Annulus leakage and distribution of the fluid flow in a liquid spout-fluid bed with a draft tube

The distribution of flow between the annulus and the draft tube sections in a liquid phase spout-fluid bed with a draft tube was studied in a flat-based semi-circular column of diameter equipped with a semi-circular draft tube of diameter at superficial fluid velocities well above the minimum fluidization velocity. The particles used were glass spheres of diameter, and the spouting medium was tap water. A sodium chloride solution was injected into the reactor and the response recorded to determine the flow distribution between the draft tube and the annulus sections. The responses of the tracer injections prove that part of the flow originating from the annulus inlet leaks into the draft tube when any amount of annulus inlet flow is present. This finding makes a significant contribution to the understanding of the entrance region below a draft tube which has been modeled as a spout-fluid bed where fluid enters the annulus region based on the pressure distribution at the spout-annulus interface while the flow is in the radial direction from the spout into the annulus. This work shows that there is cross flow in the entrance region resulting in fluid exchange between the streams originating from the spouting inlet nozzle and the annulus inlet flow.The amount of leakage is found to increase with increasing inlet flow rates. An empirical correlation is developed to predict the fraction of the leakage of the annulus inlet flow. The superficial fluid velocity through the draft tube is found to vary linearly with the total flow rate through the bed under the experimental conditions studied. The pressure distributions in the entrance region favor the leakage of fluid from the annulus inlet flow into the draft tube.     

低温甲醇洗系统贫富甲醇换热器泄漏原因分析及处理

介绍了晋煤天溪煤制油分公司低温甲醇洗系统贫富甲醇换热器的泄漏情况。分析了贫富甲醇换热器泄漏初期生产运行指标的异常现象和造成泄漏的原因,认为泄漏是不同物质对设备的腐蚀、冲刷造成的,并针对泄漏的具体情况,提出了不同的检修处理措施:管束泄漏量低于10%时,可采用焊接锲形堵头将泄漏列管两端进行封堵;管束泄漏量高于10%时,如换热器设计余量足,可将封头内部隔板取出,短接进出口;将换热管材质由10#钢换为06Cr19Ni10不锈钢。     

Corrosion and inhibition of aged 347 grade stainless steel boiler tubes

General and pitting corrosion of both service and laboratory aged 347 grade stainless steel superheater tubes have been investigated. SEM analysis showed the precipitate phases to have a double structure consisting of phase and carbide. EDA analysis of the precipitates indicated the [Cr]/[Fe] ratio to increase with exposure time. For both set of specimens, the general corrosion rate in 1m H₂SO₄ increased with aging whilst the pitting studies (in 3.5% NaCl) also indicated increased pitting susceptibility with aging. These trends are consistent with corrosion from the chromium depleted areas adjacent to the precipitates. Methoxypropylamine is effective as an inhibitor for pitting corrosion and the concentration required to prevent this form of attack increases with exposure time for both types of specimens.     

Measurements of fiber bundle interfacial properties of three-dimensionally reinforced carbon/carbon composites up to 2273 K

Experimental techniques for evaluating the interfacial properties between fiber bundles and the matrix of three-dimensionally reinforced carbon/carbon composites were examined. Specially arranged fiber bundle push-out and pull-out tests were conducted up to 2273 K in vacuum. In these tests, a fiber bundle in the specimens was extruded or pulled out by external compressive or tensile loads. Post-fracture observations revealed that a shear fracture was successfully induced within the carbon matrices at the loaded fiber bundle interface. The interfacial shear strength and initial sliding stress of the fiber bundle monotonically increased with the test temperature. The relief of residual thermal stress and increases in the frictional resistance and anchor effect at the fiber bundle interface were considered to be the major mechanisms that caused the enhancements. An increase in the heat treatment temperature during the processing of the composites resulted in a significant decrease in and .     

Tomographic measurement of breakthrough in a packed bed adsorber

The tomographic measurement technique enables the measurement of local water vapour concentrations at the exit of a packed bed adsorber using near infra-red. This is realized by extending the adsorption column with a hollow glass cylinder that serves as the measurement cross-section. The glass is transilluminated from three directions with three light-sheets. The light absorption by water vapour is measured with three InGaAs photodiode arrays and these projections are used for the tomographic reconstruction of the concentration fields. For low water vapour concentrations () an error below 8% is reported. Concentration field measurements during adsorption show the early breakthrough near the column wall due to channeling effects at a low ratio between tube and particle diameter.     

Catalytic combustion assisted methane steam reforming in a catalytic plate reactor

A theoretical study of methane steam reforming coupled with methane catalytic combustion in a catalytic plate reactor (CPR) based on a two-dimensional model is presented. Plates with coated catalyst layers of order of micrometers at distances of order of millimetres offer a high degree of compactness and minimise heat and mass transport resistances. Choosing similar operating conditions in terms of inlet composition and temperature as in industrial reformer allows a direct comparison of CPRs with the latter. It is shown that short distance between heat source and heat sink increases the efficiency of heat exchange. Transverse temperature gradients do not exceed across the wall and across the gas-phase, in contrast to difference in temperature of outside wall and mean gas phase temperature inside the tube usually observed in conventional reformers. The effectiveness factors for the reforming chemical reactions are about one order of magnitude higher than in conventional processes. Minimisation of heat and mass transfer resistances results in reduction of reactor volume and catalyst weight by two orders of magnitude as compared to industrial reformer. Alteration of distance between plates in the range 1- does not result in significant difference in reactor performance, if made at constant inlet flowrates. However, if such modifications are made at constant inlet velocities, conversion and temperature profiles are considerably affected. Similar effects are observed when catalyst layer thicknesses are increased.     

Computational fluid dynamics modeling of the flow in a laboratory membrane filtration cell operated at low recoveries

Scaled-down models of industrial filtration units are often used in laboratory studies of membrane processes. Knowledge of the flow field and shear stresses at the membrane surface is vital for the accurate interpretation of bench scale experiments. In this paper, we present results of computational fluid dynamics modeling of the flow within the SEPA CF flat sheet membrane filtration cell operated at low recoveries. The problem was formulated as the steady-state isothermal laminar flow of incompressible Newtonian fluid. Pressure, velocity, and shear stress distributions were computed with resolution for different average inlet velocities. Flow was found to be unidirectional over most of the channel area with exception of the corners of the channel. Stagnation areas in dead ends of inlet and outlet tubes and in the channel areas behind duct entries as well as local regions of high shear in duct-channel transition areas were observed. The relation between the highest shear rate created in this geometry and the average inlet velocity is given.     

Carbon-induced corrosion of MSF condenser tubes in Arabian Gulf water

The present investigation was undertaken to shed light on the failure of some 90/10 CuNi condenser tubes in a MSF distiller. The tubes showed numerous tiny pits of unusual features. Particles of C were discovered inside the tubes and were suspected of initiating micro-galvanic cells leading to pitting. The study involved the measurement of open circuit potentials (OCP) of C and 90/10 and 70/30 CuNi electrodes in stagnant-, stirred-, natural- and synthetic seawaters at various temperatures. The way these changed with time till constant values were recorded was followed over long exposure times. The EMF of the C/CuNi galvanic cells varied between 400 and 450 mV. Contact between C and the copper alloys resulted in dissolution of copper, and the development of galvanic corrosion potentials and galvanic corrosion currents. These were discussed in the light of Evans corrosion diagrams. The study was extended to include the C/316 stainless steel system under similar conditions. The OCP behaviour of the steel electrode depended upon whether measurements were carried out in stagnant-, stirred-, natural- or synthetic seawaters. Galvanic coupling with C resulted in intensified pitting attack of steel in stagnant, artificial seawater. Similar measurements were conducted on the C/Ti couple. The anode reaction led to the thickening of TiO, Insignificant corrosion was recorded with the Ti/316 stainless steel system.     

Inhibitory effect of tungstate, molybdate and nitrite ions on the carbon steel pitting corrosion in alkaline formation water containing Cl ion

The pitting corrosion of carbon steel in carbonate-formation water solution in the presence of chloride ions and the effect of addition WO₄²⁻, MoO₄²⁻ and NO₂⁻ anions on the pitting corrosion were studied using cyclic voltammetry and potentiostatic current-time measurements and complemented by scan electron microscope (SEM), energy dispersive X-ray (EDX) and X-ray photoelectron spectroscopy (XPS) investigations. Cyclic voltammograms of carbon steel in the presence of chloride ions in carbonate-formation water solution show one anodic peak, corresponding to the formation green rust carbonate and the two cathodic peaks. As the addition of Cl⁻ ions concentration increases, the anodic peak current density increases and pitting potential E_pit shifts to more negative potential. It is shown that the rate of pit initiation () decreases and the pitting potential E_pit moves to more positive direction upon the addition of inorganic anions. It was found that pitting inhibition of carbon steel increases in the sequence: (WO₄)²⁻ > (MoO₄)²⁻ > (NO₂)⁻.     

