INTRODUCTION

ABSTRACT

Some stainless steel trays were found corroded in an amine unit during a turnaround in one of the plants. Corrosion products and corrosion features were characterized on three tray samples through various techniques: XRD, SEM/EDX, TGA-FTIR, ICP-MS, head-space GS-MS. Only part of the steel trays was visually corroded, with black, grey, or rusty brown colored corrosion products. The uncorroded steel surface no longer showed original shiny grey surface, rather a slightly greenish discoloration.

Evidence of intergranular corrosion, however, was found in the visually un-corroded and discolored steel surface, which was mainly covered by FeCO3. The corrosion products on the visually corroded surface were identified as mainly FeCO3 and FeS; in addition, some organics and foreign elements were also discovered in the corrosion products.

Based on the fact part of the steel trays was corroded and the splashed S-rich deposits were still present on the visually uncorroded surface without being washed away, it was reasonably stated that these amine trays of concern were not always or completely wetted by the amine solution during process condition. Condensation or deposits, therefore, could adhere to the surface, resulting in the formation of intergranular corrosion across the steel surface. Splashing, caused by foaming and free-fall impact from the amine solution, wetted some of intergranularly corroded surface with S-containing solution with/without small particulates. The steel at these areas started to crack. The attacked grain-boundaries opened up, resulting in the disintegration of the grains. Circular pits were subsequently formed. With the expansion and coalescence of the pits, large amounts of materials were wasted, leading to severe corrosion of the amine trays.

NOVA Chemicals Corporation(1) is a major manufacturer of ethylene, polyethylene, styrene and polystyrene with manufacturing plants in Canada, the U.S. and South America. The company operates the largest ethylene and polyethylene complex in the world near Joffre, Alberta. During the turnaround at one of the plants in September and October 2008, several UNS S30403 (ASTM 304L) stainless steel trays in the amine stripper were observed to be corroded. The corroded trays were replaced and three samples were collected for investigative analysis.

The amine stripper in the Amine Regeneration Subsystem in this plant is a 12-foot 6-inch diameter, 23-tray tower that strips the acid gas (CO2 and H2S) from the amine solution (48-52% CS-PLUS). The wash section (top of the tower) has 3 bubble cap trays and the stripping section has 20 two-pass sieve trays. The tower is clad with UNS S30403 stainless steel to prevent rapid corrosion and the trays are constructed of UNS S30403 SS. The Amine Regeneration Subsystem receives the filtered rich amine solution in the CO2 Absorption Subsystem. The filtered rich amine flows through the tube side of the lean/rich amine cross exchangers and is preheated up to 96?C. The rich amine stream enters the amine stripper above tray 21 and free-flows downward through the trays; the resulted lean amine is then recirculated to the Absorption Subsystem. The pressure of the Amine Regeneration Subsystem is not to exceed 448kPa.

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