GAS HOLDUP IN UNIFORMLY AERATED BUBBLE COLUMN REACTORS

Conditions of the stable performance of gas distributing plates were studied and the effect of plate geometry on gas holdup of uniformly aerated gas-liquid beds was investigated. The ratio of plate holes opened for gas passage was determined as a function of gas hole velocity and critical values of gas hole velocity corresponding to the onset of stable performance of distributing plates were obtained.

Two regimes of bubbling were observed under conditions of stable uniform gas distribution; the regions of their existence being determined by the values of gas flow velocity and distributing plate parameters. Considerable increase of gas holdup was observed in the region of “foam” bubbling compared to the “turbulent” bubbling regime commonly encountered in bubble column reactors. The character of the bed and hence its gas holdup value were affected by the geometry of distributing plates in the “foam” bubbling region while no such effect was observed under “turbulent” bubbling conditions.     

HEAT TRANSFER IN A CONDUCTIVE-HEATING AGITATED DRYER

The heat transfers between the heating plane and the granular materials in both the “Stationary heating-plane type” and the “Moving heating-plane type” of the conductive-heating agitated dryers were discussed mainly from the view point of the scale-up of the dryer. Under the condition of complete mixing in the bed of bulk materials, the heat transfer models proposed for both the two types of dryers can predict the heat transfer coefficients in any sizes of the dryers. However, the complete mixing is not usually accomplished in the large scale of dryer. Hence, an “Incomplete mixing model” was proposed to estimate the effect of the incompleteness of mixing on the heat transfer coefficient. In this model, the incompleteness of mixing can be apparently taken into consideration only by increasing the contact time.     

Experimental study of falling film evaporation heat transfer outside horizontal tubes

The heat transfer process of falling film horizontal evaporation includes evaporation outside tubes and condensation inside tubes, the heat transfer coefficient of the former is about 50% of that of the latter. So the overall heat transfer coefficient is influenced mainly by the falling film evaporation outside tubes. An experimental study of falling film heat transfer outside horizontal tubes was carried out in order to show how the heat transfer coefficient is affected by different parameters such as flow density evaporation temperatures, temperature difference between wall and saturation water, and mass concentration of the seawater. Experiments were conducted using 14 mm outer diameter Al-brass tubes heated by internal electric heaters so that a uniform heat flux was generated on the outside surface of tubes. The results show that when flow density Γ varies between 0.013 kg/ms < Γ< 0.062 kg/ms, the heat stransfer coefficient of falling film evaporation outside horizontal tubes h increases with the increase in liquid feeding, evaporation boiling temperature and heat flux. h also increases with an increase in distributor height, however there is a maximum height in which any height above this. Besides, the amount of non-condensing gas has significant effect on h. The difference of heat transfer coefficient between freshwater and seawater is small. These results contribute to further improving the performance of heat transfer process and developing new evaporator.     

SUBLIMATION OF PURE SUBSTANCES IN GAS FLUIDIZED BEDS AT ATMOSHPERIC PRESSURE.

This paper deals with the sublimation of large bodies, or “objects”, made up from a pure substance in a bubbling gas fluidized bed of considerably smaller particles, or “fines”. The influence of such parameters as the gas velocity, the bed temperature, the size and the adsorption capacity of the fines has been investigated.

The results obtained clearly show that the rate of sublimation in fluidized beds is far higher than in air alone. It increases with increasing bed temperature, decreasing particle size, increasing powder mass capacity, and roughly varies as a parabolic function of time. It has also been observed that the temperature difference between the bed and the object surface, or “temperature depression”, depends on the fines characteristics as well as on bed temperature, but is independent of gas velocity when good solid mixing conditions are achieved.

Bed-to-object heat and mass transfer coefficients have been deduced from data points and attempts have been made to provide a reasonable theory to account for them. After a complete examination, the idea of interpreting transport phenomena based on a well-adapted “surface renewal model” has been proposed.     

