Aprovechamiento de gas no convencional en procesos GTL
- Autores: Victoria Garcilaso de la Vega González
- Directores de la Tesis: José Antonio Odriozola Gordon (dir. tes.), Miguel Ángel Centeno Gallego (dir. tes.)
- Lectura: En la Universidad de Sevilla ( España ) en 2018
- Idioma: español
- Número de páginas: 767
- Tribunal Calificador de la Tesis: María Montes (presid.), María Isabel Domínguez Leal (secret.), Anne-Cécile Roger (voc.), Sandra Palma del Valle (voc.), Fernando Bimbela Serrano (voc.)
- Programa de doctorado: Programa de Doctorado en Ciencia y Tecnología de Nuevos Materiales por la Universidad de Extremadura y la Universidad de Sevilla
- Materias:
- Enlaces
- Tesis en acceso abierto en: Idus
- Resumen
INTRODUCTION Among the types of unconventional resources of methane, biogas is of a particular interest because of its renewable character and feasibility worldwide.
The objective of the present work is to demonstrate the possibility of obtaining the required syngas to combine biogas reforming with the FT reaction. The use of biogas to produce a synthesis gas is a chance to obtain synthesis liquids fuels from a renewable resource.
The study is focused in finding a proper catalyst, active, selective and stable for biogas reforming. Then, determine the structure to function relationship of the selected catalyst to get insight in the active phase. The mechanisms and the kinetics of biogas reforming of the selected catalyst were also established. In order to establish the full perspective of the GTL process a series of Co based catalyst were selected to study the FT process.
THERMODYNAMIC CONSIDERATIONS AND SIMULATIONS ON BIOGAS REFORMING REACTIONS Thermodynamics simulations have been applied in order to establish a road map to obtain the process conditions where a syngas of a H2/CO ratio =2 is achieved, while methane and CO2 conversion are above of an 30 %. For that propose, a Gibb reactor were employ using the Peng-Robinson equations of state to calculate the equilibrium composition under different reaction conditions. In this sense, temperature, pressure, and the inlet composition were studied. The mainly side reactions, RWGS and carbon formation were also evaluated.
The biogas composition according to the CH4/CO2 ratio influences the product distribution. Methane excess results in a major production of H2, resulting in a higher H2/CO ratio. Nevertheless, in absence of water in the biogas, the required H2/CO ratio of 2 is not achieved at least below an inlet of CH4/CO2 ratio of 3.
As the temperature increase the equilibrium constant of biogas reforming increases, as well as RWGS. Below 800 ºC WGS is favored rather than RWGS. The water concentration also favors WGS, which is the side reaction that allows the production of H2.
The required composition of a syngas H2/CO =2 is obtained using a 30-35 %vol. of H2O in the temperature range of 600-650 ºC, using a model biogas of a CH4/CO2 =1,5.
The presence of H2 and CO in the inlet negatively affects conversion of biogas reforming. On the contrary, a positive effect of introducing N2 in the reactor inlet in conversion and selectivity is observed. Pressure is a disadvantage for biogas reforming reaction although favors the selectivity towards H2. Since WGS is not negatively affected by pressure. Moreover the negative effect is diminishing at high temperatures. Introducing N2 favors activity and selectivity although, more simulations are required to well understand inert effect on thermodynamics.
Solid carbon formation severely affects the final concentration at the equilibrium in all studied range of temperature biogas reforming. At low temperatures, carbon formation is mostly due to Bouduard reaction and methane cracking. While at higher temperatures carbon formation via methane cracking is predominant. Solid carbon reactivity involves mostly the revers Boudouard reaction and carbon gasification, which are activated by temperature. Using a 20 vol. % of water in the biogas inlet, carbon gasification is favored. Thus, solid carbon is being removed avoiding its influences on activity and selectivity, at temperatures above 750 ºC.
