Engineering Solutions for CO2 Conversion

Finally, the entirety of the absorber is considered at the full scale. The pressure coefficients calculated at the REU scale are the link between the latter and the full scale and are therefore introduced in the full‐scale simulations as an additional momentum sink in Eq. (2.2). The final outcome from this multi‐scale approach was the velocity field within the absorber, with the objective of avoiding the possibility of flooding and selecting the most appropriate gas distributor so as to minimize strong velocity gradients.

Other multi‐scale strategies have followed in order to model amine scrubbers, such as that presented by Sun et al. [11], who proposed a two‐scale approach partially based on CFD. At the REU‐scale three‐dimensional (3‐D) multiphase (gas and liquid), CFD simulations of a limited set of REUs of the commercial packing Mellapak 350X were used to obtain the stream split fraction coefficients and the effective wetted area ratio as a function of the liquid flow rate. The authors distinguished between four REU types, namely, inlet of the column, outlet, wall, and interior. From that stage, they mapped the entire set of REUs within the column, assigning an identifier based on their location and subsequently, introduced the parameters from the small‐scale calculations mentioned above into a mechanistic model. The final output was the liquid hold‐up distribution across the entire column, which is necessary to obtain the wet pressure drop by means of experimental correlations. Although not entirely based on CFD, the scale‐based approach reported by Sun et al. [11] proved that multiphase simulations with gas–liquid interface tracking via the VOF method are possible with the advent of computational capabilities with increased power. The use of the computationally costly VOF method was not restricted to 2‐D simulations at the corrugation level or 3‐D simulations over a simple inclined plate, but at the REU scale.

A number of other models falling under the umbrella of one of the three scales presented by Raynal and Royon‐Lebeaud [6] can be found in the literature, showcasing interesting characteristics of the flow within an amine scrubber. At the corrugation scale 2‐ and 3‐D simulations featuring the VOF algorithm have been used to study the fundamental hydrodynamics of rivulets and droplets. Some attempts to introduce the chemistry between carbon dioxide and the amine have also been reported in 2‐D by Haroun et al. [12] and in 3‐D by Sebastia‐Saez et al. [13]. At the REU scale, most of the studies have focused on obtaining the dry pressure drop of the packing, where an accurate selection of the appropriate turbulence model is crucial [14]. Finally, at the full scale, a big step forward has been taken by introducing the multiphase reactive flow on the porous Euler‐Euler approach. To accomplish this, Pham et al. [15] introduced mass and momentum source terms added to the Navier–Stokes equations. These are the terms marked as S massand S momin Eqs. (2.1)and (2.2), respectively, to account for the anisotropy of the packing. To include these source terms in the calculation, one can make use of a user‐defined function, like those used in commercial software such as ANSYS Fluent, or directly code their defining equations in an open‐source CFD software such as OpenFOAM or an in‐house code. The momentum source terms were accounted for the loss of momentum on both the liquid and the gas phase caused by the porous resistance, momentum exchange between both phases and liquid dispersion. For a complete description of the momentum source terms, the reader is referred to the work of Pham et al. [15]. Table 2.1gathers information on CFD models related to amine scrubbers in terms of the scale used and the conclusions reached.

Table 2.1Summary of published CFD studies concerning amine scrubbers.