Design and optimization of a fixed bed reactor for hydrogen production via bioethanol steam reforming

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1 Design and optimization of a fixed bed reactor for hydrogen production via bioethanol steam reforming Maria A. Goula a, Olga A. Bereketidou a,b, Costas G. Economopoulos a,c a Technological Educational Institute of Western Macedonia, Department of Pollution Control Technologies, Koila, Kozani, 50100, Greece, mgoula@kozani.teikoz.gr b University of Western Macedonia, Department of Engineering and Management of Energy Resources, Kastorias & Fleming Str, Kozani, 50100,Greece, olgitsa79@yahoo.com c University of Patras, Department of Chemistry, Rio, Patras, Greece, costas_001greek@yahoo.gr ABSTRACT: Global climate changes caused by CO emissions are currently debated around the world. Renewable sources of energy are being sought as alternatives to replace fossil fuels. Hydrogen is theoretically the best fuel, environmentally friendly and its combustion reaction leads only to the production of water. Bioethanol has been proven to be effective in the production of hydrogen via steam reforming reaction. In this research the steam reforming reaction of bioethanol is studied at low temperatures over 15,3 % Ni/La O 3 catalyst. The reaction and kinetic analysis takes place in a fixed bed reactor in C in atmospheric pressure. This study lays emphasis on the design and the optimization of the fixed bed reactor, including the total volume of the reactor, the number and length of the tubes and the degree of ethanol conversion. Finally, it is represented an approach of the total cost of the reactor, according to the design characteristics and the materials that can be used for its construction. KEYWORDS: hydrogen, bioethanol, steam reforming, reactor design 1. INTRODUCTION Nowadays there is a major interest in electricity production from renewable sources as a consequence of the environmental standards and the decline of the fossil fuel reserves. Hydrogen using as a feed in fuel cells has been proposed as a major energy source, as it reduces greenhouse gas emissions. Bioethanol produced by biomass fermentation could be an important hydrogen supplier as a renewable source [1, ].In comparison to other fuels, ethanol presents a series of advantages, since it is easier to store, handle and transport in a safe way due to its lower toxicity and volatility. Bioethanol steam reforming is one of the best alternatives to obtain hydrogen. Various catalysts are studied for the reaction of bioethanol steam reforming: oxides, oxide-supported transition metals and noble metals [3-13]. They are generally prepared by impregnation of various supports. Ni at its supported form is widely used as a steam reforming catalyst [9, 1, 14]. This is because of the fact that nickel catalysts are much cheaper than other metals. In laboratorial and industrial scale the process using supported nickel catalysts has the disadvantage of an important carbon deposition and metal incorporation. Supported nickel catalysts on Al O 3, SiO, La O 3, ZrO, and CeO are commonly used to obstruct carbon formation. The active phase and the metal oxide interaction appears as extremely important for the stability. The design of an active, selective and stable catalytic system for the steam reforming of ethanol is one of the key points. The catalyst main purpose is to optimize the hydrogen production and to discourage the by-products formation. Bioethanol steam reforming is a very complex reaction where many reaction pathways are possible [13, 15]. Some of them depend on the catalyst used for the reaction. Kinetic studies for nickel catalysts on various supports have been performed [, 15, 16]. The catalytic testing takes place in a fixed bed reactor in atmospheric pressure in the reaction temperature. In the present study a design of a fixed bed reactor in industrial scale is accomplished for a specific hydrogen production by the reforming of bioethanol. For a catalytic system of 15, 3 %Ni/La O 3 at the range of C the reaction is first order with respect to ethanol. Lanthanum oxide species, which decorate Ni 1/9

