HYSTERESIS OF GAS-LIQUID MASS TRANSFER IN A TRICKLE BED REACTOR

1 INTRODUCTIONTrickle bed reactors are widely used in the process industry,particularly in petroleumhydroprocessing operations,and have been extensively studied by chemical engineers.In atrickle bed reactor,the gas and liquid flow cocurrently down through the packed bedand undergo chemical reactions.However,there exist multiple hydrodynamic stateswhich correspond to either uniform or,in most cases,nonuniform radial distributionof the gas and liquid flows in the packed section.Moreover,the hydrodynamic state is     

MASS TRANSFER WITH CHEMICAL REACTION IN A FROTH BED REACTOR

A model to predict conversions in a froth bed reactor has been developed. The model is then compared against the available experimental data on the oxidation of sodium sulfide in a foam bed contactor. The predictions using the present model are also compared against those based on the model of a foam bed reactor developed earlier. The predictions using the present model agree fairly well with the experimental data and, in some cases, are even in better quantitative agreement than the previous single stage model of a foam bed reactor. The case of significant surface resistance due to surfactant has also been analyzed theoretically, obtaining analytical solutions for the concentration profile and fractional gas absorption in a liquid froth shell.     

THE EFFECT OF PARTICLE DILUTION ON WETTING EFFICIENCY AND LIQUID FILM THICKNESS IN SMALL TRICKLE BEDS

Partial wetting in small scale trickle bed reactors results in incorrect determination of intra-particle apparent kinetic parameters as well as in erroneous reactor scale-up. Although a dilution of catalyst particles with inert fines improves the catalyst wetting efficiency, it does not guarantee full external catalyst wetting at all superficial liquid mass velocities. In this work, a method is presented to relate the wetting efficiency obtained at different operating conditions and at different laboratories for diluted and non-diluted beds. Liquid film thickness in diluted and non-diluted beds is estimated. The effect of the operating conditions on partial wetting and liquid film thickness is discussed.     

PARTICLE-LIQUID MASS TRANSFER IN MULTI-IMPELLER AGITATED THREE PHASE REACTORS

The mass transfer coefficient between solid particle and liquid has been measured in high aspect ratio multiimpeller agitated three phase (gas-liquid-solid) reactors. Experiments were conducted in 0.15 and 0.30 m id acrylic columns of lm length each. Two types of impeller were used: disk turbine (DT) and pitched turbine downflow (PTD). Air used as the gas phase was introduced through a ring sparger of 0.8 D size located 0.1 T distance above the bottom. The interimpeller spacing was maintained at the tank diameter. The range of gas velocity used was 0-15 mm/s. A unique correlation between particle-liquid mass transfer coefficient and critical impeller speed for solid suspension has been presented.     

气液固三相循环流化床气液传质行为

<正>气液固三相流化床反应器在石油化工、湿法冶金、环境工程和煤的液化等工业领域得到了广泛应用,其基础研究也取得了很大进展.但是,传统三相床主要应用于低液速(U_L<     

NUMERICAL MODELING OF MHD CONVECTIVE HEAT AND MASS TRANSFER IN PRESENCE OF FIRST-ORDER CHEMICAL REACTION AND THERMAL RADIATION

An analysis was carried out numerically to study unsteady heat and mass transfer by free convection flow of a viscous, incompressible, electrically conducting Newtonian fluid along a vertical permeable plate under the action of transverse magnetic field taking into account thermal radiation as well as homogeneous chemical reaction of first order. The fluid considered here is an optically thin gray gas, absorbing-emitting radiation, but a non-scattering medium. The porous plate was subjected to a constant suction velocity with variable surface temperature and concentration. The dimensionless governing coupled, nonlinear boundary layer partial differential equations were solved by an efficient, accurate, extensively validated, and unconditionally stable finite difference scheme of the Crank-Nicolson type. The velocity, temperature, and concentration fields were studied for the effects of Hartmann number (M), radiation parameter (R), chemical reaction (K), and Schmidt number (Sc). The local skin friction, Nusselt number, and Sherwood number are also presented and analyzed graphically. It is found that velocity is reduced considerably with a rise in the magnetic body parameter (M), whereas the temperature and concentration are found to be markedly boosted with an increase in the magnetic body parameter (M). An increase in the conduction-radiation parameter (R) is found to escalate the local skin friction (τ), Nusselt number, and concentration, whereas an increase in the conduction-radiation parameter (R) is shown to exert the opposite effect on either velocity or temperature field. Similarly, the local skin friction and the Sherwood number are both considerably increased with an increase in the chemical reaction parameter. Possible applications of the present study include laminar magneto-aerodynamics, materials processing, and MHD propulsion thermo-fluid dynamics.     

