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Natural Gas Decarbonation with Hybrid Technologies

None of the covered technologies has flill CO, capture range and, to mitigate economic penalty from CCS, technologies must har e then capture cost reduced, by increasing capture efficiency, while reducing power, heat consumption and environmental impacts. To meet these objectives, hybrid technologies (HYB) consist of using two or more technologies, in parallel or serial arrangements. Series HYB uses one technology for bulk CO, removal and another for final polishing. The bulk technology is not in charge of meeting CO,-lean gas specifications and the polishing process faces small CO, contents, reducing energy consumption of both technologies (Al-Mamoori et ah, 2017). In parallel arrangements, CO,-rich NG is divided into two or more streams and each technology must meet CO,-lean specifications. The use of parallel arrangements increases the efficiency of the technologies, but is less common than serial configurations (Al-Mamoori et ah, 2017).

Song et ah (2017a) reviewed HYB processes, covering near 7400 publications up to 2017, with 60% using САФА, 16% MP and 4% CD. For all HYB technologies, the main research focus is on testing new combinations of technologies and materials (such as MP+CA arrangements, MP+PA arrangements, PA solvents or MP materials). Investigations into whether the use of a HYB combination is more beneficial than the use of a stand-alone technology are also critical (Song et ah, 2017a). To illustrate HYB technologies, the most promising candidates are discussed.


MP-CA and MP-PA hybrids can be used in series and parallel arrangements. In a serial arrangement. MP acts as a bulk removal process, followed by a polishing CA step, with lower CO, loading when compared to traditional CAPA (Araujo et ah, 2017). Rochelle et ah (2014) also proposed a CA-MP in a serial arrangement, with CA being the bulk remover. Absorption removes = 50% of CO, from the CO,-rich NG, and MP then removes the remaining 50%. Cost reduction derives from the lower energy required since less solvent is circulated and regenerated (Rochelle et al., 2014). In parallel arrangements, CO,-rich NG feed is divided into two streams, one being directed to MP and another to CA/PA. The main advantage is the reduction in capital costs due to smaller equipment size—e.g., the absorber column can roughly present a twofold down-scaling (Song et al., 2017a). The main issues of C'A-MP are consumption of both heat for amine regeneration and power for providing trans-membrane differential fiigacity in MP and possible membrane poisoning due to solvent cany over.

Araujo et al. (2017) compared standalone and hybrid technologies, CA. PA, MP, MP-CA and PA-MP. focused on offshore processing of CO,-rich NG under increasing CO, content in the associated gas (10%, 30% and 50%mol), and high NG flow (6 million SmVd). The authors concluded that MP-CA partially inherits MP's small footprint and is the most flexible capture in CO,-rich NG, in ultra-deepwater offshore fields having high GOR, with early CO,-EOR. Reis et al. (2017) used an optimization-based procedure to define the sendee distribution between bulk removal in MP and the CA polishing operation, subject to two constraints: Maximum CO, content in treated NG of 3% and minimum CO, content in injected gas of 75%, for raw NG with 10, 30 and 50%mol CO,. The authors concluded that slipping CO, from MP to CA considerably reduces the total footprint (i.e., the required permeation area) in MP.


CA/PA-ADS consists in the use of a sluny containing an absorbent (amines, potassium carbonate) and an adsorbent (MOF, zeolites). Both materials are regenerated with a single thermal operation at lower energy costs and present higher CO, selectivity. Liu et al. (2014) tested this concept in an amine/MOF HYB and showed that 1.25mol/L CO, loading was achieved at 1 bar with high CO, selectivity and a regeneration duty of 1.44 GJ/tCO,—standalone MEA consumes = 3.3 GJ/tCO,. CA-ADS can potentially reduce absorbent/adsorbent volume and requires less regeneration energy. Shortcomings are due to its high footprint and the initial cost of adsorbent materials.


CD-MP is the HYB scheme that has attracted the most attention. The concept is to use a CD to remove bulk CO„ water and H,S, resulting in a high-pressure gas. This high-pressure gas is then passed through a CO, selective MP for polishing. CD-MP reduces compression requirements when compared to standalone MP and refrigeration load when compared to standalone CD (Prosemat, 2017). The main advantage of CD-MP is its high flexibility, since CD is very resilient to feed changes and subsequent MP can be very CO, selective for optimal treatment. The disadvantages are MPs sensitivity to cold tempera tines and losing part of the MP modularity.