2 particles, react with CO to form La O CO 3, that reacts with surface carbon and clean the nickel surface of carbon deposits, resulting in good catalytic stability [1]. The cost evaluation refers to the total investment cost of the fixed bed reactor and the aim is to analyze the influence of feed composition, the reaction temperature and the catalytic system for the reforming reaction.. PROCESS DESCRIPTION.1. Reaction Scheme The ethanol steam reforming overall reaction C H 5 OH + 3H O 6H + CO Η r 0 = kj mol -1 (1) C H 5 OH + H O 4H + CO Η r 0 = 38.5 kj mol -1 () is generally considered as an extremely complicated reaction scheme, involving a number of sequential or parallel steps between, occurring either in the gas phase or over the catalyst s surface, the most commonly recognized of which are the following [1,3,4,7,1,14] : C H 5 OH CH 3 CHO + H (3) C H 5 OH C H 4 + H O (4) C H 5 OH CH 4 + CO + H (5) C H 5 OH + H O CH 3 CHO + H (6) C H 5 OH + 1/O CH 3 CHO + H O (7) C H 5 OH C + H O + H (8) C H 5 OH + 3 O CO + 3 H O (9) CH 3 CHO CO + CH 4 (10) CH 3 CHO + H O CO + 3H (11) CH 4 + H O CO + 3H (1) CH 4 + CO CO + H (13) CO + H O CO + H (14) CO + 1/O CO (15) CO + 3H CH 4 + H O (16) CH 4 + O CO + H O (17) H + 1/O CO (18). Catalytic systems The bio-ethanol steam reforming reaction over Ni, Co, Cu and noble metals has been extensively studied. The target is to develop an active catalyst that inhibits coke formation and CO production. Although noble metals are less sensitive in carbon deposition, nickel catalysts are also widely used for this reaction especially because of their lower prices comparing with other metals. Furthermore, in order to obstruct the carbon deposition and metal incorporation plentiful substances for activation (catalytic amplifiers) have been added to improve the performance of supported nickel catalysts. Moreover, supporting materials that are commonly used in the bioethanol steam reforming reaction are Al O 3 ZrO, SiO, La O 3 TiO CeO, zeolites or a combination of all these [1-16]. This study is based on the use of 15, 3 % Ni/La O 3 catalyst, produced by the impregnation decomposition method, which exhibits a high activity for ethanol steam reforming reaction with a conversion of ethanol to 81, 9 %. At the range of C, the reaction is first order with respect to ethanol and the rate expression is determined by the equation [15]: [ ][ ] r = k ethanol H O where n is determined. Experimental results have proved that at low temperatures and low concentrations of water and ethanol, water does not participate in the reforming reaction and the reaction is zero order with respect to water. Therefore, n = 0 and the rate expression is: [ ] r = k ethanol n /9

3 The catalytic steam reforming reaction of bioethanol for hydrogen production takes place in a fixed bed reactor in atmospheric pressure. The reactor with tubes will be designed in industrial scale, taking into account the data for the laboratorial reactor for a specific flow of hydrogen..3 Reactor design The products of the reforming reaction of bioethanol at the range of C, as expected from the predominant reaction, are methane, carbon monoxide and hydrogen. In laboratorial scale, the ethanol conversion was kept below 0 % [15].The catalyst apparent density is considered as 1 g/ml and the rate constant in the absence of water vapor is 19, 1 s -1 [15].In order to design an industrial multi tubular reactor it is essential to create a scale up system for the operating conditions and parameters []. Table 1 The operating conditions and parameters for the laboratory tubular reactor and industrial scale multi tubular reactor. Parameter Definition Bioethanol (lab. Scale) Bioethanol (ind. Scale) T in Inlet temperature, K D p Catalyst particle diameter, mm 1 4 z Catalyst bed length, m 50* E a Activation Energy of reaction, KJ/mol 1, 87 1, 87 P Total pressure, atm 1 1 W k Catalyst weight, Kg 150 * K Rate constant, s -1 19, 1 19, 1 D t Internal diameter of the tube, cm 0, 3 1 A fixed bed reactor of 3930 tubes will be designed by a pseudo homogenous one dimensional model for hydrogen production of 1100 Kg/day. The production refers to the demanded quantity of hydrogen for a gas station in order to supply 100 cars with hydrogen fuel in fuel cells with an efficiency of 66 %[17]. The fixed bed reactor consists of vertical cylindrical tubes full of catalytic particles. The operation is