**3.1. Mass transfer results for solution film in the equipment**

A series of experiments presented in this article were water vapor absorbed by the TEG solution films in the packed-bed absorber. Therefore, some results related to solution film in the mass transfer equipment were described as follows.

For packed-bed absorber, the effective interfacial area is increased for the positive Marangoni effect. Absorption of water by LiCl solution [14] found that the surface area was larger for a 30° channel angle and 6 mm thickness packing material. Therefore, the maximum heat and mass transfer performance were 0.4 for COP and 85% for dehumidification efficiency. Since the foam resulted from addition of surface additive would hinder mass transfer process, addition of surface additive into the LiCl solution did not helpful. For absorption of water vapor by TEG solution [11], results showed that the mass transfer performance was larger for concentration of TEG solution greater than 92 wt.%. Since the surface tension of water vapor was higher than that of TEG solution, the surface tension of TEG solution is increased during the absorption process. The coverage of the higher concentration of TEG solution on the packing surface was wider than the lower concentration due to the difference in surface tension. The coverage of TEG solution on the packing surface resulted from increment in surface tension, which could be termed as a positive Marangoni effect for the absorption system.

For the gradient in surface tension, the Marangoni effect in the packed-bed distillation column was divided into a positive and a negative system. Transportation of a component with lower surface tension from a liquid phase to a gas phase may increase surface tension in the surface of the transferred spot. Since the surface tension for the transferred spot is higher than the surrounding liquid, the surrounding liquid is drawn to the transferred spot. The flow phenomenon driven by this process may spread over the packing well in the packed-bed distillation column and increase mass transfer performance. Therefore, the system, making a more effective area on the packing, is termed "positive system." On the contrary, transportation of a component with high surface tension from a liquid phase to a gas phase may decrease surface tension in the surface of the transferred spot. Since the surface tension for the transferred spot is lower than the surrounding liquid, the induced stress is directed from the transferred spot to the surrounding liquid. The flow phenomenon may lead the packing surface to be contrasted. 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 increased with the stabilized froth [17].

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>

the height of packing was 45 cm. The absorption capacity could be calculated by the inlet and

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 dis-

A series of experiments presented in this article were water vapor absorbed by the TEG solution films in the packed-bed absorber. Therefore, some results related to solution film in the

For packed-bed absorber, the effective interfacial area is increased for the positive Marangoni effect. Absorption of water by LiCl solution [14] found that the surface area was larger for a 30° channel angle and 6 mm thickness packing material. Therefore, the maximum heat and mass transfer performance were 0.4 for COP and 85% for dehumidification efficiency. Since the foam resulted from addition of surface additive would hinder mass transfer process, addition of surface additive into the LiCl solution did not helpful. For absorption of water vapor by TEG solution [11], results showed that the mass transfer performance was larger for concentration of TEG solution greater than 92 wt.%. Since the surface tension of water vapor was higher than that of TEG solution, the surface tension of TEG solution is increased during the absorption process. The coverage of the higher concentration of TEG solution on the packing surface was wider than the lower concentration due to the difference in surface tension. The coverage of TEG solution on the packing surface resulted from increment in surface tension,

which could be termed as a positive Marangoni effect for the absorption system.

For the gradient in surface tension, the Marangoni effect in the packed-bed distillation column was divided into a positive and a negative system. Transportation of a component with lower surface tension from a liquid phase to a gas phase may increase surface tension in the surface of the transferred spot. Since the surface tension for the transferred spot is higher than the surrounding liquid, the surrounding liquid is drawn to the transferred spot. The flow phenomenon driven by this process may spread over the packing well in the packed-bed distillation column and increase mass transfer performance. Therefore, the system, making a more effective area on the packing, is termed "positive system." On the contrary, transportation of a component with high surface tension from a liquid phase to a gas phase may decrease surface tension in the surface of the transferred spot. Since the surface tension for the transferred spot is lower than the surrounding liquid, the induced stress is directed from the transferred spot to the surrounding liquid. The flow phenomenon may lead the packing surface to be contrasted.

outlet humidity to discuss effect of operating variables on mass transfer performance.

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

cuss mass transfer difference between the additive adding in the liquid and gas phases.

**3. Discussions for mass transfer**

**3.1. Mass transfer results for solution film in the equipment**

mass transfer equipment were described as follows.

, and

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 absorption, which was applied for absorption of CO<sup>2</sup> by aqueous MEA solution in the falling 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> . The experimental results also showed that a 3–6-fold increase in the absorption rate is observed for MEA concentrations in the range from 2 to 2.5 M.

