Sulfuric acid absorption

Gas leaving the economizer flows to a packed tower where SO is absorbed. Most plants do not produce oleum and need only one tower. Concentrated sulfuric acid circulates in the tower and cools the gas to about the acid inlet temperature. The typical acid inlet temperature for 98.5% sulfuric acid absorption towers is 70—80°C. The 98.5% sulfuric acid exits the absorption tower at 100—125°C, depending on acid circulation rate. Acid temperature rise within the tower comes from the heat of hydration of sulfur trioxide and sensible heat of the process gas. The hot product acid leaving the tower is cooled in heat exchangers before being recirculated or pumped into storage tanks. 

Only a few of the major developments can be traced here, yet these should give a fair idea of the magnitude and importance of the aliphatic petrochemical growth. It is well to remember that some of the chemistry involved in this industry is old. Four Dutch chemists, otherwise unrecalled today, prepared ethylene dichloride by addition of chlorine to ethylene in 1795, and the synthesis of ethyl alcohol from ethylene via sulfuric acid absorption was studied by Berthelot in 1855 (8). Of course, this was coal-gas ethylene, and the commercial application of this synthesis did not occur until 75 years later, in 1929, when ethylene produced from natural gas was first converted into ethyl alcohol on a practical scale (84). 

Whenever these conditions on the ratio y/y apply, the design can be based upon the physical rate coefficient kG or upon the height of one gas-phase mass-transfer unit HG. The gas-phase mass-transfer-limited condition is approximately valid for the following systems absorption of NH3 into water or acidic solutions, absorption of H20 into concentrated sulfuric acid, absorption of S02 into alkali solutions, absorption of H2S from a gas stream into a strong alkali solution, absorption of HCl into water or alkaline solutions, or absorption of Cl2 into strong alkali solutions. 

Examples absorption of S03 in dilute sulfuric acid absorption of N02 in dilute nitric acid... 

Since the spectra show little change in position or intensity as the series is ascended, jt-it transitions are excluded. Figure 6 shows that the spectra are comparatively slightly affected by solution of the chlorides in sulfuric acid absorption cannot, therefore, be due to an n-ir transition from the nitrogen atom. It may be a result of excitation of the unshared electrons on the halogen atoms, consistent with the effect of bromine substitution on the position and intensity of the absorption band. The ultraviolet spectra therefore give no direct information on the structure of the ring. 

Besides ethanol, large quantities of ethyl ether are produced by the sulfuric acid absorption process from ethylene. In the hydration of ethylene by this method the relative proportions of alcohol and ether obtained are determined by the temperature and concentration of the sulfuric acid solution at the hydration and recovery stage. The more dilute the acid, the higher the proportion of alcohol formed and vice versa. This same process has been applied to the formation of isopropyl ether from propene and this product is now available in tank car lots. 

The hydration of the higher olefins, such as the butenes and pentenes, by the sulfuric acid absorption process has been commercially practiced for a number of years in this country. In the case of the higher olefins, a more dilute acid may be used for the absorption. Consequently acid reconcentration does not present the difficulties that are present in the case of the lower olefins where a more concentrated acid must be used. 

The chromatographic characteristics of these acids suggested that they were trihydroxycholanic acids. The sulfuric acid absorption spectra of these acids determined according to the procedure of Bernstein and Lenhard (29) exhibited three maxima at 309, 368, and 418 m/ . also suggesting the presence of three hydroxyl groups [Hsia et al. (30)]. Results of elemental analyses were in agreement of the empirical formula of C24H40O5, that of a trihydroxycholanic acid. 

Figure 6.3.13 shows a modem double absorption sulfuric acid plant. Three main sections can be distinguished, sulfur combustion to produce SO2 (burner), catalytic oxidation of SO2 to SO3, and absorption of SO3 in concentrated sulfuric acid (absorption towers). 

Carbonate is measured by evolution of carbon dioxide on treating the sample with sulfuric acid. The gas train should iaclude a silver acetate absorber to remove hydrogen sulfide, a magnesium perchlorate drying unit, and a CO2-absorption bulb. Sulfide is determined by distilling hydrogen sulfide from an acidified slurry of the sample iato an ammoniacal cadmium chloride solution, and titrating the precipitated cadmium sulfide iodimetrically. 