CATALYSTS SCREENING FOR BIOGAS REFORMING Biogas reforming reaction implies a serie of disadvantages mainly focused on the endothermicity and side reactions participation. To compensate these drawbacks, reforming reactions usually operate under high temperature where the equilibrium constant of biogas reforming is favored. These considerations evidence the importance of the use of a resistant material under severe atmospheres (high temperatures, pressures and oxidant environment). But it is also convenient to be highly active and selective at moderate temperatures, where the catalyst works with an important participation of the WGS and coke formation reactions. In this chapter a number of catalysts have been tested and studied in order to select the more active, selective and stable catalyst for biogas reforming. Two major groups can be considered, cobalt based catalysts and noble metal based ones (Pt and Rh). The bimetallic combinations of RuCo and PtRh have being also studied. As support, alumina was selected to being modified by several types of oxides; ZrO2, CeO2, and MgO. In addition, it has been interesting to study the doping of ZrO2 by La and Ce cations in a 10% of molar percent. In order to characterize all the catalytic systems the following techniques were employed: XRD, XRF, N2 physisorption, SEM, TEM, Raman spectroscopy, TPR and TPO-MS.
Regarding the Co based systems, the effect of using different supports and the introduction of ruthenium as a promoter were studied. On the one hand, the following supports were studied, ZrO2/Al2O3 (named ZrAl), Ce0,5Zr0,5O2/Al2O3 (named 50CeZrAl) and CeO2/Al2O3 (named CeAl). The support nature plays a role in the maintenance of the optimal textural properties and favoring the reducibility. It is found that Zr promotion effect is related to the resistance of the textural properties, while Ce is enhancing the redox properties, more toughly noticed in the mixed oxide Ce0,5Zr0,5O2 than in CeO2. The cobalt particle size may be affected by both considerations, which results in a not clear dependence on the support nature. Under biogas reforming conditions, BR, it has being found that the reducibility of Co50CeZrAl, is important to favor activity and selectivity. However, in water rich atmospheres, as is the case of steam reforming of methane, Zr rich catalyst provides a significant improvement of activity, since the negative influence of water is recognized in the performance of CeO2. In this condition, water may favor cobalt oxidation, which is catalyzed by CeO2, thus, diminishing the number of actives sites. Hence, in steam reforming of methane, SMR, there is found a trend between activity and the specific surface of the catalyst, which represent the effectiveness of Zr promotion in this reaction. Nevertheless, a deeper analysis on the cobalt dispersion and the localization of the active phase is required in order to understand the catalytic performance of the Co serie. On the other hand, RuCo bimetallic systems present an important enhancement of the reducibility thanks to the spillover on the surface of ruthenium on the cobalt surface. In these systems, CeO2 enhances the promotion effect of Ru. It results in an increase of the catalytic activity in reaction BR, in case of Ce rich catalyst. But in case of ZrO2 the proper interaction between Ru-Co is not obtained. In this context, it may be pointed that Ru dispersion is found important to understand the promotion effect. Since Ru can be blocking the active sites of cobalt as it was found in case of Ni in RuNiMgAl catalyst [1]. Although, the contrary effect is found in MSR, where the more active performance is found in the RuCoZrAl catalyst. The promotion effect of Ru also implies an enhancement of the resistance of coke deactivation and then promoting the stability of the catalyst in BR. In conclusion, the combination of Ru and Ce0,5Zr0,5O2 promotion effect in RuCo50CeZrAl leads to a more active cobalt catalyst in BR reaction, selective at moderate temperature and significantly stable.