ESTIMATION OF FUNCTIONS IN DRYING EQUATIONS

In drying problem, particularly for drying foodstuff, modelling is very difficult. Many physical effects have to be taken into account for mass transfer ; then mass transfer coefficient varies

In different models unknown functions must be estimated. It is particularly the case in simple models of drying using average values of water content, where the mass transfer varies versus mean water content in falling rate period. On the other hand in the “diffusion model” we have the same problem concerning the diffusion coefficient which must be also estimated

The method we propose in this paper for these two models : simple and “diffusion model” of drying consists from measurements of temperature and water content of the product to search a numerical approach of the unknown function. This method uses optimization techniques on computer and least squares criterion between model values and experimental data

Results are given for the “diffusion model” applied to shelled corn drying to find the diffusion coefficient and for a simple 11107 del applied to plum drying to find the mass transfer coefficient.     

Effect of annular fins on heat transfer of a horizontal immersed tube in bubbling fluidized beds

Experiments were conducted in a bubbling air-fluidized bed to investigate the effect of annular fins of constant thickness on heat transfer. Steady state time averaged local heat transfer coefficient measurements were made by the local thermal simulation technique in a cold bubbling fluidized bed (90 mm ID, 260 mm tall) with horizontally immersed tube initially with no fin and then with three fixed annular fins of constant thickness. Silica sand of mean particle diameter 307 μm and 200 μm were used as the bed materials. The superficial velocity of air was from minimum fluidization conditions, u_mf, to approximately 3 × u_mf. The results indicate that, although the heat transfer coefficient falls with the use of fins, the total heat transfer rises as a result of the greater surface area. Increasing the particle diameter reduces the heat transfer coefficient not only for unfinned horizontal tube but also for annular finned horizontal tube at the same conditions of fluidized bed. Based on the experimental data, correlations are proposed for predicting heat transfer coefficient from fluidized bed to horizontally immersed tubes with and without fins.     

THE CONVECTIVE VELOCITY OF HEAVY PARTICLES IN TURBULENT AIR FLOWS

This paper examines the influence of the “crossing trajectories” effect on the convective velocity for heavy particles suspended in a turbulent air flow. The fluid energy spectrum “experienced” by a falling particle is deduced from experimental data and is used to evaluate the turbulent component of the convective velocity. The results indicate a significant increase in the rms value of the turbulent component due to the “crossing trajectories” effect.     

Simulation of Superheated Steam Turbulent Flows and Heat Transfer in an Impingement Dryer

Numerical simulations are presented for two-dimensional flow field and heat transfer characteristics due to a turbulent single slot jet of superheated steam discharging tangentially into a confined cylinder. A finite volumes method was used to solve the equations that describe the problem. Calculations were performed for steady state turbulent flow and unsteady state heat transfer. Constant velocities and superheated steam temperature are imposed at the inflow. Particle properties were assumed to be the same of a cubic particle of carrot. Numerical tests were performed to ensure that the model solutions were “grid independent” and also independent of the turbulence intensity of stated as boundary condition at the inlet slot. The solution procedure developed is fast and that convergence is reached after a few iterations. The results obtained are relevant to flow and heat transfer behaviors of the impingement dryer and it will be useful future studies considering particles interactions.     

OXYGEN TRANSFER IN HORIZONTAL PIPELINE AERATORS WITH NOZZLES

Horizontal pipeline and tubular loop aerators are of interest for fermentation and waste water treatment and are ideally suited for continuous processing. A major drawback is that these pipeline contactors invariably operate in the “elongated bubble and plug” regime in which the mass transfer rate is low. This article evaluates the performance of a horizontal pipeline aerator fitted with nozzles equispaced along its length to enhance mass transfer rates by promoting turbulence and augmenting effective interfacial area. Such devices can also be advantageously used in long pipe lines as in the case of treating waste while it is being transported. Pressure drop and overall liquid-side mass transfer coefficient data are reported as functions of liquid (water) and gas flow rates and nozzle size and spacing. It is shown that for all the conditions studied, k_La = 0.026(ΔP/L)^1.036 and that the pressure gradient is given by a simple correlation, provided an empirical parameter which characterises a nozzle is known. Preliminary investigations on the effect of surfactant ad the presence of suspended solids (size 75 μm) on mass transfer coefficient are also reported. Very high values of power dissipation can be achieved in such aerators without mechanically moving parts and high values of mass transfer coefficient can be realized.     

