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.

A dynamic control structure of liquid-only transfer stream distillation column

The intention of this fundamental work is to explore the manipulation of a mixture of benzene,toluene and o-xylene separated from liquid-only transfer divided-wall column(LTS-DWC).First,two control structures are clearly proposed,including seven component control loops(CS1)and seven temperature control loops(CS2).However,two control structures can handle ±10% feed disturbances rather than larger feed disturbances.Subsequently,an equivalent four-column model by introducing withdraw ratio is developed to discuss the effect of two liquid-only side-stream on the overall reboiler duty.It is indicated that the second liquid-only side-stream withdraw ratio strongly affects the overall energy consumption.Hence,six-component control loops within the fixed second liquid-only side-stream withdraw ratio(CS3)is proposed and the purity of products returns to set value even as facing ±20% feed disturbances.Finally,based on the above results,it is established a temperature control structure(CS4)for controlling fixed second liquid-only side-stream withdraw ratio,which can cope with ±15% disturbances.Inspired by these findings,an insight into the dynamic control of LTS-DWC promotes the industrial implementation of DWC through new liquid-only side-stream configurations.

Visual experimental study of nanofluids application to promote CO_(2) absorption in a bubble column

The addition of dispersed-phase nanoparticles in the liquid phase can enhance the gas-liquid transfer process as the suspended nanoparticles affect the transfer process inside the fluid through microdisturbance or micro-convection effects.In this article,a high-speed digital camera was used to visualize the bubble behavior of CO_(2) in pure water and nanofluids to examine the effects of CO_(2) gas flow rate,nanoparticle solid content and type on the bubble behavior in the fluids.The CO_(2) absorption performance in three water-based nanofluids were compared in a bubbler.And the mass transfer characteristics during CO_(2) bubble absorption and the reasons for the enhanced gas-liquid mass transfer effect of nanoparticles were analyzed.The results showed that the presence of nanoparticles affected the formation process of bubbles in the fluid,shortened the bubble detachment time,reduced the detachment diameter,effectively increased the gas-liquid contact area,and improved the bubbles detachment frequency.The system with MCM-41 corresponded to a higher overall mass transfer coefficient.Uncalined MCM-41 contained surfactant that enhanced foaming behavior in water.This prevented the transfer of CO_(2) to some extent,and the CO_(2) absorption by uncalined MCM-41/H_(2)O was 5.34%higher than that by pure water.Compared with SiO_(2) nanoparticles with the same particle size and the same composition,MCM-41 had a higher adsorption capacity and better hydrophilicity due to its larger specific surface area and rich porous structure,which was more favorable to accelerate the collision between nanoparticles and CO_(2) bubbles to cause micro-convection.Under the condition of 0.1%(mass)solid content,the enhancement of CO_(2) absorption process by MCM-41 nanoparticles was more significant and improved by 16.9%compared with pure water.

Mass transfer process in replacement-column purification device in zinc hydrometallurgy

It is important to remove the impurities, such as copper and cadmium, from leaching solution in zinc hydrometallurgy. To improve purification efficiency, a replacement-column purification device was proposed and its mass transfer characteristics and purification efficiency were experimentally studied. The results show that purification efficiency increases with the decrease of the zinc powder diameter and decreases with the increase of solution velocity. If appropriate structure and operation parameters are used, it is possible to make purification efficiency more than 99%, but the diameter of zinc powder should be larger than 0.45 mm. For the velocity of 0.05-0.7 cm/s, mass transfer coefficient kc is in the range of 3.94×10-7-2.76×10-6 m/s, and increases with the decrease of zinc powder diameter and the increase of solution velocity. Moreover, it can be derived by mass transfer correlations of Sherwood number:Sh=0.1069Re0.5Sc0.33, for 0.3<Re<6.

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.

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.

Gas-liquid mass transfer of carbon dioxide capture by magnesium hydroxide slurry in a bubble column reactor

Magnesium hydroxide(Mg(OH)2)has been considered as a potential solvent for CO2 removal of coal-fired power plant and biomass gas.The chemistry action and mass to transfer mechanism of CO2-H2O-Mg(OH)2 system in a slurry bubble column reactor was described,and a reliable computational model was developed.The overall mass transfer coefficient and surface area per unit volume were obtained using experimental approach and simulation with software assistance.The results show that the mass transfer process of CO2 absorbed by Mg(OH)2 slurry is mainly liquid-controlled,and slurry concentration and temperature are main contributory factors of volumetric mass transfer coefficient and liquid side mass transfer coefficient.High concentration of CO2 has an adverse effect on its absorption because it leads to the fast deposition of MgCO3·3H2O crystals on the surfaces of unreacted Mg(OH)2 particles,reducing the utilization ratio of magnesium hydroxide.Meanwhile,high CO3^2– ion concentration limits the dissolution of MgCO3 to absorb CO2 continually.Concentration of 0.05 mol/L Mg(OH)2,15%vol CO2 gas and operation temperature at 35℃are recommended for this CO2 capture system.

