FCC Gas Concentration Unit Stripper Revamp
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FCC gas concentration unit stripper revamp Unit charge rate and reactor conversion were increased by the elimination of stripper column flooding. Chemical-grade propylene specifications were met and2 C excursions in mixed C2/C3 streams were eliminated Dave Langdon Total Lindsey Oil Refinery Scott Golden and Edward Hartman Process Consulting Services Inc Mixed C4 C2s/C3s r e t t i C = product l 3 p s 4 C C splitter / 3 Debutaniser 3 feed C Treating r r e e s s i i n n a a t h u t e b e e D D Primary C3 product absorber Gasoline product Figure 1 Generic process flow scheme of GCU fractionation system (note: the Lindsey refinery GCU operates with two3 C splitters) he FCC gas concentration unit (C3=) requiring C3= that exceeds 95%/ the revamp, with no further flooding, (GCU) stripper column at the purity with total C2s and ethylene and is no longer the FCCU’s constraint. T Total Lindsey Oil Refinery (LOR) specifications of 400 and 50 ppmwt was revamped in 2005 to eliminate respectively, these 20 000+ ppmwt C2 Pre-revamp operation flooding, consistently maintain 2C s excursions were unacceptable, as they Prior to the revamp, the stripper column in GCU mixed C3s at 5000 ppmwt overloaded the C2 stripping capacity of limited the FCCU’s charge and or less (ie, <5000 ppmwt C2s in C3 the two downstream propane/propylene conversion for several years. During the mixture) and maximise C3 recovery (PP) splitting units. FCCU conversion previous turnaround, the column (Figure 1). Flooding in the stripper had to be reduced to the stripper internals were revamped to increase column limited the FCC unit’s (FCCU) column’s hydraulic limit to meet the capacity, but the stripper did not meet charge rate and reactor conversion C3= product specifications. Following its design objectives. Following the 2000 prior to the revamp, requiring a bypass the revamp, stripper column flooding revamp, the reactor’s outlet temperature from the main column’s overhead was eliminated, total C2s in the GCU (ROT) had to be reduced 15°F (8°C) to receiver to the bottom of the stripper mixed C3s were reduced to an average lower the gasoline and LPG yield. to manage flooding. C2s excursions of 3000 ppmwt, and the FCCU’s reactor Additionally, a wild naphtha product in the GCU mixed C3s exceeding charge rate and conversion has increased bypass line was installed from the main 20 000 ppmwt were also noted. Since to record levels. The stripper column’s column’s overhead receiver to the base LOR produces chemical-grade propylene feed rate is 33% higher than before of the stripper column to reduce the www.eptq.com PTQ REVAMPS 9 Sponge absorber Main fractionator vapour Debut. gasoline Primary absorber Comp. driver Wet gas Overhead Interstage High-pres. Stripper receiver receiver receiver Preheat LCO PA Sour water stripper Debut. feed By-pass FI line Figure 2 Stripper column bypass stripper’s feed rate (Figure 2). Even with this maximises C3 recovery. Both reboiler’s heat input drastically reduced. these changes, the stripper column columns are prone to flooding, with the Unfortunately, large reductions in the limited the FCCU’s charge rate and stripper being the most problematic stripper heat input can cause major conversion. industry-wide. Primary absorbers excursions of residual C2s in the mixed While the process flow scheme in operate at extremely high internal C3s product exiting the GCU. In this Figure 1 is a generic depiction (eg, only liquid rates, have moderate foaming case, total C2s increased to 20 000 one C3 splitter is depicted), the Lindsey tendencies, almost always flood due to ppmwt or higher. facility actually has two PP splitter units inadequate downcomer top area or The stripper experienced severe (noted as C3 splitter in Figure 1), each downcomer backup, and often flood at flooding following the 2000 revamp. In with a different arrangement. They both the intercooler draw or returns because an effort to mitigate this, LOR installed serve the same purpose, but one utilises of inadvertent restrictions. The stripper a bypass to route some of the wild three towers: one tower operates as a column also operates at extremely high naphtha around the primary absorber/ separate deethaniser, followed by a two- liquid rates, with flooding initiated by stripper columns, thereby reducing the shell PP splitter, essentially one for either downcomer choke or backup stripper’s feed rate. The wild naphtha stripping and one for rectification. The flood mechanisms. maximum bypass was about 40 m3/hr other PP splitter unit achieves the same Rarely is jet flooding a problem in out of a total stripper feed of 280 m3/hr. results in just one tower, with a C2 either the primary