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Ind. Eng. Chem. Res. 2005, 44, 4577-4585

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Hydrogen Production by Methanol Reforming in Supercritical Water: Suppression of Methane Formation Jayant B. Gadhe and Ram B. Gupta* Department of Chemical Engineering, Auburn University, Auburn, Alabama 36849-5127

The reforming of methanol is carried out in supercritical water at 276 bar and 700 °C to produce H2 along with CO, CH4, and CO2. The reactions are catalyzed by the wall of the tubular reactor made of Inconel 600, which is an alloy of Ni, Cr, and Fe. Experiments are conducted to study the effects of the pressure, residence time, and steam-to-carbon ratio on the H2 yield. The residence time is varied by changing the length of the reactor as well as the feed flow rate. Both the experimental results and equilibrium calculations show that, as the pressure is increased, methanation of CO and CO2 takes place, resulting in a loss of H2. In addition, the methane formation is favored at a high residence time and low steam-to-carbon ratio. In this study, the following three strategies are proposed for the suppression of methane formation during the production of H2 from methanol in supercritical water: (1) operation at a low residence time by having a small reactor length or a high feed flow rate; (2) addition of a small amount of K2CO3 or KOH in the feed; (3) utilization of the surface catalytic activity of the reactor made of Ni-Cu alloy. All three of these strategies resulted in a significant reduction in the methane formation and corresponding enhancement in the H2 production. Introduction

Table 1. Thermochemical Properties of Water at Various Pressures3

Over the past few years, there has been a growing interest in the development of proton exchange membrane fuel cells for various transportation applications. This has resulted in a need for compact onboard and stationary reformers, which can supply high-purity hydrogen at low cost. The development of a process to produce H2 at a very high pressure is an attractive approach for mobile and portable applications owing to its low storage volume. The major technologies1,2 for the commercial production of H2 are steam reforming, partial oxidation, and electrolysis of water. The most common technology is steam methane reforming (SMR), which catalytically converts methane to produce a mixture of H2 and CO according to reaction 1. It is followed by a water gas

CH4(g) + H2O(g) S CO(g) + 3H2(g) ∆H298K ) +206.2 kJ/mol (1) shift reaction (reaction 5) to produce a mixture of H2 and CO2 along with the unreacted CO. Some popular modifications of SMR are catalytic partial oxidation of methane (POM) and autothermal reforming (ATR), where heat required for reactions is supplied by internal partial combustion of feed with oxygen or air (reaction 2). POM consists of substoichiometric oxidation of methane, while ATR integrates POM with SMR.2 Hy-

CH4(g) + 1/2O2(g) S CO(g) + 2H2(g) ∆H298K ) -35.7 kJ/mol (2) drogen is also produced on a limited scale by the * To whom correspondence should be addressed. Tel.: (334) 844-2013. Fax: (334) 844-2063. E-mail: [email protected].

temp, °C

pressure, bar

density, g/mL

specific heat, J/mol‚K

viscosity, µPa‚s

thermal conductivity, W/m‚K

700 700 700 700 700

34 69 138 207 276

0.008 0.016 0.032 0.049 0.067

41.9 42.9 45.1 47.5 50.1

36.7 36.9 37.3 37.9 38.6

0.095 0.098 0.103 0.110 0.118

electrolysis of water (reaction 3), which however suffers from the drawback of low energy efficiency. Other new

H2O(l) S H2(g) + 1/2O2(g) ∆H298K ) +285.8 kJ/mol (3) production methods include the splitting of water by sources such as heat or light and generation of H2 by coal or biomass gasification. Supercritical water is gaining in popularity as a reaction medium owing to fast heat and mass transfers. The thermochemical properties of water3 at various pressures are summarized in Table 1. The reforming of hydrocarbons can be carried out in the presence of supercritical water instead of steam, as used in conventional technologies, to produce H2 at a very high pressure. The advantages of carrying out the reforming reactions in supercritical water over conventional technologies are as follows.4-13 The density of supercritical water is higher than that of steam, which results in a high space-time yield, and the higher values of thermal conductivity and specific heat of supercritical water are beneficial to carry out the endothermic reforming reaction (Table 1). H2 is produced at a high pressure, which can be stored directly, thus avoiding the problems associated with its compression. The process becomes economical as the compression work is reduced owing

10.1021/ie049268f CCC: $30.25 © 2005 American Chemical Society Published on Web 05/27/2005

