Sulfur particle conversion

Finally, atmospheric chemical transformations are classified in terms of whether they occur as a gas (homogeneous), on a surface, or in a liquid droplet (heterogeneous). An example of the last is the oxidation of dissolved sulfur dioxide in a liquid droplet. Thus, chemical transformations can occur in the gas phase, forming secondary products such as NO2 and O3 in the liquid phase, such as SO2 oxidation in liquid droplets or water films and as gas-to-particle conversion, in which the oxidized product condenses to form an aerosol. 

Particles are emitted Into the atmosphere from numerous natural and manmade sources and are also formed upon condensation of gases and vapors. Direct emissions of Suspended Particulate Natter (SPN) arise from a variety of human activities Including combustion. Industrial and agricultural practices the remainder Is formed from gas-particle conversions (chiefly from SOj oxidation to sulfuric acid as sulfate salts). Particles larger than about lOpm In diameter deposit In the vicinity of the sources, but smaller... 

With emission source chemical signatures and corresponding aerosol or rainwater sample measurements PLS can be used Co calculate a chemical element mass balance (CEB). Exact emission profiles for the copper smelter and for a power plant located further upwind were not available for calculation of source contributions to Western Washington rainwater composition. This type of calculation Is more difficult for rainwater Chan for aerosol samples due Co atmospheric gas to particle conversion of sulfur and nitrogen species and due Co variations In scavenging efficiencies among species. Gatz (14) has applied Che CEB to rainwater samples and discussed Che effect of variable solubility on the evaluation of Che soil or road dust factor. 

Similar data for sulfate have been reported in many studies. Figure 9.36, for example, shows overall average sulfate distributions measured in marine areas as well as at continental sites (Milford and Davidson, 1987). The marine data show two modes, a coarse mode associated with sea salt and a fine mode associated with gas-to-particle conversion. Sulfate in seawater, formed, for example, by the oxidation of sulfur-containing organics such as dimethyl sulfide, can be carried into the atmosphere during the formation of sea salt particles by processes described earlier and hence are found in larger particles. The continental data show only the fine particle mode, as expected for formation from the atmospheric oxidation of the S02 precursors. 

For fluidized beds, one essential operating parameter is that of gas throughput this is necessary for fluidization of the bed material, and depends on the diameter and density of the particles and on the density and viscosity of the gas. These studies highlight the fact that sulfur dioxide conversion is decreased at increased gas flow rate, and thus with higher energy input. As a result, a more rapid drying of the liquid contained in the bed occurs with a coupled decrease in the mass transfer surface. 

Recently, Paugam (1978) published an interesting atmospheric observation which may be related to the oxidation of DMS. He observed a substantial photolytic aerosol formation on the coast of Brittany (France) during low tide. Paugam believes that this phenomenon is due to the photooxidation of DMS emitted by algae. He also mentioned, on the basis of S02 measurement, that sulfur dioxide does not play an important part in the gas-to-particle conversion. 

In estimating the rate of gas-to-particle conversion involving S02 from anthropogenic sources, Peterson and Junge (1971) assumed that 66% is converted to sulfate and the rest is removed by dry deposition. In addition, it was assumed that sulfate is completely neutralized to ammonium sulfate. An emission rate of 160 Tg S02/yr from the combustion of fossil fuels then gives a production rate for particulate sulfate of 220 Tg/yr. In 1971 the rate of sulfur compound emissions from natural sources was less well known... 

The last anion to be discussed is nitrate. In the marine aerosol, nitrate is associated mainly with coarse particles. This fact is apparent in Fig. 7-21, but it has also been observed by Savoie and Prospero (1982). Sea water contains insignificant amounts of nitrate, so that the particulate nitrate must derive from the gas phase, that is, from gaseous nitric acid. As a gas-to-particle conversion process, one would expect the condensation of nitric acid to take place in the accumulation mode. The volatility of HN03 is much greater than that of H2S04, however. It appears that the condensation of sulfuric acid prevents the simultaneous condensation of nitric acid in the same size range. In this connection one should remember that with... 

Gillani, N. V., S. Kohli, and W. E. Wilson (1981). Gas-to-particle conversion of sulfur in power plant plumes. 1. Parametrization of the conversion rate for dry, moderately polluted ambient conditions. Atmos. Environ. 15, 2293-2313. 

