545641 Conceptual Design of a Dual Fluidized Bed System for an Intensified Steam Methane Reforming Concept Coupled with Ca-Ni Looping

Tuesday, June 4, 2019: 10:54 AM
Republic ABC (Grand Hyatt San Antonio)
Theodoros Papalas, Andy Antzara and Angeliki Lemonidou, Chemical Engineering, Aristotle University of Thessaloniki, Thessaloniki, Greece

Introduction

The severe environmental issues along with the foreseen depletion of fossil fuels have accentuated the need of finding alternative fuels and energy production technologies that reduce the dependence on fossil energy and moderate CO2 emissions. Hydrogen is regarded as an important primary industrial gas as well as a promising and sustainable energy carrier due to its combustion having high energy efficiency and zero carbon emissions. The dominant large-scale production path of hydrogen is steam reforming of natural gas which is considered an energy intensive process and requires multiple steps and complexity to achieve the generation of high-purity hydrogen. Furthermore, due to thermodynamic limitations the process is usually coupled with water-gas shift reactors downstream the reformer to recover additional hydrogen and thus not avoiding the co-production of CO2. To address the aforementioned issues, a novel technology which provides a path for intensified conversion of natural gas directly to high purity hydrogen in a single step has been developed; the so-called sorption enhanced chemical looping steam methane reforming (SE-CL-SMR) [1]. This concept combines the reformer and WGS reactors in a single unit by introducing a solid sorbent material such as CaO that can in–situ remove the produced CO2. The exothermic carbonation reaction of CaO with the produced CO2 enables the performance of both the reforming and water-gas shift reactions in a single vessel, overcoming the thermodynamic limitations of the overall reaction, while the heat generated by the strongly exothermic carbonation is consumed in–situ for the endothermic reforming reaction. When the solid sorbent material reaches saturation, it needs to be regenerated in a separate reactor for it to be re-utilized. The necessary heat for the regeneration is addressed with the introduction of a second chemical loop, where an oxygen transfer material (OTM) such as NiO is also circulating between the two reactors by undergoing continuing redox reactions. During the regeneration stage, a sweep gas such as pure oxygen can be used for the strongly exothermic oxidation of the OTM thus moderating the thermal demands of the regeneration. The oxidized OTM returns to the reformer wherein is reduced by methane and the reformate gases. In addition to the oxygen transfer properties as an OTM, NiO in the reduced form presents excellent catalytic properties for steam methane reforming and water gas shift reactions. A schematic representation of the combined process is presented in Fig. 1.

Figure 1: Schematic representation of SE-CL-SMR process

Methodology and Results

This study proposes the implementation of the reduction and regeneration cycles in a system of two interconnected fluidized bed reactors that operate under different hydrodynamic regimes. Based on previous work of our group [2], the solid materials used consist of a CaO-based CO2 sorbent (66wt% CaO/CaZrO3) and a NiO-based OTM (40wt% NiO/ZrO2) which form a mixture of Geldart A particles. The design of the reactors is performed with the assistance of the chemical process simulator Aspen Plus® V9 and all the results obtained refer to steady state simulations. The reformer is modeled as a bubbling bed, whose diameter and length are chosen to be equal to 1.5m and 3.1m respectively and thus form a total reactor volume of ~5.3m3. In order to operate in the bubbling regime, the inlet superficial gas velocity needs to be higher than the minimum bubbling velocity, with the latter being at least three times as high as the minimum fluidization velocity. In this case, the inlet gas velocity is proposed to be nearly 0.07m/s which is ~6.5 times as high as the minimum fluidization velocity. For the simulation of the bubbling bed reactor the two-phase theory is utilized, which indicates the co-existence of an emulsion and a bubble phase [3]. For the estimation of the bubbles’ diameter within the bed, a correlation is used that takes into account both the coalescence and splitting frequencies of bubbles inside the reactor [4]. An assumption is made that the bubble diameter remains constant to 11cm for all the length of the reactor, which is the value calculated in the middle of the bed. Bubbles rise with a velocity of ~0.78m/s and perform a plug flow movement within the reactor by occupying  ~5% of the total reactor volume. They also contain a small amount of solids that account for the 0.6% of the bubble volume. The rest of the reactor is comprised of the emulsion phase, where it is assumed that perfect mixing between gases and solids occur. The emulsion remains in minimum bubbling condition and thus has a solid fraction and a gas velocity of 0.53 and 0.03m/s respectively. The bed is axially divided into zones, with each being composed of a RCSTR and a RPFR model that are connected in parallel and represent the emulsion and bubble phases respectively [5]. The reactor models solve the mass balance equations for each component by taking into account kinetic equations for the rates of the reactions taking place within the reformer. A Langmuir-Hinshelwood model is utilized for the reforming and water-gas shift reactions, while the reduction of OTM is described through the shrinking core model [6,7]. The carbonation reaction is assumed to reach equilibrium. The equilibrium carbonation constant is calculated from the standard Gibbs free energies of formation of the components that participate in the reaction. The outlet streams of the reactor models of each zone are modified by taking into consideration the mass transport phenomena between the emulsion and bubble phases before entering the next pair of reactor models. The solid phase of the last outlet stream of the bubbling bed enters the regenerator which is assumed to operate as a riser. The dimensions of the reactor are chosen to be 0.17m and 4.50m for the diameter and the length respectively. The main feature of the riser is that solid particles exit from the top of the bed due to the velocity of the gas feedstock being higher than the terminal velocity of the solids. In this case, the superficial gas velocity is estimated to be 0.59m/s which is nearly 79 times as high as the minimum fluidization velocity and surpasses the value of the terminal velocity. The riser is assumed that achieves perfect mixing of gas and solid that allows its axial division into an array of RCSTR models. The kinetic expressions used both for calcination of CaCO3 and Ni reoxidation are based on the shrinking core model [7,8].

