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Ind. Eng. Chem. Res. 2003, 42, 728-735
Methanol Reforming in Supercritical Water N. Boukis,* V. Diem, W. Habicht, and E. Dinjus Institut fuer Technische Chemie, Chemisch-Physikalische Verfahren (ITC-CPV), Forschungszentrum Karlsruhe GmbH, Karlsruhe, Germany
The production of hydrogen by the reforming of methanol was studied in a continuously operated tubular reactor made of the nickel-based alloy Inconel 625. Experiments were performed at pressures from 25 to 45 MPa and temperatures in the range of 400-600 °C. The concentration of the aqueous feed varied from 5 to 64 wt % methanol. Residence times under reaction temperature conditions varied in the range from 3 to 100 s. The main component of the product gas is hydrogen, with smaller amounts of carbon dioxide, carbon monoxide, and methane. Methanol conversion is up to 99.9% without addition of a catalyst. Obviously, the heavy metals on the inner surface of the reactor influence the composition of the product gas and the conversion rate. Oxidation of the reactor inner surface before gasification turned out to enhance the reaction rate and to decrease the carbon monoxide concentration. Table 1. Critical Data for Pure Water and Methanol15
Introduction During the past decade, there has been increasing worldwide interest in methods for reducing the local emissions produced by cars. The most promising possibility for reducing emissions is the use of hydrogen for fuel-cell-powered vehicles, which produces only water vapor as off-gas. The chemical storage of hydrogen, e.g., as methanol, is one possibility for handling this fuel. Hydrogen can be produced onboard a vehicle by steam reforming or partial oxidation. The need for an onboard methanol reformer has resulted in increased R&D work on the methanol steam reforming process during the past few years.1-7 The present work focuses on the possibility of producing hydrogen from methanol via steam reforming under supercritical water conditions. In 1921, J. A. Christiansen discovered that a mixture of methanol and water vapor could be decomposed at 250 °C using copper metal as a catalyst.8-10 A later development was the use of CuO-ZnO or CuO-Cr2O3 catalysts with molar ratios of water to methanol up to 1.5 and pressures of 0.7-3 MPa.11-13 The main components of the product gas are hydrogen, carbon dioxide, smaller amounts of carbon monoxide, and even lower concentrations of methane. These catalysts are rapidly deactivated by impurities such as hydrogen sulfide and chlorine compounds and by higher temperatures. Higher alcohols, which can be found as impurities in commercial methanol, can also reduce the activity of the catalyst. The process can be described by five overall reactions, only three of which are independent: (1) methanol decomposition, (2) the water-gas shift reaction, (3) methanol steam reforming as the sum of reactions 1 and 2, (4) the methanation of carbon monoxide, and (5) the methanation of carbon dioxide.14 Reactions 1 and 3 are endothermic, the water-gas shift reaction 2 is slightly exothermic, and both methanation reactions 4 and 5 are exothermic. The overall methanol reforming reaction 3 is endothermic and thus favored at higher temperatures. CH3OH CO + H2O CH3OH + H2O CO + 3H2 CO2 + 4H2
a a a a a
CO + 2H2 CO2 + H2 CO2 + 3H2 CH4 + H2O CH4 + 2H2O
∆H°298 ) +91.7 kJ/mol ∆H°298 ) -41 kJ/mol ∆H°298 ) +50.7 kJ/mol ∆H°298 ) -211 kJ/mol ∆H°298 ) -223 kJ/mol
(1) (2) (3) (4) (5)
critical pressure (MPa) critical temperature (°C) critical density (kg/m3)
water
methanol
22.1 374 320
8.1 239 270
The equilibrium compositions for the water-gas shift reaction 2 under supercritical water conditions were calculated with ASPEN using the predictive RedlichKwong-Soave equation of state. The computation showed that the equilibrium amount of CO decreases with increasing pressure and increases with increasing temperature. The net effect of increasing pressure and temperature simultaneously on the CO concentration is small. Under supercritical water conditions (Table 1), water; methanol; and the product gases H2, CO, CO2, and CH4 form a one-phase system. Gasification of organic material in supercritical water was first investigated in the 1970s in the U.S.. Since this time, several publications on this topic have appeared.16-18,39 During the same time, even more work has appeared on the process of the oxidation of hazardous waste with oxygen or air in supercritical water.19,20 To investigate reaction kinetics and reaction mechanisms, the oxidation of the model compound methanol with airborne oxygen in supercritical water has been extensively studied.21-31 The results of the first screening experiments on methanol reforming in supercritical water were published by Boukis et al.32,33 Performing the reaction in supercritical water entails major advantages: The density of supercritical water at 600 °C and 250 bar is about 1 order of magnitude higher than the density of water vapor at 250 °C and 5 bar. This leads to a much higher space-time yield. Increased thermal conductivity and higher temperature promote the endothermic reforming reaction. The required compression work is low because of the low compressibility of the liquid fuel. The gas product on the other side is compressed to about 30 MPa, which is optimum for short-time storage. These advantages, together with the first successful experimental results, provided the motivation for the present systematic study.
