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1.
A computer simulation of a bubble column is established to determine the residual ozone concentration in the air and the dissolved ozone residual in the water. This study is aimed to improve the control of ozonation systems, both technically and economically. The program is based on mass balances along the contact column, which take into account the ozone consumption due to both the self-decomposition and the reactions with organic compounds contained in the water. The experimental measurements allow quantification of the ozone concentration in the air at the inlet and outlet of the pilot unit, as well as the dissolved ozone concentration at different heights along the column. A relation between the transfer coefficient, kLa, and the superficial velocity of the ozonated air is established. It is specific to the diffusion characteristics of the pilot unit. The kLa then is reintroduced in the program. The calculated and the measured values are shown to be similar regarding the transfer yields, the dissolved ozone concentrations at the pilot unit outlet and the profiles of dissolved ozone concentrations along the contact column. Using the program, the influence of the most important parameters of ozonation on the transfer (treatment rates, initial ozone concentration in the air, pH and organic content of the water, kLa values) have been simulated. 相似文献
2.
A computer model based on the establishment of mass balance equations and on the model of fluids flow “stirred tank in series” was developed in order to calculate the ozone transfer coefficient k La and kinetic constant k c of ozone consumption by water. On the basis of experimental data, the correlation for gas holdup ε g and bubble diameter d vs, were proposed and used to calculate the specific interfacial area a. The liquid-phase mass transfer coefficient k L for ozone was evaluated from a and the k La data. 相似文献
3.
The purpose of this research was to study the scale-up behavior of dye RB-19 ozonation in a new gas-inducing reactor, which has been used in the NTUST laboratory on various ozonation studies over the past few years. In this scale-up study, three geometrically similar gas-inducing reactors with different diameters ( D t =0.17, 0.29, and 0.51m) were employed. Three common scale-up criteria (i.e., equal liquid surface motion, equal specific power consumption, and equal impeller tip velocity) were investigated in this research. Under the equal liquid surface motion criterion, the scale-up exponent value and constant K of the modified onset Froude number were determined to be 0.5 and 0.61, respectively. The equal specific power consumption criterion was studied under gas input condition and the scale-up exponent was found to be 0.65. The regression equation for the power number of the three different scale reactors was also obtained. The scale-up exponent of equal impeller tip velocity was determined by theory to be 1.0. The scale-up investigation of dye RB-19 ozonation was then carried out in reactors with three different diameters under the same operating conditions (e.g., initial dye concentration, initial dye/ozone molar ratio, superficial gas velocity, temperature and pH value). From the experimental results, the best-fit scale-up exponent was found to be 1.18, resulting in same dye removal rate in reactors with different diameters. The enhancement factors and chemical ozone mass transfer coefficients were also obtained for these sets of ozonation experiments. 相似文献
4.
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%. 相似文献
5.
A perforated spinning disc ozone contactor is described with reference to its use as an absorber with simultaneous chemical reaction. Greatly enhanced mass transfer coefficients kL are measured whilst simultaneously maintaining low ozone loss. Comparisons of kL and volumetric coefficient, kL a values, are made with more conventional packed or bubble columns. Acetic acid, 2-propanol and 4-nitrophenol, representing a wide reactivity range, are used to elucidate the applicability of rotating contactors in effluent treatment. It has been possible to study the effects of surface activity on mass transfer with subsequent reaction and to generate design data for the next generation of rotating contactors. 相似文献
6.
Ozonation of wastewater containing azo dye has been studied to evaluate the enhancement of ozone mass transfer from O 2O 3 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. 相似文献
7.
This study aims at applying artificial neural network (ANN) modeling approach in designing ozone bubble columns. Three multi-layer perceptron (MLP) ANN models were developed to predict the overall mass transfer coefficient ( kLa, s ?1), the gas hold-up (? G , dimensionless), and the Sauter mean bubble diameter ( dS , m) in different ozone bubble columns using simple inputs such as bubble column's geometry and operating conditions. The obtained results showed excellent prediction of kLa, ? G , and dS values as the coefficient of multiple determination ( R2 ) values for all ANN models exceeded 0.98. The ANN models were then used to determine the local mass transfer coefficient ( kL , m.s ?1). A very good agreement between the modeled and the measured kL values was observed ( R2 ?=?0.85). 相似文献
8.
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. 相似文献
9.
