Bioethanol Steam Reforming on Suported Nickel Catalysts

Transcription

1 Bioethanol Steam Reforming on Suported Nickel Catalysts DORIN BOMBOS 1 *, MIHAELA BOMBOS 2, PAUL ROSCA 1, GABRIEL VASILIEVICI 2, MARIUS GHIUREA 2 1 Petroleum-Gas University of Ploiesti, 39 Bucharest Blv., , Ploiesti, Romania 2 National Research Institute for Chemistry and Petrochemistry, ICECHIM, 202 Spl. Independenþei, , Bucharest, Romania Biofuels such as ethanol, biodiesel and biohydrogen are the main alternative fuels of the future. Currently nearly 95% of hydrogen is produced by reforming hydrocarbons (natural gas), reason for what hydrogen production technologies by catalytic processing oxygenate compounds prepared from renewable resources become attractive. One way of manufacturing hydrogen consists of catalytic reforming of biomass. Catalytic reforming of bioethanol has been studied on suported nickel s. Ni / SiO 2, Ni / y Al 2 - diatomite and Ni / diatomite s were prepared by the method of incipient wetness using nickel nitrate as a metal precursor. Textural characteristics of the s were determined on a device Autosorb1 Quantacrome and acid strength distribution was determined by thermodesorption of diethyl-amine in the temperature range C. Evaluation of s surface conformation and distribution of clusters of active component on the inner surface of the s was done by SEM microscopy and hydrogen consumption of s studied depending on temperature was assessed over a range between ambient temperature and 850 C, to optimize phase activation of. Reforming of ethanol has been made in a laboratory reforming plant coupled with a gas chromatograph Varian CP The main reaction products were hydrogen, carbon dioxide and methane. Ethanol conversion was higher for the three s tested and hydrogen yield have higher values for the Ni / diatomite- y Al 2 - H-ZSM5. Keywords:bioethanol, s Ni-based, steam reforming Biofuels such as ethanol, biodiesel and biohydrogen are the main alternative fuels of the future. Currently nearly 95% of hydrogen is produced by reforming hydrocarbons (natural gas), reason for what hydrogen production technologies by catalytic processing oxygenate compounds prepared from renewable resources become attractive. One way of manufacturing hydrogen consists of catalytic reforming of hydrocarbons or biomass. Of the many raw materials bioresource, ethanol is very attractive due to relatively high hydrogen content, availability, lack of toxicity, self handling and storage. In recent years bioresources raw materials used to ethanol manufacture were diverse: biomass energy production plants, agroindustrial residues, wood residues from forest materials[1]. Hydrogen can be produced by relatively simple ecologycal technology, based on renewable raw materials available in large quantities. Virtually all derivatives from biomass can be used as raw materials, this includes liquid biofuels (bioethanol), by-products from biodiesel production (glycerol), components resulting from the processing of biomass (alcohols, fatty acids) as well as biomass directly (starch, cellulose). Thus, a number of studies for producing hydrogen from catalytic reforming of glycerol was developed. It was studied the process of obtaining hydrogen by catalytic reforming of liquid fuels derived from biomass on a commercial Ni-based (C11-NK)[2]. The effects of reforming temperature, the molar ratio of oxygen/carbon and the molar ratio of steam/ carbon were examined by a statistical approach glycerol reforming for hydrogen production using a reactor in a fixed layer with a Ni [3]. It was showed that glycerol on catalytic reforming Ru / Y 2 led to a hydrogen selectivity of 90% and total conversion of glycerol at a temperature of 600 C [4]. Hydrogen was produced by catalytic reforming of glycerol on supported cerium and showed a 100% conversion of glycerol at a temperature of 400 C[7]. The catalytic reforming of glycerol, using a metal on alumina support and varying the reaction conditions and high temperatures led to high conversions in gas and selectivity up to 70% [6]. Ni s deposited on different media: Ni / CeO 2, Ni / MgO, Ni / TiO 2 were tested in catalytic reforming of glycerol. The results showed that Ni /CeO 2 is the best, leading to a hydrogen selectivity of 74.7% and 99% glycerol conversion at 600 C [4]. The catalytic reforming of glycerol on the Ni s deposited on alumina support was tested using different