Study of Promoters to Reverse Boudouard Reaction Under Regeneration Step Conditions of FCC Process

Study of promoters to Reverse Boudouard reaction under regeneration step conditions of FCC process Sérgio Pereira1*, Filipa Ribeiro1, Marcelo Maciel Pereira2 1*[email protected] 1Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal 2Instituto de Química, Universidade Federal do Rio de Janeiro, Av. Athos da Silveira Ramos, Rio de Janeiro, RJ, Brasil ABSTRACT The fluid cracking catalytic process (FCC) contributes with about 30% of the total CO2 emissions in a refinery. The possibility to react CO2 and coke (Reverse Boudouard reaction) during the spent catalyst regeneration using a rich atmosphere in CO2 and O2, instead of air, can mitigate CO2 emissions and, simultaneously, produce CO for sequential uses. In this work, alumina catalysts modified by both alkali and alkaline earth metals in presence or not of vanadium were prepared, with the purpose of finding promoters for this reaction. Therefore, it was shown that this goal was achieved by an alumina catalyst modified with lithium and vanadium under normal regeneration temperatures, i.e. in the range of 690 to 720ºC, with the best efficiency among all catalysts, verifying a great synergy between lithium and vanadium. This synergy is noticed by comparing the values of average conversion during the first five minutes of reaction in 13 CO2/O2/He atmosphere of Li-V/Al2O3 and Li/Al2O3. At 720ºC, the first catalyst attained a value of 39% while the second attained only 6%. The first formed product was 13CO, followed by 12CO and then 12 12 by CO2. The CO was formed by both RB reaction and by burning the coke with O2. Considering the amount of coke produced in FCC units, we notice that this process could be performed in a CO2/O2 atmosphere. Keywords: CO2, FCC, Coke, Vanadium, Lithium, Reverse Boudouard reaction 1. Introduction The industry quick development in the second additional effort by the major emitting countries, half of the twentieth century led to a serious this situation will become worse [2]. This makes decline of planet’s ecological situation. A major the study of new technologies to reduce CO2 problem has been the combustion of emissions increasingly important. hydrocarbons, which during the last few decades The Fluid Catalytic Cracking (FCC) is one of the caused a significant increase of carbon dioxide in most important processes in a refinery and its atmosphere, it’s the main greenhouse gas. objective is converting vacuum gasoil from Currently, less than 1% of total CO2 production by atmospheric or vacuum distillation in high added anthropogenic sources is recycled [1] and the value products, such as gasoline. The process concentration of this gas is about 400 ppm. consists mainly of three steps: reaction, Although there are measures to reduce CO2 separation, and catalyst regeneration. In the emissions, it is estimated that, if there is no reaction step, there is the formation of coke (6- 1 1 10%) [3] in the catalyst being related to the for this work. The global RB reaction: C+CO2 → processed load quality, i.e., lower quality results in 2CO, is very endothermic, between 160 to 200 larger quantity of coke formed. This coke leads to kJ.mol-1, however the partial reaction (Scheme 1a) catalyst deactivation. However, it is easily is slightly exothermic or endothermic [5] [6]. In this regenerated from combustion reactions, releasing reaction, in addition to CO are formed species of large quantities of pollutants, mainly carbon oxidized coke (coke-O). This oxidized coke will be dioxide. It is estimated that this process is decomposed and lead to the formation of CO and responsible for about 15-30% of the total carbon CO2, Scheme 1b, though, this reaction is very dioxide emissions in a refinery. Because of the endothermic, since it involves the decomposition need to restrict emissions of this gas various of functional groups formed on the surface of the studies have been carried, especially in FCC oxidized coke [5]. Consequently for this process catalyst regeneration step. Recently it was the use of O2 is required to cause the proposed to use pure oxygen instead of air ("oxy- decomposition of these groups. The oxidized coke firing process"), where it is mixed with recycled burning with O2, represented by the Scheme 1c, CO2 from flue gases and this reaction mixture is will provide the necessary energy, since this is a used in the regeneration stage of the FCC very exothermic reaction [7]. catalysts [4]. This strategy would enable the transformation of CO2 to CO, which can Scheme 1a: CO2 + coke → CO + coke-O subsequently be converted into various products, Scheme 1b: Coke-O → CO/CO2 such as hydrogen by water-gas shift reactions, Scheme 1c: Coke-O + O2 → CO/CO2 methanol, dimethyl ether or hydrocarbons as well, Scheme 1. Reaction steps in FCC catalyst significantly reducing the CO2 emissions from regeneration in an atmosphere rich in CO2 and O2. those FCC units. The project is showed in Figure 1. Following this idea, M. Pereira and collaborators [1] [8] [9] proposed that the The reaction between CO2 and coke, known as regeneration stage of the FCC catalyst may be divided into two regeneration steps to a better performance of the reaction RB, according to Figure 2. Figure 2. Two stage FCC regeneration with diathermic wall Reverse Boudouard reaction (RB) serves study [9]. Figure 1. Simplified diagram of an FCC unit using pure O2 instead of air in the regeneration reactor [4]. 