Date of Award

Spring 2016

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Chemical Engineering - (Ph.D.)

Department

Chemical, Biological and Pharmaceutical Engineering

First Advisor

Kamalesh K. Sirkar

Second Advisor

Piero M. Armenante

Third Advisor

Boris Khusid

Fourth Advisor

S. Basuray

Fifth Advisor

Wen Zhang

Abstract

A variety of microporous hydrophobic flat sheet membranes of polyvinylidene fluoride (PVDF) and expanded-polytetrafluoroethylene (e-PTFE) are studied to evaluate the influence of membrane properties on their performance in desalination by direct contact membrane distillation (DCMD) and vacuum membrane distillation (VMD) processes. The membrane thickness is varied between 23 μm to 125 μm; the pore size is varied from 0.05 μm to 0.45 μm. The porosity is generally high in the range of 0.7 - 0.8. DCMD experiments are performed over a hot brine temperature range of 65 °C to 85 °C and distillate temperature at 25 °C for various brine flow rates and distillate flow rates in a circular stainless steel cell and a rectangular chlorinated polyvinyl chloride (CPVC) cell. Boundary layer heat transfer resistances in the membrane cell on both sides of the membrane and the two membrane surface temperatures are determined from the experimental data over a range of hot brine and cold distillate flow rates by the Wilson plot technique. Membrane properties such as the maximum pore size and tortuosity are characterized and employed in checking out model assumptions and model results for water vapor transport in the Knudsen regime and the transition region. Good agreements (within 5% deviation) of the membrane mass transfer coefficient of water vapor and the observed water vapor fluxes are obtained between the experimental values and the simulated results predicted for either the Knudsen regime or the transition region. Pore size distribution (PSD) does not matter if the membrane pore size variation falls entirely in the Knudsen diffusion or the transition regime. However, for membranes having nominal pore size ≅ 0.1 μm, Kn ≅ 1, PSD plays an important role in DCMD. The effects of membrane thickness on water vapor flux and thermal efficiency are also simulated and compared with the experimental results. The same membranes are studied in the CPVC cell for VMD behavior using the Wilson plot method over a hot brine temperature range of 65 °C to 85 °C for various feed flow rates and various vacuum levels. Liquid entry pressure (LEP) is experimentally determined. Water vapor fluxes are predicted and compared using two models: the Knudsen diffusion and the dusty-gas model (DGM). The deviation between the two models is within 1.3%. Knudsen diffusion is the dominant regime in VMD transport since the values of Knudsen number, Kn, for all membranes are larger than 1 at all temperatures. The boundary layer heat transfer resistance in the membrane cell and the membrane surface temperature are determined from experimental data via Wilson plot. Good agreements of membrane mass transfer coefficients and water vapor fluxes are found between the DGM simulations and the experimental results (deviation within 5%). The performance estimates for larger hollow fiber-based MD devices are also analyzed using mathematical models developed and numerically solved in MATLAB.

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