Vapour- and liquid-side volumetric mass transfer coefficients measured in distillation column. Comparison with data calculated from absorption correlations

Volumetric mass transfer coefficients in liquid and vapour phases in distillation column were measured by the method consisting of a fitting of the concentration profile of liquid phase along the column obtained by the integration of a differential model to the experimental one. The mathematical model of distillation process includes mass and energy balances and the heat and mass transfer equations. The film model flux expressions with the convective transport contributions have been considered in the transfer equations. Vapour and liquid phases are supposed to be at their saturated temperatures along the column. Effect of changes of phase flows and physical properties of phases on the mass transfer coefficients along the column and non-ideal thermodynamic behaviour of the liquid phase have been taken into account. The concentration profiles of liquid phase are measured in the binary distillation of the ethanol-water and methanol-ethanol systems at total reflux on metal Pall Rings and Intalox saddles 25 mm in the column with diameter of 150 mm. The distillation mass transfer coefficients obtained by the fitting procedure are compared with those calculated from absorption data using Onda's, Billet's and Linek's correlations. The distillation heat transfer coefficients calculated from the model assuming saturated temperatures in both phases are compared with those calculated from the Chilton-Colburn and penetration model analogy between mass and heat transfer. The results have confirmed an agreement neither between distillation and from absorption correlations calculated mass transfer coefficients nor between analogy and from enthalpy balance calculated heat transfer coefficients. Also the concentration profiles obtained by the integration of the differential model of the distillation column using the coefficients from absorption correlation have differed from the experimental profiles considerably.     

封存CO2裹挟可挥发性污染物的迁移模型分析

为了研究封存CO₂注入过程可挥发性污染物的释放和迁移特性,本文建立了CO₂-水-残余油多相流驱替过程中的污染物迁移模型。采用数学模拟方法分析了多相驱替过程和污染物迁移过程,并研究了相间传质特性、初始油相分布和CO₂注入速率等对污染物迁移过程的影响。研究表明:CO₂驱替过程将促使可挥发污染物进入CO₂相并随之在地层中迁移,逐渐形成相间传质区域。相间传质区域的演化反映了污染物释放和CO₂裹挟污染物迁移的特性。可挥发污染物的传质系数越大,相间传质区域越窄,油相饱和度衰减越快;油相初始饱和度较大时,其饱和度衰减相对缓慢,对应的相间传质区域也较窄。当CO₂注入速率增大时,相间传质区域增大,油相饱和度衰减变快。本文模型可用于不同地质储层环境下封存CO₂时可挥发性污染物的迁移特性分析,并用于封存CO₂的风险分析与评价。     

The performance of rotary concentric cylinder fractionating columns—1

A type of laboratory fractionating column is described, consisting essentially of two concentric glass cylinders of which the inner one rotates. This apparatus is of general use in preparative work, giving an exceptionally low hold-up, pressure drop and H.E.T.P., and having no bearings or shaft seals in the working section. It also offers a convenient means of studying the general properties of film-type rectifying systems ; it functions as a wetted-wall column in which the vapour flow regime may be changed in a predictable way without alteration of throughput by changing the form and speed of the rotor.

A technique is described for determining liquid and vapour side mass transfer resistances. In experiments at atmospheric pressure with a mixture of n-heptane and methyl cyclohexane it is shown that the separating power of the column is controlled by the vapour phase mass-transfer resistance, the liquid side resistance being negligible. Attempts are made to relate the overall column characteristics to the fluid mechanical properties of the vapour stream under various conditions of operation.     

Effect of bubble interaction on interphase mass transfer in gas fluidized beds

A non-interfering technique has been used to measure the concentration of ozone in pairs of bubbles injected into a bed of inactive 390 μm glass beads fluidized by ozone-free air. The transfer of the ozone tracer from the bubble phase to the dense phase is enhanced when compared to the transfer from isolated bubbles in the same particles and the same column. Bubble growth is also greater for the case where pairs of bubbles are introduced than when bubbles are present in isolation. Enhancement of interphase mass transfer for interacting bubbles in the present work and in previous studies incr with particle size and can be explained in terms of enhancement of the throughflow (or convective) component of transfer while the diffusive component unaltered. This mechanism leads to new equations for estimating interphase mass transfer in freely bubbling fluidized beds.     

