Solids in the feed are causing corrosion and erosion problems in our amine system. How do we remove them effectively and economically?
Responses to a question in the Q1 2021 issues Q&A feature
Various from Kurita, Axens, Sulzer and SUEZ WTS.
Viewed : 1297
Berthold Otzisk, Senior Product Manager, Process Chemicals, Kurita Europe - firstname.lastname@example.org
There are a number of methods available to clean a contaminated amine system. Dispose/replace or bleed/feed methods were often used in the past, but they are uneconomical methods due to the increased costs of fresh amines and disposal. Modern cleaning technologies such as ion exchange and electrodialysis remove ionic contaminants like heat stable salts (HSS). But all neutral degradation products, hydrocarbons, and particulates will not be removed. The use of a mobile vacuum distillation unit, which can be connected to the amine plant, or collection of lean amine and transport to an external vacuum distillation unit, is considered to be the best option to remove all contaminants from the amine solution.
It is important to know which kind of fouling is causing the problems. A good amine unit design includes a particulate contaminants filter and an activated carbon filtration to remove hydrocarbons and surface active contaminants.
Fresh amines are water white to pale yellow in appearance. Amines in service can vary in colour from yellow to red, blue, green, grey, brown, black, and so on. Most colour changes are indicators of the presence of metal salts associated with corrosion of the mild steel equipment. Metal salts may stay in the amine solution until they reach a solubility limit to precipitate as solids. Many corrosion problems are erosion problems, where the natural iron sulphide protection layer is destroyed, exposing fresh metal. A powerful water-soluble corrosion inhibitor programme can help to reduce the corrosion potential and formation of iron sulphide deposits.
In order to ensure that there are no solids in the amine system, Axens recommends an amine filtration package which filters a slipstream of the lean amine to remove solid particles. The amine filtration package also includes an activated carbon filter for hydrocarbon elimination and a polishing filter to remove any charcoal fines. Axens also recommends an amine filtration sequence for early and first start-up of the acid gas removal unit (AGRU) according to the actual level of solids: first use temporary cartridges with 100 microns absolute filtration rate, then 40-50 microns absolute filtration rate, then final grade. In order to minimise the foaming tendency of the amine solvent, solids content has to be minimised. If the source of the solids is from the feed gas where they could also cause foaming in the absorber, Axens requires installation of a feed gas coalescer upstream of the amine contactor to protect the amine contactor from foaming upsets that can reduce the capacity of the whole process chain. As the coalescing elements may plug if solids are present in the feed gas, it is advised to install solids filtration upstream of the coalescer. This solids filter protects coalescing cartridges and increases their lifetime, reducing opex and associated maintenance costs.
If a significant amount of solids is coming with the gas feed, they should be removed before they contact the amine. Once the solids contact the amine in the absorber, it will promote foam (solids stabilise tray froth) and erosion in the pipes that will accelerate corrosion.
If the solids particle size in the feed is less than 5 microns, the refiner should install a cartridge filter. If the particle size is larger than 5 microns, a conventional gas-liquid separator equipped with high efficiency internals (vane type feed inlet device followed by a combination of chevron and mesh pad mist eliminator) should remove them.
Willard Harvey, Hydrocarbon Processing Industry Product Manager, SUEZ WTS - Willard.Harvey@SUEZ.com
One of the most important aspects of treating an amine system involves the minimisation of entrained water, solids, and other foulants that impact the performance of the inlet separation process. Implementing necessary strategies throughout the process is critical in controlling corrosion and erosion challenges associated with these key systems. Suez recommends a multi-phase approach that could include water washing feed streams, dehydration, filtration, chemical inhibition (corrosion, foaming, and foulant dispersants), and monitoring. As your question centres on the solids in the feed, we will focus our discussion there.
