Hydrogen sulfide modeling

Carroll J.J. and A.E. Mather, "Phase Equilibrium in the System Water-Hydrogen Sulfide Modelling the Phase Behavior with an Equation of State", Can. J. Chem. Eng, 67, 999-1003 (1989b). 

This, naturally, results in an increased electrophilicity of the nitrile group and also creates favourable conditions for the nucleophilic attack of the mercapto anion and an easy addition of hydrogen sulfide in accordance with the above scheme. The activation energy of the thioamidation of the model compounds is much higher (for glutarodinitrile — 11,8 [49,4] and for trinitrile - 7,97 kcal/mol [33,4 kJ/mol]) than in PAN (6,18 kcal/mol [25,9 kJ/mol]). 

C09-0115. The H—O—H bond angle in a water molecule is 104.5°. The H—S—H bond angle in hydrogen sulfide is only 92.2°. Explain these variations in bond angles, using orbital sizes and electron-electron repulsion arguments. Draw space-filling models to illustrate your explanation. 

Hydrogen sulfide is a toxic gas with the foul odor of rotten eggs. The Lewis structure of H2 S shows two bonds and two lone pairs on the S atom. Experiments show that hydrogen sulfide has a bond angle of 92.1°. We can describe the bonding of H2 S by applying the orbital overlap model. 

In a report comparing community responses to low-level exposure to a mixture of air pollutants from pulp mills, Jaakkola et al. (1990) reported significant differences in respiratory symptoms between polluted and unpolluted communities. The pollutant mixture associated with the pulp mills included particulates, sulfur dioxide, and a series of malodorous sulfur compounds. Major contributors in the latter mixture include hydrogen sulfide, methyl mercaptan, and methyl sulfides. In this study the responses of populations from three communities were compared, a nonpolluted community, a moderately polluted community, and a severely polluted community. Initial exposure estimates were derived from dispersion modeling these estimates were subsequently confirmed with measurements taken from monitoring stations located in the two polluted communities. These measurements indicated that both the mean and the maximum 4-hour concentrations of hydrogen sulfide were higher in the more severely polluted community (4 and 56 g/m3 2.9 and 40 ppb) than in the moderately polluted one (2 and 22 g/m3 1.4 and 16 ppb). Particulate measurements made concurrently, and sulfur dioxide measurements made subsequently, showed a similar difference in the concentrations of these two pollutants between the two polluted communities. 

If PBPK models for hydrogen sulfide exist, the overall results and individual models are discussed in this section in terms of their use in risk assessment, tissue dosimetry, and dose, route, and species extrapolations. 

Comparative Toxicokinetics. PBPK models have not been developed to compare the toxicokinetics of hydrogen sulfide in humans and animals. Studies providing quantitative data necessary to develop PBPK models would be useful. 

The American Petroleum Industry is currently sponsoring research on the toxicology of hydrogen sulfide. These studies include examinations of neurotoxicology, reproductive and developmental toxicology (including developmental neurotoxicity), and the development of a PBPK model. 

Several attempts have been made to investigate anaerobic pathways for desulfurization. Mixed cultures of sulfate-reducing bacteria were used to desulfurize model compounds, thiophenes [12], organosulfides [12,13], and petroleum preparations [14-16], Production of hydrogen sulfide and biphenyl from DBT has been demonstrated in other studies [17-22],... 

The model is, in several aspects, a simplification of the processes that occur. As an example, it is important—and also possible when further data are available from experiments — to expand the model by including the oxidation of sulfide in a gravity sewer at low DO concentrations and the emission of hydrogen sulfide into the sewer atmosphere. From a general point of view, however, it is considered important only to deal with the most important aspects to keep the number of processes low and not include too many process parameters that are site specific. 

As far as organic matter transformations are concerned, the process rates are significantly slower compared with aerobic transformations. Basically, readily biodegradable organic matter is preserved and even, to some extent, produced opposite to the situation when aerobic processes proceed. The sulfur cycle, until now included in the sewer process model, is relatively simply described following empirical expressions for sulfide formation. Other important processes in this respect, e.g., hydrogen sulfide emission and sulfide oxidation, still need to be included, however, and, most of all, investigated from a conceptual point of view. 

Extension of the WATS model to integrate further dry-weather processes is considered important. Examples of such extensions are the description of the wastewater quality and nitrite/nitrate transformations under anoxic conditions and the emission of hydrogen sulfide into the sewer atmosphere followed by its transformation (oxidation) at the sewer walls. 

The integrated aerobic-anaerobic WATS model has changed this situation. As an example, it is possible to use the model in a gravity sewer with changing aerobic and anaerobic conditions. As previously stressed, a number of in-sewer processes still need to be dealt with. Examples are the anoxic transformations and the processes related to the extended sulfur cycle, particularly, the oxidation of sulfide and the emission of hydrogen sulfide into the sewer atmosphere, including its further oxidation at the sewer walls. Combined use of empirical and conceptual models is still needed. 

The simulations depicted in Figure 8.8 also show that arather low hydrogen sulfide concentration is predicted in the gravity sewer. Only minor problems related to hydrogen sulfide production may therefore arise. Until now, the WATS model did not include sulfide release to the sewer atmosphere, sulfide oxidation or sulfide precipitation that may further reduce the concentrations shown. The predicted sulfide concentrations are, therefore, maximum levels. In case a natural capacity of iron salts in the wastewater to precipitate sulfide is inadequate, the sulfide concentrations are considered at a level that can be relatively easily controlled. 

FIGURE 8.8. Model simulation of variations in the DO concentration, the OUR and the hydrogen sulfide concentration. The DO concentration of the wastewater inflow along the sewer line is estimated to be 2 g02 m-3. 

In most reported studies, model sulfur compounds are added to the liquid fuel to study their effect on deactivation. Sulfur in the fuel usually reacts to form hydrogen sulfide in the reformer, which is then removed downstream. In one such study, Palm et al. describe the ATR of mixtures of n-Ci3-Ci9... 

Stewart and Hack (5.) have presented operating characteristics of pressure swing adsorption systems for reducing impurities in a hydrogen stream from 40 vol percent to 1 ppm. Impurities included ammonia, water, methane, carbon monoxide, carbon dioxide, nitrogen, and several hydrocarbons. In this study heatless adsorption is used to separate hydrogen sulfide-hydrogen mixtures and the experimental results are compared with theoretical models. 

The mass balance model of Weaver and Hamrin (7) was modified to describe the operation of this system. A series of mass balances were written around columns 1 and 2 describing the varying steps in the process. The balances were written with respect to the more strongly adsorbed component, hydrogen sulfide. Due to the fact that hydrogen does not adsorb on the molecular sieve, this makes the expression less complicated and less difficult to solve than the original model. Details are presented elsewhere (8). 

For determing the concentration of hydrogen sulfide in the product gas, a Gow-Mac thermal conductivity cell (Model 10-952) was used. The cell was equipped with four matched pairs of AuW filaments, especially used because of their resistance to corrosion from the hydrogen sulfide. Layers of styrofoam were used to insulate the cell from changes in ambient temperature. This detector was found to be very sensitive to changes in the flow-rate. 

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