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to the low compressibility of liquid feed compared to that of gaseous H2.4 Hydrocarbons are completely soluble in supercritical water, which minimizes the formation of char or slag, which may otherwise lead to catalyst deactivation. This is particularly important in the generation of H2 from heavy oils such as diesel.5,6 The generation of H2 by the steam reforming of biomass leads to the formation of significant amounts of tar and char, which limits the yield of H2, and the gaseous product contains higher hydrocarbons in addition to the desired light gases. Researchers have carried out the catalytic gasification of biomass or its model compounds in supercritical water to produce H2-rich gas with effectively no tar or char formation.7-13 There have been a few studies to investigate the H2 production in supercritical water from a variety of organic feedstocks such as methane,14 methanol,4,15 ethanol,16 glucose, and glycerol.8,9,17 Methanol is a good choice as a feedstock for reforming because of its high hydrogen-to-carbon ratio and the absence of carboncarbon bonds. The high hydrogen-to-carbon ratio makes the steam reforming of methanol energetically favorable, while the absence of a carbon-carbon bond reduces the formation of soot.18,19 In addition, methanol is in the liquid state under normal conditions and hence can be stored and pumped easily. It is well-known that methanol reforming takes place at high temperatures in the presence of catalysts such as Ni, Cu, or Zn. In addition, promoters such as Cr and Zr are used to promote the activity of catalysts.20-22 Taylor et al.15 and Boukis et al.4 have used a simple tubular reactor made of a Ni alloy to carry out methanol reforming in supercritical water. In this reactor configuration, the inside wall of the tubing provides the catalytic surface area. The same reactor configuration is chosen here because of its compactness and simplicity of design. In this study, the reforming of methanol is carried out in a tubular reactor made of Inconel 600, which is an alloy of Ni, Cr, and Fe. The metals catalyzing methanation reactions are arranged in decreasing order of methanation activity as Ru > Ni > Co > Fe > Mo. This shows that Ni is a strong methanation catalyst. Most investigations of methanation are carried out using Ni-based catalysts, although Fe is also found to be active. In addition, some investigators have also used Cr-based Ni catalysts to carry out the methanation reactions.23,24 Thus, the Inconel 600 reactor has a significant methanation activity because of the presence of Ni, Fe, and Cr. The major reaction steps4,18,25,26 involved in the methanol reforming are as follows:

Methanation of CO: CO(g) + 3H2(g) S CH4(g) + H2O(g) ∆H298K ) -206.2 kJ/mol (7) Methanation of CO2: CO2(g) + 4H2(g) S CH4(g) + 2H2O(g) ∆H298K ) -165.0 kJ/mol (8) In addition, the following side reactions, which are responsible for the carbon formation, can take place.18,27,28

Methane decomposition (cracking): CH4(g) S C(s) + 2H2(g)

∆H298K ) +74.9 kJ/mol (9)

Boudouard coking (CO disproportionation): 2CO(g) S C(s) + CO2(g)

∆H298K ) -172.4 kJ/mol (10)

Coke gasification: CO(g) + H2(g) S C(s) + H2O(g) ∆H298K ) -131.3 kJ/mol (11) Reactions 4 and 9 are endothermic and hence are favored at higher temperatures. Reaction 6, which is the combined reaction of 4 and 5, is also endothermic. Reactions 7 and 8 involve a decrease in the number of moles in their stoichiometry, making them favorable at higher pressures. Conversely, reactions 4 and 9 are favored at lower pressures. The dependence of carbon formation on pressure is complicated. Carbon formation by methane decomposition (reaction 9) is hindered at higher pressures. However, methane formation is substantial at higher pressures because of reactions 7 and 8. On the basis of our equilibrium calculations performed using the Gibbs free energy minimization method (RGIBBS module) in ASPEN+ and the Peng-Robinson equation of state, carbon formation was not observed, as shown in Figure 1. The theoretical maximum value of the molar H2 yield is 3 mol of H2/mol of methanol, according to reaction 6. Figure 1 shows that, as the pressure is increased, there is an increase in the CH4 moles and a decrease in the H2, CO, and CO2 moles. A

Methanol decomposition: CH3OH(g) S CO(g) + 2H2(g) ∆H298K ) +91.3 kJ/mol (4) Water gas shift reaction: CO(g) + H2O(g) S CO2(g) + H2(g) ∆H298K ) -41.1 kJ/mol (5) Combined reaction of 4 and 5: CH3OH(g) + H2O(g) S CO2(g) + 3H2(g) ∆H298K ) +50.2 kJ/mol (6)

Figure 1. Dependence of the gas yield on the pressure using ASPEN+ equilibrium calculations (10 wt % methanol, 700 °C).

