Brazilian Journal
of Chemical ISSN 0104-6632 Printed in Brazil Engineering www.abeq.org.br/bjche
Vol. 32, No. 01, pp. 201 - 210, January - March, 2015 dx.doi.org/10.1590/0104-6632.20150321s00002820
EVALUATING HYDROGEN PRODUCTION IN BIOGAS REFORMING IN A MEMBRANE REACTOR
F. S. A. Silva*, M. Benachour and C. A. M. Abreu
Department of Chemical Engineering, Federal University of Pernambuco, CEP: 50740-520, Recife - PE, Brazil. Phone: + (55) (81) 2126-8901 E-mail: [email protected]
(Submitted: July 1, 2013 ; Revised: February 25, 2014 ; Accepted: April 29, 2014)
Abstract - Syngas and hydrogen production by methane reforming of a biogas (CH4/CO2 = 2.85) using carbon dioxide was evaluated in a fixed bed reactor with a Pd-Ag membrane in the presence of a nickel 5 catalyst (Ni 3.31% weight)/γ-Al2O3) at 773 K, 823 K, and 873 K and 1.01×10 Pa. Operation with hydrogen permeation at 873 K increased the methane conversion to approximately 83% and doubled the hydrogen yield relative to operation without hydrogen permeation. A mathematical model was formulated to predict the evolution of the effluent concentrations. Predictions based on the model showed similar evolutions for yields of hydrogen and carbon monoxide at temperatures below 823 K for operations with and without the hydrogen permeation. The hydrogen yield reached approximately 21% at 823 K and 47% at 873 K under hydrogen permeation conditions. Keywords: Membrane reactor; Biogas; Methane; Syngas; Hydrogen.
INTRODUCTION reforming (Kolbitsch et al., 2008; Lau et al., 2011). Biogas is produced from organic household waste, The increasing availability of methane has gener- industrial waste, and animal dung. Landfill gas is the ated substantial interest in alternative methods for its most important source of biogas because of its rela- conversion into synthesis gas (syngas) and/or hydro- tively high carbon dioxide content (CO2: 36-41%, gen. Research has shown that the catalytic reforming CH4: 48-65%, N2: 1-17%, and high methane emis- of methane with carbon dioxide (dry reforming) may sions. Qian et al. (2002) reported a gas generation be employed with natural gas and carbon dioxide rate of 2.50 Nm3/landfill ton/year for old landfills (> emissions rather than traditional methane steam re- 10 years old). Themelis and Ulloa (2007) estimated a forming (MSR) for syngas production (Amoro, 1999; global methane production rate of 75×109 Nm3/year Topalidis, 2007; Abreu et al., 2008; Silva et al., from 1.5×109 tons/year of solid landfill, of which 2012). Major anthropogenic sources of global carbon only approximately 10% was collected and used. dioxide emissions include flue gases from coal, oil- The expertise developed from the methane dry re- fired power stations, thermoelectric plants, FCC refin- forming process (Abreu et al., 2008) can be used to ing units, alcoholic fermentation, and several heavy increase the production of syngas and hydrogen from industries, such as those that produce iron, lime, and biogas. However, thermodynamic constraints and low cement. Biogas, a gaseous mixture of methane and catalytic performance have been identified as limita- carbon dioxide, can be directly processed by catalytic tions to achieving high methane conversions into
*To whom correspondence should be addressed
202 F. S. A. Silva, M. Benachour and C. A. M. Abreu hydrogen and carbon monoxide. Operational initia- EXPERIMENTAL SECTION tives of selective product permeation (Garcia-Garcia et al., 2013; Faroldi et al., 2013; Gallucci et al., Catalyst Preparation and Characterization 2013) can be used to overcome these restrictions when the process is in chemical equilibrium by The catalyst was prepared using nickel nitrate shifting the composition of the media to increase (Ni(NO3)2.6H2O, Sigma-Aldrich, Germany) and reactant conversions. gamma-alumina (γ-Al2O3, Degussa, Brazil) via im- In general, under the same operating conditions, a pregnation of alumina with a nickel nitrate solution. higher reactant conversion is obtained for reactor First, the impregnated solution was evaporated to operation with permeation using a selective mem- dryness. The solid was then dried at 393 K for 12 h brane than for a fixed-bed reactor (Kumar et al., and calcinated at 873 K in an argon flow for 5 h. 2008). Membranes of palladium alloys exhibit a high Finally, the catalyst was activated in a hydrogen permeability to hydrogen. Gallucci et al. (2008), Shu atmosphere at 973 K for 2 h. et al. (1991) and Dittmeyer et al. (2001) have written The nickel catalyst was characterized