Absorption and desorption of carbon dioxide in several water types

This study investigated the effect of water type on the rate of CO₂ transfer from/to an aqueous phase with varying degree of water salinity. The absorption and desorption experiments were conducted on reverse osmosis product, brackish well, and brackish water reverse osmosis reject waters as well as seawater in a mechanically agitated tank. Results show that the direction of mass transfer has a major impact on the value of the volumetric mass transfer coefficient, k_La, with the absorption experiments always rendering higher values. Furthermore, k_La values always decreased with salinity in both absorption and desorption experiments until a certain critical salinity value was reached, beyond which mass transfer increased again. However, k_La values were found to decrease continuously with an increase in the water alkalinity in absorption experiments, while no clear conclusion could be drawn for the alkalinity effect in the case of desorption experiments. These observations suggest that the effect of alkalinity should be further investigated to elucidate its impact along with the salinity on the volumetric mass transfer rate.     

Biodegradation Phenomena during Soil Vapor Extraction. III. Sensitivity Studies for Two Substrates

Abstract

In the bioventing technique, soil vapor extraction (SVE) is used to promote aerobic biodegradation of contaminants in the vadose zone. Kinetics limited by mass transport of the contaminant and/or oxygen through the aqueous phase to the microorganisms and of contaminant to the gaseous phase may be expected during field operation. Sensitivity studies were performed with a one-dimension model for two substrates showing competitive inhibition, following Monod's kinetics. The mass transfer limitations were represented by means of lumped parameters, and results for high and low values of these parameters were compared. Under kinetics severely limited by mass transport processes, biodegradation occurs at a rate given by the availability of dissolved oxygen, and important contributions of biological degradation to the overall cleanup are expected if oxygen is not utilized exclusively for the oxidation of other substances than the target contaminant. For relatively fast mass transport kinetics the system becomes quite sensitive to a rather large number of parameters, but important reductions in the remediation time will usually occur if high removal percentages are mandated.

     

Activity evaluation of a catalytic distillation packing for MTBE production

The octane enhancer methyl tertiary butyl ether (MTBE) can be produced very efficiently from methanol and isobutene in a reactive distillation column where the heterogeneous catalyst also acts as distillation packing. Some mathematical models have been published for the simulation of such a process but they focus on the physical transport processes between the vapour and liquid phases. However, the aim of this paper is to analyze the importance of the internal and external multicomponent mass and heat transfer phenomena on the catalyst under boiling conditions. Therefore, experiments were carried out in a reactive distillation column at different compositions of feed, column pressures and reflux ratios using a Raschig ring shaped acidic ion exchange resin as the catalyst. The temperature and composition of the liquid phase entering and leaving the catalytic column section were measured. These data were used to evaluate the effectiveness factor of the catalyst with a rigorous macrokinetic model. It is shown that the effectiveness factor varies significantly along the column length. Under certain operating conditions, decomposition of MTBE can occur accompanied by boiling processes inside the catalyst macropores.     

Development of design charts for fixed-bed adsorption

A mathematical model of the dynamic behaviour of non-isothermal fixed-bed adsorbers has been developed which takes into account the various mass and heat transfer resistances. Comparison of experimental and simulated results confirms that the model can predict the adsorption and desorption breakthrough curves of an adiabatically operated column, using only equilibrium data and tortuosity factors obtained from single pellet experiments. A simplified model with a reduced number of parameters was derived by investigation of the dimensionless transfer parameters under industrial conditions. It becomes evident that the main transfer mechanisms are convective heat and mass transfer in the bulk flow and diffusion within the pores of the particle. Dimensionless effluent concentration is expresses in terms of dimensionless time, a transport parameter, a non-isothermal parameter, the adsorption equilibrium and the inlet and initial concentrations and temperatures in the simplified model. For a chosen system of adsorbate and adsorbent, design charts can be developed by computer simulation, to determine graphically the breakthrough time as a function of significant process parameters, i.e. the dimensionless transfer parameter and the feed concentration.     

