Improved Absorption in GasLiquid Systems by the Addition of a Low

Ind. Eng. Chem. Res. 2008, 47, 8823–8827

8823

Improved Absorption in Gas-Liquid Systems by the Addition of a Low Surface Tension Component in the Gas and/or Liquid Phase Nai-Hsuan Yang, Yi-Jen Chen, Chien-Chih Liao, and Tsair-Wang Chung* Deparement of Chemical Engineering/R&D Center for Membrane Technology, Chung-Yuan Christian UniVersity, Chungli, Taiwan 320, R.O.C.

If a system uses the phenomenon of Marangoni convection, there will be more interfacial turbulence and this may dominate the mass transfer performance between the gas and liquid phases. Marangoni convection was applied in a gas-liquid contact device (dehumidifier) in this study to measure the effect of interfacial turbulence on the water vapor removal efficiency caused by adding a low surface tension component (99.5% ethanol) into the gas or liquid phase at room temperature. The differences in water vapor removal efficiency under different conditions with and without the addition of ethanol into the moist air or working solution (38-42 wt % aqueous lithium chloride solution) were observed. Since the Marangoni effect was more pronounced when the concentration of working solution was greater than a specific value, the trend line for water vapor removal efficiency in different solution concentrations was presented as a broken line in this study. The break in the trend line is the critical concentration of the working solution, and the value is about 40 wt % of aqueous lithium chloride solution. The experimental results show that addition of low surface tension ethanol into the gas phase was better than adding it into the liquid phase for the interfacial mass transfer performance in this gas-liquid contact device. Introduction Absorption dehumidification or desiccant cooling is a noncompressive air conditioning process widely used in large air conditioning systems for industry or residential applications. In the typical absorption dehumidifier (as shown in Figure 1), the experimental results of dehumidification efficiency are higher than predicted values from mass transfer theory. The reason for this is that most of the mass transfer theories do not consider the effect of interfacial disturbance. If the disturbance is formed by the absorption of water vapor into the surface of the working solution, it is called natural Marangoni convection or the Marangoni effect. If the disturbance occurs due to the addition of a low surface tension component into the gas or liquid phase, it is called induced Marangoni convection. The surface tension gradient on a gas-liquid interface is induced by adding a low surface tension component into the gas stream or working solution. Zarzycki and Chacuk1 showed that interfacial tension, also called lateral stress, is an important factor that affects the mass transfer mechanisms of absorption, distillation, and extraction. The factors affecting interfacial stress include molecular dynamic heat flow and interactive attraction between molecules. The key parameters of the molecular dynamic heat flow that affect the interfacial stress are the local molecular density and the absolute temperature. Depending on the location of molecular attraction in the liquid layer, two kinds of stress orientation occur. When the accumulated stress is in the bulk liquid, the stress is always in the form of equal-orientation; when the accumulated stress is in the interface, the stress is always parallel to the interface, which is called surface tension. The surface tension gradient could be produced by adding another substance on the liquid surface. This interfacial disturbance may improve the mass transfer behavior and thus the system performance of gas-liquid contact systems. In absorption dehumidification or cooling processes, the working solutions are usually divided into two categories. One * To whom correspondence should be addressed. Tel.: 886-32654125. Fax: 886-3-2654199. E-mail address: [email protected].

is aqueous organic solutions, which have the advantages of being noncorrosive and noncrystallizable. However, their viscosity and the resultant pumping costs are higher.2 The second category is inorganic aqueous salt solutions, the viscosity of which is lower and for which the consideration of crystal forming is necessary in determining the operation parameters. Therefore, the selection of a working solution should consider its physical properties.3 The vapor pressure of the working solution is one of the important properties to consider, since the vapor pressure difference between the water vapor in air and the working solution is the driving force of the absorption process.4-6 Low surface tension alcohol is soluble in both aqueous LiBr and LiCl solutions. LiBr is commonly used in absorption heat pumps, whereas LiCl is used in absorption dehumidifiers. However, in the literature, discussion of the LiCl system is not as common as that of the LiBr system,7 especially with respect to the addition of low surface tension component. The low surface tension component that can be added to the working solution in industrial systems are the long carbon chain solvents such as 2-ethyl-1-hexanol and n-octanol.8 Some studies9,10 have also found that short carbon chain solvents (methanol, ethanol,

Figure 1. Typical process lay-out of an absorption dehumidifier.

