A commonly used liquid desiccant, lithium bromide faces some shortcomings such as corrosion, crystallization at high concentrations and their need for high energy inputs to regenerate the absorbent.
Molecular dynamic simulations have recently been applied in understanding of molecular driving forces such as vapor pressure and absorption rate that influence performance of liquid desiccants. Despite knowledge provided by molecular dynamic simulations on solution behavior, little efforts have been made on microscale kinetics of liquid desiccants including current ternary working fluids.
Researchers led by Professor Gloria D. Elliott from University of North Carolina at Charlotte, published an article in Applied Thermal Engineering conducted a series of molecular dynamic simulations of the absorption of water vapor into aqueous lithium bromide and lithium bromide /sodium formate mixtures at various temperatures.
The molecular dynamic simulation was able to provide a quasi-static absorption process as lithium bromide solution absorbs water vapor at a nearly constant rate. Large number of water molecules was absorbed by 60wt% lithium bromide solution at a temperature of 443K after a simulation time of 20ns which yielded an approximate absorption rate of 38Kg·m2/s but decreased as temperature decreases. It was also seen that absorption of water molecules increases as concentration of lithium bromide increases at a minimum temperature of 373K.
Analysis on mass density profiles of lithium and bromide ions with water molecules showed a decrease in interfacial thickness layer of lithium bromide solution as its concentration increases when observed at 383K and 408K, but no effect was found with respect to temperature. It was also observed that the interfacial dipoles were most likely to lie in a plane parallel to the interface.
Effects of addition of sodium formate into the LiBr + H2O system when simulated at a temperature of 373K showed good correlation of density profile of lithium ions Li+ to that of formate anion COOH– in all the mass ratios which maintains local proximity throughout the solution.
“What we found the most interesting was that the strong interaction between Li+ and COOH– created cavities of various sizes that can accommodate water molecules,” said Dr. Lindong Weng, a former postdoctoral researcher in Elliott’s lab and the first author of the study. “Such fascinating morphology of ion placement provides a geometrical explanation for the increase in absorption capacity with added formate.”
The study also found that when the molar ratios of LiBr to NaCOOH are about 1.5:1 and 0.8:1, respectively, Li+-COOH– clusters size mainly 5< (cluster size: defined as number of lithium ions that can be linked together with each other via COOH–). This result showed that more addition of sodium formate led to more extended and interwoven lithium and formate ion clusters. The effect of addition of sodium formate also led to a decrease in water vapor absorption rate despite increase in absorption capacity.
The authors successfully optimized the advantages of lithium bromide and lowered crystallization temperature (minimal thermal energy needed via including formate salt). The molecular design and simulation methods presented in this study can aid in improved defined compositions to undergo more detailed experimental studies.
Lindong Weng 1, Wei Song2, , Donald J. Jacobs2, Gloria D. Elliott1. Molecular insights into water vapor absorption by aqueous lithium bromide and lithium bromide/sodium formate solutions, Applied Thermal Engineering 102 (2016) 125-133.Show Affiliations
- Department of Mechanical Engineering and Engineering Sciences, University of North Carolina at Charlotte, Charlotte, NC 28223, United States.
- Department of Physics and Optical Science, University of North Carolina at Charlotte, Charlotte, NC 28223, United States.
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