**2. Solution film in the mass transfer equipment**

**Table 1** shows some literature related to mass transfer equipment with continuous liquid phase. These mass transfer equipment include packed-bed absorber, packed-bed or tray distillation column, falling film absorber, concentric absorption system, and bubble absorber. Except for bubble absorber [9, 10], a continuous liquid phase was presented as a solution film in the mass transfer equipment for all others. As mentioned by Wu [11], the Marangoni effect could be triggered in mass transfer systems with continuous liquid phases. Therefore, mass transfer behaviors that occurred in the solution film are discussed in this article.

In order to discuss the spontaneous Marangoni effect in the absorption process, an absorber packed closely with cylindrical packing was designed [11]. The solution flow rate was controlled under the state of laminar flow. Since the surface tension of water vapor is larger than that of TEG solution, the spontaneous Marangoni effect is triggered by absorbing water vapor in the solution film. Although the mass transfer performance could be enhanced by adding a promoter in capturing CO<sup>2</sup> by potassium carbonate (KCO3 ), the pressure drop and holdup increased in the packed absorption column. For example, see Ref. [12]. Glycine was added to K<sup>2</sup> CO3 solution film to examine the enhancement of the CO<sup>2</sup> absorption. In addition to adding surface additives, the Marangoni instability could also be produced by the temperature dependence of the surface tension, such as nonlinear model of the instability in gas absorption was developed [13] to discuss the performance for carbon dioxide absorbed by water. Recently, the structured packings with different thickness and channel angles were designed [14] to study effect of packings and surface additives on the performance of water vapor absorbed by LiCl film.


performance and the relationship between mass transfer performance and interfacial disturbance. By measuring the surface tension of liquid solution, the surface tension of liquid solution affected by the vapor of 2-ethyl-1-hexanol (2EH) in the gas phase was demonstrated [7]. The experimental results showed that the effect of surface additives on surface tension was larger for adding in the gas phase than in the liquid phase. Subsequently, the dynamic theory for the absorption and desorption of 2EH on the surface of the LiBr aqueous solution was discussed [8]. The simulated results showed that the higher the vapor pressure of 2EH in the gas phase,

94 Heat and Mass Transfer - Advances in Modelling and Experimental Study for Industrial Applications

Therefore, the mass transfer performance could be enhanced by the interfacial disturbance resulting from adding surface additives in the liquid and gas phases while the operating variables were controlled well. Discussions of the interfacial behaviors resulted from adding surfactants to the gas phase, which were limited in the literature, and the related data were rare. Mentioned earlier, the surface tension was affected by adding surfactant to the liquid and gas phases, leading to the influenced mass transfer performance by the interfacial disturbance resulting from the surface tension gradient. Therefore, the surfactant was added in the gas and liquid phases to discuss the effect of surfactant on mass transfer performance. Besides, the mass transfer performance with and without surfactant addition to the working solution in the packed-bed absorber was also compared. Not only was the relationship between mass transfer process and interfacial phenomena described but also the enhancement of mass

transfer performance for the absorption system was demonstrated in this study.

transfer behaviors that occurred in the solution film are discussed in this article.

the packed absorption column. For example, see Ref. [12]. Glycine was added to K<sup>2</sup>

surface additives on the performance of water vapor absorbed by LiCl film.

by potassium carbonate (KCO3

film to examine the enhancement of the CO<sup>2</sup>

**Table 1** shows some literature related to mass transfer equipment with continuous liquid phase. These mass transfer equipment include packed-bed absorber, packed-bed or tray distillation column, falling film absorber, concentric absorption system, and bubble absorber. Except for bubble absorber [9, 10], a continuous liquid phase was presented as a solution film in the mass transfer equipment for all others. As mentioned by Wu [11], the Marangoni effect could be triggered in mass transfer systems with continuous liquid phases. Therefore, mass

In order to discuss the spontaneous Marangoni effect in the absorption process, an absorber packed closely with cylindrical packing was designed [11]. The solution flow rate was controlled under the state of laminar flow. Since the surface tension of water vapor is larger than that of TEG solution, the spontaneous Marangoni effect is triggered by absorbing water vapor in the solution film. Although the mass transfer performance could be enhanced by adding a promoter

the Marangoni instability could also be produced by the temperature dependence of the surface tension, such as nonlinear model of the instability in gas absorption was developed [13] to discuss the performance for carbon dioxide absorbed by water. Recently, the structured packings with different thickness and channel angles were designed [14] to study effect of packings and

O by the LiBr aqueous solution.