10.1021/ie800316n CCC: $40.75  2008 American Chemical Society Published on Web 10/24/2008

8824 Ind. Eng. Chem. Res., Vol. 47, No. 22, 2008

Figure 2. Gas-liquid contact device for this study.

n-propanol, n-butanol, and n-pentanol) may provide more interfacial disturbance than long carbon chain solvents (nhexanol, n-octanol, SDS, DTMAC, and Triton X-100). Therefore, ethanol was selected as the low surface tension component to be added into the LiCl working solution in this study for observation of the Marangoni effect. The surface tension gradient on the liquid surface is usually induced by differences in concentration11 or temperature12-15 between the interface and bulk solution. The surface tension gradient usually occurs in the continuous phase of the gas-liquid contact devices,1 and this Marangoni effect can increase the mass transfer efficiency significantly.16,17 However, in most studies, the surfactants have been added to the liquid. A vapor surfactant theory was proposed in 1996, and the authors18 believed that the surfactant might play an important role when it was added to the gas phase. Various alcohols have been employed in the study of the Marangoni effect,19-21 and the effect of the amount of surfactant used in the system has also been studied.22 In order to understand the significance of interfacial disturbance on absorption dehumidification systems, a small gas-liquid contact device, with ethanol as low surface tension component, was designed and tested in this study. The induced interfacial disturbance not only increases the mass transfer area but also raises the contact probability for gas and liquid phases. Such effects of surface tension on mass transfer performance and the underlying mechanism have been discussed in the open literature recently. The purpose of this study is to apply this Marangoni effect to the gas-liquid contact system to understand the effect of the induced interfacial disturbance and to compare the addition of surfactants in gas and liquid phase for the improvement of the mass transfer performance in a specially designed gas-liquid contact device. Experimental Section In order to induce interfacial disturbance for surface renewal and thus increase the contact area between the gas and liquid streams, a low surface tension component (ethanol) was added

to the gas or liquid phase to improve the mass transfer performance. The gas-liquid contact device developed for this study, which was modified from the literature, is shown in Figure 2.9,10 The apparatus comprised two concentric cylindrical glass tubes. The diameter and the height of the inner tube are 6 and 3 in., respectively. The diameter and the height of the outer cylindrical tube are 6.2 and 6 in., respectively. The supporting liquid (absorbent) flows upward into the inner tube from the central hole on the bottom of the inner tube. The inner tube was filled with the absorbent liquid, and the liquid continuously overflowed from the inner tube into the outer tube. It formed the interfacial area on the top of the inner tube for the contact of gas and liquid streams. In the beginning of the experimental runs, the working solution (aqueous LiCl solution) was heated to regenerate for about 2 h. The concentration of the aqueous LiCl solution was measured by reflractive index. Dried and pure N2 carrier gas was divided into two flow streams before entering the main system. One stream was connected to a series of impingers containing the water, and the other was connected to a series of impingers containing ethanol, which merged in a mixed tank to regulate the initial concentration of water vapor or the amount of ethanol added to the gas stream. Two sets of impingers were placed in the constant temperature water bath to prevent temperature perturbation and regulate the amount of evaporation of water or ethanol. The flow rates of the two gas streams were separately controlled by mass flow controllers (Brooks Co.) and then merged into a mixed tank to become the inlet gas stream. The inlet and outlet humidity (mg H2O/kg N2) of the gas stream was measured by a chilled mirror dew point meter (General Eastern Co.), and the inlet gas flow rate was monitored and controlled by the mass flow controller. For the study of the addition of ethanol into the working solution, ethanol was dropped into the liquid surface (0.044 mL/s) from the small central tube at the top of the gas-liquid contact device. The

Ind. Eng. Chem. Res., Vol. 47, No. 22, 2008 8825 Table 1. Effect of the Concentration of the Working Solution on the Water Vapor Removal Efficiency for the System without Adding Ethanol

Table 3. Effect of the Concentration of Working Solution on the Water Vapor Removal Efficiency for the System with Adding Ethanol into the Gas System

Inlet Humidity ≈ 15 mg H2O/kg N2


LiCl inlet concentration (%)

inlet humidity (mg H2O/ kg N2)

outlet humidity (mg H2O/ kg N2)

temperature (°C)

removal efficiency (%)