by atomic reviews on palladium membranes that analyze the absorption spectrophotometry (AAS), textural analy- effects of permeation on reactor performance. sis (by the B.E.T. method), and X-ray diffraction Kikuchi (1995) demonstrated that the catalytic (XRD, using CuK-alpha radiation and a Siemens activity of various metals that were used in a mem- D5000 diffractometer). brane reactor for reforming processes decreased in the following order: Ni>Rh>Pt>Pd>Ru>Ir (alumina Experimental Evaluation support). He used a membrane reactor with a nickel catalyst in methane reforming with carbon dioxide at The reforming experiments were performed in 773 K and 1.01 MPa to obtain 47% methane conver- a fixed-bed membrane reactor (Figure 1; useful -2 sion versus the 52% conversion predicted by ther- height, HR = 45.72×10 m; outside diameter, DR = -2 modynamic equilibrium calculations. Thus, the high 1.07×10 m; Pd-Ag, H2 selective membrane, height, -2 activity of nickel and its low cost make it the best Hm = 0.19 m; inner diameter, dm = 0.32×10 m; thick- -5 choice for a reforming catalyst, although there is no ness, δm =7.62×10 m; REB Research & Consulting, evidence that it is susceptible to coke formation USA) with a nickel catalyst (
(a) (b) Figure 1: (a) Scheme of the processing unit of biogas. (A) gas chromatograph, (C1, C2, C3) mass flow meter, (F) Electrical furnace, (R) membrane reactor, (S1) PC computer-mass flow meter control,(S2) PC computer- gas chromatograph, (V1, V2) valves, (M1) U manometer, (E1) vacuum pump, (b) Membrane reactor.
Brazilian Journal of Chemical Engineering
Evaluating Hydrogen Production in Biogas Reforming in a Membrane Reactor 203
At the top of the reactor, three streams of CH4, and 66.6, and the nickel metallic phase was identified CO2 and Ar were fed. The residual reagents and at 2θ = 44.1, 52.0, 77.5, and 93.4. Carbon was found in products were analyzed using on-line gas chroma- catalyst samples that were analyzed after the reaction tography (with a Saturn 2000, Varian, Carbosphere/ evaluations. Elementary carbon analysis of the used Porapak-Q, TCD) of the reactor effluent flow. catalyst indicated a carbon content ranging from After having reached a steady-state regime of 0.21% to 0.26% in weight. processing, the feed stream of methane was stopped while the flows of argon and carbon dioxide were Hydrogen Permeation Tests maintained. The residual carbon in the reactor was removed using carbon dioxide via the Boudouard Experiments on permeation through the selective reverse reaction; the effluent gas was analyzed to membrane were conducted in terms of the variables determine the amount of carbon monoxide produced. of the Sieverts equation (Figure 2). The permeation Permeation tests were performed using gaseous rate was investigated as a function of the pressure mixtures of H2 and Ar (H2:Ar:5:50,10:50, 15:50, difference in the membrane at three operating tem- 20:50, 25:50 v/v) at 723 K, 773 K, and 823 K. The peratures (723 K, 773 K, and 823 K). The hydrogen results were fitted to the Sieverts equation (JH2 = permeation rate increased with the H2/Ar ratio and 1/2 1/2 JH20.exp(-ED/RT)[(PrH2) - (PpH2) ], Rival et al., 2001); temperature. Hydrogen permeation experiments to determine the parameters JH20 and ED. Sieverts (Figure 1(b)) were also performed after the reactions tests were performed after each reaction experiment while the system was being cleaned and the catalyst to evaluate the state of the membrane. was being regenerated with carbon dioxide (i.e., corresponding to the Boudouard reverse reaction: CO2 + C → 2CO). RESULTS AND DISCUSSION Figure 2(a) presents the fits to the experimental data using the linear form of the Sieverts equation. These Catalyst Characterization linear fits were used to estimate the following orders of -5 magnitude of the parameters: JH20 = (2.21 ± 0.41)×10 2 0.5 3 The nickel content and the surface areas of the mol/m s kPa and ED = (3.37 ± 0.13)×10 J/mol. support (pre-treated Al2O3) and catalyst (Ni/Al2O3) Figure 2(b) presents the results of the hydrogen were 3.31% by weight, 226 m2/g, and 145 m2/g, re- permeation tests that were obtained after regenera- spectively, as characterized by AAS and B.E.T.-N2. tion of the catalyst in terms of the variables of the The solid phases of the catalyst used in the re- Sieverts equation. For comparison, were also in- forming reactions were detected by XRD. The γ- cluded in this figure the results of the initial tests of Al2O3 support was identified at 2θ = 37.4, 45.3, 65.8, permeation at 823 K.