Honeycomb supports with high thermal conductivity for gas/solid chemical processes

The scope of this review article is to address the use of novel monolithic catalysts with high thermal conductivity in externally cooled tubular reactors for gas/solid exothermic chemical processes in place of conventional packed beds of catalyst pellets.

After discussing the analysis and the implications of heat conduction in honeycomb monolith structures, we review herein simulation studies and experimental investigations showing that near-isothermal reactor operation can be achieved even under very high thermal loads by adopting specific materials and designs of the honeycomb supports associated with high effective radial thermal conductivities. For such monoliths, the limiting thermal resistance is located at the interface between the monolith and the inner tube wall (“gap resistance”). Recent measurements of the “gap” heat transfer coefficient point to very large values (>400 W/(m² K)), which are controlled both by the tube–monolith clearance at the actual operating conditions and by the thermal conductivity of the process gas.     

COMPREHENSIVE STUDY OF THE GAS HOLD UP AND BUBBLE SIZE DISTRIBUTION IN HIGHLY VISCOUS LIQUIDS I. GLYCEROL SOLUTIONS

The overall gas hold up, E_G, and bubble size distribution were separated into the particular gas hold up, E_GK, and Sauter diameter. d_SG. due to “small bubbles” as well as E_GG and d_SG, due to “intermediate to large bubbles.” Bubbles are defined to be “small” if they remain in the bubbling layer 15 seconds after the gas flow is turned off. The bubbles which leave the layer during this time are considered to be “intermediate to large bubbles.” The time dependences of E_G E_GK and E_GG, as well as of bubble size distribution after initiating the aeration of the liquid, is investigated. The steady state E_G, E_GK and E_GG, Sauter diameter and specific geometrical surface area of “small” and “intermediate to large” bubbles as well as of the entire bubble population were determined in bubble columns employing 50, 70, 90 and 95% glycerol solutions and perforated plates with different hole diameters (d_H = 0.5. 1.0 and 3.0 mm) respectively. In highly viscous media the “small” and “very large” bubble fractions are high. A comparison of the specific geometrical bubble surface areas with the corresponding volumetric mass transfer coefficients, k_La's, measured earlier indicate that the “small” bubbles do not contribute to k_La. The influence of the “small” bubbles on the fluiddynamics of the two phase system is discussed.     

ROTATING DRUM HEAT EXCHANGER

A Sparcely Filled Liquid/Liquid Rotating Drum Heat Exchanger (RDHE) was investigated for its heat transfer characteristics. Drums upto 10cm in diameter and 1.0m long were rotated upto 350 rpm. Liquids investigated varied in kinematic viscosity from water at 2.2 × 10^-3m²/hr to 32wt% LiCl solution at 6.3× 10^-3m²/hr. In this range data showed that the film heat transfer coefficient did not depend on kinematic viscosity. This resulted in a “best fit” correlation of Nusselt Number (100 to 800) with Peclet Number (200,000 to 2,000,000) as oppossed to Nusselt Number with Reynolds Number (2000 to 20,000). Analysis suggested that the heat transfer between two liquid films adhering to opposite surfaces of a rotating drum was controlled by conduction not a convection mechanism. The RDHE has found application in the heat transfer between two liquids of similar pressure where little to no pressure drops are allowed.     

