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.     

Indoor air quality and the emissions of VOCs from interior products

Indoor air quality (IAQ) has developed into an important environmental and health concern. IAQ legislation has been introduced in the U.S. Congress. Volatile organic compounds (VOCs) comprise a part of the total concern about IAQ. Types and sources of pollutants contributing to IAQ are identified. Some strategies for reducing VOCs in indoor air are provided, and various sources of VOCs are identified. Various means of reducing VOC emissions from products are presented. Government regulatory agencies have begun to introduce legislation focusing on regulation of IAQ. Discussed is how these regulations, some proposed legislation, and customer concerns will affect both product manufacturers and their suppliers.     

Adsorption/desorption of toluene on a hypercrosslinked polymeric resin in a highly humid gas stream

In many sources of volatile organic compounds(VOCs), large amounts of water vapor come from the air and the reactors. The relative humidity(RH) of exhaust gas is normally N60% and is supersaturated. Maintaining the property of adsorbent on VOCs in a highly humid gas stream is a serious industrial problem. In this study, the adsorption/desorption behavior of toluene in a micro-mesoporous polymeric resin was investigated in a highly humid environment to explore the influence of abound water vapor on resin adsorption and regeneration.This resin could selectively adsorb toluene at an RH of 80%, and its adsorption property was unaffected by the presence of water vapor. In the case of humidity saturation, the resin displayed a high adsorption capacity at a moisture content of b30%. Therefore, the polymer resin is an excellent water-resistant adsorbent of VOCs.In the regenerative experiment, the resin maintained its original adsorption capability after four adsorption/desorption cycles of toluene purging with nitrogen gas at 120 °C. The resin exhibited excellent regeneration performance at high humidity.     

On Vehicle VOC and NOx Emissions from the Standpoint of the Tropospheric Ozone Problem

Ozone is formed in and downwind of urban areas from urban emissions of NO_x and volatile organic compounds (VOCs) in the presence of sunlight. The main sources of VOCs in polluted air are motor vehicles, industrial solvents, processes in the petroleum and chemical industries, and vegetation. The main NO_x sources are stationary-source fuel combustion (mainly electric utilities) and motor vehicles. Recent studies have demonstrated that VOC emissions from motor vehicles have been seriously underestimated, and this may well explain why ambient O₃ has not responded well to control efforts. This review presents an overview of the sources, formation, and potential abatement strategies for O₃ pollution in the troposphere, with particular emphasis on the mobile source contribution to O₃ formation.     

Catalytic combustion of VOCs on non-noble metal catalysts

Volatile organic compounds (VOCs) are toxic and contribute significantly to the formation of the photochemical smog, which has remarkable impact to the air quality; therefore, the research on the removal of VOCs has attracted increasing interests during the last decade. This review covers the recent developments in catalytic combustion of VOCs over non-noble metal catalysts including mixed metal oxide catalysts, perovskite catalysts and Au-containing catalysts. The effect of water vapor, coke formation and the effect of supports on the catalytic combustion process will be discussed. The concept of an adsorption/catalytic combustion dual functional system is introduced and several examples of such systems are evaluated. To develop efficient and cost effective VOC removal technologies, further research in catalytic combustion needs to develop novel non-noble metal catalysts and adsorbents, and improve the understanding of catalytic mechanisms involved.     

