1 Introduction

During shutdowns at oil plants, processing and storage equipment are regularly examined. If storage tanks, processing units, production lines, and heat exchangers need to be drained, cleaning processes might take several weeks. Hazardous chemicals and hydrogen sulfide gas, as well as concentrations of combustible iron sulfide and other contaminating material, can be extremely hazardous, which is why they must be removed before the examination. Conventional mechanical washing techniques have been employed to clean flammable hydrocarbons and hydrogen sulfide gases for several years, including medium and high-pressure water jetting, steam operations, and hot or cold water cleaning techniques. Unfortunately, enormous amounts of water are needed, and the wastewater produced might have a high Total Organic Carbon. Stronger chemical cleaning and degassing chemicals have been developed in response to regulatory, safety, and environmental requirements to save downtime by minimising the need for mechanical cleaning.

The technology used and the cleaning technique used will be determined by the machinery to be cleaned. Chemical cleaning of a crude oil tank or sludge oil system employing external mobile heat exchangers, for example, will take longer than chemical cleaning of a distillation column or heat exchanger, which may be done using high-pressure cleaning equipment. Chemical cleaning removes organic and inorganic fouling particles from processing lines, distillation equipment, and metal surfaces using a mix of solvents, fluid velocity, and, in most cases, heat. The heat will be supplied by steaming, hot water injection, or heating the metal surfaces. At the same time, the fluid velocity will be provided by spraying, stirring, circulation, steam, or air addition, and the solvency will be determined by the chemical composition of the cleaning agents and their concentrations. [1]

2 Categories of cleaning chemicals

Inorganic and organic acids, chelating and complexing agents, alkaline cleansers, surfactants, organic solvents, and speciality additives are the most common chemicals used to clean equipment. Common acid cleaning chemicals such as inhibited sulphuric acid and hydrochloric acid, phosphoric acid, citric acid, or other weaker acids remove oxides and scales. The metal surface can be cleaned slowly or quickly by changing the chemical concentration, temperature, and agitation. Concentrated solutions and greater temperatures speed up the cleaning process, but they also produce hazardous and corrosive fumes, as well as an increased risk of corrosion. [2]

Caustic cleaning chemicals are less efficient and cost less than acid cleaning agents, but they are better at removing organic soils, such as oils and grease, than acids. Because of their detrimental environmental effects, chelating and complexing additives comprising EDTA, NTA, sodium diethanolglycine, or ethylenediamine play a lesser role in refinery cleaning operations. Surfactants can be anionic, nonionic, or amphoteric, and they’re sometimes used with organic solvents to boost their cleaning efficacy.

3 Degassing Chemicals

Cleaning chemicals are often used to remove scale and foul (such as viscous fouling and pyrophoric iron sulfides) from metal surfaces. In addition to these criteria, cleaning and degassing chemicals for refinery and petrochemical processes are developed to reduce benzene and other toxic hydrocarbons in the vapour phase, as well as hydrogen sulfide. These chemicals are usually injected into a liquid phase, which necessitates the hydrocarbon phase being drained first. Cleaning and degassing chemicals are typically planned for injection into the wash water phase at temperatures ranging from 60 to 100 degrees Celsius, with circulation periods ranging from 2 to 16 hours. For removing hydrogen sulfide and reactive hydrocarbons, certain cleaning chemicals generate a brief emulsion. Inside the API separator or a separate holding tank, the emulsion will eventually break down on its own. After the cleaning step is done, an emulsion breaker can speed up the separating process. Because the requisite storage tanks are employed for other uses during scheduled downtimes, and some oil companies reject to apply chemical cleaning procedures that form emulsions, the periodic storage of cleaning chemicals for emulsion breaking is limited in many circumstances. However, some chemical cleaning procedures have been tasked with providing cleaning and degassing chemicals that produce a transient emulsion and have no emulsification propensity. [ 3]

Figure 1 Degassing Process

4 Chemicals to remove flammable deposits

Refiners are highly interested in eliminating or minimising iron sulfide formations because of the volatile nature of iron sulfide and the possibility of accidental fire. Although certain chemicals may eradicate tiny, isolated iron sulfide deposits, they have limits. They are frequently less successful when dispersed across the distillation apparatus’s packing material.

Because the cleaning chemicals are designed to remove flammable compounds and hydrogen sulfide, they may approach their limitations if increasing levels of ferrous sulfide deposits are predicted. As a result, a solution for removing iron sulfide deposits quickly and effectively has been created. Because this composition does not create hydrogen sulfide or solid reaction products, it is appropriate for iron sulfide dissolving applications. It can also be used with certain unique hydrogen sulfide and benzene removal chemicals to offer a successful, all-around chemical treatment procedure. [4]

Figure 2 Iron Sulfide Deposits

5 Tank cleaning chemicals

Sludge constantly collects oil products and heavy crude storage tanks at the bottom. Traditional cleaning procedures might take many days or even weeks when these store tanks are drained and cleaned. As a result, new chemical cleaning procedures have been devised to minimise the time it takes to remove sludge from the tank.