promoters such as: Ce, Mg, Zr, La [10]. The study concluded that the use of these promoters increases selectivity in hydrogen. Higher activities of these s were observed at higher concentrations of Ni, high stability and high capacity to activate steam. Complete conversion of glycerol was achieved at a temperature of 600 C, a WHSV (weight hour space velocity) of 2.5 h -1 and atmospheric pressure. However, although several sources of hydrogen have been investigated, alcohols, and in particular bioethanol, which is a renewable material easily obtained from biomass, might be considered as a good choice. In biomass technologies, ethanol is the most attractive feedstock, availability, non-toxicity and storage and handling safety. Because of its relatively high hydrogen content, ethanol reacts with water under steam reforming conditions: C 2 H 5 OH + 3H 2 O > 2CO 2 + 6H 2 Advantages of using bioethanol, a mixture of ethanol and water to obtain hydrogen consists in the renewable character and the wide availability of that raw materials, * Tel.: (+40) REV. CHIM. (Bucharest) 62 No

2 in the fact that unlike methanol, bioethanol is not toxic and, unlike oil, does not contain sulphur which would poison the reforming s. In addition, the process of obtaining hydrogen is considered safe in terms of carbon dioxide emissions because carbon dioxide reform produced by fermentation and reforming can be used to increase biomass, considering the carbon cycle closed. Bioethanol is usually produced in the form of an aqueous solution containing between 8 and 12% wt ethanol, can be converted to fuel hydrogen fuel cell to produce electricity. Because water is required as feedstock for ethanol reforming to obtain a hydrogen-rich gas stream and excess water reduces carbon monoxide, such a process may allow the use of ethanol bioderived without the need for its distillation. This aspect is particularly important economically, because more than 90-95% ethanol purification is costly due to the formation of water-ethanol azeotrope. Direct transformation of ethanol diluted with water into hydrogen, without the need for energy-intensive to azeotropic distillation, polluting, and without undesirable by - products resulting from current processes would be an economic solution for using biomass as a renewable energy source. An important role in the reforming process of ethanol, in order to obtain hydrogen has the type of used. An active can increase the maximum selectivity to hydrogen and can inhibit the formation of carbon or carbon monoxide. Catalytic reforming of oxygenated hydrocarbons is thermodynamically favored at low temperatures, unlike the neoxigenate hydrocarbons [12] so the catalytic reforming oxygenated compounds can occur at low temperatures, because it is more exothermic [13]. However, it can result in more coke deposited than expected. For steam reforming of bioethanol for obtaining hydrogen have been studied many types of active metals inluding noble metals like Rh, Ru, Pd or Pt and transition metals as Ti, Zr, Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd, Sb. In addition there were studied different types of support as SiO 2, Al 2 and MgO, CeO 2, ZnO, TiO 2, La 2 Experimental part Preparation of s Ni / SiO 2, Ni / y Al 2 - diatomite and Ni / diatomite s were prepared by the method of incipient wetness using nickel nitrate as a metal precursor. The support materials where prepared by granulation of powdery commercial products as extrudated composite pellets. Nickel concentration deposited on catalytic supports was 5% wt. After impregnation, the s were dried at room temperature for 6h, then at 120 C for 8 h, calcined in air at 600 C for 6 h and subsequently reduced in a stream of hydrogen. tungsten filament. Powders were previously deposited on a carbon double adhesive tape. Hydrogen consumption of s studied depending on temperature was assessed over a range between ambient temperature and 850 C, to optimize phase activation of. Determinations were made on a device ChemBET TPR / TPD Quantachrome Instruments. Evaluation of s Reforming of ethanol has been made in a laboratory reforming plant coupled with a gas chromatograph Varian CP The reformer comprises a reactor electrically heated, thermostatically controlled, with possibility of programming and recording temperature. A 25 mm inner diameter fixed bed reactor was used. The volume of bed used in all the experiments was 50 