**1. Introduction**

The fluid disturbance resulting from the surface tension gradient in the interface of the gasliquid contact systems is usually called the Marangoni effect or surface tension effect. For the liquid thin film, the gas solutes are easier to dissolve into the thinner liquid film than the thicker liquid film [1], and the concentration in the thinner part would be higher than that

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

in the thicker part. Therefore, the surface tension gradient is formed in the interface. The Marangoni positive and negative systems can be defined by changes of the gas-liquid contacting area. The interfacial convection resulting from the Marangoni effect is usually called the Marangoni convection. The interfacial instability resulting from the surface tension is normal to the liquid surface, and it is always called the Marangoni instability. The surface tensions for absorption solutions could be measured by a surface tension meter (CBVP-A3, Kyowa Interface Co. Ltd.). The surface tension meter was based on the method for a Wilhelmy plate, as shown in **Figure 1**. A Wilhelmy plate is a thin plate, usually in the order of a few square centimeters in area, used to measure equilibrium surface or interfacial tension at a gas-liquid or liquid-liquid interface. The top of the plate was connected to a spring, and the spring was connected to a force sensor. The equilibrium state is reached instantaneously between the surface tension acting on the plate and the spring force pulling up the plate as the plate comes in contact with the liquid surface. According to the Wilhelmy equation, shown in Eq. (1), embedded in the surface tension meter, the surface tension for a liquid solution would be calculated and displayed on the panel.

$$\gamma = \frac{\text{F}}{\text{l} \text{cos}\theta} \tag{1}$$

ionic compounds, concentration difference, temperature difference and surfactant addition to the liquid solution. Therefore, the interfacial disturbance could be produced by solutes transferring across phases. Some studies described the relationship between the Marangoni effect

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

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The working electrodes were set in the experimental cell to detect the potential energy [2]. The concentration of the ferrocenyl surfactant was decreased with the decreased potential energy and then the surfactant became the surface active matter. The experimental results showed that the velocity of Marangoni flow decreased with the decreased desorption rate of the ferrocenyl surfactant. The concentration gradient of the desorbed ion resulted from the potential energy, and the surface tension gradient was formed by the concentration gradient. Therefore, the fluid disturbance was provoked in the surface layer of the liquid solution. In addition, the concept of the solutal Marangoni effect was always used to describe the fluid flow in the liquid surface by vaporizing the volatile materials. For example, see Ref. [3]. The mixture of ethanol/ water, heptane/decane, hexane/decane, and octane/decane was used to show that the liquid film climbs from a macroscopic reservoir as the volatile component with lower surface tension in the system. On the other hand, the studies about the thermal Marangoni effect focused on the convective phenomena for the bulk liquid layer with heating in the bottom. Since the heat source was from the bottom, the liquid density decreased along with the depth of the liquid solution. Therefore, the upward convection occurred due to the temperature gradient. Furthermore, the temperature of the upward fluid was still higher than the ambient liquid in the surface, and then the surface tension gradient was formed to make the outward flow radial. The local outward flow could be observed all over the liquid surface, and it was the so-called cellular-type flow. The small disturbance analysis was adopted to deduce that the surface tension gradient resulted from the change of temperature, which was large enough to produce the

cellular-type flow in the liquid convective cell that was heated from the bottom [4].

absorption capacities for CO<sup>2</sup>

In order to enhance the mass transfer performance for water vapor absorption by the solution in the absorption-refrigeration system, some surface additives were added to the liquid surface to activate the interfacial disturbance. N-hexanol, n-heptanol, n-octanol, 2-ethyl-hexanol, or n-nonanol was used as a surfactant to enhance the performance for water vapor absorption by the aqueous LiCl solution [5]. The experimental results showed that the interfacial disturbance occurred and the absorption capacity was enhanced in the concentration of n-octanol from 10 to 25 ppm. For the aqueous LiBr solution, adding the surfactants, except n-hexanol, induced the interfacial disturbance. The concentrations of other surfactants were controlled at 25 ppm. Besides, the

nol, n-butanol, and n-propanol added onto the liquid surface [6]. However, the interfacial disturbance could not be observed as n-hexanol and Triton X-100 were added on the surface of water. Based on the thickness of the liquid layer, the Marangoni number was also calculated to assess

