Removal of volatile organic compounds by cryogenic condensation followed by adsorption

Removal of volatile organic compounds (VOCs) from gaseous effluents by cryogenic condensation and adsorption has been studied. Mathematical models have been developed to predict the extent of removal of a binary mixture of VOCs in air by these two methods under a wide range of operating conditions. The model results are verified with the published work. A model parametric study carried out in this work suggests that if the concentrations of VOCs in the effluent stream vary over a wide range, condensation followed by adsorption is an effective technique to control the emissions. Condensation is found to be suitable if the VOCs emission levels are high (>1%). On the other hand, if the emission levels are low i.e. parts per millions (ppm) or sub ppm, adsorption is a preferred technique for removing the VOCs from the effluent stream. The model results in this work have significance from the perspective of understanding the mechanism of removal of VOCs by these two methods, determining the key operating parameters that control the removal process and also, defining an effective VOC control strategy.     

钛杂化笼型聚倍半硅氧烷吸附苯胺废水的动/热力学以及吸附机理研究

采用自行合成的一种新型钛硅固体材料——钛杂化笼型聚倍半硅氧烷,吸附处理质量浓度50 mg/L的苯胺废水。结果表明,当钛杂化笼型聚倍半硅氧烷用量2 g/L,pH值4.0,常温吸附20 min时,其吸附率达到64%。吸附符合Freundlich模型,属于优惠吸附等温线。通过计算获取ΔG0,ΔH0,ΔS0等吸附参数数据,证实了钛杂化笼型聚倍半硅氧烷吸附苯胺的趋势;同时依据参数值推测了苯胺分子在催化剂表面被吸附的机理。     

Liquid and vapor phase adsorption of chlorinated volatile organic compounds on hydrophobic molecular sieves

This work focused on the potential application of various hydrophobic molecular sieves for the sorption of four model chlorinated volatile organic compounds (CVOCs, i.e., chloroform, trichloroethylene, tetrachloroethylene, and carbon tetrachloride) from dilute liquid water streams.

Results obtained thus far have shown that silicalite-1 has a high affinity for these CVOCs, higher in fact than Centaur^® activated carbon, used as a benchmark in this study. Loading results for trichloroethylene from both liquid and vapor phase indicated that the liquid phase did not penetrate the pores of silicalite-1, while the solution did penetrate the pores of Centaur^® and a dealuminated NaY used for comparison.

Finally, three definitions from the literature for the “hydrophobicity” of molecular sieves were considered. An alternative definition for hydrophobicity is introduced here, which is easy to determine and is based on water retention.     

A mini-review on the modeling of volatile organic compound adsorption in activated carbons: Equilibrium,dynamics, and heat effects

The research on the adsorption equilibria, kinetics, and increase in process temperature of the volatile organic compound (VOC) adsorption in porous materials ensures safe production, thereby reducing production costs and improving separation efficiency. Therefore, it is critical in predicting the entire adsorption process based on minimal or no experimental input of the adsorbate and adsorbent. We discuss, in this review, the factors that affect the adsorption performance of VOCs in activated carbons, including the adsorption equilibrium, adsorption kinetics, and exotherm during adsorption. Subsequently, the existing prediction models are summarized and compared concerning the adsorption equilibrium, adsorption kinetics, and exothermic process of adsorption. We then propose a new prediction model based on intermolecular interaction and provide an outlook toward the design and manipulation of efficient adsorbents for the VOC system.     

The pH influence on photocatalytic decomposition of organic dyes over A11 and P25 titanium dioxide

The photocatalytic removal (decomposition+adsorption) of four azodyes (Reactive Red 198, Acid Black 1, Acid Blue 7 and Direct Green 99) in water was investigated using Tytanpol A11 (“Police” Chemical Factory, Poland) and Degussa P25 (“Degussa”, Germany) as photocatalysts. The effect of pH of the reaction solution has been examined. The degree of the dye removal in the solution was measured by UV-Vis spectroscopy. Photodecomposition of dye on photocatalyst surface was monitored by FTIR spectroscopy. A11 photocatalyst has lower activity in the reaction of photocatalytic decomposition of organic dyes than Degussa P25. The photocatalytic decomposition of the dyes takes place on the photocatalyst surface at pH=2 while at pH=12 photocatalytic reaction proceeds via photogenerated hydroxyl radicals for both A11 and P25.     

