Blockchain Papers

Maryam Aliabadi, Nassim Tahouni, M. Hassan Panjeshahi

No abstract is available for this record.

Hamed Abdolahimansoorkhani

Oxygen plays a central role in numerous industrial processes. Several technologies have been reported for producing oxygen from air. Among them, oxygen transport membrane (OTM) technology—based on mixed-conducting, gas-tight ceramic membranes—has attracted significant attention due to its high oxygen selectivity, relatively low capital and operating costs, and versatility for both ex-situ and in-situ applications. Mathematical modeling of OTMs offers a powerful tool to investigate internal multi-physics transport phenomena, providing deeper insight into fundamental mechanisms while serving as a cost-effective approach for optimizing membrane stack designs. After a comprehensive introduction, in the first part of this dissertation (chapter 2), a comprehensive hollow fiber membrane model was developed, grounded in an experimental system as the physical basis. The model couples Multiphysics transport processes within the membrane with large-scale thermal–fluid transport in the test assembly and furnace. Systematic parametric studies are performed to investigate fundamental mechanisms and assess membrane performance. The results show that the oxygen partial pressure on the permeate side increases asymptotically from the inlet to the outlet of the hollow fiber membrane. In contrast, the longitudinal distribution of oxygen vacancy concentration decreases along the same direction, while the oxygen flux distribution follows the profile of oxygen vacancy concentration at the permeate surface. High feed-side air pressure and low permeate-side gas pressure are found to enhance oxygen permeation performance. Among the transport resistances, surface exchange resistance at the permeate side dominates, whereas bulk diffusion resistance contributes minimally in substrate-supported thin-film hollow fiber membranes. Overall, the modeling study provides valuable insight into the underlying mechanisms and offers practical guidance for membrane design and operation to improve oxygen production efficiency. To enable practical applications, upscaling from a single hollow fiber membrane to stacks and modules is essential. However, experimental methods for evaluating upscaling strategies are both time-consuming and costly. Mathematical modeling, by contrast, offers a cost-effective and flexible tool for this purpose. In the second part of the dissertation (chapter 3), building upon experimental results from a proof-of-concept hollow fiber membrane stack, a computational fluid dynamics (CFD)-based Multiphysics stack model was developed and validated. Extensive simulations were performed to examine stack behavior under varying operating conditions, and different design strategies were evaluated to optimize stack performance. The oxygen permeation process is thermally activated. Increasing the argon sweep gas flow rate lowers the oxygen partial pressure around the shell sides of the hollow fibers and enhances the permeation flux. Along the lumen side, oxygen partial pressure rises from the inlet to the outlet, with flux significantly higher in the upstream region than downstream. A distinct gradient of oxygen vacancy concentrations is observed across upstream fiber sections, from shell to lumen surfaces, but this gradient diminishes downstream. For a fixed stack length, adding more hollow fibers increases the overall permeation rate but reduces flux. Oxygen partial pressure decreases radially from the periphery to the center of the stack, leading to lower average flux in inner layers. An appropriate packing density is therefore required to achieve compact design while limiting pressure losses. For a given total fiber length, an optimal fiber number exists that maximizes average permeation performance. Diffusive oxygen flux dominates near fiber walls, while convective flux becomes increasingly important toward the fiber center. A higher sweep gas flow rate reduces the region where diffusion dominates. To maintain the elevated temperatures required for membrane operation, a high-temperature furnace is typically used. However, this results in low heating power efficiency and makes rapid temperature changes difficult due to the large volume of the furnace. Recently, a novel strategy has been employed in which external electrical power is directly applied to a hollow fiber membrane, enabling compact self-heating. To better understand the fundamental mechanisms, a mathematical model is developed in the third part of the dissertation (chapter 4) for a self-heated hollow fiber oxygen separation membrane, assisted by vacuum conditions applied at the lumen-side outlet. Comprehensive simulations are conducted to study the effects of self-heating on Multiphysics transport processes and oxygen permeation performance. Additional simulations are performed to investigate the influence of electrical field orientations applied to the hollow fiber membrane and the vacuum levels at the lumen-side outlet. The associated fundamental mechanisms are discussed and elaborated. A higher applied voltage increases the average membrane temperature. The longitudinal temperature profile is non-uniform, with a maximum in the mid-region and steep decreases toward both ends. Oxygen permeation flux rises with applied potential and is further enhanced by higher vacuum levels (lower permeate-side oxygen partial pressure). The flux distribution shows a domed shape along the fiber length, approaching zero near the ends. At low potentials, the effect of vacuum level is negligible but becomes significant at higher potentials. On the feed side, oxygen concentration decreases from the bulk to the surface, while on the permeate side it decreases from the surface toward the lumen outlet. Both gradients intensify with increasing voltage and/or vacuum level. Across the membrane bulk, oxygen vacancy concentration increases from feed to permeate surfaces, with steeper gradients under higher potentials and stronger vacuum. The orientation of the applied potential strongly influences performance. Alignment with the permeation direction, with positive and negative electrodes connected to permeate and feed surfaces respectively, greatly enhances flux. The opposite configuration suppresses it, while a perpendicular potential has little effect on radial ion transport. Building on these advances, Chapter 5 integrates the modeling and heating strategies into a Joule-heated hollow fiber ion transport membrane reactor for methane oxidative coupling (OCM). Results demonstrate that methane plays a dual role: it is both the feedstock for conversion and a promoter of oxygen transport by lowering surface oxygen partial pressure and sustaining higher vacancy-driven flux. The coupled transport–reaction model reveals that gradual, membrane-mediated oxygen delivery significantly improves C₂ selectivity compared with conventional co-feed reactors. Methane–vacancy interactions, heterogeneous surface reactions, gas-phase chemistry, and localized Joule heating jointly shape flux, product selectivity, and thermal profiles. This chapter demonstrates the dual functionality of MIEC membranes as both oxygen separators and catalytic reactors, providing a path toward intensified, energy-efficient chemical production. Finally, Chapter 6 synthesizes the insights, highlighting how Multiphysics modeling bridges the gap between laboratory observations and industrial-scale application. By clarifying oxygen transport mechanisms, optimizing stack designs, enabling compact Joule-heating strategies, and extending membranes into reactive processes, this dissertation contributes both fundamental knowledge and practical guidance. The results position MIEC hollow fiber membranes not only as efficient oxygen separators but also as versatile platforms for low-carbon energy, 𝐶𝑂₂ management, and sustainable chemical synthesis.

