Minimise amine losses in gas and â€¨liquid treating
Even with incomplete information, a systematic approach can be used to identify the source of amine losses and the action needed to reduce them.
ERIC TELETZKE and BHAVNA MADHYANI
INEOS GAS/SPEC Technology Group
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Developed in the 1930s, the amine acid gas removal process is a highly efficient technology for removing acid gas components from gas and liquid streams. The fully regenerable amine solvent is not consumed during the acid gas removal process, allowing for prolonged operation with minimal solvent make-up. While the solvent is not consumed by the process, some amine loss is inevitable due to vaporisation and entrainment in gas treaters and solubility in liquid treaters.
Solvent losses that exceed the typically low levels expected due to vaporisation, entrainment, and solubility are a symptom of an underlying operating issue. This article will provide a systematic approach with troubleshooting decision making trees for identifying and correcting the causes of excessive amine losses. Two real-world case studies will be presented that demonstrate how this systematic approach to categorising amine losses can be used to identify and correct the source of losses in real-world applications.
Identifying pathways for â€¨amine losses
Minimising amine losses first requires knowing whether the losses are excessive. Surveys of gas processing plants have reported average amine loss rates for gas processing plants using monoethanolamine (MEA), diethanolamine (DEA), and methyldiethanolamine (MDEA) products of 3 lb amine/MMSCF (2 × 10-4 kg/Nm3).1 A comprehensive approach to reducing amine losses can reduce loss rates â€¨to <1 lb/MMSCF (<7 × 10-5 kg/Nm3) of gas treated for most gas processing systems.
Refinery losses are typically higher due to the contaminants present in refinery process streams and will largely depend on composition of those streams. While refinery amine systems can present some unique challenges, the principles and troubleshooting methods in this article can also be used to reduce losses in refinery gas and liquid treaters.
Understanding the pathways for amine to escape the system is necessary in order to troubleshoot the source of high losses. These pathways are categorised as vaporisation, entrainment, solubility, mechanical, and degradation. An explanation of each pathway will be presented along with troubleshooting methods specific to each.
Amine volatility losses are dependent on the type of amine, amine concentration, as well as the temperature and pressure of the outlet vapour streams. Process modelling software can be used to â€¨predict amine vaporisation losses for any amine solvent based on the vapour pressure data of the amine and the gas stream temperature and pressure.
When operating MDEA systems within normal operating ranges, vaporisation losses are low for both the gas contactor and regeneration still overhead. Figure 1 shows the estimated MDEA vaporisation losses for a range of temperatures and pressures.
Vaporisation losses can be significant in systems operating at low pressure or with high contactor overhead temperatures. In many cases, high overhead temperatures can be reduced by increasing the amine circulation rate, but these adjustments should only be made in consultation with your amine supplier as adjusting circulation rate can impact rich loading and acid gas removal. In situations where vaporisation losses cannot be avoided, a treated gas cooler and after-scrubber with a water wash are recommended to recover any vaporised amine. Figure 2 shows a troubleshooting decision making tree for quantifying the rate of vaporisation losses and reducing those losses.
Solubility (liquid treating)
Solubility losses apply to systems treating liquid hydrocarbon streams with an amine solvent. Depending on the temperature, pressure, and concentration of the amine, there is an equilibrium solubility in the hydrocarbon phase. The dissolved amine will be carried past the tower with treated hydrocarbon liquids. Adjusting the operating temperature and pressure of the liquid treater is typically not feasible, as these parameters are maintained within a range that ensures the hydrocarbon phase remains liquid.
The parameter that can be adjusted to reduce solubility losses is concentration. Figure 3 shows that solubility of MDEA in hydrocarbons increased rapidly when operating above a 40 wt% concentration. For this reason, operating liquid treaters with an amine concentration greater than 40 wt% is not recommended. In general, when rich loading and treating requirements are not a concern, reducing concentration can help reduce solubility losses in liquid treaters.1
An important piece of equipment for minimising solvent losses in liquid treaters is a downstream water wash system to recover any dissolved amine. While some solubility losses will occur in all systems, a downstream water wash provides an efficient recovery method to minimise these losses and remove amine from the treated natural gas liquid (NGL).1 Liquid-liquid coalescers are also effective at recovering dissolved amine when water is not available or too costly, but it is important to ensure sufficient residence time in the coalescer. Figure 4 shows a troubleshooting decision making tree for minimising solubility losses in liquid treaters.
Entrainment (gas processing)
Entrainment refers to the physical carry-over of the liquid amine solvent with the treated gas or acid gas streams. Entrainment covers a range of scenarios, from a light mist or spray to a severe foaming upset resulting in bulk liquid carry-over. Entrainment in gas processing systems can be classified as gas in liquid dispersion and liquid in gas dispersion.1
Entrainment resulting from liquid in gas dispersion accounts for most of the amine losses in a typical amine system and is typically 0.5 lb/MMSCF (4 × 10-5 kg/Nm3) in a properly designed absorber. Liquid in gas dispersion is often the result of operating an amine contactor above design gas rates or below design operating pressure. Operating under these conditions can result in the formation of small amine droplets that are lifted overhead by the upward pressure of the gas. The best way to minimise entrainment is to ensure that amine absorbers are designed and operated at <80% of their flooding velocities and have a properly sized sweet gas knock-out drum to recover any entrained amine.
The other type of entrainment that is common in amine systems is gas in liquid dispersion, or foaming. Table 1 provides a list of common foaming agents and sources. Foaming upsets will typically increase liquid hold-up in the contactor until the foam carries overhead with the treated gas. A treated gas after-scrubber is recommended for all amine systems to recover amine carry-over resulting from foaming upsets and other forms of entrainment.1
Normally, captured liquid carry- over from the contactor that collects in the after-scrubber is reintroduced into the amine circuit via the flash drum. However, caution should be taken when reintroducing this material to the system during a foaming upset. The surfactant responsible for the initial foaming incident will likely be captured in the after-scrubber liquids. Reintroducing the material to the system without filtering through mechanical and carbon filters to remove the foam initiator may prolong the occurrence of foaming. Figure 5 shows a troubleshooting tree for identifying and correcting the causes of entrainment losses in amine systems.
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