Concentric flow regime of solid-liquid food suspensions: theory and experiment

Continuous food-sterilisation processes involve the flow of solid-liquid mixtures in pipes. Prediction of particle passage times in the system is required for ensuring sterility and optimising product quality. It is important to be able to predict both the minimum and maximum passage times of particles in the heating and holding sections of the system, and ideally the whole distribution of passage times (PTD) should be known. A Positron Emission Particle Tracking (PEPT) technique was used to determine the trajectories of almost neutrally-buoyant 5- alginate spheres in viscous non-Newtonian solutions. Particle passage times were measured by Hall effect sensors or visual tracers. A wide range of experimental conditions were investigated including solids fractions from 16 to and mean mixture velocities from 20 to . Thus, the mean apparent viscosity of the carrier fluid ranged from 20 to corresponding to tube Reynolds numbers of 2.1-381. Experimental results revealed the existence of four different PTD forms depending on the flow pattern present. A theoretical two-region flow model is presented which gives good predictions of the PTD forms, the minimum and maximum particle passage times.     

氨合成系统废热锅炉管束焊接接头腐蚀原因分析及改进

氨合成系统废热锅炉投运8个月后,其管束的气体进口端联箱管与换热管间的焊接接头出现连续点状腐蚀坑样缺陷,而管束的气体出口端相应位置没有出现此类缺陷,设备其他部位未见异常.通过对氨合成系统废热锅炉管束焊接接头腐蚀部位的宏观形貌、低倍形态、化学组分、扩展方向及金相组织等特征进行分析,判定腐蚀形态为电偶腐蚀.针对腐蚀形态,采取了相应的改进措施,取得了明显的效果.     

Evolution of hydrodynamic and statistical parameters of gas-liquid slug flow along inclined pipes

The development of slug flow along two long inclined pipes (2-90° from the horizontal) with internal diameters of 0.024 and was measured by three optical fiber probes. The probes were located in a measurement module at axial distances of between the fiber tips. To measure the evolution of slug flow, the module was placed at different positions along the pipe. Instantaneous elongated bubble velocities and corresponding elongated bubble and liquid slug lengths were determined by processing the optical probe signals. The evolution of the liquid slug and elongated bubble length distributions along the pipes is characterized by a gradual growth of the mean and mode values. The growth rate decreases with decreasing inclination. Mean elongated bubble lengths have a minimum at about 60°, while mean liquid slug lengths decrease slowly with decreasing inclination angle. The coalescence rate, defined as the decrease in the ensemble size, becomes almost negligible at x/D>60, independent of pipe diameter, flow rates and inclination angle. The slug frequency has a maximum at about 60° inclination.     

On the formation of Taylor bubbles in small tubes

The formation of Taylor bubbles and resulting bubble lengths were studied in a ID vertical tube for air-water and air-octane systems. In the co-flow tube/nozzle arrangement two nozzle sizes were used as gas inlets. Superficial velocities varied between 0.001- for the liquid and 0.002- for the gas. Three different mechanisms of initial bubble formation were observed. Of the three mechanisms, mechanism 3 is periodic (with period consisting of a bubble and a liquid slug), reproducible and can be simply modelled. After initial bubble formation further modifications may occur in the formed bubble size by coalescence or pairing. Bubble pairing is encouraged by smaller nozzles and liquid flow rates, while coalescence is observed only for cases where non-Taylor bubbles form initially.Two simple models have been proposed, the first predicts the size of the Taylor bubbles formed by mechanism 3 while the second attempts to predict the condition for bubble pairing to occur. Reasonable agreement with experimental results validates the predictions of the first model for a strong dependence of the volume of Taylor bubbles formed on the gas and liquid flow rates, a moderate dependence on nozzle diameter and a weak dependence (if at all) on the surface tension of the liquid used. Mismatch with the experimental results is caused (at least in part) by the experimental setup where there was no perfect axial alignment of the gas inlet. The experiments also suffered from problems at the outlet at low flow rates where smooth bubble disengagement could not be ensured for long Taylor bubbles. The second model for pairing predicts its occurrence for concentric tube/nozzle arrangements as a function of flow rates and channel diameters. The model over-predicted the range of liquid flow rates at which pairing was observed experimentally, but it captured the form of the boundary between different bubble volume modification mechanisms when represented on superficial velocity graphs.     

Multicomponent phase equilibria of thin liquid films and their vapour

Subject of the paper are multicomponent phase equilibria between extremely thin liquid films and their vapour phase on curved or planar solid surfaces. The solid surface is either heated or cooled, so that the liquid films are evaporating or the vapour is condensing.The curvature of the film surface and long range molecular forces due to the van der Waals attraction acting over distances of about 0.2- between solid and liquid film can lead to a composition shift in liquid and vapour phase compared to the composition that would be observed at planar and not extremely thin liquid films in equilibrium with their vapour phase.The Kelvin equation for the interfacial pressure is derived, as well as the equations for the composition shift. As a numerical example shows the van der Waals forces considerably influence the pressures at the liquid-vapour interface. Responsible for the composition shift is a dimensionless thermodynamic quantity . For small values D_i→0, liquid-vapour phase equilibria become identical with those of planar surfaces. For D_i?0 the vapour phase contains less and for D_i?0 more of the lighter volatiles than planar interfaces.     

