Acid coolers anodic protection

Anodic passivation and its appHcation to sulfuric acid equipment such as stainless steel acid coolers and carbon steel storage tanks has been weU studied (102—104). More recently, sheU and tube coolers made from Sandvik SX or Saramet have been installed in several acid plants. These materials do not requHe anodic protection. 

Plate and frame coolers using HasteUoy C-276 plates have been used successfuUy. Anodically protected plate coolers are available as weU as plate coolers with plates welded together to minimize gasketing. Another promising development is the introduction of plate coolers made of HasteUoy D205 (105). This aUoy has considerably better corrosion resistance to concentrated sulfuric acid at higher temperatures than does C-276. Because of the close clearance between plates, cooling water for plate coolers must be relatively clean. 

Anodic Protection This electrochemical method relies on an external potential control system (potentiostat) to maintain the metal or alloy in a noncorroding (passive) condition. Practical applications include acid coolers in sulfuric acid plants and storage tanks for sulfuric acid. 

In sulfuric acid production involving heat recovery and recovery of waste sulfuric acid, acids of various concentrations at high temperatures can be dealt with. Corrosion damage has been observed, for example, in sulfuric acid coolers, which seriously impairs the availability of such installations. The use of anodic protection can prevent such damage. 

Anodic protection today allows safe and efficient protection of air coolers and banks of tubes in sulfuric acid plants. In 1966 the air cooler in a sulfuric acid plant in Germany was anodically protected. Since then more than 10,000 m of cooling surfaces in air- and water-cooled sulfuric acid plants worldwide have been protected. The dc output supply of the potentiostats amounts to >25 kW, corresponding to an energy requirement of 2.5 W per m of protected surface. As an example. Fig. 21-9 shows two parallel-connected sulfuric acid smooth tube exchangers in a production plant in Spain. 

To trigger off an anodic protection system for stainless-steel coolers cooling hot concentrated sulphuric acid when the potential moves towards that of active corrosion. 

Fig. 24.6. Inside an acid cooler. Fig. 9.5 gives an external view. Tubes start through the tube sheet , shown here. They extend almost to the far end of the cooler where there is another tube sheet . Cool water enters at this end and flows through the tubes to the far end. Between the tube sheets , the tubes are surrounded by warm acid moving turbulently around them. Heat transfers from the warm acid to the cool water (through the tube walls). The tube entering from the right contains a thermocouple. The polymer tubes in the foreground surround metal rods. The rods are bare between the tube sheets. An electrical potential applied between them and the water tubes anodically protects the tubes against acid side corrosion.

In the production of sulfuric acid, including heat reconvert and the reconditioning and recycling of spent acids, it is necessary to handle acids at elevated temperatures and various concentrations. Corrosion damage that considerably impairs the availability of a plant has occurred in sulfuric acid coolers, for example. Damage of this kind can be prevented by anodic protection. 

The anodic protection technique now enables air coolers and tube bundles in sulfuric acid plants to be protected from corrosion reliably and economically. Anodic protection was provided for air coolers of sulfuric acid production plants for the first time in 1966. Since then, a combined cooler surface area exceeding 10,000 m in air-cooled and water-cooled sulfuric add plants has been protected in this way worldwide. The installed initial electrical direct current output of the potentiostats is >25 kW, corresponding to an energy requirement of 2.5 W/m for the surface needing protection (Kuron and Grafen 1988). 

Figure 9.5 Acid cooler, courtesy Chemetics. Cool water flows through 1610 internal 2-cm diameter tubes, while warm acid flows cotmter currently (and turbulently) around the tubes. The tubes are 316-L stainless steel. They are resistant to water-side corrosion. They are electrochemically passivated against acid-side corrosion by an anodic protection system which continuously applies an electrical potential between the tubes and several electrically isolated metal rods ( anodic protection ). Details shell diameter 1.65 m shell material 304-L stainless steel acid flow 2000 m /h water flow 2900 m /h acid temperature drop 40 °C (green pipes=water metallic pipes = acid). Figure 24.7 gives an internal view.

Acid coolers are either anodically protected shell and tube design or non-anodically protected plate and frame design. Anodic protection can also be applied to carbon steel acid storage tanks (Corrosion Service, 2012). 

Absorption of SO3 in concentrated sulfuric acid and the formation of H2SO4 from SO3 and H2O produce heat in the absorber, as does acid dilution from makeup water addition. Process control requires that the acid be cooled before it is recirculated to the dryer or absorber towers or sent to storage. Earlier acid coolers of parallel banks of stacked, irrigated, cast iron sections have been largely replaced by stainless steel shell and tube or plate exchangers, with or without anodic protection. Hastelloy, Sandvik SX, and Saramet alloys and Teflon also are used in acid piping and coolers. ... 

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