Chemical cleaning and degassing refinery equipment The safe removal of volatile hydrocarbons, hydrogen sulphide gas and pyrophoric iron sulphide from refinery equipment before inspection is indispensable. A chemical cleaning programme reduces downtime and the need for mechanical cleaning Berthold Otzisk Kurita Europe GmbH n petroleum refineries, distillation and Types of cleaning chemicals which requires the draining of the storage equipment frequently needs The most common types of chemicals hydrocarbon phase first. The cleaning I to be inspected during turnarounds. used for the cleaning of equipment are and degassing programmes are Necessary cleaning procedures can inorganic and organic acids, chelating commonly designed for injection into require several weeks of downtime if and complexing agents, alkaline the wash water phase at temperatures storage tanks, distillation columns, cleaners, surfactants, organic solvents between 60°C and 100°C with circulation product lines and heat exchangers have and speciality additives. times of 2–16 hours. Some cleaning to be drained. The presence of volatile For the removal of oxides and scales, programmes create a temporary emulsion hydrocarbons and hydrogen sulphide common acid cleaning agents such as for scavenging the hydrogen sulphide gas, as well as deposits of pyrophoric inhibited sulphuric acid and and volatile hydrocarbons. The emulsion iron sulphide and other fouling material, hydrochloric acid, phosphoric acid, will later break down on its own in the can be particularly problematic, which citric acid or other weaker acids are used. API separator or in a separate holding is why they have to be removed before By varying the chemical concentration, tank. The addition of an emulsion inspection. temperature and agitation, the metal breaker after the cleaning procedure is For many years, traditional mechanical surface can be cleaned slowly or more completed can accelerate the separation cleaning methods such as high-pressure rapidly. Concentrated solutions and process. In many cases, the intermittent and ultra-high-pressure water jetting, higher temperatures will accelerate the storage of cleaning solutions for emulsion steaming procedures and cold or hot cleaning procedure, but leads to the breaking is limited, because the necessary water washing methods have been used generation of dangerous and corrosive holding tanks are occupied for other to remove volatile hydrocarbon and fumes as well as increased corrosion purposes during planned turnarounds, hydrogen sulphide gases. However, large potential. and some refiners refuse to apply volumes of water are required and the Caustic cleaning agents are less chemical cleaning programmes that resulting wash water can be high in TOC. efficient and less economical compared create emulsions. Kurita is asked to Based on regulatory, safety and with acid cleaning agents, but they can deliver cleaning and degassing environmental requirements, more remove organic soils, including oils and programmes that create a temporary powerful chemical cleaning and grease, better than acids. emulsion, as well as programmes without degassing products have been developed Chelating and complexing additives any emulsification tendency. to shorten downtime by reducing the containing EDTA, NTA and sodium need for mechanical cleaning. diethanolglycine or ethylenediamine Removal of pyrophoric iron The applied technology and cleaning play a less important role in refinery sulphide deposits procedure will depend on the equipment cleaning procedures because of their Due to the pyrophoric nature of iron to be cleaned. For example, the chemical negative environmental impacts. sulphide and the potential for cleaning of a crude oil tank or sludge Surfactants can contain anionic, spontaneous ignition, refiners are very oil system using external mobile heat nonionic or amphoteric compounds interested in removing or neutralising exchangers will take longer than the and sometimes they are formulated iron sulphide deposits. Although some chemical cleaning of a distillation together with organic solvents to chemical programmes are fairly effective column or heat exchanger, where improve their cleaning performance. and can eliminate small, localised iron high-pressure cleaning equipment sulphide deposits, they have limitations can be used. The chemical cleaning Cleaning and degassing and are often less effective when the itself is defined as the removal of organic chemicals iron sulphide deposits are spread and inorganic fouling material from In general, cleaning agents are designed throughout the packing material of the process lines, distillation equipment and for scale and fouling removal (ie, viscous distillation equipment. metal surfaces by a combination of