Degradation of amine solvents and the relation with operational problems

摘要

Amine solvent formulations are used for the removal of acidic components such as CO_2 and/or H_2S from natural gas to comply with sales gas specifications or to make the gas suitable for liquefaction. During operation of an amine unit degradation of the solvent occurs due to reaction with contaminants (oxygen, sulphur, CO_2) in the feed gas. As a result of these degradation reactions, amine degradation products are formed including: oxazolidone; other amines; acids and higher molecular weight products (amine dimers and oligomers). These degradation products lead to a number of operational problems in the units if allowed to build up in concentration. Build up over time is related to solvent age, use and handling and will occur in every amine solvent. Operational problems include: 1. Increased corrosion rates, particularly at the lean side of the amine unit. 2. Fouling in those parts of the unit where solvent flow velocities are low and the solvent loading of H_2S or CO_2 is high i.e. trays and bottom of the contactor and the lean/-rich heat exchanger. In "CO_2-only" systems fouling tends to occur more often in the regenerator overhead. Fouling of the heat exchanger leads to a reduction in heat transfer and an increased steam use for regeneration. Ultimately plugging of the heat exchanger may occur, restricting solvent circulation and treating capacity. 3. Decreased treating capacity of the unit due to lower free amine strength and a lower availability. Part of the amine is degraded and part is "tied up" by the acids formed (heat stable salts). 4. Increased foaming tendency of the solvent by the presence of a higher amount of suspended solids (also requiring a high rate of filter change outs). 5. A high flash gas production due to a high carry-under of gas with solvent in the main absorber. Increased corrosion rates can be explained in terms of a high acid content of the solvent decreasing the pH and in terms of high molecular weight degradation products that increase the ability of the solvent to keep iron in solution (high complexing power). The overall result is a high activity of the "iron pump" mechanism leading to high corrosion rates and associated fouling problems. Careful monitoring of the operation of the unit in combination with measuring solvent composition and solvent properties, together with good solvent maintenance practices will ensure reliable operation.