From modification to mechanism: Supercritical hydrothermal synthesis of nano-zirconia

Nano-zirconia has been widely applied due to its excellent physical and chemical properties (e.g., high strength, corrosion resistance, oxygen ion conductivity). Existing preparation methods of nano-zirconia tend to require long reaction time, and the sizes of final particles are large with uneven distributions. Sub-/supercritical hydrothermal synthesis of nanoparticles is favored by researchers owing to controllable reaction process, uniform particle size distribution, good reproducibility, short reaction time, high conversion rate and harmlessness to environment. In this paper, the characteristics and mechanisms of dissolution, crystallization and growth of nano-zirconia during sub-/supercritical hydrothermal synthesis are systematically reviewed. The influences of process and material parameters on the size and purity of particles are analyzed. Then, the reaction mechanism and product phase transition mechanism during hydrothermal synthesis of zirconia are summarized to provide a theoretical reference for the oriented preparation. Finally, the improvement and commercialization of sub-/supercritical hydrothermal synthesis technology are evaluated, and the future research topics are proposed.     

Effects of structural parameters on water film properties of transpiring wall reactor

Corrosion and salt deposition problems severely restrict the industrialization of supercritical water oxidation. Transpiring wall reactor can effectively weaken these two problems by a protective water film. In this work, methanol was selected as organic matter, and the influences of vital structural parameters on water film properties and organic matter removal were studied via numerical simulation. The results indicate that higher than 99% of methanol conversion could be obtained and hardly affected by transpiration water layer, transpiring wall porosity and inner diameter. Increasing layer and porosity reduced reactor center temperature, but inner diameter's influence was lower relatively. Water film temperature reduced but coverage rate raised as layer, porosity, and inner diameter increased. Notably, the whole reactor was in supercritical state and coverage rate was only approximately 85% in the case of one layer. Increasing reactor length affected slightly the volume of the upper supercritical zone but enlarged the subcritical zone.     

Reactivity study and kinetic evaluation of CuO-based oxygen carriers modified by three different ores in chemical looping with oxygen uncoupling (CLOU) process

In the chemical looping with oxygen uncoupling (CLOU) process,CuO is a promising material due to the high oxygen carrier capacity and exothermic reaction in fuel reactor but limited by the low melting point.The combustion rate of carbon is faster than the decoupling rate of oxygen carrier (OC).Hence,high tem-perature tolerance and rapid oxygen release rate of CuO modified by three different ores were investi-gated in this study.The kinetics analysis of oxygen decoupling with Cu-based oxygen carriers was also evaluated.Results showed that CuO modified by chrysolite had faster oxygen release rate than that of CuO.Limestone showed obvious positive effect on the oxidization process.The selected OCs could keep stable in at least 20 cycles,for about 1200 min.Shrinking core model (SCM) fitted well for the decoupling process in the temperature range of 1123-1223 K.Reduction rate kinetic information may aid in the development of chemical looping with oxygen uncoupling (CLOU) technologies during reactor design and process modeling.Ternary doped copper oxide with chrysolite and limestone could improve the reactivity of CuO in decoupling and coupling process and also improve the high temperature tolerance.     

Parameters optimization for direct contact membrane distillation based on orthogonal experiment

Parameter optimization integrating operation parameters and structure parameters for the purpose of high permeate flux,high productivity and low exergy consumption of direct contact membrane distillati...     

Pyrolysis and co-pyrolysis of lignite and plastic

The study firstly discusses the pyrolysis characteristics and kinetics by thermogravimetric analysis (TGA), and then investigates the pyrolysis of lignite and co-pyrolysis with plastic (polyethylene or polypropylene) in tube furnace. Meanwhile, the research focuses on the co-pyrolysis products under different mixing ratios as well as pyrolysis products at different testing temperatures and heating rates. The results show that higher final testing temperature and lower heating rate contribute to bond fission in lignite pyrolysis, resulting in less char product. In co-pyrolysis, lignite acts as hydrogen donor, and the yields of char and water rise with increasing amount of plastic in the mixture, while the yields of gas and tar decrease; and a little admixture of plastic will promote the production of gas and tar. Kinetic studies indicate that in temperature range of 530–600 °C, activation energies of lignite are higher than those of lignite/plastic blends, and as plastic mass ratio increases from 0% to 10%, samples need less energy to be decomposed during co-pyrolysis.     