fouling, pyrophoric iron sulphides) with If higher amounts of iron sulphide solvency, fluid velocity and, in most metal surface cleaning. In addition to deposits are expected (ie, in packing cases, heat. While the fluid velocity these requirements, the cleaning and areas), the cleaning and degassing comes from spraying, stirring, degassing agents for refinery and chemicals may reach their limits, as they circulation, steam or air addition and petrochemical applications are designed are designed mainly for volatile the heat will be supplied through for the reduction of benzene and other hydrocarbons and hydrogen sulphide steaming, hot water injection or heating volatile hydrocarbons, as well as removal. For this reason, a formulation up the metal surfaces, the solvency hydrogen sulphide in the vapour phase. for providing fast and efficient removal depends on the chemistry of the These chemical programmes are of iron sulphide deposits has been cleaning agent and its concentration. typically injected into an aqueous phase, developed. This formulation does not www.eptq.com PTQ Q1 2008 77 chemical in combination with high- pressure or ultra-high water jetting, while other techniques will require the use of a chemical in combination with mobile heat exchangers, hot water, gas oil or diesel diluents, or other mechanical technologies. In all cases, it is important to specify requirements and verify limitations in advance (ie, availability of water, heat, diluents such as gas oil or diesel). The most efficient chemical cleaning method is usually the circulation method, where the pumped cleaning solution is fed back to the tank to reduce chemical costs. In this case, the tank should be pumped down as low as possible to check the actual sludge level. The sludge level will determine the amount of chemical, water and, in some cases, diluent (ie, diesel). As a rule of thumb, a warm chemical dilution Figure 1 Fouled heat exchanger with fluid velocity will provide better results than a cold dilution with poor agitation. The success of the selected chemical cleaning programme can be determined by using a good tank survey and laboratory testing. Large storage tanks are often not homogenous, which is why samples should be taken from as many points as possible. The definition of a clean tank is that the floor is clean of hydrocarbons and dirt, and the tank is free of gas. In certain special cases where catalyst slurry has been stored in the tank, the chosen chemical cleaning technique may not be applicable. The cleaning and degassing products in tank cleaning also have their limitations if higher amounts of pyrophoric iron deposits are present. In such cases, specific chemicals for pyrophoric iron sulphide removal such as Kurita CD-5101 will skip this hurdle. Chemical cleaning of distillation equipment While tank-cleaning procedures may require dosing pumps for the chemicals, mobile heat exchangers, high-pressure or ultra-high water jetting, steam, hot water, gas oil or diesel diluents, the Figure 2 Cleaning methods application of a chemical for a tower- cleaning and degassing procedure is generate hydrogen sulphide or produce tank. When such storage tanks are more simple. During the turnaround solid reaction products, which is why it emptied to be cleaned, conventional procedure, the refinery heat exchangers is ideally suited to iron sulphide cleaning methods may require some and pumps can be used to provide dissolution applications. As a further days or even weeks. For this reason, new the heat and fluid velocity. The way advantage, it can be injected into both chemical cleaning programmes have in which the chemical is applied to hydrocarbon and aqueous product been developed to reduce the time the system depends on the process streams and combined with Kurita’s required to remove the sludge from conditions. proprietary hydrogen sulphide and the tank. It is sometimes not possible to provide benzene scavengers to provide a highly There are a number of different a system where the chemical can be effective all-round chemical-treatment standard cleaning procedures pumped back. More chemical and wash programme (Figure 1). introduced to the oil refining industry, water is therefore required. In this case, and each cleaning technique can be the chemical has to be applied to the Tank cleaning characterised according to a cleaning wash water at the top section of the In crude oil and heavy oil storage, sludge procedure. Some cleaning procedures distillation column or reflux line always accumulates at the bottom of the will require the use of a suitable (cascading method). Steam can be added 78 PTQ Q1 2008 www.eptq.com to supply more turbulence on the trays cleaning are all good indicators for or packing. The chemical will cascade evaluating