The equilibrium vapor pressure of a 42% solution of lithium chloride and water at 285 F is 14.7 psi. Therefore, one option for regenerating a weak desiccant to a concentration of 42% is to boil it in an atmospheric boiler operating at 285 F. This approach to regeneration, while conceptually simple, can be challenging to implement (although certainly not impossible as described in the section on the two-stage regenerator).
An alternative to a desiccant boiler is a regenerator in which the desiccant is heated to a temperature at which its equilibrium vapor pressure is significantly higher than the partial pressure of water vapor in ambient air. At desiccant temperatures above 160 F (preferably above 180 F if the regenerator is to supply lithium chloride at 40% or higher), a scavenging-air regenerator, in which ambient air flows over surfaces that are wetted with hot desiccant, can efficiently remove water from the weak desiccant.
Although scavenging-air regenerators can be either high-flow or low-flow devices, the low-flow version has important advantages including (1) lower pump power, (2) lower air-side pressure drop, and (3) better suppression of droplet carryover. (High-flow and low-flow technology as applied to conditioners are compared in the liquid-desiccant tutorial.) As a low-flow device, the regenerator is a heat exchanger with hot water flowing through its internal passages and ambient air flowing over its desiccant-wetted exterior surface.
Both plastic-plate heat exchangers (left photo) and wicking-fin heat exchangers (right photo) can serve as regenerators. The principal parameters that influence the regenerator's performance are (1) hot water temperature, (2) air flow rate, (3) desiccant concentration, and (4) desiccant flow rate. Ambient air temperature and humidity will somewhat affect performance, but their influence is weak as long as the hot water temperature is high.
The exhaust air from the low-flow regenerator is hot, typically 15 F to 25 F below the entering hot water temperature. A modest improvement in the regenerator's COP (on the order of 10%) can be captured by an air-to-air heat exchanger that uses the exhaust air to preheat the entering scavenging air. With a 50% effective air-to-air heat exchanger, a scavenging-air plastic-plate regenerator that concentrated a solution of lithium chloride from 39% to 43% might have the performance shown in Table 1, where COP is defined as the thermal value of the water removed from the desiccant (evaluated at 1059 Btu/lb) divided by the thermal energy supplied by the hot water. Also, in Table 1, the value for water removal applies to one plate with 6 square-feet of wetted surface area of a multi-plate regenerator.
4 point concentration change
15 F change in fluid temperature
50% effective AAHX
80% effecitve IHX
85/80 F ambient DB/WB
|Fluid Inlet T (F)||COP||WR|
|COP: coefficient of performance
WR: water removal per plate (lb/h)