In addition to adding surfactants on the liquid phase, there are also some studies adding surface additives to the gas phase to discuss the effect of surface active materials on mass transfer

the critical point for the interfacial disturbance resulting from the spreading liquid.

absorbed by water were increased as methanol, ethanol, n-propa-

and transport phenomena, which are introduced as follows.

where **γ** is surface tension, F is elastic force, *l* is the wetted perimeter, and θ is the contact angle between the liquid phase and the plate.

Mentioned earlier, the flow of interfacial fluid could be provoked by the Marangoni effect. The methods for inducing surface tension gradient include the design of electric field with

**Figure 1.** Schematic diagram for surface tension of the liquid solution exerted the plate.

ionic compounds, concentration difference, temperature difference and surfactant addition to the liquid solution. Therefore, the interfacial disturbance could be produced by solutes transferring across phases. Some studies described the relationship between the Marangoni effect and transport phenomena, which are introduced as follows.

in the thicker part. Therefore, the surface tension gradient is formed in the interface. The Marangoni positive and negative systems can be defined by changes of the gas-liquid contacting area. The interfacial convection resulting from the Marangoni effect is usually called the Marangoni convection. The interfacial instability resulting from the surface tension is normal to the liquid surface, and it is always called the Marangoni instability. The surface tensions for absorption solutions could be measured by a surface tension meter (CBVP-A3, Kyowa Interface Co. Ltd.). The surface tension meter was based on the method for a Wilhelmy plate, as shown in **Figure 1**. A Wilhelmy plate is a thin plate, usually in the order of a few square centimeters in area, used to measure equilibrium surface or interfacial tension at a gas-liquid or liquid-liquid interface. The top of the plate was connected to a spring, and the spring was connected to a force sensor. The equilibrium state is reached instantaneously between the surface tension acting on the plate and the spring force pulling up the plate as the plate comes in contact with the liquid surface. According to the Wilhelmy equation, shown in Eq. (1), embedded in the surface tension meter, the surface tension for a liquid solution would be calculated

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

where **γ** is surface tension, F is elastic force, *l* is the wetted perimeter, and θ is the contact

Mentioned earlier, the flow of interfacial fluid could be provoked by the Marangoni effect. The methods for inducing surface tension gradient include the design of electric field with

*<sup>l</sup>*cos*<sup>θ</sup>* (1)

and displayed on the panel.

γ = \_\_\_\_\_ <sup>F</sup>

**Figure 1.** Schematic diagram for surface tension of the liquid solution exerted the plate.

angle between the liquid phase and the plate.

The working electrodes were set in the experimental cell to detect the potential energy [2]. The concentration of the ferrocenyl surfactant was decreased with the decreased potential energy and then the surfactant became the surface active matter. The experimental results showed that the velocity of Marangoni flow decreased with the decreased desorption rate of the ferrocenyl surfactant. The concentration gradient of the desorbed ion resulted from the potential energy, and the surface tension gradient was formed by the concentration gradient. Therefore, the fluid disturbance was provoked in the surface layer of the liquid solution. In addition, the concept of the solutal Marangoni effect was always used to describe the fluid flow in the liquid surface by vaporizing the volatile materials. For example, see Ref. [3]. The mixture of ethanol/ water, heptane/decane, hexane/decane, and octane/decane was used to show that the liquid film climbs from a macroscopic reservoir as the volatile component with lower surface tension in the system. On the other hand, the studies about the thermal Marangoni effect focused on the convective phenomena for the bulk liquid layer with heating in the bottom. Since the heat source was from the bottom, the liquid density decreased along with the depth of the liquid solution. Therefore, the upward convection occurred due to the temperature gradient. Furthermore, the temperature of the upward fluid was still higher than the ambient liquid in the surface, and then the surface tension gradient was formed to make the outward flow radial. The local outward flow could be observed all over the liquid surface, and it was the so-called cellular-type flow. The small disturbance analysis was adopted to deduce that the surface tension gradient resulted from the change of temperature, which was large enough to produce the cellular-type flow in the liquid convective cell that was heated from the bottom [4].

