Use of axial dispersion model for determination of Sherwood number and mass transfer coefficients in a perforated rotating disc contactor

The mass transfer process in a perforated rotating disk contactor (PRDC) using a toluene-acetone-water system was investigated.The volumetric overall mass transfer coefficients are calculated in a PRDC column.Both mass transfer directions are considered in experiments.The influences of operating variables containing agitation rate,dispersed and continuous phase flow rates and mass transfer in the extraction column are studied.According to obtained results,mass transfer is significantly dependent on agitation rate,while the dispersed and continuous phase flow rates have a minor effect on mass transfer in the extraction column.Furthermore,a novel empirical correlation is developed for prediction of overall continuous phase Sherwood number based on dispersed phase holdup,Reynolds number and mass transfer direction.There has been great agreement between experimental data and predicted values using a proposed correlation for all operating conditions.     

氧化石墨烯/水脉动热管传热强化及性能预测

实验研究了氧化石墨烯（GO）纳米流体对脉动热管（PHP）传热性能的影响。结果表明：充液率、浓度及加热功率显著影响脉动热管的传热性能。在小充液率（FR=30%）时,PHP更多是在重力辅助热虹吸管以及脉动热管的共同作用下工作,热阻较低,但容易烧干;添加GO纳米颗粒可改善流体传热性能,降低PHP热阻,延缓烧干;尤其在质量分数0.05%~0.08%、加热功率10~50W时热阻可比纯水降低38.1%~74.1%;在质量分数0.08%~0.1%时,烧干极限Q_max可比纯水提高33%。在大充液率（FR=80%）时,气相空间受限,流体运动阻力较大,PHP整体运行性能较差。添加GO纳米不能明显改善PHP传热性能,在高浓度（质量分数0.1%）时还会恶化传热性能。综合考虑热阻及烧干极限,PHP在中等充液率（FR=50%）时整体运行性能最佳;且存在一个合适的工作范围（质量分数0.03%~0.08%,加热功率20~105W）,使PHP热阻比纯水下降18.9%~54.4%之间,强化作用明显。最后,在实验基础上,综合应用Ku、Pr、Ja、Bo、Mo等量纲为1数组合,拟合得到实验关联式预测GO/水PHP传热性能,适用于30%~80%充液率下,质量分数0~0.1%的GO/水纳米流体脉动热管。     

Experimental mass transfer coefficients in a pilot plant multistage column extractor

The volumetric overall mass transfer coefficients in a multistage column have been measured using axial dispersion model for toluene–acetone–water system. The effect of operating parameters on the volumetric overall mass transfer coefficients has been investigated for both mass transfer directions. The results show that the mass transfer performance is strongly dependent on rotor speed and mass transfer direction, although only slightly dependent on phase flow rates. In addition, empirical correlations to predict the overall mass transfer coefficients have been developed. The proposed correlations based on dimensionless numbers can be considered as a useful tool for the possible scale up of the multistage column extractor.     

Prediction of mass transfer coefficients in an asymmetric rotating disk contactor using effective diffusivity

Mass transfer characteristics have been investigated in a 113 mm diameter asymmetric rotating disk contactor of the pilot plant scale for two different liquid–liquid systems. The effects of operating parameters including rotor speed and continuous and dispersed phase velocities on the volumetric overall mass transfer coefficients are investigated. The results show that the mass transfer performance is strongly dependent on agitation rate and interfacial tension, but only slightly dependent on phase flow rates. In this study, effective diffusivity is used instead of molecular diffusivity in the Gr?ber equation for estimation of dispersed phase overall mass transfer coefficient.The enhancement factor is determined experimentally and there from an empirical expression is derived for prediction of the enhancement factor as a function of Reynolds number. The predicted results compared to the experimental data show that the proposed correlation can efficiently predict the overall mass transfer coefficients in asymmetric rotating disk contactors.     

