製備具光熱與全疏性質的奈米複合薄膜應用於薄膜蒸餾

Abstract

薄膜蒸餾是以熱驅動的程序主要應用於海水淡化，薄膜蒸餾具有低能耗、低操作壓力、低操作溫度等優點。即使有這些優點，傳統的薄膜蒸餾仍有三個主要問題需要被克服，包含溫度極化現象、低能量轉化效率、膜孔容易被低表面張力液體所潤濕，本研究製備光熱與全疏性質的奈米複合薄膜，以太陽能直接接觸式薄膜蒸餾來克服溫度極化現象、減緩薄膜孔洞被低表面張力液體潤濕、提升能量轉換效率，本研究主要分成三個部分，如下所示。 第一部分，製備疏水氧化鋁中空纖維薄膜，並首次以真空式薄膜蒸餾程序應用於濃縮程序，結合相轉換與燒結程序製備出多孔氧化鋁中空纖維薄膜，經由矽氟化合物改質，可製備出疏水氧化鋁中空纖維薄膜，為了評估濃縮程序效能，我們討論流體的流速、溫度、真空度與操作濃度，在真空式薄膜蒸餾進行連續濃縮程序下，以8公升10°Brix為初始濃度將液體濃縮至50°Brix，顯示本研究製備的疏水氧化鋁中空纖維薄膜可進行濃縮程序，即使在高濃度的進料濃度下仍有水通量。 第二部分，首位製備表面具全疏性質的氧化鋁中空纖維薄膜，以直接接觸式薄膜蒸餾程序處理含界面活性劑之液體，以氧化鋁中空纖維薄膜為基材，用化學浴沉積法長上氧化鋅奈米柱與奈米球，由掃描式電子顯微鏡、元素分析、原子力顯微鏡得知，氧化鋅有效的長在氧化鋁中空纖維表面，經由矽氟化合物改質後的奈米柱與奈米球，其乙醇（90 wt%）接觸角可達128.7與138.1°，於直接接觸式薄膜蒸餾實驗中，在進料端連續加入十二烷基硫酸鈉界面活性劑，長上氧化鋅奈米結構後的複合薄膜具有較佳的抵抗薄膜孔洞潤濕效果。 第三部分，首位製備光熱與全疏性質的奈米複合薄膜，應用於太陽能直接接觸式薄膜蒸餾，此光熱-全疏性質薄膜有三個優點，包含克服溫度極化現象、減緩界面活性劑潤濕薄膜表面、提升薄膜蒸餾能量轉換效率，此光熱-全疏性質的複合薄膜是沉積親水的碳黑奈米粒子，經由矽氟化合物疏水改質後可達超疏水效果，此碳黑奈米粒子可以吸收太陽光並局部加熱薄膜表面的液體，進而提升薄膜蒸餾通量25%，此外，碳黑奈米粒子可以提供凹角結構來減緩界面活性劑潤濕孔洞，比較傳統聚偏二氟乙烯與光熱-全疏薄膜，其乙醇（80 wt%）接觸角分別為0 o與104 o ，此光熱-全疏薄膜可以同時提升薄膜蒸餾通量、減緩界面活性劑潤濕薄膜孔洞以及提升薄膜蒸餾效果。

Membrane distillation (MD) is a thermal process applied to desalination under low energy, low operating pressure, and low temperature conditions. Despite these advantages, the traditional MD process has three major limitations: temperature polarization, low energy efficiency, and low-surface-tension liquid-wetting membrane pore. In this study, we prepared an omniphobic−photothermal dual-functional membrane with a direct solar MD process for simultaneously overcoming temperature polarization, mitigating membrane pore wetting by the surfactant, and significantly enhancing the energy efficiency for the MD process, which provides a potential solution for clear water using solar energy. This doctoral dissertation comprises three parts. In the first part, hydrophobic alumina hollow fiber membranes were first used for sucrose concentration in a vacuum membrane distillation (VMD) system. The alumina hollow fiber (HF) membranes were prepared using the combined phase inversion and sintering method and then grafted with 1H,1H,2H,2H-Perfluorodecyltriethoxysilane (FAS) to impart hydrophobicity. To evaluate the sucrose concentration performance of hydrophobic alumina hollow fiber membranes, the effects of various parameters on the permeate flux, such as the feed flow rate, temperature, vacuum pressure and concentration, were investigated in the VMD system. In the continuous sucrose concentration tests, an 8 L 10°Brix sucrose solution was successfully concentrated to 50°Brix using the VMD system. The results revealed that the hydrophobic alumina hollow fiber membranes are promising and feasible for concentration processes in a VMD system. In the second part, omniphobic hollow fiber membranes were first prepared and applied to direct contact membrane distillation (DCMD) with a low surface tension feed. Alumina hollow fiber membranes were prepared as the membrane substrates, and chemical bath deposition methods were applied to deposit zinc oxide (ZnO) nanorods (NR) and nanoparticles (NPs) on the HF substrate. The SEM, EDX, and AFM analyses showed that the ZnO nanostructures were effectively deposited on the membrane surfaces and able to increase the surface roughness. After surface fluorination by FAS, the HF membranes deposited by ZnO nanorods and nanoparticles demonstrated contact angles of 90% v/v ethanol/water mixture are 128.7° and 138.1°, respectively. In the DCMD experiments with the sequential addition of sodium dodecyl sulfate (SDS) from 0.2 to 2.0 mM, the HF membranes with ZnO nanostructures exhibited superior wetting resistances with low surface tension feeds compared to pristine HF membranes. In the third part, an omniphobic−photothermal nanocomposite membrane was first developed and then subjected to direct solar membrane distillation (DSMD). This membrane has three advantages: it overcomes temperature polarization, mitigates membrane pore wetting by the surfactant, and enhances the energy efficiency from 29.7% to 71.1%. The dual-functional membrane was prepared by forming a hierarchical structure of carbon black (CB) nanoparticles (NPs) modified by FAS on a polyvinylidene fluoride (PVDF) membrane surface. The fluorinated CB NPs absorb sunlight to provide localized heating. The MD flux was increased by 25% at one sun unit sunlight irradiation. The CB NPs provide a re-entrant structure with low surface energy, which prevents wetting of the membrane pores. The contact angle of 80% v/v ethanol/water mixture increases to 104°, in contrast to that of 0° on the pristine PVDF membrane. This omniphobic−photothermal dual-functional membrane simultaneously improves the flux, wetting resistance, and thermal efficiency in the MD process.