红外掩日通量遥感监测技术在石化VOCs排放监测中的应用

石化企业主要以无组织形式排放烷烃、烯烃、芳烃等挥发性有机物(VOCs),不仅污染大气,也引起加工损失。为有效控制和减少石化VOCs排放,有必要监测石化企业VOCs排放总量及分布。红外掩日通量遥感监测技术(SOF)是当前VOCs无组织排放通量监测最佳实用技术之一,简要介绍了其技术原理、测量方法、应用范围及在石化VOCs排放监控中的应用。     

A catalytic combustion technology of concentrated VOCs in textile coating process

We developed a new process employing catalytic combustion for textile coating aimed at decreasing emissions of volatile organic compounds (VOCs) and saving energy at the same time. For this purpose, the VOCs are concentrated in a temperature swing adsorption (TSA) device. A fraction of the concentrated VOCs is completely oxidized on an electrically heated (EHC) system, and its combustion gas of EHC is supplied as the heating source via heat exchangers. The remaining concentrated VOC is recycled as a renewable energy source for the drying process to dual-type catalytic burners designed specially to operate with LPG and concentrated VOC at the same time. This system minimizes the problem of VOC emission and maximizes energy conservation by reusing the VOC, toluene, from textile coating.     

工业挥发性有机污染物控制与资源化利用

综述了多种不同的工业挥发性有机污染物(VOCs)控制技术,从工程应用角度分析了其适用范围、优缺点、应用现状和研究方向等。在此基础上,针对目前工业VOCs控制过程存在的低浓度大风量VOCs处理问题、复杂VOCs废气体系处理问题及单一控制方法治理效率低等问题,提出对现有工艺技术设备的优化建议、不同处理方式集成组合、源头治理等改进手段,从宏观角度探讨了VOCs资源化的社会发展需求及解决方案。     

一步快速活化法制备生物质活性炭及其对乙酸乙酯的吸附再生

活性炭吸附法因技术成熟、简单易行、吸附效率高等优点而被广泛应用于挥发性有机化合物（VOCs）的处理中。本文以山林废弃物的野山桃核为原料,烟道废气及硝酸铁为活化剂,制备了一系列生物质活性炭,并利用固定床吸附装置对其吸附、再生性能进行了研究。利用二氧化碳和水蒸气模拟烟气,在固定流量的烟气活化氛围中进行活化,并探讨了不同硝酸铁的量对活性炭的孔隙结构及其吸附再生性能的影响。利用N₂ 吸附-脱附实验、扫描电镜、拉曼光谱和红外光谱等技术研究了活性炭详细特征。结果表明：当硝酸铁的质量分数为0.2% 时,所制备的活性炭AC-3具有最大的比表面积和平均孔径,分别为923m2/g及2.57nm。其对乙酸乙酯的饱和吸附量也最大,为973.04mg/g。利用烟气对AC-3活性炭进行活化再生处理,经过3次重复吸附-解吸再生实验,其饱和吸附能力仍可达91.5％以上,实现了废弃烟气资源化利用及活性炭的循环回收,从而达到废气治理的目标。     

Soil Cleanup by In-Situ Aeration. XX. Mass Transport of Volatile Organics in Wet Activated Carbon

Abstract

A mathematical model for the adsorption of volatile organic compounds (VOCs) from wet gas streams by granular activated carbon columns is developed, and modeling results are discussed. Capillary condensation of water in the carbon pores is found to have a very damaging effect on the rate of VOC adsorption, since diffusion rates of VOCs into the pores are greatly reduced. Implications of the results for the operation of soil vapor extraction systems for the remediation of hazardous waste sites are discussed, and suggestions are made on how to deal with the problem.     

Adsorption–desorption characteristics of VOCs over impregnated activated carbons

An activated carbon was modified by impregnating with various acids or bases. The effects of adsorption capacity and impregnated contents on the textural properties of the impregnated activated carbons (IACs) were investigated. Furthermore, VOC adsorption and desorption experiments were carried out to determine the relationship between the adsorption capacity and chemical properties of the adsorbents. The effects of various parameters such VOC concentration, aspect ratio, flow rate, and impregnated contents were investigated. High adsorption capacity for the selected VOCs was obtained over 1 wt.% H₃PO₄/AC (1 wt.% PA/AC). As a result, IAC was found to be effective for VOC removal by adsorption with the potential for repeated use through desorption by simple heat treatment.     

