Testing the Well-Mixed Model for a Gas-Liquid Stirred Ozonization Reactor

A “well-mixed” model was developed for a semi-batch gas-liquid reactor designed to study the mechanisms of ozonation of organic compounds in aqueous media, and tested against unsteady-state absorption data. In the (experimental range examined for the superficial gas velocity (v)Q < 0.30 cms(1),) this model fairly describes the fluid dynamic behavior of the reactor for stirring speeds not greater than about 400 rpm. To minimize the effects of the physical factors on the computed kinetic parameters, the kinetic experiments should be carried out in these operating conditions.     

A refined model for ozone mass transfer in a semibatch stirred vessel

The mathematical model proposed by Anselmi et al. (1984) for a semibatch stirred gas‐liquid contactor is refined to describe the mass transfer of ozone absorption and decomposition in aqueous solution with the decomposition rate expression of general reaction orders (not necessarily integers). Three system equations are employed to describe the ozone concentrations in the bulk liquid (C_ALb), the hold‐up gas (C_AGi), and the outlet gas in the free volume above the liquid surface (C_AGe), respectively. The effect of ozone decomposition on the mass transfer, which is reflected by the enhancement factor (E_r) defined as the ratio of mass absorbed per unit area in time t with chemical reaction (r) to that without chemical reaction or of the purely physical absorption, is considered in the refined model. Furthermore, the refined model also takes into account the variation of E_r with C_ALb, which changes with time during the course of gas‐liquid contacting. Thus this analysis extends the applicability of the model of Anselmi et al. (1984) and is of special importance for ozone mass transfer in the cases of basic solutions and of low mass transfer coefficients, in which the effect of decomposition on absorption is significant, and in the system with variable liquid phase ozone concentration.     

Experimental and Theoretical Studies on Hydrogenation in Multiphase Fixed‐Bed Reactors

Multiphase fixed‐bed reactors have complex hydrodynamic and mass transfer characteristics. The modeling and scale‐up are therefore difficult. The present work focuses on the role of mass transfer on the effective reaction rate. The catalytic 1‐octene hydrogenation was taken as a model reaction. The reaction rate in the trickle‐bed reactor is by a factor of 20 smaller than (theoretically) in the absence of any mass transfer limitations. For high octene concentrations (> 10 %), the effective reaction rate is limited by the H₂ consumption, above all by the gas/liquid and liquid/solid mass transfer. For lower octene concentrations the reaction is zero order with respect to H₂ and only depends on the octene consumption, i.e., on the interplay of chemical reaction, L/S and intraparticle mass transfer of octene.     

Hydrodynamic Characterization and Mass Transfer Analysis of an In-Line Multi-Jets Ozone Contactor

This research study investigates mixing and ozone mass transfer characteristics of a pilot-scale in-line multi-jets ozone contacting system. The hydrodynamic characteristics of the contactor were studied using a two-dimensional laser flow map particle image velocimetry coupled with planar laser induced fluorescence (PIV/PLIF). The PIV/PLIF system provided a combination of simultaneous whole-field velocity and concentration data in two-phase flows for different operating conditions. All measurements were conducted under a total liquid flow rate of about 10 L/s with gas flow rate ranging from 0.05 to 0.4 L/s. The gas was introduced to the system through a series of side stream injectors. The side injectors were tested under opposing and alternating modes. A mass transfer study was also conducted to estimate the overall mass transfer coefficient under the same operational conditions used for the hydrodynamics study. It was found that for the same number of jets (i.e., same gas flow rate) the liquid dispersion (D_L) was higher when alternating jets were used. Higher ozone mass transfer rates were observed when using opposing jet compared to the same number of alternating jets.     

Comparison of Concentration Profiles for Boundary Layers in Absorption with Chemical Reaction Processes

An analysis of five different systems of absorption‐with‐chemical‐reaction in gas‐liquid reactors, commonly encountered in various industrial processes, is presented. To analyze the interphase mass transfer from gas to liquid, the rate limiting parameters and the concentrations at the gas‐liquid interface were determined on the basis of pertinent theories. The calculations presented, are based on the Whitman theory for gas and liquid phase mass transfer coefficients and Henry equilibrium constants. The necessary diffusion coefficients were calculated from existing correlations, and the corresponding chemical reaction rate constants were obtained from the literature, assuming pseudo first order chemical reaction. The process parameters required (pressure, temperature, and the gas‐liquid contact time) were within the values that occur in industrial processes. The results presented, are the concentration profiles in the boundary layers for the systems studied, calculated and graphically presented, together with the gas and liquid film thicknesses and Hatta numbers, obtained from calculations for the liquid phase mass transfer. The results may contribute to a better understanding of the absorption‐with‐chemical‐reaction processes in industrial plants, thus lowering the operational costs of these processes and alleviating the ecological problems of existing technologies.     