FULLY DEVELOPED HEAT TRANSFER IN A ROTATING NON-ALIGNED STRAIGHT TUBE

Fully developed heat transfer in fluids flowing steadily through a rotating non-aligned straight tube is studied theoretically. A regular perturbation solution of the governing energy equation is developed for a constant wall flux boundary condition; this solution is valid only for “sufficiently mild” Coriolis disturbances. The asymptotic Nusselt number, complete to third order, is obtained from these results. Within the estimated region of validity of this solution the convective heat transfer coefficient depends on three independent dimensionless parameters. The first parameter serves as a Peclet number for energy transfer in the plane of the tube cross-section whereas the second and third parameters are proportional to the axial and transverse components of the Coriolis acceleration. Increasing the Coriolis acceleration will lead to transport enhancement relative to an analogous stationary straight tube exchange device.     

TRANSPORT PROCESSES AND INTERFACIAL PHENOMENA IN AN EVAPORATING MENISCUS

The chemical potential of an extended meniscus on an inclined flat plate is a function of its temperature, curvature, film thickness and height above a reference level. The meniscus thickness profile, which is related to the stress field in the liquid, was used to measure the sensitivity of the meniscus to the non-equilibrium effects associated with evaporation/condensation. The thickness profiles of a completely wetting film of a 1,1,2-Trichlorotrifluoroethane were measured using microcomputer based image processing of interferometric images. The automated data acquisition procedures were used to achieve enhanced resolution and thereby a better understanding of the transport processes occurring in the contact line region.

The interfacial properties of the system were initially evaluated in situ and then used to describe the transport processes associated with a heated meniscus. Consistent with theoretical models, the curvature increased very rapidly from a value of zero in the adsorbed film at the leading edge of the intrinsic meniscus to a high value and then decreased rapidly until a thickness of about 1 μm. Flow results from a disjoining pressure gradient in the thinner region below the maximum curvature and from a curvature gradient in the thicker portion. At a higher power input, the meniscus was found to oscillate and the differences between the “advanced” and “receded” meniscus states were measured and analyzed. The curvature profiles in these two states are significantly different. The study showed that change of phase heat transfer and fluid flow in thin films are strongly coupled because of their common dependence on the intermolecular force field and gravity.     

DYNAMIC AND HYBRID NEURAL MODEL OF THERMAL DRYING IN A FLUTDIZED BED

A preliminary study aimed at comparing Classical Dynamic Neural Modelling (CDNM) and Hybrid Neural Modelling (HNM) to describe thermal dewatering process in a fluidized bed is presented Two schemes of HN modelling were developed to find the most efficient way of combining a classical mathematical model of the process and Artificial Neural Network (ANN). CDN model was developed using “moving window” technique. In the first scheme of HNM a feed-forward ANN was trained to predict evaporation rate and heat flux in the drying process. In the second scheme of the HN model, ANN was used to determine heat transfer coefficient only. Excellent prediction of drying process by HNM is proved.     

柱形流化床传热特性的数值模拟

对Shedid等搭建的圆柱体流化床采用欧拉?欧拉法进行三维数值模拟,考察了颗粒球形度、表观进气速度和床料初始堆积高度对流化床内垂直加热壁面与流动床料之间对流传热特性的影响,采用有效导热系数分别计算气相和固相的对流传热系数。结果表明,随表观进气速度增大,流化床内颗粒物料湍流运动加剧,加热壁面平均温度和流体平均温度下降,壁面流体间传热平均温度差减小,壁面流体间对流传热系数增大;随初始床料高度增加,流化床内颗粒与加热壁面的接触面积增大,导致固相平均对流传热系数增大。     

煤油和空气的混合物水平管内冷凝换热

引　言不凝气体存在于蒸汽中会严重削弱凝结换热效果已是众所周知的事实 ,在空气的质量含量只有0 .5 %时 ,大空间凝结换热系数就下降 5 0 % [1] ,这是在蒸汽为静止状态下得到的 .罗棣等[2 ] 用水蒸气 -空气在波纹竖槽管内进行冷凝换热的研究 .Ｓｐａｒｒｏｗ等[3] 关于水蒸气 -空气在水平管外的冷凝研究表明 ,对于不同的冷凝换热表面 ,不同混合气体的流速对于含不凝气体的冷凝换热有着不可忽视的影响 .煤油是多组分物质 ,而且随温度和压力的不同气态组分也不同 ,相应的煤油蒸气的冷凝过程是不等温等压过程 .关于煤油蒸气冷凝换热的实验研…     

