Different efficiency toward the biomimetic aerobic oxidation of benzyl alcohol in microchannel and bubble column reactors: Hydrodynamic characteristics and gas–liquid mass transfer

The selective aerobic oxidation of benzyl alcohol to benzaldehyde has attracted considerable attention because benzaldehyde is a high value-added product. The rate of this typical gas–liquid reaction is significantly affected by mass transfer. In this study, CoTPP-mediated(CoTPP: cobalt(II) mesotetraphenylporphyrin) selective benzyl alcohol oxidation with oxygen was conducted in a membrane microchannel(MMC) reactor and a bubble column(BC) reactor, respectively. We observed that 83% benzyl alcohol was converted within 6.5 min in the MMC reactor, but only less than 10% benzyl alcohol was converted in the BC reactor. Hydrodynamic characteristics and gas–liquid mass transfer performances were compared for the MMC and BC reactors. The MMC reactor was assumed to be a plug flow reactor,and the dimensionless variance was 0.29. Compared to the BC reactor, the gas–liquid mass transfer was intensified significantly in MMC reactor. It could be ascribed to the high gas holdup(2.9 times higher than that of BC reactor), liquid film mass transfer coefficient(8.2 times higher than that of BC reactor), and mass transfer coefficient per unit interfacial area(3.8 times higher than that of BC reactor). Moreover,the Hatta number for the MMC reactor reached up to 0.61, which was about 15 times higher than that of the BC reactor. The computational fluid dynamics calculations for mass fractions in both liquid and gas phases were consistent with the experimental data.

Experimental determination of gas holdup and volumetric mass transfer coefficient in a jet bubbling reactor

The hydrodynamics and mass transfer characteristics of a lab-scale jet bubbling reactor(JBR)including the gas holdup,volumetric mass transfer coefficient and specific interfacial area were assessed experimentally investigating the influence of temperature,pH and superficial gas velocity.The reactor diameter and height were 11 and 30 cm,respectively.It was equipped with a single sparger,operating at atmospheric pressure,20 and 40℃,and two pH values of 3 and 6.The height of the liquid was 23 cm,while the superficial gas velocity changed within 0.010-0.040 m·s^(-1)range.Experiments were conducted with pure oxygen as the gas phase and saturated lime solution as the liquid phase.The liquid-side volumetric mass transfer coefficient was determined under unsteady-state oxygen absorption in a saturated lime solution.The gas holdup was calculated based on the liquid height change,while the specific interfacial area was obtained by a physical method based on the bubble size distribution(BSD)in different superficial gas velocities.The results indicated that at the same temperature but different pH,the gas holdup variation was negligible,while the liquid-side volumetric mass transfer coefficient at the pH value of 6 was higher than that at the pH=3.At a constant pH but different temperatures,the gas holdup and the liquid-side volumetric mass transfer coefficients at 40℃were higher than that of the same at 20℃.A reasonable and appropriate estimation of the liquid-side volumetric mass transfer coefficient(kla)in a pilot-scale JBR was provided which can be applied to the design and scale-up of JBRs.

A Comprehensive Review of the Influence of Heat Exchange Tubes on Hydrodynamic,Heat,and Mass Transfer in Bubble and Slurry Bubble

Bubble and slurry bubble column reactors(BCRs/SBCRs)are used for various chemical,biochemical,and petro-chemical applications.They have several operational and maintenance advantages,including excellent heat and mass transfer rates,simplicity,and low operating and maintenance cost.Typically,a catalyst is present in addition to biochemical processes where microorganisms are used to produce industrially valuable bio-products.Since most applications involve complicated gas-liquid,gas-liquid-solid,and exothermic processes,the BCR/SBCR must be equipped with heat-exchanging tubes to dissipate heat and control the reactor’s overall performance.In this review,past and very recent experimental and numerical investigations on such systems are critically dis-cussed.Furthermore,gaps to befilled and critical aspects still requiring investigation are identified.

