Cleaning amine units cost effectively
Heat stable salts are dissolved or avoided by chemical products and cleaning methods during operations and turnarounds.
MARCELLO FERRARA and DOMENICO FERRARA
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Formation of heat stable salts (HSS) is a common issue in amine units and is a major factor in operating costs. Accumulation of HSS can cause more energy consumption for regeneration; additionally, the degraded amine portion cannot be regenerated. This will further contribute to losses and amine degradation, as well as having economic, environmental, and operational impacts.
HSS are amine salts of ionic species such as acetate, formate, oxalate, acetate, thiosulphate, thiocyanate, and chloride that are thermally stable and are not dissociated to any great extent in the regenerator. These ionic species originate upstream of amine units as acidic byproducts of other refining processes, especially from operations in FCC and delayed coking units. Thiosulphate is the product of SO2 breakthrough in sulphur plant tail gas units. These acids are all stronger than H2S and CO2 and they react with amine in an acid-base neutralisation reaction, resulting in the corresponding amine salts.
Amine degradation is also induced by reaction with CO2. Figure 1 shows a typical degradation pathway.
HSS deposition results in the following operating problems:
• Irreversible consumption of amine with subsequent loss of acid loading capacity and increased levels of ammonia, causing corrosion in the amine unit and operating problems in the SRU.
• Increased corrosion due to HSS penetrating the protective FeS layer on the metal surface, exposing the metal to further attack. Several HSS anion species have been directly related to increased corrosion in carbon steel in amine solvent systems; in particular they appear to act as corrosion accelerators by displacing the iron sulphide film.
Corrosion rates increase with higher concentrations of all HSS species. Corrosion from HSS occurs in hot areas of the plant where liquid and vapour phases are present: the reboiler, reboiler outlet line, the regenerator column between the lean amine level, and the bottom trays.
HSS starts to cause obvious corrosion problems when the total HSS level exceeds 2%:
• Foaming due to higher concentrations of corrosion-derived particulates causes greater pressure drop across the column, unstable operations (temperature changes inside the tower), low liquid level in the absorber/regenerator causing frequent filter switching/cleaning, increased liquid level in the reflux drum, and increased H2S or CO2 levels in the treated gas. The foaming problem becomes significant when the total HSS concentration in the amine solution exceeds 1.5%.
• Fouling occurs due to a mixture of contamination products like corrosion products, salt deposits, heavy hydrocarbons, and other solid materials. Areas prone to fouling are ones with low velocities, such as the rich amine flash drum, that often accumulate lots of fouling; as fouling accumulates, it reduces the process volume of the equipment and is eventually carried over, thereby increasing the solids load of the circulating amine.
Amine unit fouling and corrosion are common problems that can adversely impact unit performance, for instance:
• Fouling of the lean/rich exchanger can ultimately lead to under- deposit corrosion causing rich amine leaks into the lean amine, thereby affecting sweet gas quality specifications and impairing environmental compliance.
• Amine contactors also foul frequently, with the heaviest fouling occurring in the bottom of the tower.
The ‘shoe polish’ fouling material is usually composed of heavy organic compounds, polymers resulting from polymerisation of unsaturated hydrocarbons, corrosion products, and silicon if silicon based antifoams are used to control foaming.
Accurate and reliable measurement of HSS concentration is essential to monitor solvent degradation and corrosion. Whenever HSS formation becomes out of control, it can lead to an unscheduled shutdown and the unit must be stopped because the reduced capacity is no longer acceptable. Eventually operating costs will increase.
Besides the concerns created by HSS during a run, they create health and safety concern during a turnaround. When left inside the equipment, HSS will release lower explosive limit contaminants when equipment is opened, thereby impairing safe entry. Because of the heavy smell, it is also common to barricade the area, which will impact turnaround operations in nearby activities. Decontamination will work only on the free vapour space above the HSS and will not affect HSS deposits. Additionally, manual cleaning and/or removal of HSS will release additional contaminants from within the deposits.
Mechanical cleaning is currently performed only where there is a stringent need for it. This implies that equipment is allowed to run at minimum acceptable performance before cleaning. The situation arises from a number of peculiarities and conventions in the hydrocarbon processing industry.
First, the industry sees cleaning as a troublesome, lengthy operation that leads to a production loss, so it is to be avoided. Also, the mechanical cleaning process involves at least 20 operations, each of which has an associated hazard.
When the production/maintenance department faces a cleaning situation, it normally reacts in one of two ways: the situation is fairly typical and managed as a routine operation; or the situation is problematic and can be postponed until it can no longer be tolerated.
Alternatives to mechanical cleaning include a number of reclamation processes which basically clean the circulating amine. Three different reclaiming technologies are currently used – thermal, ion exchange, and electrodialysis.
In a typical thermal reclaiming system, a slipstream of lean amine is taken downstream of the regenerator and lean amine pump and continuously fed to the reclaiming unit. A CO2 pretreatment step occurs to reverse the reaction between CO2 and amine that forms amine carbamate; one potential option involves heating at regeneration conditions to reverse the amine-CO2 reaction (and vaporise a small amount of water). The contaminated amine is then pretreated with caustic to reverse the reaction between acid impurities and/or degradation products and the amine. This reaction creates salts of sodium and the acid impurities/degradation products, and liberates free amine. The pretreated amine is sent to the thermal reclaimer where impurities are removed; the stripper overheads flow to a condenser and then to the main solvent circulation loop.
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