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.     

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.     

Ozonation of C.I. Reactive Black 5 and Indigo

The ozone transfer for the ozonation of the azo dye Reactive Black 5 (RB5) and indigo was investigated using a bubble column at semi-batch conditions. The results were analyzed by applying film theory and surface renewal theory. The ozonation of both dyes was so fast that an instantaneous reaction directly at the bubble surface can be assumed. The ozone balance in the gas phase was used to determine the volumetric mass transfer coefficient k_La and the enhancement factor E. Besides the measured concentrations, temperature and pressure only the Henry coefficient was required for the determination of k_La and E. By varying the ozone inlet and the dye concentration the reaction regime was identified. The Hatta number Ha which requires uncertain parameters did not have to be determined.     

Optimal Operating Conditions For Lab-Scale Ozonation Reactors

A method for the assessment of the optimal operating conditions for a mechanically stirred gas–liquid reactor is presented. The method exploits both fluid dynamic and chemical information. First, the behavior of the specific stirring power as a function of the stirrer speed allows singling out the dispersion region, in which the most efficient gas–liquid mass transfer is achieved. Inside this region, the analysis of experimental data obtained when considering a chemical system reacting at moderate Hatta numbers (i.e., Ha < about 2) allows determination of the rate constants and the fluid dynamic parameters (i.e., the mass transfer coefficient in the absence of chemical reaction and the characteristic diffusion time).     

Combined Pre-Treatment of Coagulation-Ozonation for Saline-Stabilized Landfill Leachates

Coagulated stabilized leachates were ozonated under a continuous regimen at the laboratory scale. Experimental results showed that the coagulation pretreatment allows the removal of complex organic matter related to color in stabilized landfill leachates. Infrared analysis of coagulated organic matter sludges suggests that more than 90% of leachate color is due mainly to the presence of humic acids. These acids are taken from the colloidal particles (0.34–1.0 μm) contained in landfill leachate. The effect of ozone on residual color removal displays a fast kinetic reaction. The ozone absorption process is associated to reactions with easily oxidizable organic compounds, such as those related to color in stabilized landfill leachates. The experimental results of ozone mass transfer revealed that ozone absorption depends on gas velocity and is substantially favored by chemical reaction. The estimated k′_La values (2‐7 s^?1) had the same order of magnitude as those reported in treated wastewater. The enhancement factor (E) was estimated to be in the range 3 to 11.     

Aqueous Ozonation of Pesticides: A Review

The ozonation reactions of pesticides in aqueous solution have been reviewed. Degree of reaction and reaction product identity are included. Compounds are classified into five groups: chlorinated hydrocarbons, organophosphorus compounds, phenoxyalkyl acid derivatives, organonitrogen compounds, and phenolic compounds. Experimental conditions for each study are summarized. Much work has been carried out under conditions atypical of those encountered at drinking water treatment plants. Thus, the findings of the papers reviewed cannot be applied directly to potable water treatment.     

Application of Gas Absorption Theories to o-Cresol Ozonation in Water

The reaction rate between ozone and o–cresol in aqueous solution at pH 2 has been studied. According to the absorption theories, the kinetic regime of the process in the experimental conditions investigated, corresponds to an irreversible fast pseudo-first order reaction between non-dissociated o-cresol and ozone. Similar values for the rate constant are deduced from the different theories. Therefore the film theory, having a simple mathematical basis, seems to be the most appropriate model for this kinetic study.     

Modeling of Ozonation for Dissolved Ozone Dosing

Experimental research was carried out for calibration and validation of a model describing ozone decay and ozone exposure (CT), decrease in UV absorbance at 254 nm (UVA₂₅₄), increase in assimilable organic carbon concentration and bromate formation. The model proved to be able to predict these parameters on the basis of the applied ozone dosage. The experimental ozone dosages ranged from 0.4 mg-O₃/L to 0.9 mg-O₃/L for natural water with a dissolved organic carbon concentration of 2.4 mg-C/L. The UVA₂₅₄ was found to be an effective parameter for estimation of rapid ozone decay for natural water under experimental conditions tested. The experimental setup consisted of a bench-scale plug flow reactor (approximately 100 L/h) with dissolved ozone dosing.     