Modeling of mass transfer characteristics of bubble column reactor with surfactant present

Danckwert’s method was used to determine the specific interfacial area, a, and the individual mass transfer coefficient, kL, during absorption of CO2 in a bubble column with an anionic surfactant in the carbonate-bicarbonate buffer solution and NaAsO2 as catalyst, the presence of which decreases the specific interfacial area and the individual mass transfer coefficient. The specific interfacial area and the individual mass transfer coefficient increase with increasing su- perficial gas velocity. The specific interfacial area decreases whereas the individual mass transfer coefficient increases with increasing temperature. The results of experiments were used to determine the dependence of a, kL, and kLa on the surface tension, the temperature of the absorption phase, and the superficial velocity of the gas. The calculated results from the correlation were found to be within 10% deviation from the actual experimental results.

Bubble size fractal dimension,gas holdup,and mass transfer in a bubble column with dual internals

As the scale of residual oil treatment increases and cleaner production improves in China,slurry bubble column reactors face many challenges and opportunities for residual oil hydrogenation technology.The internals development is critical to adapt the long-term stable operation.In this paper,the volumetric mass transfer coefficient,gas holdup and bubble size in a gas-liquid up-flow column are studied with two kinds of internals.The gas holdup and volumetric mass transfer coefficient increase by 120% and 42% when the fractal dimension of bubbles increases from 0.56 to 2.56,respectively.The enhanced mass transfer processing may improve the coke suppression ability in the slurry reactor for residual oil treatment.The results can be useful for the exploration of reacting conditions,scale-up strategies,and oil adaptability.This work is valuable for the design of reactor systems and technological processes.

MASS TRANSFER CHARACTERISTICS OF THE MEMBRANEOUS GAS DISTRIBUTOR

1 INTRODUCTIONIt is well known that the throughput of many gas-liquid reactors is limited by the rate atwhich a gaseous component can be transferred from the gas to the liquid.For example,it isthe oxygen transfer capacity of a fermentor that set the upper limit to the productivity ofmost aerobic fermentation.Therefore,studies have been in progress to increase the masstransfer rate between the gas bubble and the broth by acting on classical parameters of bubblesize and the turbulence of flow.However,the intensive turbulence of flow usually

Mass Transfer Characteristics of H_2 and CO in Mimicked F-T Slurry Bubble Column Reactor

The equilibrium solubilities,volumetric gas-liquid mass transfer coefficients kLa of H_2 and CO were measured as functions of temperature(298―513 K),pressure(1―3 MPa),superficial gas velocity(0.5―3 cm/s) and solid volume fraction(5%―25%) in liquid paraffin/iron-based catalyst slurry bubble column reactor.The volumetric mass transfer coefficients kLa were obtained by measuring the dissolution rate of H_2 and CO.The influences of the operation conditions,such as pressure,temperature,superficial gas velocity and catalyst concentration on kLa,were investigated.Two empirical correlations were proposed to predict kLa values of H_2 and CO in liquid paraffin/solid particles slurry bubble column reactor.The results showed that the equilibrium solubilities of H-2 and CO increased with an increasing temperature and pressure,and the solubility of CO was greater than that for H_2.It was found that the equilibrium solubility can be expressed by Henry's law.The volumetric mass transfer coefficients of H_2 and CO were of the same order of magnitude,and increased with the increase of pressure,temperature and superficial gas velocity.The presence of solid particles decreased kLa values of both H_2 and CO.

MASS TRANSFER AT GAS EVOLVING MERCURY ELECTRODE

Mass transfer at hydrogen evolving mercury electrode has been studied by determining mass transfer coefficients of indicator ions as the function of current density of hydrogen evolution for various kinds of indica tor ions in sulphuric acid solution.Mercury adhering to metal substrates of Ni,Cu or Ag is used as mercury electrode to overcome its trembling during the electrolysis.The effect of difTerent indicator ions,electrode character istic,substrate material,diameter of electrode and temperature is examined.It is found that the transfer coefficient is proportional to the mass square root of current density of hydrogen evolution.

Mass transfer in the absorption of SO_2 and NO_x using aqueous euchlorine scrubbing solution

Attempts have been made to generate euchlorine gas by chlorate-chloride process and to utilize it further to clean up SO2 and NOx from the flue gas in a lab scale bubbling reactor. Preliminary experiments were carded out to determine the gas and liquid phase mass transfer coefficients and their correlation equations have been established. Simultaneous removal of SO2 and NOx from the simulated flue gas using aqueous euchlorine scrubbing solution has been investigated. Euchlorine oxidized NO into NO2 completely and the later subsequently absorbed into the scrubbing solution in the form of nitrate. SO2 removal efficiency around 100% and NOx removal efficiency around 72% were achieved under optimal conditions. Mass balance has been confirmed by analyzing the sulfate, nitrate, euchlorine and chloride ion using ion chromatograph/auto-titrator and comparing it with their corresponding calculated values.

