Modeling Investigation of Concurrent-flow Chemical Extraction Process

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.

99 - OXYGEN TRANSFER IN THREE PHASE FLUIDIZED BEDS

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 effect of pulsed high-voltage discharge on mass transfer and reaction processes in a needle-plate reactor was studied. Bubble size distributions and characteristic parameters were determined by photography, implying that inlet air bubbles could be broken into smaller ones effectively by discharge. The mass transfer parameters including specific interfacial area (a), volumetric mass transfer coefficient (kLa), and liquid-side mass transfer coefficient (kL) were obtained by Danckwerts-plot, reflecting the mass transfer processes. In addition, 4-chlorophenol (4-CP) decomposition was determined under the same experimental conditions. The results showed that the conclusions of 4-CP removal were coincided with the statistical results of bubble images, illustrating that higher peak voltage, pulsed frequency and appropriate air flow rate are beneficial to mass transfer and reaction. However, there was a slight difference between 4-CP removal and mass transfer parameters at higher air flow rate. Therefore, it is more reasonable to optimize the operating conditions of a pulsed high-voltage discharge reactor via combining image acquisition and mass transfer parameters with the results of pollutant degradation.

In this experiment, the inert gas is used to agitate two immiscible fluids in microchannels. The mass transfer performances with or without gas agitation are investigated. 30% TBP (in kerosene)‐acetic acid‐water is chosen as testing system, and nitrogen as agitating gas. The superficial velocities of the immiscible liquid–liquid two phases and gas phase are varied in the range from 0.02 to 1.2 m/s, and 0 to 3.0 m/s, respectively. In microchannels, with enough gas agitating intensity, high dispersion between two immiscible liquid phases can be obtained. The overall volumetric mean mass transfer coefficients are two‐folds higher than those without gas agitating, which are in the range of 3.8–30.6 s−1. Some parameters which impact on the mass transfer process, such as the mixture superficial velocity of the immiscible liquid–liquid two phases, the gas superficial velocity, the microchannel structure, the gas inlet locations and the sampling time are experimentally investigated. © 2009 American Institute of Chemical Engineers AIChE J, 2009

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.

Structural optimization of deflector within air-water direct contact tank based on uniform design

Mass transfer studies in shallow bubble column reactors

In this work, gas dispersion in a two-phase partitioning bioreactor is analyzed by calculating volumetric oxygen mass transfer coefficient which is modeled using a commercial computational fluid dynamics (CFD), code FLUENT 6.2. Dispersed oxygen bubbles dynamics is based on standard "k-ε" Reynolds-averaged Navier-Stokes (RANS) model. This paper describes a three-dimensional CFD model coupled with population balance equations (PBE) in order to get more confirming results of experimental measurements. Values of k L a are obtained using dynamic gassing-out method. Using the CFD simulation, the volumetric mass transfer coefficient is calculated based on Higbie's penetration theory. Characteristics of mass transfer coefficient are investigated for five configurations of impeller and three different aeration flow rates. The pitched six blade type, due to the creation of downward flow direction, leads to higher dissolved oxygen (DO) concentrations, thereby, higher values of k L a compared with other impeller compositions. The magnitude of dissolved oxygen percentage in the aqueous phase has direct correlation with impeller speed and any increase of the aeration magnitude leads to faster saturation in shorter periods of time. Agitation speeds of 300 to 800rpm are found to be the most effective rotational speeds for the mass transfer of oxygen in two-phase partitioning bioreactors (TPPB).

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

Recently, study on a microscale-based absorption refrigeration system has sprung up motivated by the need of efficient energy utilization. Heat-driven absorption systems offer a possibility of generating both power and cooling with environment friendly refrigerants, such as ammonia/water and LiBr/water. However, these systems are often large in size and low in COP especially in single stage absorption systems. These characteristics of absorptions systems make them unattractive in most cases. This work introduces the utilization of micro-channel enhanced surfaces as heat exchangers to enhance the component and system performance, to reduce the system size and to reduce the cost of the system as well. In this work, a new concept of enhancing heat and mass transfer processes is applied in the absorber part of the absorption cycle by using a single micro-channel. Due to its merit of high area to volume ratio, microchannel technology has been well theoretically validated to be a very effective and potential choice for enhancing heat transfer performance. But there is a lack of research work on the mass transfer performance in micro-channels. This work investigated simultaneous mass and heat transfer characteristics of a novel microchannel absorber that uses LiBr/water as the working fluid. A microchannel with hydraulic diameter of 0.7mm is employed in this characterization study. Velocity distribution, pressure drop, concentration and temperature profile inside the microchannel as well as effects of the inlet absorbent concentration, flow rate and temperature together with the refrigerant flow rate on the heat/mass transfer are predicted. Investigations on the optimum inlet angle design of a single channel absorber are also presented in the end of this work. Feasibility of this novel absorber design was proved via this numerical simulation as the mass transfer taking place inside the mixing channel was observed to achieve the identical performance but with a size reduction by 1/27 compared to a conventional falling film absorber. A 7 times enhancement of the heat transfer coefficient was also achieved with the comparison of a macro-scale based absorber configuration.

Modeling investigation of mass transfer of gas–liquid concurrent flow processes

Ladle refining plays a key role in the steelmaking process. During the refining, a bubbly gas stream is used for mixing and to enhance the rate of removal of impurities from the molten steel. A numerical model has been developed to understand mass transfer and mixing behavior in a three-phase gas-stirred ladle. A two-resistance approach was used for the liquid–liquid mass transfer, while the mass transfer coefficient was determined using the Small Eddy theory. The model was validated with experimental data, obtained from a water–oil physical model simulating an industrial ladle with a scale factor of 1/17, valid for axisymmetric gas injection. Three variables were included to study the mass transfer behavior, namely gas flow rate, Q, oil (slag) thickness, h, and oil (slag) viscosity, $$ \mu_{\text{o}} $$ . The gas flow rate ranged from 2.85 L/min to 8.56 L/min to meet industrial operating conditions. It was found that: (1) the volumetric mass transfer coefficient (ka) increases when the gas flow rate (Q) increases; and (2) increasing slag (oil) thickness has a positive influence on mass transfer as it considerably increases the interfacial area and promotes turbulence at the interface. At this range of gas flow rate, the effect of slag (oil) viscosity is limited. A general correlation was established: $$ {\text{ka}} = 0.058Q^{0.459} h^{0.612} $$ . Mixing time was studied within the same flow rate range to observe its influence on the mass transfer. Mixing in the ladle is accomplished in a much shorter time than interphase mass transfer, specifically by two orders of magnitude, which indicates that mass transfer is the rate-limiting step.

Mass and heat transfer at an array of horizontal cylinders placed at the bottom of a square agitated vessel

Mass transfer intensification in a novel airlift reactor assembly with helical sieve plates