The kinetics of ozonation of the sulfur dye Methylene Blue in aqueous solution is studied. The experiments have been carried out in a bubble column, using a single nozzle as gas sparger. The influence of the operating variables (initial concentration of Methylene Blue, ozone partial pressure, temperature and pH) on the oxidation process is established. A stoichiometric ratio of 3 moles of ozone consumed per mol of dye reacted is deduced. A reaction scheme based on the direct ozone attack to the dye is suggested, and it agrees with the found stoichiometry. The overall reaction orders and rate constants as a function of pH and temperature are determined following the film theory which is applied to the experimental data. The process develops in the fast pseudo mth order kinetic regime of absorption. 相似文献
10.
A kinetic model constituted by ozone mol balance equations both in the gas and in the water phases and a total mole balance equation has been applied to predict concentrations of dissolved ozone, C o3, ozone partial pressure at the reactor outlet, P (o3)0, and remaining chemical oxygen demand, COD, for the ozonation of two industrial wastewaters released from distillery and tomato processing plants. Kinetic equations for ozone absorption rate present in the model were derived from the application of film theory to an irreversible gas-liquid reaction. Parameters involved in the model, reaction rate and mass transfer coefficients, Henry's law constant, etc., were estimated from bench-scale experiments. The model was applied to ozonation in bubble contactors of height/diameter ratio equal to that of the bench scale contactor and to a pilot plant bubble column of a height/diameter ratio about 3.6 times higher. 相似文献
11.
利用臭氧降解含较高浓度邻苯二甲酸二甲酯(DMP)的模拟废水,考察了pH、DMP初始浓度、臭氧投加量和温度对臭氧降解DMP效果的影响,并对臭氧降解DMP的效能及动力学进行了研究。结果表明,pH为5~10时DMP的降解效果较好;DMP初始浓度越大,DMP降解率越小,但DMP降解量增加;臭氧投加量的增加有利于DMP降解,但臭氧利用效率降低;温度低时,升高温度DMP降解率增大,24℃后提高温度对DMP降解率影响不大。在pH为5~10、DMP初始浓度为14~66 mg·L -1、O 3投加量为14~24 mg·min -1、20~30℃条件下反应40 min,DMP降解率达85%以上。在实验条件范围内,臭氧降解DMP符合拟一级动力学,建立了幂指数表达的动力学模型。在pH为2~12范围内,因臭氧反应机理不同,表观动力学常数k obs与pH不呈线性关系,pH为2~5时,k obs快速增加,pH为5~9时,k obs缓慢增加,pH大于9时k obs反而降低,pH为9时k obs最大,为0.0929 min -1。 相似文献
12.
In the present paper, the effect of liquid properties (surfactants) on bubble generation phenomenon, interfacial area and liquid-side mass transfer coefficient was investigated. The measurements of surface tension (static and dynamic methods), of critical micelle concentration (CMC) and of characteristic adsorption parameters such as the surface coverage ratio at equilibrium ( se) were performed to understand the effects of surfactants on the mass transfer efficiency. Tap water and aqueous solutions with surfactants (cationic and anionic) were used as liquid phases and an elastic membrane with a single orifice as gas sparger. The bubbles were generated into a small-scale bubble column. The local liquid-side mass transfer coefficient ( kL) was obtained from the volumetric mass transfer coefficient ( kLa) and the interfacial area ( a) was deduced from the bubble diameter ( DB), the bubble frequency ( fB) and the terminal bubble rising velocity ( UB). Only the dynamic bubble regime was considered in this work ( ReOR=150-1000 and We=0.002-4).This study has clearly shown that the presence of surfactants affects the bubble generation phenomenon and thus the interfacial area ( a) and the different mass transfer parameters, such as the volumetric mass transfer coefficient ( kLa) and the liquid-side mass transfer coefficient ( kL). Whatever the operating conditions, the new kLa determination method has provided good accuracy without assuming that the liquid phase is perfectly mixed as in the classical method. The surface coverage ratio ( se) proves to be crucial for predicting the changes of kL in aqueous solutions with surfactants. 相似文献
13.