Soil Clean Up by in-situ Aeration. XIII. Effects of Solution Rates and Diffusion in Mass-Transport-Limited Operation

Abstract

A model for soil vapor extraction (SVE) in laboratory columns is developed which includes the effects of mass transport kinetics of volatile organic compounds (VOCs) between nonaqueous phase liquid (NAPL) droplets and the aqueous phase, and between the aqueous and vapor phases. The model provides a detailed treatment of diffusion of VOCs through a stagnant aqueous boundary layer, and permits time-dependent gas flow rates in the vapor extraction column. Runs made with the model exhibit high initial effluent soil gas VOC concentrations typically followed by a fairly rapid decrease in concentration which in turn is followed by a prolonged tailing region in which the effluent soil gas VOC concentrations decrease quite slowly until nearly all of the VOC has been stripped from the column. The model demonstrates the futility of trying to predict SVE clean-up times on the basis of pilot scale experiments carried out for only a few days, in that these give no idea whatsoever as to the rate of VOC removal which can be expected late in the remediation. The model permits the gas flow to be varied with time; shutting off the gas flow after partial clean up results in rebounds in the soil gas VOC concentrations which can be quite large, particularly if some NAPL is still present.     

An improved adsorption–catalytic process for removing volatile organic compounds from exhaust gases

New methods are developed for conducting adsorption–catalytic processes to remove volatile organic compounds (VOCs) from exhaust gases at industrial enterprises. New flowsheets are proposed for these processes, in particular a system with localized heating of a part of the catalyst bed to initiate the combustion of adsorbed VOCs, and a system separating a full adsorption–catalytic bed into parallel sections with nonsimultaneous regeneration. Studies combine pilot-scale experiments and mathematical modeling. The flowsheet, in which the initiating heater is located directly in the catalytic adsorbent bed considerably reduces (by at least two orders of magnitude) the energy expenditures on regeneration, both in terms of specific energy consumption for purifying a unit volume of exhaust gases and in terms of the power required for the heater. Separating the bed into several sections allows a severalfold reduction in the maximum concentrations of pollutants and the gas temperature at the outlet of the adsorption–catalytic system during its operation. The proposed methods are characterized by high efficiency of gas purification and low energy consumption, so they can be widely used in protecting the atmosphere against VOC emissions.     

Advanced oxidant reactivity pertaining to granular activated carbon beds for air pollution control

The Pennsylvania State University is researching an advanced oxidation system, which includes an air-phase photolytic chamber, an air/water stripping tower, and granular activated carbon (GAC) beds, for controlling volatile organic compounds (VOCs).A laboratory-scale experimental procedure has been employed that simulated certain aspects of several full-scale installations. The apparatus has been used to characterize the loading capacity and mass transfer zone of selected VOCs on coconut shell GAC. The GAC bed has then been placed in series with an ultraviolet reactor, which generates ozone and advanced oxidants in order to regenerate the loaded GAC at intensities of advanced oxidants that were higher than full-scale installations.VOC loading tests revealed that the adsorption of methyl isobutyl ketone (MIBK) was characterized by a well-defined mass transfer zone. Upon exposure to UV/O₃, desorption and/or destruction of the MIBK and other VOCs occurred most prominently within the first inch of the GAC bed. This correlated with the penetration of advanced oxidants into the GAC bed, which also occurred most significantly in the bed's first inch. However, the amount of oxidant penetration increased with time. The removal of oxidants from air by GAC was accompanied by a decrease in mass of the GAC. The ability of oxidants to penetrate a GAC bed was altered when the bed was loaded with a VOC.     