Effect of system properties on the performance of Liquid—Liquid extraction columns—II: Oldshue—Rushton column

Mean drop size, fractional hold-up of dispersed phase and axial mixing characteristics have been determined in a 72 mm diameter mechanically agitated extraction column of Oldshue—Rushton type, using the two liquid—liquid mass transfer systems, toluene—acetone—water and MIBK-acetic acid—water. As for normal conditions of packed column operation described in Part I, solute presence and the direction of mass transfer has a significant effect on mean drop size, fractional hold-up and to a lesser extent, axial mixing in the dispersed phase. Probably the most dramatic effect however is the manner in which solute transfer affects dispersed phase behaviour. Highly coalescing conditions with transfer from the dispersed to the continuous phase can make the column practically unoperable. As for the packed column, axial mixing in the continuous phase is unaffected except in so far as solute presence and direction of mass transfer affect the hold-up of dispersed phase.     

Prediction of mass transfer coefficients in a pulsed disc and doughnut extraction column

A study of the mass transfer performance for a pulsed disc and doughnut extraction column has been presented for a range of operating conditions. The mass transfer performance has been investigated for both directions of mass transfer. This study has examined the mass transfer coefficients which has incorporated the effects of back‐mixing in the continuous phase. The effect of operating variables including pulsation intensity and dispersed and continuous phase velocities on volumetric overall mass transfer coefficient has been investigated. The experiments showed that mixer‐settler, transition and emulsion regimes exist in the column depending on the pulse characteristics. In the present work, effective diffusivity is substituted for molecular diffusivity in the Gröber equation for estimation of overall mass transfer coefficients. The enhancement factor is determined experimentally and there from a single empirical correlation is derived for prediction of enhancement factor in terms of Reynolds number, holdup and Eötvös number for all operating regimes and each mass transfer direction. The experimental results are in very good agreement with the values calculated by the proposed equation. © 2011 Canadian Society for Chemical Engineering     

Modeling the axial distribution of mass transfer coefficient in an agitated-pulsed extraction column

The dispersed phase holdup and drop size in solvent extraction columns vary along the column height and this affects the mass transfer coefficient and interfacial area. In this article, mass transfer study was performed experimentally using a 25 mm diameter agitated pulsed column. The axial distribution of mass transfer coefficient was determined by coupling population balance equation and axial dispersion model by taking the longitudinal variation in hydrodynamic performance into consideration. Feasibility of different mass transfer models in predicting concentration profiles was evaluated and a novel correlation based on effective diffusivity was developed. The results showed that both overall and volumetric mass transfer coefficients have significant change along the column height and greatly depends on the agitation speed and pulsation intensity. Increasing dispersed phase velocity also augments the overall mass transfer coefficient. The maximum number of transfer unit was measured to be 10 m⁻¹ at agitation speed of 1000 rpm.     

Gas-liquid mass transfer in a vibrating disk column

Gas—liquid mass transfer has been studied in a vibrating disk column with CO₂ gas—water concurrent flow. The overall volumetric mass transfer c     

毫米通道内氯苯-水非均相体系混合效果研究

以氯苯-水体系为对象,研究了毫米通道内液液非均相流体在不同流速下的流动形态及其混合传质效果。通过对子弹流型、子弹单滴混合流型、三相并行流型的观察分析,以流动雷诺数、毛细数、韦伯数为评价参数,讨论了流体流动表面力与流型形成的关系。以混合传质前后水相中醋酸浓度的变化为分析依据,探究了流体流型、流速对液液非均相体系传质效率与总体积传质系数的影响。结果表明:界面张力是子弹流型、子弹单滴混合流型形成的主导控制力,流动惯性力则促进三相并流的产生;随着流速的增加和流型的转变,扩大了二相传质界面积,提高了混合过程的总体积传质系数和传质效率。     

