Ammonium bisulfide has been the well known main culprit of aqueous corrosion in hydroprocessing units. A lesser understood and, at times, more menacing aqueous corrosion problem in these units has been due to chloride salts. Chlorides can be introduced into the system through the feed and/or through the make-up hydrogen gas. This paper describes case histories of chloride related problems and the means for minimizing such occurrences.
Hydroprocessing units are used in a refinery to remove undesirable impurities from feed stocks and/or to convert heavier feeds into lighter components. The catalytic conversion occurs in a high pressure, moderately high temperature reactor where the feed stock and hydrogen gas are mixed. The operating pressure range for such units is between 42 to 210 kg/cm2 (600 to 3000 psi). The operating temperatures vary from 343 to 454°C (650 to 850°F).
Since most hydroprocessing units are built under license and with proprietary technology, their process flow schemes may differ significantly from each other. Figure 1 shows a typical Chevron (hereafter referred to as Company) single stage hydrocracker unit.
Hydroprocessing units are subject to a variety of corrosion mechanisms and in-service metal degradation. Among corrosion phenomena and degradation mechanisms are high temperature sulfidation, high temperature hydrogen attack, ammonium bisulfide corrosion, sulfide stress cracking, ammonium chloride corrosion, chloride stress corrosion cracking (CISCC), and embrittlement of low alloy steels. For details of the degradation mechanisms refer to the hydroprocessing chapter of NACE Refinery Training 1. This paper deals only with the subject of chloride corrosion and CISCC.
Feed to hydroprocessing units may contain inorganic or organic chloride salts. The level of chlorides in the feed varies depending on the type of crude processed in the refinery, on the effectiveness of crude desalting, and on the particular cut of feed stock to be hydroprocessed. The heavier the cut, the higher the level of chlorides. The highest level of chlorides in the feed is normally seen in vacuum residuum hydrotreaters. Company experience indicates that the chloride level in vacuum residuum hydrotreaters can be around 3 ppm.
The other major source of chlorides to the units is hydrogen gas. Depending on process configuration, the source of hydrogen to a hydroprocessing unit can be from a hydrogen reformer unit, a catalytic reforming unit, or a combination of both. It is the hydrogen from the catalytic reformer unit that often contains chlorides and causes corrosion and cracking problems. Typically, the licensors require that the chloride in the hydrogen gas be limited to less than 1 ppm. To achieve and maintain this limit, chloride traps are often used. Frequent monitoring of chlorides in the hydrogen gas generated from a catalytic reformer unit is essential in preventing crevice corrosion and CISCC problems in hydroprocessing units.
Chlorides are also introduced into a hydroprocessing unit during shutdown soda ash washing. If the soda ash solution is not completely drained dry, it will evaporate during start-up resulting in concentration and the deposition of chloride salts.
