OVERVIEW This case study covers the chemical decontamination of the bottoms circuit of the VDU charge system. This included the base of the Vacuum column and pre-heat feed train exchangers. This work was carried out during the 2019 TAR. CHALLENGES To help reduce the duration of the TAR it w as decided to implement a trial clean of the bottoms circuit of the vacuum unit by circulation of Rezyd- HP G71™ solution in gas oil with the intention of removing fouling thus making it unnecessary to pull the exchanger tube bundles for mechanical cleaning. This would potentially save several days over conventional methods. The vacuum unit was a critical path item in the TAR schedule. The success would be determined by the performance once the unit was recommissioned. PAST PROCEDURE Previously, the bottoms circuit had been flushed with hot gas oil as the system was brought off line and the exchanger tube bundles were pulled, taken to the wash bay to be jetted clean, then re-installed. This was a time-consuming process taking several days to complete and an alternative, shorter duration cleaning solution was desired. The decontamination of the base of the Vacuum column was included in the overall column treatment but this did not fully address the removal of heavier deposits from the base. VacUnit Bottom Circuit TESTIMONIAL " Once the Vacuum Unit was in full operation the plant ran well and it was clear that the Rezyd HP circulation was a success. We are pleased with the results " Area Specialist
No.9 CASESTUDY
GRANGEMOUTH REFINERY
DECON SOLUTION It was agreed with site to conduct a trial procedure by circulating a 2% solution of Rezyd HP G71™ in gas oil at 200-220°C for a period of 18 hours, after which a short flush out with fresh oil would be implemented. During the circulation, evaluations of the circulating liquid was carried out by client to assess any appearance and viscosity changes and strainers on the circuit were monitored to ensure that no blockages or restrictions occured. The bottoms circuit was then left intact and the main column was decontaminated using a standard Zyme-Flow® UN657 Vapour Phase process followed by a Zyme-Ox® Z50 cascade flush of the column beds to neutralise any residual pyrophoric iron sulphides. RESULTS Initially, issues heating up the process stream meant that it took 14 hours to attain a temperature of 185°C and site informed us that it may not be possible to raise the temperature much higher. Rezyd-HP™ had already been dosed and circulated throughout the system during this heat up period and so a decision was taken to continue circulation and maintain this temperature for a further 12 hours to complete the process. Samples taken by site for evaluation showed the solution to be darker and more viscous than in previous gas oil flushes although this was the only means of assessing the cleaning performance. On completion of the circulation and flush of the bottoms circuit a standard Vapour Phase was carried out on the column and at the end of the 12-hour process test results showed H2S and LEL to be zero. A 1% Zyme-Ox® Z50 cascade flush was then performed and on opening the column there were no issues with H2S, LEL or pyrophoric iron sulphides.
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