挥发性有机化合物的净化处理技术

有机废气中大多含有低浓度的苯、甲苯、苯乙烯、多环芳烃等挥发性有机化合物（VOCs）。治理VOCs污染是大气污染治理的重要组成部分。叙述了吸收法、吸附法、生物法和低温等离子体4种废气净化技术的原理和国内外研究进展情况,并对其发展前景和研究方向进行了探讨。最后认为有机废气的联合协同处理方法是今后的一个重要研究方向。     

Catalytic oxidation of trichloroethylene in two-component mixtures with selected volatile organic compounds

The activity of two noble metal catalysts (Pt and Pd) on metallic monolithic support and one perovskite (La_0.5Ag_0.5MnO₃) on cordierite monolith was tested in the oxidation of selected volatile organic compounds (VOCs) and trichloroethylene (TCE), oxidized alone and in two-component mixtures of TCE with a non-halogenated compound. Only over the Pt catalyst each compound in the reaction mixtures strongly enhanced TCE oxidation. Over Pd, promoting effect on TCE oxidation was observed for toluene and ethanol only. Over perovskite, each non-chlorinated compound was found to inhibit TCE oxidation. The presence of TCE was found to inhibit the oxidation of each compound added over both noble metal catalysts, but it had no influence on the oxy-derivatives oxidation over the perovskite catalyst.     

植物释放VOOs的研究

植物源释放的挥发性有机物（BVOCs）占到了全球挥发性有机物（VOCs）排放总量约70％,在生态系统中起着重要的作用,且与人们健康息息相关,对环境安全和人类生存健康具有重要的影响。主要综述了BVOCs中异戊二烯及单萜烯等几种重要的植物释放物,概括了它们的分类、作用、释放影响因素以及对它们的研究情况。     

Modeling of volatile organic compounds condensation in fluidized bed

A model of volatile organic compound condensation in dense fluidized bed was used to simulate a new depollution process by separation in fluidized bed. The volatile compounds are condensed and captured by fluidized porous particles before being renewed in an evaporation reactor and recirculated to the condensation riser. This study shows how a large bed height, a small superficial gas velocity, a cold wall temperature provided that the condensation products that do not turn into solid state give a high and stable abatement rate as well as it provides a good use of the internal porosity of the particles. It is also demonstrated that, as already known after thermodynamics constraints, this condensation process is better adapted to high-concentrated effluent and needs a complementary separation system if regulations are severe.     

A new kinetic model for titanium dioxide mediated heterogeneous photocatalytic degradation of trichloroethylene in gas-phase

This paper focuses on the kinetics of photocatalytic removal and carbon mineralization of gaseous trichloroethylene (TCE) on near-UV irradiated TiO₂ Degussa P25. Experiments were carried out in a flat-plate photoreactor at TCE inlet concentrations of 100–500 ppmv, relative humidities (RH) of 0–62% and gas residence times of 2.5–60.3 s. Gas residence time distribution (RTD) curves revealed an axial dispersed plug flow in the photoreactor with Peclet numbers above 59.4. For all experimental conditions, the carbon mineralization efficiency (5.1–73.0%) was lower than the removal efficiency (8.6–99.9%) and dichloroacetylchloride (DCAC) was detected as a gas-phase degradation product. TCE removal efficiencies increased with lower TCE inlet concentrations, lower RH and higher gas residence times. Evaluating different kinetic models by least squares analysis, it was shown that the Langmuir–Hinshelwood (LH) model could not give an adequate fitting to the experimental results. A new kinetic model, explicitly taking into account electron–hole pair reactions, was developed based on linear TCE adsorption–desorption equilibrium and first order reaction kinetics. The new kinetic model described the experimental results in a more accurate way, as exemplified by a more randomly distributed set of residuals and by a reduction of the sum of squares (SSQ) by a factor 1.7–8.5. The effect of TCE gas-phase concentration, RH and light intensity on adsorption–desorption kinetics, electron–hole concentrations and chemical conversion rates is discussed.     

Combustion of volatile organic compounds in two-component mixtures over monolithic perovskite catalysts

Activity of monolithic perovskite catalysts in oxidation of selected hydrocarbons and oxy-derivatives, oxidized individually and in two-component mixtures, as well as the reaction selectivity were investigated. One bulk perovskite (LaMn₂O₃) catalyst and two catalysts based on LaMn₂O₃ with addition of silver (25 at.%) coated on cordierite support were prepared for the study. The efficiency of oxy-derivatives oxidation was higher than that of hydrocarbon oxidation, but some by-products (first of all aldehydes) were detected in the flue gases. Addition of silver to the active phase improved catalyst activity and selectivity to CO₂ and water. The “mixture effect” on catalyst activity and selectivity depended on composition of the reaction mixture and the catalyst used. The presence of oxy-derivative in the reaction mixture inhibited oxidation of hydrocarbons. Toluene inhibited the process of oxy-derivatives conversion and increased the concentration of incomplete oxidation products.     

