Single contact acid plants

In the early 1970s, air pollution requirements led to the adoption of the double contact or double absorption process, which provides overall conversions of better than 99.7%. The double absorption process employs the principle of intermediate removal of the reaction product, ie, SO, to obtain favorable equiUbria and kinetics in later stages of the reaction. A few single absorption plants are stiU being built in some areas of the world, or where special circumstances exist, but most industriali2ed nations have emission standards that cannot be achieved without utili2ing double absorption or tad-gas scmbbers. A discussion of sulfuric acid plant air emissions, control measures, and emissions calculations can be found in Reference 98. 

This chapter describes S02 oxidation in single contact acid plants. These plants ... 

Fig. 9.1. Single contact H2SO4 making flowsheet. SO3 rich gas from catalytic SO2 oxidation is reacted with strong sulfuric acid, Reaction (1.2). The reaction consumes H20(f) and makes H2S04(f), strengthening the acid. Double contact H2SO4 making is described in Fig. 9.6. A few plants lower the SO2 content of their tail gas by scrubbing the gas with basic solution (Hay et al., 2003).

Most sulfuric acid plants are double contact plants, Fig. 9.6, Tables 9.3, 19.3 and 23.2. They efficiently oxidize their feed S02(g) to S03(g) and efficiently make the resulting S03(g) into H2S04(f). Single contact plants (Fig. 9.1) are simpler and cheaper - but less efficient. 

Most of the mathematical chapters analyze catalytic SO2+/2O2-> S03 oxidation in single and double contact acid plants. The remainder examine temperature control and H2S04 making. 

Fig. 16.1. Schematic of single contact, 3 catalyst bed sulfuric acid plant. It is a single contact plant because it has only one H2S04 making step. Note gas cooling between catalyst beds. It permits additional S02 oxidation in the next catalyst bed.

Cool catalyst bed input gas gives high S02 oxidation efficiency in single and double contact acid plants. Low deactivation temperature Cs catalyst is beneficial in this respect, Chapters 8 and 12. 

Fig. 21.1 indicates how these requirements are achieved for a single contact sulfur-burning acid plant with 3 catalyst beds. It shows that ... 

Fig. 21.1. Heat transfer flowsheet for single contact, sulfur burning sulfuric acid plant. It is simpler than industrial plants, which nearly always have 4 catalyst beds rather than 3. The gaseous product is cool, S03 rich gas, ready for H2S04 making. The heat transfer product is superheated steam. All calculations in this chapter are based on this figure s feed gas composition and catalyst bed input gas temperatures. All bed pressures are 1.2 bar. The catalyst bed output gas temperatures are the intercept temperatures calculated in Sections 12.2, 15.2 and 16.3.

Single contact sulfuric acid plant including ... 

Fig. 23.1. Simplified single contact sulfuric acid production flowsheet. Its inputs are moist feed gas and water. Its outputs are 98 mass% H2S04, 2 mass% H20 sulfuric acid and dilute S02, 02, N2 gas. The acid output combines gas dehydration tower acid, H2S04 making tower acid and liquid water. The equivalent sulfur burning acid plant sends moist air (rather than moist feed gas) to dehydration. Appendix V gives an example sulfur burning calculation.

With the double contact process it is unnecessary to purify the tail gases to reduce their sulfur dioxide content still further, whereas tail gases from single contact plants have to be purified. This can be realized either by scrubbing with ammonia or with an aqueous solution of sodium sulfite and sodium hydrogen sulfite (Wellman-Lord process), absorption on activated charcoal (sulfacid process from Lurgi) or by oxidative gas purification such as in the peracidox process (oxidation of sulfur dioxide with hydrogen peroxide or peroxomonosulfuric acid). 

The S02-ladened off-gas from the cooler and the air from the stripper are compressed and sent to a single-contact, single-absorption sulfuric acid plant. Ninety-six percent of the SO2 is converted to 98 sulfuric acid. Ten percent of this byproduct acid is recycled and used in the MgO FGD process but most is stored onsite and eventually sold as a byproduct. The tail gas from the acid plant containing the unconverted SO2 is recycled to the flue gas ducts ahead of the FGD system. 

Existing single-contact acid plants can also be converted to doublecontact plants (63). In such cases, however, using add-on scrubber systems is an alternative, and several such systems have been used commercially. The Cominco ammonia absorption process has been used for many years (22, 64). The Lurgi Sulfacid process (65) and Wellman-Lord process (66) have had more recent and limited use. The Mitsu-bishi-JECCO process has also been applied to acid plant tail gases (27, 67, 68), but the gypsum by-product would be essentially a waste in the United States. 

The Herculaneum smelter operates a 340-ton per day Monsanto type single contact acid plant on the site. This plant treats the sulfiir-rich gases produced in the sintering process. The 93% black sulfuric acid produced is stored in tanks on site. Sulfuric acid is shipped to customers from the smelter by truck, rail or river barge. The acid plant operates and performs maintenance on a schedule similar to the sinter plant. 

The single furnace options incorporate flue gas desulphurisation (FGD) scrubbing whereas the two furnace options incorporate double contact sulphuric acid plants. 

In the 1960s the environment protection laws made it prohibitive to expand the single contact single absorption plants. The conversion efficiency of SCSA process giving 96-96.5% conversion produced SO2 emission of 16-20 kg/tonne of sulfuric acid... 

In a typical sulfur-burning, single-contact sulfuric acid plant (Figure 11.1), the molten sulfur is burned with dry... 

Figure 11.1. Sulfur-Burning Single-Contact Sulfuric Acid Plant.

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