Discrete Bubble Modeling of CO2 Absorption in a NaOH Solution in a Micro-Structured Bubble Column

Deepak Jain, J.A.M. (Hans) Kuipers, Niels G. Deen*

Gas-liquid-solid reactive flows are encountered in many industrial processes. While the transport of reactants and chemical reaction is between gas and liquid phase, solid particles are introduced as catalysts to facilitate the reaction. Major industrial processes where such flows are encountered include alkylation, hydrogenations, oxygenations, waste-water treatment and Fischer-Tropsch synthesis. Usually two reactor configurations are used on a large scale in industry, namely slurry bubble columns and trickle bed reactors. In slurry bubble column reactors, the particles are suspended in the liquid, while in trickle bed they appear as a fixed bed. While the slurry bubble column exhibits excellent mixing and heat transfer, the mass transfer suffers due to the coalescence of bubbles as they rise in the column. Bubble coalescence is unwanted as it reduces the net interfacial area for mass transfer. Trickle bed reactors are known for having good mass transfer but poor mixing and heat transfer. As the reactor performance requires advantages of both the reactor types mentioned above, a novel reactor type micro-structured bubble column (MSBC) is proposed in Jain et al. (2013).

In this reactor, it is proposed to insert static meshes of catalyst coated wires in the reactor. The wire mesh serves a number of purposes including cutting bubbles into smaller pieces (resulting in a high interfacial area), enhancing interface dynamics (increasing the mass transfer coefficient), avoiding filtration of catalyst particles (lowering costs). If needed internals like cooling/heating pipes can be inserted in the column to facilitate the local heat transfer.

Jain et al. (2013) have used discrete bubble model (DBM) to computationally study the effect of cutting in the bubble column due to presence of wires. A reduction in bubble diameter due to cutting from the mesh is reported in their work. This cutting and reduction in bubble size is sought to provide enhanced surface dynamics and more interfacial area. In the present work, the study is also extended to a industrially more relevant staged reactor with multiple meshes.

An improved version of the DBM is presented in Jain et al. (2014) in the form of a hybrid volume of fluid-discrete bubble model (VOF-DBM), where the surface dynamics of liquid at the top of the column are also included. Such a configuration helps in removing artificial top boundary layer conditions for the liquid and hence makes the model more realistic.

Chemisorption, i.e. mass transfer by absorption followed by chemical reaction, is central to various industrial processes and hence is also center of various previous researches (Darmana et al., 2007). Therefore apart from the hydrodynamic aspects in the column due to the introduction of wires, chemisorption is also studied in this work. The chemical reaction system chosen here is carbon dioxide absorption in a sodium hydroxide solution. The absorption of carbon dioxide in a sodium hydroxide solution is carried out by diffusion, which is followed by reaction of aqueous CO₂ with OH^- ions to form HCO₃^- and CO₃^2-. Since the reaction kinetics of the latter part is very fast, the total process becomes mass transfer limited and therefore it is an ideal case to study here.

In the current work, an attempt has been made to unify all these advancements in the DBM to form a single model that can be used for studying the aforementioned micro-structured bubble columns with mass transfer and chemical reaction. The results obtained from the model are analyzed and useful qualitative data has been obtained. The use of multiple meshes and the distance between consecutive meshes can be varied to get an optimum system configuration.

References:

1. Darmana, D., Henket, R. L. B., Deen, N. G., & Kuipers, J. A. M. (2007). Detailed modelling of hydrodynamics, mass transfer and chemical reactions in a bubble column using a discrete bubble model: Chemisorption of CO2 into NaOH solution, numerical and experimental study. Chemical engineering science, 62(9), 2556-2575.
2. Jain, D., Lau, Y. M., Kuipers, J. A. M., & Deen, N. G. (2013). Discrete bubble modeling for a micro-structured bubble column. Chemical Engineering Science, 100, 496-505.
3. Jain, D., Kuipers, J. A. M., & Deen, N. G. (2014). Numerical study of coalescence and breakup in a bubble column using a hybrid volume of fluid and discrete bubble model approach. Chemical Engineering Science (in press).