Nonoxidative Aromatization of CH4 Using C3H8 As a Coreactant

Article pubs.acs.org/IECR

Nonoxidative Aromatization of CH4 Using C3H8 As a Coreactant: Thermodynamic and Experimental Analysis Parisa Moghimpour Bijani,†,‡ Morteza Sohrabi,*,† and Saeed Sahebdelfar‡ †

Chemical Engineering Department, Amirkabir University of Technology, Hafez Avenue, P.O. Box 15875-4413, Tehran, Iran Catalyst Research Group, Petrochemical Research and Technology Company, National Petrochemical Company, P.O. Box 1435884711, Tehran, Iran

‡

ABSTRACT: The effect of propane addition to the feed of methane aromatization process was studied both thermodynamically and experimentally. Thermodynamic equilibrium calculations were performed within temperature, pressure, and CH4/C3H8 molar ratio ranges of 673−873 K, 1−10 bar, and 0−20, respectively, considering different constraints according to two alternative reaction network models. Experimental evaluations were conducted in a fixed-bed reactor over Zn/HZSM-5 catalyst under different operating conditions. Methane formation showed an induction period of about 4 h on stream. Aromatic yield slightly increased with increase of methane concentration in the feed. Experimental results showed the best agreement with the thermodynamic model based on a proposed mechanism of methylation of propane-derived aromatics by methane, giving equilibrium methane conversions of about 1% under the above conditions. Both thermodynamic and experimental results revealed that in the presence of propane cofeed, the contribution of methane to the overall reaction is negligibly small on a net basis.

1. INTRODUCTION Catalytic conversion of methane to value-added liquid products has drawn much attention in the past few decades. Extensive attention has been paid to the production of higher hydrocarbons from methane-containing feedstocks.1 However, the direct conversion of methane to higher hydrocarbons is still an important challenge to both science and industry, as methane is the most stable hydrocarbon because of its perfect symmetry.1,2 The effective activation of the C−H bonds in a methane molecule is extremely difficult. During the past century, basic and applied research on topics ranging from syngas production to the more recent direct oxidation of methane into methanol or formaldehyde and oxidative coupling to ethane and/or ethylene has been conducted. The latter processes were found to be hampered by unfavorable thermodynamic constraints, which led to deep oxidation of methane when conversions of practical significance were desired.3 Conceptually, direct processes should have a distinct economic advantage over indirect processes; however, to date no direct processes have been developed to a commercial stage. Product yields are generally low; consequently, operating in a single-pass mode makes separations difficult and costly.4 In early the 1990’s, Wang et al. reported that methane could be successfully converted into benzene and hydrogen over bifunctional catalysts consisting of acid type zeolite containing molybdenum carbide species.5 Dehydroaromatization of methane has definite advantages, including high selectivity to aromatics, less complicated technology, and ease of separation of the aromatic hydrocarbon products from unreacted methane. The main byproduct, hydrogen, is a vitally important element in the petroleum refining industry.6 The major limitations of this approach are the high operation cost in view of both low conversion of methane and high reaction temperature.7 In 1997, Choudhary et al.8 reported that © 2013 American Chemical Society

methane conversion to aromatic hydrocarbons could be enhanced by addition of a higher alkene and/or alkane over MFI zeolite. Thereafter, the catalytic transformation of methane mixed with light hydrocarbons to aromatics was reported by several research groups. Anunziata et al.9−11 found that, in the presence of liquefied petroleum gas (LPG), ethane, and n-pentane under nonoxidizing conditions, CH4 could be converted to aromatics over Zn/ZSM-11. Chu and Qiu12 reported a remarkable promotion of benzene formation with addition of only a few percent of ethane at 998 K over Mo/ HZSM-5. Echevsky observed the direct insertion of methane into C3−C4 alkanes in a one-step conversion of methane into heavier hydrocarbons at relatively mild pressures and temperatures (773−823 K).13 By using 13C-labeled methane, Baba reported the transformation of methane into higher hydrocarbons in the presence of C2H4 over Ag+-exchanged zeolites at 673 K.14 It has been found recently that over HZSM-5 modified by Zn, Zn−Cu, Mo−Zn, Ga, Re, and Zn−La,13CH4 could be inserted into aromatic products in the presence of C3H8 at moderate temperature (823 K).2 On the other hand, some researchers observed the opposite results. Naccache et al., for example, reported that H-galloaluminosilicate did not activate CH4 and that 13CH4 could not be inserted into the products such as aromatics in the conversion of C2H4/CH4 or C3H6/CH4 mixtures.3 Brandford believed that methane conversion was suppressed in the presence of ethane having a relatively high concentration.15 In the latest mechanistic studies provided by Luzgin et al., it was concluded that the main pathway of involving methane in Received: Revised: Accepted: Published: 572