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Figure 2. Experimental apparatus used for methanol reforming in supercritical water.

decrease in the CO moles is advantageous for the fuel cell applications.29 A decrease in the H2, CO, and CO2 moles suggests that methanation of CO (reaction 7) and CO2 (reaction 8) is favored at higher pressures. Methanation of CO results in a loss of 3 mol of H2, while methanation of CO2 results in a loss of 4 mol of H2. To enjoy the benefits of the reforming of methanol in supercritical water that are mentioned before, it is important to prevent the loss of H2 by minimizing the methanation reactions. This paper examines the strategies for the suppression of methane. The effects of various process parameters such as pressure, residence time, steam-to-carbon ratio, and catalyst are studied. Experimental Section Materials. Methanol (99.9% pure), KOH (88.7% assay), and K2CO3‚11/2H2O (99.9% assay) were obtained from Fisher Scientific and used as received. Apparatus. The reforming of methanol was carried out in a tubular reactor made of Inconel 600 (Microgroup) having a composition of 73% Ni, 18% Cr, and 9% Fe. The dimensions of the reactor were 0.125 in. o.d. and 0.085 in. i.d. The reactors having three different lengths (0.5, 1, and 2 m) were used in the study. Later the reactor was replaced with a 1-m-long tubing made of Ni-Cu alloy (Supelco) having a composition of 67% Ni and 33% Cu. The other dimensions of the tubing were kept the same. The fittings and other tubings used in the apparatus were made of stainless steel (SS) 316 (High-Pressure Equipment Company) having an approximate composition of 69% Fe, 17% Cr, 12% Ni, and 2% Mo. Figure 2 shows a schematic of the apparatus used in the study. Aqueous methanol from the feed tank was pumped to the reactor using a high-performance liquid chromatograph pump (Waters 590). The feed tank was covered on top to avoid the loss of methanol by evaporation. The reactor was heated using a tube furnace equipped with a temperature controller (Thermolyne 21100). The reactor temperature at the exit of the

furnace was measured by using a type-K thermocouple with a T arrangement. Both ends of the tube furnace were covered properly to avoid heat loss and achieve uniform temperature. The gas mixture exiting the reactor was cooled using an air-cooled heat exchanger made of SS 316 tubing. Pressure was measured by a pressure gauge P. The pressure was decreased to ambient by means of a backpressure regulator (Straval). The gas-liquid mixture was separated in a glass phase separator having gastight valves to prevent the escape of gases. The flow rate of the gases was measured using a gas flowmeter (Omega FMA-1600). A six-port injection valve (Valco) having a 100-µL sample loop was used for the online sample injection. The gas composition was measured using a gas chromatograph (GC; Varian 3700) equipped with a thermal conductivity detector and a 60/ 80 Carboxen-1000 carbon molecular sieve column (Supelco) having dimensions of 15 ft × 1/8 in. Helium was used as the carrier gas. The GC was calibrated using a standard gas mixture of known composition (BOC gases; 60% H2, 15% CO, 5% CH4, and 20% CO2). The mass flow rate of the liquid coming out of the phase separator was measured using a balance. The total organic carbon (TOC) content of the liquid was analyzed using a TOC analyzer (OI Analytical model 700). The liquid was diluted appropriately to obtain the TOC readings within the range of the instrument. The flowmeter (FMA-1600) was used in the H2 mode, and it generated 30 readings of the volumetric flow rate per second based on the built-in properties of pure H2. These instantaneous volumetric flow rates were acquired on a computer via a RS-232 port and corrected for pressure and temperature. The average volumetric flow rate was determined by totalizing the flow for a period of 15 min. This average gas flow rate, which corresponds to pure H2, was corrected to the actual gas coming out of the phase separator by estimating the viscosity of the gas mixture by Wilke’s semiempirical formula30 with the knowledge of the gas composition obtained from the GC analysis. This flow measurement

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Ind. Eng. Chem. Res., Vol. 44, No. 13, 2005 Table 3. Approximate Residence Times (τ) and Reynolds Numbers (Re) at Different Flow Rates feed flow rate 0.5 mL/min τ, s Re