In order to enhance the mass transfer performance for water vapor absorption by the solution in the absorption-refrigeration system, some surface additives were added to the liquid surface to activate the interfacial disturbance. N-hexanol, n-heptanol, n-octanol, 2-ethyl-hexanol, or n-nonanol was used as a surfactant to enhance the performance for water vapor absorption by the aqueous LiCl solution [5]. The experimental results showed that the interfacial disturbance occurred and the absorption capacity was enhanced in the concentration of n-octanol from 10 to 25 ppm. For the aqueous LiBr solution, adding the surfactants, except n-hexanol, induced the interfacial disturbance. The concentrations of other surfactants were controlled at 25 ppm. Besides, the absorption capacities for CO<sup>2</sup> absorbed by water were increased as methanol, ethanol, n-propanol, n-butanol, and n-propanol added onto the liquid surface [6]. However, the interfacial disturbance could not be observed as n-hexanol and Triton X-100 were added on the surface of water. Based on the thickness of the liquid layer, the Marangoni number was also calculated to assess the critical point for the interfacial disturbance resulting from the spreading liquid.

In addition to adding surfactants on the liquid phase, there are also some studies adding surface additives to the gas phase to discuss the effect of surface active materials on mass transfer 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, the better the mass transfer performance for absorbing H<sup>2</sup> O by the LiBr aqueous solution. 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.

**Mass transfer equipment**

Packed-bed absorber

Distillation column

Falling film absorber

Concentric absorption system

Bubble absorber NH3

CO<sup>2</sup>

H2

CO<sup>2</sup> /K<sup>2</sup>

CO<sup>2</sup>

water/acetic acid

n-heptane-toluene

H2

H2

CO<sup>2</sup>

CO<sup>2</sup>

CO<sup>2</sup>

H2

/MEA solution CO<sup>2</sup>

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

**Mass transfer material Behaviors for solution film References**

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

suddenly dropped when the surfactant is added

resulted in the presence of surface active substances

packing material covered by TEG solution was greater for the

of the solvent by adding glycine

the temperature dependence of

lower liquid flow rates promote the surface refreshment

The effective interfacial area is the same for the positive and neutral Marangoni systems at

Froth stabilization in positives systems resulted in the increased interfacial area

decreased by adding 2-ethyl-1-

tension at interface was induced

provoked by minute changes in

observed by adsorption of CO<sup>2</sup>

was induced by additive and hindered by surfactant

induced by adding ethanol

causes stable film on the

higher concentration

the surface tension

by channel angle

low vapor rates

packing

by 2EH

/NaOH solution Cellular convection was

hexanol (2EH)

surface tension

in MEA solution

[9]

95

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[10]

[11]

[12]

[13]

[14]

[15]

[16]

[17]

[18]

[19]

[20]

[21]

[22]

[23]

[24]

/water Surface tension of solution is

/water Surface tension gradient was

CO3 Reduction in the surface tension

/water Marangoni instability caused by

Water/LiCl Effective interfacial area affected

n-Heptane/methylcyclohexane The smaller packing and the

Methanol/isopropanol and methanol/water The positive Marangoni effect

O/LiBr Surface tension of LiBr was

O/LiBr Uneven distribution of surface

/MEA Cellular convection was

/water The Marangoni convection

O/LiCl The interfacial disturbance was

Methanol/water, methanol/isopropanol and

Methanol-water, methanol-2-propanol and

O/TEG The area for the surface of