SCALE ANALYSIS AND PARAMETRIC STUDY OF TRANSIENT HEAT/MASS TRANSFER IN THE PRESENCE OF NONPOROUS SOLID ADSORPTION

A transient, two-dimensional theoretical analysis of combined heat and mass transfer in the presence of adsorption/desorption is developed to study the fundamentals of heat and mass transfer dynamics. A parallel-plate rectangular channel is used as a model system. Appropriate surface boundary conditions for heat and mass transfer and adsorption/desorption interactions are formulated. A scale analysis of the governing equations is performed in order to identify the dimensionless physical parameters governing the process and to obtain the order-of-magnitude estimates for characteristic time constants of the system dynamics. Predictions of the scale analysis are validated against the results of the parametric study obtained through the numerical solutions of the governing equations. The findings demonstrate that the scale analysis is a very powerful analytical tool which allows one to evaluate the effects of different process parameters on heat and mass transfer dynamics in the presence of adsorption without performing the exhaustive numerical calculations.     

EFFECT OF FINE SOLID PARTICLES ON GAS-LIQUID MASS TRANSFER RATE IN A SLURRY REACTOR

The influence of suspended fine particles of differing adsorbing capacity (activated carbon, avicell cellulose, SiO₂, and molecular sieves) on the liquid-film mass transfer coefficient κ_L, was examined experimentally in a stirred cell of well-defined gas-liquid interfacial area by chemical methods. The greatest effect on κ_L was observed with the activated carbon (about a threefold increase). An addition of glycerine in excess of 2.5 × 10^-3 k mol/m³, which was adsorbed on the particles preferentially, removed this increase in κ_L totally. Amongst other particles, only avicell cellulose showed measureable effects. The increase in κ_L values was inversely proportional to the temperature and the stirring speed, and the particle loading was found to be immaterial after a certain value. When the reaction rate increased (hence, when the thickness of dissolved gas-rich layer decreased) by gradual addition of a homogeneous catalyst (i.e. Co⁺⁺ for sulphite oxidation), the effect of particles on κ_L decreased and eventually it disappeared.     

LIQUID-SOLID MASS TRANSFER IN A THREE PHASE DRAFT TUBE FLUIDIZED BED REACTOR

Liquid-solid mass transfer coefficients in a three phase draft tube fluidized bed reactor have been measured using spherical ion exchange particles. The particle diameters ranged from 655 to 1119μm and solids volume fractions of approximately 5 and 10% were employed in water at 28°C. The experimental data can be successfully correlated using a Reynolds number derived using Kolmogoroffs theory of isotropic turbulence, although it is doubtful whether isotropic turbulence actually prevails in the fluidized bed over the range of conditions employed. Comparison with correlations determined for bubble columns and gas-liquid fluidized beds is performed. A model which considers the draft tube reactor as comprising two distinct fluid mechanical regions is developed to explain the apparently lower values of mass transfer coefficients obtained in a draft tube as opposed to conventional fluidized bed reactor.     