), the pressure drop and holdup increased in

absorption. In addition to adding surface additives,

CO3

solution

the better the mass transfer performance for absorbing H<sup>2</sup>

**2. Solution film in the mass transfer equipment**

in capturing CO<sup>2</sup>

**Table 1.** Some literature related to mass transfer equipment with continuous liquid phase.

Based on the concept of Marangoni effect acting on the thin liquid film, the system n-heptane/ methylcyclohexane was used [15] to discuss the effect of positive and negative driving force on different packings. The criteria for determining the positive or negative driving force for the packed-bed distillation column were based on the packings; however, the criteria for determining the positive or negative Marangoni effect was decided by the mixture. The systems included methanol/water, methanol/isopropanol, and water/acetic acid, which were used to discuss the effective interfacial area for the positive, negative, and neutral Marangoni systems [16]; the systems included methanol-water, methanol-2-propanol, and n-heptane-toluene [17] to elucidate the relationship between froth stabilization and interfacial area; the systems included methanol/isopropanol and methanol/water [18] to describe the solution film affected by the Marangoni effect.

were calibrated by standard procedures. The absorbent solution was brought into the packed bed by the liquid pump and distributed over the packed bed by the nozzle. The liquid flow rates were controlled by rotameter. The air flow rates were adjusted by 0.5 HP blower and transistor inverters. The liquid film, flowed on the packing, contacted the gas phase in the packed-bed absorber and absorbed water vapor successfully. After a series of experimental tests (3–4 runs) were completed, the absorption system was heated to raise the temperature of the TEG solution. Once the heated solution contacted with the process air, the water molecules in the absorbent solution were stripped from the TEG solution. The regenerated TEG solution could be reused in the next series of experimental tests. Besides, a Rotronic IDL 20 K hygrometer with two humidity probes, which can measure the humidity from 0 to 100% RH

Discussions of Effects of Surface Tension on Water Vapor Absorbed by Triethylene Glycol…

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97

**Figure 2.** Experimental apparatus of this study.

In addition to the packed-bed absorber, the falling film or wetted wall column was also applied for the absorption process widely. 2EH was used as an additive in the vertical falling film to discuss heat and mass transfer enhanced by the Marangoni convection [19, 20]. Furthermore, the flat copper plate and the copper plate covered with a copper wire screen were also tested to observe the Marangoni convection resulting from adding 2EH to the solution film [20]. In contrast with water vapor absorbed by aqueous lithium bromide solution, carbon dioxide absorbed by aqueous monoethanolamine (MEA) solution could be regarded as a chemical absorption process. Since the surface tension of the absorbent solution was changed by a chemical absorption process, the Marangoni effect was always accompanied with this process. For example, see Refs. [21, 22]. CO<sup>2</sup> absorbed by the MEA solution was conducted, and the mass transfer enhancement and the cellular convection were discussed and observed.

Whatever gradient in surface tension resulted from spontaneous or artificial absorption process, it is difficult to observe by naked eye or scientific apparatus. Since the induced Marangoni convection and Marangoni instability were microscopic phenomena, they could be recorded or observed by scientific or special apparatus. On the basis of the difficult observation, some experimental systems were designed to demonstrate the mass transfer performance enhanced by the Marangoni effect, such as the concentric absorption system. Generally speaking, the surface additive with lower surface tension was injected by a capillary tube into the center of the absorption cell. For example, see Ref. [23]. Methanol, ethanol, propanol, and acetone were used as surface additives, respectively, to discuss absorption of CO<sup>2</sup> by water and surfactant solution in the presence and absence of Marangoni effect. Not only the liquid additive in the liquid surface but also vapor additive in the absorption system were carried out to analyze enhancement of mass transfer performance by the Marangoni effect for water vapor absorbed by LiCl solution [24].

