Revamps 2022 Issue

results, good agitation plays a very important role. The cleaning solution should be pumped at a high velocity so that dissolved components are well mobilised, preventing precipitation and fouling. With the steaming method (see Figure 1 ), a correspond- ingly smaller amount of the cleaning additive is continu- ously fed into the steam over 8 to 14 hours. To avoid thermal decomposition of active substances, the steam pressure should be <10 bar. It is recommended to dilute the cleaning additive with water, which is then dosed via the steam path and can thus be distributed more quickly on the distillation trays or packings. Case study 1: Unifining unit - aromatics plant A hydrofining unit, part of the aromatics plant, was shut down for maintenance and inspection. Previously, the plant had been flushed several times with water and treated for days with hot steam to remove the aromatic components such as the carcinogenic benzene as much as possible. Past success had always been moderate because benzene could not be removed completely from the oxidiser and, therefore, the vessel could only be entered with increased high safety efforts and an autonomous air respirator. Previously, plant management decided not to take such a risk and did not enter the oxidiser. Now the aim of the chemical cleaning and decontamination was the removal of sludge residues, complete removal of pyrophoric iron sul- phide, and elimination of hazardous gases such as H 2 S and benzene. If possible, no autonomous air respirator equip- ment should be required. For this purpose, the oxidiser was emptied according to schedule, rinsed with water, and steam applied to remove part of the benzene. After a mixing tank had been con- nected to the points preceding it using flexible pipes, about 30 m 3 of water was pumped in via the mixing tank to fill the system and heated to 95°C with MP steam. After reaching this temperature, 140 kg of Turbodispin D80 was dosed and the circulation was maintained for seven hours.

Steam

To scrubber

Oxidiser

Air

Steam

Mixing tank

Figure 2 Oxidiser cleaning circuit

rinse for some minutes with clean water to remove surface impurities. Afterwards, chemical cleaning can then be car- ried out. In practice, the recirculation method or the steam- ing method are well-known technologies for cleaning and decontamination procedures. Using the recirculation method on a closed-loop basis, 20-30% aqueous washing solution of the total volume is usually introduced at distillation columns and circulated from the top towards the bottom for at least 8 to 12 hours. Either warm condensate is used as wash water or the wash water is heated to 60 to 80°C with steam. Between 0.5 and 2.0 wt% of the cleaning additive is completely dosed into the wash water within 30 minutes, according to the amount of wash water supplied. As a rule of thumb, a warm 60-80°C chemical aqueous dilution with high fluid velocity will give far better results than a cold cleaning solution with poor agitation. The chemical cleaning solution should disperse the sludge into the aqueous diluent, providing maximum hydrocarbon recovery, minimum solid waste for disposal, and accomplish the job in the shortest possible time. To achieve excellent

Time

H₂S

NH₃

LEL (%)

VOCs (ppm)

Temp. (ºC)

Additional information

(ppm)

cleaning solution

03:00 05:00 07:00 09:00 12:00 14:00 16:00 19:00 21:00 23:00 01:00 03:00 05:00 07:00

10 14 10 12

12 10

2 5 6 8 2 5 8 7

238 320 350 380

3 2 5 2

78 90 92 94 68 85 95 95 96 95 94 96

Cleaning with

8 4

Turbodispin D80

Drainage to sour water tank

2 2 3 2

0 0 0 0 0 0 0 0

38 70 76 37 22

2 6

1 1

Cleaning with Kurita CD-5201

0 0 0 0

0 0 0

8 4

Drainage to sour water tank

Table 1 Oxidiser analytical results cleaning

Revamps 2022