Contaminants and their effects on operations
You can have better operating amine and glycol systems, gas processing plants encounter a variety of operational problems in their amine and glycol units.
Arthur L Cummings, David Street and Gary Lawson
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Some of these problems are caused by or exacerbated by contaminants —solids, hydrocarbons, salts and degradation products — that accumulate in the system. Drawing from multi-plant experience analysing and controlling these contaminants, this article addresses commonly posed questions such as:
• What contaminants accumulate in gas processing amine and glycol systems?
• Why do these contaminants accumulate in the amine and glycol systems?
• What are heat stable salts (HSS) and degradation products?
• What effects do contaminants have on amine and glycol operations?
• What do analytical reports really say about solvent condition?
• How can the contaminants — hydrocarbons, solids, salts and degradation products — be reduced, removed and controlled?
• How can lower sulphur emission specifications be met by sour gas plants and SRUs with and without SCOT units?
Gas treating applications utilising solvents such as alkanolamines and glycols rarely work trouble free. Yet their design is based on a simple concept: the solvent absorbs undesirable substances from the natural gas, circulates to the regeneration or stripping section of the process, releasing the absorbed substances so that they can be collected for further processing or disposal, then circulates the regenerated solvent back to absorbing section. The regenerative recycle principle is wonderful. The solvent can be circulated indefinitely, performing its function flawlessly. The reality is, however, that over the long term the solvent becomes contaminated and these contaminants cause operational problems.
When operational problems arise, a common response is to add something: anti-foam, corrosion inhibitor, neutraliser or fresh solvent. However, additives can change the solvent’s properties and eventually contribute to the very operational problems they are intended to solve. For example, it is well accepted that excessive anti-foam can cause foaming in amine systems. The viscosity, surface tension, thermal conductivity, electrical conductivity and water content of the solvent solution are important characteristics of the solution that can be altered by accumulated additives and contaminants.Operators become accustomed to gas treating systems with operational problems because the composition of the solvent (amine or glycol) is much different than the pure solvent solution due to contamination and excessive additive issues.
This article looks at methods of eliminating operational problems by removing solvent contaminants and minimising additives. We propose that solvent-based corrosion is best controlled by removing corrosion enhancers and corrosion products; solvent foaming is best controlled by removing hydrocarbons and corrosion products; and solvent degradation is best controlled by minimising contaminants so that degradation rates are minimised. Also, low- sulphur specifications are best achieved with clean amine solvent that can be made ultralean. Removing the common contaminants allows the gas treating application to operate with a solvent solution that approaches the theoretical design. Plants can best approach the goal of trouble-free operation by maintaining chemically and physically clean solvents. In simple terms, a clean gas treating solvent is a good gas treating solvent.
Alkanolamine solvents are used in gas plants and petroleum refineries for removing acid gases from various gas and liquid hydrocarbon streams. The acid gases removed are H2S and CO2. The low-sulphur specification of the products requires that the amine solvent work continuously and efficiently. The amine solvents have proven to be very predictable in removing acid gases until they become contaminated. Contaminated amines usually result in unstable operations and poor performance, which frequently becomes a limiting factor in the operation of the gas plant. More stringent sulphur emissions specifications increase the need for smoothly operating amine systems and may also require enhanced amine performance.
The principle contaminants of an amine solvent include:
• Iron sulphide
• Heat stable salts
• Amine degradation products
Hydrocarbons enter the amine system due to their natural solubility in the amine solvent, by mechanical entrainment, by condensation into the cooler amine solution, and by upset conditions within the amine contactor. The foaming episodes, process upsets and poor treating that result from hydrocarbon contamination are well known to amine system operators. While it is well known that hydrocarbons are related to foaming, the relationship has not really been well understood.
Surface tension effect The function of amine contactors is essentially to generate surface area between the liquid amine phase and the hydrocarbon stream being treated to facilitate mass transport of acid gas components across the surface interface. Contactors can do this either by providing a large surface area for one phase to cling to while the other phase passes by the surface (packing, FiberFilm), by generating the surface area by agitation (trays), or by creating droplets that pass through the other phase (spray towers). Regardless of the device, energy is expended in the formation of the surface area. The amount of energy required per unit of surface area generated is proportional to the surface tension of the liquid. Therefore, as the surface tension of the fluid decreases, the greater the amount of liquid surface area generated by doing work on the system.1
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