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In the process of chemical production, due to various reasons, dust and dirt such as polymers, coking, oil and dust, scale, sediment, and corrosive products may occur in equipment and pipelines. These seriously affect the use of equipment, so it is very important to clean chemical equipment.

Chemical equipment cleaning includes two types: online cleaning and offline cleaning.

 

Online cleaning

Use the cooling tower in the circulating water system as a dosing box to add chemicals to the system for natural circulation.

Advantages: The equipment does not need to be shut down and does not affect normal production and use.

Disadvantage: The cleaning effect is not very good compared to offline cleaning. Long cleaning time and significant corrosion hazards to equipment.

 

Off-line washing

It refers to the process of disassembling the components to be cleaned from equipment or pipelines and transporting them to another location (relative to the original location of the components) for cleaning

Offline cleaning can be divided into physical cleaning and chemical cleaning.

Physical cleaning: Use high-pressure running water to clean equipment. High pressure cleaning equipment is required.

Chemical cleaning: Take out the heat exchanger separately and connect the inlet and outlet pipelines of the circulating water to the cleaning vehicle for circulation. Chemical cleaning has the following characteristics:

Advantages: Reduced dosage of medication and good cleaning effect.

Disadvantages: Corresponding equipment is required, such as cleaning cars or water tanks, high-pressure pumps, various specifications of connecting valves, welding equipment, etc.

 

There are two forms of chemical cleaning: acid washing and alkali washing.

Alkali washing: mainly used to remove organic matter, microorganisms, oil stains and other attachments inside the equipment, such as rust inhibitors used during equipment installation. Alkaline washing can also play a role in loosening, loosening, emulsifying, and dispersing inorganic salts. Common cleaning agents include sodium hydroxide, sodium carbonate, trisodium phosphate, etc.

Acid washing: mainly to remove the deposition of inorganic salts, such as carbonates, sulfates, silica scales, etc. Common cleaning agents include organic acids such as hydrochloric acid, sulfuric acid, and hydrofluoric acid. Organic acids such as citric acid and amino sulfonic acid.

 

Why clean chemical equipment?
1.The necessity of cleaning before driving

Chemical cleaning before driving is essential to improve production efficiency and avoid the impact of dirt on production. Therefore, before the new chemical equipment is put into operation, it must be cleaned before starting up.

The chemical production process involves multiple chemical raw materials and requires the use of catalysts. The purity requirements for certain raw materials and catalysts are extremely high, so there are high requirements for the cleanliness of equipment and pipelines during the production process. Any impurities can cause catalyst poisoning, side reactions, and even damage the entire process. In addition, certain equipment and accessories in the device have high precision requirements or are highly sensitive to the destructive effects of impurities. Therefore, any intervention of mechanical impurities is highly likely to damage the quality of precision components and affect normal production.

2. The necessity of cleaning after starting work

Chemical equipment, when used for a long time, can produce dust such as polymers, coking, oil and dirt, water scale, sediment, and corrosive products, which seriously affect the operation of chemical equipment. Timely cleaning of chemical equipment can extend its service life, improve efficiency, ensure safety, and reduce economic losses.

So, whether before driving or after using for a period of time, equipment should be cleaned, which is an essential daily maintenance work.

 

What are the cleaning processes for chemical equipment?

Preparation before cleaning equipment

Before cleaning, components in the equipment or device that are susceptible to corrosion and damage from the cleaning solution, such as regulating valves and flow meters, should be removed, and the filter core (mesh) and one-way valve core should be removed. And measures such as adding temporary short pipes, bypasses, or blind plates should be taken to ensure that there is no leakage or damage to other components during the cleaning process, and to separate the cleaned equipment from the uncleaned equipment and pipelines.

 

Cleaning procedures and process conditions

1. Cleaning method

According to the specific situation of the equipment, soaking cycle cleaning or spray cleaning can be used.

When using soaking cycle cleaning, a low point inlet high, ammonia return cycle process can be adopted.

When using spray cleaning, a process of high point liquid inlet and low point reflux can be adopted.

2.The cleaning procedure and the degree of chemical cleaning generally include system water pressure leak detection (water flushing), degreasing, water flushing, acid washing, rinsing, neutralization, passivation, inspection, and manual treatment.

The following provides explanations for each process.

The purpose of water pressure leak detection (water flushing) is to check the leakage situation of temporary systems, and to remove dust, sediment, detached metal oxides, welding slag, and other loose and easily removable dirt from the system.

The purpose of degreasing cleaning is to remove oil stains such as mechanical oil, graphite grease, oil coatings, and rust oil from the system to ensure even acid washing.

The purpose of water washing after degreasing is to remove residual alkaline cleaning agents from the system and remove some impurities from the surface. Remove the object.

The purpose of acid washing is to remove soluble substances by chemical reaction between acid and metal oxides.

The purpose of rinsing with water after acid washing is to remove residual acid washing solution and solid particles that have fallen off from the system for rinsing and passivation treatment.

The purpose of rinsing is to use ammonium citrate to chelate with residual iron ions in the system and remove floating rust formed during the water rinsing process, reducing the total iron ion concentration in the system and ensuring subsequent passivation effect.

The purpose of neutralization and passivation process is to remove residual acid solution, while passivation is to prevent the metal surface that is in an activated state after acid washing from re oxidizing and producing secondary floating rust.

 

Cleaning after commencement of work

Chemical equipment that has been in operation for 1-2 years or more often adheres to iron oxide scale or steel containing scale. Copper scale contains copper oxide (CuO), basic copper carbonate [Cu2 (OH) 2CO3], and metallic copper.

