Liquid Absorbents

Liquid desiccants have a vapor pressure lower than water at the same temperature. The air passing over the solution approaches this reduced vapor pressure and is dehumidified. If the vapor pressure of the solution is greater than that of the water, the air is humidified. At a given concentration and temperature, liquid desiccant solutions are in equilibrium with air at a fixed humidity. The vapor pressure of liquid absorption solution is directly proportional to its temperature and inversely proportional to its concentration. If concentration is reduced or the temperature is increased, the vapor pressure increases. If concentration is increased or temperature reduced, the vapor pressure decreases.

• Hygroscopic salt solutions are water solutions with salts such as lithium chloride (LiCl) or calcium chloride (CaCl). While more costly than CaCl, LiCl is more commonly used because it is more effective and less corrosive. At 40% concentration and 70°F (21°C), LiCl is in equilibrium with air at 19.3 grains/lbm (0.0027 kg/kg). The concentration is determined by the specific gravity. Concentration limits determine the maximum and minimum allowable solutions. If cooled sufficiently, LiCl will form ice at concentrations below 33%. At concentrations above 33% it will become supersaturated and form solid salt. Compared with its use as a solid, liquid LiCl has a far greater ability to hold water molecules. In a solid state, each LiCl molecule can hold two molecules of water. As a solution in equilibrium with air at a 90% RH, each molecule can hold 26 molecules of water.

• Glycols exhibit similar characteristics to hygroscopic salt solutions, except that they require much higher equilibrium concentrations and tend to evaporate. Typical equilibrium concentrations are greater than 90%, which is more than double that of hygroscopic salts. Systems, therefore, require far more solution given that there is only a small percent of water to work with. Also due to the tendency to evaporate, requiring frequent solution replacement, glycols are more commonly used for lower temperature applications when evaporation rates are reduced. Commonly used glycols are triethylene glycol and propylene glycol. Propylene glycol is commonly used in food processing and frost-free applications due to its low toxicity. In other types of applications, triethylene glycol is used due to its lower evaporation rate.

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