Search

The equilibria and kinetics of CO2, N2, and H2O on KOH-treated activated carbon (KOH-AC) were evaluated. The isotherms for the pure components were correlated with the Langmuir, dual-site Langmuir,...

Efficient fleet configuration is a critical problem for air carrier management. This study is primarily concerned with empirical aircraft operating costs and examines the optimal fleet adapted to numerous flight routes longer than 1000 nautical miles. An aircraft-specific operating cost model is derived to estimate market average direct operating costs (DOC) of 22 aircraft types operated by 22 US airlines. It is used as a base in a fleet configuration optimization model to figure out the variability of optimal fleets for segment markets of varying sizes and lengths as well as in response to the dynamics of market circumstances. While recognizing the superior fuel burn performance of narrow-body aircraft such as Boeing 737 and Airbus A320 series, we find operating cost efficiency of wide-body aircraft (B777 and A330 series) due to the economies of scale in the non-fuel operating costs associated with aircraft size. There is a possible reduction of DOC by substituting the wide-body aircraft for smaller ones that are dominant in the current US domestic markets. And the cost efficiency of the wide-body fleet is more robust in dense and longer distance markets (particularly longer than 2000NM), especially considering fuel price fluctuations. Finally, the optimal fleet analysis with empirical traffic data suggests a mixed-size aircraft fleet, configured with narrow- and wide-body aircraft, as an alternative for a wide range of segment markets that vary in size and length.

Ammonia (NH3) has been proposed as a viable hydrogen (H2) carrier, but high reaction temperature and endothermic nature of NH3 decomposition require an efficient reaction system to maximize useable energy from NH3. Adoption of carbon-free heat sources and efficient heat transfer to the reaction bed are crucial for sustainable H2 release. Herein, the autothermal recirculating reactor (ARR) concept with the fractional utilization of the reformate H2 as a clean combustion fuel is proposed and experimentally investigated. Additionally, BN-coated Cu as a composite reactor material is developed for heat transfer enhancement of high-temperature H2 release reaction in a thermally-coupled NH3 decomposition and H2 combustion system. Coating performance against chemical degradation of Cu has been tested and confirmed. High NH3 conversion of >99.6% and reforming efficiency of 70.95%, even with high fraction of heat loss owing to small scale validation, show feasibility of the as-proposed reformer. Operation of the suggested system is envisaged with self-sustained heat supply for production of 84 W of electrical power. Also, the as-proposed reactor concept and material are suggested to serve in other endothermic H2 release reactions from various H2 carriers (methane, methanol, LOHC, etc) for potential application in power generation as well as high-purity H2 production.

Understanding separation behavior in an adsorption bed is crucial for well-designed adsorption processes. Since adsorption phenomena depend on temperature, thermal parameters, such as the internal heat transfer (hi), isosteric heat of adsorption (Qst) and axial thermal conductivity of the bed (Kw), can affect the adsorption dynamics and performance of the bed. In this study, the effects of these thermal parameters on the adsorption dynamics and breakthrough curves were analyzed with the experimental results using integrated gasification combined cycle gas from the carbon capture process (IGCC gas; H2:CO:N2:CO2:Ar = 88:3:6:2:1 mol%) as a purification gas and blast furnace gas (BFG; H2:CO:N2:CO2 = 20:0.1:44.5:35.4 mol%) as a bulk separation gas. The results were then compared with the isothermal and adiabatic results. Considering the variation of the internal heat transfer coefficient and isosteric heat of adsorption along with the propagation of gas in the bed, the temperature profiles inside the bed could be predicted better than in the case using constant values. The axial thermal conductivity of the bed significantly affected the temperature profiles as the temperature excursion was sharp. In the prediction of breakthrough curves, these variable thermal parameters could be replaced by suitable constant values estimated from experimental and theoretical approaches when well-fitted isotherm parameters were applied considering the partial pressure of each component in the feed.

Herein, we report the use of a MgCeOx-supported Cu (MgCuCe) catalyst with a unique bead structure to augment the water-gas shift (WGS) reaction. The MgCuCe catalyst exhibited an exceptionally high reaction rate of 83 μmol g−1 s‒1 at 300 °C, compared with that without MgO (30 μmol g−1 s‒1). Very few studies have focused on MgO-supported catalysts owing to the reports on the inferior activity of MgO. However, this paper reports unprecedented enhancements by introducing MgO and illustrates the WGS reaction mechanism: (1) numerous defects promoted water dissociation and subsequent associative mechanism; (2) the labile oxygen in MgO participated in redox mechanisms. The hydrogen production cost realized due to the use of the MgCuCe was 0.63 USD/kg H2, which is lower than that achieved by using commercial and CeO2-supported catalysts. This study paves the way for exploiting earth-abundant MgO in developing efficient catalysts and contributes to reducing H2 production costs.

The demand for clean energy sources has made H2 recovery from various lean hydrogen mixtures increasingly attractive. In this study, parallel and series pressure swing adsorption (PSA) processes were investigated experimentally and theoretically, and > 99% pure H2 was produced from a lean hydrogen mixture (H2:CO:N2:CO2 = 19.9:0.1:44.6:35.4 mol%) at 10 bar. A mathematical model for a PSA process using activated carbon and zeolite 13X was simultaneously validated with results from breakthrough experiments and a parallel two-bed PSA process. The parallel two-bed PSA process using a layered bed (lower bed: activated carbon, upper bed: zeolite 13X) experimentally produced H2 with a purity of 94.6–98.3% and a recovery of 33.5–63.2%; CO was not detected in the H2 product. In the parallel four-bed PSA process, the H2 recovery was drastically increased to 77.3% due to an additional pressure equalization step, but the increase in H2 purity was minute. The series PSA process, which was divided into the bulk separator and the purifier, was theoretically studied under various operating conditions. The series three- and four-bed PSA processes could produce H2 with over > 99% purity and a recovery of 62.478% and 82.643%, respectively, due to the additional pressure equalization step and the utilization of blowdown gas. The parallel four-bed PSA process showed the highest H2 productivity (33.58 molH2 kgads−1 day−1), while the series four-bed PSA process achieved an H2 productivity of 23.96 molH2 kgads−1 day−1 with > 99% H2 purity.

