This NSF award by the Chemical and Biological Separations program supports work by Professor Winston Ho and his students to synthesize advanced CO2- and H2S-selective membranes by incorporating new multi-walled carbon nanotubes (MWNTs) and silica with sterically hindered amines into the polymer membrane matrix and to study the effects of MWNTs and silica for improved resistance to membrane compression under high pressures and for enhanced transport of the acid gases.

The state-of-the-art process for the removal of CO2 and H2S from gases, including synthesis gas and natural gas, uses aqueous amine solutions, where steam is used for regeneration and the steam stream containing the acid gas is then condensed to separate/release the acid gas. This process involves cumbersome operations, high energy consumption, and capital-intensive equipment. Furthermore, it is limited by the thermodynamic equilibrium on acid gas solubility during absorption, resulting in an increased solution circulation rate and consequently large equipment. Thus, it is important to develop an effective process with both capital and energy savings. This work is aiming at developing the energy-efficient membrane process, featuring a simple pressure-driven process with no moving parts. The proposed process combines the absorption and stripping of acid gas into one step. This one-step process simplifies the acid gas separation and overcomes the thermodynamic solubility limitation.

The proposed membrane is the first of the kind capable of possessing high CO2 and H2S permeabilities and selectivities vs. hydrogen and nitrogen at relatively high temperatures (100 - 120C) and pressures (1 - 30 atm), which is needed for energy-efficient purification of syngas from coal and biomass as well as for CO2 capture. This research not only is of a great scientific interest but also may provide improved membranes of significant technological importance. We believe that it represents a significant contribution to expanding the scientific knowledge and understanding in the gas separation.

Potential impacts of the proposed research are significant as this research is aiming at the novel CO2-selective membrane and process to overcome many deficiencies of the commercial gas treating technology. This proposed one-step process not only simplifies the separation process, but also eliminates the needs for the absorber, the regenerator, two pumps for the loaded and regenerated amine solutions, and the pumping operations between the absorber and regenerator. It also eliminates the energy-intensive regeneration of the amine solution due to the high heat capacity of water and the use of temperatures swing to drive gas desorption. Thus, this membrane process will have both significant capital and energy savings. The proposed membranes have many potential applications including the purification of syngas derived from coal and biomass to produce high purity H2 for fuel cells, CO2 capture from flue gas for its sequestration, and CO2 removal from biogas, natural gas, confined space air, and ambient air. We will present and publish this research each year. Furthermore, this research provides the education of 1 Ph.D. graduate student and 2 undergraduate students for conducting not only the work on advanced membranes but also that with technological significance.

Project Report

Summary of Project Outcomes: Dr. Ho and his students have synthesized new CO2-selective membranes by incorporating sterically hindered amines and multi-walled carbon nanotubes (MWNTs) in a crosslinked polyvinylalcohol (PVA) matrix. The membranes have demonstrated impressive separation performance and durability at high temperatures of above 100°C and pressures of at least 15 atm. With 2 wt% MWNTs incorporated, the membrane performance, including a CO2/H2 selectivity of 43 and a CO2 permeability of about 1300 Barrers, showed no change for 444 h (18.5 days) when operated at 15 atm and 107°C. The nanotubes were then functionalized with surface hydrophilic groups in order to enhance the adhesion to the PVA network. The membranes with functionalized MWNTs exhibited better separation performance. The membranes developed in this work are the first of this kind, displaying exceptional CO2/H2 separation performance and high tolerance to feed gas at high pressures and high temperatures. Potentially, they could be used in a stand-alone membrane unit for energy-efficient pre-combustion carbon capture from coal-derived syngas or in conjunction with water-gas-shift reaction for CO clean-up to produce high-purity H2 for fuel cells and to simultaneously capture CO2. Intellectual Merit: The objective of this research is to manipulate polymeric and nanofiller material properties to create game-changing membrane performance for CO2 separation. The membranes have shown promising performance for applications such as fuel cell H2 purification and pre-combustion CO2 capture. Therefore, this research not only is a significant contribution to the knowledge of amine chemistry but also makes a profound impact on the rapidly developing field of membrane gas separation. Broader Impacts: The research has economic and environmental significances. The synthesized membranes provide an energy saving, environmentally friendly separation technology to purify hydrogen for fuel cell applications and to capture CO2 for restraining greenhouse gas emission. The membranes developed by this research have many potential applications including: (1) the purification of synthesis gas, derived from natural gas, shale gas, petroleum, coal or biomass, to produce high-purity H2 for fuel cells and other applications, (2) CO2 capture from flue gas for its sequestration, and (3) CO2 removal from biogas, natural gas, shale gas, confined space air, and ambient air. Background: In view of the deficiencies of the state-of-the-art process, it is important to develop an effective process with both capital and energy savings. This project is aiming at research on a novel approach using a CO2-selective membrane. This novel approach combines the absorption and stripping of CO2, carried out in 2 separate steps in the commercial technology – the state-of-the-art amine scrubbing process, into a one-step membrane process as shown in Figure 1. This one-step process not only simplifies the separation process, but also eliminates the capital-intensive equipment of the commercial technology. This one-step process also overcomes the thermodynamic solubility limit of aqueous amine solution. Findings: The requirements for pre-combustion CO2 capture and natural gas processing are challenging as feed pressures are at least 15 atm or higher. In addition, temperatures ranging from 100 to 200°C are preferred for pre-combustion CO2 capture. Dr. Ho and his students successfully synthesized nanostructured membrane by incorporating MWNTs into the crosslinked PVA/amine membrane. Figure 3(a) gives the SEM for the top view of the membrane with a textured surface, which is different from that without MWNTs, which shows a smooth surface. Figure 3(b) is the SEM for the cross-section of the membrane with the MWNTs. Near the surface, some MWNT fibers protrude out. Due to the enhanced anti-compression property by embedded MWNTs, the synthesized membranes have shown significant improvement on the stability at 15 atm in comparison with the membrane without carbon nanotubes. With the untreated-MWNT loading of 2 wt%, the membrane demonstrated stable CO2 permeability and CO2 selectivities vs. H2 and N2 for 444 h (18.5 days) using a feed gas of 40% H2, 20% CO2, and 40% N2 as shown in Figure 3. The surface and ends of MWNTs were functionalized with carboxylic acid groups and hydroxyl groups using strong acid oxidation. The presence of polar groups was confirmed by the FT-IR spectrum. The surface modification much improved the dispersibility of MWNTs in aqueous solution and, more importantly, the compatibility with the hydrophilic polymers in the membrane. After comparing all the results, it is shown that the membrane performance with acid-treated MWNTs on average was better than that with untreated MWNTs. The amino-functionalized MWNTs were obtained through chemical modification of the acid-treated MWNTs with (3-aminopropyl)triethoxysilane. The presence of these hydrophilic groups was confirmed by FTIR. The membranes incorporated with the modified MWNTs demonstrated significant improvement in CO2/CH4 separation performance, especially in term of selectivity. A CO2/CH4 selectivity of 180 along with CO2 permeability of around 1000 Barrers have been obtained at 15 atm and 107°C for a feed gas composition of 60% CH4, 20% H2, and 20% CO2.

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