OCR Text |
Show oil-soluble forms have been observed for several of the elements. No matter which form these elements are in, the important point is that they are retained essentially intact during the refining process and are found concentrated in the residual oil. All the elements listed in Table 1 are intrinsic to the crude. Sodium concentrations may increase considerably if the oil is shipped by sea. The sodium level should not exceed 50 ppm in the residual oils if properly desalted at the refinery. Asphalt-based crudes are generally rich in vanadium- particularly those from Venezuela which often contain more than 500 ppm vanadium, reported as V205. Some Middle East crudes, as well as some from California, also contain appreciable amounts of vanadium. Paraffin-based crudes are usually free of vanadium.3 A typical analysis of crudes appears in Table 2.* Corrosion and fouling of boilers fired with residual oil are generally accepted to be caused by the ash-forming constituents in the oil-sodium, vanadium, and sulfur.5-7 The literature attributes high-temperature corrosion primarily to vanadium in a molten state. The temperatures at which the attack occurs depend on the sodium-to-vanadium ratio and can be reduced from 1275°F (melting temperature of pure vanadium pentoxide) to as low as 970°F with sodium. Corrosion is particularly troublesome when the sodium-to-vanadium ratio in the ash exceeds 0.3 or the sodium concentrations exceed 20 ppm in the oil.8 At these levels sodium vanadyl vanadates release free oxygen upon condensing, increasing the corrosion potential. Deposit formation, as well as corrosion, depends on the existence of a liquid phase in the ash. The troublesome deposits usually found on the high-temperature tube surfaces have been reported to vary in appearance according to 1-6 |