Hydrogen sulfide mass transfer

The release of odorous components (i.e., the water-air mass transfer aspects) is dealt with in Chapter 4, and the behavior of sulfur (hydrogen sulfide) was, in this respect, exemplified. Figure 4.4 gives not just an understanding of the release phenomena but also an overall view of the pathways and sinks of sulfur components under sewer conditions. 

The CNG process removes sulfurous compounds, trace contaminants, and carbon dioxide from medium to high pressure gas streams containing substantial amounts of carbon dioxide. Process features include 1) absorption of sulfurous compounds and trace contaminants with pure liquid carbon dioxide, 2) regeneration of pure carbon dioxide with simultaneous concentration of hydrogen sulfide and trace contaminants by triple-point crystallization, and 3) absorption of carbon dioxide with a slurry of organic liquid containing solid carbon dioxide. These process features utilize unique properties of carbon dioxide, and enable small driving forces for heat and mass transfer, small absorbent flows, and relatively small process equipment. 

Simultaneous Mass Transfer of Hydrogen Sulfide and Carbon Dioxide with Complex Chemical Reaction in an Aqueous Diisopropanolamine Solution... 

The mathematical model was constracted on the basis of a three-phase plug-flow reactor model developed by Korsten and Hoffmaim [63]. The model incorporates mass transport at the gas-liquid and liquid-solid interfaces and uses correlations to estimate mass-transfer coefficients and fluid properties at process conditions. The feedstock and products are represented by six chemical lumps (S, N, Ni, V, asphaltenes (Asph), and 538°C-r VR), defined by the overall elemental and physical analyses. Thus, the model accounts for the corresponding reactions HDS, HDN, HDM (nickel (HDNi) and vanadium (HDV) removals), HD As, and HCR of VR. The gas phase is considered to be constituted of hydrogen, hydrogen sulfide, and the cracking product (CH4). The reaction term in the mass balance equations is described by apparent kinetic expressions. The reactor model equations were built under the following assumptions ... 

Another interesting selectivity problem arises when there are two different reactants in the supply phase, say A and C, that both react with the liquid phase reactant B, forming P and Q respectively, where the formation of Q is undesired. An example of practical importance is the selective absorption of hydrogen sulfide from an inert gas containing also carbon dioxide, in an alkaline solution (containing, e.g., alkanol amines). Conditions can be such that carbon dioxide (C) reacts rapidly with the alkanol amine, whereas hydrogen sulfide (A) reacts instantaneously. The consequence is that the absorption rate of hydrogen sulfide is practically determined by the gas phase mass transfer rate, and Ae rate of carbon... 

Desulfurization kinetics were studied with a model sulfur compound system, a dlbenzothlophene in white oil. Tests on basket agitation rate indicate that mass transfer and contacting effects are small above 750 rpm. The reaction kinetics agreed well with earlier work. The Langmulr-Hlnshelwood kinetic model was further refined to account for con5)etitlve adsorption effects due to dlbenzothlophene as well as hydrogen sulfide. 

One can also frequently choose between a purely mass-transfer operation and a chemical reaction or a combination of both. Water can be removed from an ethanol-water solution either by causing it to react with unslaked lime or by special methods of distillation, for example. Hydrogen sulfide can be separated from other gases either by absorption in a liquid solvent with or without simultaneous chemical reaction or by chemical reaction with ferric oxide. Chemical methods ordinarily destroy the substance removed, while mass-transfer methods usually permit its eventual recoveiy in unaltered form without great difficulty. 

Effect of Carbon Dioxide Partial Pressure. The presence of carbon dioxide in the sour gas has two major effects in the Stretford process. The first effect is to lower the pH of the solution. The second and more important effect is to decrease the hydrogen sulfide absoip-tion mass transfer rate. 

The most significant effect of carbon dioxide is a decrease in the absorber mass-transfer rale for H2S. In the absence of CO2, the H2S mass transfer rate depends primarily on the partial pressure of hydrogen sulfide in the sour gas. However, when the sour gas being treated contains a high concentration of carbon dioxide, the absorption efficiency of the solution may be sufficiently lowered to require an appreciable increase in the absorber height (Nicklin and Holland, 1963A). These effects are de.scribed in Figure 9-21. 

Absorber Configuration. The fundamental steps in the absorption process are the mass transfer of H2S from the gas to the liquid and the oxidation of the HS" to sulfur (Vancini and Lari, 1985). The absorption of hydrogen sulfide can take place in one or more contact stages, with the gas washed by the Stretford liquor either countercurrently or cocurrently. A subsequent delay tank allows enough residence time to convert major portions of the absorbed H2S to elemental sulfur. 

Figure 9.5 shows the concentration of sulfur in liquid phase and surface of catalyst as functions of reactor length. Typical behavior is observed, that is, there is a diminution of sulfur concentration in liquid phase as the mixture passes through the reactor. The hydrogen sulfide and hydrogen profile concentrations in liquid phase and on catalyst surface are shown in Figure 9.6. It is observed that the highest concentration of H2S in liquid phase occurs near the entrance of the reactor, while in the same section for hydrogen the lowest hydrogen concentration is observed. An explanation to this behavior is as follows near the entrance of the catalytic reactor, the mass transfer at solid-liquid interface predominates over gas-liquid mass transfer and thus the produced hydrogen sulfide remains essentially in liquid bulk and surface catalyst.

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