The noble metals tested were also supported over several supports, ZrO2-Al2O3 (ZrAl), Ce0,1Zr0,9O2-Al2O3,(10CeZrAl), La0,1Zr0,9O2-Al2O3 (10LaZrAl) and MgO-Al2O3 (MgAl). The study on monometallic systems (based on Pt and Rh) has shown that the doping of ZrO2 with the lanthanides cations (La and Ce) results in different effect. Both La and Ce present an enhancement of reducibility and dispersion of the active phase. This effect is clearly observed in Pt based catalysts since Rh particles are noticeable more dispersed in all cases. Thus Pt particle size is found greater than Rh in all cases, which is determining the activity and selectivity in BR and MSR. In addition, Pt catalyst shown deactivation related to the formation of coke. Our results demonstrate that Rh is a more active metal for biogas reforming than Pt. Using MgAl supports is was found important to establish the relation between the spinel like phase concentration MgAl2O4 and the particle size of the noble metal. In this sense, Pt catalysts present a highly performance on biogas reforming when the concentration of spinel is enhanced and thus, the interaction between Pt and support improved. Although, a stronger interaction is established Rh- MgAl2O4 giving a more actives particles in the catalyst 1RhMgAl24 due to a enhanced dispersion and the optimum SMSI related to an inversion of the spinel structure. The bimetallic systems PtRh showed an improvement of activity and deactivation resistance, in both reaction BR and MSR. The activity of the PtRh catalysts is reasonably similar due to the relevance of the synergy between Rh and Pt particles, thus the support nature plays a secondary role in the reactions performances. Since the interaction between the noble metals PtRh was not well assigned a Rh based catalyst supported on MgAl2O4 was selected to focus the following study on the manly parameters affecting the biogas reforming reaction.
BIOGAS REFORMING USING Rh MODEL CATALYSTS Two Rh based catalyst supported on MgAl2O4 were reasonable characterized by means of TEM, and Rietveld analysis, in order to get insight in the spinel structure and the relationships with the Rh particle size. A comparative study between 1RhMgAl12 and 1RhMgAl24 reflect an influence of the inverted spinel structure with a highly distributed particles. This appears to be optimum in case of 1RhMgAl24 catalyst. This system also presents a preferential grown of the family planes (311) and a porosity enhanced due to the formation of 20-100 pores, which derived in a superior average pore diameter.
To approach a correlation of the crystallography and morphology differences of the catalysts within the activity on biogas reforming, operando experiments in DRIFT-MS were employed. A preliminary study in stationary state in biogas reforming under the temperature range 450-750 ºC results in the formation of different carbonyl species in both catalyst. It has been observed a highly concentration of lineal carbonyl species, COlineal, on the performance of RhMgAl24. According to some authors [2] COlineal species are more reactives intermediates than CObridge, which are found in more extension in the less active catalyst, 1RhMgAl12. Then it is claimed that 1RhMgAl24 catalyst presents an optimum interaction between Rh-MgAl2O4, which provides a reduced Rh particles well stabilized over the catalyst surface compared with 1RhMgAl12. As a consequence of all collected results related to the interaction between Rh-MgAl2O4, an SMSI between Rh-MgAl2O4 is claimed. In addition, the SMSI is associated to an inverse structure of the spinel. The inversion grade of the formed spinel is favoring the Rh dispersion providing a more active site concentration. A more detailed DRIFT-MS operando experiments were used in order to get insight of the mechanistic importance of the Rh-MgAl2O4 interaction in 1RhMgAl24. For that, a transition state combined with steady state experiments results in an approximation of the most important stages of the reaction in biogas reforming. The following can be extracted as conclusion of this experiment: a bifuncional mechanism is favored related to a proper interaction Rh-MgAl2O4, because the interphase allows the activation of methane and CO2 but also, the proximity of support active sites to activate CH4 (Lewis acids sites Al+3 related to the (311) family planes) and CO2 (Lewis basic sites Mg+2) is improved.