微通道中气-液-液三相流流型及传质研究

使用双T型微通道,以体积分数为30%的磷酸三丁酯（TBP）的环己烷溶液-乙酸水溶液为萃取体系,研究了不同油水两相流量比及油水两相总流量条件下,气相的引入及气相流量分率α对流型及传质的影响,并获得总体积传质系数k_La与油水两相流量比q、气相雷诺数Re_g、液相平均雷诺数Re_M的关系式。研究结果表明,第1个T型接口处气相的引入所带来的气相剪切作用能促进第2个T型接口处分散相液滴的形成,可使液-液并行流转化为气-液-液三相弹状流;同时,由于气-液-液弹状流具有较高的相接触面积及内循环作用,传质系数k_La得到显著提高。     

Use of axial dispersion and plug flow models for determination of axial mixing and mass transfer coefficient in an l‐shaped pulsed packed extraction column

In this study, the volumetric overall mass transfer and phases axial mixing coefficients have been investigated in a pilot plant of an L‐shaped pulsed packed extraction column by using two liquid systems of toluene/acetone/water and n‐butyl/acetone/water. The mass transfer performance has been evaluated using two methods of axial dispersion and a plug flow model. The effect of the operational variables and physical properties, including the dispersed and continuous phases flow rates, pulsation intensity, and interfacial tension, on mass transfer and phases axial mixing coefficients have been considered. It has been found that the pulsation intensity and the continuous phase flow rate seriously affect the mass transfer coefficient, however, the dispersed phase flow rate has a weaker effect. Also, the axial mixing of a phase is strongly affected by the pulsation intensity and the flow rate of the phase itself and it is not affected by the second phase flow rate. Finally, new correlations are proposed to accurately predict the mass transfer and axial mixing coefficients.     

Effects of Contaminants on the Mass‐Transfer Characteristics of a Two‐Impinging‐Streams Gas‐Liquid Reactor

The mass‐transfer characteristics of a new type of two‐impinging‐streams reactor (TISR) was studied by means of sodium sulfite solution as the liquid phase and air as the gas phase, in the presence and absence of various types of surface‐active agents (SAAs). The influences of anionic, cationic, and nonionic SAAs on the specific interfacial area and overall volumetric mass‐transfer coefficient obtained in the TISR were investigated. It was found that the presence of a little amount of the above‐mentioned contaminants increases the specific interfacial area and decreases the overall volumetric mass‐transfer coefficient. On the basis of the experimental results obtained for various types of SAAs, correlations were derived for the interfacial area as well as the Sherwood number for the liquid phase in terms of Froude, Reynolds, Schmidt, and Morton numbers.     

Two-phase flow—IV. Gas and liquid side mass transfer coefficients

Mass transfer coefficients in gas and liquid have been obtained for the case of cocurrent gas—liquid flow through a vertical tube 6 mm i.d. by absorbing sulphur dioxide into sodium hydroxide solution and carbon dioxide into sodium carbonate—sodium bicarbonate solution respectively. The liquid side mass transfer coefficient was found to increase with the gas velocity but showed a maximum when plotted against the liquid velocity. A model based on the analogy between momentum and mass transfer has been proposed for the rate of mass transfer in the liquid phase. The mass transfer coefficient in the gas phase increases with the gas velocity but the liquid velocity has an opposite effect. A correlation in terms of dimensionless groups is presented for the gas side mass transfer coefficient.     

PERFORMANCE OF A PACKED-BED REACTOR

We examined the conversion rates in a packed bed catalytic reactor with a two phase upward flow in a wide range of operating conditions. The oxidation of ethanol to acetic acid in the liquid phase on a Pd-Alumina catalyst was chosen as the test reaction.

Global reaction rates were measured by changing gas velocities, temperature, and feed concentrations of ethanol in the liquid phase. The observed rates were compared with those calculated using two models, assuming a total external wetting of the catalyst. In the first model, a “kinetic” conversion rate was calculated by neglecting any interphase mass transfer resistance. In the second model the interphase mass transfer resistance was considered and expressed by an overall coefficient evaluated from published correlations. The results show that there is an hydrodynamic influence, probably due to the mass transfer and/or to the partial effective wetting of the catalyst. Mass transfer, on the other hand, is better than that observed in other cases. A comparison with the performances of a downflow trickle-bed reactor operating at the same tested conditions showed a much smaller influence of mass transfer and hydrodynamics on the overall conversion rate for the upflow reactor.     

