99 - OXYGEN TRANSFER IN THREE PHASE FLUIDIZED BEDS

Intensification of mass transfer in the pulsed bubble column—Comparison of the efficiency of static and pulsed aerator

The absorption mechanism of aeration and hydrophobic Volatile Organic Compounds (benzene) was investigated in a bubble column. The diffusers used in this study were rigid and flexible orifices. Tap water, and aqueous solution of commercial surfactants: cationic (DTAB), anionic (SES) and non-ionic (Dehydol LS 5 TH and Tween 80) were used as absorbents in this experiment. Moreover, the range of surfactant concentrations for aeration and benzene are 0.01, 0.1 CMC, and 0.1, 1, 3 CMC, respectively. Those were analyzed with the superficial gas velocity applied. The bubble hydrodynamic parameters were determined by using a high speed camera (340 frames/sec) and an image analysis program. The results showed the rigid orifice provides the highest mass transfer rate due to the small bubble size generated and thus the increase of interfacial area (a). The values of interfacial area and volumetric mass transfer coefficients (kLa) increased roughly linearly with the superficial gas velocity. Moreover, the kL coefficients obtained with surfactant solutions were less than those obtained with tap water: these results compensates with the increase of interfacial area. Furthermore, the parameters such as the type of surfactants, molecular weight, CMC, and surface tension were proven for significantly altering mass transfer and bubble hydrodynamic parameters in this research. Due to the application and improvement of benzene absorption in bubble column, the granular activated carbon (GAC) was filled into a bubble column reactor. It can be noted that the addition of (GAC) into the reactor with tap water can provide the highest benzene removal efficiency (up to 74.45%) compared with the non-ionic surfactant as absorbent. Moreover, the presence of non-ionic surfactant molecules can cause the negative effect onto the GAC adsorption capacity due to their attachment mechanism. Finally, it has been suggested that rigid diffuser applied with the superficial gas velocity at 0.0032 m.s-1, and water combined with granular activated carbon are the optimal design and operation for hydrophobic VOCs absorption in the bubble column used in this study.

The volumetric mass transfer coefficient is commonly used to assess the mixing effectiveness of gas–liquid bioreactor systems. Analyzing mass transfer performance in non‐Newtonian fluids inside coaxial mixers can be challenging due to the complex interaction between process variables, which requires developing robust characterization and estimation approaches. This study aims to investigate the gas dispersion in shear‐thinning biopolymers with yield stress using coaxial mixers in order to evaluate the effects of aeration and agitation on the volumetric mass transfer coefficient. A mixing configuration comprising a pitched blade turbine and an anchor was employed to disperse air into xanthan gum solutions, and the mass transfer performance was obtained at different impeller speeds and biopolymer concentrations by measuring the dissolved oxygen concentration. A dimensionless empirical correlation was proposed, and the results showed that the mass transfer was positively influenced by aeration intensity and agitation mechanism, quantified by the gas flow number and Reynolds number, respectively. Additionally, a strategy using stacking‐ensemble artificial neural networks was developed to accurately estimate the volumetric mass transfer coefficient, with a correlation coefficient of 0.998. The proposed mass transfer characterization approach overcame the complexities of analyzing aerated coaxial mixer systems and provided a reliable design model for bioreactor systems containing non‐Newtonian fluids.

In this study, modeling investigation of mass transfer in concurrent-flow chemical extraction process is carried out. Continuum Model is used to describe mass transfer process. Concentration curves in organic droplet and aqueous liquid film are firstly calculated and rate-limiting step is determined to be mass transfer of solute in aqueous phase. Mass transfer performance in aqueous phase is further calculated. Volumetric mass transfer coefficient and volumetric mass transfer coefficient during mass transfer time before solute concentration decreases to 5%, are calculated for evaluating mass transfer characteristics. The effect of organic droplet size is studied. The result demonstrates that increase in aqueous/organic volume ratio significantly decreases mass transfer performance, decrease in droplet size enhances mass transfer process through both reducing mass transfer resistance and increasing contacting area.

Gas holdup, effective interfacial area and volumetric mass transfer coefficient were measured in two and three phase downflow bubble columns. The mass transfer data were obtained using the chemical method of sulfite oxidation, and the gas holdup was measured using the hydrostatic technique. Glass beads and Triton 114 were used to study the effects of solids and liquid surface tension on the gas holdup and the mass transfer parameters a and kL a . The gas holdup in three phase systems was measured for non-wettable (glass bead) and wettable (coal and shale particles) solids. The mass transfer data obtained in the downflow bubble column were compared with the values published for upflow bubble columns. The results indicate that in the range of superficial gas velocities (0.002-0.025) m/s investigated, the values of the mass transfer coefficient were of the same order of magnitude as those observed in upflow systems, but the values of interfacial area were at least two fold greater. Also, the results showed that the operating variables and the physical properties had different influences on a and kL a in the downflow bubble column. Correlations for a and kL a for the downflow bubble column are proposed which predict the data with adequate accuracy in the range of operating conditions investigated.