adiabatic, which is accomplished in industrial reactors and the outlet temperature is lower then the inlet temperature, as the steam reforming reaction of bioethanol is endothermic. The pressure drop through bed sections must be calculated, although is not significant compared to the total pressure. The Ergun equation is commonly used to calculate pressure drop [18]. At 00 0 C the rate constant K is 0, 67 s -1 [15] according to the following equation: 0 E k = k e RT where E is the activation energy for the specific catalytic system, 1,87 KJ/mol, T the temperature, 473 K, R the gas constant, 8,314 J/mol K and Ko is calculated using the rate constant at a specific temperature. The predominant reaction CHOH H + CO+ CH 5 4 leads to a volume change of ε A = and the sum moles of ethanol output reactor can be calculated as: 1 xa CA = CA0 1+ ε AxA where C A0 represents the sum mole of ethanol input reactor and x is the degree of ethanol conversion. According to the equations of the design of PFR reactors, the total volume of the reactor can be defined as [19]: x A dx V = F r A0 0 where r A is the rate expression for first order kinetics with respect to ethanol and F A0 is the inlet flow of ethanol in feed composition, which is related to the flow of hydrogen product and the degree of the conversion through mass equilibrium. Considering the tube diameter as 1 cm and the catalyst bed length as 3 m, the total length of the fixed bed reactor will be 4 m and the internal diameter of the reactor will be 0,1 m, as the reactor is regarded as a cylindrical vessel. The total number of the tubes is calculated by the following equation: A A 3/9

4 N Q = Q tot, in tube where the total flow in the inlet of the reactor results from the feed composition of ethanol and water in the reaction temperature and the flow in the tubes is calculated by the following equation: t Qtube = U π D 0 4 where U 0 represents the superficial velocity, m/s and D t is the tube diameter The Ergun equation is commonly used to calculate pressure drop through catalyst beds[18]: ( ) ε U P µ U ε = , 75ρ 3 3 L dp ε dp ε where P is the pressure drop, atm, L is the total length of the reactor, m, dp is the catalyst particle diameter, m, ρ is the density of the gas mixture at the outlet of the reactor, g/l, µ is the mixture viscosity which is equivalent to the viscosity with the lower density in the outlet temperature, cp and ε is the bed porosity, given by the equation: D t 1+ DP ε = 0,38 + 0, 073 D t DP The results of the reactor design at reaction temperature of 00 0 C, hydrogen production of 1100Kg/day and ethanol conversion of 80% are summarised at the following table: Table Operating conditions and parameters for a fixed bed reactor for hydrogen production of 1100 Kg/day at the reaction temperature of 00 0 C Parameter Definition Industrial scale T in Inlet temperature, K 473 T out Outlet temperature, K 453 E Activation energy, KJ/mol 1, 87 X A Degree of ethanol conversion, % 80 R Gas constant, J/mol K 8,314 P Total pressure, atm 1 K Rate constant, s -1 0, 76 K 0 Constant, s -1 33, 4 ε A Volume change F H Hydrogen production, mol/day F ETOH,0 Feed ethanol flow, mol/day C A0 Initial ethanol concentration, mol/l 0, 0578 C A Total ethanol concentration, mol/l 0, r A Reaction velocity, mol/l s 0,041 V Total reactor volume, L 47, 99 D t Tube diameter, cm 1 Z Catalyst bed length, m 3 L Total length of the reactor, m 4 D Reactor diameter, m 0, 1 D p Catalyst particle diameter, mm 4 U 0 Superficial velocity, m/s 1 N Number of tubes 3930 ε Bed porosity 0,3946 P Pressure drop, atm 0, Q tube Flow in the tubes, m 3 /s 7, 8 *10-5 4/9

5 .4 Economic evaluation The objective of this part of the study is to evaluate the cost of the fixed bed reactor for hydrogen production from bioethanol steam reforming in industrial scale. The investment cost consists mainly of the total cost of the equipment, the vessel, the tubes and the cost of the catalytic system for the reaction [0]. Other apparatuses possibly included in the process simulation have not taken into account in this cost analysis. The cost analysis is based on the production of hydrogen of mol/day, resulting in the parameters that are shown in table. The purchase cost of the cylindrical vessel is calculated by the following