Selective Wettability Membranes and Surfaces for the Separation of Oil-Water Mixtures, Extractions, and Fouling Prevention

Abstract

The separation of miscible and immiscible liquid mixtures is one of the most widely utilized unit operations in the world. The energy-efficient separation of immiscible oil-water mixtures is critical for a wide variety industries including: petroleum drilling and refining, hydraulic fracturing, wastewater treatment, mining, metal fabrication and machining, textile and leather processing, and rendering. Membrane-based methods have become increasingly attractive for the separation of oil-water mixtures because they are relatively energy-efficient, can be readily used to separate a variety of industrial feed streams, and provide consistent permeate quality. In this dissertation, I discuss the design strategies for membranes with selective wettability, i.e., membranes that either selectively wet, or prevent wetting, by the non-polar or polar phase. The design strategies include the parameterization of two important physical characteristics: the surface porosity/geometry and the breakthrough pressure. On the basis of this understanding, I explore principles that allow for the systematic design of membranes with selective wettability that enable high-efficiency separation of a range of oil-water mixtures. Furthermore, I investigate and shed light on solving one of membrane technology’s greatest challenges: organic fouling. A highly versatile silanization methodology was developed, which allows water to permeate through the membranes, while repelling oil (hydrophilic and oleophobic) in air and underwater. This approach is valid for a variety of membrane pore sizes, including 5 nm diameters, and several substrate classes including: polymers, ceramics, and metals. Due to our selective wetting membranes, I accomplished complete separation of difficult-to-separate, surfactant-stabilized oil-in-water emulsions with enhanced performance due to anti-fouling surface modification, in both batch and cross-flow modes of operation. The anti-fouling ability doubled the untreated membrane’s permeation rate after >500 h of operation. The developed membranes also enable the energy-efficient separation of miscible liquids. By combining the developed membranes with liquid-liquid extraction (LLE), we engineered a new separation methodology termed CLEANS (Continuous Liquid-Liquid Extraction and In-situ Membrane Separation). CLEANS achieves >250% increases in extraction factors and avoids energy-intensive distillation, solely by addition of a common surfactant to the extractant. This is the first time anyone has intentionally created emulsions, to benefit from increased mass transfer surface area, in LLE. This was only possible due to our membrane’s unique capability to break up a variety of emulsions on contact. Additionally, I applied my knowledge of surface wetting and anti-fouling chemistry to develop a highly effective and dually functional easy-clean and anti-fog coating. The hydrophilicity of the substrate polymer prevents water droplets from condensing into light scattering features, while the post-modification imbues the surface with oil shedding ability. The coating is designed to be scalable in application by spray coating and is well bonded directly to polycarbonate substrates. Finally, by understanding liquid breakthrough pressure on chemically modified membranes, I developed a method of freeze concentration where the crystallizer and separator unit operations were combined into one. This is made possible by the membrane breakthrough pressure being high enough to hold the feed liquid in the freeze concentrator, but low enough that it can be actuated by partial vacuum to collect the concentrate. Proof of concept was provided for the freeze concentration of apple juice, ethanol solution, and dyed water. In summary, my dissertation projects resulted in several new prototype membrane-based purification systems and new anti-fog windows and glasses.