Theoretical estimation of mass transfer coefficients in solution crystallization

Generally, diffusion through film thickness is used for correlating data in the industrial crystallization processes. Furthermore, mass transfer is an important phenomenon in most chemical processes and studies involving determination of mass transfer coefficients are necessary for a better estimation of equipment performance. Solid-liquid mass transfer coefficients in stirred systems have received substantial attention in the past due to their practical applications. In contrast, little information is available on solid-liquid mass transfer in crystallization systems, despite the importance of crystallization. In this work, an expression for the mass transfer coefficients in solution crystallization has been developed based on the Stefan problem formulation. The model is able to predict a finite mass transfer coefficient when the layer thickness vanishes. The obtained mass transfer coefficients agree with previously reported experimental data. Furthermore, it is found that the stagnant film for mass transfer has oscillatory behavior, and this behavior is a function of residence time.     

用可视化方法实验研究规整填料的局部传质(英文)

A chromochemical reactive mass transfer technique has been employed to study local mass transfer characteristics of structured packing. This technology adopted by experiment is an Ammonia Adsorption Method (AAM) that yields the surface distribution of transferred mass by analyzing the color distribution on a filter paper with the results of the color chemical reaction. A digital image processing technology is applied for data visualiza-tion. The three-dimensional plot of the local mass transfer coefficients shows that there exist three peak values on different positions of a unit cell of structured packing. In order to improve mass transfer efficiency of the structured packing, one piece of baffle is added between packing sheets. As a result, the average mass transfer coefficient in-creases by (10-20)% and the pressure drop decreases by (15-55)%.     

A novel technique for local mass transfer measurements

A novel electrodeposition technique is developed to estimate time‐averaged convective local mass transfer coefficients. This method is based on the diffusion‐controlled deposition rate of copper. With an electrolyte solution consisting of H2SO₄ and CuSO₄, copper is dissolved at the anode and deposited onto the nickel cathode. The thickness of the copper, measured with an optical microscope, provides an estimate of the time‐averaged local mass transfer coefficient for the given location, and by depositing multiple layers, the coefficients under different flow conditions can be obtained from one cathode. The results for laminar flow in a smooth, round pipe showed good agreement with those calculated from the analytical Leveque solution. Results were also obtained for turbulent flow and demonstrate the potential for acquiring convective local mass transfer coefficients for various flow configurations.     

自由界面气液传质系数的旋涡作用模型

在单涡模型的基础上 ,根据各向同性湍流的能谱函数 ,考虑不同尺度的旋涡对气液传质的影响 ,建立了气液自由界面传质系数模型 .模型计算结果与管道流及搅拌反应釜的实验结果吻合较好.     

Wall shear stress and mass transfer in vertical two-component flow

Mass transfer rates in vertical gas-liquid flow were measured by an electrochemical technique. The flow regimes studied were slug, churn and annular. Average mass transfer coefficients in gas-liquid flow could be correlated by expressions similar to those for single phase flow. Fluctuations in local mass transfer coefficient could be used to indicate flow regimes. Average wall shear stress determined from the average mass transfer coefficients agreed with values calculated from measurements of pressure drop, void fraction and flow rate, provided flow reversals did not occur. The results indicate that the electrochemical technique could be used to measure wall shear stress in accelerating gas-liquid flows, such as critical flow.     

Heat transfer in co-current vertical two-phase flow

Heat transfer in co-current two-phase upflow and downflow of air–water has been investigated in a 25.8 mm electrically heated vertical pipe at 172.3 kPa for water mass velocities of 54 to 172 kg/m²s and gas flow rates of 0 to 1.322 × 10⁻² m³/s. It was found that although the injection of air in the liquid flow increased the two-phase heat transfer coefficients significantly for both systems, upflow coefficients were generally higher than those for downflow for the same liquid flow rate. This could have important implications in the design of some chemical reactors and heat engineering processes. Changes in heat transfer rates were found to occur at the flow pattern transition boundaries. Two-phase heat transfer coefficients were well correlated by an expression based on dimensional analysis for both upflow and downflow.     

Ébullition par Convection Forcée des Mélanges Eau‐Ammoniac dans un Tube Vertical

An experimental study has been conducted on the forced convective boiling heat transfer of ammonia‐water mixtures flowing inside a 6 mm inner diameter vertical smooth tube. Using a water‐heated double pipe type generator, the local heat transfer coefficients are measured inside the inner tube for a range of heat flux density (29.93 — 99.79 kW/m²), mass flux density (35.36 — 99.04 kg/m²·s), mass flow rate (0.001 — 0.03 kg/s) and ammonia mass concentration (49%, 55% and 61%). The effect of the experimental parameters on the heat transfer coefficients is analysed. Three methods are used to predict the boiling heat transfer coefficients. Experimental data were compared with the available correlations. The obtained results confirm the good performance of the Mishra et al. (1981) and Bennett‐Chen's (1980) correlations in predicting the convective boiling heat transfer coefficient of NH₃‐H₂O mixtures. These methods are able to predict the boiling heat transfer data within an average accuracy of ± 20 %.     

