Direct contact membrane distillation: analysis and application

This work investigates different aspects of direct contact membrane distillation (DCMD), from a microscopic study of membrane structure of an electrospun membrane and mass transfer mechanism across DCMD membranes, to the macroscopic picture of DCMD's design and technological viability in small-scale, localized desalination for the use in arid regions with a supply of brackish water.
Experiments were carried out to investigate the effect of membrane thickness upon transmembrane flux in DCMD. Polyvinylidene fluoride (PVDF) electrospun nanofibrous membranes (ENMs), consolidated with a heatpress process, as thin as 27 microns were fabricated and tested in a DCMD cell. A transmembrane flux as high as 60 L/h m2 was observed. A structural deviation factor considering tortuosity, which reflected the variation of structure with thickness, was introduced. The model was found to fit the data very well except for the thinner membranes operated in a system with high inlet feed temperature. Additionally an analytical model was created to estimate the optimal membrane thickness for DCMD operations. It is found that the value of optimal thickness increases with reduced heat transfer coefficients; decreased feed inlet temperature; increased membrane permeability; and increased salinity. Even for 10% NaCl the predicted optimum was 11 microns which was too thin for experimental confirmation.
Two approaches were taken to the sizing of DCMD modules. A modified effectiveness – number of heat transfer units (E-NTU) method is introduced for designing uniform flow modules. The model enables good estimation, which is within 6%. The E-NTU method is recommended for preliminary design for larger systems. A discretization model of a DCMD hollow fibre module with random packing was constructed using MATLAB. The influence on module performance of various design features, including flow configuration, fibre size, packing density and were evaluated. Four performance indicators were used to assess the performance: transmembrane flux, module production rate, thermal efficiency and overall energy efficiency. The assessments suggest a module with a packing density of about 0.5 offers the best thermal and energy efficiency. Whilst smaller diameter fibres create a moderately higher pressure drop, the extra membrane area packed into the module enhances the production rate. Moreover although transmembrane flux always decreases with a longer module, the overall module production rate justifies a bigger module in a practical desalination system. Prior to DCMD module design consideration, a reassessment of the conventional method suggested that the sum of resistance approach engages in some double counting when combining the Knudsen diffusion coefficient and the molecular diffusion coefficient. A new expression which accounts for the physical influence between molecular-molecular interactions and Knudsen diffusion is introduced.
This thesis concludes with a flowsheet design of a DCMD desalination system that can provide 500L of potable water per day, which is sufficient for a large family living in a remote arid region. The results from the MATLAB model were incorporated into the Aspen Plus using both direct and indirect methods. Considerations were given to the recycle and cooling streams and a design which avoided the need for refrigeration was found.

Advisor: Field, R