Influence of Bubble Evolution on the Effective Kinetics of Heterogeneously Catalyzed Gas/Liquid Reactions. Part I: Reactions with Gaseous Products

For heterogeneously catalyzed multiphase reactions the formation of bubbles may have an influence on mass and heat transfer as well as on the effective reaction rate. This first of two contributions deals with the Ni‐catalyzed decomposition of H₂O₂, which was used as a model system for an (almost) isothermal reaction with a gaseous product. (In part II the strongly exothermic hydrogenation of hexene will be analyzed, where gas/vapor bubbles may be generated by overheating of the catalyst.) The discharge of O₂ bubbles formed by decomposition of H₂O₂ enhances the external mass and heat transfer up to one order of magnitude. This is in analogy to the well‐known phenomena during nucleate boiling. The experiments and theoretical considerations also show that the internal mass transfer depends on the intensity of the reaction and thus on the H₂O₂ concentration, which is in contradiction to the classical Thiele approach. This discrepancy could be explained by a modified model that takes the formation of bubbles into account.     

Water electrolysis under microgravity: Part 1. Experimental technique

Water electrolysis was conducted in both alkaline (25 wt.% KOH, 2 wt.% KOH) and acid (0.1N H₂SO₄) solutions for 8 s under microgravity environment realized in a drop shaft. The gas bubble formation of hydrogen and oxygen on platinum electrodes was observed by CCD camera. In alkaline solutions, a bubble froth layer grew on the electrode surface. Hydrogen bubble size was smaller than that of oxygen. The current density at constant potential decreased continually with time. In spite of the growth of a bubble froth layer on the electrode, the electrolysis never stopped, apparently because fresh electrolyte is supplied to the electrode surface by microconvection induced by the gas bubble evolution. In acid solution, hydrogen gas bubbles frequently coalesced on the cathode surface, yielding a larger average bubble than that of oxygen. The current density did not vary at constant potentials from –0.4 to −0.8 V versus reversible hydrogen electrode (RHE), because the effective electrode surface area was significantly reduced by the larger bubble size compared to alkaline electrolyte. The present experiments indicate that, especially in a microgravity environment, the bubble evolution behavior and the resultant current–potential curves are significantly influenced by the wettability of the electrode in contact with the electrolyte.     

泡点露点与闪蒸计算

本文在教学实践与体会的基础上,提出了闪蒸过程讲授之后可深入思考的几个方面,引导同学从另一个视角认识泡点与露点状态,导得了泡点方程与露点方程,给出了基于闪蒸方程的泡点与露点的计算方法,并对已知组成与温度压力的物料状态的判别做了阐述;推导了双液相闪蒸方程,讨论了求解双液相闪蒸方程组的方法,对学生消化理解VLE重点与难点与开拓学生思维颇具裨益。可供学有余力的同学拓宽知识视野之用。     

Investigation of nanofluid bubble characteristics under non-equilibrium conditions

We report experimental and theoretical investigations of the bubble characteristics during the oscillatory growth period for several nanofluids. The nanoparticles were found to affect liquid–gas and solid surface tensions, which modulated the bubble contact angle, radius of triple line, bubble volume and the dynamics of bubble growth. To increase the accuracy of the Young–Laplace equation predictions during the bubble growth in the oscillatory period, a new method multi-section bubble (MSB) approach was developed. In this method, the bubble was divided into n sections (i.e., n = 1:N) and the Young–Laplace equation was solved for each section individually. As N increases, within each section the effects of inertia force and viscosity become reduced comparing to that of the liquid-gas surface tension. Unlike the conventional Young–Laplace approach (i.e., N = 1), the new approach is able to predict the bubble characteristics reliably in the following cases: (a) the oscillatory period when bubble is fluctuating; (b) the departure period when bubble is stretched upward, right before departure; and (c) the high shear stress condition when gas velocity is relatively high.     

Optimization of Mixing and Mass Transfer in In-line Multi-Jet Ozone Contactors Using Computational Fluid Dynamics

Typical ozone mixing and mass transfer calculations are lumped approaches based on ideal operating conditions and can misrepresent behavior in real-life installations. This article models the effect of local hydrodynamics and mixing on the overall mass transfer of ozone into water with the aid of multiphase computational fluid dynamics (CFD). CFD models were validated with measured data from a pipeline ozone contactor installation which was optimized for more rapid, uniform mixing and mass transfer. Results emphasize the sensitivity of mixing quality to nozzle placement, size, orientation and spacing relative to main pipeline diameter and flows.     