MASS TRANSFER INTO A FREE LIQUID SURFACE EFFECTED BY SUBMERGED JETS

A theoretical and experimental analysis of mass transfer into a turbulent free liquid surface effected by submerged jets has been presented. Theoretical considerations concentrated on hydrodynamic characteristics of the system which enabled us to derive the radial velocity distribution in the surface jet flow. Some conclusions have been drawn from the two-parameter models of turbulence. The results of experimental measurements of the average mass transfer coefficients have been interpreted in terms of our own model of mass transfer accounting for eddy diffusivity. The model parameters were correlated with the basic hydrodynamic parameters of the system. Radial distributions of the local values of the mass transfer coefficients were estimated. The applicability of other models of mass transfer has also been verified. It has been shown that the model presented here is the most general and applicable for interpretation of the experimental data obtained when studying the system under consideration     

Steady-state mass transfer between two opposite discs and a liquid in radial divergent flow

The paper deals with global and local mass transfer between a liquid and two opposite fixed discs. The liquid is introduced through a central hole in one of the discs and flows radially. The mass transfer coefficients, measured by the electrochemical method, are empirically correlated. The correlations are compared with corresponding empirical correlations from the literature. Local measurements using microelectrodes allowed to determine the evolution of the local mass transfer coefficients over each disc as a function of the geometrical parameters, particularly the gap between the discs.     

Liquid—liquid extraction column (rotating disc contactor)—model parameters from drop size distribution and solute concentration measurements

The practical application of an extraction column model which takes into account the influence of drop-size distribution (i.e. the ‘forward mixing’ model) is brought forward by the generation, from experimental data, of values of the mass transfer and axial dispersion coefficients required by the model. Values of these coefficients were generated from drop-size distribution and solute concentration profile measurements in a 22 cm diam. rotating disc contactor. The use of the Handlos-Baron drop mass transfer model is justified. The resulting continuous phase transfer coefficients were found to be dependent only on disc speed. Continuous phase axial dispersion coefficients were much higher than tracer-correlation predicted values at higher flows, and larger drop sizes. An explanation for this is presented.     

Electrochemical study of mass transfer in decaying annular swirl flow

This paper describes an investigation of local mass transfer behaviour at the inner rod and outer pipe wall of an annular test section in decaying annular swirl flow generated by axial vane-type swirl generators. Four swirl generators with vane angles of between 15–60° to the axis of the duct were used. The experiments were carried out in the Reynolds number range 3300–50 000 and at a Schmidt number of 1650. The axial distribution of the local mass transfer coefficients at both the inner rod and the outer wall were measured using an electrochemical technique. Current fluctuations were also recorded to gain information on the turbulence characteristics in the vicinity of the local electrodes. This paper was presented at the International Workshop on Electrodiffusion Diagnostic's of Flows held in Dourdan, France, May 1993.     

An Analytical Study of Thermophoretic Particulate Deposition in Turbulent Pipe Flows

The presence of a cold surface in non-isothermal pipe flows conveying submicron particles causes thermophoretic particulate deposition. In this study, an analytical method is developed to estimate thermophoretic particulate deposition efficiency and its effect on overall heat transfer coefficient of pipe flows in transition and turbulent flow regimes. The proposed analytical solution has been validated against experiments conducted at Oak Ridge National Laboratory. Exhaust gas carrying submicron soot particles was passed through pipes with a constant wall temperature and various designed boundary conditions to correlate transition and turbulent flow regimes. Prediction of the reduction in heat transfer coefficient and particulate mass deposited has been compared with experiments. The results of the analytical method are in a reasonably good agreement with experiments.     

Mass transfer in horizontal flow channels with thermal gradients

Mass transfer to a wall of a horizontal rectangular channel reactor was investigated by the limiting current technique for Reynolds numbers ranging from 200 to 32000. Overall mass transfer coefficients at various mass transfer surface angles were obtained while the reactor was operated under isothermal and non-isothermal conditions. Dimensionless correlations were developed for isothermal flows from 25 to 55°C and for non-isothermal flows with applied temperature differences up to 30°C. In the laminar flow range natural convection dominated, but under turbulent conditions combined natural and forced convection prevailed. Mass transfer was approximately doubled under optimum selection of channel surface rotation, temperature gradient and flow rate.     

Mixing length equation for high schmidt number mass transfer at solid boundaries

A simple modification of the Van Driest damping factor for mixing length calculations of mass and/or heat transfer coefficients at solid boundaries is proposed to increase mass eddy diffusivities in the near-wall region, without affecting velocity predictions. Calculated mass transfer rates with a constant Sc_t = 0.85 are in good agreement with present and previous experimental results obtained in pipe and duct flows at high Schmidt numbers.     