In a fixed bed reactor the catalyst 1RhMgAl24 presents an active and stable behavior. The obtained activity and selectivity agrees with the result obtained under the DRIFT experiments in the importance of the RWGS participation. The apparent activation energy obtained, Ea, in the range 650-550ºC in a fixed bed reactor is comparable to others obtained by recognized authors in the literature, using Rh based catalysts [3]. Nevertheless the overviews of the bifuncional mechanism observed must have a reflection in the kinetics of biogas reforming. This issue is more interesting to be studied using a structured catalyst. Since this strategy provides a more selective catalyst in kinetically controlled conditions, avoiding diffusion drawbacks. Rh MODEL CATALYST IN MICROCHANNEL REACTOR AND KINETIC APROACH OF BIOGAS REFORMING 1RhMgAl24 catalyst was deposited on a microchannel reactor in order to apply the process intensification. The washcoating process was applied to paint a catalyst layer onto the metallic surface of the microchannel structure by means of the preparation of a suspension or slurry. The properties of the slurry were optimized trying to favor the stability, adherence and homogeneity of the suspension. In this context, the catalyst particle size, the pH, and the viscosity were studied to ensure the appropriate suspension properties. To well characterize the coating, several techniques were used SEM-EDX, the adherence test, and the textural properties. During the impregnation of the catalyst in the metallic structure numerous modifications may occur. According to the Rietveld analysis the calcined slurry, S1, present larger particles compared to the initial catalyst. In addition, the MgO phase is removed presenting only the spinel MgAl2O4 phase with a less inverse structure. This modification results in a more active catalyst but there is also observed a preference to produce coke, resulting in a progressive deactivation.
Nevertheless the structured catalyst, where the S1 system is coating the microchannels, shows highly stability with a formation of mainly amorphous carbon. Thus, the structured catalyst present major resistant to a deactivating coke formation. Other important advantage of using the microchannel reactor is the resistance of working under high space velocity conditions. In this context, the structured catalyst demonstrates to be able of working under thermodynamic control in severest contact time range, compared with the powder catalyst. Using the structured catalyst, the proper conditions to obtain differential conversions, far away from the equilibrium, in the kinetic control were investigated. For that, different pressures, space velocities and temperatures were essayed in order to obtain selective conditions to BR. The kinetic equation was obtained in the temperature range of 650-600 ºC. The obtained value of the apparent Ea reflects an important activation of the reaction energy compared with the obtained in the powder catalyst. Other authors claim this kind of energy barriers when Rh particles well dispersed are supported on a recognized SMSI solid [4,5].
ON THE FISHER TROPSCH SYNTESIS In order to give a complete picture of the GTL process a complete chapter is dedicate to the reaction of the Fischer Tropsch synthesis. Based on the result showed by the Co serie studied in reforming of biogas, a serie of Co/Al2O3 catalyst were synthetized in which a 20% wt. of oxide promoter is introduced. The nature of the oxide is studied by using ZrO2, CeO2 and several mixed oxides Ce/Zr. All catalyst presents slightly differences in cobalt dispersion and reducibility. These small differences in characterization are amplified in the performance on FT showing larger differences between catalysts. The CoZrAl catalyst showed a promotion of the catalyst activity and selectivity compared to a reference catalyst CoAl. The long chain hydrocarbons selectivity is significantly related to the formation of olefins species. In addition, the dispersion of ZrO2 were study by comparing two catalysts synthetized by two techniques. A microemulsion synthesis (ME) provides an homogeneous dispersion of the crystals over the alumina surface with a narrow particle size distribution. Nevertheless, no pronounced differences where found neither in cobalt dispersion nor the reducibility. Although, the conventionally prepared catalyst is more active and selective than the obtained by ME, regarding the FT performance.
Bibliography [1] A. Alvarez M., L.F. Bobadilla, V. Garcilaso, M.A. Centeno, J.A. Odriozola, J. CO2 Util. 24 (2018) 509–515.
[2] H. Arai, H. Tominaga, J. Catal. 43 (1976) 131–138.
[3] J. Wei, E. Iglesia, J. Catal. 225 (2004) 116–126.
[4] A. Erdöhelyi, J. Cserenyi, F. Solymosi, J. Catal. 141 (1993) 287–299.
[5] J.F. Múnera, S. Irusta, L.M. Cornaglia, E.A. Lombardo, D. Vargas, M. Schmal, J. Catal. 245 (2007) 25–34.