Geometrical study of the three-dimensional temperature–composition diagrams. An important aid to understand the behavior of the liquid–vapour equilibrium in ternary systems

In this work the usefulness of qualitatively studying and drawing three-dimensional temperature–composition diagrams for ternary systems is pointed out to understand and interpret the particular behavior of the liquid–vapour equilibrium of non-ideal ternary systems. Several examples have been used in order to highlight the interest and the possibilities of this tool, which should be an interesting support not only for lecturers, but also for researchers interested in experimental equilibrium data determination.     

Study on multicomponent pseudo-potential model with large density ratio and heat transfer

Large density ratio multiphase flow is a persistent challenge within the field of computational fluid dynamics. This paper investigates improvements to the lattice Boltzmann based large density ratio multicomponent multiphase pseudo-potential model. The improvements include: the multiple-relaxation-time (MRT) collision operator; the exact difference method scheme; the Carnahan-Starling equation of state; and an addition of correction factor k to the equation of state. The improved model can be used for simulating large density ratio (O(1000)) multiphase flow with small spurious current and better numerical stability. The smaller spurious current can be obtained by decreasing k value, and yet interface thickness increases. The density ratio is 1284 for k = 0.1 and the spurious current is reduced to 0.0069, which is much smaller than that of 0.033 in literature. The interfacial tension can be adjusted independently from density ratio by changing k value. A thermal multiphase flow model is developed based on the large density ratio pseudo-potential model. The model is validated by using static heat conduction and dynamic flow simulations. The result of the static heat conduction of the flat interface has smaller error with the theoretical solution than that of droplet. The result of thermocapillary migration is comparable with the theoretical prediction. Finally, the heat conduction melting is simulated by coupling the enthalpy-based method.     

Integration strategies of hydrogen network in a refinery based on operational optimization of hydrotreating units

Inferior crude oil and fuel oil upgrading lead to escalating increase of hydrogen consumption in refineries. It is imperative to reduce the hydrogen consumption for energy-saving operations of refineries. An integration strategy of hydrogen network and an operational optimization model of hydrotreating (HDT) units are proposed based on the characteristics of reaction kinetics of HDT units. By solving the proposed model, the operating conditions of HDT units are optimized, and the parameters of hydrogen sinks are determined by coupling hydrodesulfurization (HDS), hydrodenitrification (HDN) and aromatic hydrogenation (HDA) kinetics. An example case of a refinery with annual processing capacity of eight million tons is adopted to demonstrate the feasibility of the proposed optimization strategies and the model. Results show that HDS, HDN and HDA reactions are the major source of hydrogen consumption in the refinery. The total hydrogen consumption can be reduced by 18.9% by applying conventional hydrogen network optimization model. When the hydrogen network is optimized after the operational optimization of HDT units is performed, the hydrogen consumption is reduced by 28.2%. When the benefit of the fuel gas recovery is further considered, the total annual cost of hydrogen network can be reduced by 3.21×10₇ CNY·a_-1, decreased by 11.9%. Therefore, the operational optimization of the HDT units in refineries should be imposed to determine the parameters of hydrogen sinks base on the characteristics of reaction kinetics of the hydrogenation processes before the optimization of the hydrogen network is performed through the source-sink matching methods.     

A closed-form solution for laminar film condensation from downward pure vapour flow in vertical tubes

An analytical solution is derived for the film thickness for simplified steady-state governing equations of laminar film condensation from laminar pure vapours flowing downward in vertical tubes. This approach yields an accurate, approximate closed-form non-marching solution for the condensate film thickness. All other relevant quantities such as the heat transfer coefficient, the vapour and liquid velocity profiles, the vapour and liquid mass flow rates, the interfacial shear stress, and the pressure gradient can be easily computed in closed-form from this solution directly at any given axial location. The present solution compares very well to other analytical works that require more complicated iterative techniques with a marching solution approach.     

Compressive behaviour of thin catalyst layers. Part II - Model development and validation

In the second part of this study, a new analytical model for catalyst layers (CLs) compression is developed using effective medium theory, using a geometric “unit cell”, to accurately predict the deformation of CLs under compression. Based on SEM images, a representative unit cell is proposed using microstructural properties of CL such as porosity, pore size distribution, and ionomer to carbon weight ratio (I/C) to simplify the random complex structure of CLs. Deformation of the ionomer film that covers carbon agglomerates is found to be the main deformation compared to other mechanisms such as Hertzian compliance of carbon particles and deformation of agglomerates. The present model is validated using the experimental results obtained for five different CL designs, presented in Part 1 of this study. The analytical model is capable of predicting the non-linear compressive behaviour of CLs with a reasonable accuracy since a continuous change of CL porosity is considered in the model. The proposed geometrical model has also been used for other properties of CL in our group and successfully predicted thermal conductivity and gas diffusivity of CL.