November 23, 2012 November 11, 2013 December 9, 2013 December 9, 2013 dx.doi.org/10.1021/ie400786z | Ind. Eng. Chem. Res. 2014, 53, 572−581

Industrial & Engineering Chemistry Research

Article

Figure 1. Equilibrium methane conversion (a), equilibrium aromatics selectivity (b), and equilibrium paraffins and olefins selectivity (c) as functions of temperature (Model I). P = 1.01 bar; CH4/C3H8 = 6/1.

catalyst was also investigated. Zn/HZSM-5 was used as the catalyst because of its favorable performance reported in the literature.9−11,17,18

coaromatization reactions is the methylation of the aromatics being produced exclusively from propane.16 The methoxy species formed by the dissociative adsorption of methane on ZnO species of the zeolite is responsible for the methylation. In view of the conflicting experimental reports, a thermodynamic analysis of the reaction could be beneficial as it could provide a basis for further experimental and computational studies aimed at increasing aromatic yields. Further, it could be useful in understanding the boundaries and constraints that are imposed by thermodynamics on process and catalyst development. However, it should be pointed out for this complicated reaction system that simple equilibrium calculations could not provide a good representation of the experimental data. Furthermore, the reaction does not reach equilibrium. Nevertheless, using additional constraints based on the reaction mechanism and catalyst type, the equilibrium calculation results could improve, especially when effective catalysts are considered. In this work, a detailed computational study on the chemical equilibria of CH4−C3H8 aromatization reactions is presented incorporating the effects of temperature, pressure, and CH4/ C3H8 molar ratios in the ranges of 673−873 K, 1−10 bar, and 0−20, respectively. To examine the effect of higher hydrocarbons presence on methane aromatization, the conversion of a mixture of methane and propane at different temperatures, space velocities, and CH4/C3H8 ratios over Zn/HZSM-5

2. MATERIALS AND METHODS 2.1. Catalyst Preparation. The Na form of ZSM-5 zeolite, supplied by Zeochem company, with a SiO2/A12O3 ratio of 40 was first converted into its ammonium form (NH4ZSM-5) by repeated ion exchange (four times, each lasting about 4 h) with a 1 M NH4NO3 aqueous solution at 358 K. These were filtered and washed and then dried at 383 K overnight. HZSM-5 catalysts were prepared by calcination of the NH4ZSM-5 at 773 K in air for 10 h. The resulting HZSM-5 zeolite was converted into the Zn/HZSM-5 form by impregnation with the required amount of zinc nitrate tetrahydrate (Zn(NO3)2·6H2O, Merck) in aqueous solution at 353 K for 2 h. The catalyst was then dried at 358 K overnight and calcined at 773 K in air for 10 h. The calcined sample was crushed and sieved to 35−70 mesh for catalytic evaluation. The zinc content of the catalyst was measured by both wet chemistry and ICP methods giving 0.4 wt % Zn loading. 2.2. Catalytic Test Runs. Catalytic reactions were carried out in a continuous flow fixed-bed stainless steel reactor (i.d. 8 mm) at atmospheric pressure. After the catalyst was pretreated with N2 at reaction temperature for 30 min, the feed (CH4, C3H8, and N2) 573

dx.doi.org/10.1021/ie400786z | Ind. Eng. Chem. Res. 2014, 53, 572−581


Article

Figure 2. Equilibrium methane conversion (a), equilibrium aromatics selectivity (b), and equilibrium paraffins and olefins selectivity (c) as functions of CH4/C3H8 molar ratio in the feed (Model I). T = 873 K; P = 1.01 bar.