T (°C), P (bar), reactor length (m) 700 °C, 276 bar, 2 m (Figures 5, 7, and 11) 700 °C, 276 bar, 1 m (Figures 6 and 7) 700 °C, 276 bar, 0.5 m (Figure 7)

Figure 3. Temperature profiles (actual points and calculated lines) along the length for 0.5-, 1-, and 2-m-long reactors. Conditions: 10 wt % methanol, feed ) 0.5 mL/min, and furnace temperature ) 700 °C.

method was checked and confirmed for accuracy by flowing the calibration gas consisting of H2, CO, CH4, and CO2 of known composition at several flow rates and measuring the actual flow by a soap-bubble flowmeter. The aqueous methanol feed was prepared before each experimental run by mixing the appropriate amounts of distilled water and methanol and used directly without sparging to remove the dissolved gases. The oxygen dissolved in water can lead to oxidation of methane (reaction 2), methanol (reaction 12), or CO (reaction 13) to form H2 and CO, H2 and CO2, or CO2, respectively.31

CH3OH(g) + 1/2O2(g) S CO2(g) + 2H2(g) ∆H298K ) -191.6 kJ/mol (12) CO(g) + 1/2O2(g) S CO2(g) ∆H298K ) -283.0 kJ/mol (13) The dissolved oxygen would most likely react with H2 to form water according to the reverse reaction of reaction 3, thus consuming H2. However, because of the very low solubility of oxygen in water (≈9 mg/L at 20 °C),32 the effect of these reactions on the H2 yield is expected to be negligible. The temperature profile along the length of the reactor is likely to have an impact on the results because the feed was not preheated. In addition to this, because the product gases were air-cooled in the SS 316 heat exchanger, instead of rapid quenching in water, the relatively higher reactor temperatures outside the furnace might also affect the gas yields. The temperature profiles along the reactor and heat exchanger are shown in Figure 3. The temperatures measured by the thermocouple at the exit of the reactor were slightly lower than the furnace temperature. To get an idea

1.0 mL/min τ, s Re

2.0 mL/min τ, s Re

59

127

29

255

15

509

29

127

15

255

7

509

15

127

7

255

4

509

about the full temperature profile, a FORTRAN program was used that was developed in-house based on the reported rate expressions and parameters for reactions 4-11. It is not discussed here, however, the information can be obtained from refs 25-28 and 3335. Figure 3 shows that the reactor temperature reaches 700 °C at about 0.5-m length and remains steady. The temperature drop after the furnace is not abrupt because the product gases were air-cooled. These relatively higher temperatures in the SS 316 environment can also affect the yields because of its catalytic activity. The Reynolds numbers (Re) and residence times (τ) at various experimental conditions are calculated by taking into account the properties of supercritical water as an approximation because the P-V-T data for the six-component system consisting of CH3OH, H2O, H2, CO, CH4, and CO2 are unavailable. The values of the Reynolds number and residence time at various experimental conditions are shown in Tables 2 and 3. The Reynolds number is found to range from 127 to 509. These low Reynolds numbers indicate the existence of laminar flow in the reactor, which may not be a serious issue because CH3OH, H2O, H2, CO, CH4, and CO2 form a one-phase mixture at the experimental conditions of supercritical water.4 However, the lack of mixing and turbulence in the reactor is likely to cause the accumulation of particles inside the reactor when aqueous methanol containing KOH or K2CO3 is fed, and this might affect the gas yields. The residence time ranged from 2 to 59 s (Tables 2 and 3). The relatively higher values of the residence time resulted in the nearequilibrium yields in most cases. Procedure. Before each experiment was started, distilled water was pumped through the system and pressurized to the desired pressure by adjusting the backpressure regulator. After a steady pressure was reached, the tube furnace was switched on to heat the reactor. After a steady temperature was achieved, the feed was replaced with aqueous methanol. The steady state was marked by a constant temperature at the exit of the reactor, the typical time of which was about 1 h. The gas analysis was done at least three times to get a constant gas composition. The gas flow was totalized for a period of 15 min, and the average gas flow rate was calculated using the method discussed before. The gas flow measurement was repeated three times. After

Table 2. Approximate Residence Times (τ) and Reynolds Numbers (Re) at Different Pressures and Temperatures pressure T (°C), reactor length (m), feed rate (mL/min)