 

The proposed system succeeds in the generation of 210 Nm3 H2 /hr with ~95% purity (dry basis). This hydrogen production is achieved with the 92.7% conversion of the methane feedstock and an estimated 79.7% hydrogen yield. Conversions of nearly 100% and 83.5% are attained by the OTM and the sorbent respectively in the reformer. The solid materials return to their initial forms in the regenerator and a recovery of ~55 Nm3 CO2/hr with more than 97% purity is attained. Assuming that the reactors operate non-adiabatically, the heat duty of the reformer and the regenerator were calculated around 68kW and 53kW respectively, which sum up to a total ~46 MJ/kmol H2 thermal demand of the whole process. The results of this study are compared to the ones predicted by the thermodynamic analysis of the combined process for the verification of the proposed model and a small deviation is witnessed between the results for the reformer as shown in Table 1. A deviation was not observed for the degrees of NiO reduction, CaCO­3 calcination and Ni oxidation, since the thermodynamic analysis that all three solid materials achieve almost 100% conversion. The above deviations can be attributed to the amount of gas that enters the reformer with the form of bubbles [5] as well as to the used kinetics data obtained from literature that does not refer to the studied materials. However, further research is required in order to confirm this proposal. Although the concept of two interconnected fluidized beds seems to be an appropriate solution to the continuous hydrogen production through the intensified steam methane reforming process, further parametric studies need to be performed to define the right number of zones of the bubbling bed reactor and the optimized values of all the variables that influence the system’s performance.

Table 1: Deviation of this model’s results to equilibrium data

 

This study

Equilibrium Data [1]

Deviation (%)

CH4 Conversion (%)

92.7

97.0

-4.4

H2 Yield (%)

79.7

83.0

-4.0

H2 Purity (%)

95

94.7

+0.3

CaO Conversion (%)

83.5

81.0

+3.1

References

 

[1] A. Antzara, E. Heracleous, D.B. Bukur, A.A. Lemonidou, Int. J. Greenh. Gas Control 32 (2014) 115-128.

[2] A. Antzara, E. Heracleous, A.A. Lemonidou, Appl. Energy 180 (2016) 457-471.

[3] D. Kunii, O. Levenspiel, Fluidization engineering, 2nd ed., Butterworth-Heinemann, Boston, 1991.

[4] J.-H. Choi, J.-E. Son, S.-D. Kim, Ind. Eng. Chem. Res. 37 (1998) 2559-2564.

[5] R. Porrazzo, G. White, R. Ocone, Fuel 136 (2014) 46-56

[6] T. Numaguchi, K. Kikuchi, Chem. Eng. Sci. 43 (1988) 2295-2301.

[7] A. Abad, J. Adánez, F. García-Labiano, L.F. Diego, P. Gayán, J. Celaya, Energy Fuels 62 (2007) 533-549.

[8] İ. Ar, G. Doğu, Chem. Eng. J. 83 (2001) 131-137.

 


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