Acetylene can be deterrnined volumetricaHy by absorption in Aiming sulfuric acid (or more conveniently in sulfuric acid activated with silver sulfate) or by reaction with silver nitrate in solution and titration of the nitric acid formed ... 

Analytical and Test Methods. o-Nitrotoluene can be analyzed for purity and isomer content by infrared spectroscopy with an accuracy of about 1%. -Nitrotoluene content can be estimated by the decomposition of the isomeric toluene diazonium chlorides because the ortho and meta isomers decompose more readily than the para isomer. A colorimetric method for determining the content of the various isomers is based on the color which forms when the mononitrotoluenes are dissolved in sulfuric acid (45). From the absorption of the sulfuric acid solution at 436 and 305 nm, the ortho and para isomer content can be deterrnined, and the meta isomer can be obtained by difference. However, this and other colorimetric methods are subject to possible interferences from other aromatic nitro compounds. A titrimetric method, based on the reduction of the nitro group with titanium(III) sulfate or chloride, can be used to determine mononitrotoluenes (32). Chromatographic methods, eg, gas chromatography or high pressure Hquid chromatography, are well suited for the deterrnination of mononitrotoluenes as well as its individual isomers. Freezing points are used commonly as indicators of purity of the various isomers. 

When sublimed, anthraquinone forms a pale yeUow, crystalline material, needle-like in shape. Unlike anthracene, it exhibits no fluorescence. It melts at 286°C and boils at 379°—381°C. At much higher temperatures, decomposition occurs. Anthraquinone has only a slight solubiUty in alcohol or benzene and is best recrystallized from glacial acetic acid or high boiling solvents such as nitrobenzene or dichlorobenzene. It is very soluble in concentrated sulfuric acid. In methanol, uv absorptions of anthraquinone are at 250 nm (e = 4.98), 270 nm (4.5), and 325 nm (4.02) (4). In the it spectmm, the double aUyflc ketone absorbs at 5.95 p.m (1681 cm ), and the aromatic double bond absorbs at 6.25 p.m (1600 cm ) and 6.30 pm (1587 cm ). 

The off-gas from the reactor contains CO2, SO, and H2SO4. The SO is removed by absorption (qv) into concentrated sulfuric acid solution or by other means. The CO2 and H2SO4 vapor is removed by absorption into water or alkaline solution. 

The Reich test is used to estimate sulfur dioxide content of a gas by measuring the volume of gas required to decolorize a standard iodine solution (274). Equipment has been developed commercially for continuous monitoring of stack gas by measuring the near-ultraviolet absorption bands of sulfur dioxide (275—277). The deterrnination of sulfur dioxide in food is conducted by distilling the sulfur dioxide from the acidulated sample into a solution of hydrogen peroxide, foUowed by acidimetric titration of the sulfuric acid thus produced (278). Analytical methods for sulfur dioxide have been reviewed (279). 

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. 

Plants producing oleum or Hquid SO typically have one or two additional packed towers irrigated with oleum ahead of the normal SO absorption towers. Partial absorption of SO occurs in these towers, and sulfuric acid is added to maintain desired oleum concentrations. Normally, oleum up to about 35 wt % free SO content can be made in a single tower two towers are used for 40 wt % SO. Liquid SO is produced by heating oleum in a boder to generate SO gas, which is then condensed. Oleums containing SO >40 wt % are usually produced by mixing SO with low concentration oleum. 

More recentiy, sulfuric acid mists have been satisfactorily controlled by passing gas streams through equipment containing beds or mats of small-diameter glass or Teflon fibers. Such units are called mist eliminators (see Airpollution control methods). Use of this type of equipment has been a significant factor in making the double absorption process economical and in reducing stack emissions of acid mist to tolerably low levels. 

Gas leaving the converter is normally cooled to 180—250°C using boiler feedwater in an "economizer." This increases overall plant energy recovery and improves SO absorption by lowering the process gas temperature entering the absorption tower. The process gas is not cooled to a lower temperature to avoid the possibiUty of corrosion from condensing sulfuric acid originating from trace water in the gas stream. In some cases, a gas cooler is used instead of an economizer. 