cm 3. The bed is supported on a 10 cm height layer of glass beads (3 mm in diameter) and the top part of the reactor was also filled with 3 mm diameter glass beads to ensure a good distribution of the fluid in the bed. A mobile metallic jacket for thermocouple was also placed in the axis of the bed, in order to measure the reaction temperature. Two wall heaters, connected to a temperature controller, were used to rise the reactor temperature. Ethanol solution was introduced through the upper flange of the reactor. Reaction products are cooled in a condenser and get into the trap. Liquid phase is discharged from the bottom of the separator through a valve and gas phase was directed to a gas chromatograph for analysis. Raw material was aqueous solutions of ethanol (7% wt). Ethanol solution was pumped into the reactor with a HPLC pump. Results and discussions Pore size distributions, determined using data obtained from nitrogen adsorption and desorbtion, are presented in figures 1-3. The pore size distribution chart when Ni/ y Al 2 - diatomite (fig. 1) shows a wide distribution with a significant proportion of mesopic and macropores. For Ni/SiO 2 - y Al 2 (fig. 2) shows a narrower pore distribution than for Ni / y Al 2 -diatomite, with a diminished percentage of macropores, reduced weight is probably due to clogging of micropores and macropores. In the case of Ni / diatomite - y Al 2 - H ZSM5 (fig. 3) shows a significant distribution with a higher share of the micropores, and much reduced mesopores and macropores. Acid strength distribution for s, determined by thermodesorption of diethyl-amine, is presented in figures 4-6 Ṫhe thermogravimetric curve of the thermodesorbtion of diethyl-amine on the Ni / y-al 2 diatomite (fig. 4) shows similar centers weak acids presence and strength Characterization of s Textural characteristics of the s were determined by an Autosorb1 Quantacrome device. Textural data were obtained by recording and automatic processing of adsorption-desorption isotherms of nitrogen. Acid strength distribution for studied s was determined by thermodesorption of diethyl-amine in the temperature range C. Thermal analyses (ATG, DSC) were performed on a DuPont Instruments device Thermal Analyst 2000/ 2100 coupled with a module 912 Differential Scanning Calorimeter and a module Thermogravimetric analizer 951. Evaluation of s surface conformation and distribution of clusters of active component on the inner surface of the s was done by SEM microscopy. Fig.1. Pore size distribution for Ni/ y Al Determinations were made on the EIF Quanta 200 with 2 - diatomite REV. CHIM. (Bucharest) 62 No

3 Fig.2. Pore size distribution for Ni/ SiO 2 Fig.3. Pore size distribution for Ni/ diatomite Fig.4. Acid strength distribution for Ni/ y Al 2 - diatomite Fig.5. Acid strength distribution for Ni /SiO 2 Fig.6. Acid strength distribution for Ni/ diatomite Fig.7 SEM image of Ni /y Al 2 diatomite average center with a wide distribution (maximum at 137 o C) and the strong centre with a maximum located in the temperature range of C. For Ni / SiO 2 - y Al 2 - diatomite (fig. 5) shows a low content of acid centers; acid centers of medium strongly show a significant higher weight and strong acid centers are missing. The content of acid centers for Ni/ diatomite y Al2O3 H ZSM5 is much higher than for the above s. Acid strength distribution shows a high concentration of weak acid centers and the average strength a much lower concentration of strong acid centers. SEM microphotographs of prepared s are presented in figures 7-9. The images presented shows homogen catalytic composition of the 3 s. In figures are shown the thermoreduction curves for the three s in the presence of hydrogen. Consumption of hydrogen depending on temperature of the Ni/y Al 2 - diatomite (fig. 10) shows similar REV. CHIM. (Bucharest) 62 No

