Improving FCCUs: bottoms system upgrades
A review of process flow scheme fundamentals and basic equipment operating and design principles that have improved unit operating profitability through better use of capacity, higher conversion and lower cost of maintenance
Dan Clark, Lawrence Pump Inc
Scott Golden, Process Consulting Services
Viewed : 7975
Many refiners continue to face significant problems with reliability of the FCCU main column bottoms (MCB) system. Poor reliability reduces unit capacity, lowers conversion and increases the cost of maintenance. Reliability troubles include MCB pump head-flow loss, exchanger tube-side fouling, debutaniser reboiler shell-side fouling, and coke accumulation in the bottom of the main column.
Because many FCCU product recovery sections are based on inherently unsound process and equipment design, only rarely can operating changes alone materially improve reliability. Thus, a revamp involving some capital expense is almost always needed to correct the basic shortcomings that lie at the root of the problem.
Superheated vapour enters the main column at 930–1025ºF (500–550ºC) where it contacts a portion of the main column bottoms stream. The MCB system removes heat and scrubs catalyst fines from the reactor effluent stream. Consequently, the system operates at high temperature and contains solids that can erode and plug equipment not specifically designed for such difficult conditions (Figure 1). MCB is pumped from the liquid pool in the bottom of the main column through an exchanger network where the temperature is reduced from 680–700ºF (360–370ºC) to 440–560ºF (225–295ºC) to meet the main column overall heat balance requirements. Typical MCB heat sinks include FCC riser feed, steam generation, gas plant reboilers, and in a few instances BFW preheat. These exchangers can be piped in parallel or series. Exchanger outlet streams (total MCB) are split into slurry pumparound, quench, and decant oil product.
Slurry pumparound, routed to the top of the heat transfer internals located directly above the reactor feed, heats up as it flows down through the column internals, while vapour cools down to approximately 650°F (343°C). Temperature of liquid exiting the heat transfer internals is about 700–770ºF (370–410ºC), depending on fractionation efficiency and operating column pressure (Figure 2).
The quench stream is fed directly into the liquid pool in the bottom of the main column. Most refiners maintain constant pool temperature by varying the quench stream flow rate. A small portion of the circulating MCB stream is yielded as decant oil product.
Process flow scheme
The process flow scheme influences total MCB circulation rate, temperature, and pressure as well as equipment sizing and selection. The exchanger network can be designed with either all-parallel or partially in series. Using exchangers configured all in parallel reduces system pressure drop and lowers pump capital and operating costs. It has the disadvantage of high MCB temperature, approximately 680–700ºF (360–370ºC) , when employed for debutaniser reboiler heat.
In this service, minimising tube wall temperature is essential to controlling shell-side fouling. Series-piped exchangers lower the MCB temperature to 560ºF (293ºC) for reboiler service but raises system pressure drop, increases pump head, pump speed, and rate of pump erosion from particulates. MCB equipment design and selection thus need to address conflicting criteria.
Historically, MCB exchangers have been designed for all-parallel exchanger configurations with tube-side velocities of 4–8ft/sec (1.2–2.4m/sec) that generate pressure drops of 25psi (1.7 bar) or less. Therefore, many refiners have MCB pumps that will operate at 1200–1400rpm, generating less than 150ft (45m) differential head. In many instances, this causes rapid tube-side and shell-side fouling.
Good design practice will specify series heat exchangers operating at 9–12ft/sec (2.7–4.0m/sec) tube velocity. This greatly reduces (and in some cases has eliminated) tube-side and shell-side fouling.
But whether the exchangers operate in series or in parallel, the MCB pump must fulfil its head-flow performance requirements while pumping erosive fluid throughout the duration of the run. Degradation of pump performance will increase the rate of exchanger fouling and limit MCB heat removal. To minimise pump erosion from catalyst and coke particles, an API fully lined pump is often required when operating with higher pump head and operating speeds in the MCB circuit.
Tube-side exchanger fouling is caused by coke and catalyst deposition inside the tubes, coke plugging the openings in the tube sheet, or by deposition of varnish-like material inside the tubes. Varnish-like material is formed in the bottom of the main column from thermal decomposition of poly-nuclear aromatics. In some instances, high MCB pool temperature causes coke to accumulate in the bottom of the main column.
Both localised MCB liquid pool operating temperature and residence time contribute to fouling, the former being the main factor. Because most refiners measure bottom temperature in the suction line to the MCB pump, they incorrectly assume that it reflects MCB liquid pool temperature. This is only true if the quench is uniformly distributed across the column cross-sectional area.
Typically, the unquenched liquid leaving the column internals and the quench are not thoroughly mixed, causing localised MCB pool temperatures equal to the unquenched stream leaving the column internals (Figure 3). Quench must be evenly distributed to suppress formation of the fouling material.
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