Composite metallic membranes comprising Pd coated on group 5 elements such as V, Nb, and Ta are actively explored as a promising material for hydrogen permeable membrane. However, such composite membranes are known to suffer from performance degradations owing to excessive surface fouling due to inter-diffusion between Pd and group 5 elements at high temperatures. In this study, mechanically stable Pd/Ta composite membranes are fabricated, and their H2 permeation flux degradation rates and mechanism of membrane degradation are investigated. Fabricated membranes had full coverage of Pd on Ta surface, with less than 5 wt% of impurities. H2 permeation rates of Pd/Ta membrane samples with different Pd and Ta thicknesses are evaluated at temperatures from 450 °C to 650 °C for a prolonged period of 20–100 h. Over the tested period, degradation in H2 permeation rate was 5% or higher, with temperature being a dominant factor. SEM, TEM, and XRD analyses of the surface and cross-sectional morphology, compositions, and crystallinity reveal that the main degradation mechanism of the composite membrane is the inter-diffusion between Pd and Ta leading to the formation of intermetallic compounds as well as morphology change of the catalytic layer. By evaluating the exponential factor in Sievert’s equation over the degradation period, it was found that the diffusion mechanism does not change over time, confirming that the bulk diffusion mechanism is affected by the inter-diffusion. H2 flux degradation was less prominent at lower temperatures with thicker Pd catalytic layers.

The adsorption equilibria and kinetics of CO 2 , CO, N 2 , CH 4 , Ar and H 2 on zeolite 13X were measured via the volumetric method at 293, 308 and 323 K and up to 1.0 MPa. Adsorption isotherms and the heat of adsorption were analyzed over a full range of pressures. Each experimental isotherm was correlated with Langmuir, Sips and temperature-dependent Sips models, with the deviations for each model being evaluated. The Sips model showed a smaller degree of deviation from the experimental data than the Langmuir model. The isosteric heat of adsorption sequence was H 2 4 ≒ N 2 2 along with surface loading. The experimental uptake curves were correlated with a non-isothermal kinetic model because the adsorption kinetics was controlled via heat generation and transfer. Adsorption rates of the gases on zeolite 13X were affected by the isosteric heat of adsorption, heat transfer rate and adsorption affinity. At the same temperature and pressure, the sequence of reciprocal of the effective diffusional time constant ( D / R P 2 ) revealed the following sequence: CH 4 ≒ N 2 2 , showing pressure and temperature dependency. While the diffusion rates of CO and N 2 were controlled via micropore diffusion, CO 2 and CH 4 were significantly affected by macropore diffusion due to high heat transfer resistance. Compared to experimental results between the powder and pellet forms of zeolite 13X, the zeolite pellet binders led to a reduction in adsorption capacity and heat of adsorption. However, the difference in adsorption kinetics was relatively minute.

The adsorption equilibria of water vapor on zeolite 3A, zeolite 13X, and dealuminated Y zeolite (DAY) were measured using a volumetric method. Equilibrium experiments were conducted at 293.15, 303.15, and 313.15 K and at relative pressure (P/Ps) up to 0.95. Experimental data were correlated using Aranovich–Donohue and Frenkel–Halsey–Hill models, using Langmuir, Toth, UNILAN, and Sips isotherms.

For catalytic reactions involving H2 extraction, the membrane reactor is an attractive option for enhancing the equilibrium and kinetics while eliminating excessive purification steps. In this study, a steam carrier adopted composite membrane reactor system is developed to produce pure H2 (>99.99%) from ammonia with high H2 productivity (>0.35 mol-H2 gcat−1 h−1) and ammonia conversion (>99%) at a significantly reduced operating temperature ( 91%) while replacing conventionally utilized noble carrier gases that require additional gas separation processes. The steam carrier presents similar membrane reactor performance to that of noble gases, and the water reservoir used for steam generation acts as an ammonia buffer via scrubbing effects. Finally, electricity generation is demonstrated using a commercial fuel cell along with process simulation, substantiating potential of the proposed membrane system in practical applications for H2 production from ammonia and on-site power generation.

As one of hydrogen purification techniques, membrane separation has a significant potential to directly obtain high-purity hydrogen from the mixed gas produced from various catalytic reactions. Specifically, dense metallic membranes exhibit several advantages including good mechanical strength under pressurized conditions, thermal stability, and high hydrogen selectivity. Conventionally, their permeation behavior is predicted from a numerical model known as the Sievert's law, which describes diffusion of hydrogen atoms through a metal layer. This study questions the validity of previous permeability trends of the Pd/BCC composite membranes and reveals the importance of surface reactions that significantly affect the permeation behavior of such membranes. A new permeation model developed, considering both the surface reactions at the catalytic layers and bulk diffusion through the metal layers, exhibits good correlation with the experimental permeation characteristics of the Pd/BCC composite membranes. Moreover, the diffusivity coefficients of BCC metals as a function of temperature is determined with higher accuracy than those reported in previous studies having temperature range inconsistency between hydrogen solubility and diffusivity. The experimental data along with the proposed model successfully accounts for the unique permeation characteristics of BCC metal membranes coated with catalytic layers and advances fundamental understanding of the permeation characteristics of the composite membrane, thereby accelerating the adoption and application of the composite membrane permeation model.