Performance enhancement of air bubbling and vacuum membrane distillation for water desalination

Membrane distillation process (MD) is a new process gaining research interest because of its efficiency. The main objective of this research is to investigate the feasibility of using air bubbling and vacuum MD for water desalination. In a brackish water desalination study, a tubular MD module was used for performing experiments on a hydrophobic tubular membrane. The membrane module was designed in zigzag mode to achieve high turbulence flow. The MD process performance in terms of permeate flux and thermal efficiency was monitored as the process variables were changed. The permeate flux and thermal efficiency increased by more than 4 and 2-fold, respectively, as the feed temperature increased from 40 to 70°C. The permeate flux and thermal efficiency at 70°C and 240 (L/h) were as high as 10.8 (kg/m''h) and 52% for pure water, and 5.4 (kg/nr'h) and 35% for 5000 mg/L of feed concentration, respectively. An air bubbling system was used to enhance. tubular MD module performance. Compared to the basic module, the mass transfer coefficient increased from 5 to 25% as air bubbling rate increased from 30 to 90 (L/h). Increasing air bubbling rate 3-fold, increased Reynolds number 3-fold, which enhanced the heat transfer coefficient 2- fold. The permeate flux and thermal efficiency were higher than that of the basic module by about 1.4 and 1.5- fold, respectively for 5000 (mg/L) feed concentration. I Vacuum Membrane Distillation (VMD) performance was investigated using a flat- sheet cross flow module with a new O-ring and two commercial PTFE and PVDF hydrophobic membranes. The design of membrane module provided high heat transfer coefficients and high Reynolds numbers. Heat and mass transfer coefficients were measured within the module in VMD experiments. The effect of process parameters on the permeate flux was investigated. At optimum operating conditions the permeate flux reached 43.8 and 52.6 (kg/nr'h) for PVDF and PTFE, respectively, with over 99% salt rejection for both.EThOS - Electronic Theses Online ServiceGBUnited Kingdo