4 Fig.8 SEM image of Ni /SiO 2 Fig.9 SEM image of Ni /diatomite Fig.10. Hydrogen consumption variation with temperature of the Ni / γal 2 - diatomite Fig.11. Hydrogen consumption variation with temperature of the Ni /SiO 2 - γ Al 2 second-largest appropriated maximum located at approx. temperatures of 500 and 600 C; because the activation starts at temperatures above 300 C is recommended to activate it in a stream of hydrogen at a controlled flow and at temperatures below 450 C to prevent its sintering. Thermoreduction curves in the hydrogen presence of the Ni/SiO 2 - Al 2 also shows two maxima, the first Fig.12. Hydrogen consumption variation with temperature of the Ni/ diatomite - γ Al 2 at a temperature of 600 C and the second at approx. 800 C (fig. 11), and when the activation starts at temperatures above 300 C is recommended also its activation in a hydrogen stream at a controlled flow and at temperatures below 450 C to prevent its sintering. Hydrogen consumption variation with temperature of the Ni/diatomite - y Al 2 -H-ZSM5 is more complex, presenting three maxim situated at temperatures of approx. 425, 550 and 650 o C respectively, activation to this flows more efficiently (pore distribution probably favours the reduction of Ni oxide), so there were recommended activation temperatures of C in hydrogen flow at a controlled flow to prevent sintering. All samples have reduced total temperature over 800 C. Operating parameters and performances from the three s tested under the same conditions of work are presented in table 1. The main reaction products were hydrogen, carbon dioxide and methane. Ethanol conversion was higher for the three s tested and hydrogen yield having higher values for the Ni / diatomite- y Al 2 - H- ZSM5 ; higher concentration of acid centers and pore distribution has favoured reforming to hydrogen formation REV. CHIM. (Bucharest) 62 No

5 Table 1 EXPERIMENTAL DATA OBTAINED FROM ETHANOL REFORMING Conclusions Textural characteristics of Ni-based s obtained by impregnation were determined from adsorption - desorption isotherms of nitrogen; thus to assess the distribution of pores, were used either BET equation or adsorption or desorption branch of isotherms by applying the BJH method.there is a wide distribution of pore size for the three s tested. Distribution of the acidic centres of s was determined by thermodesorption of diethyl-amine in the temperature range C. Thermal analysis (ATG, DSC) were performed on a DuPont Instruments device Thermal Analyst 2000 / 2100 coupled with a module 912 Differential Scanning Calorimeter and a module Thermogravimetric analizer 951. There is a lower acid strength for the Ni / SiO 2 -Al 2. Evaluation of s surface conformation on the inner surface of the s was done by SEM microscopy. Determinations were made on the EIF Quanta 200 with tungsten filament. Hydrogen consumption of s was studied depending on temperature, for the studied s over a range of temperature between ambient temperature and 850 C. Determinations were made on a device ChemBET TPR / TPD Quantachrome Instruments. Reducing to the metallic Ni begins at appropriate values of temperature for the three s; reduction curves have different shapes, vary with reduction rate the composition of support and pore size distribution. The experiments were performed in a thermally continuously regulated fixed bed reactor, with downward movement of reactants, at atmospheric pressure and temperature of 450 o C. Conversion of ethanol on Ni s tested, in operating selected conditions, has high values (close to 100%). Hydrogen yield under the chosen test conditions had higher values on s which show a higher acid strength. Main by-products of reforming ethanol were carbon dioxide and methane, the yield on the two compounds having close values. References 1. HUBER GW, SHABAKER JW, DUMESIC J.A., Science, 2003,300: p CZERNIK, S., EVANS, R., FRENCH, R., Catal. Today, 129, 3-4, 2007, p WANG, W., TURN, S.Q., KEFFERA, V., DOUETTE, A., Chem. Eng. J., 129, 1-3, 2007, p HIRAI T, IKENAGA NO, MIYAKE T, SUZUKE T., Energy & Fuels, 19, 2005, p HAISHENG CHEN, TIANFU ZHANG, BILIN DOU, V. DUPONT, P. WILLIAMS, M. GHADIRI, YULONG DING, International Journal of Hydrogen Energy, 3 4, 2009, p S. ADHIKARI, S. FERNANDO, A. HARYANTO, Catal. Today, 129 (3 4), 2007, p B. ZHANG, X. TANG, Y. LI, Y. XU, W. SHEN, Int. J. Hydrogen Energy, 32 (13),2007, p M. SLINN, K. KENDALL, C. MALLON, J. ANDREWS, Bioresour. Technol. 99 (13),2008, p S. ADHIKARI, S.D. FERNANDO, A. HARYANTO, Renew. Energy, 33 (5),2008, p A. IRIONDO, V.L. BARRIO, J.F. CAMBRA, P.L. ARIAS, M.B. GÜEMEZ, R.M. NAVARRO, M.C. SÁNCHEZ-SÁNCHEZ, J.L.G. FIERRO, Top. Catal., 49 (1 2), 2008, S. NATESAKHAWAT, O. OKTAR, U.S. OZKAN, J. Mol. Catal. A: Chem. 241 (1 2), 2005, p G.W. HUBER, J.W. SHABAKER, S.T. EVANS, J.A. DUMESIC, Appl. Catal. B, 62 (3 4), 2006, p B. PAWELEC, S. DAMYANOVA, K. ARISHTIROVA, J.L.G. FIERRO, L. PETROV, Appl. Catal. A, (323), 2007, p.188 Manuscript received: REV. CHIM. (Bucharest) 62 No