THE KINETIC THEORY OP GASES AND MASS TRANSFER IN POEES IN THE PRESENCE OF HETEROGENEOUS PHASE AND CHEMICAL CONVERSIONS

The paper presents the investigation of gas transfer in cylindrical channels in the presence of heterogeneous physicochemical conversions (filler evaporation in a channel, evaporation on the internal surface of a channel, catalytic reaction of the first order). With the use of the model kinetic equations for the gas molecule velocity distribution function, the solutions of the above problems have been obtained, suitable for the arbitrary ratios of free molecule path length to capillary radius, i.e. the Knudsen number. The comparison between the theoretical and experimental results on filler surface recession in a capillary during evaporation in the presence of a foreign gas is carried out. The applicability limits for the well-known phenomenological relations, used in the theory of drying, are established.     

流化床反应器气固传热面积模型

研究了流化床反应器内发生气固反应时间的传热机理,认为气固间的传热面积包括两部分,即气泡内所含颗粒的表面积与一气泡和泡晕间的有效传热面积 ,据此首次导出了气固传热面积模型A＝u0-umf/22.26db0.5-umf[4.5unf ρgCpg/db 5.85(λgρgCpg)0.5g^0.25/db^1.25]LmS/a将该模型 应用于半间歇的裂化催化剂烧炭再生过程,与实验数据的比较表明,模型预测是可靠的。     

IONIC MASS TRANSFER IN THREE-PHASE FLUIDIZED BEDS IN THE PRESENCE OF DISC PROMOTERS

Three-phase fluidized beds have wide applications in process industries. The present investigation is carried out to identify the enhancement of ionic mass transfer coefficients due to the presence of a disc promoter in a three-phase fluidized bed. A diffusion-controlled electrode reaction—reduction of ferricyanide ion—was employed to obtain mass transfer coefficient data. The mass transfer coefficient data were obtained by varying the geometric variables of the disc promoter (disc diameter, disc spacing) and dynamic variables (superficial liquid velocity, superficial gas velocity). The effect of particle diameter was also investigated. The investigations revealed that the mass transfer coefficients were enhanced with decreased disc spacing, increased disc diameter, increased superficial gas velocity, increased superficial liquid velocity, and increased particle diameter.     

HEAT AND MASS TRANSFER IN THE ADSORBENT BED OF AN ADSORPTION HEAT PUMP

The heat and mass transfer equations governing an adsorbent bed in an adsorption heat pump and the mass balance equation for the adsorbent particles in the adsorbent bed were solved numerically to simulate the cycle of a basic adsorption heat pump, which includes isobaric adsorption, isosteric heating, isobaric desorption, and isosteric cooling processes. The finite difference method was used to solve the set of governing equations, which are highly nonlinear and coupled. The pressures of the evaporator and condenser were 2 and 20 kPa, respectively, and the regeneration temperature of the bed was 403 K. Changes in the temperature, adsorptive pressure, and adsorbate concentration in the adsorbent bed at different steps of the cycle were determined. The basic simulated cycle is presented in a Clausius-Clapeyron diagram, which illustrates the changes in average pressure and temperature of the adsorbent bed throughout the cycle. The results of the simulation indicated that the most time-consuming processes in the adsorption heat pump cycle were isobaric adsorption and isobaric desorption. The high thermal resistance of the bed slows down heat transfer, prolonging adsorption and desorption processes.     

SOLID-LIQUID MASS TRANSFER STUDY OF THE OZONATION OF PHENANTHRENE

Phenanthrene, a priority, hazardous pollutant, is found in the vapor and paniculate phase in the atmosphere as well as in aquatic ecosystems. Water droplets can form around the particulates and dissolve the phenanthrene. Subsequent oxidation of the phenanthrene by ozone can result in the formation of secondary pollutants. Thus the mass transfer of solid phenanthrene with and without ozonation reactions in water is investigated and a simple mathematical model based on film theory is formulated to predict the changes in bulk ozone concentrations and enhancement factors. Physical mass transfer coefficients for phenanthrene in water measured at room temperature in an agitated vessel increased from 1·174 ×10^-5 to 2·932 × 10^-5 m/s as the agitation speed increased from 65 to 315 RPM. The ozonation of phenanthrene significantly increases the mass transfer rates of phenanthrene into water as indicated by enhancement factors which range from 2 to 7·8. At low agitation speeds, the film model agreed well with the experimental data regarding the rates of disappearance and concentrations of the bulk ozone. However, the calculated and measured concentrations of ozone deviated as much as 55% at high speeds of agitation probably due to turbulent effects. Nevertheless, the model is useful for estimating the enhancement factor; the theoretical and experimental values are in good agreement with the maximum deviation of 25%.     

