An investigative study on replacing the conventional furnaces of naphtha reforming with chemical looping combustion for clean hydrogen production
Graphical abstract
Introduction
Since the introduction of conventional naphtha reforming in 1940, many plants have utilized it to produce high octane reformate used for gasoline and aromatics [1,2]. The catalytic naphtha reforming units are important parts of the current oil refineries and petrochemical industries for the production of gasoline and aromatics (BTX), respectively. The produced hydrogen as a byproduct is separated and purified for various processes such as hydrotreating and hydrocracking [3]. The development of this process began when the refineries were forced to produce more high-octane gasoline. Basically, naphtha reforming reforms hydrocarbon molecules to aromatics. The conventional catalytic naphtha reforming unit consists of three or four adiabatic reactors in series [4,5]. These reactors operate at the temperature range of 700–800 , the total pressure of 4–40 bar and, hydrogen to hydrocarbon () molar ratios of 3–8 [6,7]. Catalytic naphtha reforming reactions are generally endothermic; therefore the temperature decreases along the reactor while the aromatics and hydrogen are produced. Consequently, the intermediate heaters are embedded between reactors to provide the desired inlet feed temperatures. The distribution of catalysts in reactors is variable which is lower in the first reactor but higher in the last one, because the reaction rates decrease through the reactors and become less endothermic. In the reactors, paraffins are isomerized to iso-paraffins and naphthenes. The naphthenes are dehydrogenated to produce aromatics compounds [8,9]. Iranshahi et al. investigated three configurations including radial flow spherical packed bed, radial flow tubular packed bed and axial flow spherical packed bed reactors which had lower pressure drop rather than conventional reactors [10,11]. They understood spherical packed bed reactors are a suitable configuration to minimize pressure drop [11]. Shanyinghu and Zhu studied molecular modeling and optimization of naphtha reforming by the model of Peng and Ancheyta-Juarez with 51 reactions to improve operation and profits [12]. Also, Naphtha reforming (NR) reactors were investigated by Mazzieri et al. [13], Rahimpour et al. [14], Shakeri [15] and, Zhao et al. [16].
Carbon dioxide () is one of the main greenhouse gases (GHG) that is emitted by human activities and the burning of fossil fuels. It is released to the atmosphere and remains there for a long time [[17], [18], [19]]. This causes some problems such as climate change, global warming, and ozone layer depletion [20]. Policies such as reducing fossil fuel consumption, increasing the efficiency of chemical processes, and using renewable resources are established to reduce emission [21], but their implementation might take a long time. Due to the importance of preventing emission, some processes like post-combustion capture, pre-combustion decarbonization, and oxyfuel-combustion [[22], [23], [24]] are presented to minimize emission. High costs, low efficiency, low concentration of and additional costs associated with the separation of have led to the development of the Carbon Capture and Storage (CCS) as a viable technology to minimize the emission [[25], [26], [27]]. Chemical Looping Combustion (CLC) is a two-stage combustion to generate power and heat by burning fossil fuels in chemical industries. This process was initiated by Ritcher and Knoche [28], with the aim of inherent separation by using two reactors called air (AR) and fuel (FR) reactors. Furthermore, this process is used to eliminate and emission from chemical industries and also to produce hydrogen as an important feedstock for hydroprocessing units [29,30]. Steam methane reforming (SMR) is one of the processes which can utilize the chemical looping technique for hydrogen production due to its economic interest [31,32]. Gnanapragasam et al. proposed configurations of the two chemical looping combustion systems (coal direct chemical looping (CDCL) and syngas chemical looping (SCL)) to increase the overall H2/CO2 ratio for a given amount of coal [33].
CLC has two or more reactions that are exothermic in AR and often endothermic in FR. The reactions are indicated below [34,35]:
In the oxidation reactor, the metal () receives oxygen from the air and oxidizes to the metallic oxide (MeO) while in the reduction reactor the metal oxide is reduced to provide oxygen for fuel reaction. CLC can have three different configurations: 1) two interconnected moving or fluidized-bed reactors, 2) packed reactors [36] and 3) rotating reactors. Fig. 1 shows a simplified diagram of two first configurations. In Fig. 1(a), oxygen carriers are circulated between air (AR) and Fuel (FR) reactors [37,38]. The metal in the air reactor, after being oxidized and converted to the metal oxide, is transferred to the fuel reactor to provide the oxygen needed for fuel reaction. Methane () is the main feed of the fuel reactor which reacts with metal oxide () producing and . The oxygen carrier which is reduced to metal in FR is transferred to the AR. The easy separation of carbon dioxide from the water via condensation distinguishes CLC from the other process [34].
In Fig. 1(b), oxygen carriers (OC) are assumed to reduce and oxidize through cyclic switching feed of FR and AR. Lyngfelt et al. [26] designed a fluidized fuel reactor and a riser type air reactor for CLC. In another work, Kronberger et al. [39] investigated fluidized bed reactors of CLC at atmospheric pressure. The process of NG-CLC is developed by Wolf et al. in two interconnected reactors to compare Ni and Cu as an oxygen carrier [40]. Fernandez and Alarcon [41] suggested two fixed bed reactors for the CLC process. Spallina et al. [42] investigated the CLC in packed-bed reactors.