Rust scale can generally be removed by acid washing. The method and steps of acid washing are basically the same as the method of cleaning the equipment before starting work.

When the copper content in the dirt is high, it cannot be removed by acid washing alone. It is necessary to remove the copper component with ammonia water before acid washing.

Copper and copper oxide scales often form layered attachments with iron oxides, which are difficult to clean and should be cleaned before the formation of layered attachments.

 

How to clean the heat exchanger?

The cleaning of heat exchangers is generally divided into two categories: mechanical cleaning and chemical cleaning.

 

Mechanical cleaning

The mechanical cleaning method relies on the flow of fluid or mechanical action to provide a force greater than the adhesion force of dirt, causing the dirt to detach from the heat exchange surface.

There are two types of mechanical cleaning methods: one is the strong cleaning method, such as water spray cleaning, steam spray cleaning, sandblasting cleaning, scraper or drill bit descaling, etc; Another type is soft mechanical cleaning, such as wire brush cleaning and rubber ball cleaning. Below are several types of methods:

Spray cleaning is a descaling method using high-pressure water spraying or mechanical impact. When using this method, the water pressure is generally 20~50MPa. Now there are also higher pressures of 50-70Mpa being used.

Spray cleaning, similar in design and operation to spray cleaning, is a device that sprays steam into the tube and shell sides of a heat exchanger to remove dirt through impact and heat.

Sandblasting cleaning is the process of using compressed air (300-350kPa) through a spray gun to generate a strong linear velocity on the screened quartz sand (usually with a particle size of 3-5mm), which flushes the inner wall of the heat exchanger tube, removes dirt, and restores the original heat transfer characteristics of the tube.

Scraper or drill bit descaling, this cleaning machine is only suitable for cleaning dirt inside pipes or cylinders. Install a descaling scraper or drill bit at the top of the flexible rotating shaft, and rotate the scraper or drill bit by compressed air or electricity (also using water or steam).

Rubber ball cleaning is carried out using a shot blasting cleaner. The shot blasting cleaner is composed of a sponge ball and a fluid spray gun that pushes the ball into the inside of the pipe to be cleaned. The ball is shaped like a shell and is extruded from a semi hard foam polyurethane sponge, which is elastic.

 

Chemical cleaning

The chemical cleaning method involves adding descaling agents, acids, enzymes, etc. to the fluid to reduce the adhesion between dirt and the heat exchange surface, causing it to peel off from the heat exchange surface.

The current chemical cleaning methods used are:

Circulation method: Use a pump to force the cleaning solution to circulate for cleaning.

Immersion method: Fill the equipment with cleaning solution and let it stand for a certain period of time.

Surge method: Fill the equipment with cleaning solution, discharge a portion of the cleaning solution from the bottom at regular intervals, and then reinstall the discharged liquid into the equipment to achieve the purpose of stirring and cleaning.

 

How to clean the reaction kettle?

There are three main methods for cleaning reaction vessels: mechanical cleaning, chemical cleaning, and manual cleaning.

 

Mechanical cleaning

Mechanical cleaning: Using a high-pressure cleaning device, high-pressure water flow is used to flush through the nozzle, breaking down the hard scale on the inner wall of the reaction vessel and the surface of the agitator, thoroughly peeling and removing it.

The principle of high-pressure water jet cleaning is to compress water to high pressure, and then release it through the nozzle installed on the cleaning robot inserted into the kettle. Pressure can be converted into kinetic energy of water flow, which can impact the wall dirt to achieve cleaning and removal effects.
Chemical cleaning

Firstly, it is necessary to know the composition of the scale sample inside the reactor equipment, preferably through sampling and analysis. After determining the composition of the dirt, conduct experiments first, select cleaning agents, and confirm through experiments that they will not cause corrosion to the equipment metal. Then, a temporary circulation device is set up on site to circulate the cleaning solution inside the equipment and wash away dirt.

First, rinse the mixing blade and the inner wall of the kettle with an appropriate amount of water, and drain them completely.

Flush the reaction vessel with solvent through a pressurized device.

If the cleaning effect is not achieved, add an appropriate amount of solvent to the reaction kettle, heat up, stir and reflux until the cleaning requirements are met, and then release the solvent.

Finally, rinse the inner wall of the reaction vessel with a certain amount of solvent and release it.

Manual entry into the kettle and manual cleaning

Low cost is its greatest advantage, but it requires several hours of ventilation and air exchange before entering the reactor. During the cleaning process, the oxygen concentration inside the reactor must be monitored at all times, which poses a risk of oxygen deficiency; At the same time, manual scraping not only fails to completely clean, but also causes sliding marks on the inner wall of the reaction vessel, which objectively lead to further adhesion of residues. Cleaning the kettle can also cause hygiene issues with the product. Generally speaking, it takes about half a day to a day to clean a kettle.

Each of the three methods has its own advantages and disadvantages:

Mechanical cleaning does not corrode equipment and can effectively clean hard scale, but it takes a long time and requires high labor intensity;

Chemical cleaning requires less labor, has a short cleaning time, and thoroughly cleans, but it may cause equipment to corrode;

Manually entering the kettle for cleaning is low-cost, but it carries a high level of danger and cannot be completely cleaned.

Therefore, chemical cleaning is applied in working conditions where the dirt is soft and thin, while mechanical cleaning is applied in working conditions where the dirt is hard and thick.