Reactor design and analysis of a simulated moving bed reactor for chemical-looping combustion

Clarke Palmer*, Lu Han, George M. Bollas

*Department of Chemical & Biomolecular Engineering, University of Connecticut, Storrs, 191

Auditorium Road, Unit 3222, Storrs, CT, 06269-3222, USA. Email:clarke.palmer@uconn.edu

Abstract

The control of greenhouse gas emissions is a global problem that needs an immediate solution.
To mitigate the adverse effects from fossil fuel utilization, focus is shifting towards CO2 capture and sequestration technologies. Chemical-looping combustion (CLC) is one method for energy production with low-cost CO2 separation. In this process as depicted in Figure 1, a hydrocarbon fuel (i.e., natural gas) is oxidized by a metal oxide oxygen carrier (i.e., NiO), producing CO2 and H2O. The reduced oxygen carrier (i.e., Ni) is then regenerated by oxidation in air, emitting N2 and unreacted O2. With CLC, a pure stream of CO2 can be captured after condensation of the H2O without a large energy penalty or the need for additional separation steps.

Figure 1: Basic concept of chemical-looping combustion

In the literature, CLC is implemented as interconnected fluidized bed processes (Lyngfelt, Leckner, & Mattisson, 2001), alternating flow fixed-bed processes (Noorman & van Sint Annaland, 2007), or moving bed processes (Adanez, Abad, Garcia-Labiano, Gayan, & De Diego,
2012). Each reactor configuration has advantages and disadvantages as far as their CO2 capture efficiency, the separation steps required and the need for specific oxygen carrier particles properties and addition rate. The moving bed reactor design can achieve higher efficiencies than fluidized bed processes, by utilizing countercurrent flows of fuel and solids to maximize the contact area. However, fluidization or circulation of the solids often leads to a number of operational challenges, including stream contamination, gas leakage, and particle abrasion.
An alternate version of the moving bed reactor is the simulated moving bed reactor. A basic design of this version is depicted in Figure 2. This process consists of switching the inlet and outlet ports simultaneously along the axial dimensions of a standard fixed-bed to simulate the countercurrent movement of solids. Simulated moving bed reactors have been proven to increase efficiencies and overcome equilibrium-restricted reactions in absorption, adsorption and extraction processes, such as reactive chromatography (Ray & Carrt, 1993).

B

CH4

A

CO2, H2O

Direction of gas flow and port switching

D

Figure 2: Design of simulated moving bed reactor

In this work, we explore the simulated moving bed concept for chemical-looping combustion applications. A simulated moving bed reactor design is modeled using multiple fixed-bed reactors in the configurations shown in Figure 2. A one-dimensional, homogeneous fixed-bed reactor model with axial dispersion, energy balance and momentum balance is used in concert with CLC reduction kinetics of NiO with CH4 derived previously, using literature and in-house fixed bed reactor data (Han, Zhou, & Bollas, 2013, 2014; Iliuta, Tahoces, & Patience, 2010; Zhou, Han, & Bollas, 2013). The reactor chosen in this work, mimics existing fixed bed reactors reported in the literature scaled-up to a larger scale, more relevant to industrial applications, following a model-based scale-up procedure (Zhou, Han, & Bollas, 2014). The performance of the simulated moving bed reactor is then compared to its fixed bed counterpart in terms of CO2 capture efficiency, oxygen carrier conversion, CH4 conversion and selectivity to solid carbon via methane decomposition. The reactor temperature profiles are also explored to identify
advantages in the proposed setup in terms of heat utilization within and between the reactors.
Application of the simulated moving bed technology yields two main benefits over its fixed-bed counterpart. The first is higher CO2 selectivity, which is essential for sequestration. The implications of this are evident in the conversion of oxygen carrier in Figure 3. As the inlet and outlet ports are switched along the length of the reactor, the inlet feed is constantly introduced to fresh oxygen carrier, which promotes combustion efficiency to CO2 and prevents unwanted side
reactions that yield partial oxidation products. The second benefit is the reduction of the total carbon formation during the reduction cycle. A comparison to the fixed-bed performance is shown in Figure 3. By allowing the combustion products to be recycled throughout the NiO depleted regions of the reactor that might have solid carbon buildup, the unwanted carbon is gasified. In summary, the simulated moved bed reactor design is shown to be promising for CLC applications and future work is aimed at optimization studies of the novel design. In this presentation a proof of concept analysis will be illustrated with cases studies comparing the operation of an alternating fixed bed reactor with that of a simulated moving bed, where total reactor size, oxygen carrier loading, methane capacity, temperature and pressure are kept the same.

Figure 3: Selectivity towards solid carbon vs. integrated oxygen carrier conversion (left) and CO2

gas selectivity vs. integrated oxygen carrier conversion (right) for simulated moving bed (SMB) and

fixed-bed processes

Acknowledgements: This material is based upon work supported by the National Science

Foundation under Grant No. 1054718 and the UConn Prototype Fund.

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