This research deals with the decolorization of synthetic wastewater, prepared with the acid 1:2 metal-complex textile dye C.I. Acid Blue 193, using the ozonation (O 3) and H 2O 2/O 3 processes. To minimize the number of experiments, they were performed using the 2 k factorial design. Five influential parameters were examined: initial dye concentration, ozone flow rate, initial pH value, decolorization time and H 2O 2 addition. The decolorization efficiency was 95% in 20 minutes (pH = 7; O 3 flow rate of 2 g/L.h) and a higher increase in the toxicity after the ozonation process (39%) indicates the formation of carcinogenic by-products. According to the variance test analysis, the initial dye concentration, the ozone flow rate, the initial pH value and the decolorization time and their first- and second-order interactions are significant, while the H 2O 2 addition was not important with respect to the discussed range. With the help of these significant factors a regression model was constructed and the adequacy of the model was checked. The obtained regression polynomial was used to model the relation between the absorbance and the influential parameters by fitting the response surface. This response surface may be used to predict the absorbance result from a set of influential parameters, or it can be rearranged in such a way as to predict the set of process decolorization parameters necessary to reduce the absorbance of wastewater with the given initial dye concentration, below the prescribed limit. It is also shown that the 2 k factorial design can be suitable for predicting the operating expenses of the ozonation. 相似文献
14.
This work was aimed at investigating the potential use of ozone flotation for recovered papers deinking. The mixing characteristics and the bubble size of a Venturi aerated laboratory flotation column were studied in the presence of simplified model systems and of an industrial pulp slurry. Experimental results showed that surfactants dissolved in the pulp slurry stabilized air bubbles while cellulose fibers promoted coalescence. Moreover, bubble buoyancy was sufficient to fluidize the fiber suspension generating perfect mixing. The gas–liquid transfer coefficient of ozone ( kLa) estimated from gas hold-up, air bubble size and kLa measured in water was high enough (0.84 1/s) for the complete transfer of ozone in the pulp slurry and the generation of ozone-free gas effluent. With O 3 dosage of 0.8 mg O 3/mg COD, ozone flotation allowed increasing chemical oxygen demand removal from 41 to 63% with no effect on the ink flotation efficiency, which remained close to that obtained with air flotation, i.e. ~92%. 相似文献
15.
A theoretical analysis is performed employing the film model for the isothermal absorption and self-decomposition of ozone in aqueous solutions with interfacial resistance, which is inversely proportional to the interfacial mass transfer coefficient k s. A closed-form solution has been obtained. The effects of system parameters on the ozone mass transfer rate are examined. These parameters include the interfacial resistance (1/k s), the acidic and basic self-decomposition reaction rate parameters (M m 0.5, M n 0.5.; M m = [2D Ak mC Ai m-1/(m+1)]/(k L 0) 2, M n=(2D Ak nC Ai n-1/(n+1))/(k L 0) 2, the reaction orders (m,n), the pH value of solution, and the liquid-phase mass transfer coefficient (k L 0). The results indicate that the reduction effect of the interfacial resistance on the absorption rate is most significant for the situation with the larger values of M m and M n as well as with higher pH values. Also, for any particular finite value of k L 0/k s, the reduction effect encountered is greater for a gas liquid contactor with a lower k L 0. The reduction effect should be avoided in order to maintain a higher mass transfer rate of ozone in aqueous solution. This analysis is of importance for the efficient use of ozone in water/wastewater treatment processes in the presence of interfacial resistance substances such as surface active agents. For some known special cases (for example, cases with no interfacial resistance), the present solution reduces to the previous works of other investigators. 相似文献
16.
The alcohol t-butanol has been used as a radical scavenger in the studies of ozone reactions in water and has been found to affect the gas-liquid mass transfer rates. An understanding of the effects of t-butanol on mass transfer parameters, including bubble size, gas holdup, mass transfer coefficient and the mass transfer specific surface area, is of key importance to not only improve the knowledge of this particular system but also to gain fundamental understanding about the effects of gas/liquid surface modifiers on the contact between phases and the mass transfer rates. An experimental study has been carried out to investigate the effects of t-butanol concentrations on the physical properties of aqueous solutions, including surface tension and viscosity. It was found that t-butanol reduced both properties-by 4% for surface tension and by a surprising 30% for viscosity. These reductions in the solution physical properties were correlated to enhancement in the mass transfer coefficient, kL. The hydrodynamic behaviour of the system used in this work was characterised by a homogeneous bubbling regime. It was also found that the gas holdup was significantly enhanced by the addition of t-butanol. An equation to predict the gas holdup from the gas flow rate and t-butanol concentration was proposed to describe the experimental data. Moreover, the addition of t-butanol was found to significantly reduce the size of gas bubbles, leading to enhancement in the volumetric mass transfer coefficient, kLa. Bubble mean diameter was predicted using an equation developed by the Radial Basis Function Neural Network architecture obtained from the literature, and the mass transfer coefficient, kL, was predicted using an equation based on the surface coverage ratio model. The ratio was found not to depend either on t-butanol concentration or on gas flow rate. A significant increase in the volumetric mass transfer coefficient, kLa, due to an increase in both kL and a, was obtained following the addition of t-butanol, even at low concentrations. 相似文献
18.