τ, s

34 bar Re

τ, s

69 bar Re

138 bar τ, s Re

207 bar τ, s Re

276 bar τ, s Re

700 °C, 2 m, 0.5 mL/min (Figure 4) 700 °C, 1 m, 0.5 mL/min (Figure 8) 600 °C, 1 m, 1.0 mL/min (Figure 10)

7 3 2

134 134 300

14 7 4

133 133 299

28 14 8

43 22 13

59 29 17

132 132 295

130 130 289

127 127 282

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completion of each experiment, the feed was switched back to distilled water to flush the reactor. To study the dependence of the pressure on the H2 yield, experiments were conducted at 34, 69, 138, 207, and 276 bar. The residence time behavior was investigated by conducting experiments at flow rates of 0.5, 1.0, and 2.0 mL/min. To minimize interference due to the heat effect, flow effect, and availability of the reactor surface area in the investigation of the residence time, additional experiments were conducted using three reactors of 0.5-, 1-, and 2-m length. The effect of the steam-to-carbon ratio was studied by feeding aqueous methanol having concentrations of 5, 10, 15, 20, and 30 wt %, which corresponded to the approximate steamto-carbon ratios of 34, 16, 10, 7, and 4, respectively. Later the pressure dependence was studied using the Ni-Cu reactor to investigate the catalytic activity of the Ni-Cu reactor. To study the effect of alkali promoters, aqueous methanol containing KOH and K2CO3 was fed to the reactor in separate experiments. No measurable quantity of carbon was detected during the course of the experiments over several months. This is probably due to the high steam-tocarbon ratios and relatively lower temperatures employed in the experiments. Complete conversions were obtained with the 1- and 2-m-long reactors, while the 0.5-m-long reactor resulted in incomplete conversion. Calculations. The molar flow rate of the feed was calculated based on the concentration of methanol and feed flow rate. The molar flow rates of the product gases were calculated based on the volumetric gas flow rate and dry gas composition obtained from the GC. The carbon content of the liquid stream was calculated knowing the TOC value of the liquid. The liquid was assumed to be saturated with CO2 in the phase separator, and the dissolved CO2 was calculated using the saturation solubility of 0.0391 mol/L.36,37 The calculations were checked for the overall carbon balance of the system. The error in the overall carbon balance was found to be less than 10%. Scattering in the data of the totalized gas flow rate measured by the flowmeter was less than 1%. The error in the dry gas composition obtained by the GC analysis was typically less than 2%. The overall error in the calculation of the gas yields due to the errors introduced by the individual analysis techniques and experimental error was found to be less than 5%. Results and Discussion Effect of the Pressure. The effect of the pressure on the gas yield was studied by feeding 10 wt % methanol at a flow rate of 0.5 mL/min to the 2-m-long reactor at 700 °C. This is illustrated in Figure 4. As the pressure is raised from 34 to 276 bar, the H2, CO, and CO2 moles decrease while the CH4 moles increase as anticipated, suggesting that the methanation reactions of both CO (reaction 7) and CO2 (reaction 8) are favored at the higher pressures. The H2 yield (H2 moles per CH3OH moles) has dropped from 2.75 at 34 bar to 1.50 at 276 bar, while the CH4 yield (CH4 moles per CH3OH moles) has gone up from 0.03 at 34 bar to 0.24 at 276 bar. The increase in the density at higher pressures leads to an increase in the residence time, which can be seen from Table 2. The residence time ranged from 7 s at 34 bar to 59 s at 276 bar, which resulted in the near-equilibrium yields, as shown in Figure 4. The flow conditions did not vary much with pressure because

Figure 4. Effect of the pressure on the gas yield. Experimental conditions: 10 wt % methanol, feed ) 0.5 mL/min, 700 °C, and reactor length ) 2 m.