37 38 39 40 41 42

14.8 14.9 15.0 14.8 14.7 15.1

9.3 9.1 9.0 8.7 8.1 8.3

25 25 25 25 25 25

37.16 38.93 40.0 41.22 44.90 45.03




temperature (°C)


37 38 39 40 41 42

14.9 15.4 15.1 14.8 14.8 15.0

8.8 8.2 7.0 5.5 4.7 4.3

25 25 25 25 25 25

40.94 46.75 53.64 62.84 68.24 71.33

Inlet Humidity ≈ 20 mg H2O/kg N2 LiCl inlet concentration (%)



37 38 39 40 41 42

19.3 19.4 19.6 20.3 19.5 19.4

11.4 11.0 11.0 11.3 10.4 10.2


temperature (°C)





temperature (°C)


25 25 25 25 25 25

40.93 43.30 43.88 44.33 46.67 47.42

37 38 39 40 41 42

20.5 20.0 19.9 21.0 19.3 20.5

9.6 8.9 8.4 8.3 6.3 7.2

25 25 25 25 25 25

53.17 55.50 57.79 60.48 67.36 64.88

Table 2. Effect of the Concentration of the Working Solution on the Water Vapor Removal Efficiency for the System with Adding Ethanol into the Working Solution Inlet Humidity ≈ 15 mg H2O/kg N2 LiCl inlet concentration (%)



temperature (°C)


37 38 39 40 41 42

14.9 15.0 14.8 15.0 14.9 15.1

8.5 8.5 8.1 8.0 7.6 7.7

25 25 25 25 25 25

42.95 43.33 45.27 46.67 48.99 49.01

Inlet Humidity ≈ 20 mg H2O/kg N2 LiCl inlet concentration (%)



temperature (°C)


37 38 39 40 41 42

19.9 19.4 20.5 20.2 20.1 19.4

11.2 10.4 10.8 9.4 9.2 8.4

25 25 25 25 25 25

43.72 46.39 47.32 53.47 54.23 57.22

As shown in Figures 3 and 4, the water vapor removal efficiency increased as the aqueous LiCl solution concentration was increased. This occurred because the vapor pressure of the working solution was lower for a higher concentration solution, and the vapor pressure difference between the gas stream and the working solution was the driving force of the mass transfer between the gas and liquid phases. In addition, the trend line for water vapor removal efficiency in the figures was changed significantly for LiCl concentrations in the range of 38-40 wt %. This is one of the reasons why working solutions greater than 40 wt % are commonly used in commercial systems. After the addition of ethanol into the working solution, the water vapor removal efficiency was higher than in cases without ethanol added. The reason for this was that the induced Marangoni effect occurred when the low surface tension solvent of ethanol was added to the LiCl working solution. A comparison of the water vapor removal efficiency for systems with and without ethanol added into the gas stream are presented

amount of ethanol added to the bulk liquid phase of the sorbent solution is dependent on the dropping time. Results and Discussions The experimental data with and without the addition of ethanol in different operating conditions are given in Tables 1-3. As shown, systems with higher inlet humidity have higher water vapor removal efficiency. At the same inlet humidity, the water vapor removal efficiency in the systems with ethanol added into the working solution or gas stream are higher than those without ethanol added. Due to the vaporization of ethanol, the local surface tension gradient was formed when the low surface tension ethanol was added to the working solution. This is an interfacial disturbance, which may increase the gas-liquid interfacial area and thus increase the amount of water vapor removal (or mass transfer performance).

Figure 3. Comparison of the water vapor removal efficiency for the systems with and without adding ethanol into the working solution at inlet humidity 15 mg H2O/kg N2.

8826 Ind. Eng. Chem. Res., Vol. 47, No. 22, 2008

Figure 4. Comparison of the water vapor removal efficiency for the systems with and without adding ethanol into the working solution at inlet humidity 20 mg H2O/kg N2.

Figure 6. Comparison of the water vapor removal efficiency for systems with and without ethanol added into the gas stream at inlet humidity 20 mg H2O/kg N2.

Figure 5. Comparison of the water vapor removal efficiency for systems with and without ethanol added into the gas stream at inlet humidity 15 mg H2O/kg N2.