undetermined multipliers, has been described in detail elsewhere.19 In application of Lagrange’s undetermined multipliers method for total Gibbs free energy minimization, the following equations need to be solved simultaneously:

was introduced into the reactor passing through Brooks mass flow controllers at the required temperature. N2 in the feed was used as an internal standard so that the CH4 and C3H8 conversions could be determined accurately. The reaction mixtures were analyzed by a Varian CP-3800 gas chromatograph using a flame ionization detector (FID) and a GS-GASPRO column for the separation of hydrocarbons. The thermal conductivity detector (TCD) and MS-5A column was applied for measurement of N2. The reactor outlet pipeline and the gas sampling valves were kept at a temperature above 483 K for effective sampling of all the aromatic products. The selectivities of the products were calculated based on the mass balance of carbon. 2.3. Methodology of Thermodynamic Analysis. The total Gibbs free energy of a single-phase system at specified temperature T and pressure P, (Gt)T,P is a function of gas compositions in the system and can be presented as (Gt )T , P = f(n1 , n2 , n3 , ..., nN )

ΔG of

⎛ y φî P ⎞ + ln⎜ i o ⎟ + RT ⎝ P ⎠ i

∑ k

λk aik = 0 (i = 1, 2, ..., N ) RT (2)

= ∑ ya i ik i

∑ yi = 1 i

Ak (k = 1, 2, ..., w) n

(3)

(4)

where yi is the mole fraction of species i at equilibrium conditions and φ̂i is the fugacity coefficient of species i in solution. The φ̂i values are all equal to unity if the assumption of ideal gas law is justified. λk is the Lagrange multiplier of element k, aik the number of atoms of the kth element present in each molecule of the chemical species i, Ak total number of atomic masses of the kth element in the system, as determined by the initial constitution of the system, and w the total number of elements present in the system.

(1)

where ni and N are the numbers of moles of species i and the number of chemical species in the system, respectively. At equilibrium conditions, the total free energy of the system has its minimum value. The set of ni’s which minimizes (Gt)T,P is found using the standard procedure for the calculation of gas-phase reactions and is subject to the constraints imposed by material balances. The procedure, based on the method of Lagrange’s 574



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Figure 3. Equilibrium methane conversion (a), equilibrium propane conversion (b), equilibrium aromatics selectivity (c), and equilibrium propylene selectivity (d) as functions of temperature (Model II). P = 1.01 bar; CH4/C3H8 = 6/1.

scheme, no methane formation step is considered. In other words, it is assumed that in the presence of an appropriate catalyst, methane formation by propane cracking is inhibited.

From a thermodynamic point of view, methane and propane decompositions are favored at above 830 and 380 K, respectively. Further, the Gibbs free energy of formation of higher paraffins, olefins, and aromatics are positive under typical dehydroaromatization conditions. Therefore, in both methane and propane aromatization reactions, carbon (coke) and hydrogen are the main equilibrium products with negligible amounts of aromatics and lower olefins.20,21 Coke selectivities in excess of 99% are obtained, which are not of practical interest. This reaction pathway could be best approximated by noncatalytic, high-temperature systems at sufficiently low space velocities. However, when a catalyst is employed, according to the catalyst formulation and corresponding reaction pathway, other equilibria could be established with different compositions. With appropriate catalysts, coke formation could be suppressed kinetically, which is highly desirable. However, coke-free equilibria are metastable compared to the more stable nonrestricted coke formation. In this work, two reaction networks were considered; coke formation was assumed to be hindered kinetically by the catalyst in both reaction networks. The species selected for equilibrium calculation were those that are likely to be formed in different steps of the reactions involved, namely, hydrogen, methane, C2−C5 paraffins and olefins, benzene, toluene, xylenes (BTX), and naphthalene. In the first reaction system (Model I), no limitation is imposed on reaction pathways among the reacting species. Under such a circumstance, the choice of a set of chemical species (as listed above) is equivalent to the choice of a set of independent reactions among the species. In the second reaction scheme (Model II), based on the following reaction

