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Volatile organic compounds (VOCs) emissions are regarded as a worth concerned threat to human health. The UV-Fenton coupled with mass transfer enhanced process shows promising effects on VOCs treatment. However, the detailed mechanism and mathematical model for this method have not been established. This work focuses on the hypothesis and validation of a mathematical model for UV-Fenton removal of VOCs using activated carbon particles to enhance mass transfer efficiency. Based on the mathematical model of reaction-diffusion-mass transfer, a mathematical model is established by using a series of important parameters such as ub, Dg, Dl, Kial, Kla and hydroxyl radical lifetime. The proposed model in this study introduces the key parameter of synergistic factor, which greatly improves the consistency between the model predicted results and the experimental data (the determination coefficient R2 distribution range changed from 0.71–0.98 to 0.95–0.98). Moreover, it can also explain reasonably the steady trend of outlet VOC concentration after 30 min of reaction. The mathematical model confirms that the addition of activated carbon during the UV-Fenton reaction ensures mass transfer efficiency and considerably improves (growth from 2% to 54%) the VOCs removal efficiency due to the synergy between UV-Fenton oxidation and mass transfer enhancement. Meanwhile, it provides insight into fruitful utilization of the oxidation capacity in the oxidation reaction,and achieves the purpose of predicting the efficiency of VOC removal in the Fenton process.

For the UV/Fenton treatment of waste gas, low solubility of volatile organic compounds (VOCs) was a significant factor of limiting gas–liquid mass transfer and thus restricted the removal efficiency. In this study, synergistic effect of UV/Fenton oxidation and mass transfer enhancement with addition of activated carbon (AC) was observed through a drastic improvement of n-octane removal in a bubble column reactor. Firstly, the effect of AC on mass transfer was investigated. Result revealed that the addition of AC could change bubble behavior and largely enhance the mass transfer efficiency (volumetric mass transfer coefficient ( k L a ) increased from 0.0057 to 0.14 s −1 as 0–0.1% AC was added) due to the particle shuttle effect. Then AC was introduced into the UV/Fenton reaction system, and the removal efficiency of n-octane had been remarkably improved (maintained above 50% in 60 min with addition of 0.04% AC, compared with below 10% in 20 min for the UV/Fenton alone). Moreover, we comprehensively detected the concentration of residual reactants, intermediates, final products and hydroxyl radicals in the gas and liquid phase. Results indicated that addition of AC could mildly and stably generate hydroxyl radicals (10 −8 M), and the mass balance showed that the mineralization efficiency had increased from 42.4% to 57.9%. For theoretical calculations, the value of the cooperation factor raising from 0.5 to 11.7 in 40 min could offer direct evidence for confirming the synergistic effect. Finally, the reaction pathways of n-octane in UV/Fenton or AC-UV/Fenton were proposed. It can be induced that the latter ones were more likely to first break the C C bond and then oxidize into small molecules acids or CO 2 .

Liquid dispersion in an external-loop airlift slurry reactor was experimentally studied. The effects of the superficial gas velocity, concentration of fine particle, flowing resistance, and liquid level in the gas–liquid separator on the liquid dispersion coefficient were investigated. A liquid dispersion model was proposed based on Taylor dispersion equation to predict the liquid dispersion coefficient. According to this model, the dispersion coefficient and the combination factor u L 2 e L have a linear relationship. Validation of the liquid dispersion model together with the hydrodynamic model in our previous work was made by comparing the experimental and predicted results. The good agreement showed that the models could predict the liquid dispersion coefficient and the hydrodynamic behaviors of an airlift slurry reactor in a wide range of operating conditions with a satisfactory accuracy.

Most studies on residence time distribution (RTD) have focused on the tail of the RTD curve, and very little attention has been paid to the effect of white noise on the measured results. The aim of this work is to study the effect of white noise on the calculated parameters with different data processing methods. The anti-disturbance abilities of the moment method and the least squares method are compared. The results show that the anti-disturbance ability of the least squares method was better than that of the moment method. As a result of peak overlapping in the RTD curve of a loop reactor, the moment method cannot be used to calculate the fluid velocity and dispersion coefficient. Experiments show that the least squares method is still applicable in a loop reactor.

Inter-phase mass transfer is important to the design and performance of airlift loop reactors for either chemical or biochemical applications, and a good measurement technique is crucial for studying mass transfer in multiphase systems. According to the model of macro-scale mass transfer in airlift loop reactors, it was proved that the airlift loop reactor can be regarded as a continuous stirred tank reactor for measuring mass transfer coefficient. The calculated mass transfer coefficient on such a basis is different from the volumetric mass transfer coefficient in the macro-scale model and the difference is discussed. To describe the time delay of the probe response to the change of oxygen concentration in the liquid phase, a model taking into account the time constant of response is established. Sensitivity analysis shows that this model can be used to measure the volumetric mass transfer coefficient. Applying this model to the measurement of volumetric mass transfer coefficient in the loop reactor, results that coincide with the turbulence theory in the literate were obtained.

The flow behavior and mass transfer in a three-phase external-loop airlift reactor can be improved by adding large particles. The mass transfer and liquid dispersion behavior for a three-phase external-loop reactor with large particles are studied in terms of the effect of the diameter and loading of the large particles on the liquid dispersion coefficient and mass transfer coefficient. The results showed that increasing the diameter or loading of the large particles tend to decrease dispersion and intensify mass transfer, and that an increase in the diameter of the large particles remarkably decreases the particle loop rate, while the effect of fine particles is much less notable.