0.005 0.0055 823K 723K 823K (after regeneration) .s)
2 773K 0.0050 0.004 823K .s) 2 0.0045
0.0040 0.003 0.0035
0.002 0.0030
0.0025
0.001 0.0020 Permeation rate (mol STP/m 2 Permeation Rate (mol STP/m Rate Permeation H 2 0.0015 H 0.000 0.0010 0 50 100 150 200 60 80 100 120 140 160 180 0.5 0.5 0.5 Membrane pressure difference P -P (Pa ) Membrane Pressure Difference P0.5 -P0.5 (Pa0.5) rH2 pH2 rH2 pH2 (a) (b) Figure 2: Permeation rate as a function of the differences between the square roots of pressure in the membrane: (a) effect of temperature and (b) permeation rate after regeneration at an external reaction zone pressure of 1.01 × 105 Pa0.5 and an internal zone pressure of 0.20 Pa0.5.
Brazilian Journal of Chemical Engineering Vol. 32, No. 01, pp. 201 - 210, January - March, 2015
204 F. S. A. Silva, M. Benachour and C. A. M. Abreu
Figure 3 presents the carbon dioxide and carbon (Figure 2(b)) indicated similar system performance monoxide concentrations measured during the clean- before and after the reaction. ing/regeneration operation as a function of the time on stream. Process Evaluation
0.006 CO Experimental evaluations of the reforming proc- CO2 ess of the biogas were performed in the fixed-bed 0.005 membrane reactor at three different temperatures 5 0.004 (773 K, 823 K, and 873 K) at 1.01 × 10 Pa for a feed gas composition of CH4:CO2:Ar = 0.89:0.31:1.00 vv 3 0.003 and a spatial time of τ =1,204.8 kg.s/m . The operations were performed in two steps, 0.002 without and with hydrogen permeation, followed by measuring the component concentrations in the re- Concentration (mol/L) Concentration 0.001 actor effluents. These experimentally determined
0.000 concentrations (i.e., the CH4 and CO2 conversions and the CO, H2, and H2O production) were evaluated 0 100 200 300 400 500 over a 4.5-h period. Figure 4 presents the reactant time (min) 2 conversions (i.e., Xi = [Ci0-Ci]10 /Ci0, where i = CH4 Figure 3: Carbon removal from the membrane re- and CO2 and where Ci0 denotes the initial concentra- actor. Catalyst and membrane cleaning/regeneration. tions) as a function of the time for operation with and -1 Conditions: mcat= 0.02×10 kg, feed molar ratio without hydrogen permeation at the three tempera- 3 Ar/CO2 = 1.0, flow rate = 120 cm /min, temperature tures shown. Operations at 823 K and 873 K with H2 = 823 K, and pressure = 1.01×105 Pa. permeation resulted in higher methane conversions than operation without H2 permeation under the The operational performance of cleaning/ regen- same conditions. The same trend was not observed eration of the membrane reactor, including carbon for the carbon dioxide conversions, which exhibited removal from the catalyst and/or membrane, was only a slight increase at higher-temperature opera- determined from the evolution of the reactant and tion. The permeation effect occurred during the re- product concentrations in the Boudouard reverse action step in the methane cracking process, shifting reaction (see Figure 3). After 250 min of operation the reaction equilibrium such that the conversion of under a CO2 stream, the CO level was reduced. In methane to hydrogen was increased. The carbon addition, the results of the hydrogen permeation tests dioxide concentrations remained largely unchanged.
with permeation with permeation 95 without permeation 60 90 (773 K) 85 144 min 144min 50 (823 K) 80 (773 K) (873 K) 75 (823 K) 70 (873 K) 40 65 60 144min 162 min 30 without permeation 55 50 Conversion (%) Conversion Conversion (%) Conversion 2
4 45 20
CO 40 CH 144 min 35 10 30 162 min 25 20 0 0 50 100 150 200 250 300 0 50 100 150 200 250 300 350 Time on stream (min) Time on stream (min) (a) (b) Figure 4: Reactant conversions for (a) CH4 and (b) CO2 as a function of time for operations in a membrane reactor illustrating the effects of temperature under the following conditions: catalyst Ni -1 3 (3.31% weight)/γ-Al2O3, mcat = 0.02 × 10 kg, feed molar ratio CH4/CO2 = 2.85, and τ = 1,204.8 kg.s/m at a pressure of 1.01 × 105 Pa.
