Spent sulfuric acid operations

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.

Hundreds of millions of gallons of ethanol are produced annually by this procedure. Operating in competition with other methods, this process accounts for about two-thirds of all the ethanol manufactured in the United States. The success of this process has been made possible by good yields, continuous operation on a large scale, an efficient procedure for reconcentrating the spent sulfuric acid for recycle, and, of course, a cheap source of ethylene. It will be noted that both the mono- and disulfates are converted to alcohol. A detailed description of a commercial operation, using 97.5 per cent acid, is given on p. 385. 

Direct continuous reaction of SOs with benzene, highly successful in the case of dimethyl ether as described above, is not practical because of high sulfone formation. Indirect continuous reaction with SOs by a procedure stated to yield no sulfone has, however, been achieved by the method developed by Dennis and Bull. This process is based upon an observation made by the former that, in the presence of sulfuric acid, benzene will dissolve 2-3 per cent of its own volume of benzenesulfonic acid. This process is also designed to operate in continuous countercurrent flow in a cascade system, benzene being introduced at the bottom and a benzene solution of the sulfonic acid overflowing from the top. Concentrated sulfuric acid is added continuously at the top, and spent sulfuric acid (77 per cent) is removed at the bottom of the reactor. The spent acid may be fortified to original strength with SO3 for reuse, and the benzene is recycled after the product sulfonic acid has been extracted from it with water. This procedure is, in theory, the most efficient possible, since benzenesulfonic acid is, in... 

This water, due to the dilution effect on the still unreacted sulfuric acid, causes the progressive loss of the latter s reactivity. This loss implies the necessity of continuous removal of the formed water or operating the process with excess of the sulfonating agent and eventually to separate, by physical settling, the weak-spent sulfuric acid that is not capable to comply with the desired sulfonation reaction kinetics anymore. 

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. 

In conventional alkylation operations, 98 wt. %, sulfuric acid is used as the catalyst, although some processes use HF.The spent alkylation acid, withdrawn as 88-92% acid, is not consumed in the chemical sense, but is diluted by carbonaceous material and small amounts of water. Acid reconditioning is usually completed in a separate plant. The range in makeup acid requirement and in octane quality varies with plant design, with type of feedstock, and with alkylate product rate. A wide variety of feedstocks can be processed through alkylation plants, as both low and high boiling olefins can be alkylated. 

Figure 10.17 H NMR spectra of spent acid samples spanning operational range of sulfuric acid concentrations. Shift of sulfuric acid peak with change in acid strength is indicated. Acid soluble oils and micellar water are readily quantified.

One patented process (40) was introduced in the mid- 60s to reduce the amount of sulfuric acid required by alkylation it was called the Sulfuric Acid Recovery Process (SARP) and was jointly licensed by Texaco Development Corporation and Stratford Engineering Corporation. Chemically, SARP proved all claims made for it. Utilized only with propylene/butylene alkylation the acid requirement was reduced as much as 70% actual acid dilution rates were lower than 0. 2 acid/gallon alkylate. However, the spent acid from SARP was different and could not be regenerated at the same rate as regular spent alkylation acid. This caused the chemical companies to increase the charges for regenerating the SARP spent acid to a point where there was no economic incentive to operate SARP. The two commercial SARP installations are not in use at the present time although new possibilities for SARP have arisen just in the past few months. 

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