Thermal analysis of condensation-induced vacuum and its feasibility for operating a vacuum membrane distillation system

Abstract

Vacuum pumps are primarily used in vacuum membrane distillation (VMD) for vacuum generation. However, vacuum pumps are characterized by low operational durability and a high energy demand. In this study, a steam condensation process was combined with the VMD process such that the vacuum created during the steam condensation process drove the VMD operation. A thermodynamic cycle comprised of three main processes: steam purging, vacuum creation, and water recovery was described. This paper presents a detailed thermal analysis of each process of the thermodynamic cycle based on heat transfer and thermodynamic models. The modeling results are compared with experimental data. The pressure in the vacuum chamber increased during the water recovery process because of the accumulation of non-condensable gases, which permeated the chamber along with the water vapors. Consequently, the mass flux of the VMD permeate decreased, and finally, the permeation ceased when the pressure in the chamber reached ~20 kPa; after this stage, the cycle restarted. For the current experimental conditions and vacuum chamber design, the durations of the purging, vacuum creation, and recovery processes were 30, 500, and 270 s. At least three linked vacuum chambers must be synchronized and operated simultaneously to realize continuous water recovery.