Hydrate formation in a fluid catalytic cracking off-gas line
A hydrate blockage led to sudden loss of a FCC unit’s off-gas flow and a system pressure surge. Comprehensive troubleshooting delivered remedial measures.
AQEEL AL MUHARRAQI, ASHISH DESHPANDE and GREGORY M LILBURNE
Bahrain Petroleum Company
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The fluid catalytic cracking unit (FCC) at the Bahrain Petroleum Company (Bapco) is a Model II unit designed by UOP, and was just the second FCC unit to be constructed outside the US. It was started up in May 1945, just three years after the first ever FCC unit came onstream at Exxon’s Baytown refinery, Louisiana, US. Bapco’s unit is one of very few original Model II units which remain in service today.
Bapco’s FCC unit was originally designed and operated as a two-stage conversion unit to produce high octane aviation gasoline. The original design capacity was 18 000 b/d. The unit underwent a major revamp in 1953, as a result of which the capacity was increased to 30 000 b/d with two reactors in parallel, at a conversion of 45 vol%. Over the next 10 years, the sustainable maximum capacity was gradually increased and by the mid-1960s it was about 36000 b/d, although conversions remained low, in the range 45 to 50 vol%, to meet Bapco’s objective to maximise diesel production. By early 2000, the rated capacity of the FCC unit was 43 000 b/d and the feed was mainly isomate, which was unconverted oil from the HVGO mild hydrocracking unit (2HDU).
In 2004, the FCC unit underwent a major revamp as part of the FCC unit Resid Processing Project. The main objective was to process heavy oil, such as vacuum residue and black slop gasoil (BSGO), from the upstream vacuum distillation units, using the latest available technology. The revamp included converting the unit to a single reactor unit, and installing state of the art reactor packed stripper and mix-zone temperature control (MTC) on the feed riser, which allowed processing of heavier feedstock and ultimately resulted in a reduction of the refinery fuel oil pool through the conversion of the heavy feed to lighter, more valuable products.
Throughout the history of Bapco’s FCC unit, the regenerated catalyst standpipes (RCSP) have experienced loss of fluidisation events with catalyst circulation instability. These RCSPs are unusually long at 125 ft, making them very sensitive to slight changes in aeration rate, catalyst flux and catalyst properties. Single reactor operation together with the MTC, which were part of the Resid Processing Project, resulted in an increased catalyst circulation rate in the RCSP, which has significantly improved the catalyst flux and stability, and the RCSP has now become much less sensitive to variations in catalyst properties and aeration air rates.
In 2007, the new and revamped units associated with the Low Sulphur Diesel Production Project came onstream. A new HVGO hydrocracking unit (1HCU) was commissioned and 2HDU was revamped from a mild HVGO hydrocracking unit to an ultra low sulphur diesel hydrotreater, which significantly reduced feed availability to the FCC unit. At that time, the available feed to the unit was 25000 b/d, and it was a mix of unconverted oil from 1HCU and ‘surplus’ HVGO from the refinery’s HVGO pool. This shortfall in the FCC unit feed was an opportunity to process more heavy oil, consistent with the objectives of the Resid Processing Project.
In late 2011, Bapco’s lube base oil unit (LBOU) was commissioned, which processed the unconverted oil from 1HCU. This resulted in another substantial reduction in the FCC unit feed rate, and the opportunity to further increase heavy oil inclusion. The unit is now operating continuously in resid processing mode, at 13 000 b/d, with more heavy oil being processed than ever before. The heavy oil inclusion in the feed is as high as 60%, compared to 8% 10 years ago, with feed Conradson carbon as high as 6.5 wt%, and the metals content on the circulating catalyst typically 10 000 wppm. This is the most severe feed in the MENA region for a FCC unit operating without a catalyst cooler. The main benefit has been a further sustained reduction in refinery fuel oil production and increased refinery profit.
As expected when processing heavier feed with a higher sulphur content, the hydrogen sulphide (H2S) content in the FCC unit off-gas increased. However, although rigorous HAZOP reviews and management of change were conducted as part of the Resid Processing Project, the project team did not appreciate the impact of the higher H2S content in the FCC unit off-gas. It was not envisaged that the higher H2S content would ultimately lead to hydrate formation in the off-gas lines downstream of the gas absorber, causing repeated blockages and process upsets. These incidents reduced process unit availability, resulting in financial loss and putting process equipment at potential risk due to overpressure.
FCC unit process
The FCC unit raw oil feed, which is mainly HVGO, is pumped through the feed preheat system where it is heated by the hot product streams from the main fractionator (C601, see Figure 1). The preheated raw oil feed is then mixed with the heavy oil feed, and the combined feed stream is injected into the reactor feed riser, with atomising steam, through five feed nozzles. A recycle stream of heavy catalytic naphtha (HCN) product is also injected into the feed riser, to increase the riser mix zone temperature, reduce delta coke and allow processing of more heavy oil.
The reactor overhead vapour, which is a mixture of hydrocarbon product vapour and steam from the feed riser and the reactor stripper, flows into the bottom of C601, where it is partially condensed and is fractionated into heavy cycle gasoil (HCGO), light cycle gasoil (LCGO), whole catalytic naphtha (WCN), and wet gas.
The WCN and the wet gas are routed for processing in the gas concentration unit (GCU). WCN is pumped to the GCU feed drum (V509). Wet gas, which is saturated with water from the FCC unit reaction section and the stripping steam used in the C601 HCGO and LCGO side strippers, is also routed to V509, via the wet gas compressor (WGC).
FCC unit off-gas system
The gas from V509 flows into the GCU absorber (C701) and contacts lean oil (unstripped LCGO), which absorbs the heavy hydrocarbons from the gas and recycles them to C601 (see Figure 2). The naphtha from V509 is routed to the depropaniser column (C702) to remove light hydrocarbon gases, before being processed in a series of distillation columns to recover LPG and light, medium and heavy naphtha products.
C702 operates at a normal pressure of 260 psig. C702 off-gas pressure is let down across a pressure control valve (PV3) to the C701 overhead pressure of 135 psig. The C702 off-gas stream is combined with the off-gas from C701, and the combined off-gas pressure is let down through the C701 pressure control valve (PV1) to 67 psig before being routed to the combined off-gas knock-out drum (V738). Off-gas from V738 flows to the olefinic gas treating unit for H2S removal and then into the refinery fuel gas system.
The pressure downstream of PV1 is essentially constant because it floats on the fuel gas system pressure. The off-gas also undergoes Joule-Thomson cooling as it is throttled across PV1.
Any sudden disruption to the off-gas flow rate may cause an upset in the fuel gas system and upsets in the downstream fired heaters.
Off-gas compositions and H2S
The H2S content in the combined off-gas increases as the sulphur content in the FCC unit feed increases; the greater the heavy oil inclusion in the FCC unit feed, the higher the H2S content in the off-gas. After starting up 1HCU in 2007, the FCC unit off-gas’s H2S content increased from 3 mol% to 4.4 mol% and it further increased to 8.5 mol% after commissioning of the LBOU in Q3 2011. A summary of the FCC unit feed and off-gas qualities is shown in Table 1.
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