Direct contact condensation in packed beds

Abstract

A diffusion driven desalination process was recently described where a very effective direct contact condenser with a packed bed is used to condense water vapor out of an air/vapor mixture. A laboratory scale direct contact condenser has been fabricated as a twin tower structure with two stages, co-current and countercurrent. Experiments have been operated in each stage with respective saturated air inlet temperatures of 36, 40 and 43 °C. The temperature and humidity data have been collected at the inlet and exit of the packed bed for different water to air mass flow ratios that vary between 0 and 2.5. A one-dimensional model based on conservation principles has been developed, which predicts the variation of temperature, humidity, and condensation rate through the condenser stages. Agreement between the model and experiments is very good. It is observed that the countercurrent flow stage condensation effectiveness is significantly higher than that for the co-current stage. The condensation heat and mass transfer rates were found to decrease when water blockages occur within the packed bed. Using high-speed digital cinematography, it was observed that this problem can occur at any operating condition, and is dependent on the packing surface wetting characteristics. This observation is used to explain the requirement for two different empirical constants, depending on packing diameter, suggested by Onda for the air side mass transfer coefficient correlation.

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.