equation [1, ]: C = f C + C m b a where f m is a factor for the used material, which plays an important role for the resistance in corrosiveness. In this study, the material is stainless steel 316 and f m =, 1. The factor C a is defined as: 0,7936 0,7068 C = 46D L a where D is the diameter of the vessel, in and L is the total length of the reactor, in. The weight of the vessel, lbm, is given by the equation []: W = ( th + thd) π Lρ m where t h is the thickness of the vessel, in and ρ m is the density of the construction material, lbm/in 3 The factor C b is defined as: Cb The cost of a tube can be calculated as: ( W) ( W) = exp 9,1 0, 889 ln + 0, ln C tube = 1,1d where d is the tube diameter, ft and z is the bed length, ft. The total cost of the tubes is estimated as: C tubes = NC using the total number of the tubes in the reactor, N. The total cost of the catalyst can be estimated, taking into account the percentage of the metal in the surface of the support (15,3 % Ni/La O 3 ) and the quantities that are essential for the impregnation method in order to produce the catalytic system. The results of the economic evaluation of a fixed bed reactor for hydrogen production of mol/day are summarized in table 3. Table 3 Economic evaluation of a fixed bed reactor of mol/day hydrogen production from bioethanol steam reforming in 15, 3 % Ni/La O 3 Parameter Definition Industrial scale D Reactor diameter, in 4, 7 L Reactor length, in 157 F m Factor for SS 316, 1 C a Factor, 9776 t h Thickness of the vessel, in 0,5 p m Density for SS 316, lbm/in 3 0,9 W Weight of the vessel, lb 37 C b Factor, 8015 C Cost of the vessel, D t Tube diameter, ft 0,033 Z Catalyst bed length, ft 9, 84 C tube Cost of a tube, 4, 08 N Number of tubes 3930 C tubes Total cost of tubes, C Ni(NO3) 6HO Cost of Ni(NO 3 ) 6H O 34 /50 g C LaO3 Cost of La O 3, /Kg 155 W k Weight of catalyst, Kg 50 ρ κ Catalyst density, Kg/L 1,3 C catalyst Total cost of catalyst, C total Total cost, C total, 007 Cost in the year 007, /9 0,99 tube z

6 The study emphasizes on the evaluation of the investment cost of the reactor, consisting of the vessel, the tubes and the catalyst. The production cost is not an issue of this study and generally it can be calculated by counting the project cost, the operating cost, the raw material cost and the utility cost. An escalation of the ethanol price during the past is shown in figure 1 [3]. 3. RESULTS AND DISCUSSION Figure 1: Ethanol market price 3.1The influence of the desired hydrogen production and the feed composition In order to evaluate the change in hydrogen production and feed composition, three different values of hydrogen production are studied. The first one, mol/day, refers to the demanded quantity of hydrogen for a gas station in order to supply 100 cars with hydrogen fuel in fuel cells with an efficiency of 66 %. The second refers to a unit of hydrogen production that is able to supply a prefecture of citizens. Considering that there is a correlation of a car/3 people, in the prefecture will be approximately cars. Hydrogen will be used to supply these cars in fuel cells and efficiency of 66 %. The heat combustion of hydrogen is estimated in 33, 3 KW/KgH, resulting in a flow of hydrogen about *10 3 mol/day [17]. The third value refers to an industry of hydrogen production by steam reforming of bioethanol, with a production of 100Kt/year = 135 *10 6 mol/day. The results of the economic evaluation and the design of the three reactors for the three values of hydrogen production are shown in table 4: Table 4 Parameters and economic evaluation for the three values of hydrogen production Parameter Hydrogen (100 cars) Hydrogen (3.000 cars) Hydrogen Industry F H, mol/day * *10 6 Reactor volume, L 47, , Cost of vessel, Number of tubes Cost of tubes, Weight of catalyst, Kg , Catalyst cost, Total investment cost, /9

