MASS TRANSFER AND PHASE HOLDUP CHARACTERISTICS IN THREE-PHASE FLUIDIZED BEDS

The effects of liquid surface tension (42.6 ∼ 72,4 mN/m) and viscosity (1 ∼214mPa • sⁿ), liquid (0.01 ∼0.12m/s) and gas (0.01 ∼0.20m/s) velocities and particle sizes (1 — 8 mm) on phase holdup and mass transfer coefficient ( k_La) have been determined in a 0.142 m-I.D. × 2.0 m-high Plexiglas column. The gas phase holdup increases with liquid velocity, and the rate of increase in gas phase holdup sharply increases with gas velocity in the bed of surfactant solutions. In the beds of 1.0 and 1.7 mm glass beads, the bed contraction occurs whereas in the beds of 2.3 mm glass beads the bed contraction does not occur with an aqueous soltuion of ethanol (σ = 50.4 mN/m). The value of k_La increases with decreasing surface tension (σ ) but it decreases exponentially with increasing liquid viscosity in continuous bubble columns and three-phase fluidized beds. In three-phase fluidized beds with surfactant solutions, k_La increases with gas and liquid velocities and particle size. In three-phase fluidized beds of viscous or surfactant soltuions, k_L,a can be estimated in terms of the energy dissipation rate based on the isotropic turbulence theory and a flow regime map is proposed based on the drift flux theory.     

微通道内离子液体/乙醇混合溶液吸收CO2的传质特性

采用高速摄像仪对微通道内离子液体/乙醇混合溶液吸收CO₂的传质行为进行了实验研究。考察了弹状流型下气液两相流量比和离子液体浓度对液侧体积传质系数k_La和液侧传质系数k_L的影响。当离子液体浓度不变时,k_La、k_L均随气液流量比的升高而增大并逐渐趋于恒定。当液相流量不变时,对于不同浓度的离子液体溶液,液侧体积传质系数k_La和液侧传质系数k_L随气液流量比的变化曲线出现了交叉点。在交叉点之前,k_La和k_L均随着离子液体浓度的增大而减小;在交叉点之后,k_La和k_L均随着离子液体浓度的增大而增大。提出了用于预测液侧体积传质系数k_La的新的量纲1经验关联式,预测效果良好。     

水力喷射-空气旋流器中微粒强化气液传质及其机理

在一种新型高效的气液传质设备--水力喷射-空气旋流器(WSA)中,研究了第三相固体粒子对气液传质的影响。分别采用化学吸收法(CO₂-空气-NaOH体系)和物理吸收法(CO₂-空气-H₂O体系)测定了不同固含率c_s、进口气速u_g、液体喷射速度u_L下的有效相界面积a和液膜传质系数k_L,并由此得到总体积传质系数k_La和增强因子E。结果表明,随着粒子固含率增大,k_L、a、k_La和E先增大后减小,存在一适宜固含率。在不同进口气速和液体喷射速度下,加入微粒后,k_L、a、k_La均增大,但E随进口气速和液体喷射速度增加而减小。微粒加入后,主要从a、k_L和表面更新频率S这3方面强化了气液传质,但主要是通过增强表面更新频率S而实现的。     

水力喷射-空气旋流器中气液传质特性及其机理

对一种新型高效的气液传质设备--水力喷射-空气旋流器(WSA)的传质机理进行了研究。分别采用化学吸收法(CO₂-空气-NaOH体系)和物理吸收法(CO₂-空气-H₂O体系)测定了不同进口气速、不同液体喷射速度下的有效相界面积a和液膜传质系数k_L,并由此得到体积传质系数k_La。结果表明,由于WSA中气液间的强交互作用,a、k_L以及k_La均随进口气速和液体喷射速度增大而增大。采用量纲分析法对实验数据进行了归纳,拟合出了a、k_L和k_La随气相Reynolds数Re_g、液相Weber数We_L之间的经验公式:a=0.0024Re_g^1.25We_L^0.079,Sh_L=35.31Re_g^0.2303We_L^0.13,k_La=6.52×10^-8Re_g^1.48We_L^0.21,这些关联式能较好地预测WSA的传质性能。研究还表明,在WSA中的气液射-旋流传质体系中,传质过程符合双膜理论、表面更新理论和溶质渗透理论,但以表面更新机理为主。     