A Genetically-Based Latitudinal Cline in the Emission of Herbivore-Induced Plant Volatile Organic Compounds

The existence of predictable latitudinal variation in plant defense against herbivores remains controversial. A prevailing view holds that higher levels of plant defense evolve at low latitudes compared to high latitudes as an adaptive plant response to higher herbivore pressure on low-latitude plants. To date, this prediction has not been examined with respect to volatile organic compounds (VOCs) that many plants emit, often thus attracting the natural enemies of herbivores. Here, we compared genetically-based constitutive and herbivore-induced aboveground vegetative VOC emissions from plants originating across a gradient of more than 10° of latitude (>1,500 km). We collected headspace VOCs from Asclepias syriaca (common milkweed) originating from 20 populations across its natural range and grown in a common garden near the range center. Feeding by specialist Danaus plexippus (monarch) larvae induced VOCs, and field environmental conditions (temperature, light, and humidity) also influenced emissions. Monarch damage increased plant VOC concentrations and altered VOC blends. We found that genetically-based induced VOC emissions varied with the latitude of plant population origin, although the pattern followed the reverse of that predicted—induced VOC concentration increased with increasing latitude. This pattern appeared to be driven by a greater induction of sesquiterpenoids at higher latitudes. In contrast, constitutive VOC emission did not vary systematically with latitude, and the induction of green leafy volatiles declined with latitude. Our results do not support the prevailing view that plant defense is greater at lower than at higher latitudes. That the pattern holds only for herbivore-induced VOC emission, and not constitutive emission, suggests that latitudinal variation in VOCs is not a simple adaptive response to climatic factors.     

冷凝和吸附集成技术回收有机废气

有机废气治理的难度在于石化、石油、化工等领域的工艺不同,导致排放的废气组分及浓度相差很大。根据有机废气的特点,选择合适的工艺进行有效治理并实现资源回收是非常必要的。目前,冷凝和吸附集成工艺回收有机废气成为人们的研究重点。冷凝法回收有机废气应用于高浓度场合,尤其适合应用在集成工艺的前端。吸附法回收技术更适合于低浓度油气吸附,作为集成技术的后端处理。有机废气冷凝和吸附集成技术,既发挥冷凝法在冷凝高浓度油气方面高效的优势,以及吸附法在吸附低浓度油气时可以将油气浓度控制在很低范围的优势,同时又可避免单纯冷凝技术由于低温冷凝而引起的成本及操作费用剧增,以及吸附法由于吸附高浓度油气而产生的安全隐患。通过对冷凝和吸附段的工艺及结构参数进行优化,并选择合适的制冷剂及吸附剂,以期最终达到回收率、设备投资、运行能耗及安全性等综合技术经济指标的最优化。     

Plant Volatile Organic Compounds (VOCs) in Ozone (O<Subscript>3</Subscript>) Polluted Atmospheres: The Ecological Effects

Tropospheric ozone (O₃) is an important secondary air pollutant formed as a result of photochemical reactions between primary pollutants, such as nitrogen oxides (NO_x), and volatile organic compounds (VOCs). O₃ concentrations in the lower atmosphere (troposphere) are predicted to continue increasing as a result of anthropogenic activity, which will impact strongly on wild and cultivated plants. O₃ affects photosynthesis and induces the development of visible foliar injuries, which are the result of genetically controlled programmed cell death. It also activates many plant defense responses, including the emission of phytogenic VOCs. Plant emitted VOCs play a role in many eco-physiological functions. Besides protecting the plant from abiotic stresses (high temperatures and oxidative stress) and biotic stressors (competing plants, micro- and macroorganisms), they drive multitrophic interactions between plants, herbivores and their natural enemies e.g., predators and parasitoids as well as interactions between plants (plant-to-plant communication). In addition, VOCs have an important role in atmospheric chemistry. They are O₃ precursors, but at the same time are readily oxidized by O₃, thus resulting in a series of new compounds that include secondary organic aerosols (SOAs). Here, we review the effects of O₃ on plants and their VOC emissions. We also review the state of current knowledge on the effects of ozone on ecological interactions based on VOC signaling, and propose further research directions.     

