•Critical review and analysis of 3947 flow regime data points for vertical flows.•Assessment of the available models for prediction of upward flow regime transition.•Generation of a universal flow ...map for upward flows based on Reynolds numbers.
The accurate prediction of two-phase gas and liquid flow regimes is important in the proper design, operation and scale-up of pressure management and fluid handling systems in a wide range of industrial processes. This paper provides a comprehensive review of 3947 published experimental data points for gas-liquid flow maps in vertical pipes and annuli, including a critical analysis of state-of-the-art measurement techniques used to identify bubble, slug, churn and annular flow regimes. We examine the critical factors of pipe geometry (diameters, deviation from vertical), fluid properties and flow conditions that affect the transition from one flow regime to another. The review surveys the theoretical models available to predict flow regime transitions, and we validate the accuracy of these models using the published experimental data. The most reliable flow regime transition models for upward co-current flows are analytically shown to be: (i) Barnea 1987 for dispersed bubble to bubble flow, (ii) Taitel et al. 1980 for bubble to slug flow, (iii) Barnea 1987 for slug to churn flow, and (iv) Mishima and Ishii 1984 for churn to annular flow regime transition.
Moreover, based on the review we provide an outlook on the research needs and important developments in prediction of two-phase flow in vertical pipes including the use of computational fluid dynamics (CFD) techniques to simulate gas-liquid flows in vertical geometries.
Activated carbon monoliths with hierarchical pore structures were prepared from petroleum tar pitch and powdered coal in a low pressure foaming process with potassium hydroxide activation. The ...effects of coal to tar pitch ratio and of the amount of potassium hydroxide on the stability of tar pitch during the foaming process, the product's density and the micropore structure were studied. The carbon monolith prepared with adding 50%wt coal to pitch retained the shape of the cylindrical foam mould. This carbon monolith featured an open-cell structure with cell widths of around 2 μm and a well-developed microporosity that presented a BET specific surface area of 1044 m2 g−1. The apparent density of this structure was 0.42 g cm−3. The adsorption capacity of the carbon monolith for CO2, N2 and CH4 were evaluated by the volumetric sorption method at pressures up to 130 kPa and by a gravimetric sorption method for pressures up to 4500 kPa. At 298 K and pressures close to 3500 kPa the adsorption capacities of the carbon monolith prepared with 50%wt coal to pitch were 7.398 mol kg−1 CO2, 5.049 mol kg−1 CH4 and 3.516 mol kg−1 N2. The sorption results suggest these activated carbon monoliths have potential as monolithic adsorbents for gas separation or storage applications.
Activated carbon was produced from waste coffee grounds by treatment with ZnCl
2. Supercapacitor electrodes prepared from this coffee grounds carbon exhibited energy densities up to 20
Wh
kg
−1 in 1
...M H
2SO
4, and excellent stability at high charge–discharge rates. In a two-electrode cell a specific capacitance as high as 368
F
g
−1 was observed, with rectangular cyclic voltammetry curves and stable performance over 10,000 cycles at a cell potential of 1.2
V and current load of 5
A
g
−1. The good electrochemical performance of the coffee grounds carbon was attributed to a well developed porosity, with a distribution of micropores and mesopores 2–4
nm wide, and the presence of electrochemically active quinone oxygen groups and nitrogen functional groups. This work highlights the potential to utilize waste biomass to produce electrode materials for cost-effective energy storage systems.
Activated carbons for supercapacitor electrodes are prepared from sugar cane bagasse using chemical activation with ZnCl
2. The ZnCl
2 activation of bagasse is studied using thermogravimetric ...analysis and the carbon pore structures are characterised using N
2 and CO
2 adsorption. In two-electrode, sandwich-type supercapacitor cells containing 1
M H
2SO
4 the sugar cane bagasse carbons (SCCs) exhibit specific energy up to 10
Wh
kg
−1 and specific capacitance close to 300
F
g
−1. The electrochemical performance of the SCCs is attributed to their high specific surface area and the development of mesopores with ZnCl
2 impregnation ratios of 1 or greater. By contrast, the pyrolysis of bagasse without ZnCl
2 produces a carbon with low specific capacitance. The SCC prepared with a ZnCl
2 ratio of 3.5 shows the most stable electrochemical performance at fast charge–discharge rates.