Gas-Liquid Mass Transfer in a Slurry Bubble Column Reactor under High Temperature and High Pressure

The gas-liquid mass transfer of H2 and CO in a high temperature and high-pressure three-phase slurry bubble column reactor is studied. The gas-liquid volumetric mass transfer coefficients kLa are obtained by measuring the dissolution rate of H2 and CO. The influences of the main operation conditions, such as temperature, pressure, superficial gas velocity and solid concentration, are studied systematically. Two empirical correlations are proposed to predict kLa values for H2 and CO in liquid paraffin/solid particles slurry bubble column reactors.

Optimization of Dividing Wall Column with Heat Transfer Process Across the Wall for Feed Properties Variation

This paper investigates the thermal-coupled effect across the wall and the optimal heat transfer region of the wall for enhancing the energy saving effect of dividing wall column (DWC), and also studies the effects of feed thermal condition (q) and middle component composition of feed (cB) on the heat transfer process, the optimal heat transfer region, and the maximum heat transfer quantity across the wall. The simulation results show that the maximum heat transfer quantity across the wall and the potential for energy saving increase with the increase of q, while with the limitation of temperature difference across the wall, the beneficial heat transfer effect between certain range of stages, which are involved in the optimal heat transfer region, cannot be realized completely for a specific value of q. Besides, compared with q, a changing cB does not change the degree of realizing the beneficial heat transfer effect, but can bring about the variation of liquid split ratio (RL) and vapor split ratio (Rv). Thus, for achieving a maximum energy-saving effect of DWC, different q and cB need to find its own corresponding suitable heat transfer process across the wall.

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.

Evaluation of the clenbuterol imprinted monolithic column prepared by reversible addition-fragmentation chain transfer polymerization

To make more homogenous organic monolithic structure, reversible addition-fragmentation chain transfer （RAFT） process was employed in the synthesis of the clenbuterol imprinted polymer. In the synthesis, the influence of synthetic conditions on the polymer structure and separation efficiency was studied. The result demonstrated that the imprinted columns prepared with RAFT process have higher column efficiency and selectivity than the columns prepared with conventional polymerization in the present study, which may result from the higher surface area, smaller pore size and the narrower globule size distribution in their structures. The result indicated that RAFT polymerization provided better conditions for the clenbuterol imprinted monolithic polymer preparation. 2009 Xiang Chao Dong. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.

Mass Transfer and Axial Mixing Characteristics in a Coalescence-Dispersion Pulsed-Sieve-Plate Extraction Column

A new configuration of coalescence-dispersed pulsed-sieve-plate extraction column (CDPSEC) was developed, and the mass transfer and axial mixing characteristics were evaluated with the two-point dynamic method.The influence of operation conditions was discussed with experimental results, showing that the mass transfer performance of CDPSEC mainly depends on the energy input and the holdup of dispersed phase. Higher energy input results in higher holdup of the dispersed phase, the axial mixing of the continuous phase is suppressed, and the true height of mass transfer unit decreases markedly. On the other hand, higher energy input leads to more serious forward mixing of the dispersed phase, so the energy input should be limited. Accordingly the operation conditions were divided into two regions, and empirical correlations for predicting the mass transfer and axial mixing characteristics in different regions with a satisfactory accuracy were suggested.

Effect of seismic design details on hysteresis performance of SRC-RC transfer columns

Four types of seismic design details were tested using 11 transfer column specimens and one comparison specimen of RC under low cyclic reversed loading. Test results show that diagonal cracks control the failure pattern and damage occurs mainly in the RC section with weak shear capacity in the transfer columns. There is a large difference in the bearing capacity and ductility of the transfer columns according to the test results, which indicates that the strengthening effect of diverse structural measures is quite different. The section ratio of I-section-encased steel and the axial compression ratio also have a great influence on the bearing capacity and ductility. Although the bearing capacity of transfer columns with additional longitudinal bars and additional X bars is relatively large, they have poor deformation capacity. Setting more stirrups along the columns is the best structural measure to enhance the seismic performance. The studs on the I-sectionencased steel by welding can help to complete the stress transfer between the steel and concrete, and avoid performance degradation of the two materials due to bonding failure.