Inter-phase mass transfer is an important issue for design and development of airlift loop reactors of high performance in either chemical or biochemical applications. In this work, the axial dispersion in both gas and liquid phases was taken into account for modeling the macro-scale mass transfer in airlift loop reactors. Finite difference method was used to numerically solve the differential equation system of the mass transfer model. For oxygen, the numerical results showed that the solute concentration of the gas phase can be treated as constant and the flow pattern of the liquid phase as a plug flow. Based on the conclusion obtained from numerical solution, the mass transfer model was simplified and the analytical solution of the simplified model was obtained. Comparison between the numerical and analytical solutions showed that the simplification of the model was reasonable and there was almost no influence on the calculated results. Experimental measurements on the mass transfer rate were carried out to verify the mathematical model. The comparison between the experimental and calculated results showed that the mass transfer model has satisfactory prediction ability and can be used to describe the mass transfer process in airlift loop reactors.

The axial dispersion model was applied to the riser and down-comer, respectively, of an airlift loop reactor and a mathematical model was developed to describe its residence time distribution (RTD). A numerical solution was first obtained for the case where there is no gas–liquid separator. A tracer impulse was considered to flow in an infinite tube comprising alternating riser, gas–liquid separator and down-comer. The tracer concentration at a point in the reactor is the sum of all the tracer concentrations at the corresponding points in the infinite tube. Based on this model, an analytical solution was obtained for the mathematical model. The comparison between the analytical solution and experimental data shows that the model can be used to simulate the RTD in airlift loop reactors. By using the analytical solution in a loop reactor with partial recirculation, the solution with a recycle factor is obtained. A comparison with data in the literature shows that the solution with the recycle factor can be used for the RTD in a loop reactor with partial recirculation.

One important challenge in chemical industry, especially for processes of synthesis fuels, is the development of effective three-phase reactors for high pressure and temperature. In the present study, a novel airlift internal-loop reactor (ALR) was proposed and its hydrodynamic behaviors were experimentally studied. The local bubble rise velocity and liquid velocity are measured under different operating conditions. The influence of operating conditions including the superficial gas velocity and solid holdup on the bubble rise velocity and liquid velocity in the riser was studied. A mathematical model was proposed based on the energy balance to describe the multiphase flow characteristics in the internal-loop airlift reactor. The comparison between the calculated and experimental values shows that the proposed model can predict the flow behavior reasonably.

Small bubbles and flow uniformity are important for gas–liquid and gas–liquid–solid multiphase reactors. A reactor internal was designed and installed in an external-loop airlift reactor (EL-ALR) to enhance bubble breakup and flow redistribution and improve reactor performance. Hydrodynamic parameters, including local gas holdup, bubble rise velocity, bubble Sauter diameter and liquid velocity were measured. A radial maldistribution index was introduced to describe radial non-uniformity in the hydrodynamic parameters. The influence of the internal on this index was studied. Experimental results show that The effect of the internal is to make the radial profiles of the gas holdup, bubble rise velocity and liquid velocity radially uniform. The bubble Sauter diameter decreases and the bubble size distribution is narrower. With increasing distance away from the internal, the radial profiles change back to be similar to those before contact with it. The internal improves the flow behavior up to a distance of 1.4 m.

Gas holdups and the rising velocity of large and small bubbles are measured using the dynamic gas disengagement approach in a pressured bubble column of 0.3 m in diameter and 6.6 m in height. The effects of superficial gas velocity, liquid surface tension, liquid viscosity, and system pressure on the gas holdups and the rising velocity of small and large bubbles are investigated. The holdup of large bubbles and the rising velocity of small bubbles increase with increasing liquid viscosity and liquid surface tension. Meanwhile, the holdup of small bubbles and the rising velocity of a swarm of large bubbles decrease. Moreover, the holdup of large bubbles and the rising velocity of a swarm of small bubbles decrease with increasing system pressure. A correlation for the holdup of small bubbles is obtained from experimental data.

The local hydrodynamic behaviors, including the gas holdup, bubble size and bubble rise velocity were measured in an external loop airlift reactor (EL-ALR) with two types of distributor—porous sinter plate and perforated plate, using an optic fiber probe. The radial and axial evolutions of these parameters and the influence of the gas distributor on the flow hydrodynamics were studied. Core-peaking and wall-peaking radial profiles of the gas holdup were found in the experiments corresponding to different bubble sizes. A mechanism model based on the assumption of the equilibrium of lateral forces acting on a bubble was proposed to interpret the different radial profiles of the gas holdup.

Detailed local three-phase hydrodynamic parameters, namely, the liquid velocity, bubble rise velocity, and gas holdup, were obtained for an external-loop airlift reactor using a fiber-optic probe and laser Doppler anemometry techniques. The radial and axial evolutions of these parameters and the effect of the solid fraction on the flow hydrodynamics were studied. The experiments showed that the bubble size has a large influence on the evolutions of these parameters because of radial forces acting on the bubbles. Bubble coalescence prevails near the distributor, while bubble breakup is more important in the fully developed flow. The liquid velocity and gas holdup decrease and the bubble rise velocity increases as the solid fraction increases.