Energy & Fuels 2000, 14, 1139-1142
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Dimethyl Ether Steam Reforming to Feed Molten Carbonate Fuel Cells (MCFCs) V. A. Sobyanin,† S. Cavallaro,*,‡ and S. Freni*,§ Boreskov Institute of Catalysis, 5 Lavrentieva ave, 630090 Novosibirsk, Russia, Dipartimento di Chimica Industriale, Universita` di Messina, P.O. Box 29, 98166 Sant’Agata di Messina, Italy, and Istituto C.N.R.-T.A.E., via Salita S. Lucia sopra Contesse 5, 98126 Santa Lucia, Messina, Italy Received September 20, 1999. Revised Manuscript Received August 7, 2000
Thermodynamic consideration for dimethyl ether (DME) steam reforming to evaluate the equilibrium products distributions as a function of temperature, pressure, and H2O/DME molar ratio has been performed. The possibility of using DME as a feedstock for hydrogen production has been analyzed. Products distributions have been compared for DME, ethanol, and methanol steam reforming reactions.
1. Introduction DME as well as methanol are produced from syngas, which comes from coal, petroleum, biomass, and natural gas.1-5 It is a useful intermediate for producing many valuable chemicals. DME is used as an environmentally benign aerosol propellant instead of ozone-depleting Freons. Recently, DME has been recognized as a substitute diesel fuel and as a potential fuel for power generation and domestic use.3-5 According to ref 3, DME is relatively inert, noncorrosive, and noncarcinogenic. Its physical properties are similar to those of LPG, so DME can be handled and stored as LPG. It seems that the demand for DME will increase rapidly and DME will play an important role in energy transfer. In fact, a novel single-step process for largescale DME production has been developed.2-4 Moreover, DME synthesis is more economically efficient than methanol synthesis. In particular, the production cost per heating value in DME synthesis is reduced by 20% with respect to methanol synthesis.5 If so, hydrogen production by DME steam reforming instead of other fuel (e.g., methanol) steam reforming could be more attractive for fuel cells and other applications. This paper is a first step of our current research activity in this direction. It is devoted to thermodynamic consideration of DME steam reforming for estimating the equilibrium product distributions as a function of tem* Corresponding authors. E-mails:
[email protected]; scavall@ scirocco.unime.it. † Boreskov Institute of Catalysis. ‡ Universita ` di Messina. § Istituto C.N.R.-T.A.E. (1) Brown, D. M.; Bhatt, B. L.; Hsiung, T. H.; Lewnard, J. J.; Waller, F. J. Catal. Today 1991, 8, 279. (2) Rouhi, A. M. Chem. Eng. News 1995, May 29, 37. (3) Dybjaer, I.; Hansen, J. B. Stud. Surf. Sci. Catal. 1997, 107, 99. (4) Fleisch, T. H.; Basu, A.; Gradassi, M. J.; Masin, J. G. Stud. Surf. Sci. Catal. 1997, 107, 117. (5) Shikada, T.; Ohno, Y.; Ogawa, T.; Ono, M.; Mizuguchi, M.; Tomura, K.; Fujimoto, K. Stud. Surf. Sci. Catal. 1998, 119, 515-520.
perature, pressure, and H2O/DME feed molar ratios. Besides, the findings, including hydrogen yield and carbon formation, are compared with the results on methanol and ethanol steam reforming. Note that hydrogen production by methanol steam reforming has been extensively studied due to its fuel cell applications.6-16 Ethanol steam reforming has been studied owing to worldwide interest given to fuel ethanol produced from corn or sugarcane by biochemical conversion.17-20 Moreover, recently the use of ethanol as a fuel for both direct and indirect internal reforming MCFC has been suggested.21-23 To our knowledge, however, there are as yet no reports in the literature on hydrogen production by DME steam reforming. (6) Amphlett, J. C.; Evans, M. J.; Jones, R. A.; Mann, R. F.; Weir, R. D. Can. J. Chem. Eng. 1981, 59, 720. (7) Takezawa, N.; Iwasa, N. Catal. Today 1997, 36, 45. (8) Kobayashi, H.; Takezawa N.; Minocki, C. J. Catal. 