A study on flow through hydroentangling nozzles and their degradation

Hydroentangling is a technique for mechanically bonding loose filaments or fibers arranged in a web. The efficiency with which the web is entangled depends on the peculiar properties of laminar high-speed waterjets used. The characteristics of such waterjets strongly depend on the operating pressure and the nozzle inlet sharpness which influence the dynamics of fluid flow. In this study, we report on experiments and CFD simulations aimed at improving our knowledge of such two-phase flows. In particular, we simulate the formation and growth of the cavitation cloud inside a sharp-edge hydroentangling nozzle at pressures ranging from 10 to . Our experimental results run at the same pressures, confirm that nozzle cavitation will cause “hydraulic flip”. Once hydraulic flip occurs, atomizing waterjets will turn into constricted laminar waterjets with long intact lengths—a necessary condition for hydroentangling. It has been observed that the nozzle inlet deteriorates under high pressures. Our CFD simulations show a striking similarity between the contours of shear stress at the nozzle inlet and the nozzle wear pattern. These findings together with the SEM elemental analysis at the nozzle inlet reveal the potential for metal oxidation around the inlet, implicating stress-induced corrosion as a major contributor to the nozzle wear. Cavitation might also be one of the mechanisms responsible for the above-mentioned wear at the inlet edge. Additionally, our water-borne solid particle tracking, confirms SEM experimental results that particle deposition can potentially play a considerable role in the deterioration of the nozzle inlet shape.     

Macro- and micromixing studies in an unbaffled vessel agitated by a Rushton turbine

Macromixing characteristics, power number and visual observation of the vortex behaviour and micromixing in an unbaffled tank agitated with a Rushton turbine are reported. The latter has also been compared in detail with earlier results from an identical tank containing baffles. The maximum mean specific energy dissipation rate, , in the unbaffled tank that can be utilised without severe air incorporation is compared to with baffles. However, at this lower , the micromixing efficiency is always significantly greater without baffles except when addition is made onto the top of the liquid or into the trailing vortex very close to the impeller. In these latter cases, they are approximately the same but even a small submergence of the feed tube below the liquid surface greatly enhances micromixing in the unbaffled case whilst it is still very poor with baffles. This good micromixing performance of the unbaffled vessel was very unexpected. Furthermore, an established method of estimating the local ε_T gave values of at every feed position where measurement was undertaken. Since the spatially averaged value of φ=1, this result suggests the possibility that the accepted concept of micromixing being totally controlled by the local ε_T at the feed point may not be valid for such swirling flows.     

The catalytic heat exchanger using catalytic fin tubes

The characteristics of a catalytic heat exchanger which integrates heat generation and heat exchange into one equipment have been investigated by the experiment and numerical simulation. The surface of the fin tubes was catalyzed by the formation of the oxide layer and the subsequent washcoating of ZrO₂, followed by the impregnation of Pd catalyst. The experimental results showed that the performance of catalytic combustion in the catalytic heat exchanger was more significantly affected by the inlet velocity of the mixture than by its inlet temperature and equivalence ratio. It was also found that the catalytic surface area was a critical parameter to obtain the complete conversion of the mixture. Numerical simulation has been performed with a commercial software FLUENT. The calculated results indicated that the performance of the catalytic combustion was influenced by the catalytic fin configuration as well as the flow pattern of the mixture over the catalytic fins. The results recommend that the number and thickness of catalytic fins should be designed above 6 pieces/inch and less than to achieve the best performance in the catalytic heat exchanger.     

Influence of reaction parameters on the synthesis of hyperbranched polymers via reversible addition fragmentation chain transfer (RAFT) polymerization

The synthesis of hyperbranched poly(methyl methacrylate) (PMMA) via reversible addition fragmentation chain transfer (RAFT) polymerization was investigated by varying the ratio chain transfer agent (CTA): monomer (methyl methacrylate, MMA): brancher (ethylene glycol dimethyl methacrylate, EGDMA): free radical initiator (AIBN) at various temperatures (50, 55, 60, 65, 70 °C). The rate of polymerization was observed to increase with temperature and concentration in brancher, whilst it was lowered by an increase in chain transfer agent concentration. The molecular weight of the samples increased with the ratios brancher: CTA and monomer: CTA. The polydispersity of the samples increase with conversion, as the level of branching increases. At fixed concentration in brancher, an increase of CTA concentration led to polymers with lower PDI. The variation of enthalpy and entropy relative to the monomer reaction were calculated, and it was observed that an increase in the brancher concentration induced an increase in both and , whilst lower CTA concentrations led to an increase in . The variation in Gibbs energy for the monomer reaction was calculated at 60 °C, and results confirmed the presence of a retardation effect when increasing CTA concentration generally observed in RAFT polymerization.