Evaluation of Solubility and the Gas-Liquid Equilibrium Coefficient of High Concentration Gaseous Ozone to Water

Solubility and the gas-liquid equilibrium coefficient of gaseous ozone to water were examined under higher concentrations of supplied gaseous ozone up to 100 mg/L. The experimental and modeling approach was employed to evaluate the gas-liquid equilibrium coefficients and mass transfer of ozone. The gas-liquid equilibrium coefficients were evaluated as 0.35, 0.31 and 0.25 (mg/L-liquid)/(mg/L-gas) at 15, 20 and 30 °C, respectively. These gas-liquid equilibrium coefficients are applicable for the wide concentration range of supplied ozone gas up to 100 mg/L. The calculation result by a model which has terms of the mass transfer of ozone, the gas-liquid equilibrium coefficient and the rate of ozone self-decomposition, was examined and had a good agreement with the experimental data over the wide range of temperatures, pHs, inorganic carbon concentrations and supplied ozone gas concentrations. The rate of ozone self-decomposition evaluated separately from this study was employed for the calculation. We can conclude that absorption of gaseous ozone to water is expressed by the three terms mentioned above when the rate of ozone self-decomposition is evaluated properly. In sensitive analysis, we elucidated that the rate of ozone self-decomposition affected strongly on the concentration of dissolved ozone at steady-state under higher concentration of supplied gaseous ozone.     

Ozone and Oxygen Absorption in Downflow Bobble Columns

During the development of the downflow bubble column, knowledge of the specific interfacial area was a precondition for assessing the mass transfer performance. A comparison with other gas/liquid contactors shows that, in the downflow bubble column, higher mass transfer areas can be obtained than, for example, in a bottom-sparged bubble column or in a loop-type reactor in the region of high liquid throughputs. Examples for practical performance of downflow bubble columns are given, based on the derived correlations.     

Ozone Mass Transfer in Stirred Vessel

A method is proposed and a model is developed which are capable of providing a correlation of the mass transfer coefficient k_La, with stirrer speed and gas superficial velocity. The method can be adopted for deriving a correlation which can be profitably used for ozone gas–liquid reaction both for assessing the absorption regime and for the simulation of oxidation processes which evolve according to slow reaction regime.     

Maleic Acid Ozonation: Reactor Modeling and Rate Constants Determination

This work is related to ozone treatment of organic wastes, a gas-liquid reaction. A model has been developed that accounts for mass transfer from the gas to the liquid and for the kinetics of chemical reactions. The theoretical approach of the model firstly refers to the film theory enhanced by a new parameter, the depletion factor. This method allows the part of the reaction occurring within the film and that occurring within the bulk to be distinguished. In relation with the first part of the model, the second part allows the overall behavior of a gas-liquid reactor with time to be described. The model is validated by a set of experiments on maleic acid ozonation in a semi-batch reactor.     

Mass Balance Analysis of Ozone in Conventional Bubble Contactors

Ozone transfer into potable water was studied in a commercial scale contactor. Ozone mass balances have been calculated to determine ozone utilization in the contactor. Gas and liquid flowrates, as well as inlet ozone concentrations in the gas were varied. From these data it was possible to determine the mass transfer coefficient, the ozone consumption and the transfer efficiency. Procedures for calculating the design of contactors and the optimal operating conditions are proposed.     

Mass Balance Analysis of Ozone in a Conventional Bubble Column

Ozone transfer into potable water was studied in a conventional bubble column, and ozone mass balances have been calculated to determine ozone utilization efficiencies. Liquid and gas flow rates, as well as inlet ozone concentrations in the gas phase were varied. Using these data, it was possible to determine the ozone mass transfer coefficient, ozone transfer efficiency, and ozone consumption. A model of ozone transfer was established, and procedures for calculating the optimum design parameters and operating conditions are proposed.     