Double-Absorption Plants. In the United States, newer sulfuric acid plants ate requited to limit SO2 stack emissions to 2 kg of SO2 per metric ton of 100% acid produced (4 Ib /short ton Ib = pounds mass). This is equivalent to a sulfur dioxide conversion efficiency of 99.7%. Acid plants used as pollution control devices, for example those associated with smelters, have different regulations. This high conversion efficiency is not economically achievable by single absorption plants using available catalysts, but it can be attained in double absorption plants when the catalyst is not seriously degraded. 

Oleum Ma.nufa.cture, To produce fuming sulfuric acid (oleum), SO is absorbed in one or more special absorption towers irrigated by recirculated oleum. Because of oleum vapor pressure limitations the amount of SO absorbed from the process gas is typically limited to less than 70%. Because absorption of SO is incomplete, gas leaving the oleum tower must be processed in a nonfuming absorption tower. 

As worldwide attention has been focused on the dangers of acid rain, the demand to reduce sulfur dioxide [7446-09-5] emissions has risen. Several processes have been developed to remove and recover sulfur dioxide. Sulfur can be recovered from sulfur dioxide as Hquid sulfur dioxide, sulfuric acid, or elemental sulfur. As for the case of hydrogen sulfide, sulfur dioxide removal processes are categorized as adsorption, absorption, or conversion processes. 

Two colorimetric methods are recommended for boron analysis. One is the curcumin method, where the sample is acidified and evaporated after addition of curcumin reagent. A red product called rosocyanine remains it is dissolved in 95 wt % ethanol and measured photometrically. Nitrate concentrations >20 mg/L interfere with this method. Another colorimetric method is based upon the reaction between boron and carminic acid in concentrated sulfuric acid to form a bluish-red or blue product. Boron concentrations can also be deterrnined by atomic absorption spectroscopy with a nitrous oxide—acetjiene flame or graphite furnace. Atomic emission with an argon plasma source can also be used for boron measurement. 

Ca.ta.lysts for Acetylation. Sulfuric acid is the preferred catalyst for esterifying cellulose and is the only known catalyst used commercially for this function. The role of sulfuric acid during acetylation has been discussed (77,78). In the presence of acetic anhydride, sulfuric acid rapidly and almost quantitatively forms the cellulose sulfate acid ester (77). Even in the absence of anhydride, the sulfuric acid is physically or mechanically retained (sorbed) on the cellulose. The degree of absorption is a measure of the reactivity or accessibiUty of different celluloses. 

Flue Ga.s Desulfuriza.tion. Citric acid can be used to buffer systems that can scmb sulfur dioxide from flue gas produced by large coal and gas-fired boilers generating steam for electrical power (134—143). The optimum pH for sulfur dioxide absorption is pH 4.5, which is where citrate has buffer capacity. Sulfur dioxide is the primary contributor to acid rain, which can cause environmental damage. 

Cyclohexanol can be deterrnined colorimetricaHy by reaction with -hydroxy-ben2aldehyde in sulfuric acid (18). This method can be used in the presence of cyclohexanone and cyclohexane. Cyclohexanol and cyclohexanone both show a maximum absorbency at 535 nm but at 625 nm the absorption by cyclohexanone is negligible, whereas cyclohexanol shows appreciable absorption. 

Absorption of ethylene in concentrated sulfuric acid to form monoethyl sulfate (ethyl hydrogen sulfate) and diethyl sulfate ... 

The absorption is carried out by countercurrent passage of ethylene through 95—98% sulfuric acid in a column reactor at 80°C and 1.3—1.5 MPa (180—200 psig) (41). The absorption is exothermic, and cooling is required (42) to keep the temperatures down and thereby limit corrosion problems. The absorption rate increases when ethyl hydrogen sulfate is present in the acid (43—46). This increase is attributed to the greater solubiUty of ethylene in ethyl hydrogen sulfate than in sulfuric acid. 

The effects of various catalysts (47,48), contaminants (49,50), acid concentration (51), temperature (52), and pressure (53—57) on the rate of absorption have been studied. The patent Hterature indicates that absorption can be improved by making the contact between the gaseous ethylene and hquid sulfuric acid more efficient (58—61), by suitable design of the absorption tower (62), and by various combinations of absorption and hydrolysis (63-68). 

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