In the oil refining sector, there are a variety of conventional cleaning processes, and every cleaning method may be classified according to the cleaning procedure. Some cleaning techniques may necessitate using a chemical in conjunction with medium to high-pressure water jetting. In contrast, others will necessitate using a chemical in conjunction with transportable heat exchangers, hot water, gasoline,  petroleum chelating agents, or other mechanical techniques. It is critical to clarify needs and check restrictions ahead of time in all circumstances.

A thorough tank inspection and lab testing can be used to assess the efficacy of the chosen cleaning chemical. Because huge storage tanks are rarely homogeneous, samples should be gathered from as many locations as feasible. A cleaned tank is one in which the floor is free of hydrocarbon and debris, as well as the tank has no gas. The selected chemical cleaning approach may not be suitable in some exceptional instances when catalytic sludge has been kept in the tank. If there are large volumes of combustible iron deposits in the tank, the cleaning chemicals have limitations. In such circumstances, specialist chemicals for eliminating combustible iron sulfide will get over this problem.

6 Distillation equipment cleaning chemicals

While clean-up techniques may need chemical dosing pumps, portable heat exchange equipment, high-pressure water jetting, hot water, gasoline, or diesel chelating agents, the use of the chemicals for a tower cleaning and degassing operation is relatively straightforward. The heat exchangers and compressors in the refinery can be employed to supply heat and fluid velocity during the shutdown phase. The process conditions determine how the chemical is applied to the system.  It isn’t always practical to install a mechanism that allows the chemical to be injected into the system.

It is not always practical to build a method that allows the chemical to be pumped back into the system. As a result, additional chemicals and wash water are necessary. In this scenario, the chemical must be added to the washing water at the top of a distillation process or reflux column (cascading method). Additional instability on the plates or packing can be achieved by adding steam. The chemical will be washed down the drain with the washing water.

The most effective approach is the circulation technique, which allows the chemical to be circulated back into the system, lowering chemical expenses. First, the items must be drained. Hydrocarbon or water is used to wash and fill the system, based on the composition of the cleaning chemical. Firstly, the cleaning chemical may be injected at the prescribed temperature range as soon as possible; the unit must be supplied to one-third of its capacity. It’s critical to keep an eye on the cleaning procedure. During the cleaning process, the stream of the cleaning chemical, the temperature conditions, the chemical activity, and the change in colour of the sample were all monitored. The cleaning solution should be emptied, and the system cleansed again after cycling for a few hours (cleaning time varies depending on the chemical and process conditions). In some circumstances, the chemical must be dispensed constantly in the vapour phase (steaming technique) for aqueous applications, requiring condensing in the distillation tower. Small pumps can keep the chemical flowing at a steady rate.

While the circulation approach is already the most cost-effective, it has another significant benefit. The chemical may be injected at various points throughout the system, reducing the time it takes for the chemicals and the wash liquid to combine. Small, strong pumps could administer the chemical in places where an electric pump would be difficult to install.

The entire duration for one wash cycle will vary depending on whether simply chemical washing of the distillation apparatus is necessary or whether flammable hydrocarbon or hydrogen sulfide must also be removed. Discharging the distillation byproducts, rinsing with an appropriate wash liquid, refilling with the wash solvent, injecting the chemicals, recirculating for a few hours, and discharging within three to four hours are all part of a cleaning cycle. It is usually preferable to apply a 1 per cent dilution thrice than a 2per content dilution once in cleaning efficiency. Due to time constraints during turnarounds, however, this is not always possible. There should be no combustible iron deposit, combustible hydrocarbons, or hydrogen sulfide once the maintenance holes and pipework have been opened. [5]

Figure 3 Cleaning Process

7 Fuel gas system cleaning chemicals

A refinery typically has a large number of fuel gas combustion equipment. They’re linked to a major fuel gas distribution network that gathers gas streams from various refinery areas and routes them to refining heaters or furnaces. The fuel gas’s composition and quality vary depending on the crude oil, but the primary constituents are methane, ethane, and ethylene, along with surplus hydrogen. Carryover of hydrocarbon condensate to the fuel gas burners is typical, and the hydrocarbon condensation can flow into the firebox, posing a dangerous condition. Ammonia spillover with fuel gas is also prevalent. The possible creation of ammonium salt deposits, in addition to the condensing water or heavy compounds in the fuel distribution network, is regarded as a key element for the fuel gas system’s continuous and efficient functioning. [6]

Block burner nozzles take four to five person-hours of mechanical cleaning, and a typical burner cleaning cycle lasts three to four months. Mechanical cleaning might cost a hundred million euros per year if you have 30–50 heaters with many burner nozzles. Even though ammonia salts are water-soluble, steam-cleaning treatments are typically ineffective. Advanced chemical cleaning is an alternative to mechanical and steam cleaning, in which the advanced chemicals are injected into the fuel gas system to eliminate ammonium salt deposits. The fouling material may be removed in a matter of minutes, restoring the burner flames’ appearance and extending the burner tips’ life.