° = 131 kJ/mol; C3H8 ↔ C3H6 + H 2 ; Δr H873 ° = 8.21 kJ/mol Δr G873

(5)

° = 28.2 kJ/mol propylene; 2C3H6 ↔ C6H6 + 3H 2 ; Δr H873 ° = − 41.0 kJ/mol propylene Δr G873

(6)

° = 51.0 kJ/mol; C6H6 + CH4 ↔ C6H5CH3 + H 2 ; Δr H873 ° = 39.8 kJ/mol Δr G873

(7)

° = 103 kJ/mol benzene; C6H6 + 2CH4 ↔ C8H10 + 2H 2 ; Δr H873 ° = 96.5 kJ/mol benzene Δr G873

(8)

° = 7.88 kJ/mol; C6H5CH3 + C3H6 ↔ C10H8 + 3H 2 ; Δr H873 ° = − 56.7 kJ/mol Δr G873

(9)

The latter assumption is based on the maximum possible methane conversion. Methane is not produced in any of the above reactions, and naturally methane conversion could not attain a negative value. According to the above scheme, benzene is solely produced from propylene, whereas naphthalene could be formed subsequently from benzene derivatives. Methane serves as the alkylating agent. This model is proposed based on the recent studies of Luzgin et al.16 To converge to the desired model, appropriate additional constrains were imposed on the above equilibrium calculations. 575



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Figure 4. Equilibrium methane conversion (a), equilibrium propane conversion (b), equilibrium aromatics selectivity (c), and equilibrium propylene selectivity (d) as functions of pressure (Model II). T = 873 K; CH4/C3H8 = 6/1.

may be neglected because of the fact that these are less than 10−4% under the given conditions. The equilibrium calculations indicate that the formation of both higher paraffins and olefins is not favorable within the temperature range of 673−873 K. It can be accounted for by the fact that the C−C bond strength (about 246 kJ mol−1) is much lower than those of the C−H bond (about 363 kJ mol−1) and carbon−carbon bond in aromatic rings (ca. 520 kJ mol−1). Consequently, cracking and coke formation reactions are thermodynamically more favorable than formation of higher saturated and unsaturated aliphatic compounds.24 3.1.1.2. Effect of Operating Pressure. Pressure only slightly affects equilibrium compositions. In the range of 1−10 bar, equilibrium conversion of methane decreases from −22 to −26 while that of propane is close to 100%. Selectivity to alkylbenzenes hardly increases with pressure while those of benzene and naphthalene slightly reduce. An increase in the pressure up to 10 bar promotes the selectivity to ethane from 0.65% to 2%. A decrease in the selectivity to aromatics and enhancement of selectivity to lower alkanes at higher pressures has been observed in practice.25 3.1.1.3. Effect of CH4/C3H8 Molar Ratio. Figure 2a shows the equilibrium methane conversion as a function of CH4/C3H8 molar ratio. It can be observed that at low CH4/C3H8 molar ratios, methane is produced; however, with increasing CH4/C3H8 molar ratio, its formation declined. With increasing

The required thermodynamic functions were obtained from ref 23. All calculations were performed using MATLAB R2008a software.

