The waste and loss caused by the heat exchanger fouling is very huge, which has attracted the attention of the scientific community and engineering technology circles in various countries. It is an urgent problem to be solved in many industries and departments of the national economy, and it is also the main problem that the heat transfer academics have not really solved. one. Nostrand has investigated the economic losses caused by the formation of scales (excluding water cooling, air cooling and boiler fouling) in some refineries in the United States and the world, resulting in fouling caused by US refineries due to scaling. The economic losses totaled 1.36 billion U.S. dollars per year, and the world totaled 4.4 billion U.S. dollars. 111 According to relevant reports in China, for the heat exchange equipment, the energy consumption per cubic meter of scale should be 7%~9%, and the thermal efficiency is reduced by 10%. ~20%. There are about 420,000 industrial boilers in China, and about 1.75 million tons of raw coal is wasted every year due to scale. Due to improper boiler water quality management, scaling will lead to local expansion deformation and scale corrosion, which directly affects the safety of chemical production. 23. Heat exchanger fouling detection and analysis Heat exchanger equipment fouling refers to the components or impurities in the fluid. A layer of solid matter that gradually accumulates on the heat exchange surface of the contact, which is widely found in various heat exchange equipment in the fields of engineering, such as chemical engineering, power and refrigeration.
1.1 Classification of fouling From the perspective of the fouling mechanism, the liquid side dirt can be classified into the following six categories: crystallization fouling, particulate fouling, chemical reaction fouling, corrosive dirt, biofouling and solidified fouling. It should be noted that the usual fouling formation process may be the result of several fouling formation mechanisms. For example, crystallization fouling and corrosive dirt often mix and coexist in the same heat exchange surface, and several kinds of dirt are often generated at the same time on the heat exchange wall surface and interact with each other. Therefore, for each basic fouling type, it is important to understand the formation mechanism to prevent fouling.
1.2 Scale-like components and analytical scales include soft scales, hard scales, corrosion products and biological slime.
Hard scales are generally strong on the pipe wall and are difficult to scrape off with a spoon. The soft dirt is relatively loose, deposited on the pipeline, and adheres to the pipe wall. It is easy to wash down with water, and the feeling of fine particles is pinched by hand, and it is powdery when dry. If there is slime, the color is darker and there is a certain stickiness when the hand is pinched. Corrosion products are mostly corrosive rusts produced by electrochemical corrosion or microbial corrosion. The biological slime is generally composed of bacilli wrapped in a dense layer of mucus, which is grayish black in appearance, smells astringent, and has a slippery feel on the hand. 14. Hydroelectric Technology, 2001, 37 (Materials and Powder Metallurgy Materials) .
Responsible editor Wang Yakun's scale analysis, as the name implies, is to take qualitative and quantitative analysis of the components of the dirt sampled by the cleaning equipment, usually with qualitative analysis. Scale analysis is an important prerequisite for determining the formulation of the cleaning solution. The analysis results can provide the main components of the dirt and are the main basis for determining the range of cleaning agents. The result of the qualitative analysis is the approximate composition of the scale, and the content of each component in the scale is unknown, and quantitative analysis is needed to determine the content of each component. Quantitative analysis should be performed on the basis of qualitative analysis, which is advantageous for the selection of complex soils that are determined to clean certain important large equipment. Quantitative analysis can be carried out by volumetric method, gravimetric method or instrumental analysis. 151. The first step of the scale analysis is sampling. The basic requirement is that the sample taken should be representative. The amount of rice is generally 100~200g, and a certain amount of retention is reserved while satisfying the test requirements, so as to review and verify.
Qualitative analysis usually uses chemical testing methods to analyze scale samples.
Instruments and reagents: drug balance, commonly used glassware; HCl 5% ether.
Test procedure: 5 scales weighing about 5 g were weighed in a balance and placed in a 100 mL beaker with serial numbers. First, add 5% HCl 30mL to the No. 1 beaker, observe the scale, record the change; then add 10% BaCl230mL in the No. 2 beaker to observe the scale and record the change; then add d5%NaF in the No. 3 beaker. The scale was observed at about 30 mL, and the change was recorded. The scale was ground in a mortar with a No. 4 beaker, and then placed in a beaker, and 50 mL of an ether solvent was added to observe the scale, and the change was recorded.