Suez feels that utilising clean feed to amine systems greatly impacts the performance of solvent solution, the protection of assets, and the longevity of the amine solvent solution. An initial step in removing the solids that cause the corrosion and erosion in an amine system might include appropriate filtration. The sizing and designing of an appropriate feed filtration unit is paramount for control and protection of the amine system. This step may include a coalescer or a filter housing that is designed to handle 100% of the influent prior to the inlet separation. The filtration unit(s) should be designed to remove at least 50% of the solids. The size of the unit and filters can be determined through a particle size distribution and solids analysis of your feed. A full analysis of the amine and the particles in the feed will help determine what is causing the amine degradation, corrosion, and erosion challenges. The particle size distribution is critical as you try to economically and effectively control the solids throughout the system (smaller micron filters that remove greater than 50% involve significant cost per filter replacement in the pre-separation phase; too large a micron filter causes challenges downstream as it will impact the filtration in the rich and lean amine portions of the amine system). Suez suggests that you work with a filter company that specialises in solids removal.
There are multiple feed coalescing or feed filtration designs on the market – each with their pros and cons – and you must determine which is best for each application at your facility. Determining and designing which system is best for your streams is an essential part of effective and economical control. A key learning over the years suggests that a filter housing system that can utilise polypropylene filters without end-caps can reduce the cost of the cartridge consumable and simplify the filter cost. (Our experience suggests that the cost of the end-caps often overshadows ease of handling and the benefits in overall performance.)
Often Suez has encountered erosion and corrosion byproducts that impact amine system quality even with what would be considered ‘good’ feed solids control. We consider appropriate filtration to include the filtration of your rich and lean amine as well. These filter packages include cartridge filters for the rich amine, lean amine, and carbon beds.
The rich amine circuit should be filtered and a good design filters 100% of the stream as well. The 100% filtration of the rich amine reduces the fouling associated with corrosion and erosion byproducts. A particle size distribution analysis and solids analysis should be performed on the rich and lean amine streams as well. Some manufacturers suggest starting at 20-micron filters and reducing to 5-micron filters over time. Our rule of thumb is to start with a filter size that removes 50% of the solids and then reduce. If there is a history of ‘black shoe polish’ then increase filtration to 5-micron filters as the system cleans up. Again, starting at the 50% rule reduces the need for constant filter changing (which on rich systems can lead to operational challenges due to the richness of the acid gases present).
The lean amine stream should also be treated the same with particle size distribution and solids analysis. The system can be designed to filter 10-30% of the lean amine flow. If design filters 20-30% of the lean amine, then 10-micron filters should be incorporated into your clean amine programme. If design filters 10-20% of the lean amine, then 20-micron filters should be incorporated into your clean amine programme. Suez recommends the use of 10-micron filters in the lean amine to recover from upsets.
A good carbon bed should be employed as part of a complete filtration package. The carbon bed should incorporate suitable carbon media to accommodate at least 10% of the lean amine flow. The bed should be sized for 15-minute contact time. Superficial velocity should equal 2-4 gpm/ft2. It is recommended to change the carbon bed every six months or sooner if the amine shows significant colour or you see increased foaming.
Solids in the system can also form inside the absorber due to corrosion in the hot lean amine zone. A quick field method for determining whether all your solids are coming from your feed to the unit or whether there is contribution from internal corrosion and erosion can be employed. Simply blend 100 ml of lean amine with 100 ml of reflux from the regenerator (stripper) column. If the sample immediately blackens then corrosion is taking place in the amine circuit (particularly the hot amine zone). If the sample blackens then the addition of a proven corrosion inhibitor at the rich/lean exchanger should be considered to control the corrosion taking place in the system by keeping the iron concentrations low. (There are corrosion inhibitors that are specific to H2S rich systems, specific to H2S and CO2 rich systems, and specific to CO2 rich systems – use the one that is appropriate for your loading.) In addition, an inhibitor designed to address the degradation of the amine solvent solution due to oxygen ingress can benefit many amine systems. This type of inhibitor may also need to be employed in conjunction with the appropriate corrosion inhibitor as oxygen can affect formation of corrosive acids that can severely impact your asset reliability.
If fouling is occurring due to solids in the system, the use of an antifoulant may be necessary to help disperse the solids to keep them in solution longer so they can be removed in your filtration circuits throughout the system.
As you are aware, there are costs associated with the maintenance and reliability of your amine system assets (equipment, solvents, and personnel). There are no ‘magic or silver bullets’ to address the challenges as stated. However, following good hygiene habits, corrosion and erosion mitigation filtration procedures and chemical assistance can impact the longevity of your amine systems.
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