The indirect effect of aerosols on climate is exemplified by the processes that link S02 emissions to cloud albedo. Sulfur dioxide is oxidized in gas and aqueous phases to aerosol sulfate. Although increased S02 emissions can be expected to lead to increased mass of sulfate aerosol, the relation between an increased mass of aerosol and the corresponding change of the number concentration of aerosol is not well established. Yet, it is the aerosol number concentration that is most closely related to the cloud drop number concentration. Aerosol mass is created by gas-to-particle conversion, which can occur by growth of... 

Their sizes range from a few nanometers to a few micrometers often with pronounced concentration modes around a few tens of nm Aitken mode), in the range 100-500 nm (accumulation mode) and at a few (im (coarse mode) see Fig. 4.17. Coarse mode particles typically originate from the dispersion of solid and liquid matter such as soil, dust and sea spray (Chapter 2.6.4). Submicron particles are usually mixtures of primary particles (almost combustion products from biomass smoke and diesel soot) and secondary produced particles via the gas-to-particle conversion (most important is sulfuric acid from SO2 oxidation and organics from oxidation of VOCs). 

Another problem was the plugging of the feed gas header with sulfur particles. This problem was caused by swings in gas flow rate, which resulted in backflow of the catalyst solution into the header during low flow operation periods. This allowed conversion of H2S to solid sulfur to take place inside the header. This problem was solved by installing an air sparge line at the inlet of the feed gas header and periodically blowing the header. 

Although most of the atmospheric particulate mass is confined to the troposphere (region below an altitude of 11 km), the stratospheric aerosol can have significant effects on climate. This subject has been reviewed by Pueschel (1996). The primary source of particulate in the stratosphere (altitude fix>m 11 to SO km) is the formation of sulfuric acid droplets by gas-to-particle conversion of SO2 injected into the stratosphere by major volcanic eruptions. These droplets are formed by homogeneous nucleation involving photochemical reactions of SOj and water vapor. They spread widely over the hemisphere (north or south) in which they originated. 

Both heterogeneous and homogeneous nucleation appear to play roles in ice formation in the upper troposphere. Liquid droplets several microns in radius can be found at temperatures down to about T —35°C [38] (Fig. 2). These are probably concentrated solutions of sulfuric acid, ammonium sulphate, and possibly other nitrogen compounds formed on deliquescent aerosols. Some of the aerosols on which the ice particles nucleate are formed aloft by gas-to-particle conversion in the clear air surrounding clouds ([39]) some originate from volcanos, and some arise from gas and/or particle anthropogenic sources. 

It was shown in laboratory studies that methanation activity increases with increasing nickel content of the catalyst but decreases with increasing catalyst particle size. Increasing the steam-to-gas ratio of the feed gas results in increased carbon monoxide shift conversion but does not affect the rate of methanation. Trace impurities in the process gas such as H2S and HCl poison the catalyst. The poisoning mechanism differs because the sulfur remains on the catalyst while the chloride does not. Hydrocarbons at low concentrations do not affect methanation activity significantly, and they reform into methane at higher levels, hydrocarbons inhibit methanation and can result in carbon deposition. A pore diffusion kinetic system was adopted which correlates the laboratory data and defines the rate of reaction. 

Fig. 4.13 Electrodeposition of Cd nanoparticles on the graphite surface is followed by electrochemical oxidation and conversion of the oxidized intermediate to CdS or core-shell sulfur-CdS particles. (Reproduced from [125])...

Scale prevention methods include operating at low conversion and chemical pretreatment. Acid injection to convert COs to CO2 is commonly used, but cellulosic membranes require operation at pH 4 to 7 to prevent hydrolysis. Sulfuric acid is commonly used at a dosing of 0.24 mg/L while hydrochloric acid is to be avoided to minimize corrosion. Acid addition will precipitate aluminum hydroxide. Water softening upstream of the RO By using lime and sodium zeolites will precipitate calcium and magnesium hydroxides and entrap some silica. Antisealant compounds such as sodium hexametaphosphate, EDTA, and polymers are also commonly added to encapsulate potential precipitants. Oxidant addition precipitates metal oxides for particle removal (converting soluble ferrous Fe ions to insoluble ferric Fe ions). 

The extent of gas-to-aerosol conversion of secondary pollutants can be estimated by measuring gas particle distribution factors for carbon, nitrogen, and sulfur species. For example, /c = P/ P + G), where P = particulate organic carbon ng/m as carbon) and G = gas-phase... 

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