8 Evaluation criteria of the chemicals

A proper assessment and lab tests before and after tank cleaning helps assess the effectiveness of the cleaning process. It’s critical to learn about the bottom sludge’s composition, which might include oil, water, volatile substances, ash, and sediments. The amount of water and sediment in crude oils is important since it might cause the distillation equipment to corrode. To assess the net amounts of natural oil in sales, an assessment of bottom sediments is necessary. Recovered oil may be sent to the refining operation using the cleaning technique, improving refinery profit.

When adequate samples are collected, the optimum results are produced. Because the oil tanks are not homogeneous, samples should be gathered from as many locations as feasible. When it comes to taking samples from oil tanks, many refineries employ a variety of sample takers. The water intake can be utilised for sampling if there are no sample points are. Any sludge-like substance can be collected when the valve is opened. Although this selection method may not be accurate, it is preferable to take no sample at all.

Distillation equipment will need to be chemically cleaned for around 10 hours. SamToSamco monitors the cleaning’s progress; samples can be obtained before and through the treatment or refinery cleaning operations; common characteristics include the concentration of active ingredients, the heat of the washing solvent, colours, acidity, iron content, chloride ions, ammonia, and sulfides. The samples appear to get increasingly unclean during cleaning, and the increase in analysed iron, chlorides, sulfides, and ammonia suggests that the contaminated material is being mobilised. When a mobilised rise in concentrations of these variables slows down, it means the cleaning is coming to a conclusion or that additional or new cleaning chemicals are needed.

The heat-transfer factor and pressure drop in heat exchangers, a sample of contaminated materials, ultrasonic measurement equipment, and previous shutdown reports on observed contamina are useful resources for determining the quantities of cleaning chemicals required. Cleaning is more difficult in tanks and vessels with inadequate agitation and distillation units with packing. It’s a good sign that the right cleaning chemical and techniques were chosen if these locations end up looking very clean.

Figure 4 Distilattion Column after chemical cleaning

9 Conclusions

Heating systems, oil tanks, heat exchange systems, coolers, hydraulic systems, reactors, compressors, piping systems, refinery towers, and cooling systems are examples of refinery equipment often cleaned chemically. Much of the labour involved in a refinery turnaround is maintenance, and the shutdown is meticulously scheduled months in advance. Regulatory, safety and ecological standards increase the demands on oil refineries and other petroleum facilities’ turnaround processes. The selection of the cleaning process and the chemicals depend upon the following critical factors:

  • Material to be cleaned.
  • Waste disposal
  • Application technique
  • Current results
  • Contact time available
  • Chemical composition
  • Work-in-progress
  • Desirable results

The cleaning chemical must be properly chosen. Concentrated solutions of blocked acid cleansers at higher temperatures can produce harmful and corrosive vapours and stress corrosion cracks in high nickel alloys. Only a few scale varieties can be removed using alkaline cleansers, and greater temperatures are necessary.

Degassing and decontamination chemicals, including patented solutions to eliminate combustible iron sulfide deposits, have been created and may be solvent or water-based. Refineries prefer Water-based and solvent-based cleansers that do not generate emulsions. These products will clean metal surfaces more quickly and safely while lowering benzene and other volatile hydrocarbons, hydrogen sulfide, and ammonia levels. This will assist cut down on shutdown time, air pollution, wastage, labour expenses, and environmental costs.

10 References

[1]          R. A. Shank, S. Gharaibeh, T. R. McCartney, and M. Panunzio, “Decontamination of a Vacuum Distillation Unit: Mechanical Versus Chemical Cleaning; A Case Study,” in CORROSION 2017, 2017: OnePetro.

[2]          R. Haydu, W. E. Brantley III, and J. J. R. K. M. E. i. C. L. R. H. Casenhiser, “INDUSTRIAL CHEMICAL CLEANING METHODS,” 2008.

[3]          A. Groysman, “Corrosion problems and solutions at oil refinery and petrochemical units,” in Corrosion problems and solutions in oil refining and petrochemical industry: Springer, 2017, pp. 37-99.

[4]          A. J. K. a. o. m. Groysman, “Corrosion problems and solutions in oil, gas, refining and petrochemical industry,” vol. 61, no. 3, pp. 100-117, 2017.

[5]          B. J. P. t. q. Otzisk, “Chemical cleaning and degassing refinery equipment,” vol. 13, no. 1, p. 77, 2008.

[6]          M. K. Karmakar, P. Chandra, and P. K. J. J. o. E. C. E. Chatterjee, “A review on the fuel gas cleaning technologies in gasification process,” vol. 3, no. 2, pp. 689-702, 2015.