3. RESULTS AND DISCUSSION 3.1. Equilibrium Calculation Results. 3.1.1. Model I. 3.1.1.1. Effect of Operating Temperature. Figure 1 shows the equilibrium conversion of methane and selectivity to the main products. It is noteworthy that under the conditions applied, calculated equilibrium conversions of propane (not depicted) were close to 100%. In contrast, within a temperature range of 673−873 K, the conversions of methane attain negative values (Figure 1a). This implies that methane is produced on a net basis. Nevertheless, with an increase in the temperature, methane is ultimately consumed. Therefore, the net conversion of methane is enhanced in elevated temperature. The effect of temperature on equilibrium selectivity of aromatics, paraffins, and olefins is shown in Figure 1b,c. As can be observed, the selectivity of naphthalene decreases with temperature, while that of benzene improves. Selectivity of xylene isomers is not only extremely low but also reduces with temperature. While in the case of toluene, although the selectivity is rather low, it slightly improves. The equilibrium calculations also indicate that the formation of olefins increases with temperature, while that of paraffins is only slightly affected. Selectivities of C4+ olefins and paraffins 576



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Figure 5. Equilibrium methane conversion (a), equilibrium propane conversion (b), equilibrium aromatics selectivity (c), and equilibrium propylene selectivity (d) as functions of CH4/C3H8 molar ratio in the feed (Model II). T = 873 K; P = 1.01 bar.

resulting in a decrease of methane and propane equilibrium conversions. The effects of pressure on equilibrium aromatics and propylene selectivities are demonstrated in panels (c) and (d) of Figure 4, respectively. In general, aromatic yield decreases with increasing pressure. Although propylene selectivity enhances with pressure, its yield reduces with pressure. 3.1.2.3. Effect of CH4/C3H8 Molar Ratio. Figure 5 illustrates the effects of CH4/C3H8 molar ratio in the feed on equilibrium compositions according to Model II. Methane and propane

CH4/C3H8 molar ratio, methane formation decreases almost linearly. Equilibrium propane conversions are close to 100% under these reaction conditions. Equilibrium aromatics selectivity slightly decreases with CH4/ C3H8 molar ratio, while selectivites of ethane and ethylene both increase (Figure 2b,c). 3.1.2. Model II. 3.1.2.1. Effect of Operating Temperature. Within the temperature range of 673−873 K, conversion of both methane and propane increases with temperature, as illustrated in panels (a) and (b) of Figure 3, respectively. Unlike Model I predictions, equilibrium methane conversions are positive and equilibrium propane conversions promotes from 74% to 100% at higher temperatures. The effects of temperature on equilibrium selectivity of aromatics and propylene are presented inpanels (c) and (d) of Figure 3, respectively. As it may be observed from the figure, the selectivity of benzene decreases with temperature while that of naphthalene increases. As expected, the aromatic yield at higher temperatures is larger than those at lower temperatures. The equilibrium calculations also indicate that formation of propylene decreases with temperature. 3.1.2.2. Effect of Operating Pressure. As the system pressure is raised, equilibrium methane conversions and propane conversions decrease (Figure 4a,b). With regard to reactions 5−9, except for reactions 7 and 8, the rest of the reactions exhibit positive total stoichiometric coefficient values, ν. Increase in the system pressure shifts the reactions with positive ν to the left,

Figure 6. Total Gibbs free energy for different models as functions of temperature. P = 1.01 bar; CH4/C3H8 molar ratio = 6/1. 577



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Figure 7. Methane conversion (a), propane conversion (b), and aromatic selectivity (C6−C10) (c) as functions of time on stream. Feed: CH4/C3H8/ N2 = 6/1/1.3. P = 1 atm; GHSV = 1500 mL gcat−1 h−1.