Brazilian Journal of Chemical Engineering
Evaluating Hydrogen Production in Biogas Reforming in a Membrane Reactor 205
Figure 4 illustrates that initiating hydrogen re- membrane reactor under the same conditions as this moval by permeation significantly increased the study. methane conversion at the two highest temperatures. Although the effects of the operating conditions In the stages following the permeation step, higher (i.e., the feed flow rate, feed ratio, temperature, pres- steady-state methane conversions were attained than sure, and catalyst weight) on the CH4 and CO2 con- for operation without permeation. versions should be considered, the biogas conversion Figure 5 presents the hydrogen and carbon monoxide in a membrane reactor is generally expected to be 2 yields (Yj = [Cj/ΣCi0] × 10 ), j = CO and H2) for the greater than that in a fixed-bed reactor. process. The hydrogen production increased for Munera et al. (2003) evaluated the methane dry operation with H2 permeation at 823 K and 873 K, reforming process in a membrane reactor (with a whereas the carbon monoxide level remained steady. Pd/Ag membrane) using a 0.6% weight Rh/Al2O3 Tables 1 and 2 provide the experimental conver- catalyst at 823 K and atmospheric pressure. The sions and yields obtained under steady-state condi- study obtained methane and carbon dioxide conver- tions at the three temperatures considered. sions of 33.9% and 41%, respectively, for a feed Under hydrogen permeation at 873 K, the meth- molar ratio of CH4/CO2 = 1.0. The methane and car- ane conversion increased by 83% and the hydrogen bon dioxide conversions obtained in the present yield was approximately 113% higher compared to study were 14.5% and 48%, respectively, under the operation without permeation. Galuszka et al. (1998) same conditions. The lower methane conversion may obtained a methane conversion of 48.6% and a hy- be attributed to the applied feed molar ratio drogen yield of 46.5% using a nickel catalyst in a (CH4/CO2 = 2.85) of the biogas.
with permeation 50 50 144 min 144 min (773 K) 40 (823 K) 40 (773 K) 162 min (873 K) (823 K) (873 K) 30 30 without permeation 162mmin yield (%) 2 144 min H 20 CO Yield (%) Yield CO 20 without permeation 144 min 10 with permeation 10
0 0 50 100 150 200 250 300 0 50 100 150 200 250 300 Time on stream (min) Time on stream (min) (a) (b) Figure 5: Product yield for (a) H2 and (b) CO as a function of time for operation in a membrane reactor illustrating the effects of temperature under the following conditions: catalyst, Ni (3.31% weight)/γ- -1 3 Al2O3, mcat = 0.02 × 10 kg, feed molar ratio CH4/CO2 = 2.85, and τ = 1,204.8 kg.s/m at a pressure of 1.01 × 105 Pa.
Table 1: Component conversions of the biogas Table 2: Product yields for the biogas reforming reforming process in a membrane reactor at the process in a membrane reactor under the follow- following steady-state conditions: Ni (3.31% ing steady-state conditions: catalyst Ni (3.31% -1 -1 weight)/γ-Al2O3, mcat = 0.02 × 10 kg, feed molar weight)/γ-Al2O3, mcat = 0.02 × 10 kg, feed molar 3 3 ratio CH4/CO2 = 2.85, and τ = 1,204.8 kg.s/m at a ratio CH4/CO2 = 2.85, and τ = 1,204.8 kg.s/m at a pressure of 1.01 × 105 Pa. pressure of 1.01 × 105 Pa.