The individual phase holdups and mass transfer characteristics in three-phase fluidized beds with different floating bubble breakers have been determined in a 2.0 m high Plexiglas column of inner diameter 0.142 m. The bubble breaking phenomena by the breakers have been studied via a photographic method in a two-dimensional Plexiglas column. The volumetric mass transfer coefficient kLa in three-phase fluidized beds with hexagonal-shaped breakers is up to 40% greater than that in beds without floating bubble breakers. The bed porosity ε L + ε g, gas-phase holdup ε g, and volumetric mass transfer coefficient kLa increase with an increase in the volume ratio of floating bubble breakers to solid particles, Vf/ Vs, up to around 0.15, and thereafter decrease with Vf/ Vs in three-phase fluidized beds with floating bubble breakers. Also, kLa increases with increasing breaker density, projected area and contact angle between the floating bubble breakers and the water. The volumetric mass transfer coefficients in terms of the Sherwood number in three-phase fluidized beds with the various floating bubble breakers have been correlated with the volume ratio of floating bubble breakers to solid particles, the particle Reynolds number based on the local isotropic turbulence theory and the modified Weber number. 相似文献
19.
In spite of their simple construction the scale up of bubble columns of industrial size demands application of models which account for dispersion effects and variations of pressure and gas flow rate. However, using such models and parameter values obtained from other studies it was not possible to describe successfully measured conversions of the absorption of isobutene in a 7 m bubble column though the interfacial area was determined separately. The measurements were carried out under such conditions at which the absorption takes place in the slow reaction regime of mass transfer. A sufficient agreement between experimental and predicted conversions could be obtained merely if a lower value of kL was used. A more detailed analysis of bubble size distributions indicated that the decrease of kL may be apparently only since the interfacial areas determined photographically must not necessarily be the area which is effective to mass transfer. kLa-values in larger bubble columns with gas spargers which are common in industry are considerably lower than kLa-data found in smaller columns with porous gas distributors. 相似文献
20.
This paper focuses on the effect of surfactants on the mass transfer parameters (volumetric mass transfer coefficient kLa and liquid-side mass transfer coefficient kL). Tap water and aqueous solutions with surfactants (anionic, cationic and non-ionic at concentrations up to are used as liquid phases. The bubbles are generated into a small-scale bubble column having an elastic membrane with a single orifice as gas sparger. To understand the effects of the surfactants on the mass transfer, not only the static surface tension is used, but also the characteristic adsorption parameters like the surface coverage ratio at equilibrium Se. The liquid-side mass transfer coefficient is obtained from the ratio of the volumetric mass transfer coefficient (measured by a chemical method) and the specific interfacial area. These two parameters are obtained simultaneously. The methods used to obtain these parameters are described in Painmanakul et al. [2005. Effects of surfactants on liquid-side mass transfer coefficients. Chemical Engineering Science 60, 6480-6491].Whatever the liquid phase, three zones are found on the liquid-side mass transfer coefficient variation with the bubble diameter. For bubble diameters less than 1.5 mm, whatever the liquid phases, the kL values are roughly constant at . For bubble diameters greater than 3.5 mm, the kL values do not vary much with the bubble diameter, but depend on the surfactant concentration. For bubble diameters between 1.5 and 3.5 mm, the kL values increase from to the value reached at 3.5 mm. This increase depends on the surfactants. Higbie's model does not represent the kL values for bubble diameters greater than 3.5 mm, even though there is a small amount of surfactant in the liquid phase. Thus, a model is proposed for each zone described above. Explanations are also proposed for the effect of the surfactant on the kL values for each of the above zones. 相似文献
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