Chemical Cleaning and Degassing Refinery Equipment

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 the progress of the cleaning. down together with the wash water to After circulating the cleaning solution the sewer. for some hours (cleaning time depends Again, the most efficient method is on the type of chemistry and process the circulation method, where the conditions), it should be drained off and chemical can be pumped back, which the system flushed again. will reduce chemical costs. The products For aqueous applications, in some first have to be drained off. Depending cases the chemical has to be dosed on the chemistry of the cleaning continuously in the steam phase chemical, hydrocarbons or water are (steaming method), which requires applied to flush and fill the system. Figure 3 Injection points D1-D4 at a condensation in the distillation column. Ideally, the system should be filled vacuum distillation unit Small air-driven pumps can be used, one-fifth to one-third of its volume which provide a constant flow of the before the cleaning and degassing cleaning process. The flow of the chemical (Figure 2). chemical can be injected at the cleaning solution, temperature While the circulation method is defined temperature range as soon as conditions, chemical activity and colour already the most economical method, it possible. It is important to monitor the change of the samples during the has another big advantage, provided the chemical can be injected at different areas of the system to shorten the mixing of the chemical and the washing liquid. Small suitable and powerful air- driven pumps can be used to inject the chemical at locations where it is more difficult to install an electrical dosing pump (Figure 3). Depending on whether only the chemical cleaning of the distillation equipment is required or volatile hydrocarbons or hydrogen sulphide also have to be eliminated, the total time for one cleaning cycle will generally be 2–16 hours. A cleaning cycle is defined by draining the distillation products, flushing with a suitable washing solvent, filling with the washing solvent, injection of the chemical, circulating for some hours and draining within two to three hours. From the point of cleaning efficiency, it is always better to apply a 1% dilution twice than a 2% dilution once. However, due to time limitations during turnarounds, this cannot always be realised. After opening of the manholes and piping, no pyrophoric iron deposits, volatile hydrocarbons and hydrogen sulphide should be detectable. Cleaning of fuel gas systems A refinery generally operates many fuel gas combustion devices. They are connected to a centralised fuel gas distribution system, which collects the gas streams from different parts of the refinery to direct the gases to the refinery heaters or furnaces. The composition and quality of the fuel gas vary with the crude oils, but the main components are methane, ethane and ethylene in combination with excess hydrogen. A hydrocarbon condensate carryover to the fuel gas burners is common, and the hydrocarbon condensate can go all the way in the firebox by creating an unsafe and hazardous situation. Amine carryover with the fuel gas is also common and, besides the condensation of water or heavier hydrocarbons in the fuel gas

80 PTQ Q1 2008 www.eptq.com amount of chemicals needed for cleaning. Tanks and vessels with poor agitation, or distillation units with packing, are more difficult to clean. If these areas eventually appear very clean (Figure 4), it is a good indication that the correct cleaning programme and technology were selected.

Conclusions Refinery equipment that is commonly cleaned chemically includes boilers, tanks, heat exchangers, fin-fan coolers, hydraulic lines, reactors, condensers piping systems, refinery towers and cooling systems. During a refinery turnaround, much of the work involves cleaning, and the turnaround is carefully planned months in advance. Regulatory, safety and environmental requirements are increasing the demands on turnaround operations in refineries and other petrochemical plants. Figure 4 Packing area of a distillation column after chemical cleaning Whenever a piece of equipment has to be cleaned, there are key parameters distribution system, the potential representative samples are taken. Storage to be considered for a proper cleaning formation of ammonium salt deposits is tanks are not homogenous, which is method selection: considered to be a critical factor for the why samples should be taken from as — The surface to be cleaned continuous and efficient operation of many sampling points as possible. If — Waste removal the fuel gas system. samples have to be taken from tanks, — Application method The mechanical cleaning of blocked most refineries use different types of — Present results burner nozzles requires three to four sample thieves. A cage with a sample — Available contact time man-hours and an average burner bottle, or a graduated glass cylinder in a — Chemical content restrictions cleaning cycle is three to four months. sampling container with a trap door — Process conditions Having 30–50 heaters with many burner that closes on the bottom of the — Final conditions of work. nozzles installed, mechanical cleaning graduated cylinder are two examples of The type of cleaning chemical has to may cost some hundred thousand sampling devices. If there are no be chosen carefully. Concentrated euros per year. Although ammonium sampling points installed, the water solutions of inhibited acid cleaners can salts are water-soluble, steam-cleaning draw can be used for sampling. After generate dangerous and corrosive fumes procedures often show a limited success. opening the valve, any sludge-type at higher temperatures, and can cause An alternative to mechanical cleaning material can be collected. This way stress corrosion cracking at high nickel and steam cleaning is the ACF of sampling may not be representative, alloys. Alkaline cleaners can remove technology developed by Kurita, where but it is better than having no sample only limited types of scale, and higher the ACF product is injected into at all. temperatures are required. the fuel gas system to remove The chemical cleaning of distillation Suitable chemicals including ammonium salt deposits. Within a few equipment will require 2–16 hours. proprietary products to remove minutes, the fouling material can be Before and during the procedure, pyrophoric iron sulphide deposits have removed, and burner flames will samples can be taken to observe the been developed for degassing and look good and the burner tips will progress of the cleaning. The decontamination, and can be solvent- live longer. concentration of “actives”, the or water-based products. Refineries temperature of the washing solution, generally prefer water-based cleaners Sampling and monitoring colour, pH, iron concentration, and solvent cleaners that do not For tank cleaning, the success of the chlorides, ammonia and sulphides are form emulsions. These products will cleaning technology can be determined typical parameters for refinery cleaning provide faster and safer results on by a good survey and laboratory testing procedures. During cleaning, the cleaning the metal surface, reducing before and during cleaning. It is samples appear to look more and more benzene and other volatile important to obtain information on the dirty, and the increase in analysed hydrocarbons, hydrogen sulphide and composition of the bottom sludge, iron, chlorides, sulphides and ammonia ammonia levels. This will help reduce which can contain oil, water, volatile indicates the mobilisation of the fouled the turnaround time, air emissions, solids, ash and sediments. The material. When the increase in waste, labour costs and pollution- concentration of water and sediment concentration of these parameters control costs. content of crude oils is significant, slows down, it is an indication that because it can cause corrosion of the the end of the cleaning is imminent distillation equipment. A determination or more or new cleaning chemical is of water and sediments is required to required. measure accurately net volumes of The heat-transfer coefficient and the actual oil in sales. Based on the cleaning pressure drop of heat exchangers, the Berthold Otzisk is a technical engineer at technology, recovered oil can be directed sampling of fouled materials, ultrasonic Kurita Europe GmbH, Viersen, Germany. to the refining process again, increasing measuring equipment and former Otzisk has more than 15 years’ experience refinery profit. turnaround reports on observed fouling in sales, service and research of new The best results are obtained when are all good tools for estimating the technologies. Email: [email protected]

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