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 absorption of CO<sup>2</sup> was enhanced by the interfacial disturbance. Sodium lauryl sulfate (SLS) 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 absorption system [24]. Since the Marangoni effect was more pronounced for the concentration of LiCl greater than 40 wt.%, the experimental results showed that the removal efficiencies were increased significantly beyond 40 wt.%. Absorption enhancement was better for ethanol vapor than for ethanol droplets, which was also demonstrated.

exposure time and to decrease the absorption capacity. According to these points of view and

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

when the liquid flow rate was increased, as shown in **Figure 6**. Since the TEG solution was used as the working solution for absorption of water vapor in the gas phase, the higher mass transfer performance would be accompanied with the larger liquid flow rate. On the other hand, the larger liquid flow rate would promote the phenomenon of surface renewal. It could be thought that the absorption site increased in the packed-bed absorber and the mass transfer

increased with the increased liquid flow rate. Since the explanations of the effect of operating variables on mass transfer performance for TEG solution with surfactants were similar as that for the TEG solution without surfactants, the descriptions for the mass transfer performance affected by operating variables were neglected under the condition of adding surfactants.

As shown from **Figures 3**–**6**, the removal amounts were always larger for the TEG solution with ethanol than without ethanol. The results can be explained by two points of views. Ethanol molecules would vaporize from the TEG thin film in the packed-bed absorber. The gradient in surface tension would result from the vaporization of ethanol, and the interfacial disturbance would be formed by the gradient in surface tension. The probability of water vapor contacted with the TEG solution film would increase with the formed interfacial disturbance. Therefore, the mass transfer performance would be enhanced by adding ethanol to the TEG solution. On

performance was increased. Therefore, **Figure 6** showed that the removal amount of H<sup>2</sup>

O by the TEG solution decreased with the

O by the TEG solution was increased

http://dx.doi.org/10.5772/intechopen.76148

O was

101

the experimental results, the removal amount of H<sup>2</sup>

**3.3. Influences of additives for the presented study**

**Figure 4.** Effect of TEG concentrations on removal amount.

larger air flow rate. In contrast, the removal amount of H<sup>2</sup>

#### **3.2. Influences of operating variables for the presented study**

As shown in **Figure 3**, the removal of H<sup>2</sup> O by the TEG solution was decreased with the increased liquid temperature. Since the driving force for H<sup>2</sup> O absorbed by the desiccant solution is determined by the depression of water vapor pressure, the water vapor will be transferred from the bulk gas phase with the higher vapor pressure to the gas-liquid interface with the lower vapor pressure. The vapor pressure of desiccant solution was lower for the lower liquid temperature, and the depression of water vapor pressure would be larger for the lower liquid temperature. Therefore, the lower the liquid temperature, the higher the mass transfer performance. Similarly, the vapor pressure of the TEG solution was decreased with the increased concentration, and the depression of vapor pressure was larger for the higher TEG concentration. The larger depression of vapor pressure would lead to the larger driving force for H<sup>2</sup> O absorbed by the TEG solution. Therefore, the removal amount of H<sup>2</sup> O increased with the increased TEG concentration, as shown in **Figure 4**. **Figure 5** showed that the removal amount of H<sup>2</sup> O by the TEG solution decreased when the air flow rate increased. Since the amount of treated H<sup>2</sup> O was increased with the increased air flow rate, the mass transfer performance was lowered with the increased air flow rate. On the other hand, the higher air flow rate reduced the time for water vapor exposed to the liquid surface so as to reduce the

**Figure 3.** Effect of liquid temperature on removal amount.

exposure time and to decrease the absorption capacity. According to these points of view and the experimental results, the removal amount of H<sup>2</sup> O by the TEG solution decreased with the larger air flow rate. In contrast, the removal amount of H<sup>2</sup> O by the TEG solution was increased when the liquid flow rate was increased, as shown in **Figure 6**. Since the TEG solution was used as the working solution for absorption of water vapor in the gas phase, the higher mass transfer performance would be accompanied with the larger liquid flow rate. On the other hand, the larger liquid flow rate would promote the phenomenon of surface renewal. It could be thought that the absorption site increased in the packed-bed absorber and the mass transfer performance was increased. Therefore, **Figure 6** showed that the removal amount of H<sup>2</sup> O was increased with the increased liquid flow rate. Since the explanations of the effect of operating variables on mass transfer performance for TEG solution with surfactants were similar as that for the TEG solution without surfactants, the descriptions for the mass transfer performance affected by operating variables were neglected under the condition of adding surfactants.

### **3.3. Influences of additives for the presented study**

absorption system [24]. Since the Marangoni effect was more pronounced for the concentration of LiCl greater than 40 wt.%, the experimental results showed that the removal efficiencies were increased significantly beyond 40 wt.%. Absorption enhancement was better for ethanol

tion is determined by the depression of water vapor pressure, the water vapor will be transferred from the bulk gas phase with the higher vapor pressure to the gas-liquid interface with the lower vapor pressure. The vapor pressure of desiccant solution was lower for the lower liquid temperature, and the depression of water vapor pressure would be larger for the lower liquid temperature. Therefore, the lower the liquid temperature, the higher the mass transfer performance. Similarly, the vapor pressure of the TEG solution was decreased with the increased concentration, and the depression of vapor pressure was larger for the higher TEG concentration. The larger depression of vapor pressure would lead to the larger driving force

the increased TEG concentration, as shown in **Figure 4**. **Figure 5** showed that the removal

formance was lowered with the increased air flow rate. On the other hand, the higher air flow rate reduced the time for water vapor exposed to the liquid surface so as to reduce the

O by the TEG solution decreased when the air flow rate increased. Since the

O was increased with the increased air flow rate, the mass transfer per-

O absorbed by the TEG solution. Therefore, the removal amount of H<sup>2</sup>

O by the TEG solution was decreased with the

O absorbed by the desiccant solu-

O increased with

vapor than for ethanol droplets, which was also demonstrated.