Temperature Conversion (%) Conversion (%) Temperature Yield (%) Yield (%) (K) without permeation with permeation (K) Without permeation with permeation CH4 CO2 CH4 CO2 H2 CO H2 CO 773 4.61 35.11 7.79 34.09 773 7.52 7.14 9.23 15.41 823 11.53 42.56 14.54 48.16 823 15.49 30.06 21.14 25.22 873 19.25 92.61 35.33 92.87 873 22.80 53.21 47.00 46.82
Brazilian Journal of Chemical Engineering Vol. 32, No. 01, pp. 201 - 210, January - March, 2015
206 F. S. A. Silva, M. Benachour and C. A. M. Abreu
The experimental results for biogas reforming in conditions employed (biogas mixture diluted in ar- this comparative evaluation were explained using a gon, with low content in carbon dioxide, and opera- set of reaction mechanisms for the reforming process tions with low methane conversions). developed by Abreu et al. (2008) for the same cata- The resulting differential equations were ex- 3 lyst, which consisted of the following major reaction pressed as dCi/dτ + ri= 0, where τ (kg.s/m ) was the steps: methane catalytic cracking, with carbon depo- modified spatial time. The differential equation for sition and hydrogen production (I: CH4 ↔C + 2H2); the mass balance of hydrogen for the permeation hydrogen consumption via a water gas-shift reverse operation was as follows: reaction (II: CO2 + H2↔ CO + H2O); and carbon consumption by carbon dioxide (III: CO2 + C ↔ dc −E/RTD H2 je0 p 2CO, i.e., the Boudouard reverse reaction). Thus, in − −−+=S(C.R.TP)r0H2 (6), dτ mHH2 2 operation with hydrogen permeation, the methane conversion increased to maintain the methane cata- where S = A /m = (4d .[(D 2- d 2)ρ (1-ε)]-1) is the lytic cracking equilibrium (step I). Accordingly, the m m cat m R m cat ratio between the membrane surface area and the mass removal of hydrogen from the reaction medium de- of the catalyst and ε denotes the bed porosity. The cor- layed the reverse water gas-shift reaction (step II), decreasing the carbon dioxide consumption. There- responding initial conditions (τ = τ0) were as follows: fore, the available carbon dioxide partially cleaned C()C;C()Cτ= τ= ; the carbon deposited on the catalyst (step III), main- CH 0 CH CO 0 CO 44o22o taining the activity level of the nickel catalyst during the process. C()C()Cτ= τ= ()0. τ= CO0H0HO0 22 Kinetic and Reactor Modeling The isothermal conditions of the operations were The relations rji (j = I, II, III), which correspond guaranteed by the feed, the convective heat dis- to the rate laws of the reaction steps of the biogas charge, the heat released with hydrogen permeation, reforming process, were expressed as follows: the reaction enthalpies (∆HJ.rJi), and the heat trans- ferred from the oven through the reactor wall. The kK C thermal behavior of the reactive operations was r = 1CHCH44 (1) ICH4 1K+ C modeled by a steady-state energy balance incorpo- CH44 CH rating the aforementioned effects. The following differential equation (Equation (7)) describes the CC rk(CC=−CO H2 O ) (2) temperature evolution as a function of the modified IICO222 2 H CO Keqwgs spatial time:
dT rkCIIICO= 3 CO (3) .ρ =−(ΔHr +ΔHr +ΔHr ) 22 pM gdτ I CH4 II CO2 III CO2
The global reaction rates of each component (r ; 4U i −−(T T ) i = CH , CO , CO, and H O) were written as follows: RE 4 2 2 d.Rρ cat (7) rr=− ; r(rr)=− + (4) Sdmm CH44 ICH CO22 IIICO −−δ−(1 )J (H H ); IICO2 ρ D H2 H2 rH2 mH2 H2 R T(τ )T= rrCO=+ 2r; IIICO 00 IICO2 2 (5) 3 r2rr;rr=− = where ρg = 15.28 mol/m , U (the overall heat transfer H24222 ICH IICO H O IICO 2 -2 coefficient) = 2.41 J/m .s.K, DR = 1.07× 10 m,
ρ = 1,200 kg/m3, and ρ = 71 kg/m3. The reformer The evolution of the effluent concentrations for cat H2 the membrane reactor was obtained from mass bal- and sweep gas enthalpy are denoted by HrH2 and HmH2, ances incorporating component reaction rates based respectively. The temperature TRE = T0 = 750 K and −−62 93 on the three aforementioned reaction steps. In the CpM =+ 29.30 0.023T − 8.96x10 T − 1.40x10 T mass balance equation a constant flow rate along the J/mol.K, where CpM denotes the heat capacity of the reactor was considered based on the experimental mixture of CH4, CO2, and Ar. The enthalpies of re-
Brazilian Journal of Chemical Engineering
