Knowledge on glass lining failure and repair of glass-lined reactors

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This post was last edited by yinkuilin6868 on 2015-11-17 at 20:33. Due to the physical properties of the enamel glaze used in glass-lined reaction vessels, the enamel becomes brittle and unable to withstand impacts. As a result of improper use by users, local cracking of the enamel in the reaction vessel can occur. There’s no need to panic; simply stop the vessel immediately and carry out repairs. It’s similar to when a new car gets a minor scratch on its paint at times. Below, we will introduce the reasons for porcelain cracking in glass-lined reaction vessels, as well as the methods for dealing with this issue. What are the causes of ceramic cracking in glass-lined reactors? Case of glass cracking in reaction vessels: Due to process requirements, a chemical plant needed a large number of glass-lined reaction vessels, glass-lined pipes, and other glass-lined equipment for its entire production line. A 2000L electrically heated glass-lined reactor that is in normal use has a heat transfer oil as the medium in its jacket, with a temperature of around 250 degrees. The reaction medium inside the reactor is sulfuric acid at a concentration of 75%; the process requirements dictate that the reaction temperature should be around 180 degrees Celsius. Recently, the reactor has been experiencing frequent cases of large-scale cracking of the glass lining! After on-site inspection by the technicians specializing in glass-lined equipment, it was found that the interval between the addition of the two batches of material was relatively short; the temperature inside the glass-lined reactor remained high even after the previous batch of material had been emptied, while the sulfuric acid with a concentration of 75% added immediately afterwards had a temperature of around 70 degrees. Based on this analysis, the porcelain cracking in the reactor is likely caused by thermal shock. 　　Analysis of porcelain cracking and recommended handling methods: In this case, the temperature of the heat transfer oil in the jacket of the reactor was 250 degrees. After discharging the material, the temperature of the reactor vessel should have been 180 degrees higher than the reaction temperature. However, the limit of the enamel-coated reactor’s tolerance to thermal shock is 110 degrees Celsius. Although the sulfuric acid material added again was preheated to 70 degrees, the temperature difference between the material and the reactor vessel is likely to exceed 110 degrees, which led to porcelain cracking. It is recommended to increase the interval between two material feedings and raise the preheating temperature of sulfuric acid, which will help prevent porcelain cracking. 　　When ceramic cracking occurs in glass-lined reaction vessels during normal use, it affects the proper operation of the entire reactor, thereby impacting production and resulting in direct economic losses. 　　Ceramic cracking refers to the phenomenon of the enamel lining in a glass-lined reactor breaking, which results in the integrity of the enamel being compromised; this, in turn, can lead to corrosion of the reactor’s main structure during reactions. 　　There are two main reasons for porcelain cracking. One is uneven heating during the operation process, with large temperature changes in a short period of time, which causes the enamel to expand and contract rapidly and leads to porcelain cracking. This type of porcelain cracking can be controlled, and it can also be avoided. 　　The second issue is defects that occur during the firing process, resulting in spalling. The main reasons are factors such as the quality of the metal blank, the composition and uniformity of the glaze, as well as the pickling time, firing time, and firing temperature during the enameling process; the primary culprit is hydrogen. 　　When metal is enamel-coated, the metal blank after firing is in an austenite state, and this state facilitates the absorption of hydrogen; in other words, a large amount of hydrogen is already absorbed by the blank at the firing stage. During the cooling process, austenite gradually transforms into another state – ferrite. At this stage, the metal’s ability to weld in the presence of hydrogen is **reduced**, and the hydrogen that was previously dissolved begins to be released. Due to the barrier posed by the enamel coating, this hydrogen accumulates in the depressions between the enamel layer and the metal substrate. As the substrate undergoes phase transformation, more hydrogen is released. The greater the amount of hydrogen, the greater the pressure exerted on both the substrate and the enamel layer. When the pressure generated by this hydrogen becomes high enough to exceed the physical strength of the enamel layer, even minor physical impacts can cause the pressure of the hydrogen inside to build up, resulting in the enamel cracking. Special repair methods for glass-lined reaction vessels The common repair methods for glass-lined reaction vessels include glaze firing, corrosion-resistant metal repair, and mixed repair using corrosion-resistant metal coatings. Here, a special repair method is introduced: the non-metallic repair method. 　　Synthetic resin materials possess high chemical stability and excellent physical and mechanical properties. Using these materials to repair chemical enamel equipment results in a simple process, low costs, short repair times, as well as good resistance to corrosion and wear. 　　Generally, synthetic resin materials have low adhesion to enamel surfaces; for example, phenolic resin has an adhesion strength of 1.7 MPa after curing at room temperature, while the thermal stress generated in an enamel-lined reactor at 100°C reaches 1.5 MPa. Therefore, when using adhesives to repair enamel reaction kettles, two issues need to be addressed: improving the adhesive strength itself and reducing thermal stress. 　　1. Improve the adhesive strength of the material itself. It is necessary to first enhance the physical and mechanical properties of the adhesive. When repairing glass-lined reactors, epoxy-phenolic composite mortar is commonly used; phenolic resin serves as an active toughening agent for epoxy resin, thereby improving the mortar’s resistance to heat and corrosion. Adding 17%–20% of nitrile rubber or butyl rubber to the mixed clay improves the flexibility of the clay. To reduce the viscosity of the paste, taking into account the evaporation of the thinner, 690 or 501 reactive thinners were used in place of xylene, and 5%–8% aluminum oxide powder was added to improve the bonding strength of the resin. 　　2. Reducing thermal stress: At high temperatures, the resin further cures and develops shrinkage forces, which significantly reduces the bonding strength. The use of prestressing in construction helps to reduce thermal stress, yielding good results – the service life of the repairs is increased by more than 4 months. The so-called prestressed construction is a method of preheating the equipment before bonding it. 　　In this way, when the device cools down to room temperature, differences in the contraction coefficients of the housing and the adhesive result in certain stress—a preload. When the temperature of glass-lined equipment changes during use, this stress is offset first; as the temperature continues to rise, the new stress generated is relatively small, thereby reducing the impact of temperature difference stress. 　　For small areas of damaged enamel, adhesive repair is usually sufficient; the damaged area should generally be cleaned of rust, oil, and dust. If metal is exposed at the damaged area, an alkaline-curing epoxy resin can be used as a barrier layer. The equipment is then heated to 40–60°C, and a corrosion-resistant paste is prepared according to the formula specified for the chosen adhesive. This paste is applied in thin layers to the damaged enamel area; after that, the area is kept warm and dry. The process is repeated, with more layers being applied and dried, until the desired thickness is achieved. Finally, gradually raise the temperature of the reactor to 20°C above its operating temperature, and maintain it at that level for 24 hours. This method has low repair costs and holds great practical value in the chemical industry.