methane conversion profile (Figure 7a). The “initial activity” characterized by a rapid drop of methane conversion, lasting about 4 h, and a “gradual increase” in methane conversion approaching to zero in longer TOS. A similar trend for methane conversion has been also observed in aromatization of a CH4− C2H6−CO2 mixture over Mo/SiO2/H-ZSM-5 catalyst.15 The initial drop could be attributed to a transition from an induction period to maximum methane-forming activity of the catalyst followed by a decrease in methane formation activity due to gradual catalyst deactivation. As a consequence, for the purpose of comparison of the results, the data after 2 h TOS were employed. The higher the temperature, the lower is methane conversion or the higher is its formation. This illustrates a transition from a Model II reaction scheme to Model I upon increasing temperature. At higher temperatures, noncatalytic, nonselective conversion routes become increasingly predominant, and the system tends to approach the more stable state rather than the desired Model II. This is reflected in increased coke and CH4 formation (on fresh catalysts) as shown in Figure 7. As expected from kinetic and thermodynamic considerations, high temperature is advantageous to aromatization reaction. This is reflected in an increase of propane conversion and aromatic selectivities with temperature. Although the aromatization reaction sequence involves certain exothermic steps, cracking and dehydrogenation reactions make the overall process highly endothermic. Table 1 shows the methane and propane conversions together with individual product selectivity after 2 h on stream at different temperatures. It illustrates that the aromatic cut consists mainly of benzene and its methylated homologues. If the BTX (mononuclear aromatics) are lumped as “benzenes”, the conversions and selectivities are well-correlated with Model II predictions. At 873 K and CH4/C3H8 = 6 from Model II, equilibrium propane conversion, BTX selectivity, and naphthalene selectivity are 99.9, 83.6, and 16.3%, respectively, compared to

conversions increase with CH4/C3H8 molar ratio in the feed. According to reactions 7−9, addition of CH4 to the propane shifts the reactions to the right, resulting in the increase of equilibrium methane conversion and equilibrium selectivity to naphthalene, toluene, and mixed xylenes. Selectivites to benzene and propylene both decrease with increasing CH4/C3H8 molar ratio (Figure 5d). In summary, by addition of methane to the feed, aromatic yield increases. 3.1.3. Comparison of the Models. A comparison of the results of Models I and II reveals that equilibrium methane and propane conversions as well as product distributions as predicted by the models differ markedly. From the former model, extensive methane formations and high selectivity to naphthalene are predicted. In contrast, predictions from the latter model indicate that methane is consumed with a low rate and that the main aromatic products are benzene and its derivatives. Figure 6 compares the free energy of the different systems studied. It may be observed that Models I (metastable A) and II (metastable B) are both metastable compared to unrestricted coke formation model (stable). The least stable system is the desired case (Model II), which highlights the importance of catalyst formulation. Models I and II tend to converge at higher temperatures. However, the rate of cracking (coke and methane formation) in comparison with that of desired aromatization reaction is further enhanced at the elevated temperatures. The results of thermodynamic calculations are compared with those determined experimentally in the following section. 3.2. Experimental Results. 3.2.1. Effect of Temperature. Figure 7 shows the trends of methane and propane conversions together with the aromatic selectivity versus time-on-stream (TOS) over Zn/HZSM-5 catalyst. The profiles are smooth and monotonically decreasing for propane conversions and aromatic selectivities which can be attributed to catalyst deactivation. However, two distinct stages could be distinguished in the 578



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Table 1. Conversion and Selectivities of the Key Products (P = 1 atm, TOS = 2 h, CH4/C3H8/N2 = 6/1/1.3) temperature (K) 753 823 873 a

product selectivity (%)a

conversion (%) =

C3=

B

T

X

C9

N

aromatics

15.3 7.3 2.6

17.5 26.6 33.2

20.7 30.3 28.6

9.2 9.0 5.4

0 2.6 7.9

0 2.1 6.5

47.4 70.6 81.6

C1

C3

C2

C2

−2.5 −4.2 −7.0

15.7 65.0 96.4

6.2 5.8 8.0

25.2 14.4 7.6

B, benzene; T, toluene; X, xylenes; N, naphthalene.

Figure 8. Variation of methane conversion (a), propane conversion (b), and aromatics selectivity (C6−C10) (c) with time on stream. GHSV = 1500 mL gcat−1 h−1; P = 1 atm; T = 873 K.

