This paper compares the suitability of 22 Cr duplex stainless steel (UNS S31803), low alloy steel, and Alloy 625 (UNS N06625) for MEG reclamation systems. Oxygen is of particular interest. Corrosion of duplex stainless steel in the presence of high oxygen concentrations under MEG reclamation conditions is manifested as preferential ferrite dissolution and/or localized corrosion at crevices. Oxygen entry must be minimized to prevent corrosion of duplex stainless steel. Corrosion most likely occurs where salts deposit. Low alloy steels pit at low oxygen concentrations. Duplex stainless steel (with oxygen restrictions) and Alloy 625 are suitable for the MEG reclamation service conditions.
An ever increasing number of gas condensate fields are being operated with Mono Ethylene Glycol (MEG) as the hydrate inhibitor. The MEG is reclaimed and recycled. The MEG reclamation system comprises a rich MEG flash drum and storage vessel, flash separator with a recycle loop, heat exchanger, salt removal equipment, and a distillation column. A simplified flow diagram is shown in Figure 1. The reclaim system is operated under vacuum with an absolute pressure of about 150 mbar.
Full Stream MEG Reclamation (Evaporation of the Total Rich MEG Feed) (available in full paper)
The service conditions in the MEG flash separator and, in particular, the recycle loop heat exchanger, present a challenging materials selection combination. The conditions include temperatures of 120°C, salt-supersaturated 95-99-wt% MEG, accumulation of non-volatile compounds such as acetate, and possible oxygen entry through vacuum leaks in seals, chemical injection, and nitrogen gas blankets. Data on non-aqueous corrosion under these conditions is not available in theliterature and there are only limited MEG reclaim plants operational in the world, several of which have only just begun operation. This work was undertaken to confirm the selection of duplex stainless steel (UNS S31803) material for this critical service. Oxygen is of particular concern.
Oxygen contamination from overhead vapor space in tanks can be reduced by the use of inert blanketing gas. Inert gas is typically used to prevent explosion or combustion in the presence of hydrocarbons. However, the critical oxygen concentration for combustion is much higher than the concentration to cause corrosion. Use of inert gas with relatively high oxygen content (from a fraction of a percent to a few percent) may have an adverse impact on MEG reclamation process equipment due to the oxygen absorbed from the "inert" gas. An alternative to inert gas is fuel gas. This oxygen-free gas is typically available but has application consequences due to the presence of hydrocarbons.
Even corrosion resistant materials may corrode in brines that contain oxygen. Prolonged exposure of 316 SS and duplex stainless steel to 50-300 ppb oxygen in brine causes pitting corrosion. The pitting susceptibility is greatest in the welds, and depends on the welding practice. Rogne et al1 found pitting corrosion of duplex steels in this oxygen range. They performed tests on duplex stainless steel welds in CO2 saturated salt solutions at 80°C and 120°C.
