Falling film flow of ionic liquid–MEA solution on vertical cooling flat plate and channel
Introduction
Carbon dioxide is the largest contributor to the increasingly serious greenhouse effect. It is an urgent task to develop carbon dioxide capture and storage technologies with environment-friendly and economic benefits. The most viable near-term approach is chemical absorption capture. Falling film reactor is one kind of attractive and prospective reactors in the application to carbon dioxide (CO2) absorption due to its excellent heat and mass transfer performance as well as effective gas–liquid reaction. Meanwhile, in recent years, ionic liquid (IL) is strongly proposed to work as an environmentally friendly [1], [2], [3], [4] absorbent for CO2 absorption considering its negligible volatility and significant thermal stability. Unfortunately, the application of ionic liquid has been hindered by its high viscosity and cost. Recent researches indicated that admixture of ionic liquid and amines will head its way to the practical application [5], [6], [7], [8], [9]. Camper et al. [8] mixed ionic liquid with amines as absorbent for CO2 capture and found that using this unprecedented and industrially attractive mixing approach, the desirable properties of room temperature ionic liquids (i.e., nonvolatility, enhanced CO2 solubility, lower heat capacities) can be combined with the performance of amines for CO2 capture. Zhang et al. [9] mixed ILs with N-methyl-diethanolamine (MDEA) aqueous solutions to form new solvents for the uptake of CO2. The results indicated that IL greatly raised the absorption rate of CO2 in aqueous MDEA solutions. However, when the mixed ionic liquid solution is applied into the falling film reactor for CO2 absorption, different from the stagnant solution, the flow characteristics that results in various film flow pattern will significantly affect heat and mass transfer process in the film, hence the performance of CO2 absorption. Meanwhile, the physical properties of the mixed ionic liquid solution also give rise to different film flow characteristics [10] from that of the common media. Additionally the thermal Marangoni effect induced by spatially inhomogeneous temperature field at the gas–liquid interface may also deform the falling film during the operation. The Marangoni effect dramatically influences the falling film flow dynamics and thus the heat and mass transfer. On account of this, the Marangoni effect has been widely studied. Sternling et al. [11], [12] found the criteria for the Marangoni flow by solving the governing equations of the interfacial turbulence caused by longitudinal variations of surface tension. Schatz and Neitzel [13] summarized recent experimental studies of instabilities in free-surface flows driven by thermocapillarity. Kabov et al. [14] used the solution of 25% ethyl alcohol in water as working liquid and experimentally investigated the effect of a local heat source on a liquid film falling down a vertical plate. The results showed that the non-uniform temperature distribution at the liquid–gas interface because of non-uniform heating led to a thermocapillary counterflow which caused a deformation of the film surface having a horizontal bump-shape. This shape deformed into vertical downstream rivulets when the imposed heat flux was over a critical value. Zaitsev et al. [15] made an investigation on the wetting angle on the thermocapillary breakdown of a falling liquid film and showed that no effect of the equilibrium wetting angle on the non-isothermal breakdown of the film was revealed. Zaitsev and Kabov [16] studied the thermocapillary effect on a wavy falling liquid film. Katkar and Davis [17] investigated the bifurcation in a thin liquid film flowing over a locally heated surface for a non-volatile and a volatile liquid. The results showed that the bifurcation is universally observed for both, a non-volatile film and a volatile film. Peng et al. [18] studied the falling film flow dynamics of ionic liquid-water binary solutions on a uniformly heated vertical plate, the results showed that the lateral Marangoni effect influenced the heated film flow significantly. Additionally, a novel flow pattern called as “bifurcate flow pattern” was observed when the mass fraction of IL was 30%. Chinnov [19]studied the temperature distributions and wave characteristics of the water film flowing down a vertical plate with a heater at high Reynolds numbers. It is noted that the existing studies on Marangoni effect mainly focused on the falling film flow over an infinite plate (open plate). However, the falling film flow usually happens on a confined space with restricted sidewalls [20], [21] in the practical applications. The falling film flowing over a vertical plate with sidewalls will present quite different flow characteristics from the flow without sidewalls due to the corner effect. The studies on the Marangoni effect in falling liquid film of ionic liquid mixed solution on a vertical plate with sidewalls have not been reported so far.
In the present study, the tetramethylammonium glycinate ionic liquid ([N1111][Gly]) and monoethanolamine (MEA) mixed solution was chosen as the working media, and experiments on falling film flow of heated mixed solution on vertical cooling plate were performed to investigate the influence of Marangoni flow. Considering the practical application, comparably, both a vertical plate and a vertical channel were applied. Furthermore, the effects of wall-liquid temperature difference and liquid flow rate of mixed solution were discussed.
Section snippets
Experimental system and reagents
The schematic diagram of experiment system is shown in Fig. 1(a). The system consisted of an absorbent supply system, a cooling water supply system, a plate falling film reactor and a temperature control and measurement system. The mixed solution of ([N1111][Gly]) (Lanzhou Institute of Chemical Physics-Chinese Academy of Sciences) and MEA (Chongqing Dong Fang Hua Bo Company) with mass fraction of 5% and 15%, respectively, was adopted as the absorbent. The plate falling film reactor made of
Falling film flow of ionic liquid-MEA solution on a vertical cooling plate
The thermographic images of falling film of the 5% IL+ 15% MEA mixed solution on the vertical plate under various wall-liquid temperature differences are shown in Fig. 2, where the flow rate QL was of 500 mL/min. The temperature distribution in the film can be obtained from the thermographic images by reading the color bar. It was found that a string film was developing along central area of the vertical plate for all cases. The center area of the falling film was of approximately uniform
Conclusions
Visualization experiments with the help of IR thermography were carried out to investigate the falling film flow characteristics of heated ionic liquid-MEA mixed solution on a vertical cooling plate and a channel. The effects of the wall-liquid temperature difference and the mixed solution flow rate on the film flow pattern, temperature distribution in the film, film width and film area were discussed. The results showed that the characteristics of the falling film flowed in the channel was
Acknowledgements
We are grateful to the financial support from the National Natural Science Foundation of China (No. 51276205), the State Key Program of National Natural Science Foundation of China (No. 51136007), Research Project of Chinese Ministry of Education (No. 113053A), the Natural Science Foundation of Chongqing, China (Grant No. cstc2013jjB9004) and Fundamental Research Funds for the Central Universities in China (No. CDJZR12140035).
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