1981, 69, 487. (9) Agaras, H.; Cerrella, G.; Laborte, H. Appl. Catal. 1986, 45, 53. (10) Idem, R. O.; Bakhshi, N. N. Ind. Eng. Chem. Res. 1994, 33, 2056. (11) Amphlett, J. C.; Mann, R. F.; Peppley, B. A.; Stokes, D. M. Proc. Intersoc. Energy Convers. Eng. Conf. 1991, 26 (3), 642. (12) Vehara, I. Tenth Anniversary Conference of Hydrogen Industry Council, Kananaskia, Alberta, Canada; Hydrogen Industry Council 700, 4th Ave. S. W., Calgary, 1992, Canada. (13) Jiang, C. J.; Trimm, D. H.; Wainwright, M. S. Chem. Eng. Technol. 1995, 18, 1. (14) Jiang, C. J.; Trimm, D. H.; Wainwright, M. S. Appl. Catal., A: General 1993, 93, 245. (15) Jiang, C. J.; Trimm, D. H.; Wainwright, M. S. Appl. Catal., A: General 1993, 97, 145. (16) Amphlett, J. C.; Mann, R. F.; Weir, R. D. Can. J. Chem. Eng. 1988, 66, 950. (17) Garcia, E. Y.; Laborde, M. A. Int. J. Hydrogen Energy 1991, 16, 307. (18) Vasudeva, K.; Mitra, N.; Umasankar, P.; Dhingra, S. C. Int. J. Hydrogen Energy 1996, 21, 13. (19) Haga, F.; Nakajima, T.; Miya, H.; Mishima, S. Catal. Lett. 1997, 48, 223. (20) Marino, F. J.; Cerrella, E. G.; Duhalde, S.; Jobbagy, M.; Laborde, M. A. Int. J. Hydrogen Energy 1998, 23, 1095. (21) Cavallaro, S.; Freni, S. Int. J. Hydrogen Energy 1996, 21, 465. (22) Maggio, G.; Freni, S.; Cavallaro, S. J. Power Sources 1996, 74, 17. (23) Freni, S.; Maggio, G.; Cavallaro, S. J. Power Sources 1998, 62, 67.
10.1021/ef990201s CCC: $19.00 © 2000 American Chemical Society Published on Web 10/27/2000
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2. Theoretical Approach We assumed H2, CO, CO2, CH4, C (graphite), and unreacted H2O and DME to be the products of DME steam reforming. Then according to general thermodynamic principles, equilibrium concentrations of each species can be derived from a combination of atomic balance (H, O, and C) equations and equilibrium constants for four independent chemical reactions:
(CH3)2O + H2O ) 2CO + 4H2
(1)
CO + H2O ) CO2 + H2
(2)
2CO + 2H2 ) CO2 + CH4
(3)
2CO ) Cs + CO2
(4)
The equilibrium constants for reactions 1-4 and atomic balances can be represented by the following equations: (i) equilibrium constants:
k1 )
-1 4 4 n2CO‚nH ‚n-1 DME‚nH2O‚p 2
(5)
-1 k2 ) nCO2‚nH2‚n-1 CO‚nH2O
(6)
-2 -2 k3 ) nCO2‚nCH4‚n-2 CO‚nH2 ‚p
(7)
-1 k4 ) nCO2‚n-2 CO‚p
(8)
(ii) atomic balances: carbon balance:
2n°DME ) 2nDME + nCO + nCO2 + nCH4 + nc
(9)
hydrogen balance:
3n°DME + nH2O ) 3nDME + nH2 + nH2O + 2nCH4 (10) oxygen balance:
n°DME + n°H2O ) nDME + nH2O + nCO + 2nCO2 (11) where p is the total pressure; n°DME and n°H2O are the numbers of DME and H2O moles in the feed, respectively; nDME, nH2O, nH2, nCO, nCO2, nc, nCH4 are the numbers of DME, H2O, H2, CO, CO2, C, and CH4 moles at equilibrium, respectively. Evidently, the solution of this nonlinear system of seven equations gives equilibrium composition of reaction mixture as well as the contents of H2, CO, CO2, C, CH4, DME, and H2O. To solve the system, a multidimensional globally convergent method for nonlinear system of equations supported by subroutine “Mathematica 3.0” (Wolfram research Ed.) has been used. Thermodynamic data for calculating the equilibrium constants (k1-k4) were taken from ref 24. The equilibrium products distributions were computed over the temperature range from 600 to 1000 K, pressure range of 1-5 atm, and H2O/DME feed molar ratios 1-20. For all conditions, the ideal gas-phase system was assumed. (24) Stull, D. R.; Westrum, E. R., Jr.; Sinke, G. C. The Chemical Thermodynamics of Organic Compounds; John Wiley & Sons: New York, 1969.
Table 1. Effect of Pressure on Equilibrium Compositions for DME Steam Reforming at Feed Molar Ratio H2O/DME ) 2 700 K
1000 K
speciesa
1 atm
5 atm
1 atm
5 atm
CH4 CO CO2 H2 C H2O
1.175 0.025 0.550 0.776 0.251 1.875
1.344 0.011 0.520 0.362 0.126 1.950
0.117 1.514 0.369 4.017 no 0.749
0.565 0.952 0.483 2.787 no 1.082
a The species content is expressed by moles of the species per mole of DME in the feed.
