Spent sulfuric acid decomposition

Fig. 5.1. Spent sulfuric acid regeneration flowsheet. H2S04(f) in the contaminated spent acid is decomposed to S02(g), 02(g) and H20(g) in a mildly oxidizing, 1300 K fuel fired furnace. The furnace offgas (6-14 volume% S02, 2 volume% 02, remainder N2, H20, C02) is cooled, cleaned and dried. It is then sent to catalytic S02 + Vi02 —> S03 oxidation and H2S04 making, Eqn. (1.2). Air is added just before dehydration (top right) to provide 02 for catalytic S02 oxidation. Molten sulfur is often burnt as fuel in the decomposition furnace. It provides heat for H2S04 decomposition and S02 for additional H2S04 production. Tables 5.2 and 5.3 give details of industrial operations.

Thennal decomposition of spent sulfuric acids to give sulfur dioxide is achieved in a furnace at temperatures around 1,000 °C. Spent acids come from processes where H2SO4 or oleum is used as a catalyst (alkylation, nitration, sulfonation, etc.) or from other processes where H2SO4 is used to clean, dry, and remove water. 

Thermal decomposition of spent acids, eg, sulfuric acid, is required as an intermediate step at temperatures sufficientiy high to completely consume the organic contaminants by combustion temperatures above 1000°C are required. Concentrated acid can be made from the sulfur oxides. Spent acid is sprayed into a vertical combustion chamber, where the energy required to heat and vaporize the feed and support these endothermic reactions is suppHed by complete combustion of fuel oil plus added sulfur, if further acid production is desired. High feed rates of up to 30 t/d of uniform spent acid droplets are attained with a single rotary atomizer and decomposition rates of ca 400 t/d are possible (98). 

Spent acid burning is actually a misnomer, for such acids are decomposed to SO2 and H2O at high temperatures in an endothermic reaction. Excess water in the acid is also vaporized. Acid decomposition and water vaporization require considerable heat. Any organic compounds present in the spent acid oxidize to produce some of the required heat. To supply the additional heat required, auxiUary fuels, eg, oil or gas, must be burned. When available, sulfur and H2S are excellent auxiUary fuels. 

Relatively high (typically 980—1200°C) temperatures are required to decompose spent acids at reasonable burner retention times. Temperatures depend on the type of spent acid. A wide variety of spent acids can be processed in this way, but costs escalate rapidly when the sulfuric acid concentration in spent acid (impurity-free basis) falls below about 75%. A few relatively uncontaminated spent acids can be reused without decomposition by evaporating the excess water in concentrators, or by mixing in fresh sulfuric acid of high concentration. Weak spent acids are frequently concentrated by evaporation prior to decomposition. 

Sulfur burning furnace Sulfide mineral smelters and roasters Spent acid decomposition furnace... 

Fig. 5.2. Spent acid decomposition furnace. It is brick lined steel 4 m diameter and 20 m long. The energy for decomposing H2S04(< ) into S02(g), 02(g) and H20(g) and for evaporating water is provided by burning molten sulfur and natural gas with hot air. Industrial details are given in Table 5.2.

Sulfonation of LAB. The sulfonation of alkylbenzenes leads to sulfonic acid tyre product, which is then neutralized with a base such as sodium hydroxide to produce sodium alkylbenzene sulfonate. The sulfonation reaction is highly exothermic and instantaneous. An efficient reactor heat removal system is used to prevent the decomposition of the resultant sulfonic acid. The sulfonation reaction takes place by using oleum (SO3H2SO4) or sulfur trioxide (SO3). Although, the oleum sulfonation requires relatively inexpensive equipment, the oleum process has major disadvantages compared to sulfur trioxide. The need for spent acid stream disposal and the potential corrosion owing to sulfuric acid generation increased the problems related to oleum process [1]. 

Preparation. Coned, nitric acid (300 ml.) is chilled in ice and treated with cooling with 300 ml. of coned, sulfuric acid. In a second flask 150 ml. of methanol is cooled in ice to keep the temperature below 10° during cautious addition of 50 ml. of coned, sulfuric acid. One third of the cold nitric-sulfuric acid is placed in each of three 500-ral. Erlenmeyers, and each portion is treated with one third of the methanol-sulfuric acid mixture, added in 2-3 min. with constant swirling. Methyl nitrate separates as an almost colorless oily upper layer. After standing for 15 min. the lower layer of spent acid is separated and quenched with a large volume of water to avoid vigorous decomposition. The combined ester is washed with 25 ml. of ice-cold 22% sodium chloride solution, and the process is repeated with addition of enough alkali to produce a faintly alkaline reaction. The ester is washed free of alkali with ice-cold salt solution, then washed twice with 15 ml. of ice water, dried over calcium chloride, decanted, and used directly. Yield 190-230 g. (66-80%). Distillation is not recommended the crude ester is satisfactory for synthetic purposes. 

Sung, S. Szechy, G. Albright, L.F. Decomposition of spent alkylation sulfuric acid to produce... 

One-third of the cold nitric-sulfuric mixture is placed in each of three 500-cc. Erlenmeyer flasks (Note 3), and each portion is treated separately with one-third of the methyl alcohol-sulfuric acid mixture, with constant shaking and thorough mixing (Note 4). The temperature is allowed to rise fairly rapidly to 40° and kept at this point by external cooling. During the addition of the methyl alcohol-sulfuric acid, most of the ester separates as an almost colorless oily layer above the acid. The time required for completion of the reaction is two to three minutes for each flask. The reaction mixtures are allowed to stand in the cold for an additional fifteen minutes but not longer. The lower layer of spent acid is separated promptly and poured at once into a large volume of cold water (about i 1. for each portion) to avoid decomposition which quickly ensues with copious evolution of nitrous fumes. 

Current processes for the manufacture of trinitrotoluene (TNT) produce atmospheric and water pollution that is only partly relieved by mechanical clean-up methods. TNT is currently produced from toluene by successive mono-, di-, and trinitrations with mixed aqueous nitric and sulfuric acids in the first two steps and anhydrous mixed acid in the last. Each stage in the current process is conducted at elevated temperatures, and side reactions in the overall process directly produce thousands of pounds of oxides of nitrogen, sulfuric acid aerosols, and volatile nitro organic products (such as tetranitromethane and nitroaro-matics). These pollutants derive from the thermal decomposition of the aqueous nitric acid solutions, from oxidative side reactions that produce as many as 40 by-product compounds, and from formation of unsymmetrlcal "meta" Isomers. Since symmetrical TNT is inevitably accompanied by meta isomers as well as oxidation products, the crude material is treated with sodium sulfite solutions to remove the undesirable Isomers and nitroaromatics by derivatization. The spent sulfite solution, known as "red water, is then disposed of by combustion to an inorganic ash. Itself a disposal problem. 

The organic solvent is recovered for reuse in the production plant. Excess sulfuric acid mother liquor is conveyed via the internal piping system for spent acid to the existing sulfuric acid plant, where concentration, decomposition to sulfur dioxide, and oxidation to sulfur trioxide occur. Sulfur trioxide is then withdrawn and fed to the sulfonation reaction through piping. 

---