7 The results in table 4 show that the reactor volume and the total cost depend on the production of hydrogen. An increase in the value of hydrogen production results in an increase in the quantity of bioethanol in feed composition, an increase in the volume of the reactor for the reforming of bioethanol and consequently an increase in the total investment cost of the fixed bed reactor. 3. The influence of the reaction temperature Considering that the rate expression used in this study refers to low temperatures [15], there is an approach to evaluate the influence of the temperature variation at the range of C.For reaction temperature from C, with a catalyst of 15,3 % Ni/La O 3 and hydrogen production of mol/day, the total volume of the reactor and the total investment cost variations are shown in figures, 3: React Vol (L) 47, ,5 46 Vol (L) 45, , , React Vol (lt) 44,41 44,9 45,4 45,91 46,4 46,94 Temperature ( 0 C) Figure: The influence of the reaction temperature on the reactor volume Total Cost Total Cost ( ) Total Cost Temperature ( 0 C) Figure 3: The influence of the reaction temperature on the total cost 7/9

8 From the above diagrams it can be concluded that an increase in the reaction temperature leads to an increase in the reactor volume and the total investment cost. Increasing the reaction temperature, there is an increase in the rate constant, K and the total volumetric supply of ethanol in the feed composition. The reactor volume is proportional to the supply of ethanol, resulting in an increase in the total volume with a parallel increase in the reaction temperature, considering that the degree of ethanol conversion is constant. The higher the reactor volume, the higher and the total cost and increasing the temperature, the total cost of the reactor also increases. 3.3 The influence of the catalytic system The rate expression in this study is the same for three different catalytic systems [15]. The whole study is based on the catalyst 15,3 % Ni/La O 3, but it is essential to take the two other catalysts into account and evaluate the influence in the reactor volume and the investment cost for hydrogen production of mol/day. The rate of the reaction is for the three nickel catalysts first order with respect to ethanol and the rate constants are given in the absence of water vapour. For ethanol conversion of 80 % and reaction temperature of 00 0 C the results of the reactor design and economic analysis are presented in table 5. Table 5 Reactor design and economic analysis for three different nickel catalysts in the same rate expression in reaction temperature of 00 0 C and ethanol conversion of 80 % Parameter 15, 3%Ni/La O 3 16, 1%Ni/Al O 3 0, 6%Ni/Y O 3 K, s -1 0, 76 4, 93 4, 0 E, KJ/mol 1, 87 16, 88 7, 04 r, mol/l s 0,041 0, ,0079 V, L 47, 99 0, 66 47, 97 C vessel, C tubes, C catalyst, C total, 007, The data from table 5 show that the total investment cost depends on the catalytic system for the reforming reaction of bio ethanol. The nickel catalyst supported in Y O 3 is very expensive because of the high cost of the support and the high percentage of the metal on the supports surface. From the three catalysts, nickel supported on La O 3 shows great stability and activity in the reaction process and seems to be the best catalytic system for an industrial reactor in order to achieve the desired selectivity in hydrogen production and avoid carbon formation. Nickel supported on alumina is frequently used, but it leads to an undesired coke formation. The selection of the optimum catalyst must include a research of the production method, the activity and stability in the reaction and the total cost of production in order to use it in great quantities in industrial scale. 4. CONCLUSIONS A fixed bed reactor design was developed for the catalytic steam reforming of bioethanol for hydrogen production. The design was based on a pseudo homogenous model and a scale up system was created for the comparison of laboratorial and industrial scale. The PFR design equations were used to accomplish the design of the reactor. The cost analysis put emphasis on the total investment cost of the reactor and specifically the cost of the vessel, the total cost of the tubes and the cost of the catalytic system. From the cost analysis it can be concluded that an increase in the production of hydrogen, increases the reactor volume and the total cost. Fluctuations of the reaction temperature also influence the reactor volume and the reactor cost. The use of three different supported nickel catalysts for the same rate expression, have a great effect on the total investment cost, because of the cost of the support and the percentage of the metal on the surface of the support. As a result, the optimum conditions for the reactor, originate from the parameters that lead to the minimum cost, a high selectivity to hydrogen and a high activity of the catalyst for the reforming reaction of bioethanol. 8/9

