LARTC 2026
Mitigating fouling in a DCU sponge absorber
An Indian refinery operating a 2.8 MMTPA delayed coking unit (DCU) includes a gas concentration section with a sponge absorber to maximise light hydrocarbons recovery, essentially liquefied petroleum gas (LPG) from coker gas, using light coker gasoil (LCGO) as the absorbent (i.e., lean oil). The sponge absorber tower has been fac- ing operational challenges since initial unit start-up, such as fouling build-up in the mass transfer components, high pressure drop (ΔP), lean oil carryover, and slippage of light hydrocarbons into the overhead fuel gas, causing frequent unplanned shut- downs and loss of production. The main troubleshooting goal was to recover sponge absorber operating through- put of 25 TPH coker gas processed at the tower while preventing lean oil carryover; minimising pressure drop across the inter- nals; minimising C3+ slippage to the fuel gas; and a run length at least equal to origi- nal design. In view of a future coking section capacity expansion, the feasibility of pro- cessing more coker gas from the current 25 TPH to 32.2 TPH was reviewed by Sulzer. Major issues The sponge absorber was originally designed to process a coker gas flow rate of 20.8 TPH, well aligned with process require- ments at the time of installation. However, operational demands have increased signif- icantly over the years, necessitating higher throughput and improved performance. Several trials were carried out to push sponge absorber capacity up to a feed gas flow rate of 25 TPH. However, the system exhibited several critical limitations. To correct these limitations and make the tower suitable for processing an even higher coker gas flow rate (32.2 TPH), a 2018 revamp retrofitted the existing con- ventional floating valve trays to fixed valve trays. However, the planned capacity increase could still not be achieved. Tower performance remained below expectation, even at a capacity marginally higher than 25 TPH. Giuseppe Mosca Sulzer Italy srl Unmesh Bhagwat Sulzer India Private Limited Prasanna Ramani Sulzer Chemtech USA Inc
ily consisted of inorganic solids that could accumulate on the tray deck. The acid-insoluble fraction typically con- tributes to hard scale formation, while the iron oxide content promotes the forma- tion of hard fouling layers that could stick on the tray deck. It was concluded that the reduction of the free passage area at the installed trays, responsible for the exces- sive pressure drop and hydraulic tower limitation, was mainly due to fouling accu- mulation on the tray deck. It was proposed to upgrade the existing fixed valve trays with Sulzer’s high-perfor- mance VG AF™ trays ( Figure 2 ) equipped with the XVG™ valve, combined with a push valve to enhance the liquid plug-flow regime; removal of stagnant zones; min- imised residence time on the tray deck; and an enhanced outlet weir to minimise solids accumulation on the tray deck. The unit was shut down in the 2021 turna- round. Inspection of the sponge absorber revealed significant fouling deposits on the tray deck. Plant feedback The performance guarantee test run was successfully conducted in March 2021. The unit was running satisfactorily for more than four years without any symp- toms of fouling accumulation on the tray deck. The upgraded sponge absorber can safely process a coker gas flow rate higher than 30 TPH (i.e., 20% higher than pre- revamp) with a lean oil flow rate of 22 m³/h, for the highest LPG recovery and the high- est fuel gas quality. The ΔP ranging around 0.23 kg/cm² (almost one-third of that before revamp) is well within the calculated values for the current throughput. The ultimate capacity is not yet tested because the coking sec- tion has not been pushed further. Trend lines show no potential bottleneck, while the hydraulic rating of the VG AF trays provides spare capacity for process- ing at least 10% additional coker gas and an extended unit run length. Furthermore, C3+ recovery from coker gas increased along with improved fuel gas quality.
Bypass
Flue gas to amine absorber Lean oil from main fractionator
Stabilised naphtha from debutaniser
Wild naphtha from main fractionator
Sponge absorber
Primary absorber
Bypass
Coker gas from wet gas compression
Rich oil to main fractionator
To stripper
Figure 1 Process flow diagram of primary and sponge absorbers
Enhanced outlet weir
Push valve
XVG valve
Figure 2 Sulzer VG AF High Performance Anti-Fouling trays
pushed up to its hydraulic limitation, was made available to Sulzer. The data included feed gas flow rate and composition (gas chromatographic analysis); lean oil flow rate and composition (specific gravity and distillation curve); and pressure and tem- perature profile. A process simulation based on plant data matched the existing running tower perfor- mance. Vapour and liquid loadings across the sponge absorber were also provided, enabling accurate hydraulic rating of the retrofitted fixed valve trays. The sponge absorber ΔP measured at the plant was as high as 0.6 Kg/cm² across 21 fractionation trays, almost three times Suzer’s calculated value, indicating a severe reduction of the free passage area at the installed trays. The column was hydraulically flooded at a high ΔP, preventing further lean oil feed to the tower. In this case, the flooding was a
phenomenon where liquid was accumulated on the trays and could not flow down to the next tray. The lean oil was routed to the bot- tom rich oil stream, bypassing the tower. Consequently, no LPG could be recovered. It was lost to the overhead stream, deterio- rating fuel gas quality ( Figure 1 ). As a fast-track remedy, the refinery tem- porarily bypassed the column, shutting off lean oil circulation, and off-gas from the primary absorber was routed directly to the amine absorber. Water washing of the fractionation trays was performed, and normal operation was restored after a few hours. Fouling materials High loss of ignition (LOI) at 800°C was observed, which highly contributed to coke-like deposits. The ash content was also significantly high, meaning that after organics combustion, the residue primar- PUBLISHED BY Publisher of PTQ/Digital Refining/ Decarbonisation Technology Business Development Director Paul Mason tel: +44 7841 699431 sales@petroleumtechnology.com
Troubleshooting The existing running tower operating data,
Contact: unmesh.bhagwat@sulzer.com
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