The concept of packed-bed absorber was also the solution film that flowed over packing materials so that a series of experiments were performed to discuss absorption of water vapor by TEG solution and to elucidate the relationship between interfacial disturbance and mass transfer behaviors. TEG solution was used as a working solution to absorb water vapor in the packed-bed absorber, as shown in **Figure 2**, and the packing materials were 5/8-inch polypropylene flexi rings. The system can handle air flow rates from 1.35 to 1.58 kg/m<sup>2</sup> s and liquid flow rates from 0.6 to 0.9 kg/m<sup>2</sup> s. The flow meters and flow controller used in this study were calibrated by standard procedures. The absorbent solution was brought into the packed bed by the liquid pump and distributed over the packed bed by the nozzle. The liquid flow rates were controlled by rotameter. The air flow rates were adjusted by 0.5 HP blower and transistor inverters. The liquid film, flowed on the packing, contacted the gas phase in the packed-bed absorber and absorbed water vapor successfully. After a series of experimental tests (3–4 runs) were completed, the absorption system was heated to raise the temperature of the TEG solution. Once the heated solution contacted with the process air, the water molecules in the absorbent solution were stripped from the TEG solution. The regenerated TEG solution could be reused in the next series of experimental tests. Besides, a Rotronic IDL 20 K hygrometer with two humidity probes, which can measure the humidity from 0 to 100% RH

**Figure 2.** Experimental apparatus of this study.

Based on the concept of Marangoni effect acting on the thin liquid film, the system n-heptane/ methylcyclohexane was used [15] to discuss the effect of positive and negative driving force on different packings. The criteria for determining the positive or negative driving force for the packed-bed distillation column were based on the packings; however, the criteria for determining the positive or negative Marangoni effect was decided by the mixture. The systems included methanol/water, methanol/isopropanol, and water/acetic acid, which were used to discuss the effective interfacial area for the positive, negative, and neutral Marangoni systems [16]; the systems included methanol-water, methanol-2-propanol, and n-heptane-toluene [17] to elucidate the relationship between froth stabilization and interfacial area; the systems included methanol/isopropanol and methanol/water [18] to describe the solution film affected

96 Heat and Mass Transfer - Advances in Modelling and Experimental Study for Industrial Applications

In addition to the packed-bed absorber, the falling film or wetted wall column was also applied for the absorption process widely. 2EH was used as an additive in the vertical falling film to discuss heat and mass transfer enhanced by the Marangoni convection [19, 20]. Furthermore, the flat copper plate and the copper plate covered with a copper wire screen were also tested to observe the Marangoni convection resulting from adding 2EH to the solution film [20]. In contrast with water vapor absorbed by aqueous lithium bromide solution, carbon dioxide absorbed by aqueous monoethanolamine (MEA) solution could be regarded as a chemical absorption process. Since the surface tension of the absorbent solution was changed by a chemical absorption process, the Marangoni effect was always accompanied with this pro-

the mass transfer enhancement and the cellular convection were discussed and observed.

used as surface additives, respectively, to discuss absorption of CO<sup>2</sup>

Whatever gradient in surface tension resulted from spontaneous or artificial absorption process, it is difficult to observe by naked eye or scientific apparatus. Since the induced Marangoni convection and Marangoni instability were microscopic phenomena, they could be recorded or observed by scientific or special apparatus. On the basis of the difficult observation, some experimental systems were designed to demonstrate the mass transfer performance enhanced by the Marangoni effect, such as the concentric absorption system. Generally speaking, the surface additive with lower surface tension was injected by a capillary tube into the center of the absorption cell. For example, see Ref. [23]. Methanol, ethanol, propanol, and acetone were

solution in the presence and absence of Marangoni effect. Not only the liquid additive in the liquid surface but also vapor additive in the absorption system were carried out to analyze enhancement of mass transfer performance by the Marangoni effect for water vapor absorbed

The concept of packed-bed absorber was also the solution film that flowed over packing materials so that a series of experiments were performed to discuss absorption of water vapor by TEG solution and to elucidate the relationship between interfacial disturbance and mass transfer behaviors. TEG solution was used as a working solution to absorb water vapor in the packed-bed absorber, as shown in **Figure 2**, and the packing materials were 5/8-inch polypropylene flexi rings. The system can handle air flow rates from 1.35 to 1.58 kg/m<sup>2</sup> s and liquid flow rates from 0.6 to 0.9 kg/m<sup>2</sup> s. The flow meters and flow controller used in this study

absorbed by the MEA solution was conducted, and

by water and surfactant

by the Marangoni effect.

by LiCl solution [24].

cess. For example, see Refs. [21, 22]. CO<sup>2</sup>

at −20 to 60°C, was used in this study. The concentration of the TEG solution was measured by a refractometer. The cross-section area of the packed bed and air tunnel was 15\*15 cm<sup>2</sup> , and the height of packing was 45 cm. The absorption capacity could be calculated by the inlet and outlet humidity to discuss effect of operating variables on mass transfer performance.