Ozone Absorption in Water: Mass Transfer and Solubility

Mass Transfer of ozone absorbed by water in a semi-comtinuous stirred reactor is studied at the lab scale. Experimental investigation using a complete factorial scheme shows a predominant effect of agitation speed and gas flow and results in a correlation for the mass transfer coefficient, k₁a. Solubility of ozone in water is estimated by evaluation of an apparent Henry's law constant for different temperatures (20* and 50*C), pH values (2 and 7) and a t constant ionic strength (0.13).     

The Effect of Superficial Gas Velocity on Wavy Films and its Use in Enhancing the Performance of Falling Film Reactors

Mass transfer in co‐current downward annular flow depends on the amount of liquid carried by the waves. The thickness of this portion of the liquid film increases with the superficial gas velocity up to about 20–25 m s^–1 for two‐phase air/water flow. The maximum apparent friction factors observed in air‐water annular flow also appear at superficial air velocities about 20–25 m s^–1. Organic compounds, like fatty alcohols and alkylbenzenes, show a maximum apparent friction factor at lower superficial gas velocities. The gas velocity at which a maximum friction factor occurs is dependent on the surface tension, appearing at lower gas velocities for liquids with lower surface tensions. Progressive increases of the superficial gas velocity can be used to graduate the mass transfer along a falling film device.     

Effect of Chemical Reaction and Mass Transfer on Ozonation of the Azo Dyes Reactive Black 5 and Reactive Orange 96

Ozonation of wastewater containing azo dye has been studied to evaluate the enhancement of ozone mass transfer from O₂O₃ gas into water with the presence of chemical reactions in a bubble column reactor. Experiments were performed at different initial dye concentrations and at various gas flow rates. C.I. Reactive Black 5 (RB 5) and C.I. Reactive Orange 96 (RO 96) have been chosen as representative model substances being found in wastewater from textile-finishing wastewater. Results show that the rate of ozone mass transfer increases with increasing initial dye concentration and gas flow rate. Consequently, an enhancement factor E for ozone mass transfer with chemical reaction could be calculated which increases with dye concentration. The chemical reaction between ozone and dye enhanced the mass transfer within the liquid film of the gas liquid boundary. The greatest enhancement factor for wastewater containing RO 96 of 2050 mgL^?1 is E = 15.4 compared with E = 9.1 for RB 5 of 3800 mgL^?1, both for gas flow rates of 19 Lh^?1. For lower gas flow rates, higher enhancement factors were observed, particularly for RO 96.     

Efficiency of a Gas‐Liquid Contactor for the Oxygenation of Activated Sludge: Assessment of Mass Transfer Coefficient

The aim of this work is to investigate a co‐current air‐liquid downward flow bubble column with air entrainment by liquid injection nozzle in order to use it as an aerator in activated sludge treatment plants. The study concerns the determination of mass transfer efficiency by measuring the mass transfer coefficient, k_La, both in clean water and in activated sludge. In clean water, this parameter is determined by three methods, i.e., gassing out method, absorption with chemical reaction and off‐gas method. In activated sludge medium, k_La values are measured by two methods, i.e., sludge reoxygenation and the hydrogen‐peroxide method. The values of k_La obtained in clean water are compared to those obtained in sludge, enabling the assessment of the α factor, i.e., ratio of oxygen transfer coefficient sludge/clean water. The results are in good agreement with those reported previously in the literature.     

Ozone Mass Transfer in the Mixing Zone of a Confined Plunging Liquid Jet Contactor

