Oxidation of VOCs

E. J. Dowd, W. M. Sheffer, and G. E. Addison, "A Historical Perspective on the Euture of Catalytic Oxidation of VOCs," paper 92-109.03, 85th Jinnual Meeting of Air and Waste Management A.ssociation, Kansas City, Mo., June 21—26,1993. 

Supported palladium oxide is the most effective catalyst used in total methane oxidation and in catalytic oxidation of VOCs [1-5]. However, the activity of the conventional catalysts is not sufficient [5-6]. Recently, the Pd-zeolite catalysts have attracted considerable attention due to their high and stable CH4 conversion efficiency [4-8]. In this work, the effect of the preparation method, the nature of the charge-balancing cations, the palladium loading and the pre-treatment gas nature on the texture, structure and catalytic activity of the Pd-ZSM-5 solids are investigated. 

Incineration systems are based on the principle that all volatile organic compounds are combustible and can, in principle, be eliminated simply by being burned. Combustion can be achieved without catalysts (thermal systems) or with catalysts (catalytic systems). In either case, flue gases are passed into a chamber where they are heated in an excess of air, resulting in the oxidation of VOCs. Thermal systems operate at temperatures of 750°C-1,000°C, while catalytic systems operate at temperatures of about 350°C-500°C. 

A second reason for the response of 03 to NOx at low VOC/NOx ratios is that at these high NOx concentrations, N02 competes with VOC for the OH radical by forming HN03 (reaction (13)). This terminates the chain oxidation of VOC and removes N02 from the system without forming 03. This chemistry has been confirmed by direct OH radical measurements thus, Eisele et al. (1997) report that OH concentrations increase with the NO concentration up to l-2 ppb but decrease thereafter due to the OH + N02 reaction. This is consistent with model calculations of isopleths of OH concentrations as a function of the VOC/NOx ratio, similar to the ozone isopleths in Fig. 16.14 (e.g., Kley, 1997) in the high NOx-low VOC... 

Various reaction rate laws for the catalytic oxidation of VOCs (Poulopoulos et al., 2003)... 

A waste gas stream containing 500 ppm of a VOC has to be treated in a fixed-bed reactor so that the concentration of VOC reduces to 50 ppm. A solid catalyst will be employed for the oxidation of VOC to C02, at a temperature of 900 °C and a pressure of 1 atm. The reaction is first order with respect to die VOC concentration with a reaction rate coefficient kv = 1.5 x 10 8 m3/m2 s. Given that... 

The modification recommended by Bechtel eliminates the need for steam to heat an evaporator or for stripping eliminates the need for a condenser and storage tank to provide liquid for the ultraviolet oxidation process and eliminates the need for hydrogen peroxide to augment ultraviolet oxidation of VOCs. With the revised process, the plant will be... 

Table 3.2 Performance of supported Au catalysts for the oxidation of VOCs and hydrocarbons. 

In the wake of the spectacular application of monoliths in the treatment of automobile exhaust gas, the potential of monoliths in other applications was studied. Gas-phase reactions were the major area. Catalytic oxidation has received a lot of attention. Low-NOA. burners based on monoliths were designed, catalytic oxidation of VOCs also benefits from structured catalysts, basically because of the low pressure drop and the resistance against dust. 

Several compounds were also found to have a seasonal distribution. Kubatova et al. (2002) found that concentrations of lignin and cellulose pyrolysis products from wood burning were higher in aerosol samples collected during low-temperature conditions. On the other hand, concentrations of dicarboxylic acids and related products that are believed to be the oxidation products of hydrocarbons and fatty acids were highest in summer aerosols. PAHs, which are susceptible to atmospheric oxidation, were also more prevalent in winter than in summer. These results suggest that atmospheric oxidation of VOCs into secondary OAs and related oxidative degradation products are key factors in any OA mass closure, source identification, and source apportionment study. However, additional work is much desirable to assess the extent and seasonal variation of these processes. 

