Thin-Film Composite Membranes and Their Modules for Post-Combustion CO2 Capture

Abstract

This dissertation introduces the development of membrane-based CO2 separation system for post-combustion CO2 capture. Conventionally, amine-based chemical absorption technologies are widely used in industrial fields, but the chemical absorption methods require high energy consumption to reproduce the absorbents, which has been a serious drawback in terms of cost efficiency. Membrane technology is one of the most promising technologies to replace the conventional amine-based absorption technology for its simplicity, compactness and high energy efficiency. However, despite its great potentials, membranes have been hardly used in CO2/N2 separation due to the difficulties of scale-up the membranes with thin selective layers. Herein, two approaches for preparing thin-film composite (TFC) membranes are proposed to control defects in membrane preparation. Defect control is the key to develop large-scale membrane preparation, and we successfully demonstrated a bench-scale membrane-based CO2 separation system. The overall research progresses from material development to bench-scale process design are described in this thesis. In Chapter 2, the development of a mixed-matrix membrane material is introduced using chemically modified graphene oxide (GO) and a commercial polymer. The incorporation of nano-particles and a polymer matrix can result in improved gas transport properties by the assistant of nano-particles. GO is one of the most promising nanocarbons that enable the incorporation of graphene and related materials into bulk materials and nanocomposites, especially for CO2 selective membranes due to its high CO2 sorption property. GO has hydrophilic nature that enables straightforward dispersion in aqueous solution by sonication, but GO show poor dispersibility in common organic solvents, which prevent preparing solution-mixing polymer nanocomposites. In this chapter, we introduce an approach to prepare highly soluble, functionalized GO in both aqueous and non-aqueous solvents. This was achieved by reacting polyetheramine consisting of amphiphilic components, e.g., polypropylene oxide and polyethylene oxide, with carboxylic acid groups at GO edges. We used the functionalized GO in a GO-polymer nanocomposite material for CO2/N2 separation, and confirmed the enhanced CO2 permeability and CO2/N2 selectivity. In Chapter 3, the development of a TFC membrane using Teflon gutter layer is described. TFC membranes with less than 100 nm of selective layers are highly desired to maximize the permeance of gas separation membranes for high energy efficiency. For membranes with ultrathin selective layers, a gutter layer is usually required to prevent pore penetration in the selective layers. The introduction of a gutter layer strongly improves TFC membrane performance by increasing the overall membrane performance up to an order of magnitude. Unfortunately, this improvement causes an undesired decrease in selectivity unless the gutter layer is properly designed. However, the most commonly used material for a gutter layer, polydimethylsiloxane, does not meet the requirements for high-performance membrane materials. Thus, we prepared a Teflon gutter layer with over sixfold higher CO2 permeance than polydimethylsiloxane, and prepare a TFC membrane with ultrathin gutter (75 nm) and selective layer (70 nm) for CO2/N2 separation using O2 plasma treatment method. Finally, Chapter 4 introduces an approach to control defects in membranes for large-scale membrane preparation using a protective layer. Despite huge advances in CO2/N2 separation membrane materials, very few membranes were successfully demonstrated in large-scale due to the difficulties to control micro-defects which are easily generated in TFC membranes. Using a protective layer can be helpful to control the defects, but the effectiveness as well as the limitation of a protective layer have never been studied systematically. In this chapter, we propose an approach to estimate the membrane performances depending on the protective layer properties as a function of the defect ratios. We proved that using a protective layer is quite effective in large-scale membrane preparation by the experiments. The membrane prepared by using a protective layer exhibited about 1200 GPU of CO2 permeance with CO2/N2 selectivity over 50 under mixed-gas conditions. In addition, we demonstrated the membrane-based CO2/N2 separation system in bench-scale using as-prepared membranes, and the result and the process design are also represented in this last chapter.