The simple identification method of the dirt component is described below.
After adding 5% HCl, the scale blocks are dissolved, and a large amount of bubbles are generated at the same time, and the amount of residual residue of the solution is extremely small. The dirt component is carbonate scale.
There was almost no bubble generation in HCl, and a large amount of white precipitate was produced after the addition of 10% BaCb solution. The dirt component is sulfate scale.
%NaF can be slowly dissolved. The dirt component is silicate scale.
After adding 5% HCl, the acid solution is yellowish green. The dirt component is iron oxide scale.
Diethyl ether was added to the grind scale, and the ether layer was light yellow in color, and the insoluble scale was grayish white. The dirt component is grease stain.
The composition of the dirt is very complicated, and in addition to the single type of dirt, mixed scale is also common. Therefore, it is usually necessary to carry out a few steps of the identification test to determine the main components of the scale, but for some simple single-type scale samples after a one-step or two-step test is basically determined, it is not necessary to do the identification test every step.
2 The choice of cleaning agent is usually determined by the sample analysis or the investigation of the material passing through the equipment, and the selection of the cleaning agent is carried out in combination with the material of the equipment. The cleaning agent mainly refers to the acidic cleaning agent and the common inorganic acid. And organic acids). According to the composition of the scale, the nature of the material to be cleaned, the range of the optional cleaning agent is determined first, and then the scientific test method is used to select the type, concentration, and use temperature of the cleaning agent used.
The following principles should generally be mastered when selecting a cleaning agent.
Under the premise of ensuring the dissolution of scale for a certain period of time, the cleaning agent with low concentration, normal temperature, safety and economy is selected.
The cleaning agent for the test should be determined according to the material of the equipment, the condition and composition of the scale, etc. It is important to point out that the cleaning of stainless steel equipment is not allowed to use hydrochloric acid, because the residual chlorine ions in the pickling of hydrochloric acid will Causes stainless steel to produce stress corrosion and intergranular corrosion cracking.
Under normal circumstances, two kinds of cleaning agents can be selected. Each cleaning agent uses two kinds of high and low concentrations commonly used in cleaning, and a total of four kinds of formula cleaning reagents.
For some complex and special scales, the range of choice should be wide. Some test cleaning agents can be used. For some complex organic scales, special targeted cleaning agents should be used.
The same quality was weighed by the drug balance (5 samples and 4 pieces were placed in 4 100mL beakers marked with numbers, 60~70mL of different selection acids were added respectively, and the start time of the test was recorded on the test report according to the serial number. Then, carefully observe the change of the scale pattern in the cup, and record the time after the scale sample is basically dissolved in the beaker and other test results on the report sheet.
If the scale is not dissolved after a certain period of time, then the method of warming and stirring may be considered. The beakers are respectively placed on the magnetic heating mixer, the magnetic stirring rods are placed, the stirring is turned on, and the appropriate rotation speed is adjusted (if the stirring is difficult, The scale can be broken. Turn on the heater to adjust the temperature from low to high and gradually increase, but not exceed the maximum use temperature of the commonly used corrosion inhibitor. Otherwise, the use of the corrosion inhibitor will cause problems. The test procedure is still as follows. The temperature of the test, etc., was performed and recorded. If the scale is not dissolved after heating, it is necessary to consider the use of special targeted chemical solvents, which are mostly complex organic scales. The cleaning agent should be selected according to the molecular structure of the materials taken by the equipment.
According to the results of the above test and the selection principle of the cleaning agent, and in combination with the actual situation, the cleaning agent is determined. Under normal circumstances, the time of the dissolution is the scale, and the shorter the dissolution time, the better the solubility of the cleaning agent on the scale, so the cleaning agent with the shortest dissolution time should be selected. If the dissolution time is not much different, considering the time of dissolution, it is mainly measured from the principle of cleaning agent selection and ease of operation. After comprehensive analysis, the appropriate cleaning agent is determined, and the determination of some special cleaning agents is also determined. There are other factors to consider.