Kinetic study for photocatalytic degradation of volatile organic compounds in air using thin film TiO2 photocatalyst

The present paper examined the kinetics of photocatalytic degradation of volatile organic compounds (VOCs) including gaseous trichloroethylene (TCE), acetone, methanol and toluene. Variable parameters were initial concentration of VOCs, water vapor content and photon flux of ultra-violet (UV) light. A batch photo-reactor was specifically designed for this work. The photocatalytic degradation rate increased with increasing the initial concentration of VOCs, but maintained almost constant beyond a certain concentration. It matched well with the Langmuir–Hinshelwood (L–H) kinetic model. For the influence of water vapor in a gas phase photocatalytic degradation rate, there was an optimum concentration of water vapor in the degradation of TCE and methanol. And, water vapor enhanced the photocatalytic degradation rate of toluene, whereas it inhibited that of acetone. As for the effect of photon flux, it was found that photocatalytic degradation occurs in two regimes with respect to photon flux.     

Coordination polymers: Opportunities and challenges for monitoring volatile organic compounds

Coordination polymers (CPs) are emerging as the next generation of macromolecules for many industrial and technological applications. The highly porous nature of these CPs offers the opportunity to exploit them as very effective adsorbents for gaseous molecules, including volatile organic compounds (VOCs). Release of VOCs into the environment is highly undesirable as they can be extremely harmful to general public health and environmental quality. Lately, a large volume of the scientific literature has pointed toward the potentially important role of CPs in the monitoring and analysis of VOCs, offering unprecedented detection limits. This review discusses the opportunities and challenges for the use of CP materials in such applications, describing their general working principles, analytical performance, advantages, and limitations. Recent progress in the application of CPs in the detection, monitoring, and analysis of VOCs is critically reviewed. The discussion is further extended to cover future applications and current research activities in this emerging analytical field.     

Support effect on complete oxidation of volatile organic compounds over Ru catalysts

Catalytic combustion of ethyl acetate, acetaldehyde, and toluene was investigated on various supported Ru catalysts prepared by the impregnation method, and the effect of reduction treatment on the activity was examined. Among the as-calcined catalysts tested, Ru/CeO₂ showed the highest activity for all tests regardless of the pre-treatment in hydrogen atmosphere. The catalytic activity of Ru/SnO₂ was significantly degraded by the reduction treatment, whereas the activity of Ru/ZrO₂ and Ru/γ-Al₂O₃ was enhanced. To reveal these phenomena, the as-calcined and reduced catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR), transmission electron microscopy (TEM), and BET surface area. The dispersion of ruthenium on the supports was evaluated by chemisorption methods of carbon monoxide. The catalytic activity was strongly related to ruthenium species easily oxidizable and reducible at low temperatures. Such ruthenium species were loaded on CeO₂ in a highly dispersed state, resulting in the highest activity.     

Concentration control of volatile organic compounds by ionic liquid absorption and desorption

Volatile organic compounds (VOCs) are difficult to be eliminated safely and effectively because of their large concentration fluctuations. Thus, maintaining a stable concentration of VOCs is a significant study. In this research, H₂O, Tween-80,[Emim]BF₄,[Emim]PF₆, and[Hnmp]HSO₄ were applied to absorb and desorb simulated VOCs. The ionic liquid[Emim]BF₄ demonstrated the best performance and was thus selected for further experiments. As the ionic liquid acted as a buffer, the toluene concentration with a fluctuation of 2000-20000 mg·m^-3 was stabilized at 6000-12000 mg·m^-3. Heating distillation (90℃) was highly efficient to recover[Emim]BF₄ from toluene. The regenerated[Emim]BF₄ could retain its initial absorption capacity even after multiple cycles. Moreover,[Emim]BF₄ had the same buffer function on various aromatic hydrocarbons.     

Catalytic combustion of volatile organic compounds on gold/iron oxide catalysts

Catalytic oxidation of 2-propanol, methanol, ethanol, acetone and toluene was investigated on coprecipitated Au/Fe₂O₃ catalysts in the presence of excess of oxygen. Gold catalysts have been found to be very active in the oxidation of tested volatile organic compounds (VOCs). The high activity of these systems has been related to the capacity of highly dispersed gold to weaken the Fe–O bond thus increasing the mobility of the lattice oxygen which is involved in the VOCs oxidation probably through a Mars–van Krevelen reaction mechanism.     