Since the mass transfer performance would decrease with the decreased effective area, such a system is termed "negative system." Therefore, the surface refreshment was affected by the smaller packing and the lower liquid flow rates more significantly [15]. Three systems include organic and aqueous systems, Marangoni positive (methanol/water), neutral (methanol/isopropanol), and negative (water/acetic acid) systems, which were used to develop a mass transfer model for a distillation column packed with the structured packing [16]. The results showed that the effective area was larger for the positive system than that of the negative system due to the more stable liquid film. Besides, the experimental results also demonstrated that the better liquid distribution or more stable liquid film on the packing surfaces resulted from the positive effect, methanol/water, to increase mass transfer performance [18]. In addition to the packed distillation column, the interfacial area for the positive system in a tray distillation column also

Discussions of Effects of Surface Tension on Water Vapor Absorbed by Triethylene Glycol…

2EH was used as an additive to enhance absorption of water vapor by the LiBr solution film in the falling film system [19, 20]. Enhancement of heat transfer could be caused significantly by small amounts of additives during absorption process, and the enhanced degree was decided from the additive concentration and Reynolds number [19]. Besides, 2EH was also used as an additive in the system of the vertical falling film, and flat copper plate and the copper plate covered with a copper wire screen were tested by LiBr solutions with and without 2EH [20]. The experimental results showed that twice the heat transfer was enhanced by adding 2EH in LiBr-water films on the bare copper surface and approximately 2.5 and 3.5 times the mass transfer was enhanced by adding 2EH in LiBr-water films on the bare copper surface in adiabatic and water-cooled absorption, respectively. The Marangoni effect resulted from chemical

ing film systems [21, 22]. A model was assumed that the cellular convection was driven by the gradient in surface tension, which was induced by infinitesimally small perturbations of concentration [21]. The numerical results demonstrated that the minimum gas-liquid contact time was necessary for the convection to occur, and the time turned out to be below 0.01 s. In order to measure the mass transfer rate affected by the Marangoni effect in a microreactor and to compare this rate with the value for the analogous process without Marangoni effect, a falling film microreactor (FFMR) with 29 microchannels was designed and investigated for the gas-liquid mass transfer process [22]. The appearance of the Marangoni effect in a falling film microreactor was observed, which was accompanied with absorption enhancement when increasing amine concentrations under the condition of lower partial pressures of CO<sup>2</sup>

experimental results also showed that a 3–6-fold increase in the absorption rate is observed

For the concentric absorption system, methanol, ethanol, propanol, and acetone were added, respectively, to the water surface to induce interfacial disturbance [23]. The results showed that

and cetyltrimethylammonium bromide (CTMAB) were used as a surfactant, respectively, to test the performance of carbon dioxide absorbed by water. Enhancement of mass transfer performance for carbon dioxide absorbed by water was demonstrated for water surface adding 20–100 wt.% aqueous solution of methanol, ethanol, and 2-propanol. Increment of mass transfer performance with the increased surfactant concentration was also observed. In addition, the ethanol vapor and the ethanol droplets from capillary were added, respectively, to the

was enhanced by the interfacial disturbance. Sodium lauryl sulfate (SLS)

by aqueous MEA solution in the fall-

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99

. The

increased with the stabilized froth [17].

absorption, which was applied for absorption of CO<sup>2</sup>

for MEA concentrations in the range from 2 to 2.5 M.

absorption of CO<sup>2</sup>

The water vapors were absorbed by 93 wt.% TEG solution and 93 wt.% TEG solution with 5 wt.% ethanol, respectively. Therefore, effect of operating variables on mass transfer performance was discussed, and absorption capacities with and without surface additives added to the TEG solution were compared. On the other hand, the ethanol vapor was injected in the gas phase to discuss mass transfer difference between the additive adding in the liquid and gas phases.