LIQUID PHASE TRANSPORT PROPERTIES IN A HIGH PRESSURE PACKED COLUMN

A mathematical model of fluid flow and mass transfer in a packed bed was derived and used to evaluate the liquid phase axial dispersion and mass transfer coefficients under high pressure conditions. The least-squares method was used to evaluate the rate parameters from experimental breakthrough curves, and the agreement between the concentration curves predicted from rate parameters and those measured experimentally was good. Experiments were performed at 20 and 200°C with water as a solvent and nonporous soda-lime glass beads as packing. Although the axial dispersion coefficient was independent of temperature and pressure, the mass transport parameters were found to be pressure dependent.     

A kinetics model for interphase formation in thermosetting matrix composites

During the cure of thermosetting polymer composites, the presence of reinforcing fibers significantly alters the resin composition in the vicinity of the fiber surface via several microscale processes, forming an interphase region with different chemical and physical properties from the bulk resin. The interphase composition is an important parameter that determines the micromechanical properties of the composite. Interphase development during processing is a result of the mass‐transport processes of adsorption, desorption, and diffusion near the fiber surface, which are accompanied by simultaneous cure reactions between the resin components. Due to complexities of the molecular‐level mechanisms near the fiber surface, few studies have been carried out on the prediction of the interphase evolution as function of the process parameters. To address this void, a kinetics model was developed in this study to describe the coupled mass‐transfer and reaction processes leading to interphase formation. The parameters of the model were determined for an aluminum fiber/diglycidyl ether of bisphenol‐A/bis(p‐aminocyclohexyl)methane resin system from available experimental data in the literature. Parametric studies are presented to show the effects of different governing mechanisms on the formation of the interphase region for a general fiber–resin system. The interphase structure obtained from the model may be used as input data for the prediction of the overall composite properties. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 3220–3236, 2003     

Hydrodynamic and mass transfer performance of multiple upcomer extraction trays

The multiple upcomer (MU) extraction tray was specifically developed for aromatics extraction processes. Hydro-dynamic and mass transfer experiments have been carried out with n-butanol/succinic acid/water as an experimental system in a MU tray extraction column of 0.1 m diameter. The dispersed phase hold-up, drop size and overall efficiency were measured and the hydrodynamic and mass transfer parameters were correlated. The flooding velocities and the overall efficiency predicted from the correlation for this column agreed well with the experimental data.     

Adsorption of Volatile Organic Compounds on Soil and Prediction of Desorption Breakthroughs

Abstract

Vapor extraction of volatile organic compounds from soil was investigated by performing adsorption and desorption experiments of trichloroethylene, trichloroethane, and chlorobenzene on soil particles. The adsorption breakthrough curves were obtained using a dynamic response technique based on frontal analysis chromatography. The shape of the breakthrough curves indicated that the adsorption process was close to an ideal adsorption system of no mass transfer resistance, no axial dispersion, and infinitesimal width of mass transfer zone. The adsorption isotherms were BET Type I for trichloroethylene and trichloroethane, and BET Type II for chlorobenzene. Two types of desorption profiles were observed depending on the compounds, i.e., the continuously decreasing profiles of trichloroethylene and trichloroethane and the stepwise decreasing profiles of chlorobenzene. The desorption profiles of trichloroethylene and trichloroethane were simulated using a local equilibrium theory which indicated that desorption behavior was independent of the number of adsorption layers on soil. For chlorobenzene, the monolayer desorption was the rate-controlling step of the overall desorption process. The effect of moisture on desorption efficiency was significant for chlorobenzene, which showed more unfavorable desorption behavior than trichloroethylene and trichloroethane.     

