Study of gas-liquid mass transfer in bubble and spray column adding solid media

pH influence on oxygen mass transfer coefficient in a bubble column. Individual characterization of kL and a

Swirling gas injection is a well-known technique to improve mass transfer in bubble columns. It can be used to create small bubbles with a high surface area-to-volume ratio, which is beneficial for mass transfer. Swirl gas injection can also be used to create a more uniform bubble size distribution and improve the mixing of gas and liquid in the column. This study aims to determine the impact of swirl gas injection on bubble properties, including bubble shape, size, and velocity. A bubble detection approach has been developed for quick and precise determination of bubble size distributions in gas-liquid systems. Advanced digital image processing, including edge detection and bubble edge recognition, is used in this method. The experiment is conducted in a bubble column at a height of 57 cm and 61 cm. The column had a ring sparger and was made of Plexiglas. Tap water was used as the liquid, while air from an air compressor was utilized as the gas phase. The shape, size, population, and velocity of the bubble are measured using a high-speed digital camera. According to this study, the average bubble size reduced as the impeller speed increased, while the population of bubbles increased when the sparger rotation speed increased from 30 to 150 rpm.

Mass transfer in bubble columns operating at high gas throughputs

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%.

The emission of hydrogen sulphide is a major problem associated with anaerobic treatment of sulphate and sulphite containing wastewaters. Conventional absorbing processes, such as packed towers, spray towers or bubble columns, are all constrained by factors such as flooding and foaming. Membrane systems, on the other hand, enable independent control of the liquid and gas flow rate and a step change order of magnitude increase in the specific surface area of the contact process. The membrane acts as a gas absorber with a design similar to a shell and tube heat exchanger. On the other hand, they are limited by facets of the membrane such as its resistance to mass transfer and permselectivity, as well as its cost. The work presented in this paper refers to an absorption process based on a non-wetted hollow fibre membrane for the scrubbing of hydrogen sulphide from air, with water as the contact solvent. Results presented describe the performance of the unit in terms of overall transfer and outlet liquid concentration as a function of circulation regime, gas flow rate, liquid flow rate and specific surface area. In particular, results are presented using traditional plots of Sherwood number (Sh) against Graetz (Gr) number for the liquid flowing in the lumens, such that experimental and available empirical descriptions of the process performance are directly compared. Results suggest that, as expected, very efficient mass transfer is obtained. However, the mass transfer was found to reach a maximum value against Gr, contrary to available empirical models.

The mechanism of the gas–liquid flow and mass transfer in the bubble column has always been the hotspot for scholars due to the complex gas–liquid interphase interaction. This work established a three-dimensional computational fluid dynamics model for simulating the physical absorption of CO2 to study the gas–liquid flow and mass transfer characteristics in a bubble column, combining the single equation Wray–Agarwal turbulence model, population balance model, and gas–liquid mass transfer model. Compared with the eddy cell model and the slip penetration model, the predicted values of the modified eddy cell mass transfer model were more consistent with the experimental results, and the errors were within ±10%. The flow field characteristics with and without mass transfer were studied, and the results showed that the gas holdup, bubble diameter, and gas–liquid interface area changed by more than 250, 100, and 120%, respectively. Furthermore, the effects of the gas flow rate and static liquid height on the gas–liquid flow field and mass transfer characteristics were analyzed. These results could provide a solid foundation for further research and development of the bubble column.

