Date of Award

Spring 2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Environmental Science - (Ph.D.)

Department

Chemistry and Environmental Science

First Advisor

S. Mitra

Second Advisor

Tamara M. Gund

Third Advisor

Edgardo Tabion Farinas

Fourth Advisor

Haidong Huang

Fifth Advisor

Pradyot Patnaik

Abstract

Membrane distillation (MD) is a newer technology that is being investigated for applications such as seawater desalination and concentration of fruit and sucrose solutions. The major advantage of MD over traditional thermal distillation is that it requires a substantially lower thermal energy requirement to power the process. This allows low grade energy sources such as waste heat or solar energy to be used with MD. Compared to concentration processes such as reversed osmosis or ion-exchange, MD does not require specialized equipment, high electrical consumption, the use of strong acids and bases nor does it generate hazardous waste as a by-product.

In membrane distillation, a heated solution is passed through the lumen of a hollow fiber porous hydrophobic membrane. The vapor pressure differential between the cool and hot side of the membrane allows the vapor to pass across the pores but prevents passage of the liquid phase. Unlike pervaporation, which also relies on differential vapor pressure, MD also involves the transfer of a significant amount of heat across the membrane. MD processes to date have demonstrated several inefficiencies that cause it to be a relatively low yield process. These inefficiencies include conductive heat loss through the membrane material, temperature polarization at the bulk feed-membrane interface, pore wetting and effective use of available membrane surface area.

In this investigation, traditional membrane distillation was compared to membrane distillation using the same starting membrane material but which had carbon nanotubes (CNTs) incorporated into the membrane pores. The modified membrane is referred to as carbon nanotube immobilized membrane (CNIM). It was demonstrated that several properties of CNTs aided in improving the performance of MD. These include high thermal conductance, rapid sorption-desorption ability, ability to transport water in a rapid ordered manner and hydrophobic characteristics.

Experiments were conducted where MD was used as a preconcentration technique to analyze trace quantities of drug substance in water. CNIM provided much higher levels of enrichment for the analytes of interest than did preconcentration using the plain membrane. Another set of experiments was then successfully conducted that demonstrated that CNIM-MD was applicable to the preconcentration of drug products in a polar solvent. Desalination experiments were completed that showed that CNIM provided significantly higher levels of salt reduction and flux at a lower energy requirement than did the standard membrane. Finally, MD-CNIM was investigated as a means to remove or concentrate trace levels of inorganic impurities from an aqueous matrix.

Overall, it was demonstrated that MD using CNIM provided a more efficient process with significantly higher solvent reduction and levels of enrichment than did MD using plain membranes.

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