(41c) Process Intensification of CO2 Capture in a Gas-Liquid Vortex Reactor

1. Introduction

In 2021, the energy sector experienced an unprecedented surge in emissions [1], showing the pressing need for the global transition to a net-zero, circular economy. This transition is an ongoing, yet challenging process, especially for sectors facing significant hurdles in decarbonizing. Therefore, developing Carbon Capture (CC) technologies to efficiently capture CO₂ from point-source gases is imperative. Among various CC technologies, chemical absorption stands out as a mature, industrially employed technique due to the high absorption efficiency and the relatively low cost of the solvents employed. However, significant hurdles persist, stemming from the high capital expenditure (CAPEX) associated with large columns and the management of extensive gas volumes, alongside substantial operational expenditure (OPEX) incurred during solvent regeneration.

Process intensification emerges as a crucial strategy to overcome these challenges. Process intensification refers to any chemical engineering development that leads to a substantially smaller, cleaner, and more energy-efficient technology [2], holding the promise for substantial cost reductions.

The Gas-Liquid Vortex Reactor (GLVR) is an intensified reactor developed at the Laboratory for Chemical Technology (LCT, UGent) for gas-liquid applications. The reactor consists of a static, cylindrical, fluidization chamber where gas is injected tangentially creating a high slip velocity and gravity field (Figure 1). This gas injection creates a high turbulence that induces the momentum transfer from the gas to the liquid phase, resulting in intensified heat and mass transfer. Notably, GLVR achieves high throughput in a small reactor volume, leading to high energy efficiency. Previous experimental studies have confirmed the favorable hydrodynamics and micromixing efficiency resultant from intensified momentum transfer within the reactor [3]. Computational Fluid Dynamics (CFD) simulations have also been performed in parallel for the GLVR targeting to evaluate the hydrodynamics and optimize the reactor design and performance [4, 5]. The current study endeavors to evaluate the performance of the GLVR in reactive scenarios, with a speciffic focus on the area of CO₂ capture technologies. The recently constructed and optimized GLVR will be evaluated for CO₂ absorption and desorption conditions using liquid solvents. The gas flow acts both as the energy input and as the feed that contains CO₂ in the absorption or as the gas carrier in the desorption process. The GLVR technology obviates the need for mechacinal rotation, resulting in an simplified design that induces durability and operating safety.

2. Methods

The investigation of the GLVR performance for CC technologies is conducted through two stages: evaluation of CO₂ absorption and CO­₂ desorption experiments using liquid solvents. Monoethanolamine (MEA) diluted to 30 wt.% serves as the benchmark solvent under atmospheric conditions. For CO₂ absorption experiments, a gas mixture containing CO₂ in the range of 5-10 vol.% along with nitrogen (N₂) is injected into the GLVR. A various range of operating conditions is tested, while an non dispersive infrared (NDIR) sensor provides on-line data for the absorption efficiency. The amount of CO­₂ absorbed in the liquid phase is validated using titration techniques. To enhance absorption efficiency, blended amines will be further investigated. Specifically, a mixture of MEA and piperazine (PZ) is chosen due to PZ’s rapid kinetics, aiming to leverage the reactor's short residence time optimally. Furthermore, the absorption efficiency will be evaluated under various initial CO₂ loadings of the lean solvent entering the reactor, mimicking industrial conditions more closely.

Regarding the CO₂ desorption experiments, the pre-heated liquid solvent is initially loaded with CO₂ resulting in a loading in the range of 0.28-0.5 mol CO₂/mol MEA. A carrier gas, injected into the GLVR at elevated temperatures, provides the energy and heat necessary for the desorption to take place. Preliminary studies involve the evaluation of the desorption efficiency as an indicator of the solvent regeneration performance and the heat transfer efficiency in the GLVR. Subsequently, a steam generator will be employed to assess the energy consumption during the desorption process.

3. Results and discussion

Figure 1 shows the first assessment results obtained from the CO₂ absorption and desorption (solvent regeneration) experiments in the GLVR. The experiments took place under various gas and liquid flow rates. In the context of CO₂ absorption, an efficiency of up to 80% was attained, with potential for further enhancement by utilizing blended amines with advanced properties. It was observed that the absorption efficiency increases with increasing liquid flow rate and/or decreasing gas flow rate. However, this correlation introduces a tradeoff, as higher liquid flow rates lead to lower CO₂ loadings, while lower gas flow rates result in decreased capture rates. Smart and efficient process design holds promise in mitigating this challenge. Nonetheless, the GLVR exhibits superior absorption capacity compared to conventional reactors, as it achieves a high absorption rate within a comparatively small reactor volume. This positions the GLVR as a promising option for reducing the capital cost associated with conventional absorption columns.

Furthermore, the GLVR exhibited notable regeneration efficiency, reaching up to 70%, demonstrating the potential for advanced heat and mass transfer. The CO₂ desorption efficiency boosts significantly for a low liquid flow rate and a high gas flow rate. Finally, the CO₂ release rate per unit volume of the GLVR was found to be one to two orders of magnitude higher than a rotating packed bed (RPB), another intensified process technology, and a conventional packed bed (PB) respectively. This highlights its potential for enhanced, intensified performance that can lead to substantial OPEX reductions.

4. Conclusions

Experimental investigation of the CO₂ absorption and desorption performance in the Gas-Liquid Vortex Reactor (GLVR) was carried out. The study reveals promising findings, with the absorption and desorption efficiency reaching up to 80% and 70% respectively. These outcomes demonstrate the ability of the GLVR to achieve intensified mass and heat transfer at high throughputs, highlighting its potential for substantial capital and operational cost reductions. The superior absorption and desorption capacity suggests the reactor’s suitability for efficient scale-up, placing it as a competitive technology compared to other conventional or intensified reactors. Further research will focus on including various solvents to improve the absorption efficiency and optimizing the energy consumption for solvent regeneration. Overall, the findings suggest that the GLVR appears as a promising process intensification technology for CO₂ capture, towards a sustainable, low carbon future.

References

[1] IEA, World energy outlook 2022, IEA Paris, France, 2022.

[2] A.I. Stankiewicz, J.A. Moulijn, Process intensification: transforming chemical engineering, Chemical engineering progress 96(1) (2000) 22-34.

[3] Y. Ouyang, M. Nunez Manzano, S. Chen, R. Wetzels, T. Verspeelt, K.M. Van Geem, G.J. Heynderickx, Chemisorption of CO2 in a gas–liquid vortex reactor: An interphase mass transfer efficiency assessment, AIChE Journal 68(5) (2022) e17608.

[4] S. Chen, P. Malego, K.M. Van Geem, Y. Ouyang, G.J. Heynderickx, Design and Optimization of Gas–Liquid Vortex Unit Using Computational Fluid Dynamics (CFD) Simulation, Industrial & Engineering Chemistry Research 62(42) (2023) 17068-17083.

[5] S. Chen, Y. Ouyang, L.A. Vandewalle, G.J. Heynderickx, K.M. Van Geem, CFD analysis on hydrodynamics and residence time distribution in a gas-liquid vortex unit, Chemical Engineering Journal 446 (2022) 136812.