Chemical cleaning and fouling removal technologies

Chemical cleaning technologies for the removal of hazardous gases and problematic fouling deposits are explained.

Berthold Otzisk and Luis Del Castillo

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Article Summary

A refinery or petrochemical plant shutdown can last several weeks if storage tanks, vessels, distillation equipment, and pipework have to be cleaned extensively. The reduction of cleaning and shutdown times helps minimise cleaning and maintenance costs. The safety of the employees who have to enter the columns, vessels, and tanks is always the top priority. Under no circumstances should they come into contact with hazardous gases or other dangerous substances. The use of chemical cleaning programmes is part of very well-developed safety concepts. Chemical cleaning and degassing concepts have proven very successful in recent years.

Cleaning and decontamination
A shutdown is a very labour-intensive time and requires perfect organisation and scheduling. Often hundreds of workers are on site for mechanical cleaning, repair, or replacement of equipment, where every delay means high production losses. Persons in charge try to minimise exposure of workers to any situations where health risks or pyrophorically induced fires could be initiated. The formation of iron sulphide (FeS) is very common in oil refineries and ethylene production plants.

Iron sulphide deposits accumulate in distillation columns, pipes, trays, structured packings, vessels, and heat exchangers. During shutdowns, there is a high potential for spontaneous ignition in the presence of oxygen due to the pyrophoric iron sulphide nature. Keeping such deposits wet to avoid unwanted fire hazards is not a good option. Pyrophoric materials must be eliminated and removed safely.

Meanwhile, very efficient cleaning and degassing programmes have been established where chemical additives can be used to clean the equipment quickly in a proper way. Pyrophoric iron sulphide species are neutralised and eliminated, reducing the risk of unwanted ignition in contact with the ambient air. Distillation columns and heat exchangers no longer need to be mechanically cleaned, and packings can often remain in the columns during shutdowns if they are clean and no longer contain pyrophoric iron sulphides.

Dangerous emissions, including benzene, H2S, NOx, CO2, SOx, and VOCs, can be reduced to a minimum. This is becoming increasingly important in today’s world. The high partial pressure of light hydrocarbons makes it more difficult to separate them from other volatile gases in the vapour phase. To trap (scavenge) light hydrocarbons into the wash water, the gas molecules need to be polarised. This polarisation can be achieved by electric induction or by using chemical cleaning programmes. Downtimes can be shortened, resulting in higher productivity, reliability, and operability.

Chemical cleaning methods
After distillation columns, vessels, heat exchangers, and piping have been drained for cleaning, it is common to rinse for some minutes with clean water to remove surface impurities. Afterwards, chemical cleaning can then be carried out. In practice, the recirculation method or the steaming 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 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 correspondingly smaller amount of the cleaning additive is continuously 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 sulphide, and elimination of hazardous gases such as H2S and benzene. If possible, no autonomous air respirator equipment 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 connected to the points preceding it using flexible pipes, about 30 m3 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.

Serving as a dispersant, the additive brought petrochemical 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 circulation was refilled with 25 m3 of water. After these dirty cleaning solutions were completely dried up, the system was flushed with 5 m3 of water to remove the remaining dissolved sludge. The system was then refilled with 30 m3 of water and brought to a temperature above 90°C.

At 15:45, 400 kg of Kurita CD-5201 was dosed and circulated 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 solution could be drained at 7:00 to flush the remaining residues with some water afterwards.

The subsequent control measurements showed no benzene 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 product quality. Sulphur compounds are separated in the Merox process and oxidised with excess air to form alkyl disulphide. 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 washing 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 H2S, mercaptans, benzene, and VOCs. Overall, total columns and vessel 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.

Circuit No. 1:     Absorber (54 m3)
Circuit No. 2:     Pre-washer (35 m3)
        Extractor (47 m3)
        and filter (28 m3)
        Accumulator drum (35 m3)
Circuit No. 3:     Oxidiser (7 m3)

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