A kinetic study of organic compounds (acetone,toluene, n-hexane and n-decane) adsorption behavior on activated carbon under supercritical carbon dioxide conditions at temperature from 313 to 353 K and at pressure from 4.2 to 15.0 MPa

Adsorption kinetics of four volatile organic compounds (VOCs) (acetone, toluene, n-hexane and n-decane) on activated carbon under supercritical carbon dioxide (scCO₂) conditions was studied. Breakthrough curve measurements of VOCs in scCO₂ were performed with a fixed bed method for activated carbon (ca. mean particles diameter: 100 μm, specific surface area: 1300 m²/g and mean pore diameter: 0.687 nm, respectively). The measured breakthrough curves could be correlated with a kinetic model by using only one fitting parameter (effective diffusion coefficient in pore) within 10% of average relative deviation. The determined effective diffusion coefficient decreased with decreasing temperatures and increasing pressures at all conditions. Additionally, a generalized model of the determined effective diffusion coefficients was developed, and the proposed model could satisfactorily describe temperature and pressure dependence at all VOCs conditions.     

不同孔径活性炭吸附挥发性有机物的分子模拟

借助Materials Studio软件建立了0.902nm、1.997nm、3.000nm、4.000nm孔径的活性炭狭缝孔模型,采用巨正则蒙特卡洛模拟（Grand Canonical Monte Carlo）的模拟方法计算了其对挥发性有机物（VOCs,如异己烷、苯、甲苯、丙酮和甲醇）的吸附数据,考察了活性炭孔径的变化对VOCs吸附性能的影响,并对实际应用进行指导。模拟结果显示：活性炭对VOCs的吸附受孔径和吸附能的共同影响,在293.15K、各物质饱和蒸气压p₀下,随着孔径的增大,吸附质吸附剂之间的亲和力呈下降趋势。活性炭孔径由0.902nm增加到4.000nm对异己烷、苯、甲苯的饱和吸附量逐渐增大,而4.000nm孔径活性炭对丙酮饱和吸附量小于3.000nm孔径活性炭,3.000nm、4.000nm孔径活性炭对甲醇饱和吸附量小于1.997nm孔径活性炭。在工业废气VOCs吸附回收中选择0.902～1.997nm孔径活性炭能够达到最佳效果。     

Influence of catalyst pretreatments on volatile organic compounds oxidation over gold/iron oxide

This paper reports a study on the influence of calcination pretreatments on the catalytic behaviour of the Au/iron oxide system towards the combustion of some representative volatile organic compounds (2-propanol, ethanol, methanol, acetone and toluene). The catalytic activity of Au/Fe₂O₃ samples towards the total oxidation to CO₂ has been found to be strongly dependent on the catalyst pretreatment, decreasing on increasing the calcination temperature. On the basis of characterisation data (XPS, FT-IR, XRD, BET surface area) it has been proposed that the catalytic behaviour is related to the gold state and/or the iron oxide phase. It appears plausible to suggest that the gold oxidation state and/or the particle size play a key role in the catalytic combustion of volatile organic compounds.     

Kinetic study of the direct causticization reaction involving titanates and titanium dioxide

The solid state reaction between titanium dioxide and sodium carbonate forming sodium titanates was investigated. Reactions between sodium carbonate and titanium dioxide and/or sodium tri-titanate play a key role in the direct causticization of kraft black liquor. Experiments were carried out in a microdifferential reactor made of quartz glass at varying temperatures up to a maximum of 880 °C. Kinetic data were obtained by measuring the release of carbon dioxide. The physical and chemical properties of the reactants and products were analysed in order to obtain a maximum understanding of the reaction path. Scanning electron microscopy (SEM) and the specific surface area of the reactants and products were applied for morphology determination. X-Ray diffraction (XRD) was employed to characterize the phase composition of the product. The results showed that 100% conversion can be obtained at temperatures above 830 °C. Different kinetic models were taken into consideration, such as the Jander and Valensi-Carter models for diffusion-controlled reaction rates and the phase-boundary model for first-order reaction kinetics. One model was based on the theory that the kinetics can be described by the phase-boundary theory at the beginning of the reaction but, as the reaction proceeds, the rate becomes diffusion controlled. This model gave a good fit to the experimental data collected at 840 °C. However, this model did not work as well at high temperature (880 °C) or at lower temperatures as at 840 °C. This was explained by the fact that the reaction path is different at these temperatures, i.e. other reaction products are involved.