Turbulent fluid flow and electrochemical mass transfer in an annular duct with an obstruction

A so-called blockage geometry consisting of a rod with a fin positioned concentrically within a pipe is used to asses the capabilities of numerical turbulent flow and mass transfer models to predict the turbulent mass transfer coefficients. Measurements of the mass transfer coefficient have been performed for a range of fin diameters and flow rates. The limiting diffusion current measurements were performed using the ferri-ferrocyanide system and nickel electrodes. Different mass transfer turbulence models are used for the calculations and the results are compared with the measurements. The influence of flow rate and fin diameter on the mass transfer rate is examined.     

Heat and mass transfer in a boundary layer with the evaporation and combustion of ethanol

Experimental data on heat and mass transfer in a boundary layer upon ethanol evaporation from a porous surface and its combustion in an air flow are reported. It has been established that variations in the flow velocity in the flow core weakly affect the temperature and concentration of substances on the reactor wall. The flame temperature and the distribution of mass flows over the wall depend essentially on the flow velocity. It has been observed that heat-and mass-transfer coefficients decrease in combustion. The representation of experimental data using overall enthalpies and generalized concentrations as transfer potentials suggests an analogy between the processes of heat and mass transfer in a reacting boundary layer.Novosibirsk. Translated from Fizika Goreniya i Vzryva, Vol. 30, No. 1, pp. 8–15, January–February, 1994.     

Hydrodynamics and Mass Transfer in Gas‐Liquid‐Solid Circulating Fluidized Beds

Although extensive work has been performed on the hydrodynamics and gas‐liquid mass transfer in conventional three‐phase fluidized beds, relevant documented reports on gas‐liquid‐solid circulating fluidized beds (GLSCFBs) are scarce. In this work, the radial distribution of gas and solid holdups were investigated at two axial positions in a GLSCFB. The results show that gas bubbles and solid particles distribute uniformly in the axial direction but non‐uniformly in the radial direction. The radial non‐uniformity demonstrates a strong factor on the gas‐liquid mass transfer coefficients. A local mass transfer model is proposed to describe the gas‐liquid mass transfer at various radial positions. The local mass transfer coefficients appear to be symmetric about the central line of the riser with a lower value in the wall region. The effects of gas flow rates, particle circulating rates and liquid velocities on gas‐liquid mass transfer have also been investigated.     

Experimental Study on the Heat Transfer Enhancement of Oscillating-Flow Heat Pipe by Acoustic Cavitation

Experimental investigations on the heat transfer characteristics of an oscillating-flow heat pipe with acoustic cavitation in comparison with the ordinary oscillating-flow heat pipe are given in this article. The experimental results showed that the heat transfer rate of an oscillating-flow heat pipe with an acoustic cavitation field imposed on the evaporator section was higher than that without a cavitation field by 8–24.5%. It has been proved that acoustic cavitation can enhance the heat transfer performance of an oscillating-flow heat pipe. However, for the case of acoustic cavitation applied on the condenser section, not all ultrasonic fields applied were effective, and the heat transfer rate increased from ?39 to 77%. Further discussion on the experimental results is provided.     

DIRECT CONTACT CONDENSATION OF DILUTE STEAM/AIR MIXTURES ON WAVY FALLING FILMS

Condensation from air-steam mixtures on falling water layers is investigated experimentally and theoretically. The thin film flows on the inner surface of a 5cmi.d. vertical pipe. This film is wavy turbulent while the gas phase is kept saturated with steam. Experiments are conducted with the gas mixture effectively stagnant, compared with the fast moving liquid film. Measurements are also made under a mild vacuum applied on the gas phase. Heat transfer coefficients averaged over the entire length of the condensing surface, tend to increase by decreasing the liquid flowrate, by increasing the steam fraction, and by applying a mild vacuum on the gas phase. However, for the cases examined, there is a liquid flowrate above which the heat transfer coefficient becomes almost constant.

Numerical predictions are made for a fully developed turbulent film using an eddy diffusivity model. The results indicate that for a system with a large amount of noncondensable gases-as in this study-the temperature profile in the liquid film is nearly uniform and that the major resistance to condensation resides in the gas phase. The analysis also shows that the relative contribution of sensible heat transferred through the gas phase is small relative to the latent heat released upon condensation. Comparison of predictions with experimental data suggests that a significant parameter in these analyses is the gas diffusion boundary layer thickness which seems to be comparable in size with the liquid film thickness. Finally, the possibility is discussed of correlating condensation heat transfer coefficients with already available statistical characteristics of the falling wavy layer. Theoretical predictions based on this idea are in good agreement with data.     

规整填料表面点传质的可视化研究

采用显色化学反应的流体可视化方法研究规整填料表面上的点传质效果。并且用高精度彩色扫描仪进行后处理,把颜色值转化为数字值。通过风洞的标定特性来建立数字值和点传质量之间的关系,进而得到填料表面的点传质系数。由填料表面点传质系数的三维分布图,可清晰地看到在每两片填料交叉形成的任一传质单元当中,存在3个传质高峰。