During planned shutdowns, the equipment in refineries and petrochemical plants has to be checked. The downtime can require several weeks when tanks, vessels, distillation columns, and pipes have to be drained and cleaned for i maintenance or repair. Shutdowns are labour-intensive and require accurate scheduling and organisation between all projects, as often hundreds of workers are on-site. A variety of materials such as greases, tars, ratty oils, polymers, or coke and iron sulfide deposits have to be removed. There is a particular duty of care to protect the health of personnel when carcinogenic benzene, toxic hydrogen sulfide, volatile hydrocarbons, or other hazardous gases can be released. Senior level personnel try to minimise exposure to any situation where health risks or accidents could be initiated. This article describes chemical cleaning and decontamination technologies to achieve a significant reduction in downtime and health hazards.
Coke fouling on furnace tubes can be taken off by controlled ccmbustion with steam and air, but higher amounts of coke deposits, or polymers, have to be removed manually. Iron sulfide (FeS) is one of the most common fouling materials in oil refineries. It easily accumulates in heat exchangers, vessels, pipes or columns equipped with valve trays or packings. If FeS deposits are not removed, they must be kept wet all the time. Otherwise there is a high potential for spontaneous ignition in the presence of oxygen due to its pyrophoric iron nature. FeS oxidises exothermally when in contact with air, so most pyrophoric iron fires occur during shutdowns as this is when distillation equipment is open for maintenance and inspection.
There are different possibilities to form FeS during operation and the term 'iron sulfide' covers a range of crystallographic forms, which are both non-magnetic and magnetic.
The reaction (1) describes the conversion of iron oxide to FeS in an oxygen-free atmosphere and presence of hydrogen sulfide (HjS). Reactions (2) and (3) occur as a corrosion product between iron (Fe° or Fe1*) and hydrogen sulfide species:
Fe2O3 + 2H2S --> 2FeS + 3H2O + S (1)
Feo + H2S --> FeS + H2 (2)
Fe2+ (aq.) + H2S --> FeS + H2+(aq.) (3)
Reactions (4) and (S)can occurwhen vessels, columns, heat exchangers or pipes are opened for maintenance and inspection. Due to its pyrophoric nature, it is a rapid exothermic oxidation process and will ignite in the presence of combustible hydrocarbons. Therefore, it is of great importance to avoid self-ignition of the accumulated fouling material that could cause a fire.
4 FeS + 302 --> 2Fe2O3 + 4S (4)
4 FeS + 702 --> 2Fe2O3 + 4SO2 (5)
Chemical programmes are required when the equipment has to be cleaned as quickly as possible to minimise downtime costs. In most cases, wash water is used as the diluent and additional solvents such as dlesel or light cycle oil (LCO) are only required if that is part of the normal cooling process prior to the cleaning and decontamination process. Many refinery customers renounce the use of acid cleaning, chelating agents or strong oxidising chemicals as they can cause numerous threats, such as unwanted chemical reactions or corrosion on the steel equipment, while benzene, hydrogen sulfide or volatile gases cannot be eliminated simultaneously.