Ammonia (NH3) is widely used for the production of nitrogen-containing chemicals, especially in agriculture. It has also gained recognition as an important energy carrier for H2. It is conventionally produced by the Haber-Bosch (HB) process, an energy demanding method that relies on high pressure (10-20 MPa) and temperature (400-500 oC) for NH3 synthesis and heavily depends on fossil fuels. The complex thermodynamics and kinetics of the reaction results in low reaction yields and requires the separation of NH3 through energy intensive cryogenic condensation for NH3 removal from the reactor effluent stream before further reaction of the recycled H2 and N2 can occur. In this work, we seek to improve NH3 synthesis by using Na+-gated nanochannel membranes, which are able to selectively remove NH3 at close to the HB process conditions. The first part of my research work is focused on using Na+-gated nanochannel membranes (on α-Al2O3 supports with O.D. = 1.5 mm) for the selective separation of NH3 from a mixture gas containing NH3, H2 and N2, at temperature and pressures up to 250 oC and 35 bar, respectively. Membrane technology is attractive because of its simplicity, continuous operation, and low energy consumption. However, previously reported membranes were either evaluated under conditions far below the HB process conditions of elevated temperature and pressure, or their separation performance declined quickly with an increase in temperature or pressure. Additionally, some studies did not report NH3/H2 selectivies, where the NH3 and H2 molecules are closely related in size, demonstrating how challenging it is to separate NH3 at high temperature and pressure.
We investigated the potential of the Na+-gated nanochannel membranes for the selective separation of NH3 from a mixture gas containing NH3, H2 and N2, at temperatures and pressures of 100-250 oC and 4-35 bar, respectively. The membrane demonstrated NH3/H2 and NH3/N2 selectivies as high as 4,280 and >10,000, respectively. The membrane stability was investigated and confirmed through long-term stability testing and characterization of the membrane by XPS, XRD, and FTIR-ATR. A preliminary technoeconomic analysis for the membrane, using the experimental separation results, demonstrated the great potential of the Na+-gated nanochannel membranes in the HB process, where 80% energy savings and 20% lower NH3 production cost could be achieved.
Having demonstrating potential of the membrane, the Na+-gated nanochannel membranes were then synthesized on more scalable α-Al2O3 supports (O.D. = 5.7 mm), where we investigated the effects that various synthesis parameters had on the growth of the Na+-gated nanochannel membranes. The synthesis parameters investigated include i) the number of seeding steps, ii) gel aging time, iii) membrane growth time, and iv) the composition of the growth gel. The effects of these synthesis parameters on the membrane performance and quality were investigated by single and mixed-gas testing, as well as characterization by SEM, XRD, and DLS. Our results indicated that Na+-gated nanochannel membrane quality is sensitive to these parameters and reduced membrane quality could lead to declining membrane performance due to increasing defect concentration.
We also investigated the adsorption of NH3 on NaA by obtaining NH3 adsorption isotherms from 373-523 K and 0-500 kPa in an attempt to optimize Na+-gated nanochannel membranes. The NaA zeolite was ion-exchanged with K+ and Cs+ ions, to investigate the role of the cations in the zeolite structure on NH3 adsorption. After first characterizing the zeolites by SEM, XRD, ICP-OES, and ICP-MS, NH3 adsorption isotherms were obtained. The results showed that the ion size and polarity influence NH3 uptake, while adsorbate-adsorbate interactions, zeolite pore/cavity size, and framework Al content affect the heat of adsorption. These adsorption data are critical for understanding and predicting membrane transport properties.
The final aim of this work was to begin developing an analytic solution of the Maxwell Stefan diffusion model, using the Toth adsorption model, to better understand NH3 transport through Na+-gated nanochannel membranes and facilitate future process design.;
May2023; School of Engineering
Dept. of Chemical and Biological Engineering;
Rensselaer Polytechnic Institute, Troy, NY
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