gas streams by installing a water wash (with a large circulation rate) or one that includes an ammonium polysulphide (APS) addition. When gas is bubbled through this water wash, the APS removes cyanides. Although this is effective at protecting the absorber, when these contaminants enter the sour water system, they can lead to problems. Few refineries employ these technologies to manage HSAS. Controlling heat stable salts As HSAS form, the conductivity of the amine will increase. It is recommended to use a regular measurement of the conductivity of the amine solution as an indicator of HSAS levels. For MDEA, a conductivity of 2-3 milliSiemens per centimetre (mS/cm) typically translates into 0.5-1 wt% HSAS, which is tolerable. If the conductivity is more than this, or if the trend is increasing, the amine should be subjected to a more detailed analysis by a qualified laboratory. When HSAS exceeds around 2.0 wt%, action should be considered. There are three common methods for reducing the HSAS concentration: • Reclamation : Third-party services connect to the system and, using various technologies, remove either the entire salt or the acid anion portion. Examples of reclamation technologies: AmiPur by Kovalus, TL2/TL3 by ElectroSep, or HSSX by MPR. • Purge and replace : If immediate relief is required, a portion of the amine inventory could be removed from the system and replaced with fresh. • Caustic neutralisation : Sodium or potassium hydroxide can be added, which will neutralise the salts by releasing the amine portion and replacing it with caustic. The salt’s corrosiveness is minimised. The solubility limit of caustic-based salts in amine is approximately 5 wt%. Beyond this limit, salts will precipitate in cooler areas, such as the lean amine cooler. The amount of caustic addition should not exceed 80% of the HSAS to prevent negative impacts on the amine regeneration. Design options to minimise fouling There are trays and packing that are resistant to fouling. If the absorber is going to be ‘dirty’
despite the engineer’s best efforts, installing a more suitable type of internals may be practical. One such example is Sulzer’s ‘V-grid’ valve tray. Although this may prevent (or delay) fouling, the solids will then be carried by the amine through the rest of the system and likely cause fouling elsewhere unless captured and removed. Regarding packing, its flow characteristics enable it to act as an effective filter. Solids can accumulate in the narrow openings of the packing, blocking flow paths for both gas and liquid. Operations that minimise fouling Increase the liquid level in the regenerator. Increase steam traffic through the regenerator. Until the heat stable salts can be managed, the amount of corrosion can be minimised by protecting the metal with amine. Increase the amine level in the bottom of the regenerator as high as possible and increase steam to the reboiler, increasing the jet flooding inside the regenerator. The Jet flood should be at least 15% to prevent maldistribution. However, to properly ‘spray’ amine and wet the metal between trays, a >30% jet flood is better. Jet flood levels can reach as high as 85% before amine carryover with the steam becomes a concern. Increase circulation rate. The greater the velocity of the amine, the less likely solids are to settle onto the absorber internals. So, as a short- term strategy, increasing the circulation rate can help prevent fouling or a continuation of fouling if it has already started. There are, however, many disadvantages to this; notably, more energy consumption in the heat exchangers and higher risk of erosion in the piping. The maximum liquid velocities are 2 m/s in carbon-steel piping and 1 m/s in carbon steel exchanger tubes. Be cautious when adjusting the amine circulation rate as it is a critical operating parameter. Use a simulation program to determine the effect on the rest of the amine plant.
Steven Ayres Steve.Ayres@SGS.com Benjamin Spooner Ben.Spooner@SGS.com
Refining India
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