气体喷射-气液旋流式冷氢箱的传质特性

开发了一种具有多重旋流结构的气体喷射-气液旋流式冷氢箱,并通过大型冷模实验对其进行传质性能测试。在气体喷射速度10～80 m·s^-1和液体进口速度0.2～0.7 m·s^-1条件下,采用氧吸收法测量了液相体积传质系数k_La,采用空气-Na₂SO₃溶液化学吸收法测量了相界接触面积a。结果表明:k_La和a均随气体喷射速度和液体进口速度增大而增大,其中受气体速度变化影响更为明显;由测定的传质参数数值可知新型冷氢箱传质效果较传统冷氢箱大幅提高,与机械输入设备处于相同量级,具有优异的气液混合性能。还采用量纲分析法对数据进行了归纳拟合,得到传质参数与气相Reynolds数、液相Weber数之间的关联式,可以较好地关联预测冷氢箱的传质性能。     

VOLUMETRIC MASS TRANSFER COEFFICENTS OF GAS LIQUID JET LOOP REACTORS BY OXIDATION OF HYDRAZINE

Oxidation of hydrazine in the presence of a homogeneous catalyst (CuTeSP) has been proved as a suitable reaction for the determination of volumetric mass transfer coefficients k_La of stirred tank reactors. This method is applied to a gas liquid jet loop reactor (JLR).At power inputs of N/V_R≤ 8 kW/m³ k_La values of about 0·1 and 0·4s^-1 were achieved. The evaluated values agree well with those determined by a stationary physical method.     

超重力旋转填充床氧解吸过程的数值模拟

基于旋转填充床流体流动的可视化结果,建立了超重力旋转填充床气液传质过程的数学模型,模拟氮气解吸水中溶解氧的传质过程。模拟结果表明,缩短液相停留时间、提高液相扩散系数都能增大液相传质分系数k_L;总体积传质系数K_La随超重力因子的增加而增大、随温度的上升而增大、随气相流率的增加略有下降、随液相流率的增加明显增大;空腔区传质贡献率随空腔区的增大而增大,随超重力因子的增大而减小;且短暂的停留时间是超重力旋转填充床对传质过程强化的本质原因。模型较好地符合文献的实验数据,误差在±16%以内。     

LIQUID-FILM TIME CONSTANT ASSESSMENT FOR kLa MEASUREMENTS IN RESPIRING FERMENTATION BROTHS

To determine the liquid-film time constant (τ_F) of the dissolved oxygen electrode for k_La measurements with the dynamic method in respiring fermentation broths, an approach using the steady-state readings of the electrode in the broths containing respiring and non-respiring cells, respectively, has been established. The results showed that τ_F in microbial suspensions with high volumetric oxygen uptake rates could be very large under the conditions of low aeration and agitation, indicating the slower response of the oxygen electrode caused by the more difficult penetration of oxygen molecules through the thicker films with active cell respiration. The significance and adequacy of the τ_F corrections for k_La measurements were evaluated experimentally.     

MULTIPHASE HYDRODYNAMICS AND SOLID-LIQUID MASS TRANSPORT IN AN EXTERNAL-LOOP AIRLIFT REACTOR—A COMPARATIVE STUDY

The solid-solid mass transfer performance of an external-loop airlift reactor was measured by dissolution of benzoic acid coated on nylon-6 particles, and the hydrodynamics of the gas-liquid-solid multiphase system in the airlift reactor were investigated. The solid-liquid system was designed to simulate the micro-carrier culture of animal cells, and some typical suspensions of immobilized enzyme particles.

The solid-liquid mass transfer coefficient remained constant below a superficial air velocity of 0.04 ms^-1 for the particles examined, but increased rapidly with further increase in gas velocity. Solids loading (0.3-3.5% w/w) did not affect the mass transfer coefficient in turbulent flow.

The mass transfer coefficient was correlated with energy dissipation rate in the airlift reactor. The mass transfer coefficient in stirred vessels, bubble columns, fluidized beds, and airlift reactors was compared.

Over an energy dissipation Reynolds number of 4-400, the solid-liquid mass transfer coefficient in the airlift device was comparable to that obtainable in fluidized beds. The performance of the airlift was distinctly superior to that of bubble columns and stirred tanks.     