10.1021/ie020557i CCC: $25.00 © 2003 American Chemical Society Published on Web 01/24/2003
Ind. Eng. Chem. Res., Vol. 42, No. 4, 2003 729
Figure 1. Test bed used for the methanol reforming experiments under supercritical water conditions.
Experimental Section The experiments described in the section Subsequent Experiments were performed with reactors made of Inconel alloy 625. The tubing and the fittings were made of stainless steel SS316. The reactor was a pressure tube with a length of 1000 mm, an outer diameter of 14.3 mm, and an inner diameter of 8.3 mm (see Figure 1). To achieve the required short residence times, the volume of the reactor was reduced by a cylindrical displacer made of stainless steel SS316 (d ) 8.0 mm, l ) 1000 mm), resulting in an annular-gap-shaped reaction space. A movable thermocouple placed inside a capillary tube (1/16 in. diameter) passed through the central bore of this cylindrical displacer recorded the internal temperature profile of the reactor. The temperature gradient between the center of the displacer and the fluid phase was assumed to be negligible. Three separately regulated electrical heating coils were used to adjust the temperature profile. Several additional thermocouples were fixed to the outside of the reactor for temperature measurements. The ends of the tube were cooled to 15 °C. In the preheating zone of the reactor, the temperature increased steeply from 150 to 600 °C within a section of 10 cm length. The reaction zone was isothermal to within an accuracy of (10 °C; its length was 55 cm. At the end of the reaction zone, the temperature dropped from 600 to 200 °C within a distance of 15 cm. The reactors used for the experiments described in the sections First Experiments and Second Set of Experiments were different in terms of the length of the cylindrical rods. The displacers were only installed in the preheater and in the cooling section of the reactor. The residence times in the different sections of the reactor for these experiments (flow rate 1 mL/min) were about 0.8 min at temperatures lower than 450 °C (preheater section), about 0.4 min at temperatures in the range from 450 to 594 °C, 1.1 min in the reaction zone at 594 °C ((21 °C), and about 1.3 min in the cooling section at 590 ( 20 °C. The volume of the isothermal reaction zone was about 20 mL. An HPLC pump (Bischoff model 3010) compressed the aqueous methanol solution at a flow rate of 0.1-4.5 mL/ min (6-270 mL/h). The experimental pressure was in the range of 25-45 MPa. After reaction, the product mixture was expanded to atmospheric pressure by a
back-pressure regulator, and the liquid and gaseous phases were separated in a simple phase separator. Before the first experiment, the reactor was unintentionally used for oxidation work in supercritical water. By this operation, the reactor was preoxidized by a treatment with a solution of H2O2 in water (3 wt %) at 600 °C and 25 MPa for about 50 h. Later, this treatment turned out to be crucial for high conversion rates and high hydrogen yields. The experimental parameters varied were temperature, pressure, residence time, feed concentration, and composition of the reactor inner surface. The product gas composition, the methanol conversion, and the composition of the aqueous effluent were measured. The temperature was increased from 400 °C in 50 °C steps to 600 °C. The lower limit for pressure variation was 25 MPa; the upper limit was 45 MPa. The composition of the aqueous solution varied from 5 to 64 wt % methanol, corresponding to a molar ratio varying from 34 to 1 (mol of H2O)/(mol of CH3OH). The residence time varied in the range of 3-100 s. Experimental measurements of the residence time were not performed. The residence time values were calculated from the flow rate, the density of pure water under reaction conditions, and the free inner volume of the reactor. The physical properties of the mixture, of course, changed during reaction. This change was largest for solutions with