Revamps 2022 Issue

Time

H₂S

NH₃

LEL (%)

VOCs (ppm)

Temp. (ºC)

Additional information

(ppm)

Cleaning solution

13:00

Preperation of the cleaning solution with circulation and heating by means of steam

16:00 17:00

0 0

48 20

4 9

Reaching the recommended minimum temperature

18:00 19:00 20:00 21:00 22:00 23:00 00:00

0 0 0 0 0 0 0

0 0 0

4 0 0 0

67 67 66

5 3

0 0 0

180

76 70 70

0 0

Finishing the chemical cleaning

Table 2 Analytical results absorber cleaning

Serving as a dispersant, the additive brought petro- chemical impurities into a form suitable for blowdown so that they can be easily removed. Starting at 10:00, some contaminated cleaning solutions were drained, and the cir- culation was refilled with 25 m 3 of water. After these dirty cleaning solutions were completely dried up, the system was flushed with 5 m 3 of water to remove the remaining dissolved sludge. The system was then refilled with 30 m 3 of water and brought to a temperature above 90°C. At 15:45, 400 kg of Kurita CD-5201 was dosed and cir - culated through all flexible hose routes. At 21:00, another 200 Kg of product was dosed. At 1:00, another 200 kg was injected, and the cleaning solution was circulated until 6:00. The cleaning was then completed. The dirty cleaning solu- tion could be drained at 7:00 to flush the remaining resi - dues with some water afterwards. The subsequent control measurements showed no ben- zene or VOCs were detectable after the last flushing with water. This was the first time after 30 years of operation that the oxidiser could be entered directly. No autonomous air respirator equipment was required. Case study 2: Merox unit cleaning Mercaptans are undesirable components, impairing prod- uct quality. Sulphur compounds are separated in the Merox process and oxidised with excess air to form alkyl disul - phide. During a planned shutdown, a Merox plant was chemically cleaned, with a special scope on cleaning the extractor column. It is filled with Raschig rings and still contained high concentrations of VOCs after water wash- ing or steaming. For chemical cleaning of this process unit, three independently operating circulation circuits were set up to clean the metal surfaces and remove pyrophoric iron sulphide and hazardous gases such as H 2 S, mercap- tans, benzene, and VOCs. Overall, total columns and ves- sel volumes were not large, so small wash circuits were used. The numbers in brackets describe the total volume of the columns in each case and, based on one-third of the total volume, a 2 wt% aqueous Kurita CD-5201 solution was prepared. Adding medium pressure (MP) steam, the

cleaning circuits were operated in a temperature range of 60-80°C.

Absorber (54 m 3 )

Circuit No. 1: Circuit No. 2:

Pre-washer (35 m 3 ) Extractor (47 m 3 ) and filter (28 m 3 ) Accumulator drum (35 m 3 )

Oxidiser (7 m 3 )

Circuit No. 3:

All three cleaning cycles were started on the same day, but since only one column of the cleaning solution was applied to circuit No. 1 (absorber) and circuit No. 3 (oxi - diser), these cleanings were completed after about 10 hours of treatment. The benzene and LEL concentrations were 0% after drainage and flushing with clean water at the end. VOC concentrations were brought down from peak values of 140 and 180 ppm, respectively, to values <1 ppm. H₂S and ammonia gases were not detected during chemical cleaning due to standard pre-washing and steam- ing of the systems. Circuit No. 2 was more complex, with several columns and pipelines. During the preliminary planning, it was decided to flush the pre-washer and extractor together with the cleaning solution over 8-10 hours. In practice, it turned out that it was not technically possible to keep the level in the mixing tank stable if both columns were flushed with clean - ing solution at the same time. After 11 hours – due to the interruption for exclusive treatment of this column – clean - ing of the extractor was completed (see Table 2 ). The unpolluted solution from the extractor was passed on to the pre-washer, and this part was also flushed for several hours. After about eight hours of circulation, the cleaning solution was then transferred to the sand filter system. After approximately 10 hours, this section was also properly cleaned, and the cleaning solution was finally passed to the accumulator drum and circulated for five hours. After that, the used unpolluted solution was drained, and the system was flushed with clean water. In total, due to the interruption, the cleaning of the entire Merox system

Revamps 2022