“closed homogeneous batch reactor”model,and the mixing of heat carrier and steam was simulated by the“non-reactive gas mixer”model.Before being mixed with ethane,the composition and temperature of heat carrier were recalculated by the “closed homogeneous batch reactor”model with addition of steam.Ethane is mixed with heat carrier and then undergoes pyrolysis at high temperature in “Reactor-2”.The quenching process and the post processing were not discussed in this work.

2.2.3-D CFD simulations

In the 0-D simulations,the mixing and pyrolysis processes are separated completely,while in a real reactor they occur simultaneously.Therefore,3-D CFD simulations were carried to investigate the highly coupled mixing,chemical reactions,and heat transfer using ANSYS Fluent 16.0.The turbulence was described by the realizablek-ε model considering that it is more suitable than the standardk-ε model for jet flows with strong streamline curvature[24].The realizablek-ε model satisfies mathematical constraints on Reynolds stresses and makes the calculation results realizable,because it contains an alternative formulation for turbulent viscosity.The calculation of the temperature and composition distributions near the wall was carried out by using standard wall functions,as the realizablek-ε model is not suitable for low turbulence conditions.

Conventional methods cannot describe the complex finite-rate chemical network well.The eddy-dissipation-concept(EDC)model was used to describe the interaction between turbulence and reaction,which has been successfully used in simulations of the methane POX process [25] and steam cracking process [26,27].In addition,the computational cost and accuracy in combustions simulations can be balanced well by using the EDC model [28].The in situ adaptive tabulation (ISAT) algorithm was used to reduce computational cost,which could accelerate the simulations by 2-3 orders of magnitude[29].The discretized equations were solved by the steady implicit solver.The detailed settings of the CFD simulations are listed in Table 1.

Table 1 Detailed settings of CFD simulations

2.3.Detailed reaction mechanism

In our previous works,the modified GRI 3.0 was validated [30]and was used to simulate the ethane PDP in an FIB reactor without steam addition [23].This detailed reaction mechanism was also used in the present work to study the effect of adding steam to heat carrier.The original GRI 3.0 mechanism has been widely used in the calculations of C1-C3combustion reactions [31].To account for the fuel-rich characteristics,in the modified GRI 3.0 the rate constant of OH+C2H2=CH2CO+H was regressed based on experimental data as [25]:

The modified GRI 3.0 used for simulating the conversion of ethane and the yields of acetylene and ethylene was verified in our previous work [21].

3.Numerical Settings

3.1.0-D simulations

The molar composition of coke oven gas is set as 65% H2,25%CH4,and 10%CO.The coke oven gas is preheated to 873 K and then combusts with stoichiometric oxygen,and then the product mixture is mixed with steam of 373 K and the composition and temperature are recalculated.Ethane is mixed with the heat carrier and undergoes pyrolysis under adiabatic conditions.

3.2.3-D CFD simulations

3.2.1.Computational domain

The FIB reactor,which was demonstrated to have good performance in our previous work [23],is adopted to efficiently realize the ethane PDP with addition of steam.Typical parameters,including the inner diameterD1,outer diameterD2,and length of the reverse jet tubes,and diameter of the single computational unitD3,are shown in Fig.2.To reduce computational cost,a repeating unit of the whole reactor was used as the computational domain,with the side boundary condition set as “symmetry”.The simulation results of the repeating unit and whole reactor were found to be similar in our previous work on scale-up effect of the FIB reactor.Fig.3 shows the schematic map of FIB reactor mesh.

Fig.2.Schematic structure of FIB reactor.

Fig.3.Schematic map of FIB reactor mesh.

3.2.2.Boundary conditions

The heat carrier in this work was obtained by combustion of coke oven gas with stoichiometric oxygen and further mixing with steam of 373 K.The temperature and composition of heat carrier were calculated by 0-D simulations.The heat carrier without steam addition was fed into the reactor at a constant mass flow of 3.6 kg·h-1,and the mass flow of ethane was changed according to mixing ratio (φ),which was defined as the mass ratio of ethane to that of heat carrier without steam addition.The thermal boundary condition for the reverse jet tube was set as“coupled”between the flow and solid phases.The reactor outlet was set as pressure outlet.Other detailed settings of the FIB reactor are listed in Table 2.