A comprehensive study on co2-interphase mass transfer in vertical cocurrent and countercurrent gas-liquid flow

Cocurrent and countercurrent absorption and desorption of CO₂ in water was investigated in tall bubble columns (length 440 and 720 cm, diameter 15 and 20 cm, respectively). Operating conditions were applied which provided for high interphase mass transfer rates. Under these circumstances the relative gas holdup varies considerably with axial position whereas the mean bubble diameter measured at two points was found to be approximately constant. The measured data permit the calculation of local values of interfacial areas, superficial gas velocities, and frequency factors for bubble coalescence and break up. A dispersion model which takes into account the hydrostatic head and a variable gas velocity was applied to describe the measured concentration profiles in both phases. If increased mass transfer coefficients at the column bottom and measured local values of the hold up were used a striking agreement between experimental and predicted profiles could be obtained. The findings lead to a more sophisticated picture of the complex behaviour of gas-liquid dispersions at high interphase mass transfer rates.     

Mathematical modeling of gas-liquid mass transfer rate in bubble columns operated in the heterogeneous regime

In this paper, a multi-scale approach is followed to study gas-liquid mass transfer in bubble columns. First, a single bubble of equivalent diameter d is considered. Its morphology and its gas to liquid relative velocity are related to the bubble diameter through the use of known correlations. Then, the gas-liquid mass transfer between the bubble and the surrounding liquid is studied theoretically. An equation describing the transport of the transferred species in the viscous boundary layer around the bubble is solved. In a second step, a bubble column of 6-10 m height is studied experimentally. The gas phase in the column is characterized experimentally by means of a gammametric technique. Finally, the two studies are linked, yielding a 1D mathematical model able to predict the gas-liquid mass transfer rate in a bubble column operated in the heterogeneous regime.     

Soil Clean-Up by Surfactant Washing. IV. Modification and Testing of Mathematical Models

Abstract

Three models for the operation of surfactant washing/flushing columns or test beds are developed. These differ in the manner in which hydrophobic contaminant is held in the soil and, therefore, in the nature of mass transfer of contaminant from the stationary phase to the advecting surfactant solution. The fitting of parameters to experimental results is addressed, following which the parameters obtained are used to simulate operation of laboratory columns and a pilot-scale test bed. The results are compared which experimental data from the column and test bed. The air stripping of biphenyl from spent surfactant solution is modeled using a local equilibrium approach to see if air stripping could account for observed losses. The air stripping of toluene from the surfactant solution is modeled using a local equilibrium approach or a lumped parameter method to model diffusion-limited kinetics.     

Design and Modeling of a Staged Downflow Bubble Reactor

A staged downflow bubble column for ozonation of drinking and wastewater was developed. Equations for the calculation of head loss, the mass transfer coefficient, and power dissipation were proposed For mathematical modeling, the stagewise backmixing model was used. The validity of the model was verified by experiments with raw lake water Ozone absorption in a staged downflow bubble column was shown to represent a high capacity process for ozone uptake in water and wastewater treatment.     

SCALING IN SITU BIOREMEDIATION PROBLEMS BY APPLICATION OF MULTIPHASE, MULTICOMPONENT TRANSPORT THEORY

A methodology for scaling in situ bioremediation problems is presented. This methodology is based on multiphase, muiticomponent transport theory and employs inspectional analysis and numerical sensitivity studies. A general mathematical model that describes subsurface aerobic biotransformation of organic chemical species in a multiphase setting is first presented. This general model is applied to the specific case of microbial enhanced vapor-vacuum extraction (MEVVE) in a one-dimensional zone of immobile liquids. The resulting simplified MEVVE model considers rate-dependent interphase mass transfer, a flowing gaseous phase, a single hydrocarbon pseudo-component, and either oxygen or hydrocarbon limited microbial activity. By inspectional analysis a set of dimensionless groups are derived that represent the various model parameters. A scries of numerical sensitivity studies are presented that examine the impact of selected dimensionless groups on overall system biotransformation rates. This analysis demonstrates that overall biotranformation rates can be significantly limited not only by insufficient transport of oxygen in the gaseous phase, but also by interfacial mass transfer resistance between nonaqueous phase liquid globules and adjacent fluids. Finally, an examination of the selected dimensionless groups reveals the parameter requirements for properly scaled MEVVE tests. These results indicate the need for further investigation of the importantance of fluid distributions in the pore space and their impact on the design of laboratory and field tests.