Evaluating Hydrogen Production in Biogas Reforming in a Membrane Reactor 207 action can be expressed in the following form: steady-state conditions at 773 K, 823 K, and 873 K T (see Figures 6 and 7) are shown on the same graph ΔHCdT;CabTcT=ν =++2 . for comparison with the predictions. JPJPJiii∫ ∑ i To Figures 6 and 7 illustrate that the experimental The mass balance equations and Equation 7 were component concentrations exhibited the same trends solved for the effluent concentrations and the tem- predicted by the model equations, except for hydro- perature of the reaction medium using the fourth- gen concentrations at temperatures higher than 823 order Runge-Kutta method. The values of the kinetic K (Figure 7(a)). The concentrations of carbon diox- and adsorption parameters were estimated from our ide and carbon monoxide in the output gas were previous work (Abreu et al., 2008). The Arrhenius and similar with and without the permeation of hydrogen. van’t Hoff correlations were expressed as follows: However, the concentrations of methane and hydro- 9 gen were approximately 23% and 51% lower, re- - methane cracking reaction: k1 = 3.58×10 exp -11 spectively, when the system operated with permea- (-248.55/RT) mol/kg.s and KCH4 = 31.39×10 exp (167.32/RT) m3/mol; tion. Methane was further consumed to maintain the 13 equilibrium, which was temporarily displaced by - water gas-shift reverse reaction: k2 = 1.07×10 exp (-350.08/RT) (m3)2/mol/kg.s; and hydrogen permeation. The small amount of available - carbon dioxide-carbon interaction (i.e., the Bou- hydrogen in the reaction medium could be processed 5 by the water gas shift reverse reaction (step II). Thus, douard reverse reaction): k3 = 1.16×10 exp(-115.86/ RT) m3/kg.s. residual carbon dioxide, not consumed by the reac- The equilibrium constant of the water gas-shift tion with hydrogen, was used to process the carbon reaction was given as follows: into carbon monoxide by the Boudouard reverse reaction (step III). −−27 The predicted component concentrations had ap- Keqwgs =− exp( 6.31x10 − 1.86x10 ln(T) proximate values at temperatures lower than 823 K for operations with and without hydrogen permea- ++2.11x10−−−441 T 9.37x10 T (8) tion. When hydrogen permeation was used, lower methane, carbon monoxide and hydrogen concentra- −−5.44x10−−62 (T 298.15)T tions were predicted in the reactor effluent gas for temperatures above 823 K. The component concentrations at the reactor exit Figure 8 presents the model predictions and the were calculated as a function of the temperature over evolution of the carbon monoxide and hydrogen 2 the 750-895 K range for operations with and without yields (Yj = [Cj/ ΣCi0] × 10 , where j = CO and H2) hydrogen permeation under different steady-state with and without hydrogen permeation (CH2 = conditions. The experimental results obtained under CH2permeated + CH2reactor exit). The hydrogen yield was
model without permeation 13.5 model with permeation model with permeation 5 model without permeation 13.0 experimental results without permeation experimental results without permeation 12.5 experimental results with permeation )
3 experimental results with permeation 12.0 4 ) 3 11.5
11.0 3 10.5 10.0 2 9.5 9.0 Concentration (mol/m Concentration 2 8.5 1 concentration (mol/m concentration 4 8.0 CO CH 7.5 0 7.0
6.5 760 780 800 820 840 860 880 900 760 780 800 820 840 860 880 900 Temperature (K) Temperature (K) (a) (b) Figure 6: Model predictions and experimental concentrations as a function of temperature for steady- state operation with and without hydrogen permeation for methane-carbon dioxide biogas reforming in a membrane reactor for reactants (a) CH4 and (b) CO2, under the following operating conditions: mcat = -1 5 3 0.02 × 10 kg, P = 1.01 × 10 Pa, CH4/ CO2 = 2.85, and τ = 1,204.8 kg.s/m .
Brazilian Journal of Chemical Engineering Vol. 32, No. 01, pp. 201 - 210, January - March, 2015
208 F. S. A. Silva, M. Benachour and C. A. M. Abreu
model without permeation 6 model with permeation experimental results without permeation 10 5 experimental results with permeation ) 3 8 )
3 4
3 6
2 4
model without permeation 1 2 Concentration (mol/m Concentration
CO Concentration (mol/m Concentration CO model with permeation 2
H experimental results without permeation 0 0 experimental results with permeation
-1 760 780 800 820 840 860 880 900 760 780 800 820 840 860 880 900 Temperature (K) Temperature (K) (a) (b) Figure 7: Model predictions and experimental concentrations as a function of temperature for steady- state operation with and without hydrogen permeation for methane-carbon dioxide biogas reforming in a membrane reactor, showing products (a) H2 and (b) CO under the following operating conditions: mcat = -1 5 3 0.02 × 10 kg, P = 1.01 × 10 Pa, CH4/CO2 = 2.85, and τ = 1,204.8 kg.s/m .