Modeling bubble column reactor with the volume of fluid approach:Comparison of surface tension models

This work aims at comparing surface tension models in VOF(Volume of Fluid) modeling and investigating the effects of gas distributor and gas velocity. Hydrodynamics of a continuous chain of bubbles inside a bubble column reactor was simulated. The grid independence study was first conducted and a grid size of 1.0 mm was adopted in order to minimize the computing time without compromising the accuracy of the results. The predictions were validated by comparing the experimental studies reported in the literature. It was found that all surface tension models can describe the bubble rise and bubble plume in a column with slight deviations.

Experimental and Numerical Study of Gas-Liquid Flow in a Sectionalized External-Loop Airlift Reactor

The external loop airlift reactor(ELALR)is widely used for gasliquid reactions.It’s advantage of good heat and mass transfer rates compared to conventional bubble column reactors.In the case of fermentation application where a medium is highly viscous and coalescing in nature,internal in riser helps in the improvement of the interfacial area as well as in the reduction of liquidphase back mixing.The computational fluid dynamic(CFD)as a tool is used to design and scaleup of sectionalized external loop airlift reactor.The present work deals with computational fluid dynamics(CFD)techniques and experimental measurement of a gas holdup,liquid circulation velocity,liquid axial velocity,Sauter mean bubble diameter over a broad range of superficial gas velocity 0.0024≤UG≤0.0168 m s 1.The correlation has been made for bubble size distribution with specific power consumption for different plate configurations.The effects of an internal on different mass transfer models have been completed to assess their suitability.The predicted local mass transfer coefficient has been found higher in the sectionalized external loop airlift reactor than the conventional ELALR.

MASS TRANSFER TO LIQUID-LIQUID INTERFACE

Experiments have been done on mass transfer to a liquid-liquid interface on which inert gas bubbles are sparged.To simulate the pyrometallurgy system of melten slag-metal(or matte),aqueous solution-mercury(or zinc amalgam) system was used.The mass transfer coefficients of indicator ions as a function of bubble parameters have been determined.The experimental results show satisfactory agreement with the mass transfer model proposed Previously.

低冷量下强化CO_(2)吸收的甲醇基纳米流体性能

冷甲醇洗法捕集CO_(2)技术要求低温环境,需要大量的冷冻能量,能耗较大。在吸收剂中引入纳米颗粒,可以有效提高气液传质速率,降低能耗。本研究旨在开发冷量需求少,吸收、解吸性能提升的甲醇基纳米流体。实验分别考察了纳米颗粒种类、固含量、尺寸,表面活性剂含量、操作温度和初始CO_(2)浓度等因素对吸收剂捕集CO_(2)性能的影响及机理。结果表明,在0.2~1.0g/L的TiO_(2)、Al_(2)O_(3)、SiO_(2)纳米流体中,0.4g/L的TiO_(2)-甲醇纳米流体的CO_(2)吸收、解吸增强效果最佳。向其中添加0.10%(质量分数)聚乙二醇辛基苯基醚(Triton X-100)后,纳米流体的吸收、解吸增强效果达到最大,且在5次循环后仍呈现良好增强效果。此外,本文对纳米流体增强吸收的机理进行了深入分析,提出了一个经验公式来预测TiO_(2)-甲醇纳米流体的增强因子E和最佳固含量。

多孔板鼓泡塔流动与传质特性数值模拟

采用Euler-Euler双流体模型对安装不同数量的水平多孔板的鼓泡塔内气液两相流动及传质特性进行数值模拟研究,并探究了不同表观气速条件对鼓泡塔内气含率、气泡直径分布和气液传质系数的影响。结果表明,不同数量的多孔板和不同位置的多孔板都会影响气含率的分布;随着多孔板的数目增加,鼓泡塔液相上方区域的气含率增加,但影响区域有限;安装多孔板后,鼓泡塔内径向位置的平均气含率变化明显,出现“M”形状的分布。不同表观气速下,未安装多孔板的鼓泡塔内直径为1~2mm的微小气泡占比超过30%;安装多孔板后,微小气泡占比明显增加;气液传质系数在中心区域(径向无量纲为-0.5~0.5)较为平缓,波动不大。最后将模拟计算得到的气液体积传质系数与Akita的关联式计算值进行比较,本文计算结果略高。