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

**3.2. Influences of operating variables for the presented study**

increased liquid temperature. Since the driving force for H<sup>2</sup>

As shown in **Figure 3**, the removal of H<sup>2</sup>

**Figure 3.** Effect of liquid temperature on removal amount.

for H<sup>2</sup>

amount of H<sup>2</sup>

amount of treated H<sup>2</sup>

As shown from **Figures 3**–**6**, the removal amounts were always larger for the TEG solution with ethanol than without ethanol. The results can be explained by two points of views. Ethanol molecules would vaporize from the TEG thin film in the packed-bed absorber. The gradient in surface tension would result from the vaporization of ethanol, and the interfacial disturbance would be formed by the gradient in surface tension. The probability of water vapor contacted with the TEG solution film would increase with the formed interfacial disturbance. Therefore, the mass transfer performance would be enhanced by adding ethanol to the TEG solution. On

**Figure 4.** Effect of TEG concentrations on removal amount.

the other hand, the packing area was always not wetted completely in the absorption process. Vaporization of ethanol would be easier in the interface between the TEG-solution thin films and non-wetted packing surface, that is, the rate of vaporization of ethanol for the thinner liquid film would be larger than the thicker liquid film. Furthermore, the surface tension of the TEG solution was larger than ethanol so the surface tension of the thinner liquid film would be larger than the thicker liquid film. Therefore, the gradient in surface tension would be provoked by vaporization of ethanol, and the liquid film would be spread on the packing surface more widely. Therefore, the gas-liquid contacting area was increased and the mass transfer performance would be enhanced. Since the surface tension affected by temperature was not significant, the difference of the removal amounts with and without ethanol was smaller than other

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

http://dx.doi.org/10.5772/intechopen.76148

103

Since the interfacial fluid flow could be provoked by the surface tension gradient, the volatile matter was used as an additive in this study to discuss the effect of additive on mass transfer performance. Therefore, not only experimental mass transfer rates for water vapor absorption into the TEG solutions under different operating conditions were reported but also mass transfer performance with and without adding additives were also compared in this study.

flow rate. Besides, the interfacial disturbance that resulted from addition of surface additive made the absorption ability of the TEG solution with ethanol higher than that without ethanol. According to the following deductions, the mass transfer performance was enhanced by adding ethanol to the TEG solution. The first is that the vaporizing ethanol would result in the surface tension gradient to form the interfacial disturbance, and the probability of gas-liquid contact is increased. The second is that the surface tension gradient would result from the vaporization of ethanol, and the formed surface tension gradient would prompt the TEG solution thin film to spread on the packing surface more widely. Therefore, the better removal amount of H<sup>2</sup>

for the TEG solution with ethanol was demonstrated by the experimental results in this study.

\* and Tsair-Wang Chung<sup>2</sup>

1 Department of Fashion Styling and Design, Chungyu University of Film and Arts,

2 Department of Chemical Engineering, Chung Yuan Christian University, Taoyuan City,

\*Address all correspondence to: hungta.wu@msa.hinet.net

O increased with the higher TEG concentration and liquid flow rate;

O decreased with the lower liquid temperature and air

O

variables, as shown in **Figure 3**.

**4. Conclusions**

The removal amount of H<sup>2</sup>

**Author details**

Honda Wu (Hung-Ta Wu)<sup>1</sup>

Keelung, Taiwan, ROC

Taiwan, ROC

however, the removal amount of H<sup>2</sup>

**Figure 5.** Effect of air flow rate on removal amount.

**Figure 6.** Effect of liquid flow rate on removal amount.

the other hand, the packing area was always not wetted completely in the absorption process. Vaporization of ethanol would be easier in the interface between the TEG-solution thin films and non-wetted packing surface, that is, the rate of vaporization of ethanol for the thinner liquid film would be larger than the thicker liquid film. Furthermore, the surface tension of the TEG solution was larger than ethanol so the surface tension of the thinner liquid film would be larger than the thicker liquid film. Therefore, the gradient in surface tension would be provoked by vaporization of ethanol, and the liquid film would be spread on the packing surface more widely. Therefore, the gas-liquid contacting area was increased and the mass transfer performance would be enhanced. Since the surface tension affected by temperature was not significant, the difference of the removal amounts with and without ethanol was smaller than other variables, as shown in **Figure 3**.
