Sulfuric acid, velocity effect

Two complementai y reviews of this subject are by Shah et al. AIChE Journal, 28, 353-379 [1982]) and Deckwer (in de Lasa, ed.. Chemical Reactor Design andTechnology, Martinus Nijhoff, 1985, pp. 411-461). Useful comments are made by Doraiswamy and Sharma (Heterogeneous Reactions, Wiley, 1984). Charpentier (in Gianetto and Silveston, eds.. Multiphase Chemical Reactors, Hemisphere, 1986, pp. 104—151) emphasizes parameters of trickle bed and stirred tank reactors. Recommendations based on the literature are made for several design parameters namely, bubble diameter and velocity of rise, gas holdup, interfacial area, mass-transfer coefficients k a and /cl but not /cg, axial liquid-phase dispersion coefficient, and heat-transfer coefficient to the wall. The effect of vessel diameter on these parameters is insignificant when D > 0.15 m (0.49 ft), except for the dispersion coefficient. Application of these correlations is to (1) chlorination of toluene in the presence of FeCl,3 catalyst, (2) absorption of SO9 in aqueous potassium carbonate with arsenite catalyst, and (3) reaction of butene with sulfuric acid to butanol. 

A brick lining has the secondary effect of protecting the lead from abrasion by contained slurries or suspended matter. The protective surface film of insoluble salts can be thinned or removed by such abrasion, and so the resistance to acids seriously affected. Figure 12-7 shows graphically how the velocity of a 20% sulfuric acid solution at 77°F, without any entrained solids, passing over the face of a lead lining can cause increasing corrosion as velocity increases. 

Figure 12-7 Effect of velocity on corrosion of lead in 20% sulfuric acid at 20°C. Courtesy of Lead Industries Association,...

Type 304 stainless steel is basically an alloy of 18 to 19 wt% Cr and 8 to 10 wt% Ni. Its corrosion behavior in sulfuric acid is sensitive to both alloy composition and the sulfuric acid environment. Variables with respect to alloy composition include whether the Cr and Ni concentrations are high or low within the allowed range and the concentrations of residual elements such as sulfur, phosphorus, copper, and molybdenum. Thermal and mechanical treatments are also variables but are not considered in the following. Important variables with respect to the sulfuric acid environment include degree of aeration and agitation (velocity effect) and small concentrations of species such as nitric acid, cupric ions, and ferric ions. The net influence of these variables is to find corrosion rates varying from <25 pm/year (1 mpy) to >2500 pm/year (100 mpy) (Ref 3 9). 

Figure 6.5 shows the velocity distributions in a boundary layer of a liquid with Pr , = 100 (e.g., sulfuric acid at room temperature). For this Prandtl number, the thermal boundary layer penetration into the liquid is much less than the flow boundary layer, and the regions where viscosity variations occur are confined close to the surface. The curve corresponding to p /pe = 1 is the Blasius solution (see Fig. 6.1). The curve labeled p ,/pe = 0.23 corresponds to a heated surface where the low viscosity near the surface requires steeper velocity gradients to maintain a continuity of shear with the outer portion of the boundary layer. The heated free-stream cases reveal the opposite effects. In general, the outer portions of the flow boundary layers act similarly to the velocity distribution of the Blasius case except for their being displaced in or out by the effects that have taken place in the thermal boundary layer.

Figure 7.5. Effect of velocity on corrosion of mild steel (0.12% C) in 0.33 A/sulfuric acid under air and oxygen [19], 23 2°C, 45-min test, rotating spec., 18 mm diameter, 56mm long and 0.009% C steel under nitrogen [20].

Fig. 5-12. The effect of sulfuric acid concentration on the critical velocity and on the extent of corrosion at low flow rates at 250°C. Solution composition 0.04 m UO2SO4 and 0.005 m CUSO4.

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