The Analysis of Distillation Tray Column Efficiency by Fluid Dynamics and Mass Transfer Computation

It has long been found that the flow pattern of the liquid phase on distillation tray is of great importance on distillation process performance. But until now, there was very few published work on quantitative investigation of this subject. By combining the computational fluid dynamics (CFD) with the mass transfer equation, a theoretical model is proposed for predicting the details of velocity and concentration distributions as well as the tray efficiency of distillation tray column. Using the proposed model, four different cases corresponding to different assumptions of liquid and vapor flowing condition for a distillation tray column were investigated. In Case I, the distributions of velocity and concentration of the incoming liquid from the downcomer and the uprising vapor from the underneath tray spacing are uniform. In Case n, the distribution of the incoming liquid is non-uniform but the uprising vapor is uniform. In Case HI, the distribution of the incoming liquid is uniform but the uprising vapor is non-uniform. In Case IV, the distributions of both the incoming liquid and the uprising vapor are non-uniform. The details of velocity and concentration distributions on a multiple sieve tray distillation column in four different cases were simulated using the proposed model. It is found that the shape of the simulated concentration profiles of vapor and the liquid is quite different from case to case. The computed results also show that the tray efficiency is highly reduced by the maldistribution of velocity and concentration of the incoming liquid and uprising vapor. The tray efficiency for Case I is higher than Case Ⅱ or Case Ⅲ, and that for Case Ⅳ is the lowest. It also reveals that the accumulated effect of maldistribution becomes more pronounced when the number of column trays increased. The present study demonstrates that the use of computational method to predict the mass transfer efficiency for the tray column, especially for the large one, is feasible.

Measurement of Mass Transfer Coefficients and Their Modelling of Continuous Counter-Current Aqueous Two-Phase System in a Packed Extraction Column

Overall dispersed side volumetric mass transfer coefficients for protein and amino acids were measured in continuous countercurrent PEG4000/KHP aqueous two-phase systems in a 57mm I.D. packed extraction column. A model for overall dispersed side volumetric mass transfer coefficients was derived by describing the motion of the drops based upon Navier-Stokes equation combined with the relationship between mass transfer coefficients and the drop velocity. The model provides good predictions and can be successfully used in aqueous two-phase extraction. The average relative deviation between calculated values and experimental data ranges from 8% to 14%.

Effect of surfactant type on interfacial area and liquid mass transfer for CO_2 absorption in a bubble column

The effect of different surfactants(n-octyltrimethylammonium bromide(OTABr),sodium dodecyl benzene sulfonate(SDBS) and Tween 80) with different critical micelle concentrations(CMC) on the CO_2 absorption into aqueous solutions in a bubble column is analyzed in the present work.The presence of these surfactants increased the gas-liquid interfacial area,and decreased the liquid phase mass transfer coefficient,but with significant different extent.The results indicated that the CMC can be a key parameter affecting the mass transfer of CO_2 absorption into a dilute aqueous solution of a surfactant.Sardeing's model was used to fit the experimental data successfully by re-correlating the parameters.

Heat Transfer Investigation and Modeling of Heat Integrated Distillation Column

The high degree of reversibility of heat integrated distillation column(HIDiC) has been thermodynamically interpreted by the entropy method. In this paper, a heat transfer model and a more universal method were proposed, through which the overall heat transfer coefficient at different height of column under different operating conditions could be obtained before the experiment. Then the separation of a binary ethanol-water system was carried out experimentally as a case study to verify the heat transfer model and the aforementioned calculation method. The close results between the calculation, the simulation, and the experiments suggested that the proposed model and the calculation method in this paper were accurate and applicable. Meanwhile, it was demonstrated that the HIDiC shows obvious effect of reducing entropy increase and improving thermodynamic efficiency as compared to conventional distillation column.

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.

A General Equation for Calculating the‘True’Height of Transfer Unit in Extraction Columns

Based on single drop mass transfer models and two phase flow equation,a general equationfor calculating the‘true’height of transfer unit of extraction columns was derived and tested withfour types of extraction columns with some different working systems.The calculated results fittedwell with those obtained by experiments.

HYDRODYNAMICS AND MASS TRANSFER IN A NEW TYPE(FG)STACK PACKED EXTRACTION COLUMN

Hydrodynamics and mass transfer experiments were carried out in a 0.1m diameter extraction column with a new type (FG) stack packing developed recently in our laboratory.The working systems used were n-Butano1/Succinic acid/water with low interfacial tension of 1.50×10-3N·m-1 and 30% TBP (Kerosene)/Acetic acid/water with interfacial tension of 10.0×10-3N· m-1. Experimental results obtained indicated that (FG) stack packed extraction column was proved to possess high throughput capacity and excellent mass transfer efficiency. Replacing rotary discs with FG packings in a lube furfural extraction tower was quite success with significant economical benefits being claimed.