The influence of molecular diffusion on mass transfer between turbulent liquids

The influence of molecular diffusion on liquid—liquid mass transfer in a stirred transfer cell has been found by measuring the rates transfer of helium and iso-butane from water to toluene and dekalin. These solutes have very different diffusion coefficients, their presence does not alter the physical properties of the liquids and, because their equilibrium distributions strongly favour the organic phases, the water phase mass transfer coefficient could be determined and was found to depend on the square root of the diffusion coefficient.The results are compared with the predictions of a model for liquid—liquid mass transfer under turbulent conditions, based on the approach of an eddy to the interface being restrained by interfacial tension and gravitational forces and taking into account eddy pressure fluctuations in both phases. This model provides a correlation for these results, as well as water phase mass transfer coefficients for the transfer of iso-butane from water to n-octanol, and previous stirred transfer cell results.     

气相中碘分子的吸收工艺研究

采用H_2O_2将湿法磷酸中的碘离子氧化成碘分子,之后采用空气萃取法将碘分子以气相吹出,气相中碘分子经还原剂循环洗涤吸收后便于富集回收。气相中碘分子的吸收最佳工艺条件为:以亚硫酸为还原剂,亚硫酸溶液质量分数为8%,反应温度为35℃,吸收塔内含碘气相停留时间为15 s。最佳工艺条件下,气相中碘分子的吸收率为88.37%。50 t/a碘回收装置中,含碘气相冷凝器换热面积为3 000 m~2,吸收塔规格为ф4 300 mm×15 000 mm,材质为钢衬PO,吸收液喷淋量为175 m~3/h。     

Analysis of concentration polydispersity in mixed liquid–liquid systems

Inter-phase mass transfer for each chemical component is typically modelled with one material balance for the continuous and one for the dispersed phase. This approach contains inherently an assumption that the phases are well mixed at least locally. For the dispersed phase, this assumption requires that breakage and coalescence are significantly faster compared to the mass transfer, which is not necessarily true. It is important to carry out preliminary assessment whether the dispersed phase segregation is important and should be considered in subsequent modelling efforts, before embarking heavy multidimensional simulations where all possible dispersed phase variations are considered. In this work, relevant time scales are first defined and used for analyzing dispersed phase mixedness in liquid–liquid systems with mass transfer between the phases. Then appropriate dispersed phase modelling tools for the purpose are evaluated. Simple droplet number density based analysis is shown to estimate mixedness reasonably well. Furthermore, the drop number density approach is also shown to predict the average drop sizes with almost comparable accuracy than the full population balances.     

A simple model describing the effect of axial mixing on countercurrent mass exchange

By means of a model the effect of axial dispersion on mass transfer in countercurrent contactors is described. The mixing is represented by a cascade of a number of ideal mixers in each phase. The separation performance—characterized by the number of external transfer units, ∫ dC/(C*  C)—is in a simple analytical way related to the extraction factor, mF_x/F_y, to the number of internal transfer units, k_ox.a.L/v_x, and to the number of ideal mixers in each phase.The number of external transfer units calculated with this model is in good agreement with the results of the longitudinal dispersion model as presented by Miyauchi and Vermeulen.The simplicity of the model enables rapid slide rule calculations and provides rapid recognition of the factors limiting the separation performance.     

Experimental study of Gas—liquid transfer rates using an inclined perforated tray without downcomer

Gas—liquid interfacial area, liquid phase and gas phase mass transfer coefficients are measured for an inclined perforated plate without downcomer. The influence of geometric and hydrodynamic parameters is investigated experimentally.The existence of optimal geometries and hydrodynamic conditions which yield maximal mass transfer rates is shown. A techno—economic study is carried out which compares usual equipment with the present technique. Possible applications are suggested.