Figure 5. Effect of the residence time (feed flow rate) on the gas yield. Experimental conditions: 10 wt % methanol, 276 bar, 700 °C, and reactor length ) 2 m.

there is not any appreciable change in the Reynolds number (Table 2). Effect of the Residence Time. To investigate the residence time effect, the feed flow rate was increased from 0.5 to 2.0 mL/min in the 2-m-long reactor at 700 °C and 276 bar. Figure 5 shows that there is not much change in the gas yields obtained at the flow rates of 0.5 and 1.0 mL/min. As the flow rate is increased further to 2.0 mL/min, there is an increase in the H2, CO, and CO2 moles, which is coupled with a slight decrease in the CH4 moles. The residence time decreased from 59 s at 0.5 mL/min to 15 s at 2.0 mL/min, which is responsible for the reduction in CH4 (Table 3). The residence time was decreased further to 7 s by conducting the above experiment in the 1-m-long reactor while keeping other conditions the same. Figure 6 shows that there is a further increase in the H2 yield from a value of 1.89 to 2.51 and a decrease in the CH4 yield from a value of 0.10 to 0.03 as the flow rate is increased from 0.5 to 2.0 mL/min. The reduced amounts of CH4 at 2.0 mL/min suggest that there might be a decrease in the extent of methanation reactions due to the lower residence time.

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Figure 6. Effect of the residence time (feed flow rate) on the gas yield. Experimental conditions: 10 wt % methanol, 276 bar, 700 °C, and reactor length ) 1 m.

Figure 7. Effect of the residence time (reactor length) on the gas yield. Experimental conditions: 10 wt % methanol, feed ) 1 mL/ min, 276 bar, and 700 °C.

However, this might also be due to the changes in the flow pattern inside the reactor. Table 3 shows that the Reynolds number increased from 127 to 509 as the flow rate was increased from 0.5 to 2.0 mL/min. The increased mixing at the higher flow rates can affect the reactions and gas yields. The higher flow rates could also result in the lower reactor temperatures because of insufficient heating, which however would increase the CH4 yield considering the exothermic nature of the methanation reactions. The changes in the initial heating profiles at the different flow rates could also be another reason because the feed was not preheated. To minimize the interference due to the abovementioned factors, the residence time was varied by feeding 10 wt % methanol at a constant flow rate to the reactors having different lengths at 276 bar and 700 °C. Figure 7 shows that the H2 yield is low for the 0.5-mlong reactor. This is due to the incomplete conversion, which was confirmed by detecting unreacted methanol in the liquid. As the reactor length is increased to 1 m, the H2 yield increased because of the complete conversion and reached a value that is well above the equilibrium yield. A further increase in the reactor length

Figure 8. Effect of the pressure on the gas yield. Experimental conditions: 10 wt % methanol, 700 °C, reactor length ) 1 m, and feed flow rate ) 0.5 mL/min.

to 2 m, however, resulted in a decrease in the H2, CO, and CO2 moles and and increase in the CH4 moles. The identical flow conditions (Re ) 255) and initial heating profiles ensured minimal interference from the factors other than the residence time. The high residence time of 29 s for the 2-m-long reactor resulted in increased CH4 yield and near-equilibrium H2 yield. These results provide additional evidence to the hypothesis that the extent of methanation increases at the higher residence time. The low residence time can prevent the methanation reactions from reaching equilibrium. This makes it possible to control the extent of methanation and H2 loss by operating at a sufficiently low residence time. With the knowledge of the residence time behavior, the pressure dependence on the H2 yield was studied again in the 1-m-long reactor. Figure 8 shows the effect of the pressure when 10 wt % methanol was fed to the 1-m-long reactor at a flow rate of 0.5 mL/ min at 700 °C. The drop in the H2 yield with an increase in the pressure is not drastic, and the H2 yields are higher than the equilibrium yields. A comparison with the earlier results (Figure 4) reveals that the H2 yields are higher in this case. The lower values of the residence time, which ranged from 3 s at 34 bar to 29 s at 276 bar, resulted in reduced CH4 yields. Effect of the Steam-to-Carbon Ratio. The steamto-carbon ratio is an important parameter as far as the economics of the process is concerned, which depends on the concentration of methanol. To study the impact of the steam-to-carbon ratio on the H2 yield, experiments were conducted at methanol concentrations of 5, 10, 15, 20, and 30 wt %. Figure 9 shows that there is a sharp decrease in the H2 and CO2 moles and an increase in the CH4 moles as the steam-to-carbon ratio was decreased. It is obvious that methanation is favored at higher methanol concentrations. The equilibrium of methanation reactions (reactions 7 and 8) is affected by the presence of steam. The higher amounts of steam shift the equilibrium of reactions 7 and 8 backward, leading to a decrease in the CH4 yield. Alternately, the higher amounts of steam shift the equilibrium of the water gas shift reaction (reaction 5) to the right to produce more H2 and CO2. Ni-Cu Alloy Reactor. Ni is a strong methanation catalyst, while Cu is a much weaker hydrogenation catalyst yet active in the water gas shift reaction. Cu is