Evaluating Dissolved Ozone in a Bubble Column Using a Discrete-Bubble Model

Ozonation has been widely applied in water treatment plants, so it is essential to figure out mass transfer and reaction kinetics in ozone contact tanks. In this study, a discrete-bubble model was established to calculate the dissolved ozone concentration in a countercurrent flow system. Results from batch experiments showed that an exponential relationship was well fitted between ozone reaction rate coefficient and ozone consumption amount, and the relationship was not affected by the ozone dosage. Therefore, it was applied in a discrete-bubble model, which was validated by experiments in a countercurrent bubble column with error less than 15%.     

Kinetic Regime Changes in the Ozonation of 1,3-Cyclohexanedione in Aqueous Solutions

The reaction between ozone and 1,3-cyclohexanedione in aqueous solutions at different pH was investigated from the point of view of the film theory. The rate of ozone absorption is accompanied by a second order irreversible reaction with 1,3-cyclohexanedione. The kinetic regime of absorption was found to change from fast (the reaction being of pseudo first order) to instantaneous, according to the ozone partial pressure applied. The former kinetic regime allows the determination of the reaction rate constant, while the latter leads to the volumetric mass transfer coefficient of the system.     

A new way of enhancing transport process –The hybrid process accompanied by ultrafine particles

A brief review is presented on the enhancing effects of fine particles on multiphase mass transfer in the first part of this paper. Some experimental results on the following aspects are shown and discussed: influence of fine particles on mass transfer parameters (k_La, k_L, a), enhancement due to particles adsorption and catalyzing a chemical reaction, and effects of particle size. The second part of this paper proposes two kinds of hybrid process accompanied by ultrafine particles: adsorptive distillation and slurry catalytic distillation. Finally, prospective fields for additional research 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).     

Ozonation of Crotonic Acid Solutions: Comparison of Two Methods for the Determination of the Rate Constant

This work is related to the determination of the rate constant during direct ozonation of crotonic acid which is limited by the transfer rate of ozone from the gas to the liquid. According to the literature, this constant would be 150 10³ L/mol/s at pH 7, but this estimation is supported only by experimental results which are characteristic of the “instantaneous reaction regime.” It is demonstrated in this article that such a regime does not allow the correct determination of this rate constant. On the basis of the film model and by writing the mass balances for the reactor, a computer method was developed that enables the model results to be fitted with the experimental results and the rate constant to be determined. This method is used to analyze a series of results obtained in a bubble column batch reactor which are characteristic of the “intermediate reaction regime,” where the determination of the rate constant is possible. The value of 500 10³ L/mol/s gives an excellent fitting.     

Henry And Mass Transfer Coefficients In The Ozonation Of Wastewaters

A procedure for the determination of Henry and mass transfer coefficients in an ozone-industrial wastewater system is presented. The method is applied to the ozonation of a tomato plant industrial wastewater, developed in the slow kinetic regime. In so doing, molar balances of ozone (in gas and water phases) are used together with gas-liquid reaction kinetic theory. While Henry's coefficients obtained are similar to those corresponding to ozonation in organic-free water, significant deviations are observed regarding the mass transfer coefficient.     

Mathematical Modeling of Theoretical Cryptosporidium Inactivation in Full-Scale Ozonation Reactors

Conditions for theoretical inactivation of Cryptosporidium by ozone could be achieved at full-scale facilities if their design is appropriate. To perform this task correctly the chemical engineer's approach for process design must be applied. This paper discusses the basic equations the estimation of the disinfection efficiency of different ozone reacting systems. Available kinetic data have been integrated in a global model accounting for the hydrodynamics and mass transfer performances of the ozonation reactor. Thus the proposed method allows one to predict Cryptosporidium inactivation level in a given ozonation system. However, if a specified disinfection goal is to be achieved for Cryptosporidium with the developed model it is also possible to choose and optimize the design of the ozone reactor.     