Even bubble column reactors (BCR) and airlift reactors (ALR) have been developed in terms of various related aspects towards mass transfer enhancement, the effective analysis of gas diffuser types on mass transfer and gas–liquid hydrodynamic characteristics is still limited. Therefore, the present study aims to analyze the relative effect of different types of air diffusers on bubble hydrodynamics and mass transfer performance to understand their behaviors and define the best type. The experiments were conducted by varying different diffuser types, reactor types (BCR and ALR), and superficial gas velocity (Vg) (0.12 to 1.00 cm/s). Five air diffusers including commercial fine sand (F-sand) and coarse sand (C-sand) diffusers, and acrylic perforated diffusers with orifice sizes of 0.3 mm (H-0.3), 0.6 mm (H-0.6), and 1.2 mm (H-1.2), were used in this study. For every condition, it was analyzed in terms of bubble hydrodynamics and oxygen mass transfer coefficient (KLa). Lastly, the selected diffusers that provided the highest KLa coefficient were evaluated with a solid media addition case. The results of both reactor classes showed that F-sand, the smallest orifice diffuser, showed the smallest air bubbles (3.14–4.90 mm) compared to other diffusers, followed by C-sand, which larger about 22–28% on average than F-sand. ALR exhibited a better ability to maintain smaller bubbles than BCR. Moreover, F-sand and C-sand diffusers showed a slower rising velocity through their smaller bubbles and the tiny bubble recirculation in ALR. Using F-sand in ALR, the rising velocity is about 1.60–2.58 dm/s, which is slower than that in BCR about 39–54%. F-sand and C-sand were also found as the significant diffusers in terms of interfacial area and gas hold-up. Then, the KLa coefficient was estimated in every diffuser and reactor under the varying of Vg. Up to 270% higher KLa value was achieved from the use of F-sand and C-sand compared to other types due to their smaller bubbles generated/maintained and longer bubble retention time through slower rising velocity. After adding 10% ring shape plastic media into the reactors with F-sand and C-sand diffusers, a better performance was achieved in terms of KLa coefficient (up to 39%) as well as gas hold-up and liquid mixing. Lastly, ALR also had a larger portion of mixed flow pattern than BCR. This eventually promoted mass transfer by enhancing the mixed flow regime.

Instantaneous mass transfer coefficients were obtained for the absorption of carbon dioxide bubbles rising in an aqueous solution of sodium carboxymethylcellulose. The rheological character of the solutions was well described by the Ellis model.Mass transfer coefficients were high initially but trailed off rapidly with bubble age. Exceptions were found at specific diameters where the bubble shape went through a transition. At about 0.2 cm. a transition from ellipsoidal to a sphere shape occurred, which has also been observed in Newtonian fluids. At a larger diameter, however, the non‐Newtonian fluid showed a shape change from a spherical “cap” to a “top” shape and finally to an ellipsoid. A sudden increase of mass transfer coefficient accompanied each shape transition.Drag coefficient data were correlated successfully with a new Reynolds number. The Newtonian and power law portion of the Ellis model each contributed a component to the Reynolds number, which, when added together, correlated drag data for the bubbles as well as for glass spheres.Attempts to account for transition shape changes and bubble tailing in the non‐Newtonian fluid and their effects on bubble mass transfer are included.

Experimental study of the SO2 removal efficiency and volumetric mass transfer coefficients in a pilot-scale multi-nozzle spray tower

Mass transfer studies in shallow bubble column reactors

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 performance of a penicillin fermentation was assessed in a laboratory-scale bubble column fermentor, with mycelial growth confined to the pore matrix of celite beads. Final cell densities of 29 g/L and penicillin titres of 5.5 g/L were obtained in the confined cell cultures. In comparison, cultures of free mycelial cells grown in the absence of beads experienced dissolved oxygen limitations in the bubble column, giving only 17 g/L final cell concentrations with equally low penicillin titres of 2 g/L. The better performance of the confined cell cultures was attributed to enhanced gas liquid mass transfer rates, with mass transfer coefficients (k(L)a) two to three times higher than those determined in the free cell cultures. Furthermore, the confined cell cultures showed more efficient utilization of power input for mass transfer, providing up to 50% reduction in energy requirements for aeration.

Experimental mass and heat transfer at a serpentine tube heat exchanger located at the wall of a square stirred tank reactor

Modelling mass transfer properties in a biotrickling filter for the removal of isopropanol

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.