3.2.3.Mesh independence test

In this section,the appropriate mesh density was investigated so that the computational cost can be reduced while keeping enough accuracy.Under the same mesh density,structural mesh could give better predictions than nonstructural mesh.Hence,structural mesh was used in the following simulations.Considering that the purpose of this work was to investigate the influence of changing the composition and temperature of heat carrier on the PDP of ethane in an FIB reactor,the mesh density used here was the same as that used in our previous work [23].Therefore,the mesh with 495,697 nodes having mesh independence was adopted in this work.

4.Results and Discussion

The conversion of C2H6and the selectivity and yield of major products are calculated by

whereFiis the molar flow rate of speciesi,ziis number of carbon atoms in molecule,andFC2H6,0is molar flow rate of C2H6in feed.It should be noticed that the calculation of selectivity and yield is based on carbon atoms in C2H6.As the carbon atoms in heat carrier are almost converted to CO2,they are not considered in the selectivity and yield calculations.

4.1.Adding steam to heat carrier

The temperature of coke oven gas combusting with stoichiometric oxygen could reach 3100 K.At such a high temperature,there is significant cracking of H2O and CO2.As listed in Table 3,the molar fractions of O2,OH and O are 7.64%,10.7% and 4.61%,respectively.For the PDP of ethane,the main byproducts are CH4and CO.The splitting reactions of C-C and C-H bonds are competitive.Because the bond energy of C-C is larger than that of C-H,a high temperature will promote the split of C-C and the formation of CH3and CH2radicals,and these radicals combine with H radical to generate CH4byproduct.During the pyrolysis process of ethane,hydrocarbons can be oxidized by O and OH radicals to oxygenate such as CH3CO and CH2CO,which will further compose to produce CO.Therefore,the CO yield can be reduced by decreasing the concentration of oxidizing radicals.Since the oxidizing radicals are produced by cracking of H2O and CO2at high temperature,the key to increase the C2yield is to decrease the temperature of heat carrier.Steam is a promising candidate as it has a large specific heat capacity and does not introduce new substances.In addition,steam can be condensed into water easily at low temperatures,which is beneficial for separation.

The steam addition ratio β is defined as mass ratio of the added steam and the product of coke oven gas combustion with stoichiometric oxygen.The calculated molar fractions of main species of heat carrier with different steam addition are summarized in Table 3.With increasing addition of steam,the temperature and concentrations of oxidizing species decrease.When β=1.8,the molar fractions of strongly oxidizing species such as O and O2decrease to～0.01% and～0.5%,respectively,and the molar fraction of OH decreases to 0.42%.

Table 2 Detailed boundary conditions of the FIB reactor

Table 3 Molar fractions of main species in heat carrier with addition of steam

The cracking ratioCis defined as the ratio of cracked molecules to the original molecules without cracking.With increasing addition of steam,the temperature and cracking ratio of CO2and H2O continue to decrease,as shown in Fig.4.At β=1.8,the cracking ratio of H2O is close to 0,the cracking ratio of CO2is lower than 10%,and the temperature is lower than 2200 K.Further addition of steam does not significantly reduce the concentration of oxidizing species,but it decreases the temperature,which will decrease the reaction rate and increase the reaction time.Hence,the studied range of steam addition is 0-1.8 in this work.The cracking ratio of H2O is much lower than that of CO2,showing that H2O is more stable than CO2at high temperatures and produces a lower amount of oxidizing species.The influence of the ratio of CO2to H2O in heat carrier will be investigated in Section 4.3.

4.2.Influence of steam addition

4.2.1.0-D simulations

Fig.5 shows the simulation results of ethane PDP at different steam addition under ideal mixing conditions.As shown in Fig.5(a),when the steam addition ratio increases from 0 to 1.8,the yield of C2rises from 75.3%to 84.7%.The conversion of C2H6is～100%at different steam additions,indicating that the increase of C2yield is mainly attributed to the increased selectivity.As shown in Fig.5(b),with increasing steam addition,the CH4selectivity decreased from 16.1% to 11.8% and the CO selectivity decreases from 5.9%to 1.7%.The rapid decrease in CH4and CO selectivities confirms that it is feasible to increase C2yield by adding steam to the heat carrier to avoid excessively high temperature while providing enough heat for subsequent pyrolysis reactions.