After determining the formulation of the cleaning agent, the last test work is the choice of corrosion inhibitor. It is an important step in determining the formulation of the cleaning solution. The choice is very important for the cleaning of the equipment, because an ideal cleaning solution can not only remove Scale, more importantly, does not damage or destroy the equipment being cleaned while descaling, otherwise the cleaning itself loses its meaning. The corrosion inhibitor usually determines the selection range of the corrosion inhibitor according to the type of cleaning agent, the use temperature, the concentration, etc., and then determines the type and concentration of the corrosion inhibitor according to the results of the test, combined with various factors and actual conditions.
After the above steps, the formulation of the cleaning solution can be scientifically determined, that is, the type, concentration, and type and concentration of the cleaning agent are determined.
3 Heat exchanger online cleaning technology The heat exchanger can be divided into online and non-online cleaning according to whether it is shut down during cleaning or disassembled. In general, online cleaning is chemical cleaning, and non-online cleaning is high pressure water washing. Online cleaning does not require downtime, no need for disassembly and transportation of the heat exchanger, short cleaning time, small amount of engineering, obvious cleaning effect and less corrosion to the equipment. The discharge of the cleaning agent after the qualified treatment will not affect the environment. Non-online cleaning process has high consumption, low cleaning capacity, high labor intensity, long cleaning cycle, great impact on environmental pollution, and there are major safety hazards in the disassembly and assembly of heat exchangers. 167. Online cleaning technology can not only solve the non-destructive cleaning of equipment. The problem can also be cut into the heat exchange system for online cleaning of single or multiple devices. The cleaning cost is low, the time is short, and the overall design and equipment movement are convenient. Promoting the application of online cleaning technology can not only improve the heat exchange effect of cold-exchange equipment, extend the maintenance cycle of cold-exchange equipment, but also save a lot of maintenance costs for the petrochemical enterprises and losses caused by maintenance and shutdown. 181. Online cleaning is mainly applied to tubes. The shell condenser and the heat exchanger are used for cleaning microbial slime and general soft dirt. The online cleaning device is composed of a washer, a power pump, a matching pipeline, a valve and a control cabinet. The principle of on-line cleaning is to arrange cleaning elements of appropriate materials in each tube of the heat transfer device. During the production operation, under the driving of the heat transfer medium in the tube, the cleaning element slides back and forth in the tube for a predetermined period of time. Since the cleaning element and the inner wall of the tube are ground, rubbed and collided, the inner wall of the heat transfer tube is kept clean. Therefore, the purpose of online cleaning and anti-scaling is achieved. 191. The online cleaning has wide application range and strong adaptability to fluid medium. The corresponding material can be used for different media; the same is suitable for high-viscosity media, and the operation is simple and can be operated manually. It can also be controlled automatically. The wear of the heat transfer tube can be artificially controlled, as long as the selection of the cleaning element is appropriate, the working time and frequency are properly set, and the heat transfer tube wear is almost zero.
The device is simple to install, can be installed on new equipment, and can be modified on old equipment. When the modification is carried out, only the old equipment can be partially modified, and the general technician can operate.
The online cleaning operation is reliable, the device has a simple structure, and the probability of failure is small, and even if a failure occurs, the normal operation of the system will not be affected. Different from traditional mechanical and chemical cleaning, it will not cause damage or corrosion to the equipment, which will not only prolong the service life of the equipment, but also save a lot of cleaning costs, and can eliminate the loss of production caused by shutdown cleaning and ensure the smooth production process. get on.
4 Conclusion In the chemical cleaning process of the heat exchanger dirt, first of all to determine the type of scale, analyze the composition of the scale, determine the appropriate cleaning formula for the dirt components, and finally determine the cleaning device and the complete cleaning process, to obtain a complete and feasible implementation plan . The online cleaning (descaling) device of the heat exchange equipment provides an effective and feasible way to solve the problem of the dirt of the heat exchanger, improving the efficiency of the heat transfer equipment, improving the use conditions of the heat transfer equipment, and saving energy and reducing consumption. It has great economic significance.
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