9 5. ACKNOWLEDGMENTS This study was supported by the Ministry of National Education and Religious Affairs, The operational program for education and initial vocational training (O.P. education), project ARXIMIDIS II,.6.1. ιd, Athens, Greece 6. REFERENCES [1] Athanasios N. Fatsikostas, Dimitrs I. Kondarides, Xenophon E. Verykios, Production of hydrogen for fuel cells by reformation of biomass derived ethanol, Catalysis Today, 75, , 00 [] Ahmed Adoudheir, Abayomi Akande, Raphael Idem, AJAY Dalai, Experimental studies and comprehensive modelling of hydrogen production by the catalytic reforming of crude ethanol in a packed bed tubular reactor over a Ni/Al O 3 catalyst, Inter. Journal of Hydrogen Energy [3] Prakash D.Vaidya, Alirio E. Rodrigues, Insight into steam reforming of ethanol to produce hydrogen from fuel cells, Chemical Engineering Journal, 117, 39 49, 006 [4] Dimitris K.Liguras, Dimitris I. Kondarides, Xenophon E. Verykios, Production of hydrogen for fuel cells by steam reforming of ethanol over noble metal catalysts, Applied Catalysis B: Environmental, 43, , 003 [5] M.A. Goula, S.Kontou, W. Zhou, Xin Qin and P.Tsiakaras, Hydrogen Production over a Commercial Pd/Al O 3 Catalyst for Fuel Cell Utilization, Ionics, 9, 3&4, 48, 003 [6] Maria A. Goula, Sotiria K. Kontou, Panagiotis E. Tsiakaras, Hydrogen production by Ethanol Steam Reforming Over a Commercial Pd/γ-Al O 3 Catalyst, Applied Catalysis B:Environmental, 49,, , 004 [7] J. Liorca, N. Horms, J. Sales, J.G. Fierro and P.Ramirez de la Piscina, Effect of sodium addition on the performance of Co-ZnO based catalysts for hydrogen production from bio ethanol, Journal of Catalysis, 1,, , 004 [8] J.R. Salge, G.A. Deluga and L.D. Scmidt, Catalytic partial oxidation of ethanol over noble metal catalysts, Journal of Catalysis, 35, 1, 69-78, 005 [9] Yu Yang, J. Ma and Fei Wu, Production of hydrogen by steam reforming of ethanol over a Ni/ZnO catalyst, Inter. Journal of Hydrogen Energy, In Press, Corrected Proof, 005 [10] J.C. Vargas, S. Libs, A. Roger and Alain Kiennemann, Study of Ce Zr Co fluorite type oxide as catalysts for hydrogen production by steam reforming of bio ethanol, Catalysis Today, In Press, Corrected proof, 005 [11]V.S. Bergamashi, F.M.S. Carvahlo, C. Rodrigues and D.B. Fernandes, Preparation and evaluation of zirconia micro spheres as inorganic exchanger in adsorption of copper and nickel ions ad catalyst in hydrogen production from bio ethanol, Chem. Eng. Journal, 11, 1 3, , 005 [1] V. Fierro, O. Akdim, H. Provendier and C. Mikrodatos, Ethanol oxidative steam reforming over Ni-based catalysts, Journal of Power Sources, 145,, , 005 [13] M. Benito, J.L. Sanz, R. Isabel, R. Padilla, R. Arjona and L. Daza, Bio ethanol steam reforming: Insights on mechanisms for hydrogen production, Journal of Power Sources, in press, Corrected Proof, 005 [14] Athanasios N. Fatsikostas and Xenophon E. Verykios, Reaction network of steam reforming of ethanol over Ni-based catalysts, Journal of Catalysis, 5, , 004 [15] Jie Sun, Xin - Ping Qiu, Feng Wu, Wen-Tao Zhu, H from steam reforming of ethanol at low temperature over Ni/Y O 3, Ni/La O 3 and Ni/Al O 3 catalysts for fuel cell application, Inter. Journal of Hydrogen Energy, 30, , 005 [16] Abayomi Akande, Ahmed Aboudheir, Raphael idem, Ajay Dalai, Kinetic modelling of hydrogen production by the catalytic reforming of crude ethanol over a co- precipitated Ni-Al O 3 catalyst in a packed bed tubular reactor, Inter. Journal of Hydrogen Energy [17] [18] Ergun, S., CEP, 48,, 89 94, 195 [19] Froment G.F. and Bishoff, K.B., Chemical reactors analysis and design, john Wiley, N.Y., 1979 [0] Q. Smejkal, D. Linke, M. Baerns, Energetic and economic evaluation of the production of acetic acid via ethane oxidation, Chemical engineering and processing, 44, 41 48, 005 [1] Walas, M.S., Chemical process equipment. Selection and design, Butterworth-Heinemann, 1990 [] Peters, M.S., and Timmerhaus, K.D., Plant design and economics for chemical engineers, McGraw Hill, N.Y., 4 edition, 1991 [3] price.com 9/9