reaction of C1−C3 is between 823 and 873K, at which a stable propane conversion higher than 66% can be achieved. 3.2.2. Effect of CH4/C3H8 Molar Ratio. Regarding the results as discussed in previous sections, the low net participation of methane in the reactions implies that it acts largely as an inert gas. Therefore, its little effect on the reaction sequence is expected. Such an expectation is confirmed from Figure 8. A notable observation is the conversion of methane. Figure 8a shows significant methane formation when its content in the feed is low. However, at higher concentrations of methane in the feed, formation of methane from propane is suppressed, which can be explained by the Le Chatelier principle. This could be considered an advantage of the presence of methane in the feed. Similar to thermodynamic predictions, the aromatics including benzene selectivity enhance slightly with increasing CH4/C3H8 molar ratio (Figure 8c). Ethane selectivity decreases with increase in methane in the feed; however, ethylene and propylene selectivites do not show significant changes. 3.2.3. Effect of Feed Space Velocity. The feed space velocity exerts significant influences on the reaction. Figure 9a,b shows the effect of space velocity on the methane and propane conversions. Similar to the previous observation, methane

the corresponding experimental values of 96.4, 67.2, and 6.5%, which are in fair agreement. The main difference is the higher portion of methylated benzene derivatives in experimental results which can subsequently be dealkylated to benzene or converted to naphthalene. Further, gradual catalyst deactivation by coke formation indicates that coke formation could not be totally eliminated kinetically. While the aromatic selectivity was found to improve with temperature, the C2= and C3= selectivity decreased. Selectivity to ethane remained approximately unaffected (see Table 1). Generally speaking, higher temperatures promoted formation of the less saturated aromatic compounds (especially naphthalene) at the expense of olefins. However, the formation of higher polyaromatic hydrocarbons and coke was largely hindered by the shape selectivity of the ZSM-5 support. The ratios of benzene/ alkylbezenes and naphthalene/BTX increased with temperature, showing dealkylation of aromatics at higher temperatures. However, at elevated temperatures, e.g., 873 K or higher, severe carbon deposition occurred, and as such, the conversion of propane and the selectivity of aromatics decreased rapidly with the reaction time (Figure 7b,c). As a comprehensive consideration, the proper temperature range for the aromatization 579



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Figure 9. Variation of methane conversion (a), propane conversion (b), and aromatics selectivity (C6−C10) (c) with time on stream. Feed: CH4/C3H8/N2 = 6/1/1.3. P = 1 atm; T = 873 K.

which are more consistent with noncatalytic reactions and Model I predictions. The fact that equilibrium methane conversions are extremely low and sometimes attain even negative values under various operating conditions implies that the relatively high methane conversions reported by certain authors may not be sustainable or reproducible. Such a view has been also pointed out by Naccache et al.3 following a comprehensive review of coaromatization literature. The thermodynamic study carried out in the present work may confirm such a conclusion. Therefore, effective conversion of methane to aromatics remains a challenge requiring further investigations.

conversions attain negative values; however, methane formation is reduced with increasing space velocity. As expected, propane conversion promotes with decreasing space velocity (increasing contact time). The following common trends in product selectivity are observed with increasing space velocity: (a) Selectivity for aromatics continually decreases. (b) Selectivity for propylene and ethylene increases. (c) Selectivity for ethane decreases. These imply that olefins are the primary products that undergo aromatization in longer times on streams. Furthermore, at higher space velocities the reactions are far from equilibrium. It is important to note that with increase in space velocity, the drop in conversion of propane and aromatic selectivity accelerate with the time on stream. This could be due to the shift of the reaction regime from a thermodynamically controlled to a kinetic controlled one, rendering the conversion more sensitive to catalyst deactivation at higher space velocities. In conclusion, it may be noted that the effect of space velocity on the system behavior is opposite to that of temperature. This can be explained by the fact that lower space velocities act similar to high temperatures in shifting the equilibrium to more stable conditions. 3.3. Summary. The experimental data determined for catalytic aromatization of the CH4−C3H8 mixture over Zn/HZSM-5 catalyst are better correlated with Model II under proper operating conditions. Conditions favoring the approach to equilibrium such as high temperatures and low space velocities lead the reaction to the formation of higher portions of coke and methane