high methanol concentrations and high conversions. Methanol in the aqueous solution decomposed, and the gases H2, CO, CO2, and CH4 formed. Because the pVT data for the six-component system are not available, the true residence time cannot be computed. The residence time calculated for pure water is used instead as the best alternative. Gas analysis was carried out using HP 5880 and HP 6890 gas chromatographs. One instrument was operated with nitrogen as the carrier gas to achieve good sensitivity for hydrogen; for the other chromatograph, helium was used as the carrier gas. In the liquid effluent, the residual total organic carbon (TOC) content was measured (Rosemount Dohrmann DC-190 instrument). The CO2 concentration in the aqueous phase was not measured. The amount of CO2 in the aqueous phase can be calculated from the saturation data for CO2 in water under ambient conditions. It is about 0.035 (mol of CO2)/ (kg of H2O) at 20 °C and 100 kPa.34 Analysis of the liquid product of some experiments, which showed only partial conversion by size-exclusion chromatography, indicated that the methanol content is decisive for the TOC value. Organic acids (formic acid and acetic acid) and aldehydes could be detected only in traces. The amounts of feed and liquid product were measured continuously by two balances (Sartorius AG). The gas flow was determined using a gas meter (TG 05 or TG 3, Ritter Apparatebau GmbH, depending on the actual flow rate). Analysis of the reactor inner surface, after reaction, was performed by SEM (scanning electron microscopy), EDX (energy-dispersive X-ray analysis), AFM (atomic force microscopy), and SNMS (secondary neutral particle mass spectroscopy). The EDX equipment (Link Isis 300, Oxford Microanalysis Group, Wiesbaden, Germany) was operated in connection with a field-emission scanning electron microscope (LEO Corp., Oberkochen, Germany). A Nanoscope IIIa-AFM (DI) instrument was used for the measurement of the surface roughness. The SNMS instrument was developed and operated at IFIA
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Ind. Eng. Chem. Res., Vol. 42, No. 4, 2003
Table 2. Comparison of Results for the Gasification of a 5 wt % Methanol Solution Depending on the Pretreatment of the Reactora reactor
product gasb (vol %) CO2 CO
H2O2 pretreatment
flow rate (mL/min)
gas volume (L/h)
conversion (%)
H2
yes no no
1.03 0.91 0.94c
9.1 6.5 5.8
99.8 86.5 86.4
71.9 74.2 71.8
24.4 10.1 9
0.8 10.2 12.2
0.4 0.3 1
no
0.84c
6.6
90.0
66.3
22.7
4
0.3
yes yes
0.93 1.05
8.2 9.1
99.9 99.6
75 71.2
16 20.2
1.1 1.6
0.4 0.3
first (used) reactor reactor 1 new reactor 2 new after 4.5 working hours reactor 2 new after 97 working hours reactor 1 reactor 2
CH4
a [H O ] ) 3 wt %, T ) 600 °C, p ) 25 MPa, exposure longer than 40 h. The T, p, and flow rate conditions were the same for the 2 2 oxidative pretreatment and for the subsequent gasification. b Product gas concentration values are original values not corrected for leaking air. c The set point of the pump was the same, but because of wear, the flow rate declines with time.
(Institute for Instrumental Analysis, Forschungszentrum Karlsruhe GmbH, Karlsruhe, Germany).35 Additionally, the metal concentrations in the effluent were measured with an ICP (inductively coupled plasma) emission spectrometer (Varian Liberty 150). Chromate in this solution has been measured photometrically with diphenylcarbacide as an indicator. Results and Discussion First Experiments. Experimental work started with a 5 wt % methanol in water solution. The maximum experimental temperature was 600 °C, and the maximum pressure was 25 MPa. The volume of the isothermal reaction zone of the reactor used for this very first experiment was 20 mL, resulting in a residence time of about 1.1 min (assuming a density of the reaction mixture of 0.07 g/mL). The experimental results (see Table 2, first row) were a conversion of 99.8% (based on TOC concentrations in the educt and product), a 71.9 vol % H2 concentration, and a
