## 3 Model Development
### Steady-State Model Development
#### 3.2.4 Absorber Design
The solvent stream enters the absorber above the third stage, while the first two stages are reserved for the water washing section. A washing stream of water enters the absorber for two main reasons: (1) removing any entrained MEA that may be carried out along with the vented gas, forcing them down back to the absorber, (2) cooling of the vented gas before being released to the atmosphere. The washing section helped in reducing the entrained MEA in the flue gas five times compared to the scenario without a washing section. In commercial operation, absorbers have several components to ensure efficient contact between the two phases, such as liquid distributor, to ensure a uniform liquid distribution over the entire column cross-sectional area. Redistributors are also installed to prevent any unwanted build-up that might occur due to exceeding liquid misdistribution. Additional components include liquid collectors, wall wipers, support grids, hold down plates, and gas distributors [38]. The column diameter (D) is then determined to achieve the desired performance and separation after calculating the solvent flow rate as well as the packing material. The diameter is highly dependent on the vapor and liquid flow in the column, thus, it is calculated based on an 80% approach to flood. The flood point is defined as the operating point where the pressure drop rises rapidly with a parallel decrease in mass-transfer efficiency [40]. Column diameter is initially calculated using Equations (A1)-(A3) listed in Appendix A to provide us with an initial accurate first guess before further improvement could be made using Aspen Plus(r). An interactive sizing tool in Aspen is another approach to aid the user when sizing the equipment. Aspen uses information about the vapor and liquid flows to determine the diameters for different column sections. An initial diameter of 7.6 m was calculated for the absorber tower, a sensitivity analysis shown in Figure 12, was conducted to further validate and improve the obtained diameter. A final reduction of almost one meter was possible, maintaining both the Jet flooding and the desired capture rate. The Design specs tool available in the Flowsheeting options, enabled us to change the column diameter within a specified range while maintaining the flooding constraint. The design specs tool then performs several iterations until the condition is met and then automatically changes the value in the flowsheet. The column height needed to achieve the desired absorption was determined using the height equivalent to the theoretical plate (HETP) concept and is preferred over the height of transfer unit (HTU) approach [19]. Any increase in the absorber height will cause the rich loading to increase and, in turn, decrease the reboiler duty. The study conducted by [PERSON] [11] proved similar behavior up to a certain height. Beyond that height, there was not much effect on the reboiler duty. As there is a limit to the rich loading that could be achieved, increasing the height has to be optimized based on the capital cost of the equipment and the power required to blow the flue gas overcoming the increasing pressure drop with increasing height. According to the KOCH vendor, the HETP for the chosen packing was 315 mm, an initial value of 27 stages was used in the simulation model resulting in a packing height of 8.5 m. After several iterations, three more stages were added to the absorber tower to have a final height of 9.45 m achieving the desired separation. Resistance in the vapor and liquid film were calculated in different ways in Aspen. Without discretizing the film, the fast reaction rates will be calculated based on the interface concentration. As the reaction rates are rapid in the liquid film, the discretize film option was chosen to calculate the liquid film resistance. The number of discretized points is chosen to ensure solution stability as well as reasonable computation speed to allow for several iteration runs.