Shuai Cao, Yibo Lu, Yijian Tang, Yangyang Sun · 8 authors

No abstract is available for this record.

Fei Yu, Xiaochen Zhang, Peng Liu, Bingbing Chen · 5 authors

Abstract Metal‐organic frameworks (MOF) have attracted extensive attention due to their ultra‐high specific surface area and tunable structure, the mechanism of direct utilization for capacitive deionization (CDI) defluorination remains undefined. Here, MIL‐101(Cr) with ultra‐high specific surface area, high water stability, and open metal sites (OMSs) is prepared by a hydrothermal method for defluorination of CDI. Carbon black is used as a “chain” to connect F‐stored in the holes of MIL‐101(Cr) (Cr‐MOF)as “blocks” to enhance the conductivity and ion storage capacity of MIL‐101(Cr)/carbon black electrodes (Cr‐MOF electrodes). This simple construction method avoids the process complexity of in situ synthesis and performs better. These easily constructed “blockchain‐like” Cr‐MOF electrodes exhibit excellent defluorination capacity (39.84 mg NaF g electrodes −1 ), low energy consumption (1.2 kWh kg NaF −1 ), and good stability. The coupling of the electrochemical redox reaction of Cr 3+ /Cr 4+ with confined water is investigated using in situ and ex situ analysis methods combined with density functional theory (DFT), resulting in an unprecedented defluorination mechanism for Cr‐MOF electrodes. This study opens up new ideas for the application of MOF in CDI, clarifies the removal mechanism of MOF, and lays a foundation for further promoting the application of raw materials with poor conductivity in the field of CDI.

S. Nangle-Smith

A short, 30cm, test section was used to study the effect of electrohydrodynamic (EHD) forces on flow redistribution in a horizontal, shell and tube heat exchanger subject to both boiling and condensation. The use of a short test section allows for a consistent flow pattern across the test section length which provides further insight into the true effect of EHD. It was found that the voltage polarity of the applied voltages influences the flow distribution. For the current geometry studied, it was found that positive polarity voltages tend to pull liquid away from heat transfer surface and that negative voltages tended to repel more liquid toward the heat transfer surface. Using this knowledge we were able to show that positive voltages were more effective for convective condensation heat transfer enhancement, whereas negative voltages were more effective for convective boiling heat transfer enhancement. A twofold enhancement of convective boiling heat transfer was achieved for positive voltages and a 4fold enhancement was achieved for negative voltages. Similar pressure drop penalties were seen for both cases, approximately twice that of the no EHD case. Furthermore, the effect of DC level, peak to peak voltage, frequency and duty cycle waveform parameters on convective boiling enhancement were studied to explore the range of controllability for the current set of flow parameters. It was found that these various waveform parameters can induce different flow patterns and consequently different heat transfer and pressure drop configurations. In general the heat transfer is enhanced by EHD, but different pressure drop penalties can be achieved for a given enhancement ratio using different waveforms. High heat transfer for relatively low pressure drop was achieved using either negative DC signals or 50%duty cycle pulse waveforms. In some cases the enhancement is quite little compared to the pressure drop, for example the zero DC level, varying peak to peak voltage data. It is suggested that in a system where the heat exchanger pressure drop due to EHD is more dominant than the system pressure drop, it may be possible to use EHD as a method of retarding the system rather than enhancing it thereby broadening the scope of controllability. Finally we showed the proof of concept of using DC EHD as a rapid control mechanism for the load conditions. Using -8kVDC the water side heat flux could be varied by approximately ±3.2 kW/m² within 5 seconds. As a comparison, the same experiment was repeated using the refrigerant flow rate to control the load. Response times were similar for both experiments and although the power required for the flow rate control was less, the minimal variability in flow parameters for the EHD control make it a more attractive method of load control.