In order to investigate the effect of addition steam in the heat carrier,the variation of the temperature and ethane concentrationwith reaction time under ideal mixing conditions are compared for β=0 and 1.4,as shown in Fig.6.It can be seen from Fig.6(a) that ethane converts slowly at β=1.4 due to the low concentration of strong oxidizing species.The conversion of ethane starts at 10-6s and 10-8s at β=1.4 and 0,respectively.Thus the requirement of fast mixing decreases with steam addition.The temperature profiles at β=0 and β=1.4 are shown in Fig.6(b).The temperature without steam addition (β=0) increases initially and then decreases since some ethane is oxidized by strong oxidizing species in the initial period.With steam addition(β=1.4),the temperature keeps decreasing with reaction time due to endothermic reactions of ethane pyrolysis and low concentration of strong oxidizing species.Note that the variation of temperature is similar at β=0 and 1.4 after 10-6s of reaction.

Fig.4.The temperature and cracking ratios of CO2 and H2O at different steam addition.

4.2.2.CFD simulations

The 0-D simulation results show that the C2yield can be improved by adding steam to the heat carrier.The ethane PDP with steam addition is further studied by CFD simulations in a FIB reactor.Fig.7 shows the comparison between CFD and 0-D simulation results.As shown in Fig.7(a),with the increase of steam addition,the difference of C2yield between 0-D simulations at ideal mixing and 3-D CFD simulations becomes smaller.At ideal mixing conditions,the C2yield is 75.3%without steam addition and it is 82.9%at steam addition ratio of 1.4.In comparison,the C2yield in an FIB reactor is 62.4% without steam addition and it increases to 78.5%with steam addition (β=1.4).The difference in C2yields between ideal mixing and FIB reactor is much smaller at β=1.4 than that at β=0.This is because the concentration of strong oxidizing species and reaction temperature decrease with increasing steam addition.Furthermore,the addition of steam decreases the reaction rate of C2H6,which lowers the demand for high mixing efficiency.For further investigation,the yields of CH4and CO are shown in Fig.7(b).The difference of CH4yield between 0-D and CFD simulations is similar at different steam addition ratios,while the difference of CO yield decreases significantly with increasing addition of steam,showing that CO formation is much more sensitive to mixing efficient than CH4formation.In the CFD simulation results,the CO yield decreases from 16.1% with steam addition to 4.6% at β=1.4,which contribute the significant increase in C2yield.

The mixing ratio is defined as the mass flow ratio between heat carrier without steam addition and ethane feed.It will significantly affect the chemical reactions by changing temperature of the system.The amount of oxidizing species and the pyrolysis temperature decrease with an increase in the mixing ratio.Fig.8 shows the effects of the mixing ratio on the conversion of C2H6,selectivity to C2and yields of C2H2and C2H4.As shown in Fig.8(a),the C2yield reaches its maximum of 74.2% at a mixing ratio of 1.2 and steam addition ratio β=1.0.With the increase of mixing ratio,the conversion of C2H6decreases while the C2selectivity increases,due to lower concentration of oxidizing species.However,the fractions of C2H2and C2H4significantly change with the mixing ratio.The yield of C2H2decreases while that of C2H4increases with the increase of mixing ratio.The major product is C2H2at low mixing ratios while it is C2H4at high mixing ratios.For example,the C2H2and C2H4yields are respectively 47% and 24.4% at a mixing ratio of 0.8,while they are 14.7%and 57%,respectively,at a mixing ratio of 1.4.This is because C2H2is more stable than C2H4under high temperature.With steam addition at β=1.4,as shown in Fig.8(b),the C2yield reaches its maximum of 78.5% at a mixing ratio of 1.1.With the increase of mixing ratio,the conversion of C2H6decreases and the C2selectivity increases,which is similar to that at β=1.0.

Fig.5.Simulation results of ethane PDP at different steam additions under ideal mixing conditions:(a)C2H6 conversion and C2(C2H2+C2H4)yield;(b)selectivities of CH4 and CO.

Fig.6.Effect of steam addtion on the variation of (a) normalized mass fraction of C2H6 and (b) temperature with reaction time under ideal mixing conditions.

Fig.7.Comparison of 0-D and 3-D CFD simulation results of (a) C2 yield and (b) CH4 and CO yield at different steam addition ratios.

Fig.8.Effect of mixing ratio on C2H6 conversion,C2 selectivity and yields of C2H2 and C2H4 (a) β=1.0 and (b) β=1.4.