LIQUID-PHASE MASS TRANSFER COEFFICIENT IN SLURRY BUBBLE COLUMN REACTORS: THEORY AND EXPERIMENTAL DATA IN SIMULATED FERMENTATION MEDIA

Mass transfer in a slurry bubble column reactor was examined. A theoretical correlation for liquid-phase mass transfer coefficient was developed using a combination of Higbie's penetration theory and the Einstein-Li periodic viscous sublayer model. The proposed correlation predicts that the liquid-phase mass transfer coefficient decreases with an increase of the yield stress of slurries. The measurement of mass transfer rates was conducted in a 40-L bubble column reactor. The slurries containing low-density particles were simulated by aqueous carboxypolmethylene solutions. Satisfactory agreement was found between the proposed correlation and the experimental data.     

环流反应器和搅拌槽传质特性的分析

本文测定了提升管为单段和提升管分为三段的气升式环流反应器中的液相体积氧传质系数，在螺带式搅拌桨区应器中测定了液相体积氧传质系数和功率消耗，实验体系为模拟生物发酵液非牛顿流体特性的核甲基纤维素水溶液，本文还从液相体积氧传质系数及单位液相体积的功耗所产生的氧传质效果方面对提升管不分段和提升管分为三段的气升式环流反应器与螺带式搅拌反应器进行了比较。     

SOLID-LIQUID MASS TRANSFER AT THE WALLS OF A RECTANGULAR AGITATED VESSEL

Rates of mass transfer between the walls of a rectangular agitated vessel and solution were studied by measuring the limiting current of the cathodic reduction of ferricyanide ion at the vessel wall. Variables studied were rotation speed of 45° pitched blade turbine impeller, physical properties of the solution and the presence of drag reducing polymer in lhe solution (Polyox WSR-301). The data were correlated for polymer free solution by the equation:

Sh= 1.925 Sc^0.33 Re^0.5

Comparison of the present data with previous heat and mass transfer studies conducted in cylindrical vessels has shown that the mass transfer behaviour of agitated rectangular vessels lie between baffled and unbaffled agitated cylindrical vessels. Polyox addition was found to reduce the mass transfer coefficient by an amount ranging from 5.6 to 37% depending on polymer concentration. Practical implications of using drag reducing polymers in agitated vessels were discussed.     

浓度对传质系数的影响及多元物系传质

在筛板塔中对甲醇-乙醇-水三元物系进行精馏传质过程实验,考虑浓度对传质系数的影响建立了一个数学模型,对各组分在塔中的浓度分布进行模拟计算,计算结果能正确反映各组分在塔中浓度分布的变化规律.实验与计算结果都表明在某些浓度范围内乙醇呈现出比较复杂的变化规律.     

MASS TRANSFER WITH CHEMICAL REACTION IN PARTIALLY WETTED FLAT PLATE CATALYST PELLETS: RIVULET FLOW ANALYSIS

Mass transfer with chemical reaction is analyzed in a system formed by a flat plate solid catalyst, partially wetted by a flowing rivulet of a liquid in contact with a stagnant pure gas. The paper solves the fluid dynamic problem of the liquid phase first, and afterwards incorporates the mass transfer and the chemical reaction. The system is assumed to be isothermal and at steady state, with a first order kinetics whose limiting reactant is in the gas phase. This work studies the influence of the gas-liquid surface tension, the liquid reactant flow rate, the liquid viscosity and the angle of inclination of the solid, upon the wetting factor. The model proposed also predicts the effect of these parameters and the Thiele modulus on the overall effectiveness factor and the molar flux of the limiting gaseous reactant at the catalytic solid-liquid interface in a direct way. This approach makes the wetting factor a non-manipulated variable.     

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 means