Note at last that if no carbon forms in the equilibrium system, only reactions 1-3 should be taken into consideration for calculating the system composition. Equilibrium composition in this case is calculated via solving the set of eqs 5-7 and 9,10, and nC in eq 9 should equal zero. 3. Results and Discussion When calculating the equilibrium composition of any system, it is critically important to make an assumption regarding what chemical species are present in the equilibrium mixture. We assumed that under all conditions of DME steam reforming the equilibrium mixture contains H2, CO, CO2, CH4, DME, H2O, and possibly C. Besides, we excluded the presence of oxygenated compounds such as methanol, formaldehyde, methylformiate, ethanol, acetaldehyde, and C2+ hydrocarbons, e.g., ethylene, ethane, propane. The latter assumption based on our primary calculations revealed the equilibrium concentrations of these species to be negligible. Moreover, according to calculations, DME conversion was almost complete in all cases, i.e., nDME ) 0 at equilibrium. Therefore in this work we consider the dependencies of concentration on temperature, pressure, and H2O/DME feed molar ratio only for those species present in the equilibrium mixture in essential quantities, that are H2, CO, CO2, CH4, H2O, and C (graphite). 3.1. Coke Formation Region. It is known that the risk of coke formation during catalytic steam reforming may cause serious operational problems. In this regard, it seems important to examine the thermodynamic possibility of carbon (graphite) formation during DME steam reforming. Typical product distributions as a function of temperature, H2O/DME ratio, and pressure in coke-formation region are presented in Figure 1 and Table 1. Figure 1 shows the temperature effect on the equilibrium composition at H2O/DME ratios equaling 1 and 2, and total pressure of 1 atm. It is seen that the amounts of H2 (Figure 1a) and CO (Figure 1b) increase, and the amount of CH4 (Figure 1a) decreases with increasing temperature. The amounts of CO2 (Figure 1b) and coke (Figure 1c) attain flat maximum at ∼800 K. The increase of H2O/DME ratio from 1 to 2 leads to the increase of H2 yield (Figure 1a) and the decrease of coke yield (Figure 1c). The increase of total pressure causes the decrease of both H2 and coke yields at T ) 700 K and H2O/DME ) 2 (see Tab. 2). Figure 2 shows the range of conditions (in terms of temperature and H2O/DME feed molar ratio) under which carbon appears in the system. It is clearly seen
Dimethyl Ether Steam Reforming for MCFCs
Energy & Fuels, Vol. 14, No. 6, 2000 1141
Figure 2. Coke-formation and coke-free regions as a function of temperature and H2O/DME feed molar ratio. Total pressure 1 atm.
Figure 1. (a-c) Temperature dependencies of equilibrium compositions at DME steam reforming in carbon-formation region; feed molar ratios H2O/DME ) 1 and 2, total pressure 1 atm. Circles, H2O/DME ) 1; squares, H2O/DME ) 2.
that as the total pressure equals 1 atm, the higher is the ratio H2O/DME, the lower is the temperature at which coke formation occurs. Indeed, at 600 K the coke forms at H2O/DME < 2.6, while at 1000 K this limit is lower than 1.4. This result is quite natural for the process of steam reforming and is described in the literature for methane,25 methanol,6 and ethanol,17,18 steam reforming.
