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2022-08-07
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Using the powerful function of analytical technology to solve the corrosion problem in refineries

the corrosion problem in refineries has been the subject of many studies, papers, courses and forums from the past to the present. Although many literature records show that people have made great progress in the understanding of corrosion, it also shows that this problem still exists and has a growing trend

it is estimated that the cost of corrosion in refineries around the world is about US $15billion per year. More accurate figures are not available because refineries do not disclose the extent of corrosion problems they face, which is understandable given the increasing number of environmental regulations they face. It must be pointed out that these expenses do not include profit loss and lost effective production time. NACE international 13. Relative error of beam speed: an analysis report of ± 1% of the set value shows that in the United States alone, the annual profit loss caused by Refinery Corrosion may be as much as 12billion US dollars

although extensive research has been carried out on this issue and a large number of literatures have been compiled, many aspects of the corrosion mechanism are not fully known. The problem related to oil refining is that there are not only one corrosion source, but many. Compounding the problem is that many corrosives interact and increase or inhibit corrosivity among themselves. In addition, physical process conditions also play a role; Therefore, temperature, flow rate and Reynolds number must also be considered. The refinery's own infrastructure is equally important. Obviously, pipes, vessels, weldments and instruments can not be ignored. Since there are so many variables, corrosion is obviously a complex problem

sulfur

one of the reasons why this situation cannot be improved in a short time is the increase in the processing quantity of high sulfur and poor quality crude oil. High sulfur crude oil is a kind of crude oil with high sulfur content (different from low sulfur crude oil). Due to the low raw material cost of high sulfur crude oil, it is favored by refineries for economic reasons. In addition, as the low sulfur crude oil is almost exhausted, the available quantity is reduced. In high sulfur crude oil, sulfur exists in the form of mercaptan, H2S, sulfate, elemental sulfur, etc. Many of these components will produce chemical reactions, which will lead to stress cracking and sulfuric acid corrosion in the whole oil refining process

acidity

in addition to sulfur, crude oil also contains many components quantified by total acid number (tan). This value is not specific to a certain acid, but refers to all possible acidic components in the crude oil, determined by the amount of potassium hydroxide required to neutralize the acidic substances in 1 gram of oil. Common acidic substances in petroleum include not only organic acids such as naphthenic acid, but also mineral acids, H2S, HCN, CO2, etc., all of which will cause serious corrosion of equipment. Under such attack of corrosive compounds, even materials suitable for acidic conditions cannot escape the consequences of damage. In addition, based on cost considerations, crude oil with high Tan is increasingly favored

desalting

crude oil desalting is the first step in oil refining, which will have a direct impact on corrosion and scaling. By mixing and flushing the crude oil with water, salt and solid substances will be transferred to the aqueous phase deposited in the tank. The electrostatic field accelerates the separation of oil and water, which will remove a large number of inorganic salts that cause scaling or hydrolysis to form corrosive acids

chemicals are usually added in the form of demulsifiers to eliminate oil/water emulsification. In addition, chemicals such as caustic soda are added to neutralize the acidic components. However, the addition of caustic substances without control can have adverse consequences. Excessive caustic substances will form soap due to the presence of fatty acids and other reasons. Soap helps to stabilize the oil-water mixture and hinders the separation process. In addition, an excessively strong mixture of crude oil and water will form an emulsion that is very difficult to break. Crude oil often arrives at the refinery in the form of emulsion, because there is water used to extract oil from the oil tank to the maximum extent, and water may also exist naturally in the oil tank. It has been proved that the emulsion cannot be broken when it is too strong. When this happens, a large number of pollutants will appear in the downstream process, resulting in serious consequences

a process parameter that can play an important role in neutralizing acidic substances and demulsifying is process pH. Careful monitoring of the pH value in the demineralized wastewater can ensure efficient quantification of caustic or acidic substances, thereby saving a lot of costs. The stability of the oil/water emulsion depends in part on the pH value. Maintaining the pH value of the mixture within a certain range can help the demulsifier to break the emulsion through direct interaction with water droplets. Therefore, the speed and quality of the separation process can be improved, which will reduce the water carrying capacity, thus greatly reducing the downstream corrosion and scaling rate

distillation

even if a good desalting operation is carried out, there will still be a large number of corrosive agents in the downstream processing. A good example is the acid water corrosion in crude oil distillation unit. In this process, a large amount of acid gas will be formed, of which the typical gas is hydrogen sulfide. The steam injected into the crude oil tower to improve the fractionation method will condense above the unit. Hydrogen sulfide dissolves in the condensate and forms a weak acid that will cause stress corrosion cracking in the top area of the crude oil tower and the condenser above. This will lead to frequent replacement of condenser pipe fittings and, in severe cases, the replacement of the entire top of the crude oil tower

