High Efficiency Liquid-Desiccant Regenerator for Air Conditioning and Industrial Drying

Over 2 quads of fossil fuels are used each year for moisture removal. This includes industrial and agricultural processes where feedstocks and final products must be dried, as well as comfort conditioning of indoor spaces where the control of humidity is essential to maintaining healthy, productive and comfortable working conditions. Desiccants, materials that have a high affinity for water vapor, can greatly reduce energy use for both drying and dehumidification. An opportunity exists to greatly improve the competitiveness of advanced liquid-desiccant systems by increasing the efficiency of their regenerators. It is common practice within the chemical process industry to use multiple stage boilers to improve the efficiency of thermal separation processes. The energy needed to regenerate a liquid desiccant, which is a thermal separation process, can also be reduced by using a multiple stage boiler. In this project, a two-stage regenerator was developed in which the first stage is a boiler and the second stage is a scavenging-air regenerator. The only energy input to this regenerator is the natural gas that fires the boiler. The steam produced in the boiler provides the thermal energy to run the second-stage scavenging-air regenerator. This two-stage regenerator is referred to as a 1?-effect regenerator. A model of the high-temperature stage of a 1?-effect regenerator for liquid desiccants was designed, built and successfully tested. At nominal operating conditions (i.e., 2.35 gpm of 36% lithium chloride solution, 307,000 Btu/h firing rate), the boiler removed 153 lb/h of water from the desiccant at a gas-based efficiency of 52.9 % (which corresponds to a COP of 0.95 when a scavenging-air regenerator is added). The steam leaving the boiler, when condensed, had a solids concentration of less than 10 ppm. This low level of solids in the condensate places an upper bound of about 6 lb per year for desiccant loss from the regenerator. This low loss will not create maintenance problems nor will it significantly increase operating expenses. An energy balance on the boiler showed that heat loss through the insulated jacket was 10%. This value is much higher than the 2% to 5% that is typical of most boilers and indicates a need to better insulate the unit. With insulation that brings jacket losses down to 5%, a 1?-effect regenerator that uses this boiler as its high-temperature stage will have a gas-based COP of 1.05. The estimated cost to manufacture a 300-lb/h, 1?-effect regenerator at 500 units per year is $17,140. Unfortunately, the very high cost for natural gas that now prevails in the U.S. makes it very difficult for a gas-fired LDAC to compete against an electric vapor-compression air conditioner in HVAC applications. However, there are important industrial markets that need very dry air where the high price of natural gas will encourage the sale of a LDAC with the 1?-effect regenerator since in these markets it competes against less efficient gas-fired desiccant technologies. A manufacturer of industrial dehumidification equipment is now negotiating a sales agreement with us that would include the 1?-effect regenerator.