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Cosorption processes of triethylene glycol in a packed-bed liquid desiccant dehumidifier.

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INTRODUCTION

The environmental awareness that occurred in the 1980s has led to the public demand for healthy outdoor and indoor environments. Today, the common approach adopted for cleaning contaminated indoor air is diluting indoor air with clean outdoor air in order to achieve acceptable indoor air quality. Air cleaning may be an attractive alternative when the outdoor air quality is poor or when there is a desire to reduce energy costs associated with high outdoor air exchange rates. Accordingly, substantial research efforts has been carried out to identify effective methods to clean indoor air while minimizing energy consumption.

Although water vapor is not considered to be an air contaminant, it has a significant impact on both comfort and health of human occupants. Unlike other contaminants in the air, water vapor cannot be completely removed from the airstream. Extremely low relative humidity can lead to eye irritation, mucous dryness, and other health problems (Arundel et al. 1992). On the other hand, high indoor relative humidity can lead to condensation on cold faces, such as ducts, dryers, windows and various building materials, and can also promote the growth of various micro-organisms. Therefore, the control of the indoor relative humidity is an important design parameter for HVAC industry practitioners and researchers.

In conventional vapor-compression systems, the process of dehumidification and humidity control is brought about by cooling the air below its dew point and reheating the air to the desired relative humidity. This method of humidity control is extremely energy intensive. In contrast, systems employing desiccants can help control the humidity by removing water from the air through a sorption process.

Desiccants are broadly classified as solid and liquid. Solid desiccants include molecular sieves and silica gel, whereas liquid desiccants include both inorganic salt such as lithium bromide and an organic solution like triethylene glycol (TEG). Generally, systems that employ solid desiccants can achieve a higher degree of dehumidification than systems that use liquid desiccants.

Although liquid desiccant systems have lower drying capacities than solid desiccant systems, systems that use liquid desiccants do have some advantages. These include the possibility of cooling the air by manipulating the air and liquid flow rates, the lower regeneration temperatures required by liquid desiccants, and the fact that liquid systems generally do not require complex dehumidifier geometries and thus can be easily applied to various commercial and industrial applications.

In liquid desiccant systems, air and desiccant can be brought into contact in several absorber configurations. This can be achieved by either (a) bubbling the air through the liquid, as in a tray column; (b) spraying the liquid in a fine dispersion in an upward airstream, as in a spray tower; (c) spraying the liquid over a bank of cooling tubes past which air is blown; or (d) passing the air and liquid streams through a packed bed. In this study, we focus on a packed bed, since it usually offers lower pressure drop but has the greatest interfacial area between the air and the liquid streams. This leads to high interchange rates of heat and mass between the liquid desiccant and the airstream.

Apart from studies on the heat and mass transfer characteristics of desiccants, desiccant potential for removing indoor air contaminants has also begun to be recognized. Hines and Ghosh (1993) revealed the capabilities of solid desiccants such as silica gel and a molecular sieve for removing contaminants from the air. Activated carbon fiber has been shown to have a strong capability of removing volatile organic compounds (VOCs) (Das et al. 2004). Hydrpohobic zeolites, in membrane form, have also been found have an ability to selectively remove one or more organic pollutants from humid airstreams (Chitawar and Greene 1997; Aguado et al. 2004). Consequently, solid absorbents have been employed in many industrial applications in the US and Asia for independently controlling humidity and VOCs.

On the other hand, Moschandreas and Relwani (1990) demonstrated that a liquid desiccant-based, gas-fired dehumidification system using lithium chloride solution (LiCl) had the capacity to remove indoor pollutants. However, no systematic study has been conducted to assess the full adsorption potential of LiCl solution. It was not until the work done by Hines et al. (1992), a comprehensive study conducted to determine the removal capabilities of both organic and inorganic liquid absorbents, that organic compounds such as TEG solution were found to have a stronger capability to remove the organic contaminants from the airstreams than inorganic salts such as lithium chloride. The researchers attributed this phenomenon to the fact that the removal capacity of the LiCl solutions depend to a great extent on the solubility of the particular pollutant in the water portion of the solution while the pollutant removal capacities of the TEG solution depends on the solubility of the pollutants in the absorbent portion of the solution.

Although research on the absorption properties of TEG for water and various VOCs have been ongoing for some time (Peng and Howell 1981; Ng et al. 1983; Grasso et al. 1994), there is no published experimental or numerical data on the simultaneous absorption of water vapor and air contaminants by TEG. Chau and Worek (2007) developed a numerical model to simulate the cosorptive characteristics of TEG solutions for both water vapor and VOC air contaminants under various operating conditions through packed-bed absorbers. Toluene, which has widely been recommended as a reasonable surrogate and representative for indoor total VOCs in desiccant adsorption studies (Liu 1990, 1993), has been selected as another component in...

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