Acid gas removal

Increasingly, industry is being required to limit its emissions of acidic gases. Limestone reacts with the most common acidic gases (i.e. SO2, SO3, HCl and HF), and is considerably less expensive than alternative alkaline materials, such as lime (see chapter 29), sodium carbonate/bicarbonate and caustic soda. It is, therefore, not surprising that considerable effort has been put into developing processes using limestone as an absorbent. 

The processes which have been developed may be divided into wet and dry scrubbing. The choice of process depends on several factors, including  

Commercially, the most important application is the use of wet scrubbing with limestone for flue gas desulfurisation (FGD) at electricity generating stations. For example, a 2000 MW station operating on a 2 % sulfur coal, at a 70 % load factor, requires about 300,000 tpa of limestone to remove 90 % of the oxides of sulfur. The majority of generating stations fitted with FGD use limestone, and about half of those oxidise the calcium sulfite produced to gypsum (CaS04 -2H20) [12.18]. Some stations use lime (see chapter 29), while others use a variety of other processes [12.3]. 

Dry scrubbing with limestone is also used to remove hydrogen fluoride from the exhaust gases from kilns in which heavy clay goods and ceramic products are fired. It is also employed for FGD on small to medium sized boilers. 

Coals typically contain 0.5 to 4 % by weight of sulfur in the form of iron sulfide (FeS2), organic sulfur compounds and inorganic sulfates. Many other fuels contain significant amounts of sulfur (e.g., heavy fuel oil and PET coke). During combustion the sulfur is oxidised to sulfur dioxide, with smaller amounts of sulfur trioxide also being produced. Sulfur dioxide is only moderately soluble in water. It dissolves more rapidly in a suspension of limestone (and still more rapidly in alkaline solutions/suspensions — see chapter 29). 

The physical wash processes can be designed so that H2S and CO2 are absorbed separately, meaning that both gases can be reeovered at high concentration. The CO2 stream may be compressed for CCS (refer to Section 1.4.2). The H2S stream is converted either into elementary sulphur by the Claus process or into concentrated sulphuric acid by the WSA process [291]. 

Giammarco-vetrocoke-sulfur Stretford Tekahax Ferrox 

The removal of acid gases from gas streams can be generally classified into two categories (1) chemical absorption processes and (2) physical absorption processes. There are several such processes that fit into these categories (Tables 23.3 and 23.4) the features of the individual process may vary (Table 23.4 van den Berg and de Jong, 1980 Bodle and Heubler, 1981). 

In more general and simple process terms, acid gas removal is considered to be hydrogen sulfide and CO2 removal the ranoval of SOx and NOx is often achieved by contact of the gas with 

Simple Classification System for Acid Gas Removal Processes 

Chemical Absorption (Chemical Solvent Processes) Alkanolamines MEA 

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The Eastman Chemicals from Coal faciUty is a series of nine complex interrelated plants. These plants include air separation, slurry preparation, gasification, acid gas removal, sulfur recovery, CO /H2 separation, methanol, methyl acetate, and acetic anhydride. A block flow diagram of the process is shown in Eigure 3. The faciUty covers an area of 2.2 x 10 (55 acres) at Eastman s main plant site in Kingsport, Teimessee. The air separation plant is... 

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. 

Extensive work has been done on corrosion inhibitors (140), activated carbon use (141—144), multiple absorption zones and packed columns (145,146), and selective absorption and desorption of gas components (147,148). Alkan olamines can also be used for acid gas removal in ammonia plants (149). 

The Rectisol process is more readily appHcable for acid gas removal from synthesis gas made by partial oxidation of heavy feedstocks. The solvents used in Purisol, Fluor Solvent, and Selexol processes have low vapor pressures and hence solution losses are minimal. Absorption systems are generally corrosion-free. 

Hot potassium carbonate is generally considered for bulk acid gas removal applications. Often a secondary removal step using an alkanolamine is also used to meet product specifications. 

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. 

Cold methanol has proven to be an effective solvent for acid gas removal. Cold methanol is nonselective in terms of hydrogen sulfide and carbon dioxide. The carbon dioxide is released from solution easily by reduction in pressure. Steam heating is required to release the hydrogen sulfide. A cold methanol process is Hcensed by Lurgi as Rectisol and by the Institute Francaise du Petrole (IFP) as IFPEXOL. 

A bleed from the scmbbing system is sent to a sour slurry stripper. The water is then clarified and can be recycled to minimize the volume of effluent to be biotreated and discharged or evaporated. The acid gas from the acid gas removal system and from the sour slurry stripper is fed to a Claus plant, where salable elemental sulfur (qv) is produced. For maximum sulfur recovery and minimal sulfur emissions, the Shell Claus off-gas treating process (SCOT) is used. 

After acid gas removal the pyrolysis gas from the last stage of compression is cooled by propylene refrigerant and sent to a condensate stripper. This tower separates the C and heavier products, which exit the bottom, from the and lighter components. 

Diethanolamine Systems. Diethanolamine (DEA) is a secondary amine that has in recent years replaced MEA as the most common chemical solvent., s a secondary amine, DEA is a weaker base than MEA, and therefore DEA systems do not typically suffer the same corrosion problems. In addition, DEA has lower vapor loss, requires less heat for regeneration per mole of acid gas removed, and does not require a reclaimei. DEA reacts with H iS and COt as follows ... 

Figure 1-1. The Selexol process for acid gas removal (1) absorber, (2) flash drum, (3) compressor, (4) low-pressure drum, (5) stripper, (6) cooler.

Membrane transport models, 22 638-639 Membrane units, for acid gas removal, 22 377... 

Solvent-resistant elastomers, 9 560-562 Solvent-resistant membranes, 27 656 Solvent-resistant rubber, 22 583-584 Solvent selection, in liquid-liquid extraction, 70 746-749 Solvent-solute interactions, 26 855,23 91-96 acid/base interactions in, 23 96 dispersion in, 23 92-93 electrostatic forces in, 23 91-92 hydrogen bonding in, 23 94-95 hydrophobic interactions in, 23 95 polarization in, 23 92 repulsion in, 23 93-94 Solvent strength, of pure fluids, 24 3-4 Solvent systems, for acid gas removal, 72 376-377... 

Acid gas removal is the removal of sulfur compounds and CO2 (acid gas) from process gas streams. The following sections describe available process alternatives, design options, and guidelines for selection among alternatives. 

Process Alternatives. Acid gas removal processes have been extensively surveyed in the published literature (1,8, 3) ... 

Water is often used as a medium for removing particulates from process gas streams (3). In the process, significant quantities of CO2 and H2S may be removed particularly if NH3 is present in the process gas. Because water is not generally used commercially for the express purpose of acid gas removal, It will not be discussed further. 

Process Selection. Table I presents general performance data to guide process selection of acid gas removal facilities. Several surveys have recently appeared in the literature ( 4, 5). 

Christensen, K.G. and Stupin, W.J. "Merits of Acid-Gas Removal Processes," Hydrocarbon Processing, February 1978. 

Ammonia. Ammonia interferes with existing acid gas removal processes because it can pass on through the scrubbers and then solidify on cyrogenic surfaces or it can go with the acid gases and poison the sulfur conversion catalysts. If ammonia is absorbed into an aqueous stream, then this aqueous stream must be... 

The physical absorbents for acid gas removal from natural and synthetic gases. 

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