Significance of Acoustic and Hydrodynamic Cavitations in Enhancing Ozone Mass Transfer

Assessment of both acoustic and hydrodynamic cavitations for intensifying ozone mass transfer was conducted simultaneously. Four process schemes were arranged to evaluate the effect of application of each kind of cavitation as well as both of them, on the ozone mass transfer process. All processes were conducted at pH of 3 to avoid ozone natural self-decomposition initiated by hydroxide ions (OH^?). The chemical and mechanical effects of cavitation were distinguished by using radical scavengers to suppress radical formation. The result showed that hydrodynamic and acoustic cavitations enhanced ozone mass transfer synergistically. The enhancement obtained from the acoustic cavitation was higher than that obtained from the hydrodynamic cavitation, and the chemical effects of cavitation were much significant than that of mechanical effects. The enhancement obtained due to chemical effects of cavitation was about twice the enhancement obtained due to mechanical effects when only one type of cavitation was combined with ozonation. Combination of both type of cavitation and ozonation gave the enhancement obtained due to chemical effect of 3.68 times that obtained due to mechanical effects.     

Ozonation Dynamics and Its Implication for Off-Gas Ozone Control in Treating Pulp Mill Wastewaters

Ozonation of biologically pretreated pulp mill wastewaters was studied using both bench and pilot scale fine bubble contactors to determine the oxidation efficiencies, mass transfer coefficients (k_La) and enhancement factors (E) due to the occurrence of chemical reactions. A sensitivity analysis based on the measured process parameters was then used to reveal the interrelated effects of key factors on off-gas ozone concentrations. It was shown that the removal efficiencies of color and AOX were simply related to the amount of utilized ozone, regardless of variation of other operating conditions. Furthermore, the rate of absorption fell within the fast or instantaneous kinetics regimes due to the occurrence of rapid chemical reactions. The Ek_La values were found to vary substantially during the course of ozonation, indicating that the enhancement factors were not only affected not only by operating conditions but also by wastewater characteristics. To effectively control the off-gas ozone emission, measures should be taken to minimize the backmixing, use a counter-current flow arrangement and provide adequate contact time.     

Ozone Application during Coagulation of Wastewater: Effect on Dissolved Organic Matter

The aim of this work was to determine the effect of ozone on dissolved organic matter (DOM) during wastewater coagulation using alum. Adding ozone to the coagulation treatment (O₃ECT) enhanced the quality of the final effluent in comparison to conventional coagulation treatment (CT). Final effluents were analyzed by Fourier transform infrared Spectroscopy (FTIR) and UV-Vis spectroscopy. The hydrophilic polar fraction was separated from the hydrophobic fraction using fractionation resins XAD-8 and XAD-4. Wastewater hydrophilic DOM was characterized by molecular weight distribution (MWD), and, BOD₅, TOC and DOC parameters. Results show that FTIR spectra for both effluents were very similar in the frequency range 7000 to 1400 cm^?1; however, some differences occurred in the O₃ECT effluent in the range 1100 to 700 cm^?1, which indicated that amines and aromatics groups were affected. In the same way, the UV-Vis spectra showed that there was an increase in absorbance within the wavelength range 285 to 300 nm for the O₃ECT effluent (when compared with CT) while the absorbance decreased in the range 210 to 455 nm, particularly at 380 nm. With regards to the hydrophilic polar compounds of DOM, the most abundant fraction in raw wastewater was found in the 3–10 kDa range. After CT, the effluents fraction concentrations decreased in all cases, but for O₃ECT the 10–30 kDa range exhibited a marked increase while the smaller (<3 kDa, 3–10 kDa) and larger fractions (>30 kDa) decreased. The ozone application during coagulation slightly increased TOC and DOC percentage removals, 84.4% to 89.1% and 80% to 82.5%, respectively. This suggests that the small dose of ozone transferred (1.47 mg/L) caused only minor changes in DOM. Finally, the O₃ECT effluent exhibits 10 mg/L more of biological oxygen demand (BOD₅) than a single CT.     

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.