The metal of the oxygen carrier has a significant effect on the CLC performance. These metals are typically Nickel (), Cobalt (), Copper () and Iron () which their important characteristics are listed as follows [37,[43], [44], [45], [46], [47]]:
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High conversion rates and reactivity in redox reactions
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High oxygen transport capacity
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No agglomeration on the metal such as carbon in the fuel reactor
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Low sintering and attrition rate
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Resistance to temperature and pressure variations
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Good mechanical stability whilst switching between two reactors
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Environmentally benign materials
Since metal () alone does not have the capability to provide the above specifications, a porous support is considered to increase the surface contact between the gas and solid phases. For example, nickel () supported on alumina () generally shows good thermal and physical stability [[48], [49], [50]]. Nickel-based catalysts are also widely used in the chemical looping reforming (CLR) process to produce hydrogen [51,52]. Some researches demonstrate that Ni is an active catalyst in hydrolysis reactions to generate hydrogen [53,54]. In addition, the comparison of NiO with other oxygen carriers such as CuO illustrates that the NiO can be used at higher temperatures without evidence of sintering [55].
Process integration (PI) is currently one of the most important methods to increase the efficiency of processes. One of the ways of PI is the coupling of energy intensive endothermic reactions with suitable exothermic reactions for energy saving and reducing the size of plants. In this regard, Ghani et al. investigated the thermal coupling of methyl ethyl ketone (MEK) and ammonia processes. In their suggested scheme, the required heat of methyl ethyl ketone production was supplied by the ammonia synthesis reaction, in contrast, the required hydrogen of ammonia synthesis was provided by the dehydrogenation reaction of 2-butanol to MEK [56]. Dehnamaki et al. investigated the effect of flow direction in a novel bifunctional reactor containing methanol oxidation and cyclohexane dehydrogenation to produce formaldehyde, benzene, and hydrogen simultaneously. They compared concurrent and counter current flows and observed the former is a better choice [57]. The mathematical modeling of CCR type of naphtha reforming thermally coupled with hydrogenation of nitrobenzene to aniline was done by Jafari et al. to enhance aromatics [58]. They employed a kinetic model including 32 pseudo-components with 84 reactions for naphtha reforming reaction. Aboosadi et al. studied thermally coupled reactors having methane-steam-reforming and hydrogenation of nitrobenzene to aniline [5]. Their results showed that the thermal coupling technique increases the conversion of methane.
In the conventional naphtha reforming (CNR) process, catalysts () are packed in three or four tubular reactors in which gas flows downward. Considering the quality of gasoline, researchers are seeking solutions to increase yields of high octane species in it [59,60]. Thermally Coupled Reactor (TCR) is an appropriate technique to improve the efficiency of the naphtha reforming process with the aim of minimizing energy consumption. In TCR the furnace and pre-heaters of CNR are replaced by an appropriate exothermic reaction. Chemical Looping Combustion (CLC) is one of the exothermic processes which can be used as a heat source to provide the required heat of naphtha reforming process. In previous research papers, the axial−flow fluidized bed and fixed-bed reactors of CLC were studied [61,62]. In this work, novel radial flow fixed bed reactors containing naphtha reforming and CLC are proposed. Based on the following results, the yield of aromatics is increased and the concentration of unreacted methane is not observed in the output of the fuel reactor.
Section snippets
Process descriptions
Fig. 2 (a) shows a schematic of the naphtha reforming thermally coupled with CLC with series feeding. In the first two reactors, naphtha reforming is coupled with CLC and the last reactor is considered alone. Moreover, a CLC assisted furnace (CLC-furnace) is designed to provide the temperature of the inlet feed of the first reactor. The product of naphtha reforming from the last reactor is sent to the separation units after passing through a heat exchanger. In that heat exchanger, reformate and
Mathematical model
The equations of energy and mass balances were derived for AR, FR, and NR reactors. The homogeneous model was considered. The main assumptions made are [81,82]:
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Plug flow stream is assumed.
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Heat losses are negligible.
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The ideal gas is considered in AR, FR, and NR.
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The heat and mass transfer of gas into the catalyst particle is ignored (NR).
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Carbon formation in FR is ignored.
Table 4 represents the governing equations and auxiliary correlations.
Results and discussion
The results of AF-CNR and RF-CNR have been compared with plant data under the same condition in Table 5 [83]. As can be seen, there is a good agreement between model prediction and plant data. Moreover, the output results of CLC-NR are presented. As one can see, the higher molar flow rate of aromatics is achieved in the CLC-NR configuration compared to the AF-CNR and RF-CNR.
The rate of heat transfer for the inward flow pattern was obtained higher than the outward pattern. Also, due to the
Conclusions
The thermally coupling of naphtha reforming process and CLC (CLC-NR) was proposed in order to enhance the production rate of aromatics and hydrogen. A series reactor configuration with a radial flow pattern was considered in this work. In CLC-NR configuration, reactors were divided into some subsections (e.g., 15, 30) to to provide the heat required for endothermic reactions of naphtha reforming. In this research, an average dimensionless time, (), is considered for comparative studies.
Acknowledgements
The authors would like to thank Iran National Science Foundation (INSF) for supporting the research (Grant number: 98012467).
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