70 model without permeation 65 model with permeation 60 60 experimental results without permeation 55 experimental results with permeation 50 50 45 40 40 35 30 30 25 Yield (%) Yield 2 20 (%) Yield CO 20 H 15 model without permeation 10 10 5 model with permeation experimental results without permeation 0 0 experimental results with permeation -5
760 780 800 820 840 860 880 900 760 780 800 820 840 860 880 900 Temperature (K) Temperature (K)
(a) (b) Figure 8: Model predictions and experimental product yields for (a) H2 and (b) CO for operations with and without hydrogen permeation for methane-carbon dioxide reforming in a membrane reactor under the -1 5 following operating conditions: mcat = 0.02 × 10 kg, P = 10.1 × 10 Pa, CO2/CH4 = 0.35, and τ = 1,204.8 kg.s/m3.
predicted to increase strongly with the temperature CONCLUSIONS and more so for operation with permeation. A higher carbon monoxide yield was predicted for A fixed-bed reactor with a Pd-Ag/H2 selective operation without permeation than with permeation membrane was used to convert biogas into syngas by at 873 K. a reforming process. The performance of a nickel An increased sensitivity to thermal effects was catalyst (3.31% weight)/γ-Al2O3) was evaluated at predicted using Sieverts equation for hydrogen mass 773 K, 823 K, and 873 K and 1.01 × 105 Pa with and transfer by permeation for operation at temperatures without hydrogen permeation. Operation with per- above 840 K. Thus, hydrogen permeation increased meation at 873 K increased the biogas methane con- rapidly for operations at temperatures above 840 K, version to approximately 83%, and the hydrogen and the hydrogen production (Figure 8(a)) increased yield was 113% higher than that for operation with- via methane cracking. out hydrogen permeation.
Brazilian Journal of Chemical Engineering
Evaluating Hydrogen Production in Biogas Reforming in a Membrane Reactor 209
A mathematical model was formulated to predict X Conversion % the evolution of the effluent concentrations of the Y Yield % membrane reactor as a function of the operating tem- perature. At temperatures lower than 823 K, similar Greek Letters evolution profiles were predicted for the hydrogen and carbon monoxide yields for operations with and ε Bed porosity (-) ρ Density kg/m3 without hydrogen permeation. The hydrogen yield 3 reached approximately 21% at 823 K and 47% at 873 ρg Density of gaseous mixture kg/m K under hydrogen permeation conditions. τ Spatial time kg/s.m3 γ Gamma (-) Δ H Heat of reaction J/mol ACKNOWLEDGMENTS δm Thickness m
The authors acknowledge the financial support pro- Suffix vided by CAPES, FINEP, and PETROBRAS, Brazil for this study. i Components (CH4, CO2, CO, H2, and H2O) j Reaction steps NOTATIONS rH2 Hydrogen from reaction side pH2 Hydrogen partial pressure A Membrane area m2 from permeation side C Concentration mol/m3 R Reactor Cp Heat capacity J/mol.K RE Reference cat Catalyst dm Inside membrane diameter m M Mixture DR Outside reactor diameter m ! dp Catalyst pore diameter μm
E D Diffusivity activation energy J/mol REFERENCES hm Membrane height m HR Reactor height m Abreu, C. A. M., Santos, D. A., Pacífico, J. A., Filho, 2 JH2 Permeation flux mol/m .s N. M. L., Kinetic evaluation of methane-carbon 2 0.5 JH20 Pre-exponential factor for mol/m .s.Pa dioxide reforming process based on the reaction permeation flux steps. Ind. Eng. Chem., 47, p. 4617-4622 (2008). k1 Kinetic constant for the step mol/kg.s Amoro, J. N., The multiple roles for catalysis in pro- I reaction duction of H2. Appl. Catal. A: Gen., 176, p. 159- 3 2 k2 Kinetic constant for the step (m ) /mol.kg.s 176 (1999). II reaction Dittmeyer, R., Volker, H., Daub, K., Membrane re- 3 k3 Kinetic constant for the step m /kg.s actors for hydrogen and dehydrogenation process III reaction based on supported palladium. J. Mol. Catal. A: 3 KCH4 Adsorption equilibrium m /mol Chem., 173, p. 135-184 (2001). constant of methane Faroldi, B., Bosko, M. L., Múnera, J., Lombardo, E., Keqwgs Equilibrium constants for (-) Cornaglia, L., Comparison of Ru/La2O2CO3 per- the RWGS reaction formance in two different membrane reactors for KeqCH4 Equilibrium constants for (-) hydrogen production. Catal. Today, 213, p. 135- the methane decomposition 144 (2013). reaction Gallucci, F., Fernandez, E., Corengia, P. and van Sint m Catalyst mass kg Annaland, M., Recent advances on membranes P Pressure Pa and membrane reactors for hydrogen production. rji Rate of consumption/ mol/kgcat.s Chemical Engineering Science, 92, p. 40-66 production of component (2013). R Gas constant J/mol.K Gallucci, F., Tosti, S., Basile, A., Pd–Ag tubular 2 SBET Specific surface area m /g membrane reactors for methane dry reforming: A T Temperature K reactive method for CO2 consumption and H2 2 U Overall heat transfer J/s.m .K production. Journal of Membrane Science, 317, p. coefficient 96-105 (2008).
