An economically attractive carbon capture solution for FCC
Reducing the cost of FCC carbon capture along with increasing FCC throughput and facilitating wider range feedstock processing.
Jan de Ren, Sakthivelan Durai, Raul Zavala and Erick Bennet
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Refiners are facing increasing pressure to reduce the carbon (CO2) intensity of their fluid catalytic cracking (FCC) units due to the ongoing energy transition and rise of environmental, social, and governance (ESG) initiatives. Refinery emissions contribute approximately 3% of the total anthropogenic Scope 1 and 2 carbon dioxide (CO2) emissions, amounting to roughly 1,124 million tonnes per year (tpy).1 Within FCC-based refineries, the FCC unit typically accounts for 15-25% of these emissions,2,3 primarily stemming from the FCC regenerator. These emissions are a result of the coke burn operation required to maintain the unit’s heat balance and restore catalyst activity.
Implementing post-combustion carbon capture in FCC units presents a three-fold challenge from an economic standpoint:
Œ The CO2 concentration in the flue gas is low compared to pre-combustion applications.
The volume of flue gas that requires treatment is substantial.
Ž The flue gas contains a notable amount of contaminants, necessitating significant upfront investment and continuous operating expenses for pretreatment. Pretreatment is required to prevent high solvent make-up rates in the downstream solvent-based carbon capture unit (CCU).
Honeywell UOP’s proprietary Synthesized Air FCC technology offers several advantages, including reducing the cost of the FCC carbon capture step, enabling increased FCC throughput, and facilitating the processing of a wider range of feedstocks. This encompasses not only conventional feedstocks but also opportunity feeds, bio-renewable feeds driving sustainability, and plastic-derived feeds driving circularity by partially converting to light olefins (typical precursor for polymer production). Synthesized Air FCC technology presents an avenue for refiners to enhance the CO2 capture process in FCC units. By reducing costs, increasing throughput, and accommodating various feedstocks, this technology aligns with the industry’s drive towards a more environmentally conscious and economically viable future.
Oxy-combustion principle and concept evaluation at a European refiner
The FCC process breaks low-value, long-chain hydrocarbon molecules into higher-value, smaller molecules. One of the major by-products of the cracking reactions is coke, which gets deposited on the catalyst surface, resulting in catalyst deactivation. To restore the catalyst activity, the coke on the catalyst is combusted in a regenerator, where the traditional FCC process uses air as oxidising media. Typical atmospheric air contains 78 mol% nitrogen (N2), 21 mol% oxygen (O2), 0.93 mol% argon (Ar), and other gases like carbon dioxide (CO2), carbon monoxide (CO), and neon (Ne) make up the rest.4 For simplicity, air is composed of 4 moles of N2 for every mole of O2. The O2 is consumed by the combustion process in the regenerator, whereas N2 remains inert and carries a substantial amount of heat released during the combustion process. This heat is typically recovered to a large extent in the flue gas heat recovery section of the FCC process.
In an ‘oxy-combustion’ (referred to as ‘synthesised air’ going forward) mode of operation, the combustion is facilitated by synthesised air, which is a mixture of O2 and CO2. The CO2 replaces N2 as the inert heat carrier to avoid excessively high temperature in the regenerator, as per Figure 1.
In the simplest form of synthesised air operation, every mole of N2 in the air is replaced with an equal mole of CO2. This results in the number of moles of regenerator flue gas remaining the same as typical air operation. Hence, the critical velocities in the regenerator, including vessel superficial velocity and cyclone velocity, can be maintained in a similar manner to base air operation. This is the ideal ‘constant velocity operation’ in synthesised air operation.
The complexity of synthesised air operation arises from the fact that the molecular weight of CO2 is approximately 1.6 times that of N2. Due to higher molecular weight and consequent higher mass flow, heat carried away by CO2 will be significantly higher for the same moles of N2, which results in a substantial drop in regenerator temperature (assuming reactor temperature and feed quality are held constant). To achieve a ‘constant heat balance operation’ of the regenerator, the number of moles of CO2 in synthesised air operation needs to be reduced without compromising the critical velocities. See Figure 2 for a pictorial representation.
UOP performed a case study in 2023a on the application of Synthesized Air FCC system for a European refiner operating its FCC unit at a feed throughput of 65,000 bpsd, with nearly 75% residue in the feed blend. The blended feed Conradson Carbon (CCR) concentration of the feed was 3.6 wt% (see ‘Case Study’ section).
Based on the findings of this case study, conversion from normal air combustion to synthesised air operation is estimated to reduce the regenerator temperature from 738°C to 709°C. The drop in regenerator temperature and the reduced flue gas flow rate create additional coke burn capacity in the regenerator. This additional coke burn capacity provides an opportunity to increase residue content in the feed blend from 75% to 100%. The resulting regenerator temperature for this operation was estimated to be 738°C, like the base case. The results of the 2023 case study produced by Honeywell UOP proprietary process models demonstrate the inherent potential of its Synthesized Air FCC technology to increase FCC profitability, apart from reducing direct CO2 emissions (Scope 1) via an installed CCU, as shown in Figure 3 and detailed in the ‘Case Study’ section.
The principal components and general arrangement of the Synthesized Air FCC system are presented in Figure 3. It is comprised of a third-stage separator (TSS), power recovery turbine (PRT) section, heat recovery steam generator (HRSG), nViro FCC section, novel, proprietary, non-solvent Honeywell UOP FCC carbon capture section (CCS), and recycle blower. Note that an existing ESP/bag filter can be revamped to an nViro FCC unit, and a wet gas scrubber can be used in lieu of an nViro FCC unit.
Flue gas from the FCC regenerator enters the TSS, where separation of the larger catalyst fines from the flue gas stream is accomplished. The TSS provides the necessary particulate removal to protect the expander internals from blade deposition or erosion. The clean flue gas exiting the TSS flows to the expander for power recovery. The larger-sized catalyst fines, separated from the main flow of flue gas in the TSS, are carried out of the bottom of the TSS with a slip stream of flue gas. The larger-sized catalyst fines are removed from the slip stream via, for example, a fourth-stage separator (FSS) before the slip stream merges with the main flue gas line upstream of the HRSG. The expander inlet valve will throttle to control the regenerator-reactor differential pressure. From the expander, the flue gas flows to the HRSG, where high-pressure steam is generated from the cooling of the flue gas stream. In some cases, provisions can be made to the HRSG to accommodate the removal of NOx contaminants. For units where HRSG cannot accommodate a NOx removal system, the same can be designed as part of the nViro FCC system or wet gas scrubber.
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