The occurrence of this problem in a large gas purification plant and its solution by the use of stainless steel weld overlay has been described by Ferguson and Stutheit (1991). This can result in pitting corrosion of carbon steels. This is effective at very low concentrations of H 2S however, at moderate concentrations (about 100–1000 ppm) a much less protective scale containing iron sulfide is formed. The vanadate inhibits corrosion of carbon steel by forming a tightly bonded protective iron-oxide scale on the surface. In order to maintain the metavanadate in its fully oxidized (pentavalent) state, the licensors of the Benfield process recommend that potassium nitrite be injected into the solution on a carefully controlled basis ( Sorell, 1990). For the corrosion inhibitor to be effective, the plant must be designed so that exposed carbon steel surfaces are wetted by the inhibited solution. If desired, vanadium pentoxide (V 2O 5) can be added directly to the potassium carbonate solution, forming potassium metavanadate by reaction with potassium carbonate. In commercial units today, it is preferred to use the alkali metal metavanadates, particularly potassium metavanadate (KVO 3). The metavanadate salts also have the additional advantage that they tend to increase the rate of CO 2 absorption in the absorber column. It has been shown in the laboratory, pilot plants, and in commercial plants, that small amounts (0.01–2.0 wt.%) of metavanadate salts are not only highly effective in inhibiting corrosion on steel surfaces in potassium carbonate systems, but also retain their solubility and anti-corrosion effectiveness in the presence of CO 2 and H 2S ( Bienstock and Field, 1965). Fortunately, H 2S itself appears to inhibit CO 2 corrosion somewhat so that carbon steel can still be used for most of the equipment in plants handling gas containing both CO 2 and H 2S. In addition to increasing operating costs by destroying the inhibitor, the reduction reaction results in the formation of insoluble precipitates, which cause erosion of equipment, fouling of heat exchanger surfaces, and other operating difficulties. Since H 2S and possibly other sulfur compounds rapidly reduce the chromate ion, the use of this inhibitor is uneconomical in the presence of appreciable quantities of such impurities. Concentrations of 1,000 to 3,000 ppm have been found satisfactory for commercial installations. This material is recommended as a corrosion inhibitor only for plants handling sulfur-free gas. Potassium dichromate was found to be an effective corrosion inhibitor, and 0.2% was used in the solution for subsequent CO 2-absorption tests. Bureau of Mines, severe corrosion of carbon steel was encountered, especially where the conversion to bicarbonate was high or where carbon dioxide and steam were released by pressure reduction ( Bienstock and Field, 1961). Nielsen, in Gas Purification (Fifth Edition), 1997 Operating Problems.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |