Hydrogen sulfide removal selectivity

Process selectivity indicates the preference with which the process removes one acid gas component relative to or in preference to another. For example, some processes remove both hydrogen sulfide and carbon dioxide, whereas other processes are designed to remove hydrogen sulfide only. Thus it is important to consider the process selectivity for hydrogen sulfide removal compared to carbon dioxide removal, ie, the carbon dioxide-to-hydrogen sulfide ratio in the natural gas, in order to ensure minimal concentrations of these components in the product. 

The two primary approaches to removal of sulfurous compounds from process gas streams are 1) the direct-oxidation processes and 2) the acid-gas removal systems9,10. in the direct-oxidation processes, hydrogen sulfide is selectively removed from the process gas, and the sulfur is recovered in its elemental form. The acid-gas removal processes, as a group, remove acidic components (H2S, CO2, HCN, etc.) and regenerate these species into a separate side stream. 

Figure 5.28. How diagram of rectisol process for selective hydrogen sulfide removal, followed by carbon dioxide removal.189...

Figure 5.28 shows a flow scheme of the Rectisol process for selective hydrogen sulfide removal, followed by carbon dioxide removal.189 H2S and C02 can be removed respectively, but two absorbers and two regenerators are required. The chief drawback of the Rectisol process is its complex scheme and the need to refrigerate the solvent due to its low boiling point, which results in high capital and operating costs. 

Since hydrogen sulfide is absorbed much more rapidly in potassium carbonate solutions than carbon dioxide, it would be expected that the process could be made at least partially selective for hydrogen sulfide. It is claimed that selective hydrogen sulfide removal can be... 

Use of an aqueous methyldiethanolamine (MDEA) solution as a selective solvent (see Chapter 2) for hydrogen sulfide removal in tail gas cleanup has been described by Meissner (1983). With this solvent an overall conversion, including the Claus unit, of 99.9% of the H2S fed to the Claus unit is attainable. However, the treated gas contains some residual H2S, which may require incineration before discharge to the atmosphere. 

The SulFerox process is highly selective for hydrogen sulfide removal because carbon dioxide does not react chemically with the solution. Carbon dioxide removal is limited by the CO2 solubility in the circulating solution. When the system is operated at a pH of 8, the... 

Even though it is highly selective for hydrogen sulfide removal, SulFerox can also remove between SO and 90% of the mercaptans present in the sour gas. The initial reaction step is thought to proceed through the formation of an iron-thiol intermediate complex (Bedell et al., 1988), which leads to the generation of a thiyi radical ... 

Figure 14-8. Flow diagram of SELEXOL process for selective hydrogen sulfide removal and complete carbon dioxide removal.

Adsorption systems employing molecular sieves are available for feed gases having low acid gas concentrations. Another option is based on the use of polymeric, semipermeable membranes which rely on the higher solubiHties and diffusion rates of carbon dioxide and hydrogen sulfide in the polymeric material relative to methane for membrane selectivity and separation of the various constituents. Membrane units have been designed that are effective at small and medium flow rates for the bulk removal of carbon dioxide. 

Gas purification processes fall into three categories the removal of gaseous impurities, the removal of particulate impurities, and ultrafine cleaning. The extra expense of the last process is only justified by the nature of the subsequent operations or the need to produce a pure gas stream. Because there are many variables in gas treating, several factors must be considered (/) the types and concentrations of contaminants in the gas (2) the degree of contaminant removal desired (J) the selectivity of acid gas removal required (4) the temperature, pressure, volume, and composition of the gas to be processed (5) the carbon dioxide-to-hydrogen sulfide ratio in the gas and (6) the desirabiUty of sulfur recovery on account of process economics or environmental issues. 

The solvent can be tailored to provide selective acid gas removal based on the Hquid—gas solubiHties. For example, the Selexol process, Hcensed by Union Carbide Corporation, uses the dimethyl ether of polyethylene glycol (DMPEG) to provide high hydrogen sulfide selectivity. The solubiHty of hydrogen sulfide in DMPEG is 8—10 times that of carbon dioxide. 

Sulfides with widely different solubilities and solubility products can be selectively precipitated by adding S2 ions to the solution removed from the chlorides in the first step (see Fig. 11.20). Some metal sulfides (such as CuS, HgS, and Sb2S3) have extremely small solubility products and precipitate if there is the merest trace of S2" ions in the solution. Such a very low concentration of S2 ions is achieved by adding hydrogen sulfide, H2S, to an acidified solution. A higher hydronium ion concentration shifts the equilibrium... 

Biodesulfurization (BDS) is the excision (liberation or removal) of sulfur from organosul-fur compounds, including sulfur-bearing heterocycles, as a result of the selective cleavage of carbon-sulfur bonds in those compounds by the action of a biocatalyst. Biocatalysts capable of selective sulfur removal, without significant conversion of other components in the fuel are desirable. BDS can either be an oxidative or a reductive process, resulting in conversion of sulfur to sulfate in an oxidative process and conversion to hydrogen sulfide in a reductive process. However, the reductive processes have been rare and mostly remained elusive to development due to lack of reproducibility of the results. Moderate reaction conditions are employed, in both processes, such as ambient temperature (about 30°C) and pressure. 

COMBISULF A process for removing sulfur compounds from the gases from combined-cycle power plants with integrated coal gasification. Carbonyl sulfide is removed by catalytic hydrolysis hydrogen sulfide is removed by selective absorbtion in aqueous MEDA and the sulfur is recovered from both processes by a modified Claus unit. Developed by Krupp Koppers and first operated in Spain in 1996. See PRENFLO. 

Purisol A process for removing hydrogen sulfide from gases by selective absorption in N-methyl-2-pyrrolidone (NMP). Developed and licensed by Lurgi, particularly for desulfurizing waste gases from IGCC plants. Seven units were in operation or under construction in 1996. 

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