although the refinery operators are familiar with the causes of this corrosion according to the special organization principle of the foam granulator, they can not always take effective countermeasures. Corrosion inhibitor and many neutralizers such as caustic soda or caustic ammonia are usually injected to improve the pH value of acidic water. Although this is a typical problem-solving method, the solution is more serious than the problem itself. There are many kinds of acid gases and ammonia, which will lead to solid salt deposition. Among them, ammonium sulfide is one of the main causes of alkaline acid water corrosion. When the pH value exceeds 7.6, the corrosion of ammonium sulfide will be seriously improved. Excessive addition of caustic substances will easily lead to excessive pH value. Therefore, as with desalting, the key to reducing corrosion is to accurately control the pH value. By measuring the pH value at the water inlet of the condenser above the crude oil distillation tower, adding an appropriate amount of neutralizer will not only reduce the corrosion, but also reduce the consumption of chemical substances. It is reported that the use of corrosion inhibitor has been reduced by more than 15%

in addition to the crude oil distillation tower, many other downstream processes also use acidic water, resulting in corrosion problems

generally affected processes are:

- vacuum distillation

- fluid catalytic cracking

- hydrocracking

- hydrotreating

- coking

- sour water stripping

few design engineers will install pH control devices in advance in any of the above processes. This does not seem to be a common phenomenon, but few people do it all the time. In fact, many devices have been and are equipped with pH control loops at some stage. However, in terms of reliability, most pH analyzers exhibit extremely poor performance in sour water environments in refineries. Most pH electrodes are not specifically designed for refinery applications and exhibit poor performance under high sulfur concentrations and large hydrocarbon loads. Since maintenance personnel and manufacturers often solve these problems, most pH analyzers in these "sub standard" applications will be ignored by operators

however, proper pH control can yield significant benefits if done properly. It can not only reduce the consumption of chemicals and reduce the corrosion to the equipment, but also reduce the number of maintenance downtime and prolong the operation time of the equipment

xerolyt extra

although there are many kinds, almost all pH electrodes will encounter difficulties in the adverse environment of acid water application. The most common cause of electrode failure is the pollution and scaling of the reference electrode system. Sulfide diffuses into the electrode from the process, and then reacts with the silver/silver chloride reference electrode to change the potential of the reference electrode. This will cause the pH measurement results to drift. Oily substances and solid pollutants may cover or block the reference end diaphragm, which obviously will also adversely affect the performance of the electrode. In order to solve these problems, the electrode manufacturer is unique in selecting the reference diaphragm. Today's pH electrodes are equipped with ceramic, plastic, rubber or even wooden reference diaphragms. Their common problems are: severe performance (some functions will fail immediately), high maintenance requirements and short service life in sour water applications

pH electrodes with flowing reference ends have long provided excellent results. The flow reference value is obtained by pressurizing the electrode. Excessive pressure causes the electrolyte to enter the process through the diaphragm. This outflow of electrolyte will reduce the speed of pollutants entering the electrode reference system, and inhibit the fouling and plugging of the diaphragm. These types of electrodes, including the Mettler Toledo INPRO 2000 I, are ideal for refinery applications. However, these electrodes require frequent refilling of electrolytes, which is not always acceptable from a maintenance point of view. Mettler Toledo has a wealth of knowledge on this issue and has designed INPRO 4260 I pH electrode using xerolyt extra solid polymer electrolyte. The INPRO 4260 I has an open liquid connection, which is in fact a small hole that transports solid electrolytes in direct contact with the process medium. Unlike the micro capillaries of any other type of diaphragm in the traditional pH electrode, the open liquid connection has a large diameter, so the possibility of plugging or scaling is reduced. Another big difference is the choice of polymer electrolytes. Xerolyt extra is specially designed for use in hydrocarbon environments and also provides a powerful and durable barrier to prevent sulfide contamination

automatic cleaning and calibration

whether or not it is an innovative design, even the best pH electrode needs regular maintenance. Among all process instruments, pH electrode may have the widest adjustable range and extraordinary sensitivity. Depending on the process application, calibration is required after a period of operation to ensure that the instrument will maintain its reliability and accuracy. Obviously, the maintenance work in the aforementioned sour water application is still very demanding. Therefore, no matter how good the electrode is, it still needs to be cleaned and calibrated more frequently than in applications such as boiler feed water. It has proved that in many cases, manual cleaning and calibration every two to four weeks will be sufficient. However, when refining high sulfur crude oil or when the above hydrocarbon/water separation effect is poor, it is necessary to maintain the electrode in case of problems or inaccurate measured values. In such applications, automatic cleaning and calibration systems are often used

Mettler Toledo's EasyClean 400 system has proved very suitable for a variety of "polluting" applications. The device is fully certified for use in hazardous areas and can perform the most difficult pH measurement tasks. When necessary, EasyClean 400 will pneumatically shrink the electrode and automatically clean it thoroughly and calibrate it at two points in turn. The system is equipped with cleaning solution and buffer solution, which only needs the

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