**1. Introduction**

The global atmospheric concentration of carbon dioxide and its contribution to global warming requires technical measures to limit the emission of this greenhouse gas. One possibility is by separating the carbon dioxide from the waste gas stream of fossil power plants by applying absorption technologies. The absorption process of CO2 can be performed using solvents with different additives. Amine solvents such as monoethanolamine (MEA), diethanolamine (DEA), and methyldiethanolamine (MDEA) are common for CO2 capture (Wilk et al. 2017) [1]. The reversible reactions and the moderate reactivity between CO2 and amine solutions enable efficient CO2 capture (Yamada 2016) [2]. Barzagli et al. (2014) [3] studied nonaqueous amine solvents such as 2-(isopropylamino)ethanol (IPMEA), 2-(tert-butylamino)ethanol (TBMEA) and N-methyl-2,2'-iminodiethanol(MDEA) for CO2 capture and found that CO2 removal efficiency was in the range of 87–95% at equilibrium, depending on the operational conditions. Although amine absorption processes are widely used for CO2 capture, they have some disadvantages related to equipment corrosion, amine degradation (Sanna 2014) [4], and high energy consumption (Wilk et al. 2017) [1]. The absorption technology for CO2 capture consists mainly of the absorber column and the regeneration unit. The absorber column can be a tray or packed column. The absorbent enters the absorber from the top, and the waste gas, which contains CO2, enters the absorber from the bottom. The gas and the liquid phases contact each other on the trays or the packing materials. The trays or the packed material enhance the gas/liquid interfacial area, which increases the mass and heat transfer between the contact phases. The CO2 component transforms from the gas phase to a liquid phase and then is absorbed.

#### *Studying the E*ff*ect of Pressure on Gas*/*Liquid Interfacial Area*

Literature reviews reveal that there are various studies concerning the influence of pressure on the hydrodynamics and mass transfer in gas/liquid systems. Some of the studies are devoted to the influence of pressure on the creation of bubbles. These studies used a capillary tube or single orifices connected to a gas chamber. Other studies have investigated the influence of pressure on interfacial areas in a bubble column, packed column, or tray column. Kling et al. (1962) [5] were the first to observe that an increase in pressure at a single gas inlet orifice and constant superficial gas velocity creates a decrease in the initial bubble volume (Oyevaar 1989) [6]. Kling et al. (1962) [5] suggested that the increase in energy content causes the gas to enter further into the liquid, causing elongated bubbles, which separate more easily from the orifice, leading to smaller bubbles at higher pressures. LaNauze et al. (1974) [7] studied the influence of pressure and gas flow rates on the creation of CO2 bubbles in the water at different diameters of orifices photographically. They published the results of the behavior of bubble volume over pressure up to 2.1 MPa at different gas flow rates. They found that the bubble volume is increased when the gas flow rate is increased. Furthermore, it was shown that the bubble volume decreased significantly when the pressure was increased between 0.1–1 MPa, whereas it slightly decreased when the pressure was increased between 1–2.1 MPa. Bier et al. (1978) [8] studied the influence of operating pressure on an initial bubble volume, by sparging N2 or He through a capillary tube into the water or ethanol. The authors concluded that the influence of the operating pressure is much smaller compared with sparging the gas through an orifice connected to a gas chamber since the gas chamber limits pressure vibrations that happen in close gas supply lines (Oyevaar 1989) [6]. Idogawa et al. (1987) [9] noted that the diameter of the initial bubble decreases to 25% when pressure is increased from 0.1 to 15 MPa. Oyevaar et al. (1989) [10] determined interfacial areas at pressures up to 1.85 MPa in a bubble column and a packed column. The authors found that the interfacial areas are unaffected by pressure in the packed bubble column, but that the influence of the pressure on the interfacial areas in the bubble column arises from the generation of smaller bubbles at the gas distributor. Badssi et al. (1988) [11] investigated the effect of the pressure and superficial velocity of gas and liquid on the interfacial area in a laboratory column equipped with cross-flow sieve trays; they checked that each tested variable has an independent influence on the interfacial area. They investigated the effect of the pressure on the total interfacial area in two different gas–liquid systems, CO2-DEA and CO2-NaOH. They reviewed that the total interfacial area decreases when the pressure is increased. Benadda et al. (1996) [12] studied the effect of pressure on the interfacial area in a counter-current packed column. Their experiment conditions were conducted at specific gas mass flow rates of 0.1 kg/m2s, and specific liquid mass flow rates of 5.52 kg/m2s. They concluded that the interfacial area decreases when the pressure is increased between 0.1 and 1.2 MPa. Molga et al. (1996) [13] determined the gas–liquid interfacial during CO2 absorption by using DEA as well as DEA-ETG aqueous solutions. Their experiment device was a bubble column reactor with an inner diameter of 156 mm. The results obtained by Molga et al. (1996) [13] are different; they reviewed that there is no observed influence of pressure on the measured interfacial areas.

Most available studies in the literature are interested in the proportional correlation between the pressure and the absorbed amount of gas in a liquid. In contrast, studies on the effect of pressure on the gas/liquid interfacial area are still limited. One can summarize the objectives of this study as follows:

