Environment & Safety Gas Processing/LNG Maintenance & Reliability Petrochemicals Process Control Process Optimization Project Management Refining

2021 AFPM Annual Meeting Virtual Edition: FCCU revamps—Reduced CAPEX for increased petrochemicals

GARY R. MARTIN, Sulzer GTC Technology

Modern fluid catalytic cracking units (FCCUs) provide an important link between a fuel refinery and the production of light olefins and aromatics for petrochemical use. FCC licensors are designing units to produce 10 wt%–20 wt% (on fresh feed) propylene yield from the more historically typical 5 wt%. Realistically, economics are driving the designs to the lower end of this range, with optimum designs usually somewhere between 10% and 15% and more commonly between 7 wt% and 10 wt% for revamps.

Economic factors influence the optimum propylene production with one factor being the CAPEX associated with recovering the propylene. Revamps of existing FCCUs can provide for increased propylene production at the expense of gasoline; however, a constraint to increased production is the reuse of existing equipment to minimize CAPEX. Increasing FCCU severity, the addition of ZSM-5 and varying the reactor partial pressure, total pressure and catalyst-to-oil ratio enable increasing propylene production, but this can have a significant effect on the gas concentration section (Gascon).

A simplified process flow diagram of a common conventional FCCU Gascon configuration is shown in FIG. 1. Variations of this flow scheme have been used but most Gascons are based on a similar configuration. The shift in additional propylene production tends to produce more dry gas, making it more difficult to recover the propylene. To maintain high propylene recovery along with the shift in product slate can lead to much higher loadings in the sponge absorber, primary absorber, deethanizer, debutanizer and the C3/C4 splitter. These columns are often shell limited, and it is normally uneconomical to replace or parallel these five columns by conventional means.

While shifting product slate to petrochemicals makes sense for the forward-looking refiner, it must also make economic sense for today’s business.

FIG. 1. Conventional FCCU Gascon simplified process flow diagram.

FCCU licensors have shown how the reactor section design/operation can be modified to increase petrochemicals production; however, the technologies do not provide the technological improvements in other areas of the FCCU needed as an economical means of retooling the process with a revamp that results in a good ROI. There have been advancements in the modification of FCCU main fractionators that enable higher throughput and a shift in yield structure. However, the shift to propylene production has a large influence on the FCCU Gascon. High-capacity trays have enabled some capacity gain in the Gascon, but this gain is inadequate to handle a significant increase in propylene production.

Turning to divided wall column. As shown in FIG. 2, divided wall column (DWC) technology provides a lower CAPEX, more simplified construction and a decreased plot space revamp design to overcome major limitations in the Gascon. These changes can be utilized to increase propylene production and alkylation unit feedstock.

Increased production of lower mole weight components makes it more difficult for the Gascon to be capable of processing the feed while maintaining high recoveries and desired product purities. As dry gas production increases with the increased operating severity, this loads up the sponge absorber, primary absorber and deethanizer. Higher propylene production primarily increases the loads to the primary absorber, deethanizer, top of the debutanizer and C3/C4 splitter. If these existing columns are shell limited, a lower cost design option is needed to provide a revamp that produces a good ROI.

FIG. 2. FCCU Gascon revamp utilizing GT-LPG Max℠ technology.

The DWC design shown in FIG. 2 illustrates the installation of a single vessel that unloads the sponge absorber, primary absorber, deethanizer and C3/C4 splitter. This design can unload the existing Gascon to enable improved recoveries in the existing unit along with handling additional load from shifting to increased propylene production.

Depending on the existing capacity of the wet gas compressor discharge cooler and high-pressure receiver, there may be a need to add a separate cooler and high-pressure receiver in front of the new DWC. The DWC design can produce equal or higher propylene recovery and product purities than that of a conventional design configuration and add further capacity, as needed. Most likely, no modifications to the internals of the existing columns are required. The only potential limitation is the bottom of the debutanizer and bottom of the C3/C4 splitter, but other solutions exist if this is the case.

Typically, this will not be a problem, as changes in the product slate result in these areas not being a limitation, or at least allow for changes to handle the new design conditions. The shift in product slate decreases gasoline yield, which helps unload the bottom of the debutanizer. The DWC can also be used to overcome limitations in plot space and the construction can be completed prior to a shutdown with only tie-ins required.

The DWC FCCU Gascon design can easily operate over the typical operating range of FCCUs and is a lower CAPEX solution. The simplistic design configuration provides for achieving equal or better product recoveries and purities, as a conventional system, at lower CAPEX. For typical FCC reactor yields, in maximum gasoline mode, this FCCU Gascon design can be used “without” the addition of a high-cost refrigeration system to provide propylene recovery of greater than 97 LV%.

However, in reactor designs approaching 20 wt% propylene yield, it will be necessary to use a chilled system to maintain propylene recovery > 97%; however, this would be required for a conventional Gascon design, as well. As with a conventional system, intercoolers and other design elements on the primary absorber can be utilized as necessary to obtain the desired propylene recovery.

Pending patents cover the utilization of the DWC gas plant process flow scheme presented. Using conventional designs no longer makes sense when compared to the benefits of DWC gas plants. This technology not only improves business economics but also helps reduce fossil fuels consumption, reduces carbon emissions and lowers the impact on the environment. The use of DWCs to increase the yield of petrochemicals from crude helps refiners position themselves to remain viable.

To learn more about utilizing this DWC technology for FCC, SGP and coker gas plants, contact the author at gary.martin1@sulzer.com..

About the author:

GARY MARTIN is Business Segment Leader of dividing wall column technology for Sulzer GTC Technology, based at the company’s Irving, Texas office. Previously, he was President of Process Consulting Services. His experience has included conceptual process design and process design packages for large capital projects, optimization and troubleshooting services to the refining industry worldwide. Martin earned a BS degree in chemical engineering from Oklahoma State University, and has authored more than 60 technical papers. Additionally, he has developed new licensed process technology, including processes utilizing dividing wall column technology.

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