PTQ Q1 2024 Issue

Solvent purification With a solvent-based CO₂ capture process, the process efficiency and operating expenses of the entire unit hinge on the cleanliness of the solvent and equipment. On a positive note, recommended filtration and separa - tion steps are well-studied due to the longevity of these processes in gas processing plants. Solid feed contaminants such as fine fly ash particulates (as small as <1 µm diameter) that can bypass feed pretreat - ment steps due to their small size can build up and foul the lean/rich heat exchanger, the reboiler, the absorber internals and require more frequent solvent change-out over time. Contaminants can also alter the surface tension of the sol - vent, causing an increased tendency to foam and increased foam stability, requiring the use of anti-foam. Finally, fine particulates can form aerosol nuclei, which contribute to solvent emissions, resulting in solvent losses out of the absorber vent, as found from tests at the post-combustion carbon capture plant at Niederaussem.² Corrosion products from stainless steel and similar equipment can also precip - itate in the rich side of the solvent loop into solid particu - lates such as iron compounds, causing similar issues. To remove these solids, particulate filtration of the sol - vent is recommended at a minimum of 10% slipstream. The target level for solids after filtration is 1-5 ppmw. Five or 10 µm-rated absolute particle filters are recommended, based on the diameter of the solid particulates. It is important to understand the differences between how particulate filters are rated. Nominal ratings are arbi - trarily assigned by the filter manufacturer, and there is no regulation for the value of the nominal ratings to indicate the performance of removing certain particle sizes. In con - trast, absolute particle filter ratings must meet rigorous ‘ISO or ASTM’ standards. The absolute rating of a particle filter directly corresponds to the largest diameter of particle that the filter will allow through – all larger particulates will be captured. An example of the difference between solvent cleanliness after using no filter, a nominally rated filter, and an absolute-rated filter is shown in Figure 3 . Rich side filtration is commonly recommended to remove precipitated corrosion like iron sulphide and to protect the lean/rich heat exchanger. Significant improvements in the removal of solvent contaminants have been demonstrated using Pall absolute-rated filters, with extensive data prov - ing the removal of precipitated corrosion products and pro - cess equipment protection from the gas treating industry.³ Lean filters can also be added to the process scheme to prevent fine particulates from entering the absorber. Lean filtration is particularly recommended for polishing and removing adsorbent fines if there is a carbon bed on the lean solvent side. Carbon beds are often installed to remove solvent deg - radation products and have been found to remove some metal ions. Degradation products such as organic acids, formed by the solvent degrading through oxidative and thermal mechanisms, can be corrosive, cause foaming, solvent losses, and reduced absorber capacity. Metals are common from internal metallurgy and can catalyse amine degradation. Not all activated carbon targets the same

contaminants, so the product must be selected carefully to ensure that it does not prematurely plug. Other concerns in the solvent loop include heat-stable salts, which are produced when amines react with acidic components such as O₂, CO, and SO₂. Concerns with heat- stable salts are that they render the amine inactive and can make the solution corrosive if allowed to reach a level above 3%. Ion exchange techniques are commonly recommended for treating heat-stable salts. Finally, there is increasing concern about nitramines and nitrosamines in the carbon capture industry due to their nature as a potential carcinogen. These compounds are produced from NOx in the flue gas reacting with amines. Water wash prevents nitramines and nitrosamines from venting out of the absorber, but they must still be removed from the water wash before disposal to avoid environmen - tal contamination. Processes to remove nitrosamines and nitramines, such as selective catalytic reduction (SCR) for NOx removal and use of activated carbon, are ongoing areas of study to ensure that these compounds remain below desired levels.⁴ Flue gas pretreatment The top three contaminants commonly present in post-com - bustion flue gas are NOx, SOx, and particulates such as fly ash. All three of these contaminants should be removed prior to CO₂ capture, regardless of the capture technol - ogy used. NOx levels are reduced in the pretreatment step before the absorber with a selective catalytic reduction process (SCR); SOx levels are also reduced during pretreat - ment with a wet or dry scrubbing process. Particle filtration is commonly employed in pretreatment steps with cyclones, electrostatic precipitators, and bag filters. Cyclones use rotation to separate solids but have difficulty removing small particulates. Electrostatic pre - cipitators (ESPs) remove fine particulates by applying an electric charge but can be expensive and associated with an increased safety risk. Furthermore, wet ESPs have been found to break up large contaminants, increasing the total number of contaminants in some cases. Finally, bag filters can be temperature-limited due to the use of polymeric material. They can have a shorter lifetime and lower partic - ulate removal efficiency when compared to absolute-rated filters with inorganic (metallic or ceramic) filter media. One key requirement of filtration pretreatment is that it operates at a low pressure drop due both to the excessive

Figure 3 Amine cleanliness after no filtration, nominal filtration, and absolute filtration

PTQ Q1 2024