22 Several studies have been done to promote the Aldrich). The supports already impregnated by reaction between coke and CO2 [10] [11]. Group I group I and II metals were placed in contact with and II elements are excellent promoters for this the aqueous solution of vanadium precursor in a reaction, for taking into account the acidity of the rotary evaporator at 60ºC under reduced pressure CO2 molecule and the basicity of these metals, for 2 hours. Finally, drying was carried out in an they are able to create basic sites on alumina for oven at 120°C for a period of approximately 12 carbon dioxide adsorption thereby promoting their hours. After drying, the samples were calcined at reaction with the coke. As regards the vanadium, 600°C for 3 hours with a heating rate of 5°C/min. it is widely used as a catalyst for oxidative The catalyst preparation was performed to set up dehydrogenation reactions in the presence of O2 a loading of 1 wt.% of vanadium on all supports and CO2. The mobility along the constituents of and the catalyst were named as V/Al2O3, Li- the catalyst can be used in favor of RB reaction, V/Al2O3, Na-V/Al2O3, K-V/Al2O3, Mg-V/Al2O3, Ca- since the catalyst can be formed in-situ during the V/Al2O3 and Ba-V/Al2O3. regeneration of the spent catalyst. Herein, we explore the RB reaction over alumina modified by 2.1.3. Catalyst coking alkali (Li, Na, K) and alkaline-earth (Mg, Ca, Ba) For coke formation, the prepared catalysts were metals, with and without vanadium [12]. impregnated with vacuum gas oil (VGO) at a proportion catalyst/VGO of 2:1. In order to 2. Experimental facilitate the dissolution of VGO was used n- hexane (99%, Vetec) as solvent. The coking was 2.1. Catalyst preparation carried out in a quartz reactor under nitrogen flow of 60 mL/min. The reactor was introduced into a 2.1.1. Support preparation preheated oven at 530˚C and the reaction The precursor alumina exhibited the boehmite occurred during 15 minutes. structure, according to X-ray diffraction pattern (JCPDS 74-1895). 2.1.4. Catalyst characterization The preparation of modified alumina was carried The textural characterization was carried out by out by performing a physical mixture of alumina nitrogen adsorption on an ASAP 2020 and aqueous solution of chlorides salts of lithium, Micromeritics analyzer. The implementation of this sodium, potassium, magnesium, calcium and technique involved a step where gas was barium. The amount of group I and II chloride removed from the samples at a temperature of salts in the mixture was determined to obtain a 5 220°C for 4 hours. The specific surface area was wt.% final mixture in alumina. After drying, the determined through the BET (Brunauer-Emmett- modified support was obtained by the calcination Teller) method and pore volume and pore at 800°C for 20 hours in air with a heating rate of diameter distribution were calculated form the N2 5°C/min. The catalysts were named as Li/Al2O3, adsorption curve using the BJH (Barrett-Joyner- Na/Al2O3, K/Al2O3, Mg/Al2O3, Ca/Al2O3 and Halenda) method. Ba/Al2O3. Additionally, a pure alumina sample was The X-ray diffraction (XRD) patterns were prepared to use as comparison. obtained using a Rigaku Ultima IV diffractrometer. This equipment uses CuK radiation (1.542 Å), 2.1.2. Vanadium addition voltage 40 kV and 20 mA current. The Vanadium was introduced by incipient wetness measurements were performed according to a impregnation using an aqueous solution of variation of the angle 2θ vanadyl acetylacetonate (VO(C5H7O2)2, 95%, 33 Table 1. Catalysts textural properties. Metal and coke amount. Metal SBET Coke Metal SBET Coke Catalysts 2 Catalysts 2 (wt.%) (m /g) (wt.%) (wt.%) (m /g) (wt.%) Al2O3 - 127 3,0 V/Al2O3 nd 122 7,0 1,7 (Li) Li/Al O 1,5 48 1,9 Li-V/Al O 46 3,9 2 3 2 3 0,9 (V) 2,7 (Na) Na/Al O 2,5 131 2,3 Na-V/Al O 134 5,1 2 3 2 3 0,9 (V) K/Al2O3 nd 149 2,6 K-V/Al2O3 nd 148 5,4 1,6 (Mg) Mg/Al O 1,8 110 6,0 Mg-V/Al O 112 5,4 2 3 2 3 0,9 (V) 3,2 (Ca) Ca/Al O 3,7 104 4,1 Ca-V/Al O 105 5,5 2 3 2 3 0,8 (V) Ba/Al2O3 nd 138 3,0 Ba-V/Al2O3 nd 134 6,0 between 5° and 80° using a 0.02° step with a The first catalytic tests were performed under 10°/s.

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