UV/H202 oxidation of VOCs has also been studied in detail and several studies reported kinetic models to predict the efficiency of the process. For example, Liao and Gurol [75], Glaze et al. [113], De Laat et al. [155] and Crittenden et al. [74] studied the UV/H202 oxidation of VOCs such as n-chlorobutane, 1,2-dibromo-3-chloropropane and tri- and tetra-chloroethanes in batch photoreactors with low-pressure mercury vapor lamps. Effects of pH, concentration of hydrogen peroxide, UV intensity and the presence of carbonates or fulvic substances were variables studied. 

In summary, the rate of oxidation of VOCs and therefore by inference the production of ozone is governed by the concentration of the catalytic HO c radicals. There are a large variety of VOCs with a range of reactivities therefore this remains a complex area. 

Table 4 Operating Temperatures for Catalytic Oxidation of VOCs...

Figure 13 Fabrication of individual square metal monoliths into catalyst panels that are inserted into an industrial SCR unit for con-trol of NO, emissions. Similar reactors are used for oxidation of VOCs...

Mn02 oxygen lability. Indeed, vacancies locally affect the O lability the eoordination number of the 6 O surrounding the Mn vacancy is decreased. As admitted for battery electrode applications, Mn ions neighbours become thus more reducible. The complete oxidation of VOC is a reaction demanding many O. So, Mn vacancies (if they are situated close to the surface) offering 6 labile O are likely to take an active part in the catalytic process. 

Table 2. Sources of glyoxal from the oxidation of VOCs.

Although Chen et al. focused on CO oxidation in gas turbine exhausts with noble metal catalysts, much of the deactivation data that they presented is also relevant to oxidation of VOCs in other air pollution control applications. They reported that 100 to 200 ppm SO2 in the exhaust will require 150 to 200 C higher catalyst temperatures for the same CO conversion as that without SO2. However, above -350 C the effect of SO2 disappears with these catalysts because the CO reaction rate becomes mass-transfer controlled. The inhibition by SO2 is attributed to the strong adswption of the sulfur compounds on both the catalyst and carrier, limiting adsorption of CO. These adsorbed sulfur compounds can be removed with time and high temperatures in the absence of SO2, restoring catalyst activity. 

Alkoxy (RO) radicals are formed in the reaction of alkyl peroxy (R02) radicals with NO. Subsequent reactions of alkoxy radicals determine to a large extent the products resulting from the atmospheric oxidation of VOCs (Orlando et al. 2003). Alkoxy radicals react under tropospheric conditions via a variety of processes unimolecular decomposition, unimolecular isomerization, or reaction with 02. Alkoxy radicals with fewer than five carbon atoms are too short to undergo isomerization for these the competitive processes are unimolecular decomposition versus reaction with 02. The general alkoxy radical-02 reaction involves abstraction of a hydrogen atom by 02 to produce an H02 radical and a carbonyl species ... 

Oxidation of VOCs leads to the formation of more highly substituted and therefore lower volatility reaction products. The reduction in volatility is due mainly to the fact that adding oxygen and/or nitrogen to organic molecules reduces volatility (Seinfeld and Pankow 2003). Addition of carboxylic acid, alcohol, aldehyde, ketone, alkyl nitrate, and nitro groups to the precursor VOC can reduce its volatility by several orders of magnitude (see Section 14.5.1). The reactions of VOCs with 03, OH, and N03 can all lead to SOA formation in the atmosphere. 

Figure 8 Simple parallel-consecutive reaction network describing the oxidation of VOCs by a supported hopcalite catalyst [86]...

The use of zeolite catalysts in the total oxidation of VOCs has been previously highlighted [58-59], zeolite based catalysts have particularly been used for the destruction of a range of chlorinated organics. Early studies by Chatterjee and... 

Konova, P, Stoyanova, M. Naydcnov, A. Christoskova, S. and Mchandjiev, D. Catalytic oxidation of VOCs and CO by ozone over alumina supported cobalt oxide Appi. Catai A General, 2006, 298 109-114... 

---