Oct-2008

Online cleaning of 
process equipment

In the interest of time and cost savings, online cleaning prior to shutdown can reduce the duration and complexity of refinery turnarounds, as demonstrated in the following case studies

Marcello Ferrara 

ITW Technologies

Article Summary

It is possible to online clean a crude distillation unit or another major section of a refinery in just 24 hours, without any airborne emissions or spills or the need for any large pieces of equipment on-site such as a crane. Online cleaning technology should be evaluated on a cost/performance basis rather than a cost/cost basis.

Case study 1
Desalter sludge removal
In a previous turnaround, it took a refiner ten days to make a desalter gas free for workers to enter and manually remove sludge. There was 40 m3 of sludge, so it took an additional four to five days to have it ready for maintenance work. Using online cleaning, however, the desalter was ready for worker entry in two days. At the manway opening (Figure 1), only 1 m3 sludge was found, as 98% of the original sludge had been transformed into a fully reusable product. The original sludge level was above the manway (>120 cm). After cleaning, most of the vessel was sludge free, with less than 5 cm of fluid sludge left on the bottom. In less than one day, the desalter was ready for maintenance/repair work, so online cleaning shaved 12 days off the previous turnaround.

Case study 2
Heavy fouling in slop distillation unit
A slop distillation unit processes, on a campaign basis, the slop collected on-site at a refinery that processes about 280 000 bpd. The facility has four topping/vacuum units, one FCCU, one residue hydrocracker and two ethylene units. After the one- to two-month campaign, the slop distillation unit is shut down for cleaning, as it suffers from very heavy fouling and coke accumulation.

To demonstrate the effectiveness of online cleaning, the unit was run as usual. It was then shut down and opened to check the fouling conditions in the main fractionator and two bottom exchangers. The unit had about 1 m of coke-like sludge at the bottom of the main fractionator, and the fractionator’s trays and bottom exchangers were also heavily fouled.

The online cleaning lasted 24 hours. Degassing/decontamination was then carried out to reduce the steam-out time, eliminate any noxious emissions and pyrophoric ignition. The main fractionator bottom and trays were free of any deposits, and some 1–2 kg deposits were found in the bottom exchangers. However, these deposits were loose and easily removed using the unit’s service water at a pressure of 2–3 bar.

Case study 3
Simultaneous cleaning of vacuum and visbreaker units
Online cleaning was applied to a vacuum unit integrated with a visbreaker unit at an EU refinery. The reason for considering online cleaning was an increase in the visbreaker’s quench temperature, which was limiting throughput. Mechanical cleaning of the four shells (with the hottest shell at the bottom) had not helped to recover the temperature.

After being online cleaned, the two units were started up in 36 hours. The recovery of the quench temperature was about 50°C, and as the units were integrated a recovery of 18°C on the vacuum furnace inlet temperature was also achieved.

Case study 4
Visbreaker vacuum section 
bottom train
Online cleaning was applied to the vacuum section of a visbreaker due to an increase in this section’s bottom train outlet temperature. The refinery usually cleaned the exchangers when the 
outlet temperature was approaching 280°C. It had to mechanically clean two exchangers to reach the targeted run length. However, this did not recover the temperature, so the visbreaker was facing an unscheduled shutdown to clean the vacuum section’s bottom.

To solve the problem, the vacuum section’s bottom was online cleaned in 48 hours. The visbreaker was then started up immediately. The temperature recovery was on average 45°C. The unit could easily reach scheduled turnaround and run for five more months.

Improved unit operations
Scheduling online cleaning while a unit is running or shut down during a turnaround should not only be viewed as an alternative to mechanical cleaning, but also as a tool for achieving operational excellence. The economics of such applications could give 30 times more revenue than mechanical cleaning.