Removal of Sulfur Compounds and Carbon Dioxide

ALkazid Process Alkali Salts of Amino Acids (N-Methylaminopropionic Acid) 

Giammarco-Vetrocoke Process Potassium Carbonate with Arsenite 

Hi Pure Process Two-Stage Hot Potassium Carbonate Washing 

Purisol Process from Lurgi N-Methyl-2-Pyrrolidone (NMP) 

Selexol Process Dimethyl Ethers of Polyethylene Glycol 

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Removal of Sulfur Compounds and Carbon Dioxide. If sulfur is present as H2S or COS... 

In a typical gas oil design, the lighter products overhead from the quench tower/primary fractionator are compressed to 210 psi, and cooled to about 100°F. Some Q plus material is recovered from the compressor knockout drums. The gases are ethanolamine and caustic washed to remove acid gases sulfur compounds and carbon dioxide, and then desiccant dried to remove last traces of water. This is to prevent ice and hydrate formation in the low temperamre section downstream. 

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. 

Carbon Dioxide Condensation, Sulfurous Compound Absorption. Carbon dioxide is condensed by cooling the gas from its dew point to about -55°C the condensate is contaminated with sulfurous compounds. Substantial amounts of carbon dioxide can be condensed if the dew point is relatively warm. A synthesis gas at 1000 psia with 30 mol % carbon dioxide has a dew point of about -30°C, and approximately 65% of the carbon dioxide condenses to a liquid in cooling to -55°C. Removal of carbon dioxide by condensation reduces the amount which must be removed subsequently by absorption. Condensation is preferred over absorption because it is more reversible and hence is more energy efficient and less capital intensive. 

Carbon Dioxide and Water Removal from Ethylene, 1076 Carbon Dioxide Removal from Cryogenic Plant Feed Gas, 1076 Removal of Sulfur Compounds, 1078 Hydrogen Purification by Pressure Swing Adsorption (PSA), 1081... 

In cases where both selective sulfur removal and complete carbon dioxide removal are required, such as with coal-derived substitute natural gases, two successive independent absorption-regeneration cycles are used as shown in Figure 14-8. Various flow schemes are discussed in. some detail by Van Deraerschot and Valentine (1976) and Sweny (1980). These authors claim that, with proper plant design, removal of sulfur compounds to concentrations as low as a few parts per million can be achieved with relatively low co-absorption of carbon dioxide. 

During the liquefaction process, usually much of the oxygen, carbon dioxide, sulfur compounds and water are removed so that liquefied natural gas (LNG) IS nearly 100 percent methane. LNG takes up one-six-hundredth the volume of natural gas, with a density less than half that of water. 

The CNG acid gas removal process is distinguished from existing AGR processes by three features. The first feature is the use of pure liquid carbon dioxide as absorbent for sulfurous compounds the second feature is the use of triple-point crystallization to separate pure carbon dioxide from sulfurous compounds the third feature is the use of a liquid-solid slurry to absorb carbon dioxide below the triple point temperature of carbon dioxide. Pure liquid carbon dioxide is a uniquely effective absorbent for sulfurous compounds and trace contaminants triple-point crystallization economically produces pure carbon dioxide and concentrated hydrogen sulfide for bulk carbon dioxide absorption the slurry absorbent diminishes absorbent flow and limits the carbon dioxide absorber temperature rise to an acceptable low value. The sequence of gas treatment is shown in Figure 1, an overview of the CNG acid gas removal process. 

If sulfur is present as H2S or COS, it is a poison for many catalysts and will partly or completely inhibit the catalyst activity.46 Carbon monoxide (CO) and carbon dioxide (C02) can poison the ammonia synthesis catalyst so both of these compounds must be removed.53... 

Once removed from the raw gas, the question arises as to what should be done with the acid gas. If there is a large amount of acid gas, it may be economical to build a Claus-type sulfur plant to convert the hydrogen sulfide into the more benign elemental sulfur. Once the H2S has been converted to sulfur, the leftover carbon dioxide is emitted to the atmosphere. Claus plants can be quite efficient, but even so, they also emit significant amounts of sulfur compounds. For example, a Claus plant processing 10 MMSCFD of H2S and converting 99.9% of the H2S into elemental sulfur (which is only possible with the addition of a tail gas clean up unit) emits the equivalent of 0.01 MMSCFD or approximately 0.4 ton/day of sulfur into the atmosphere. Note that there is more discussion of standard volumes and sulfur equivalents later in this chapter. 

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