The jetting velocity of heat carrier affects the turbulence energy greatly during the mixing process,and its effect was studied by adjusting the flow rate of the heat carrier without steam addition while keeping the mixing ratio at 1.1 and steam addition ratio at 1.4.Fig.9 shows that the C2H6conversion and C2H2and C2H4selectivities are insensitive to the heat carrier jetting velocity.Considering the pressure drop,the maximum of the jetting velocity is set as～100 m·s-1.As shown in Fig.9(a),the C2yield increases from 78.7% to 79.9%.The C2selectivity also rises slightly from 80% to 81.4% and the C2H6conversion remains at～98% in all cases.The C2yield increases with the increase of jetting velocities due to enhanced mixing.This is why the C2yield in 0-D simulations is higher than in 3-D CFD simulation.To further discuss the influence of the jetting velocity of heat carrier,the yields of CH4and CO are shown in Fig.9(b).The CO yield decreases slightly from 4.6% to 3.5% as the jetting velocity increases from 34 to 97 m·s-1,while the CH4yield is almost unchanged.The high jetting velocity,which makes the oxidizing species distribute more uniformly,decreases the CO yield,because the generation of CO is more sensitive to mixing efficiency than that of CH4.

Fig.9.Effect of heat carrier jetting velocity on (a) C2H6 conversion and C2 (C2H2+C2H4) yield and selectivity,and (b) yields to CO and CH4.

4.3.Influence of C/H in hear carrier

The molar fraction of the fuel gas in our previous work was set as 65%H2,25%CH4,and 10%CO.The cracking ratio of H2O and CO2in the combustion product mixture is different,as shown in Fig.4,leading to different concentrations of oxidizing species.The influence of C/H in fuel was studied by setting the fuel as a mixture of CO and H2and changing their ratio at constant operating conditions.

The fuel gas combusts with stoichiometric oxygen and then mixed with steam addition ratio β=1.0.Ethane is mixed with the heat carrier and the maximum C2yield is obtained by changing the mixing ratio.Fig.10 shows the 3-D CFD simulation results at different C/H ratios.It can be seen from Fig.10(a) that the C2yield decreases from 81.3% when pure CO was used as fuel gas to 71.2% with pure H2as fuel gas.At different C/H ratios,the C2H6conversion is close to 100%,while the C2selectivity decreases with decreasing C/H.The main reason is that the calorific value of H2is larger than that of CO,and the temperature of heat carrier increases from 2138 K with pure CO to 2633 K with pure H2.

Fig.10.Effect of different CO/H2 ratios in the fule gas on (a) C2H6 conversion and C2 (C2H2+C2H4) yield and selectivity and (b) selectivity to byproducts CO and CH4.

Under high temperatures,ethane splits into CH3radicals,which will produce CH4by reacting with H radicals [32].Thus the increase of temperature promotes the formation of CH4and reduces the yield of C2.As discussed in Section 4.1,at a high temperature of heat carrier more oxidizing species is generated,leading to enhanced formation of CO.Fig.10(b) shows that the selectivity of byproducts increases with an increase in H2proportion in the gas fuel.Therefore,a higher steam addition ratio is needed when the H2proportion in the fuel is increased to lower the temperature of the heat carrier.

5.Conclusions

The influence of changing the heat carrier composition on the partially decoupled process of ethane was investigated by simulations with detailed chemistry.The heat carrier was changed by adding steam and adjusting the C/H ratio of the gas fuel to enhance the C2yield.By adding steam to the heat carrier,the molar fraction of strong oxidizing species decreases from～10-2to～10-3.In the heat carrier,H2O is more stable than CO2at high temperature,having a lower cracking ratio and produces a lower amount of oxidizing species.At ideal mixing conditions,the C2yield increases from 75.3% without steam addition to 84.7% at steam addition ratio of β=1.8,because steam addition decreases both CO and CH4yields.CFD simulations of an FIB reactor shows that the maximum C2yields at β=1.0 and 1.4 are 74.2% and 78.5%,respectively.The requirement of high mixing efficiency is diminished by steam addition,which is favorable for reactor scale-up.With increasing H2proportion in gas fuel,C2yield decreases and a higher steam addition is needed to lower the temperature of heat carrier.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21276135),and by Project of Chinese Ministry of Education (113004A).