Regeneration of spent sulfuric acid

About 10% of sulfuric acid is used as catalyst in petroleum refining and polymer manufacture. The acid is not consumed during these uses, but its effectiveness is diminished by contamination with water, hydrocarbons and other chemicals. 

The contaminated spent is made into new strong acid by  

S03(g) + H20(f)in strong sulfiiric acid + H2S04(f)jn strengthened sulfuric acid- 

The steps are all continuous. This chapter describes steps (a) to (d). Steps (e) to (g) are described in Chapter 6 onwards. 

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Subsequently, instead of the sodium salt, the free sulfonic acid [149] is precipitated by addition of sulfuric acid or recycled mother liquor, followed by cooling. The mother liquor, which remains after filtering off the product, can be supplied directly to existing plants for regeneration of spent sulfuric acid. 

In multiple-tube boilers (usually horizontal) the fire may be on the tube side. As long as the fire tubes are kept submerged in water, the tubes do not overheat. Boilers of this type are widely used in the regeneration of spent sulfuric acid, and in the production of elemental sulfur from hydrogen sulfide. 

In the sulfuric acid process, the sulfuric acid removed must be regenerated in a sulfuric acid plant which is generally not a part of the alkylation unit and may be located off-site. Spent sulfuric acid generation is substantial typically in the range of 13 to 30 pounds per barrel of alkylate. Air emissions from the alkylation process may arise from process vents and fugitive emissions. 

Both of these processes direct the SO2 absorbed from the FCCU flue gas to the refinery SRU, where it is converted to elemental sulfur and added to the marketable sulfur that is generated by the SRU from H2S. Alternately, the SO2 can be converted to sulfuric acid in a dedicated sulfuric acid plant, or in combination with an existing refinery spent acid regeneration unit. When the SO2 is directed to the SRU, 1 ton of SO2 captured in the scrubber is converted to 0.5 tons of marketable elemental sulfur and less than 0.1 ton of sodium sulfate waste is generated per ton of SO2 absorbed. In an acid plant, 1 ton of SO2 generates 1.5 tons of 98% sulfuric acid. Steam is also generated from the conversion of SO2 in both the SRU and the acid plant, which moderates somewhat the steam consumption rate of the solvent regenerator for both the LABSORB and CANSOLV systems. 

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.

The relative location of refinery and acid plant is one of the most important factors in the economic decision between sulfuric acid and anhydrous hydrogen fluoride as a catalyst for alkylation. Besides the distance, other factors such as regeneration of spent acid, energy costs, the nature of the feed and increasingly stringent regulatory constraints play an important role in the selection of alkylation catalyst. Sulfuric acid is selected for alkylation if feed is rich in pentenes or n-butene. HF is selected if the feed is rich in propenes or isobutane. 

But let me be completely honest. 1 once had a radiation level detector on a spent sulfuric acid tank. 1 was the operating superintendent at this plant, which regenerated sulfuric add. We had a not-so-small fire at this tank. Rather than repair the tank, pumps, and lines, 1 decided to dismantle and haul away the entire mess of steel. After all, 1 had much larger and more modern storage tanks at my disposal. 

Sulfuric acid is produced by burning sulfur and H S to SO with air. Then more air is added and reacted in a catalyst bed of vanadium pentoxide to produce SO. The SO, is mixed with water to make sulfuric acid. The reaction is exothermic and takes place in the vapor phase. Sulfuric acid is, by weight, one of the main chemicals produced in the world. I spent 1974 to 1976 in charge of a sulfuric acid regeneration plant in Texas City. In 1975, I had 34 shutdowns and start-ups. I ve kept a piece of lead wire, part of that awful plant, in my desk drawer. When things seem bad, that memento reminds me that it could be a lot worse. 

Inorganic chemical processing Regeneration of contaminated (spent) sulfuric acid catalyst... 

Boron trifluoride catalyst may be recovered by distillation, chemical reactions, or a combination of these methods. Ammonia or amines are frequently added to the spent catalyst to form stable coordination compounds that can be separated from the reaction products. Subsequent treatment with sulfuric acid releases boron trifluoride. An organic compound may be added that forms an adduct more stable than that formed by the desired product and boron trifluoride. In another procedure, a fluoride is added to the reaction products to precipitate the boron trifluoride which is then released by heating. Selective solvents may also be employed in recovery procedures (see Catalysts,regeneration). 

Use closed-loop systems for pickling regenerate and recover acids from spent pickling liquor using resin bed, retorting, or other regeneration methods such as vacuum crystallization of sulfuric acid baths. 

The last reaction cited above as shown is very effectively catalyzed by bacterial action but is very slow chemically by recycling the spent ferrous liquors and regenerating ferric iron bacterially, the amount of iron which must be derived from pyrite oxidation is limited to that needed to make up losses from the system, principally in the uranium product stream. This is important if the slow step in the overall process is the oxidation of pyrite. The situation is different in the case of bacterial leaching of copper sulfides where all the sulfide must be attacked to obtain copper with a high efficiency. A fourth reaction which may occur is the hydrolysis of ferric sulfate in solution, thus regenerating more sulfuric acid the ferrous-ferric oxidation consumes acid. 

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