Ariuelli et al. (2019) applied the SS process in offshore NG processing with ultra-high CO, content (> 60%mol) for dew-point adjustments and decarbonation on a floating-hub processing 50 MMsrnVd of CO, ultra-rich gas, reinjecting 96% of treated CO,-rich gas for enhanced oil recovery, while reserving 4% as fuel-gas upgraded to 20%mol CO, for power production. The best economic performance of SS alternative compared to SS followed by MP reflects its highest revenues from recycling condensate from the 1st SS unit, entailing 18% higher oil production.

Technology Readiness Level and Research Status in Natural Gas Decarbonation

Technology Readiness Level (TRL) is a method to estimate technology maturity that is widely used in literature (Baklitiaiy-Davijany and Myhrvold, 2013). TRL usually varies from 1 to 9, in which lower values represent low maturity. In this section, TRLs for the addressed CO, capture technologies are

Table 2. Technology readiness levels and cutoffs.


Development stage completed

Definition of the development stage



Unproven Concept

Basic R&D, paper concept

Basic scientific.1engineering principles observed and reported, paper concept with no completed tests or design history.


Proven Concept

Proof of concept through papers or R&D experiments

Existing studies that contain experiments or formulated applications but do not include physical models.


V alidated Concept

Experimental proof of concept using physical model tests

Concept design validated with physical models. A mock-up or dummy equipment is functionally tested with reliability tests on the major components.



Prototype Tested

System function, performance and reliability tested

A prototype is built and passed through generic functional and performance tests with benefits and risks being experimentally demonstrated.


Environment Tested

Pre-production system envuonment tested

Prototype has passed all other TRL categories and is tested on a realistic environment (either simulated or real).


System Tested

Production system mterface tested

A full-scale equipment is built and tested/integrated into the intended operating system with fiill mterface and functionality bemg fully tested.



System Installed

Production system mstalled and tested

A TRL 5 (fiill-scale equipment with intended operational conditions) operating for over 3 years in which the first years may need additional support due to the learnmg curve.


Field Proven

Production system field proven

System is installed for over 3 years with proven reliability, economic and environmental results.

suggested. Table 2 presents the proposed TRL cutoff values—low, medium and high maturity—applied to CO, capture technologies suitable to CO,-rich NG processing on ultra-deepwater rigs. The attributed classification is specific to the NG application and not an overall TRL.

Mamie technologies for CO, removal from CO,-rich NG include CA. PA. MP and CD. These four technologies have TRL = 7 since they all har e commercial applications with over 3 years of operation. CA and PA are the oldest technologies, with many licensors. OP. Honeywell is one of the main players, offering both amine and carbonate-based solvents with their AmiueGuard and Modified Benfield Process products. Other interested parties in CA are ExxonMobil, Basf, Schlumberger and Shell, offering full absoiption packages (operation, solvent and columns), with Ineos, Evouik and Dow Chemical Company offering only solvents.

PA, depending on the solvent, has more licensors than CA, with the main contributors being UOP/ Honeywell, Luigi and Shell with their respective Selexol, Recistol Piuisol and Sulfinol processes. PA and CA were the first carbon capture technologies to reach commercial application. For example, the Oklahoma Natural Gas Processing plant (120 MMscf/d of NG) and the Freer Texas Refinery (12 MMscf/d of a 15% CO, NG) in the USA use CA. More recently, a plant in Fort Nelson Canada, for geological storage of CO„ uses CA to purify 2.2 Mt/yr of NG. As for PA, the Century Plant in the USA uses Selexol to remove CO, from a 5 Mt/yr NG stream and the Shenfu Dongselm plant in China uses Rectisol to decarbonate 1,065 t/hrNG.