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Cu with Ni results in a sharp drop in the power of catalysts to hydrogenate CO2 to CH4. Araki and Ponec41 found that the addition of Cu to Ni strongly reduces the rate of methanation of CO. This behavior of the Ni-Cu alloy can be explained based on the mechanism of methanation reactions. It is reported that methanation of CO proceeds via dissociative chemisorption of CO.41,42 Dissociation of CO and H2 (reactions 14 and 15) takes place to form the intermediates, which combine in steps to form CH4 (reaction 16). Cs, Os, and Hs denote the

Figure 9. Effect of the steam-to-carbon ratio (feed concentration) on the gas yield. Experimental conditions: feed ) 1 mL/min, 276 bar, 700 °C, and reactor length ) 2 m.

CO f Cs + Os

(14)

H2 f 2Hs

(15)

Cs + Hs f (CH)s

(16)

dissociated species of C, O, and H, respectively, which are adsorbed on the active sites on the surface of the catalyst. Alloying Ni with Cu results in the dilution of active Ni in an inactive matrix, which diminishes the number and size of Ni clusters that are necessary for the dissociation of CO and deposition of Cs on the surface. Thus, diluting Ni with Cu reduces the number of places where Cs can be formed and held and decreases the extent of methanation of CO.41 It is reported that methanation of CO2 proceeds via its reduction to CO either by the reverse water gas shift reaction (reaction 5) or CO2 dissociation (reaction 17).41,43,44 Hence, alloying Ni with Cu also results in a decrease of CO2 methanation.

CO2 f COs + Os

Figure 10. Effect of the pressure on the gas yield with a Ni-Cu reactor. Experimental conditions: 10 wt % methanol, feed flow rate ) 1 mL/min, 600 °C, and reactor length ) 1 m.

reported as a catalyst for steam reforming of methanol.20,38,39 Lindstrom and Pettersson20 have reported increased H2 yields with catalysts having higher Cu content. Hence, experiments were conducted using the Ni-Cu alloy reactor in this work. Figure 10 shows the effect of the pressure on the molar gas yields for the experiments conducted using 10 wt % methanol fed at a flow rate of 1 mL/min to the 1-m-long Ni-Cu reactor at 600 °C. The pressure variation was studied at 600 °C because of the lower operating temperature of the Ni-Cu tubing. The equilibrium H2 yields at 600 °C are lower than those at 700 °C because of the exothermic nature of methanation reactions. Figure 10 shows that an increase in the pressure has very little effect on the gas yields. The H2 yields, which are higher than the equilibrium H2 yields, remain fairly constant over the entire pressure range. The CH4 yield is found to be almost negligible. This could be the combined effect of the low residence time, which ranged from 2 to 17 s in this case, and possibly the catalytic activity of the NiCu reactor. Cratty and Russell40 have studied the activity of Ni, Cu, and some of their alloys in catalyzing the hydrogenation of CO2 to form CH4 and reported that alloying

(17)

K2CO3 or KOH Doping. Kruse and Dinjus14 reported an increase in the H2 yield with the addition of K2CO3 in the generation of H2 from CH4 in supercritical water. We conducted experiments with the addition of 0.83 wt % K2CO3 in 10 wt % methanol fed to the Inconel 600 reactor (Figure 11). The H2 yield increased considerably with the addition of K2CO3. Onsager et al.45 have reported the generation of H2 from water and CO in the presence of K2CO3. It was proposed that the reaction mechanism proceeds via the formation of an alkali metal formate intermediate (reactions 18-21).

K2CO3 + H2O f KHCO3 + KOH

(18)

KOH + CO f HCOOK

(19)

HCOOK + H2O f KHCO3 + H2

(20)

2KHCO3 f H2O + K2CO3 + CO2

(21)

We conducted experiments with a stoichiometrically equivalent amount of KOH (0.68 wt %), which also resulted in an increase in the H2 yield (Figure 11). However, if the above mechanism alone were responsible for the increase, it would have resulted in a decrease in the CO moles, which is not quite apparent from Figure 11. Sinag et al.46 studied the hydropyrolysis of glucose in supercritical water in the presence of K2CO3 and reported an increase in the H2 yield and a significant decrease in the CO yield. To understand the mechanism of the process along the length of the reactor, experiments were carried out using reactors of different lengths. Table 4 shows the molar flow rates (µmoles/s) of the individual gases at various lengths.