absorber or the Therefore, the stripper’s feed rate stripping section provided at the top of stripper column. Moreover, the stripper unloaded by up to 15%. However, as a the tower and the C3= drawn as a can entrap water because the top liquid proportion of the main liquid to side product. temperature is too cold for all the water the primary absorber, the reduction was to go overhead, and the bottom 25% or more, which reduced the Absorber/stripper systems temperature is too hot to allow the primary absorber’s internal liquid/ Primary absorber/stripper systems are water to leave the bottom. This forces a vapour (L/V) ratio, thus reducing C3 the heart of the FCC GCU. These water phase in the top and middle recovery. Furthermore, the wild naphtha columns and the high-pressure (HP) sections of the column, leading to contained more than 2000 ppmwt C2s receiver control C3 recovery and C2 severe flooding. When the stripper and high H2S, forcing the stripper rejection. Ideally, the stripper bottoms floods, it is not uncommon to rapidly column to overstrip its feed to meet the product’s C2 content is controlled at the fill the HP receiver, because the column mixed C3s product’s C2s specifications. maximum required to meet the GCU feed rate is higher than the tray capacity. While this improved the overall mixed C3s total allowable C2s of the When the stripper floods, the FCC’s performance of the FCC, the C2s previously noted 5000 ppmwt, because charge rate must be lowered and the excursions still occurred. www.eptq.com PTQ REVAMPS 11 Primary absorber/stripper system revamp Condenser outlet LOR’s 2005 revamp installed new three-phase Gas to primary internals in the primary absorber/stripper absorber columns, as well as a riser on the stripper feed draw nozzle in the HP receiver to improve oil water separation. Primary absorber modifications included new High-pressure separator trays with a larger downcomer top area Riser Oil and modifications to the intercooler Water draws to eliminate restrictions. These changes have allowed the FCC’s feed rate Ideally only IC and conversion to be increased without Water soluble water having to operate the wild naphtha bypass. Since startup, the unit has been tested at a maximum charge rate and Stripper feed conversion without any indications of Sour water either columns flooding. However, stripper further charge rate increases have pushed the absorber/stripper system to a cooling limit. Thus, during summer operation, Figure 3 HP receiver oil and water separation the GCU’s offgas C3+ content has increased from 8.0 to 12 mol%. Since this top downcomer area must be large HP receiver, or low-temperature feed offgas is fed to a cryogenic gas plant enough to allow the entrained vapour to forcing a water phase to form even where most of the C3+ is recovered, the flow out at the same time as the liquid though there is only soluble water in impact of reduced GCU C3= recovery on and vapour flow in. In an atmospheric the feed. Poor oil/water separation in the overall C3= product yield is less crude column, it is possible to have the HP receiver results in free water in discernible. This said, the GCU’s liquid loadings based on the top area of the feed. The HP receiver may be too operating objective is still to try to the downcomer higher than 200 gpm/ small or there may be problems 2 minimise C3+ from the sponge absorber, ft , whereas stripper downcomer controlling the boot interface. Prior to 2 because C3= recovery efficiency on the loadings should not exceed 130 gpm/ft . the 2005 revamp, LOR’s HP receiver’s cryogenic unit is only 70% maximum. So Higher loadings choke the inlet, hydrocarbon was withdrawn from the directionally more C3= leaving the preventing additional flow into the bottom of the receiver because the draw sponge absorber will result in more C3= downcomer and causing liquid to nozzle did not have a riser. Hence, loss to fuel, with significant economic accumulate on the tray deck. when the water level reached the penalty. Given the overall FCC Downcomers also flood via a backup hydrocarbon draw, the stripper feed economics, the C3= losses do not impact mechanism. Once the froth height in contained free water. During the 2005 the feed rate/ROT policy despite the the downcomer reaches tray spacing, the turnaround, a riser was added to the HP GCU’s efforts to drive the losses from column will flood. Since vapour is receiver’s hydrocarbon draw nozzle to 12% back down to 8.0%. entrained with the liquid entering the prevent water from inadvertently being All the stripper column’s internals downcomer, there is a density difference withdrawn (Figure 3). were replaced. The existing top from the bottom of the downcomer to A water phase forms inside the downcomer area was too small and the the top.