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Effectiveness and performance of a counterflow liquid desiccant regeneration tower in a hot-humid climate.

Article Excerpt
INTRODUCTION

Conventional energy sources continue to diminish, but the voracious demand for energy by a growing global population continues to increase. The accompanying [CO.sub.2] and other green house gas emissions have been identified as primary causes of global warming. New energy-efficient,

environmentally friendly air conditioning systems are therefore urgently required to ensure a sustainable built environment and also to meet the current standards for indoor air quality. Solar assisted desiccant regeneration systems could have potential contribution to [CO.sub.2] reduction. The United Arab Emirates is one of the hottest and most humid regions where solar energy will be very useful for desiccant regeneration. Air conditioning systems are sized for a combination of latent and sensible loads. The latent cooling load can account for as much as 30% to 50% of air conditioning requirements. Conventional refrigeration-based air conditioning is costly to install and to operate. Desiccant dehumidification removes humidity from ventilation air resulting in reduced air conditioning requirements to meet the demands of sensible cooling and hence smaller air-conditioning plants are required. Reducing humidity in the air handling systems and the building spaces during the cooling -season will improve indoor air quality by preventing condensation in equipment and thus hinder the growth and propagation of micro-organisms. There are two types of desiccant systems: liquid and dry. Liquid desiccant systems commonly use two chambers with air/liquid contact surfaces. In the conditioning chamber, ventilation air is dehumidified as the concentrated desiccant absorbs moisture from the air. In the regeneration chamber, the air is humidified as moisture is transferred from the dilute desiccant to the scavenging air. The desiccant or exhaust air is usually heated to promote desiccant regeneration. A desiccant pump, level controls and heat exchanger are typically included in the system. The heat required for regenerating the desiccant can be supplied by fossil fuel, waste heat or solarenergy.

Several liquid desiccants, including aqueous solutions of organic compounds (e.g. triethylene glycol) and aqueous solutions of inorganic salts (e.g. lithium chloride), have been employed to remove water vapor from air. The process equipment utilized for liquid-gas contact is generally falling film, spray or packed towers. Many researchers worked on packed regenerators and compared the results with theoretical models. Although random packed towers facilitate more mass transfer by providing a larger area in a relatively smaller volume.

Factor and Grossman (1980) compared the experimental and theoretical model of a packed regenerator using LiBr and preheated air. Gandhidsan (1990) developed a simplified theoretical model for regeneration of a desiccant in a packed bed using solar heated air. Patnaik et al. (1990) conducted experiments on a packed bed tower for the regeneration of aqueous lithium bromide. They studied the influence of the type of liquid distribution system on the performance of the regenerator and presented correlations based on experimental results for the rate of water evaporation as a function of inlet air temperature, humidity, inlet desiccant concentration, and flow rate. Radhwan et al. (1993) presented a mathematical model to predict the performance of a packed bed operating with counterflows of air and calcium chloride. Elsayed et al. (1993) developed a finite difference model to calculate the effectiveness of heat and mass transfer in packed beds. Extras et al. (1994) investigated the influence of the performance variables on a packed regenerator performance. Potnis and Lenz (1996) tested a packed regenerator using random polypropylene and structured packings. Martin and Goswami (1999) assessed the effectiveness of a liquid desiccant regenerator based on different design variables. Martin and Goswami (2000) developed two novel performance correlations for the effectiveness of a packed bed liquid desiccant dehumidification and regeneration. Fumo and Goswami (2002) assessed the effectiveness of the dehumidification and regeneration processes under the effects of design variables using lithium chloride solution. Gandhidsan (2005) investigated the influence of the heating source on the evaporation rate of a packed bed regenerator. Longo and Gasparella (2005) represented experimental and theoretical analysis on a packed desiccant regeneration using three chemicals. Elsarrag (2007) investigated experimentally the effect of different design parameters on the evaporation rate and humidity effectiveness of a structured packed regenerator using triethylene glycol (TEG). Gommed and Grossman (2007) investigated the performance of a liquid desiccant system for solar cooling and dehumidification. They used thermal storage for desiccant regeneration. Elsarrag (2008) tested a regeneration system modified from solar tilted still for calcium chloride regeneration.

This paper reviews the packed desiccant regenerators' humidity effectiveness and the performance variables used in the previously published studies. The influence of the design parameters on the humidity effectiveness is assessed. A novel definition of counterflow packed desiccant regeneration towers effectiveness is deduced and an empirical effectiveness correlation is presented. The results...