July 2025

Process Optimization

RFCCU revamp to improve main fractionator gascon column performance and minimize propylene losses to refinery offgas

This article will discuss the critical parameters that could potentially affect the performance of a residue fluid catalytic cracking plant’s main fractionator and gas concentration (gascon) section columns. It will also elaborate on how the systematic troubleshooting methodology helped to solve the problems and improve column performance. 

IP: 10.3.182.34

The residue fluid catalytic cracking (RFCC) plant’s main fractionator (MF) and gas concentration (gascon) section columns are critical in refining operations, as these sections separate various valuable light ends and middle distillate products such as propylene, light-cycle naphtha, heavy-cycle naphtha, light-cycle oil and fuel gas.  

ADNOC operates an RFCC unit (RFCCU) in the Ruwais Refinery. It was observed that the MF slurry pumparound bed was fouling frequently: this resulted in increased pressure drop across the slurry section, leading to a shorter run length. Refinery personnel sought to evaluate and optimize the MF column slurry bed’s performance from a fouling perspective.  

Due to the severe fouling of the slurry section, cleaning/replacing the slurry section’s packing and internals becomes a difficult task for the refinery during each turnaround (TAR). In addition, the refinery was experiencing more propylene slippage in the secondary absorber overhead—approximately 8.5 mol%. Therefore, the goal of the revamp was to minimize propylene losses in the gascon overhead section and to identify the reason for propylene carryover from the secondary absorber overhead. 

This article will discuss the critical parameters that could potentially affect the performance of these columns. It will also elaborate on how the systematic troubleshooting methodology helped to solve the problems and improve column performance. 

RFCCU: Problem statement schematic. A schematic diagram of the RFCCU’s MF and gascon columns are detailed in FIG. 1. 

FIG. 1. RFCCU schematic diagram of the MF and gascon columns. 

Troubleshooting methodologies used. The following are various troubleshooting methods used.  

MF: High pressure drop in the slurry pumparound bed. The distillation section of the unit contains a large MF—the slurry pumparound was equipped with a bed of conventional grid packing at a height of 1.8 m. The distributor above the slurry pumparound bed was a trough type with a combination of drip tubes and small notches.  

During a TAR in 2020, the refinery conducted inspections of the existing packing and distributor. The main observations from the inspections are described in FIG. 2. 

FIG. 2. Observations of the MF slurry pumparound bed after a 2020 TAR. 

Refining personnel cleaned the existing packing and distributor during the 2020 TAR; however, after startup, the pressure drop across this section was still on the higher side at approximately 19 mbar. 

To identify the reason for the high pressure drop in the bottom slurry bed, the co-authors’ company^a gathered operating data from the existing MF and performed simulations to generate vapor-liquid (V-L) traffic across the slurry section by matching all operating parameters and product yields. The co-authors’ company then performed a hydraulic analysis of the existing column internals at current operating loads.  

It was found that the plant’s operational differential pressure (dP) was very high, compared to the hydraulics calculations conducted by the co-authors’ company. This indicated severe fouling in the slurry pumparound bed and its distributor.  

The co-authors’ company investigated the existing grid packing design and concluded that the existing packing may not have been thoroughly cleaned due to its constructional features. Furthermore, pressure drop across the bed increased from 19 mbar to 30 mbar within 1.5 yr of operation. The existing dP measurement of the slurry pumparound section before the revamp is shown in FIG. 3.  

FIG. 3. The slurry pumparound bed’s dP before the revamp. 

Therefore, the co-authors’ company recommended replacing the existing conventional grid packing with advanced anti-fouling mellagrid packing^b (FIG. 4) as well as the existing drip-tube-type distributor with the co-authors’ specialized slurry-notch distributor^c. 

FIG. 4. A comparison of conventional grid packing (left) vs. the co-authors’ company’s proprietary mellagrid packing^b (right). 

The primary features of the mellagrid anti-fouling packing^d include:  

• High resistance to fouling and/or coke buildup 

• Mechanical structure that allows for easy cleaning (i.e., suitable for cleaning via water and/or stream jetting) 

• Mechanically robust structure  

• High heat transfer efficiency 

• Smooth surface.  

In addition to the advanced packing, the co-authors’ company proposed a sandwiched-bed design with tie-rods to ensure that its mechanical integrity can withstand high uplift forces. Features of the sandwiched-bed assembly include the following:  

• The assembly is used for packed beds that are subject to high uplift loading, typically 1 psi [7 kilonewtons per square meter (kN/m2)], but sometimes up to 2 psi (14 kN/m2). 