3.2. Coke-Free Region. Evidently, the carbon-free operational region is more attractive for hydrogen production by steam reforming of DME or other fuel. Figure 3 and Table 1 show typical effects of temperature, H2O/DME ratio, and total pressure on the content of major species in equilibrium mixture at DME steam reforming in this region. It is seen (Figure 3a) that the hydrogen amount increases both with increasing temperature and H2O/DME ratio. In particular, at the highest H2O/DME ) 20 and atmospheric total pressure, about 5.6-5.7 mol of hydrogen per DME mole are present in equilibrium mixture within a wide range of temperature (800-1000 K). Methane content (Figure 3a) decreases with increasing temperature and H2O/ DME ratio. Carbon monoxide (Figure 3b) content increases with increasing temperature and decreases with increasing H2O/DME ratio. The content of carbon dioxide (Figure 3b), in contrast to carbon monoxide, comes through maximum at ca. 800 K and increases with increasing H2O/DME. The data of Table 1 show that at 1000 K and H2O/ DME ) 2 the yields of hydrogen and CO decrease with increasing pressure, whereas the yields of methane and carbon dioxide increase. The calculation results permit us to conclude that in our model of DME steam reforming, implying the coke and methane formation, high temperatures, low pressures, and high H2O/DME feed molar ratios facilitate hydrogen production. For example, the molar fraction of hydrogen in dry outlet gas mixtures attains 0.75-0.80 at temperatures above 800 K, H2O/DME ) 20, and total atmospheric pressure, whereas molar fractions of CH4, CO, and CO2 are equal to 0.02, 0.03, and 0.2, respectively. In principle, this indicates the possibility of producing hydrogen-rich mixtures by DME steam reforming. 3.3. Comparison of Equilibrium Compositions for DME, Ethanol, and Methanol Steam Reforming. As mentioned above, to our knowledge there are no reports in the literature on hydrogen production by (25) Rostrup-Nielsen, J. R. Catalysis, Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: Berlin, 1984; Vol. 5, Chapter 1,
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Sobyanin et al. Table 3. Equilibrium Compositions for DME (H2O/DME ) 3) and Methanol (H2O/MeOH ) 1) Steam Reforming at P ) 1 atm and T ) 600 K molar fraction, % fuel
(CO + H2)a
H2O
CO2
CH4
refs
CH3OH DME
5 5.6
56 55.6
11 11.1
28 27.7
[6] this work
a The data from ref 6 allows determination of only sum CO and H2 content in the equilibrium mixture.
Figure 3. (a, b) Temperature dependencies of equilibrium compositions at DME steam reforming in carbon-free region; feed molar ratios H2O/DME ) 4 and 20, total pressure 1 atm. Triangles, H2O/DME ) 4; rhombs, H2O/DME ) 20. Table 2. Equilibrium Compositions for Ethanol and DME Steam Reforming at P ) 1 atm and Feed Molar Ratio H2O/Fuel ) 2 800 K
1000 K
speciesa
ethanol
DME
ethanol
DME
CH4 CO CO2 H2 C H2O
0.873 0.188 0.652 1.746 0.286 1.508
0.855 0.184 0.644 1.767 0.315 1.523
0.112 1.511 0.376 4.037 no 0.739
0.117 1.514 0.369 4.017 no 0.749
V
a
The species content is expressed by moles of the species per mole of ethanol or DME in the feed.
DME steam reforming. Meanwhile, hydrogen production by methanol6-16 and ethanol17-23 steam reforming has been studied extensively. To estimate the possibility of DME using as a feedstock for hydrogen production, it seems reasonable to compare equilibrium product distributions for DME, ethanol, and methanol steam reforming. Table 2 shows the equilibrium product distributions at DME and ethanol steam reforming, both for coke-
formation and coke-free operational regions. The data for ethanol steam reforming are taken from ref 18. It is seen that with both fuels and in both regions the equilibrium compositions are very similar. This result is quite regular, since both DME and ethanol convert in steam reforming almost completely and feed atomic ratios C:O:H is equal in both systems (C:O:H ) 1:1:5). Comparison of equilibrium compositions at DME and methanol steam reforming is a more complicated problem, since the selection of molar feed ratio H2O/fuel, at which comparison should be performed, is not so obvious as in DME vs ethanol experiments. To our mind, to be correct, the comparison should be performed at the same C:O:H feed atomic ratios, i.e., C:O:H ) 1:2:6, that corresponds to H2O/CH3OH ) 1 and H2O/DME ) 3. Table 3 shows equilibrium compositions in terms of molar fractions for DME and methanol steam reforming at 600 K in coke-free operational region. As expected, the equilibrium product distributions at DME and methanol steam reforming are almost the same. However, equilibrium hydrogen yield is very low in both cases. Thermodynamically, methane is the main hydrogen-containing product of the steam reforming at moderate temperature. So, methane formation consumes hydrogen atoms and produces decreasing hydrogen content in the equilibrium mixtures. According to experimental results,7-16 methanol can be catalytically steam reformed to hydrogen-rich mixture (∼70 vol % H2) at temperatures below 600 K. This considerable difference between calculated and experimental hydrogen yield values is attributed to the fact that methanol steam reforming catalyst completely inhibits methane formation at moderate temperatures. In this regard we suppose that the DME steam reforming catalyst allowing production of hydrogen-rich mixtures at moderate temperatures could also be designed. Conclusions Thermodynamic consideration proved feasible the production of hydrogen-rich mixtures by DME steam reforming at high temperature, low pressure, and high H2O/DME feed molar ratio. Comparison of equilibrium product distributions for DME, ethanol, and methanol steam reforming demonstrated similar behavior of these systems. To provide production of hydrogen-rich mixtures by DME steam reforming at moderate temperatures, it is necessary to develop the catalyst possessing low selectivity toward methane. EF990201S
