Choosing the ideal CO2 drying solution for CCS applications
Aluminosilicate adsorbents provide a reliable, low-energy solution for CO2 dehydration prior to transport, storage or usage.
Kirstie Thompson, Margaret (Peg) Greene and Manish Mehta
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Carbon capture and storage (CCS) is a rapidly growing market and will continue to grow as stakeholders emphasise the implementation of sustainable practices. In the CCS realm, much focus has been on the CO2 capture itself, but dehydration of CO2 for transportation and storage is also a key step. Many CO2 capture technologies utilise aqueous amine solutions, saturating the CO2 during the capture process. This wet CO2 is extremely corrosive, causing concern for pipelines and other surfaces it may contact. Thus, before the captured CO2 can be transported, stored, or utilised, a dehydration step is necessary.
A robust and efficient method for the dehydration of CO2 will be necessary for the emerging CCS market. BASF has been providing materials for the dehydration of CO2 in the beverage industry and enhanced oil recovery (EOR) applications for decades. Based on this experience and an extensive study comparing available technologies, BASF has concluded Sorbead, a specialty aluminosilicate, is best suited for the dehydration of CO2.
This article will discuss the CO2 dehydration technologies currently available and the criteria that should be considered when making technology selection decisions. It will also detail the benefits of choosing aluminosilicates such as Sorbead for CO2 dehydration, including longer material lifetimes, lower energy duties, smaller bed sizes, and lower Capex/Opex costs compared to glycol and other adsorbent solutions.
Glycol — the old guard of dehydration
The archetypical dehydration solution for natural gas, and most industrial plant-based operations, is a triethylene glycol (TEG) solvent-based system. In these operations, ensuring efficiency and employee/environmental safety is of utmost concern. The solvent-based nature of TEG systems necessitates the use of circulating equipment, frequent chemical quality checks, and chemical make-up adjustments. In these complex plants, additional maintenance and chemical storage requirements can be very burdensome and limiting in some cases, i.e. off-shore operations. Also, the addition of any liquid-based chemical increases safety concerns by introducing the possibility of chemical spills and emissions.
Along with water, TEG will co-adsorb heavy hydrocarbon components such as benzene, toluene, and xylenes. After adsorption, these components would then be released into the atmosphere along with desorbed water in the regenerator off-gas stream. Additionally, recoveries greatly depend on the system used, and without enhancements such as a vapour recovery system, additional contaminants such as CO2 and Hâ‚‚S can also be present in the off-gas vapour stream. These emissions cause serious concern for the plant and the surrounding environment.
Standard TEG systems can achieve outlet Hâ‚‚O contents of <100 ppmv. Though these systems are considered standard practice, they struggle to keep up with ever-changing pipeline specifications (<<50 ppmv Hâ‚‚O), often requiring additional modifications or add-ons such as enhanced stripping and vapour recovery systems. It is also increasingly common for pipelines to dictate very strict glycol specifications, often <15 ppbv. With TEG systems alone, this specification is unattainable and requires the addition of an adsorbent guard bed. This raises the question: can adsorption alone be applied for CO2 dehydration, eliminating the many complexities of TEG systems?
Adsorption dehydration options
Solid adsorption-based systems are another proven dehydration technology. These materials include activated alumina, molecular sieves, and silica gels. This article will closely compare activated alumina to amorphous aluminosilicate gel, two products in the BASF portfolio sold as F200 and Sorbead, respectively. BASF has decades of experience designing temperature swing adsorption units with these materials, so it is well positioned to offer the optimum solution for each project. The adsorption options available share some common benefits, including no maintenance between turnaround, quick start-up times (0-5 hours), and easily achieved pipeline glycol specifications. Also, the solid nature of these adsorbents eliminates the safety and storage concerns associated with the liquid chemical nature of TEG systems. While adsorption in general has many benefits, each of these materials has unique properties, making some better suited for CO2 dehydration than others.
Molecular sieves have traditionally been employed in the dehydration of natural gas for liquefied natural gas (LNG) production. BASF supplies molecular sieves into numerous natural gas dehydration applications annually, but as much as these materials are well suited for the dehydration of natural gas, they are unsuited for that of CO2. Molecular sieves are very unstable in acidic environments, like wet CO2. To account for molecular sieve degradation in acidic conditions, much larger bed sizes are required, along with more difficult and frequent bed changeouts. Additionally, when compared to the other adsorbent solutions available, they have some of the highest regeneration temperatures (â‰¥250°C depending on the specific material chosen) and thus the highest energy requirement. For these reasons, molecular sieves are not recommended for CCS applications. Activated alumina maintains many of the same pitfalls as molecular sieves, although to a lesser degree. They are not acid resistant, leading to shorter bed lifetimes, larger bed sizes, and the corresponding higher Capex and Opex costs. They also require a higher heat of regeneration. While these pitfalls exist, in some instances activated alumina may still be a good option for CCS dehydration applications.
BASF is a leading supplier of aluminosilicate gels, having served natural gas and CO2 treatment applications with the Sorbead portfolio for over 60 years. While other standard silica gel options with cheaper initial material costs are available on the market, these materials do not exhibit all the same benefits, robustness, and lifetime cost savings. Sorbead aluminosilicate gel is made using a specialised production process that imparts increased durability and adsorbent surface area, resulting in an overall higher-performing material. There is a clear benefit to using an advanced aluminosilicate for CO2 dehydration (see Table 1), as these provide the best combination of material properties for CO2 dehydration.
Sorbead aluminosilicate — the best adsorbent for CO2 dehydration
The acidic chemistry of an aluminosilicate makes it the only adsorbent stable to the acidic conditions in CO2 dehydration. This stability ensures long lifetimes of 5-10 years and bed sizes up to 75% smaller than activated alumina. This further results in lower Capex costs (smaller bed sizes) and lower Opex costs (fewer bed changeouts, lower heat duty, less regeneration gas). While the acidic nature of an aluminosilicate provides stability in CO2 streams, it makes it susceptible to instability in the presence of basic contaminants, such as ammonia, amines, and olefins. However, if these impurities are present in low concentrations, they can be easily accounted for in the design of the CO2 dehydration unit. If such contaminants are present in higher quantities, many upstream treatment methods are available. There are also many references of Sorbead units successfully operating downstream of acid gas removal units (AGRUs), where amine carryover could be a concern. Overall, the benefits of Sorbead stability in the presence of the primary, acidic CO2 streams outweigh any instability in the presence of minor, basic impurities.
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