The co‐doping of heteroatoms has been regarded as a promising approach to improve the energy‐storage performance of graphene‐based materials because of the synergetic effect of the heteroatom ...dopants. In this work, a single precursor melamine phosphate was used for the first time to synthesise nitrogen/phosphorus co‐doped graphene (N/P‐G) monoliths by a facile hydrothermal method. The nitrogen contents of 4.27–6.58 at % and phosphorus levels of 1.03–3.00 at % could be controlled by tuning the mass ratio of melamine phosphate to graphene oxide in the precursors. The N/P‐G monoliths exhibited excellent electrochemical performances as electrodes for supercapacitors with a high specific capacitance of 183 F g−1 at a current density of 0.05 A g−1, good rate performance and excellent cycling performance. Additionally, the N/P‐G electrode was stable at 1.6 V in 1 m H2SO4 aqueous electrolyte and delivered a high energy density of 11.33 Wh kg−1 at 1.6 V.
Monoliths for supercapacitors: A single precursor melamine phosphate is used to synthesize nitrogen/phosphorous‐co‐doped graphene (N/P‐G) monoliths by a facile hydrothermal method. The resultant N/P‐G monoliths not only exhibit excellent capacitive performance in supercapacitors but are also stabilized at 1.6 V in aqueous electrolyte, delivering a high energy density.
Abstract
Electrochemical reduction of CO
2
presents an attractive way to store renewable energy in chemical bonds in a potentially carbon-neutral way. However, the available electrolyzers suffer from ...intrinsic problems, like flooding and salt accumulation, that must be overcome to industrialize the technology. To mitigate flooding and salt precipitation issues, researchers have used super-hydrophobic electrodes based on either expanded polytetrafluoroethylene (ePTFE) gas-diffusion layers (GDL’s), or carbon-based GDL’s with added PTFE. While the PTFE backbone is highly resistant to flooding, the non-conductive nature of PTFE means that without additional current collection the catalyst layer itself is responsible for electron-dispersion, which penalizes system efficiency and stability. In this work, we present operando results that illustrate that the current distribution and electrical potential distribution is far from a uniform distribution in thin catalyst layers (~50 nm) deposited onto ePTFE GDL’s. We then compare the effects of thicker catalyst layers (~500 nm) and a newly developed non-invasive current collector (NICC). The NICC can maintain more uniform current distributions with 10-fold thinner catalyst layers while improving stability towards ethylene (≥ 30%) by approximately two-fold.
Titanate perovskite (ATiO
) semiconductors show prospects of being active photocatalysts in the conversion of CO
to chemical fuels such as methanol (CH
OH) in the aqueous phase. Some of the ...challenges in using ATiO
are limited light-harvesting capability, rapid bulk charge recombination, and the low density of catalytic sites participating in CO
reduction. To address these challenges, Ga-doped NiTiO
(GNTO) photocatalysts in which Ga ions substitute for Ti ions in the crystal lattice to form electron trap states and oxygen vacancies have been synthesized in this work. The synthesized GNTO was then loaded with Ru nanoparticles to accelerate charge separation and enable excellent CO
photoreduction activity under visible light. CO
photoreduction was conducted in a batch photoreactor charged with a 0.1 M NaHCO
aqueous solution at room temperature and a 3.5 bar pressure using a 1.0 wt % Ru-GNTO photocatalyst to yield methanol at a rate of 84.45 μmol g
h
. A small amount of methane was produced as a side product at 21.35 μmol g
h
, which is also a fuel molecule. We attribute this high catalytic activity toward CO
photoreduction to a synergistic combination of our novel heterostructured 1.0 wt % Ru-GNTO photocatalyst and the implementation of a pressurized photoreactor. This work demonstrates an effective strategy for metal doping with active nanospecies functionality to improve the performance of ATiO
photocatalysts in valorizing CO
to solar fuels.
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