Analysis of fouling of shell-and-tube acid cooler

Date:2020-06-01

The shell-and-tube acid cooler is used for cooling and cooling the concentrated sulfuric acid in the dry suction system. The concentrated sulfuric acid goes through the shell and the circulating water goes through the tube. After running for a period of time, it was found that the acid temperature at the inlet of the dry suction tower remained high, and the temperature difference between the acid side and the water side was small. After reasoning, it is determined that there are scaling problems on the acid side and the water side. After disassembling the equipment, it was found that the scale on the acid side and the water side was very serious. The causes of scaling on the acid side and water side are analyzed one by one.

 

The concentrated sulfuric acid cooler is cooled by circulating water. Most domestic sulphuric acid production companies use industrial water without softening the circulating water. So after running for a period of time, the heat exchange tubes of the cooler are prone to fouling. This is mainly because Ca contained in industrial water easily forms CaCO and sticks to the wall of the pipe. The analysis of the scaling component proves that the main component is calcium. There are many factors affecting calcium scaling, such as temperature, pressure, and flow rate , PH, etc.

 

1.1.1 The effect of temperature on scaling

 

The effect of temperature on scaling mainly changes the solubility of scaling salts. Generally, the solubility of scaling salts in water decreases with increasing temperature. Salt scale is mainly carbonate. When the temperature rises, Ca (HCO3)) decomposes to produce CaCO, which leads to scaling. The reaction equation is: Ca (HCO3),-→ CaCO, l + CO, + H, 0 (1) This reaction is an endothermic reaction. As the temperature rises, the equilibrium moves to the right, which is beneficial to the precipitation of CaCO.

 

1.1.2 The effect of pressure on scaling

 

Gas is generated due to CaCO and scaling reaction, which is greatly affected by pressure. When the pressure decreases, reaction (1) proceeds to the right, promoting CaCO and scaling.

 

1.1.3 Effect of flow rate on scaling

 

The fouling growth rate decreases with increasing fluid velocity. When the flow rate

 

When it increases, the scaling is not easy to deposit; when the flow rate is reduced, the probability of solid particles and microbial excreta carried in the cooling medium increases, and the probability of pipeline fouling also increases significantly, especially at the location of structural mutation.

 

1.1.4 The effect of pH on scaling

 

The results of the study indicate that increasing the pH of the solution will accelerate the crystallization speed of the carbonate, increase the thermal resistance of progressive fouling, shorten the induction period of fouling and promote the growth of fouling "; but when the pH of the solution is too low, it is easy to accelerate the Corrosion leads to corrosion and scaling. Therefore, it is necessary to comprehensively consider the problems between the above two aspects, reasonably choose the plHE value of the medium, and the recommended medium pH value is 6.5 ~ 7.5.

 

1.2 Scaling on the acid side

 

Through the analysis of the acid side scaling components, it is found that the main component is silicon. Analyze the reason and analyze the production water and find that the silicon content in the scale is high, indicating that the main component of the acid side scale is silica gel.

 

The scaling on the acid side is mainly due to the formation of silica sol by the dry suction system supplementing the SIO contained in the water. Silica sol is a dispersion of nano-scale silica particles in water or solvent, also known as silica sol, or silica hydrosol. Since SiO in silica sol contains a lot of water and hydroxyl groups, the molecular formula of silica sol can be expressed as msi0, · nH, 0. Most of the particles are aggregated in single or multiple spheres. The diameter of colloidal particles is generally 10 ~ 100mm. Due to the concentrated sulfuric acid environment, the moisture in the silica sol is absorbed by the concentrated sulfuric acid. When the silicon solution loses moisture, the monomer silicic acid gradually polymerizes into a high-polymer silica gel. As the water evaporates, the colloidal molecules increase and finally form -SIO-0-SIO-coating film. It has good adhesion and will firmly adhere to the surface of the heat exchange tube. Because the particle size of silica sol is small, it has a large specific surface area (50 ~ 400m² / g). Silica sol has greater adsorption. The numerous network structure voids produced by the numerous micelles in silica sol can absorb inorganic and organic substances in solution under certain conditions.

 

An acid absorption cooler of a dry suction system uses a shell and tube acid cooler without anode protection. The amount of water supplied by the first suction tower is 8m² / h, and the content of SO in the water is 0.004%. After running for 1 year, the acid side of the cooler is taken out for fouling analysis, S0. The quality score is 78.61%. Due to the strong adsorption capacity of silica sol, it can absorb impurities in equipment and pipelines, and form deep-colored dirt on the acid side tube wall.

 

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