Application of microchemical technology in mass transfer behavior contrastive research of rare earth extraction

Carbon dioxide absorption in glucosamine aqueous solutions

Aerated stirred tank reactors are widely used in bio-process engineering and pharmaceutical industries. To supply the organisms with oxygen and control the pH value, oxygen is transferred from air bubbles into the liquid phase, and, at the same time, carbon dioxide is stripped from the liquid phase with the same gas bubbles. The volumetric mass transfer coefficients for oxygen and carbon dioxide are, therefore, of crucial importance for the design and scale-up of aerated stirred tank reactors. In this experimental work, the volumetric mass transfer coefficients for oxygen and carbon dioxide are investigated simultaneously to study their mutual influence. The mass transfer performance for oxygen and carbon dioxide is conducted in stirred tank reactors on the 3 L laboratory scale, 30 L pilot scale, and 15,000 L production scale. First, the influence of dissolved carbon dioxide on the oxygen mass transfer performance is investigated in a 30 L pilot scale stirred tank reactor. The results show that the volumetric mass transfer coefficient of oxygen is not affected by the concentration of dissolved carbon dioxide, but the total mass flux of oxygen decreases with increasing carbon dioxide concentration due to the decreasing partial pressure difference. With rising gassing rate and volumetric power input, both mass transfer coefficients for oxygen and carbon dioxide show the same increasing trend. Although this trend can also be observed when scaling down to the 3 L laboratory scale reactor, a significantly different effect must be considered for the scale-up to the 15,000 L industrial scale reactor. The limited absorption capacity for carbon dioxide of the gas bubbles during the long residence time in the industrial scale reactor is noticeable here, which is why the specific interfacial area is of negligible importance. This effect is used to develop a method for independent control of oxygen and carbon dioxide mass transfer performance on an industrial scale and to increase the mass transfer performance for carbon dioxide by up to 25%.

Limiting diffusion currents were measured at the inner surface of sintered nickel electrodes sheets and a model adapted from heterogeneous catalysis was used to quantify the minimum mass transfer within the porous material. External mass transport from the bulk electrolyte was induced by rotating a disc facing the sintered nickel disc electrode located at the bottom of a cylinder. Values of the volumetric mass transfer coefficient at two sintered materials were thus obtained. The method and its limitations are discussed.

Impact of operating parameters on values of a volumetric mass transfer coefficient in a single-use bioreactor with wave-induced agitation

Gas–liquid mass transfer in the gas–liquid–solid mini fluidized beds

Until now the oxygen transfer in viscous non‐Newtonian solutions has been studied only in bubble columns of about 0.14‐m diameter. Recently Godbole et al. (1982) reported much smaller gas holdups in Carboxy Methyl Cellulose solutions (CMC) for a large‐diameter column. Therefore, the gas holdups, volumetric mass transfer coefficients, and specific gas‐liquid interfacial areas are measured in CMC solutions using a bubble column of diameter 0.305 m and height 3.4 m. The transition from churn‐turbulent to slug flow regime occured at higher viscosities and the gas holdups and volumetric mass transfer coefficients were lower in both flow regimes than reported for smaller column diameters. Empirical correlations are presented for the gas holdup, volumetric mass transfer coefficient, and specific gas‐liquid interfacial area which would be suitable for the design of fermentors.

Liquid‐solid mass transfer behaviour of a rectangular stirred tank reactor lined with single screens and stacks of closely packed screens was studied in relation to catalytic and electrochemical reactor design by the electrochemical technique. Variables studied were impeller rotation speed, mesh number, and wire diameter of the screen, as well as physical properties of the solution, number of screens per stack, and impeller geometry. The rate of mass transfer increased with increasing impeller rotation speed and decreased with increasing screen mesh number and number of screens per stack. Radial flow turbine impellers produced higher rates of mass transfer than axial flow impellers. The data were correlated by dimensionless mass transfer equations. A comparison between the volumetric mass transfer coefficient at a stirred single‐screen electrode and at a stirred flat plate electrode shows that the volumetric mass transfer coefficient at the single screen is higher than that at the flat plate by a factor ranging from 7.3–22.5, depending on the operating conditions. For screen stacks, the ratio between the stack volumetric mass transfer coefficient and the flat plate value ranges from 2–46 depending on the operating conditions.The importance of the present results in the design and operation of catalytic and electrochemical reactors used to conduct diffusion‐controlled reactions was highlighted. The possibility of using screens as turbulence promoters to enhance the rate of heat transfer between the reactor wall and the surrounding cooling jacket was noted.

Mass transfer studies in shallow bubble column reactors

An experimental investigation of the flow and mass transfer behavior in a vertical aeration process with orifice ejector