Bubble Size Distribution in Oil‐Based Bubble Columns

A practical population balance model was used to evaluate the bubble size distribution in a bubble column. In addition, the bubble size distribution in the bubble column was measured at different gas velocities by photography and analysis of the pictures. Four types of liquid, i.e., water and three petroleum‐based liquids, were used in the experiments. The gas phase was air. It was found that the existing models in the literature are not able to satisfactorily predict the experimentally measured bubble size distribution. The model can be corrected by applying a correction factor to the energy dissipation rate. The corrected model fits the experimental bubble size distribution considerably better than the existing models. The variation of this correction factor is reported for different systems at different gas velocities.     

A simple method for the inclusion of external and internal supports in the spline finite strip method (SFSM) of buckling analysis

The SFSM is an attractive numerical technique for the buckling analysis of folded-plate structures where general loading regimes and boundary conditions need to be modelled. In implementing splines as interpolation functions in the longitudinal direction of the strip, amended splines have been used conventionally to model the variety of end conditions that may occur. These amended splines are fairly difficult to implement, particularly so if internal restraints are also to be specified. A simple technique for replacing the specification of dedicated amended splines is presented in this paper. The method is then employed to study the local buckling of flat plates under longitudinally and transversely varying compression and bending with different boundary conditions at the ends.     

空间生长GaAs材料多孔部分的俄歇能谱分析与阴极荧光？…

在空间生长ＳＩ－ＧａＡｓ的某些部位有汽泡产生，经俄歇分析，汽泡表面约有１０ｎｍ的砷层，它从半绝缘砷化镓内部逸出，导致其成为半导体。用阴极荧光形貌观测了其多晶结构。     

Structure of effective catalyst layers around bubbles in a fluidized catalyst bed

In a fluidized catalyst bed, the reactant gas transfers from the bubble phase to the emulsion phase and reactions proceed in the emulsion phase. The catalyst particles around the bubbles should contact the gases containing a high concentration of the reactants. Therefore, the effect of the catalysts around the bubbles is very important for estimating the conversion and selectivity in the reactor. In order to study the role of these catalysts, the hydrogenation of carbon dioxide was carried out in a fluidized catalyst bed. Based on the results, the amount of the catalyst that was effective for the reaction was calculated. In addition, the shape of the bubbles ascending in the fluidized catalyst bed was observed using a fast X-ray computer tomography (CT) scanner. The structure of the bubbles in the fluidized catalyst bed was very complicated and the surface area of the bubbles was much greater than the obtained when assuming spherical shaped bubble. By assuming that effective catalysts existed around the bubbles, the thickness of catalyst layer was obtained. Finally, the 3-dimensional images of the catalyst layers around the bubbles were reconstructed.     

Development of support vector regression (SVR)-based correlation for prediction of overall gas hold-up in bubble column reactors for various gas-liquid systems

The objective of this study was to develop a unified data-driven correlation for the overall gas hold-up for various gas-liquid systems using support vector regression (SVR)-based modeling technique. Over the years, researchers have amply quantified the hydrodynamics of bubble column reactors in terms of the overall gas hold-up. In this work, about 1810 experimental points were collected from 40 open sources spanning the years 1965-2007. The model for overall gas hold-up was established as a function of several parameters which include superficial gas velocity, superficial liquid velocity, gas density, molecular weight of gas, sparger type, sparger hole diameter, number of sparger holes, liquid viscosity, liquid density, liquid surface tension, operating temperature, operating pressure and column diameter of the gas-liquid system. For understanding the hold-up behavior, the data used for training the model was grouped into various gas-liquid systems viz., air-water, gas-aqueous viscous liquids, gas-organic liquids, gas-aqueous electrolyte solutions and gas-liquid systems operated over a wide range of pressure. A generalized model established using SVR was evaluated for its performance for various gas-liquid systems. Statistical analysis showed that the proposed generalized SVR-based correlation for overall gas hold-up has prediction accuracy of 97% with average absolute relative error (% AARE) of 12.11%. A comparison of this correlation with the selected system specific correlations in the literature showed that the developed SVR-based correlation significantly gives enhanced prediction of overall gas hold-up.