Mechanism study on magnitude of mass transfer coefficients in liquid desiccant dehumidification and regeneration

Liquid desiccant systems have attracted extensive attention due to huge energy-saving and environmental potential. Mass transfer coefficients are key parameters for system design and performance evaluation. Contrast experiments between liquid desiccant dehumidification and regeneration were conducted here to investigate the magnitude of mass transfer coefficients by using LiBr aqueous solutions. Mass transfer coefficients of dehumidification were found to be much higher than regeneration even under the condition that the mass transfer driving force and inlet parameters except solution temperatures were kept the same. To obtain microscopic explanation, free energy curves of water transport were simulated by molecular dynamics based on umbrella sampling. Results show that large energy obstacle should be overcome for water moving into air, which is unnecessary for water transported into aqueous solution. Lower mass transfer coefficients of regeneration should be attributed to the mass transfer direction rather than higher solution temperatures of regeneration than dehumidification. Dimensionless correlations of mass transfer coefficients using water diffusivity in LiBr aqueous solution and correlations using surface tensions show better prediction performance than dimensionless correlations using water diffusivity in air. This study discloses changing mechanism of mass transfer coefficients and helps develop correlations with high accuracy.