Economics of Treating Waste Gases from an Air Stripping Tower Using Photochemically Generated Ozone

Air stripping towers have been recommended for the removal of volatile organic compounds (VOCs) in drinking water supply and industrial waste treatment systems. This technique removes VOCs economically in the liquid phase. It can, however, create adverse secondary environmental impacts by removing VOCs from the water and discharging them to the air.

A commonly proposed method for controlling .VOC emissions is filtration of the off-gas through adsorption of the stripped organics in the off-gas by granular activated carbon. The high incremental cost of this alternative has produced an interest in alternative control technologies.

One alternative currently available is based on short wavelength ultraviolet (UV) radiation. This technique combines the effects of ozone generation, free radical formation and photolysis of the contaminants to effectively control the VOC emissions. This technique is known as Advanced Photo Oxidation (APO)^R.

The cost for APO is $0.27/m³ for a 3.8 m³/hr contaminated water system. A system of this size is adequate for a groundwater decontamination project where a moderate length of time is available for restoration of the site. The cost of a conventional air stripping tower with Granular Activated Carbon (GAC) adsorption emissions control in this size range would be $0.40 to $0.45/m³ (J.M. Montgomery, 1986).

Additional testing will be required to fully develop design guidelines for different contaminants and larger systems. Another area for additional technical documentation is the application of this technique to the liquid phase oxidation of VOCs.     

Influence of Water Vapor on the Adsorption of VOCs on Lignin‐Based Activated Carbons

Abstract

Activated carbons with a wide range of burn‐off degrees obtained from Eucalyptus kraft lignin have been used to study the influence of the presence of water vapor on VOCs adsorption. The amount adsorbed and the rate of adsorption of both benzene and water vapor increase with activated carbon burn‐off as a consequence of an increase of micropore volume, broadening of micropore size distribution and increasing development of meso‐ and macroporosity. Similar results were found for MEK and methanol. Benzene is only partially desorbed at the adsorption temperature and an appreciable amount of it remains in the carbon, most likely in the narrow micropores. On the contrary, water vapor is completely desorbed at the adsorption temperature and its adsorption profile clearly exhibits two steps with different adsorption rates, associated to water molecules adsorbed on the active sites given rise to cluster formation and further migration and filling of the micropores. Adsorption with mixtures of VOC and water vapor has been carried out. The total amount adsorbed by the carbon, near the equilibrium point, is higher than in the case of the stream containing only the VOC. The adsorption rates for the mixtures streams are similar to that for the corresponding streams containing only the VOC in the case of carbons with a well developed porous structure. However, the presence of water vapor increases the rate of adsorption on the activated carbons with narrower microporosity. Saturation of the activated carbon with water vapor prior to the adsorption of a mixture containing benzene and water vapor has shown little effect on the amount of benzene adsorbed, suggesting that water and benzene molecules are adsorbed in different sites on the carbon surface.     

Removal of volatile organic compound by activated carbon fiber

Experiments were carried out to study adsorption/desorption of volatile organic compound (VOC) on the activated carbon fiber (ACF) under dynamic conditions. The primary objective was to experimentally demonstrate the suitability of ACF in effectively adsorbing VOCs from inert gaseous stream under varying operating conditions, and compare its performance vis-à-vis that of the other commercially available adsorbents, such as granular activated carbon (GAC), silica gel, and zeolites. The adsorption experiments were carried out in a fixed tubular packed bed reactor under various operating conditions including temperature (35-100 °C), gas concentration (2000-10,000 ppm), gas flow rate (0.2-1.0 slpm) and weight of the adsorbent (2-10 g). A mathematical model was developed to predict the VOC breakthrough characteristics on ACF. The model incorporated the effects of the gas-particle film mass transfer resistance, adsorbent pore diffusion and the adsorption/desorption rates within the pore. The experimental data and the corresponding model simulated results were compared and found to be in good agreement. The ACF repeatedly showed a good regeneration capability following desorption by DC electrical heating.