4. CONCLUSIONS Thermodynamic considerations support the view that cofeeding of methane with higher hydrocarbons cannot successfully induce methane to participate in aromatization reactions. Over the range of practical interest, equilibrium methane conversion could not exceed 1% when propane is cofed. Experimental results showed even some net methane formation during most of the Zn/HZSM-5 catalyst lifetime. Higher temperatures enhance aromatic formation but also increase coke formation by pyrloysis of the thermally less stable propane. Therefore, effective conversion of methane to aromatics remains a challenge requiring further research.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Chemical Engineering Department, Amirkabir University of Technology, No. 424, Hafez Avenue, P.O. Box 15875-4413, Tehran, Iran. 580



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Notes

catalysts: A start to the development of new one-step catalytic processes for the gas-to-liquid transformation. Appl. Catal., A 2004, 258, 159. (14) Baba, T. Conversion of methane over Ag+-exchanged zeolite in the presence of ethane. Catal. Surv. Asia 2005, 9, 147. (15) Bradford, M. C. J.; Te, M.; Konduru, M.; Fuentes, D. X. CH4− C2H6−CO2 conversion to aromatics over Mo/SiO2/H-ZSM-5. Appl. Catal., A 2004, 266, 55. (16) Luzgin, M. V.; Rogov, V. A.; Arzumanov, S. S.; Toktarev, A. V.; Stepanov, A. G.; Parmon, V. N. Methane aromatization on Zn-modified zeolite in the presence of a co-reactant higher alkane: How does it occur? Catal. Today 2009, 144, 265. (17) Zheng, L.; Xuan, D.; Guo, J.; Lou, H.; Zheng, X. Non-Oxidative Aromatization of CH4-C3H8 over La-Promoted Zn/HZSM-5 Catalysts. J. Nat. Gas Chem. 2006, 15, 52. (18) Feng Liu, J.; Liu, Y.; Fu Peng, L. Aromatization of methane by using propane as co-reactant over cobalt and zinc-impregnated HZSM5 catalysts. J. Mol. Catal. A: Chem. 2008, 280, 7. (19) Smith, J. M., Ness, H. C. V.; Abbott, M. M. Introduction to Chemical Engineering Thermodynamics, 6th ed.; McGraw-Hill: New York, 2001. (20) Moghimpour Bijani, P.; Sohrabi, M.; Sahebdelfar, S. Thermodynamic Analysis of Nonoxidative Dehydroaromatization of Methane. Chem. Eng. Technol. 2012, 35 (10), 1825. (21) Moghimpour Bijani, P.; Sohrabi, M.; Sahebdelfar, S. Thermodynamic analysis of propane aromatization. Petrol. Sci. Technol. 2012. (22) Lima da Silva, A.; De Fraga Malfatti, C.; Lourdes, M. Thermodynamic analysis of ethanol steam reforming using Gibbs energy minimization method: A detailed study of the conditions of carbon deposition. Int. J. Hydrogen Energy 2009, 34, 4321. (23) Yaws, C. Y. Chemical Properties Handbook: Physical; McGrawHill: New York, 1999. (24) Resasco, D. E. Dehydrogenation-Heterogeneous. In Encyclopedia of Catalysis, Horváth, I. T.; John Wiley & Sons: New York, 2003. (25) Zhou, L. BP-UOP Cyclar Process.In: Meyers, R. A. Handbook of Petroleum Refining Processes (3rd ed.); McGraw-Hill:: New York, 2004.

The authors declare no competing financial interest.

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NOMENCLATURE Ak total number of atomic masses of the kth element in the system, as determined by the initial constitution of the system aik number of atoms of kth element present in each molecule of the chemical species i (Gt)T,P total Gibbs energy of a single-phase system with specified temperature T and pressure P (J mol−1) o ΔGfi standard Gibbs energy change of formation for species i (J mol−1) n total number of moles at equilibrium condition (mol) P system pressure (N m−2) o P pressure at the standard state (N m−2; in this case, 100 000 N m−2) R universal gas constant (J mol−1 K−1) T system temperature (K) W total number of elements present in the system yi mole fraction of species i at equilibrium conditions

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GREEK SYMBOLS φ̂i fugacity coefficient of species i λk Lagrange multiplier of element k (J mol−1)

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REFERENCES

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