MP is commercially available and widespread in certain applications, such as in FPSOs, and can increase its participation in the NG chain. Most commercialized membranes in the world are produced by UOP Honeywell (Separex Polysep Composites), Schlumberger (Cynara, Apura and Semple) and Ah Liquide (ALaS MEDAL). Acetate-based polymers are the membranes mostly applied in processing plants. The Lula oil field operated by Petrobras uses a Separex type membrane to treat 1 Mt/yr of NG having 20-40%mol CO„ Cakerwala Gas in the Gulf of Thailand uses Cynara to treat 1,280 MMScfd of NG bearing 36%mol CO,, demonstrating the high potential of NIP for CO,-rich NG. Castor Storage in Spain uses Separex in a 5%mol CO, NG, showing MP capacity to treat gases with low CO, content.

In contrast with РАСА and MP, CD only has 3 commercially available technologies—Cool Energies CryoCell®, ExxonMobil’s Controlled Freeze Zone™ and the Ryan/Holmes Process. Ryan/Holmes is the oldest, with uses registered as early as 1980 by the Seminole Unit (operated by Amerada Hess Company), the Willard Unit (operated by ARCO oil and gas company), the GMK South Field (operated by Mobil Oil Corporation), and the Wasson Denver Unit (operated by Shell Oil Company) (Lastari, 2009). CryoCell® has been in field test and demonstration scale plants since 2006 and Controlled Freeze Zone™ has been operating in commercial scale field tests since 2008. CD can be considered at TRL = 7, as all available technologies are field proven at large scales and have been operating for over 3 years.

Cool Energies technology consists in a cryogenic CO, separation in which solid CO, is obtained. The process requires a prior dehydration (under 5 ppm of water is allowed into cryogenic unit) and the main innovation is in the CryoCell® three phase separator of methane and CO,. The main disadvantage is that solid CO, must then be liquified to be transported or stored. Controlled Freeze Zone™ attempts to simplify CD using a single-step separation in one large column. To achieve desired separation in a single stage, very low temperatures and pressures are required and CO, freeze-out (diy-ice formation in distillation column) will occur. Since solids are unwanted in a distillation unit—can clog tower nozzles— the ExxonMobil CD technology has two special areas inside the column in which solid CO, is re-liquified due to interactions between the solid and descending liquid droplets at higher temperatures. Considering that CD is already widespread in other applications (e.g., industrial gases), well-stablished actors could move into CO, removal from NG and a few, like Air Liquide, have already shown interest in the decarbonation market. Figure 8 summarizes the high TRL technologies providing the main stakeholders and the materials used.

For medium and low maturity technologies, a distinction in interests is made between academy and industry. ADS has medium maturity technology with TRL = 3. It must be noted that a different TRL could be given to each of the sub processses within ADS, such as PSA, TSA or ESA, since each has its own maturity level. PSA is the most advanced and represents ADS in the present analysis. Since PSA has various laboratory cycle tests reported for NG applications, it is considered TRL = 3.

SS research is mostly focused around computational simulation of possible scenarios. The main interested parties are in the academic sphere of research with low industrial interest, despite some applications of SS already existing-mainly in WDPA and HCDPA of NG and LNG processing by Twister B.V. (2017). Twister B.V. commercializes its SS model, which has been submitted to field tests, hence, its TRL = 5.

Summary of mam players m lugh TRL technologies

Figure 8. Summary of mam players m lugh TRL technologies.

CO, management technologies vs range of CO, content in CO,-rich NG

Figure 9. CO, management technologies vs range of CO, content in CO,-rich NG.

Research in GLMC is focused on determining the best membrane/solvent combinations while reporting on basic parameters, e.g., CO, flux and CO, recovery. Tests mainly use pure CO, feeds, and do not integrate GLMC with upstream/downstream operations. GLMC has a TRL = 2, despite coal, flue gas and syngas applications already evolving to field testing. For CO, removal from NG, GLMC has been mainly confined to laboratory-scale tests. Figure 9 presents the range of operation for each technology covered in this chapter with respect to the content of CO, in CO,-rich NG.

CA-based technologies are applicable to low CO, content but lose their edge to CD and SS that have their application niche in very high CO, content scenarios. CD is an excellent technology for bulk CO, removal but the CO,/C,H6 azeotrope and high energy consumption are strong drawbacks in low CO, content. Since every HYB combo has a different range of action they are not represented in Figure 9.

Lastly, Table 3 summarizes the technologies for CO, separation from NG reviewed in this chapter, pinpointing technological gaps, main interests, TRL, availability and flexibility in variable CO, scenarios.

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