Brazilian Journal of Chemical Engineering Vol. 32, No. 01, pp. 201 - 210, January - March, 2015
210 F. S. A. Silva, M. Benachour and C. A. M. Abreu
Galuszka, J., Pandey, R. N., Ahmed, S., Methane- Pompeo, F., Nichio, N. N., Souza, M. M. V. M., conversion to syngas in a palladium membrane Cesar, D. V., Ferretti, O. A., Schmal, M., Study of reactor. Catal. Today, 46, p .83-89 (1998). Ni and Pt catalysts supported on α−Al O and 23 García-García, F. R., Soria, M. A., Mateos-Pedrero, ZrO2 applied in methane reforming with CO2. C., Guerrero-Ruiz, A., Rodríguez Ramos, I., Li, K., Appl. Catal. A: Gen., 316, p. 175-183 (2007). Dry reforming of methane using Pd-based mem- Qian, X., Koerner, R. M., Gray, D. H., Geotechnical brane reactors fabricated from different sub- Aspects of Landfill Design and Construction. strates. Journal of Membrane Science, 435, p. First Ed., Prentice Hall, New York (2002). 218-225 (2013). Rival, O., Grandjean, B. P. A., Guy, C., Sayari, A., Kikuchi, E., Palladium/ceramic membranes for se- Larachi, F., Oxygen-free methane aromatization lective hydrogen permeation and their application in a catalytic membrane reactor. Kinet. Catal. Re- to membrane. Catal. Today, 25, p. 333-337 (1995). act. Eng., 40, p. 2212-2219 (2001). Kolbitsch, P., Pfeifer, C., Hofbauer, H., Catalytic steam Shu, J., Grandjean, B. P. A., Van Neste, A., Kaliaguine, reforming of model biogas. Fuel, 87, p. 701-706 S., Catalytic palladium based membrane reactors: (2008). A review. Can. J. Chem. Eng., 69, p. 1036-1060 Kumar, S., Agrawal, M., Kumar, S., Jilani, S., The (1991). production of syngas by dry reforming in mem- Silva, F. A., Hori, C. E., da Silva, A. M., Mattos, L. brane reactor using alumina-supported Rh cata- V., Munera, J., Cornaglia, L., Noronha, F. B., lyst: A simulation study. J. Chem. React. Eng., 6, Lombardo, N. E., Hydrogen production through A-109, p. 1-39 (2008). CO2 reforming of CH4 over Pt/CeZrO2/Al2O3 Lau, C. S., Tsolakis, A., Wyszynski, M. L., Biogas catalysts using a Pd–Ag membrane reactor. Catal. upgrade to syn-gas (H2–CO) via dry and oxida- Today, 193, p. 64-73 (2012). tive reforming. Int. J. Hydrogen Energ., 36, p. Themelis, N. J., Ulloa, P. A., Methane generation in 397-404 (2011). landfills. Renew. Energ., 32, p. 1243-1257 (2007). Munera, J., Irusta, S., Cornaglia, L., Lombrado, E., Topalidis, A., Petrakis,D. E., Ladavos, A., Loukatzikou, CO2 reforming of methane as a source of hydro- L., Pomonis,P. J., A kinetic study of methane and gen using a membrane reactor. Appl. Catal. A: carbon dioxide interconversion over 0.5%Pt/ SrTiO3 Gen., 245, p. 383-395 (2003). catalysts. Catal. Today, 127, p. 238-245 2007).
Brazilian Journal of Chemical Engineering