Predicting Thermal Efficiency in Timber Radio Frequency Vacuum Drying

In this work, the efficiency of transforming dielectric energy into evaporated water is analyzed for the case of timber radio frequency vacuum drying. Based on well-known heat and mass transfer equations, a simplified mathematical model is proposed that estimates the drying efficacy in regards to the thermo-physical properties of wood. Although not exact, the theoretical results are close to the experimental observations and elucidate some phenomena like the tendency of the timber to dry from inside to outside, and the drying rate increase with the rise of the timber gas permeability. The theoretical efficiency model also predicts a range of wood permeability values for which the drying efficiency changes from 100 to 0%, thus providing a quantitative scale for classifying the spectrum of “difficult-to-dry” all the way to “easy-to-dry” wood species when using radio frequency vacuum technology.     

PREDICTION AND RATIONAL CORRELATION OF THERMOPHORETICALLY REDUCED PARTICLE MASS TRANSFER TO HOT SURFACES ACROSS LAMINAR OR TURBULENT FORCED-CONVECTION GAS BOUNDARY LAYERS†

An approach originally developed to predict and correlate the thermophoretically-augmented submicron particle mass transfer rate to cold surfaces is shown here to account extremely well for the thermophoretically reduced particle mass transfer rate to “overheated” surfaces experiencing either a forced boundary layer (BL)-flow of laminar or turbulent dusty gas. This laminar BL/hot wall situation occurs, e.g., in hot surface/cold envelope chemical reactors used for growing epitaxial silicon layers from mainstreams containing, say, silane vapor and inadvertent submicron dust particles. “Thermo-phoretic blowing” is shown to produce effects on particle concentration BL-structure and wall mass transfer rates identical to those produced by real blowing (transpiration) through a porous wall. Indeed, a “blowing parameter additivity” relationship is proposed to account for the simultaneous effects of both phenomena should they be acting in concert (or in opposition). Exact numerical BL calculations covering the parameter ranges: l≤T_w/T_e6, (particle thermophoretic-/gas thermal- diffusivity ratios between )0·1 and 0·8 and particle Schmidt numbers between 10⁰ and 2 × 10³ are used to establish the validity of the basic forced convection mass transfer correlations for self-similar laminar BLs and law-of-the-wall turbulent BLs. This includes parametric combinations of immediate engineering interest for which the deposition rate is thermophoretically reduced by no less than 10-decades! The applicability of our correlations to developing BL-situations is then illustrated using a numerical example relevant to wet-steam turbine technology.     

State of hydration of the proton and the transition between molten-salt and aqueous-solution double-layer and kinetic behaviour

The influence of the state of hydration of the proton, from H₃O⁺ in the room-temperature melt, H₃O⁺. CF₃SO⁻₃, to 1 M aqueous in CF₃SO₃H solution in water, on the interfacial capacitance behaviour of the Au electrode is reported. The results allow the transition from “molten-salt” to “aqueous-solution” double-layer capacitance behaviour to be evaluated.

At positive potentials, near the potential for onset of surface oxidation of Au by OH, the interfacial capacitance is dominated by a pseudo-capacitance associated with chemisorption of the anion CF₃SO⁻₃, with charge transfer. At negative potentials in the range in which cathodic H₂ evolution occurs, the capacitance varies from ca 23 to ca 50 μF cm⁻², with the maximum arising around a degree of proton hydration corresponding to H₇O⁺₃.

Cathodic H₂ evolution kinetics have been studied from the same proton sources and show characteristic dependences of log i_o and the heat of activation on the hydration-state of H⁺. The Tafel slopes decrease with increasing temperature, giving a further example of non-conventional behaviour of the transfer coefficient.