• The top layer of the bed is sandwiched to the bottom layer by tie rods passing through the packing and/or grid elements. 

• Tie rods are linked to the hold-down grid to support it. 

• Depending on the type of packing, different types of support and hold-down grids can be used.  

An illustration of the slurry pumparound bed’s sandwiched design after the revamp is shown in FIG. 5. 

  

FIG. 5. The slurry pumparound bed’s sandwiched design after revamp. 

After replacing the existing slurry pumparound bed and its distributor with the co-authors’ company’s mellagrid anti-fouling packing^d and distributor^c, the pressure drop across the slurry pumparound section lowered from 30 mbar to an average of < 7 mbar (FIG. 6). 

FIG. 6. The slurry pumparound dP after the revamp. The pressure drop across the slurry pumparound bed decreased to an average < 0.007 kg/cm2 (7 mbar).  

Secondary absorber: Propylene slippage at the overhead. Before the revamp, the refinery was experiencing high propylene slippage in the secondary absorber overhead—approximately 8.5 mol%. The goal of the revamp was to minimize propylene losses in the gascon overhead section and identify the reason for the high propylene carryover. 

The co-authors’ company worked closely with the refinery’s personnel to study the performance of the existing trays and column internals of the primary and secondary absorbers and the stripper. Several case studies were performed to improve the primary absorber’s (FIG. 7) operating scenarios: 

• Case 1: Increased the wild naphtha (from the debutanizer bottom) 

• Case 2: Increased the wild naphtha flow, along with chilled reflux and chilled pumparound at 18°C 

• Case 3: Wild naphtha flow to match the operating case, along with chilled reflux and pumparound at 18°C. 

FIG. 7. Scheme of the primary absorber column.  

TABLE 1 is a sensitivity analysis of the primary absorber’s operating parameters.  

Based on all the above cases and the columns’ hydraulic evaluation, it was concluded that the primary absorber’s existing RV-1 valve trays were sufficient to handle the above proposed case loadings; therefore, the existing trays were retained.  

Based on the simulation analysis, increasing the WCN lean oil flowrate to the primary absorber, along with applying chiller to the primary absorber’s intercoolers, were expected to increase C3 recovery. 

Secondary absorber’s hydraulics analysis. Based on a careful review of the existing tray geometry and hydraulics performance (TABLE 2), it was noticed that the secondary absorber’s trays were showing a very high pressure drop. This secondary absorber was equipped with 25 conventional floating valve trays. The reason for the high pressure drop could be fouling of the existing floating valve-type trays.  

After replacing the existing trays with the co-authors’ company’s proprietary anti-fouling trayse (TABLE 3), C3 losses in the overhead were reduced from 8.5 mol% to < 7.5 mol% post revamp (TABLE 4). 

CASE STUDIES SUMMARY 

FIG. 8 details a summary of the RFCCU’s MF before and after revamp.  

FIG. 8. RFCCU MF summary: Before and after revamp.  

Takeaways. This article states the following:  

1. The systematic troubleshooting methodology involving operational data analysis, simulation and hydraulics validation helped to solve the problem of high dP in the MF slurry pumparound section and led to a performance improvement of the gascon section absorbers.  
2. The co-authors’ company’s mellagrid anti-fouling packing^d and proprietary distributor^c offered better anti-fouling features vs. conventional grid packings and the existing small-notched/drip-tube distributor, respectively.  
3. Primary absorber simulation sensitivity analysis and secondary absorber anti-fouling trays^e have helped to reduce the propylene slippage in overhead gas to the refinery offgas. 

FIG. 9. RFCCU secondary absorber summary: Before and after revamp.

NOTES  

^a Sulzer  

^b Sulzer’s Mellagrid™ packing  

^c Sulzer’s VES™ slurry-notch distributor  

^d Sulzer’s Mellagrid™ 40AF anti-fouling packing 

^e Sulzer’s VG AF™ trays 

REFERENCES 

1 Mosca, G., “Upgrading an FCC main fractionator to improve operational reliability and flexibility,” RefComm, Budapest, Hungary, October 2017. 

2 Mosca, G., S. Bhise and S. Costanzo, “Debottlenecking an FCC main fractionator with high-performance mass transfer components,” AIChE Spring Meeting, New Orleans, Louisiana (U.S.), April 2004. 

The Authors

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