微通道内MEA/MDEA混合溶液吸收CO2的传质特性

研究了T形微通道内N-甲基二乙醇胺（MDEA）和单乙醇胺（MEA）混合水溶液吸收CO₂的传质过程。考察了弹状流型下气液两相流量、MEA和MDEA浓度对液侧传质系数k_L和体积传质系数k_La的影响。液侧传质系数和体积传质系数均会随着MEA浓度的升高而升高。与MEA相比,MDEA浓度的提高对传质影响较小。传质系数会随着液体流量的增大而增大,但气体流量的变化对其影响较小。体积传质系数随液体流量的增大而增大,但随气体流量的增大先增大,之后趋于稳定。考虑到化学反应对传质的强化作用,引入了Hatta数,提出了一个新的体积传质系数预测式,预测效果良好。     

The plug flow model for mass transfer in three-phase fluidized beds and bubble columns

The plug flow model (PFM), overwhelmingly used to describe mass transfer in bubble columns and three-phase fluidized beds, has never been critically tested. This study analyzes the PFM single parameter, K_La, to quantify mass transfer in the forementioned systems. Particular attention is paid to the mass transfer features of the zone near the distributor (grid zone) largely ignored until now. This study, carried out under the largest gas and liquid flow rates ever published, for similar types of systems, indicates the presence of two well defined mass transfer zones. These features invalidate, for design purposes, the use of the PFM. However, it still can be used as a qualitative mass transfer indicator. This has permitted a comparison between the mass transfer efficiency of bubble columns and three-phase fluidized beds with the conclusion that three-phase fluidized bed of 0.5 cm particles can compete successfully with bubble columns.     

OXYGEN TRANSFER IN THE WATER-JET VESSEL

This is an investigation of the gas holdup and the volumetric mass transfer coefficient of a plunging water jet in an air-water system. We sound k_La to be directly proportional to gas holdup in two regions. For the first time, this has been clarified in the plunging liquid jet system. The volumetric mass transfer coefficient and the gas holdup have been well correlated in terms of the Froude number, liquid jet length, nozzle diameter and vessel diameter.     

HEAT TRANSFER CHARACTERISTICS OF THREE PHASE FLUIDIZED BEDS

Heat transfer characteristics of two (liquid-gas, liquid-solid) and three (liquid-gas-solid) phase fluidized beds have been studied in a 15.2 cm-ID column fitted with an axially mounted cylindrical healer. Effects of gas velocity (0-12 cm/s). liquid velocity (0-16cm/s), particle size (1.7-8.0 mm) and liquid viscosity (0.001-0.039 Pa s) on heat transfer coefficient were determined. The heat transfer coefficient increased with fluid velocities and particle size and it decreased with liquid viscosity in two and three phase fluidized beds. The bed porosity at which the maximum heat transfer occurred decreased with particle size but increased with liquid viscosity. The coefficient were correlated in terms of experimental variables. Modified Nusselt number from the present and previous studies has been correlated with modified Prandtl and Reynolds numbers.     

Bubble breakage phenomena,phase holdups and mass transfer in three-phase fluidized beds with floating bubble breakers

The individual phase holdups and mass transfer characteristics in three-phase fluidized beds with different floating bubble breakers have been determined in a 2.0 m high Plexiglas column of inner diameter 0.142 m. The bubble breaking phenomena by the breakers have been studied via a photographic method in a two-dimensional Plexiglas column. The volumetric mass transfer coefficient k_La in three-phase fluidized beds with hexagonal-shaped breakers is up to 40% greater than that in beds without floating bubble breakers. The bed porosity ε_L + ε_g, gas-phase holdup ε_g, and volumetric mass transfer coefficient k_La increase with an increase in the volume ratio of floating bubble breakers to solid particles, V_f/V_s, up to around 0.15, and thereafter decrease with V_f/V_s in three-phase fluidized beds with floating bubble breakers. Also, k_La increases with increasing breaker density, projected area and contact angle between the floating bubble breakers and the water. The volumetric mass transfer coefficients in terms of the Sherwood number in three-phase fluidized beds with the various floating bubble breakers have been correlated with the volume ratio of floating bubble breakers to solid particles, the particle Reynolds number based on the local isotropic turbulence theory and the modified Weber number.     

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。     

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.     