Figure 7. Comparison of the water vapor removal efficiency for systems with ethanol added into the working solution and gas stream at inlet humidity 15 mg H2O/kg N2.

in Figures 5 and 6. The trend of the water vapor removal efficiency in the figures was changed significantly in the cases of concentrations of aqueous LiCl solution in the range of 38-40 wt % as well, and the water vapor removal efficiency was higher at a working solution concentration greater than 40 wt %. The trend of the increasing rate of the water vapor removal efficiency was different in these two concentration ranges. In Figures 7 and 8, a comparison of the systems with ethanol added into the working solution and gas stream is presented. With ethanol added into the gas stream, the water vapor removal efficiency in the system was better than that with ethanol added to the working solution. Since the ethanol in the gas stream may allow the ethanol to contact more of the interfacial area of the system in a short time, the Marangoni effect in the gas-liquid contact area was more significant. This observation can be applied to absorption dehumidification systems and absorption cooling systems. For example, Patberg et al.23 found that the transfer of a component with lower surface tension from the liquid to the gas phase results in an increase in the surface tension of the remaining liquid, which is termed the Marangoni positive, such that the packing materials

can be completely wetted by the liquid. On the other hand, when the component that evaporates has the higher surface tension, the remaining liquid can form spots, which is termed the Marangoni negative, and as a consequence, channeling phenomenon can occur in the packed bed. Instead of being added into the working solution of the packed bed systems, especially in the case of Marangoni positive, the low surface tension component is added to the gas stream. This may increase the mass transfer area and mass transfer performance between the gas and liquid phases further. Discussions of the relationship between the water vapor removal efficiency and the interfacial disturbance resulting from the surface tension gradient are one of the purposes of this study. When the surface tension gradient resulting from absorbing water vapor and/or adding low surface tension component is large enough to exhibit the Marangoni effect, the disturbance in the interface takes place. In this study, the Marangoni effect was observed when the concentration of the working solution exceeded a specific value and the trend line for water vapor removal efficiency in different solution concentrations was shown as a broken line in this study. The break in the trend line is the critical concentration of the working solution,

Ind. Eng. Chem. Res., Vol. 47, No. 22, 2008 8827

under grant NSC96-2221-E033-032 and by Center-of-Excellence Program on Membrane Technology, the Ministry of Education, Taiwan, R.O.C. Literature Cited

Figure 8. Comparison of the water vapor removal efficiency for systems with ethanol added into the working solution and gas stream at inlet humidity 20 mg H2O/kg N2.

and the value is about 40 wt % for aqueous lithium chloride solution. It is noted that the Marangoni effect in the system where the low surface tension component was evaporated to mix with the gas stream to contact with the sorbent solution is more significant than in the system where the component was added to sorbent solution directly. Conclusions Interfacial disturbance is one of the important factors in the mass transfer performance of the gas-liquid interface. The surface tension gradient is the key factor that induces the interfacial disturbance. This can cause variation of the gas-liquid interfacial area and changes in the mass transfer mechanism that affect the mass transfer efficiency. In addition, the interfacial area can be renewed by adding a volatile component, which will cause a disturbance in the gas-liquid interface and improve the probability of gas and liquid contact. In this work, a gas-liquid contact device was used to study the dehumidification process. LiCl desiccant solution was used as the sorbent to absorb the water vapor, and low surface tension ethanol was added to improve the interfacial mass transfer performance. On the basis of this study, the water vapor removal efficiency increased as the inlet humidity is increased. Since the amount of working solution used in this study was much greater than the equilibrium flow rate of the system, the working solution was still sufficient to absorb more water vapor. It was observed that the water vapor removal efficiency increased as the working solution concentration increased. Since the vapor pressure of the desiccant solution is lower for the higher concentration working solution, the vapor pressure difference between gas and liquid phases, which is the driving force for mass transfer, is increased. As expected, the water vapor removal efficiency was improved with the addition of ethanol in both the gas and liquid phases. Addition of ethanol to the gas phase resulted in better mass transfer efficiency than addition to the liquid phase, as observed in this study. This is important for systems designed to improve the mass transfer efficiency by adding a low surface tension component to cause interfacial disturbance. Acknowledgment This work was supported by Chung Yuan Christian University (Project number CYCU-97-CR-CE), National Science Council,

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ReceiVed for reView February 26, 2008 ReVised manuscript receiVed June 30, 2008 Accepted September 8, 2008 IE800316N

Improved Absorption in GasLiquid Systems by the Addition of a Low

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