Coking, corrosion effect

Interior tube deposits. Coke, salts, and corrosion products can adhere to the inside wall of the tube. The deposits precipitate out of the flowing fluid, due to low tube-side velocity. Once the solids stop moving, they bake onto the tube wall. The flowing fluid can no longer effectively cool that portion of the tube wall, covered with these internal deposits. The minimum acceptable tube-side velocity to retard solids... 

Often, relief valves do not open because their piping connection to the vessel is plugged with corrosion products, salts, or coke. Even if this connection is only partially plugged, the effective capacity of the relief valve is greatly diminished. 

Additives have been shown to effect decreases in coke deposits. Small amounts of organometallic compounds and commercial fuel-oil additives have been shown to decrease coke weight. Metallic compounds in excess can result in increased deposition because of the metal oxide, and some of these compounds have also been reported to cause metal corrosion, as have sulfur compounds. Results of investigations with additives to inhibit coke formation are not conclusive enough at present to justify their acceptance. 

Fe(CN)6]3, as the free acid, has been used as a corrosion inhibitor for metal surfaces 55 K2[Fe(CN)6] has also been reported as being effective for this purpose.56 Both hexacyano ions act as combustion inhibitors for aromatic polymers.57 In the production of carbon monoxide by coke gasification for use in blast furnaces, additives are needed to enhance the reduction of any carbon dioxide formed back to the monoxide. Trisoxalatoiron has been found effective for this... 

C/OKE FORMATION IS AN UNDESIRABLE SIDE EFFECT of many chemical operations because it leads to costly decoking steps, reduced efficiency of operation, and increased rates of metal corrosion and erosion. There is, therefore, a tremendous incentive to minimize or at least better control coke deposition in numerous commercial units. 

Equipment improvements of the coke drum include an overhead elliptical head instead of a spherical head, thus increasing the effective volume of the coke drum. The transition section between the skirt and shell connection uses a forged piece structure instead of overlay structure, thereby extending fatigue life. Alloy steel and cladding are used instead of carbon steel, thus improving corrosion resistance. 

The reader may wish to perform the following experiment to dramatize this effect. Bring a pint of water to a rapid boil. Add a cup of particulates to the boiling water (finely crushed iron sulfide cocoa coke fines etc.). The resulting foamover is an example of what happens to the reboiler effluent when corrosion products circulate through the reboiler. 

A worksheet for estimating the probability of corrosion damage to metallic structures in soils has been published, based on European work in this field. The worksheet consists of 12 individual ratings (R1 to R12), listed in Table 2.29. This methodology is very detailed and comprehensive. For example, the effects of vertical and horizontal soil homogeneity are included, as outlined in Table 2.30. Even details such as the presence of coal or coke and other pollutants in the soil are con-... 

Metal dusting is a special form of catastrophic corrosion that takes place in carbon-containing environments where the carbon activity exceeds a value of 1 [26,27]. For Fe-based materials the principles are shown schematically in Figure 13.17. The catalytic effect of the metal or cement-ite surface leads to a dissociation of the carbon-containing gaseous species (CO , C ,Hy) with the effect of carbon uptake into the metal subsurface zone and carbon deposition (amorphous coke) on the surface. This mechanism is evidently influenced by the crystallographic situation of the metal grains on the material surface [28]. For carbon monoxide (CO) adsorbed on metal... 

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