STUDY OF DOWNFLOW LIQUID JET LOOP REACTOR

The hydrodynamic and mass transfer characteristics of a downflow liquid jet loop reactor (D-JLR) were studied experimentally with water/air and CMC (carboxymethyl cellulose) solution/air systems. The effects of the geometry, the operating parameters and the physical properties of the liquid phase on gas hold-up and mass transfer coefficient were measuered. Compared with other types of gas-liquid reactor, D-JLR shows higher mass transfer coefficient and lower energy dissipation rate, the optimum diameter ratio was found to be about 0.42-0.6. A model for gas hold-up in D-JLR with Newtonian and non-Newtonian fluids has been developed on the basis of the equation of motion and the concept of average mixing length. The prediction of gas hold-up with the model agreed with the experimental results of this work.

水动力传质在臭氧氧化水处理工艺的研究进展

臭氧氧化具有氧化能力强、反应速率快、对环境友好等特点,在水处理工艺中受到广泛关注。但臭氧在水处理中的应用存在利用率低、能源和运营成本高等问题。根据反应器的复杂程度以及规模扩大化应用,水动力传质技术是提高气液传质和臭氧氧化效率的重要手段,这已成为臭氧强化传质发展的趋势。文中阐述了传统反应器和微气泡反应器面临的问题,介绍了基于填料式反应器、振荡流反应器、气-液膜接触器、水动力空化等多种水动力传质技术在臭氧氧化水处理工艺的研究进展,并讨论了水动力传质技术在臭氧氧化工艺未来的发展方向以及待解决的问题。

气体分布器对气升式反应器内流场和传质特性的影响

在气升式反应器冷模实验装置中,研究了气体分布器类型及结构参数对流场和传质的影响规律。结果表明:与环形分布器和四喷嘴分布器相比,采用半球形分布器时平均气含率更高、气含率径向分布更均匀、气泡直径更小、比表面积更大、下降段液速更大、液相体积传质系数更高。气升式废水处理反应器中选用半球形气体分布器可获得更好的混合和传质效果。进一步考察了半球形分布器开孔率和开孔角度对气含率、下降段液速和体积传质系数的影响,确定了最优开孔率为0.457%、最优开孔角度为45°。研究结果可为气升式反应器的工业设计和操作优化提供依据。

气液反应器中气泡有效利用率(Ⅰ)——新概念的提出和理论计算式

针对气液反应器,提出了一个新概念——气泡有效利用率。通过理论分析及推导,得到在假定条件下的单气泡有效利用率η_s与准数φ_b的关系式。准数φ_b关联了物性条件、设备条件和操作条件,它间接反映了这三个条件对气泡有效利用率η_s的影响。并推导出不同大小和不同停留时间分布的气泡群的气泡利用率的计算式,对气液反应器的设计有重要意义。

微量醇及电解质强化气液传质的研究

在气升式内环流反应器中考察了微量醇类物质(乙醇、叔丁醇、季戊四醇)及电解质(CaCl_2)对氧的液相体积传质系数和气含率的影响。结果表明,向空气-水体系中添加微量醇类物质可以明显改善气液传质性能,同时醇类物质强化气-液传质存在一极限浓度,超过此浓度后,氧的液相体积传质系数略有降低;当表观气速增加时,去离子水体系和含醇体系的气含率都增加,但醇体系的气含率明显高于去离子水体系。此外,添加电解质 CaCl_2也会使氧的液相体积传质系数有所提高,并从离子强度的角度对其抑制气泡聚并的机理进行了分析。

鼓泡床反应器内流动与传质行为的研究进展

总结了有关浆态鼓泡床反应器内流动、混合用气液传质特性的研究成果,详细地介绍了鼓泡床反应器内气含率、液速、液体轴向扩散系数、传质系数的测量方法,阐述了鼓泡床反应器性能的主要影响因素,如系统压力、温度、气体表观气速、液体性质及固含率等对流动、液相混合和传质特性的影响,并对鼓泡床反应器的应用前景进行了详述。