Modelling and Simulation and Economic Evaluation of CO2 Capture Using Downflow Gas Contactor (DGC) Process (Flexible Funding 2022)

Dr Tohid N.Borhani, University of Wolverhampton, was awarded funding in the UKCCSRC’s Flexible Funding 2022 call to look at the “Modelling and Simulation and Economic Evaluation of CO2 Capture Using Downflow Gas Contactor (DGC) Process”.

Carbon capture is recognised as one of the most effective technologies to reduce the environmental impact of human activities [1–4]. There are several processes that can be used in the post-combustion and pre-combustion conditions. Processes such as absorption, adsorption, membrane, cryogenic, etc. Between these processes, absorption using different solvents, especially using amine solvents, is the most mature process in the world [5]. This method consists of removing carbon from exhaust gas using solvent before processing or releasing to the atmosphere. The performance of this technology relies on the equipment/unit configuration and type, solvent types, and operating system of the process [6,7]. Therefore, by changing and modifying each of these parameters, we can change the performance of absorption-desorption systems.

One of the most promising parameters that can improve the performance of CO2 absorption process in aspect of cost and solvent utilisation is changing or modifying equipment/unit in the process. So far, different types of unit operations have been developed for CO2 absorption such as rotating packed bed (RPB), packed column, trayed column and Bubble column. Each of these unit operations has their own advantages and disadvantages. Thus, novel concept designs are currently developed to increase the overall performance of the system while minimising cost. One of them is the Downflow Gas Contactor (DGC) which can be a promising alternative for numerous applications. Indeed, DGC can be classified as a mass transfer device preliminary designed for contacting liquid and gas.

Figure 1: Batch DGC unit used in this study

In this study, we have examined the batch DGC to capture CO2 suing water and MEA solution (Figure 1). The result of experimental work was promising, and DGC can be considered as a high potential unit operation for carbon capture, and capture the CO2 by more than 90% carbon capture level (Table 1). Primary cost analysis showed that this unit can be cheaper than conventional packed column. By considering an industrial base case the size of DGC to do the same level of carbon capture is 0.89 m for diameter and 7 m length of column. If we need to use packed column to do the same job, we will need a column with 12.8 diameter and 18.4 m length. More research and work to convert batch DGC to steady state system is required. In addition, steady state stripper DGC should be designed.

 

Table 1: Carbon capture from mixture of N2 and CO2 using 5 wt% MEA solution

A model of the batch DGC system was developed to investigate the effectiveness of its CO2 absorption with no packing. The DGC is characterized by bubbly flow, and mass transfer is controlled by its hydrodynamics, which affects the holdup, bubble size and mass transfer coefficient. The interfacial area which is directly proportional to the mass transfer flux is in turn dependent on bubble diameter and gas fractional holdup. The model development consists of two parts: (1) first principle mathematical equations development, and (2) implementation in Aspen Custom Modeler (ACM) and validation (Figure 2). The two parts were carried out and here we report the key findings. Although, there are yet-to-be resolved numerical problems in the validation of the model for comparison with experiments, a useful conclusion can be drawn as follows.

We found that under the operating regimes and reactor dimensions of the DGC, the control of gas holdup and interfacial area is essential. The interfacial area was as high as 757.2 m2/m3 for 5 mm-size gas bubbles but it reduced with larger bubbles. Superficial velocity or volumetric gas flow rate directly influences the holdup. The findings were presented at a larger meeting with collaborators/partners. Thus, in theory, the DGC reactor operating in downward concurrent flow with no packing can be used to capture CO2. We propose that for more accurate inferences, more robust future investigation can consider the mapping of reactor’s physical and operating parameters to the gas holdup data to be acquired by experiments using laboratory or pilot sized reactor. These would provide an experimental basis for the creation of predictive and scale-up rules for the DGC reactor.

Figure 2: Model structure in this study

More experimental work is ongoing to find out the mass transfer coefficient and holdup parameters in the column. This study was a primary feasibility study to show that DGC can be used for carbon capture, and we are working on more research proposals to scale up this system in the future.

Read more on Tohid’s Flexible Funding 2022 project page.

Acknowledgments

We extend special appreciation to both Membracon UK Ltd [8] and WRK Ltd [9] as key collaborators within this project. WRK provided the patented DGC technology and Membracon as their UK delivery partner. They provided invaluable technical support, without which this research would not have been feasible.

References

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[2] Shu DY, Deutz S, Winter BA, Baumgärtner N, Leenders L, Bardow A. The role of carbon capture and storage to achieve net-zero energy systems: Trade-offs between economics and the environment. Renewable and Sustainable Energy Reviews 2023;178:113246. https://doi.org/https://doi.org/10.1016/j.rser.2023.113246.

[3] English JM, English KL. An overview of carbon capture and storage and its potential role in the energy transition. First Break 2022;40:35–40.

[4] Cormos A-M, Dinca C, Petrescu L, Chisalita DA, Szima S, Cormos C-C. Carbon capture and utilisation technologies applied to energy conversion systems and other energy-intensive industrial applications. Fuel 2018;211:883–90.

[5] N.Borhani T, Wang M. Role of solvents in CO2 capture processes: The review of selection and design methods. Renewable and Sustainable Energy Reviews 2019;114. https://doi.org/10.1016/j.rser.2019.109299.

[6] Aghel B, Janati S, Wongwises S, Shadloo MS. Review on CO2 capture by blended amine solutions. International Journal of Greenhouse Gas Control 2022;119:103715. https://doi.org/https://doi.org/10.1016/j.ijggc.2022.103715.

[7] Borhani TN, Wang M. Role of solvents in CO2 capture processes: the review of selection and design methods. Renewable and Sustainable Energy Reviews 2019.

[8] Membracon UK | Global Leader In Water Treatment Since 2002 n.d. https://www.membracon.co.uk/

[9] WRK Design and Services – Consulting Chemical Engineers n.d. http://wrkdesign.co.uk/