In a Confined Plunging Liquid Jet Contactor (CPLJC) a jet of liquid is introduced into an enclosed cylindrical column (downcomer) that generates fine gas bubbles that are contacted with the bulk liquid flow. The region where the liquid jet impinges the receiving liquid and expands to the wall of the downcomer is called the Mixing Zone (MZ). In the MZ, the energy of the liquid jet is dissipated by the breakup of the entrained gas into fine bubbles, and the intense recirculation of the two-phase mixture. The study presented here was undertaken to quantify the ozone-water mass transfer performance of the MZ through the determination of the volumetric mass transfer coefficient, k_La (s^?1), and to produce a model for predicting k_La based on the specific energy dissipation rate. It was found experimentally that k_La in the MZ increased with increasing superficial gas velocity. A maximum experimental k_La value of 0.84 s^?1 was achieved which compares well to other contactors used in water treatment. Such a large k_La value combined with the small volume of the reactor, favorable energy requirements and safety features of the system, suggests that the CPLJC provides an attractive alternative to conventional ozone contactors. The relatively large mass transfer rates were found to be a function of the high gas holdup and fine bubble size generated in the MZ, which results in an almost froth-like consistency. A model based on the specific energy dissipation rate of the water jet, E (kg · m^?1· s^?3), and MZ bubble size was used to predict k_La in the MZ. Using E, the number average bubble size was predicted which was then used to calculate the liquid phase mass transfer coefficient k_L. The bubble size was also used with the predicted mixing zone gas holdup to calculate the specific interfacial area, a (m^?1), which was then combined with k_L to determine a predicted value of k_La. The average deviation between experimental and predicted k_La was 6.2%.     

Toward Understanding of Two‐Phase Eccentric Helical Reactor Performance

Mass transfer investigations in a two‐phase gas‐liquid Couette‐Taylor flow (CTF) reactor and a numerical flow simulation are reported. The CTF reactor is characterized by high values of the mass transfer parameters. Previous mass transfer investigations have yielded high values of the volumetric mass transfer coefficients (of the order of 10^–1 s^–1) and the specific interfacial area, compared to those obtained in a stirred tank (10³ m² m^–3). In order to intensify mass transfer in the CTF reactor, an eccentric rotor (rotating inner cylinder) was used. In the eccentric annulus with rotating inner cylinder, due to frequent variation of the hydrodynamic flow field parameters, nonlinear hydrodynamic conditions occurred. These conditions can influence the rate of mass transfer. The experimental results of benzaldehyde oxidation in an eccentric CTF reactor confirmed an increase in mass transfer, as against a concentric CTF reactor. Numerical simulation of the Couette‐Taylor (helical) flow was performed in a concentric and in an eccentric annulus. Calculation of parameters such as velocity, static pressure, kinetic energy and energy dissipation rate revealed a significant effect of gap eccentricity on the flow behavior.     

Ozonation of Cycloalkenes Dissolved in Ethanol

The ozonation of cyclooctene and cyclododecene in ethanol was studied at 278 K in a stirred reactor with a flat gas/liquid interface. The parallel reactions with ethanol in the gas and the liquid phase had to be accounted for. At high cyclooctene concentrations, ozone transfer into the liquid phase was enhanced by up to a factor of E = 325. Due to the high ozone solubility and the high enhancement factors, up to half of the mass transfer resistance was on the gas-side. The reaction rate constant k₂ is more than threefold higher for cyclooctene as compared to cyclododecene.     

EXPERIMENTAL DETERMINATION AND MODELLING OF LIQUID-SOLID MASS TRANSFER COEFFICIENTS IN A THREE-PHASE REACTOR

Apparent mass transfer coefficients for solid dissolution in a liquid with and without a chemical reaction were experimentally determined in a fixed bed three phases reactor with downward cocurrent gas and liquid flows. The chemical system selected was benzoic acid, sodium hydroxide in aqueous solution, and atmospheric air. Continuous gas, pulse and dispersed bubble regimes were studied and the results were correlated obtaining apparent mass transfer coefficient as a function of liquid and fluid volumetric flow. It was found that gas flow effect on mass transfer coefficient was small over continuous gas and dispersed bubble regimes, but appreciable over pulse regime. Additionally, it was found that the mathematical model that best described the mass transfer process under pulse regime, by using the increment factor due to the instantaneous chemical reaction, is the film theory     

Application Of A Single-Bubble Model In Estimation Of Ozone Transfer Efficiency In Water

A single-bubble model of mass transfer in gas-liquid systems enables the estimation of transfer efficiencies under different process conditions. In particular, it can be applied to simulate the effects of bubble size, value of the mass transfer coefficient, kinetics of reactions taking place in water and depth of the contact chamber. The results of such modelling in terms of transfer efficiency are presented for physical and chemical absorption of ozone in water at different hydrodynamic conditions (bubble size, water temperature, water depth in the contact chamber, and initial ozone concentration in the bubbles). The results of computations are compared with some reference data on ozone absorption in water in industrial-scale contact chambers.