CONTINUOUS PRODUCTION OF DISSOLVED OXYGEN-FREE WATER USING RAMOND SUPERMIXER®

The process of continuous production of dissolved oxygen free water was investigated by means of desorption of dissolved oxygen into fine nitrogen bubbles generated in a novel motionless mixer named Ramond Supermixer® (RSM). The pressure drop across the mixer (ΔP_SM) and the volumetric mass transfer coefficient (k_La) during the desorption of oxygen from water were measured for various combinations of process variables; liquid and gas velocities and number of mixing units. The k_La during absorption was smaller by less than approximately 6% than that during desorption. Specific gas-liquid interfacial areas (a) were measured by the chemical absorption method for a CO₂-aqueous sodium hydroxide solution system. Such process characteristics as ΔP_SM, k_La, and a were correlated with the process variables and compared with those available in the literature. To further confirm the mass transfer characteristics in RSM, the desorption of oxygen from aqueous electrolyte solutions was also investigated. All the correlations obtained for k_La during the desorption of oxygen from water and aqueous electrolyte solutions were compatible to a great higher extent.     

EXPERIMENTAL OBSERVATIONS OF FLUIDIZED BEDS AT ELEVATED PRESSURES

The object of the work described here was to elucidate the effects of operation under pressure on the physical behaviour of gas fluidized beds. Extensive measurements of various bubble properties such as size, shape and rise velocity in beds of coarse powders (mean particle diameters of 184 μm and 450μm) operated at pressures of up to 81 bar were made by photographing the images created by irradiation of the bed with X-rays, and analysing the bubble silhouettes thereby obtained. Most of the results presented here are averages of some 200 individual measurements.

Experimental evidence to support the following picture of the effect of pressurization on the behaviour of freely bubbling gas fluidized beds is presented. Both bubble interaction (tendency to coalesce) and the incidence of bubble splitting increase with increasing pressure; the two are intimately connected. The nett results are a decrease in bubble size with increasing pressure over most of the pressure range and an increase in the tendency for bubbles to distribute non-uniformly in a radial direction. This latter tendency probably causes gross solids circulation in the bed, and this in turn leads to higher bubble rise velocities than those observed for single bubbles under similar conditions. The splitting mechanism accounting for the decrease in bubble size was found to be intrusion of the wake into the bubble void by the flow of gas through the wake region of a leading bubble during pair coalescence.

An updated review of other published work relating to the subject of experimental observations of the effects of pressure on gas fluidized beds is included in the form of a table.     

壁面润湿性对微通道内二氧化碳-水两相流流动及传质性能的影响

在1 mm×1 mm矩形截面下微通道内,以二氧化碳-水为工作流体,研究壁面润湿性和气液表观流速对气-液两相流型和气液传质的影响,并研究了气、液表观流速对弹状流流体力学性质的影响。在亲水微通道中观测到了泡状流、泡状-弹状流、弹状流;在疏水微通道中观测到了非对称弹状流、拉长的非对称弹状流、分层流。实验表明亲水微通道中弹状流区域下气泡长度大体上随气相表观流速的增大而增大,随液相表观流速的增大而减小;液弹长度大体上随气相表观流速的增大而减小,随液相表观流速的增大先增大后减小;液侧体积传质系数k_La均随气、液相表观流速的增大而增大,随通道壁面润湿性的增强而增大。     

The Effect of Bed Temperature on Mass Transfer between the Bubble and Emulsion Phases in a Fluidized Bed

The rate of interphase mass transfer between the bubble and emulsion phases of a bubbling fluidized bed is of primary importance in all models for fluidized bed reactors. Many experimental studies have been reported, however, all these investigations have been carried out in fluidized beds operated at room temperature. In this work, the effect of the bed temperature on the interphase mass transfer is reported. Single bubbles containing argon – used as a tracer – were injected into an incipiently fluidized bed maintained at the required temperature. The change in argon concentration in the bubble was measured using a suction probe connected to a mass spectrometer. The effects of bed particle type and size, bubble size, and bed temperature on the mass transfer coefficient were examined experimentally. The interphase mass transfer coefficient was found to decrease with the increase in bed temperature and bubble size, and increase slightly with increase in particle size. Experimental data obtained in this study were compared with some frequently used correlations for estimation of the mass transfer coefficient.