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Table 4. Lengthwise Molar Flow Rates of the Gas Components with K2CO3 and KOH Doping molar gas flow rate, µmol/s feed

feed flow rate, mL/min

10 wt % methanol 10 wt % methanol 10 wt % methanol 10 wt % methanol + 0.83 wt % K2CO3 10 wt % methanol + 0.83 wt % K2CO3 10 wt % methanol + 0.68 wt % KOH 10 wt % methanol + 0.68 wt % KOH 10 wt % methanol + 0.68 wt % KOH

0.5 1.0 2.0 1.0 2.0 0.5 1.0 2.0

reactor length ) 0.5 m H2 CO CH4 CO2

reactor length ) 1 m H2 CO CH4 CO2

reactor length ) 2 m H2 CO CH4 CO2

46 93 219 81 82

46 114 246 109 240

33 65 190 108 222 43 125 267

It was surprising to see a decrease in the molar flow rates of the gases for the 0.5- and 1-m-long reactors. This can be explained with the formation of K2CO3 particles because K2CO3 is insoluble in supercritical water.47 The particles might have deposited on the reactor wall as a result of the lack of turbulence in the reactor because the Reynolds number in this case ranged from 127 to 509. It would expose the lesser surface of the wall to the methanol molecules, leading to lower conversions. As the reactor length is increased to 1 m, the molar flow rates approach the values that are closer to those without K2CO3 addition. There is a dramatic difference in the molar flow rates of the gases for the 2-m-long reactor. There is an increase in the molar flow rates of H2, CO, and CO2, while the molar flow rate of CH4 drops down dramatically with the addition of K2CO3. Schoubye23,48,49 has studied the effect of the carrier and promoter on the methanation activity of Ni catalysts and reported that the K2O promoter in particular had an adverse effect on methanation activity. Similar observations are made by Campbell and Falconer,50 who investigated hydrogenation on K-promoted Ni catalysts and found that K loading decreases the rates of CO2 and CO hydrogenation. A possible change in the catalytic activity of the reactor wall due to its corrosion in the presence of KOH or K2CO3 could also lead to higher H2 yields. Researchers have observed corrosion phenomena in this highpH environment in supercritical water. Habicht et al.51 have investigated Ni-based alloys exposed to supercritical water environments. They observed corrosion on Inconel 625 (Ni-Cr-Mo) alloy exposed to 5 wt %

1 3 6 3 2

4 5 6 1 0

16 32 73 26 32

2 5 13 4 9

3 3 3 1 1

16 37 86 35 78

1 1 7 4 8 1 5 10

6 11 19 1 1 2 1 1

12 25 65 35 74 15 42 90

aqueous methanol containing 3000 ppm K2CO3 at 400 bar and 700 °C, leading to the depletion of Ni and Mo due to leaching. Antal et al.12 have reported that the inside wall of the Hastelloy (Ni-Cr-Mo-Co) reactor corroded and the metals were leached out when it was used in the production of H2 from biomass in supercritical water. Conclusions The reforming of methanol was carried out in supercritical water in a tubular reactor made of Inconel 600 to produce H2 along with CO, CH4, and CO2. The factors favoring the methanation reactions are high pressure, high residence time, and low steam-to-carbon ratio. Methanation can be reduced greatly by lowering the residence time so that the equilibrium is not allowed to be reached. The reactor made of Ni-Cu tubing minimizes the formation of CH4. Methanation can also be reduced with the addition of K2CO3 or KOH in the aqueous methanol feed. The reasons to account for the decrease in the CH4 yield by these methods are discussed. Acknowledgment We acknowledge the financial support of this study by the Consortium of Fossil Fuel Science and the United States Department of Energy (DOE Cooperative Agreement DE-FC26-02NT41594). The authors also acknowledge Dr. Pradeep Prasad for the FORTRAN code that was used to generate the temperature profiles in Figure 3 and Mr. Joe Aderholdt for constructing the experimental apparatus. Literature Cited

Figure 11. Effect of K2CO3 and KOH addition. Experimental conditions: 10 wt % methanol, feed flow rate ) 1 mL/min, 700 °C, and reactor length ) 2 m.

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Received for review August 11, 2004 Revised manuscript received April 20, 2005 Accepted May 3, 2005 IE049268F