International Journal of Heat and Mass Transfer
Direct contact condensation in packed beds
Introduction
A seawater distillation process that has drawn interest over the past two decades is humidification dehumidification desalination (HDH). Numerous investigators, including Bourouni et al. [1], Al-Hallaj et al. [2], Assouad and Lavan [3], Muller-Holst et al. [4], Abdel-Salam et al. [5], Xiong et al. [6], Shaobo et al. [7], Xiong et al. [8], El-Dessouky [9], Goosen et al. [10], and Al-Hallaj and Selman [11] have shown that this process has advantages when operating off of low thermodynamic availability energy such as waste heat. However, they utilize film condensation, which is ineffective in the presence of non-condensable gas, and thus they are not typically cost competitive. Klausner et al. [12], [13] recently described an economically feasible diffusion driven desalination (DDD) process to overcome this shortcoming. DDD is a distillation process driven by waste heat derived from low pressure condensing steam within a thermoelectric power plant and is viable for inexpensive large-scale fresh water production (>1 million gallons per day). To enhance the heat transfer rate in the presence of non-condensable gas, a direct contact condenser approach, initially described by Bharathan et al. [14], is utilized. The packed column is well known as an efficient device for gas–liquid direct contact mass transfer such as absorption, stripping, and distillation.
Distillation with the DDD process occurs via humidification of a flowing air stream and dehumidification of that air stream. A characteristic of the DDD process is that the air flow rate through the system is significantly higher than the vapor flow evaporated into the air stream and liquid condensed out. In order for the DDD process to be cost effective, an efficient and low cost method is required to condense water vapor out of the air stream. With a large fraction of the air/vapor mixture being non-condensable, direct contact condensation is considerably more effective than film condensation. In addition, direct contact condensation within a packed bed is more effective than droplet direct contact condensation.
While a significant amount of literature is available on droplet direct contact condensation, considerably less information is available for packed bed direct contact condensation. In analyzing direct contact condensation through packed beds, Jacobs et al. [15] and Kunesh [16] used a volumetric heat transfer coefficient for the rate of convective heat transport and penetration theory [17] to relate the heat and mass transfer coefficient. The volumetric approach does not account for local variations in heat and mass transfer. Penetration theory assumes the liquid behind the interface is stagnant, infinitely deep, and the liquid phase resistance is controlling. As suggested by Jacobs et al. [15] these may or may not be reasonable assumptions, depending on the liquid film condensate resistance. Bharathan and Althof [18] and Bontozoglou and Karabelas [19] improved the analysis of packed bed direct contact condensation by considering conservation of mass and energy applied to a differential control volume. Local heat and mass transfer coefficients were used. Both analyses relied on penetration theory to relate heat and mass transfer coefficients.
The motivation for this work is to experimentally explore the heat and mass transfer process within a packed bed direct contact condenser and develop a robust and reliable predictive model from conservation principles that is useful for design and analysis. A fresh approach is used that does not rely on penetration theory. One of the difficulties encountered is that the interfacial temperature between the liquid and vapor cannot be directly measured, and thus the liquid and vapor heat transfer coefficients cannot be directly measured. Klausner et al. [20] have already developed a detailed evaporative heat and mass transfer analysis for the evaporator section (diffusion tower) of the DDD process. The extensively tested Onda [21] correlation was used to evaluate the mass transfer coefficients on the liquid and gas side. A heat and mass transfer analogy was applied to evaluate the liquid and gas heat transfer coefficients. Excellent results were obtained, and a similar approach will be pursued here.
A laboratory scale packed bed direct contact condenser has been fabricated. The condenser is constructed as a twin tower structure with two stages, co-current and countercurrent. The performance of each stage has been evaluated over a range of flow and thermal conditions. As expected, the countercurrent stage is significantly more effective than the co-current stage. It is found that the manner in which the packing is wetted can significantly influence the heat and mass transfer performance. Visual observations of the wetted packing have been made and a discussion relating the wetting characteristics to the different empirical constants suggested by Onda [21] is provided.
Section snippets
Formulation
A physical model is developed for direct contact condensation by considering that cold water is sprayed on top of a packed bed while hot saturated air is blown through the bed from the bottom. The falling water is captured on the packing surface and forms a thin film in contact with the saturated turbulent air stream. Energy transport during the condensation process is accomplished by a combination of convective heat transfer due to the temperature difference between water and air and the
Experimental facility
In order to test the efficacy of the analytical models described in Section 2, an experimental packed bed direct contact condenser has been fabricated. Fig. 3 shows a pictorial view of the laboratory scale DDD facility, and Fig. 4 provides its schematic diagram. Dry air is drawn into a centrifugal blower equipped with a 1.11 kW motor. The discharge air from the blower flows through a 0.102 m inner diameter vertical PVC pipe in which a thermal flowmeter is inserted to measure the air flow rate.
Experimental and computational results
The effective packing diameter dp for the structured polypropylene packing is 0.017 m. In Onda’s original work [21] he suggested that the coefficient in Eq. (14) should be C = 5.23 for dp > 0.015 m and C = 2.0 for dp ⩽ 0.015 m. However, careful scrutiny of the data shows that the change in the coefficient is smooth, and the abrupt change represented by a bimodal coefficient is only an approximation. The 0.017 m effective packing diameter used in this work is very close to the threshold suggested by Onda.
Conclusion
A laboratory scale direct contact condenser with packed bed has been fabricated with co-current and countercurrent flow stages. Corresponding experiments reveal the heat and mass transfer characteristics for different flow configurations. A comparison between the two stages demonstrates that countercurrent flow generally has 15% higher condensation effectiveness than co-current flow. The condenser effectiveness is strongly dependent on the water to air mass flow ratio and not sensitive to the
Acknowledgements
This paper was prepared with the support of the US Department of Energy under Award no. DE-FG26-02NT41537. However, any opinions, findings, conclusions, or recommendations expressed herein are those of the authors and do not necessarily reflect the views of DOE.
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