In allusion to a chemical plant in Guizhou Province，it′s pointed the situation that scaling jam and pitting corrosion phenomena inside graphite tube during phosphoric acid condensation，and thus the tube graphite heat exchanger loses its efficiency.Using the test methods of macroscopic examination and scanning electronic microscope observation，chemical composition analysis，XRD analysis，thermogravimetric analysis and open porosity test to analyze the graphite tube scaling layer and the graphite tube，so as to comprehend the mechanism of the inefficacy of the tube graphite heat exchanger and control it effectively.Results showed that the main contents of scaling layer were CaCO3，CaF2 and SiO2；scaling jam was due to high content of impurities，such as CaCO3，SiO2 and some metallic oxide（MgO and Fe2O3）in phosphoric acid and un-timely scaling removal in graphite tube bundle．Pitted corrosion mainly caused by corrosion under the scale，and the size，quantity and distribution of corrosion pits varied according to the porous degree of scaling layer．The looser of scaling layer，the easier of phosphoric acid to penetrate the scaling layer to form localized corrosion on the inner wall，and inner wall would appear pitted corrosion phenomena with the increase of etching time．
Tubular graphite heat exchanger is widely used in heat exchange of corrosive medium such as phosphoric acid, sulfuric acid and hydrochloric acid because of its strong corrosion resistance, good thermal conductivity and high heat transfer efficiency. Graphite tube bundle is the heat exchange place of corrosive medium, which is called tube side. Heat and cold carriers pass through shell side. Both of them exchange heat by temperature difference. With the prolongation of working time of graphite tube bundles, failure accidents of graphite tube bundles caused by scaling, blockage and pitting corrosion become more and more frequent, which not only seriously threaten the safety of workers, but also cause waste of materials and environmental pollution, and affect the normal operation of production plants and the economic benefits of enterprises. A major failure may lead to a catastrophic accident, through failure analysis can avoid and prevent similar failure, thereby improving the safety of equipment. Therefore, the failure analysis of scale blockage and pitting corrosion of graphite tube bundles in the production process environment has important reference value and practical significance.
In January 2017, it was found that the concentration of phosphoric acid could not meet the production requirements. It was initially inferred that the problem was caused by graphite tube bundles, so it was checked and repaired. It was found that most of the graphite tube walls had been seriously scaled and blocked and pitting corrosion. In order to ensure the representativeness, coverage and comprehensiveness of sample selection, three failure graphite tubes were intercepted from near to far diameters in the most prone position (near the inlet) of heat exchanger. Macroscopic examination, scanning electron microscopy, chemical composition analysis, X-ray diffraction analysis, thermogravimetric analysis and open porosity test were carried out and chemical composition of dilute phosphoric acid was tested. Through the above characterization and analysis methods, the causes of scaling, blockage and pitting corrosion of graphite tubes are revealed, and scientific support is provided for the operation and maintenance of the equipment.
The failure of the tube type graphite heat exchanger tubes used for phosphoric acid concentration is related to the production medium and environment. Phosphoric acid concentration process: First of all, low pressure saturated steam was introduced into the tubular graphite heat exchanger to heat phosphoric acid. The phosphoric acid entered the evaporation chamber through the driving force provided by the axial flow pump of concentration, and the water content was taken away by vacuum evaporation. It was concentrated from w (H3PO4) = 32% ~ 37% to w (H3PO4) = 62%. In this process, phosphoric acid moves along the tube side of the tubular graphite heat exchanger, and the low pressure saturated steam takes the shell side. According to the chemical composition test of dilute phosphoric acid, it can be inferred that the scaling and clogging phenomenon of graphite tube is caused by the high content of impurities CaCO3, SiO2 and metal oxides (MgO and Fe2O3) in dilute phosphoric acid and the graphite tube bundle is not cleaned regularly. When phosphoric acid is concentrated, as the concentration of phosphoric acid increases gradually in the cycle of tubular graphite heat exchanger and evaporation chamber, the solubility of many ions (such as Na2O, K2O) and other acid insoluble substances (such as calcium, magnesium, fluorine and other salts) in phosphoric acid decreases sharply and precipitates, forming a hard scale shell attached to the inner wall of the graphite tube. The hard scale shells consist mainly of calcium carbonate and silicate, such as calcium silicate. The results of chemical composition analysis and X-ray diffraction analysis showed that the scale layer on the inner wall of the three failed graphite tubes is mainly composed of CaCO3, CaF2 and SiO2 (known CaCO3 and SiO2 react to produce CaSiO3 at high temperature), which is consistent with the theoretical analysis.
It is known that when phosphoric acid is concentrated, phosphoric acid is more likely to penetrate the porous scale layer and react with the inner wall of the graphite tube to form local corrosion, that is, the “blocked zone”. And with the extension of corrosion time, pitting corrosion pits will appear in the inner wall of the tube, and the inner wall of the tube has a tendency to gradually thinning, and even the possibility of corrosion perforation. The corrosion condition of the graphite tube closer to the feed port is more serious.
1) Scaling and clogging in the inner wall of graphite tube heat exchanger is caused by the high content of impurities CaCO3, SiO2 and metal oxides (MgO and Fe2O3) in dilute phosphoric acid, and graphite tube bundlesis not cleaned regularly. With the increasing concentration of phosphoric acid, the concentration of soluble impurities in phosphoric acid increases correspondingly. When it reaches supersaturation state, crystallization will occur and some crystals will adhere to the inner wall of graphite tube to form a hard scale shell. These salt scales will gradually crystallize, harden and thicken layer by layer under steam heating until finally they reach supersaturation state. Block the inner wall of the pipe. From macroscopic examination and microscopic analysis, the pitting pit is mainly caused by scale corrosion.
2) According to the chemical composition analysis and X-ray diffraction analysis, the scale layer in the inner wall of the graphite tube is mainly composed of CaCO3, CaF2 and SiO2, while the corrosion products near the pit of the inner wall of the graphite tube are mainly composed of CaCO3, CaF2, MgF2, SiO2 and FeS2. According to the size, number and distribution of pits in the inner walls of three failed graphite pipes, phosphoric acid is more likely to react with the inner walls of graphite pipes through the relatively loose scale layer and form local corrosion, and pits will appear in the inner walls of the pipes with the extension of corrosion time. Compared with other graphite pipes, the graphite pipes which are nearest to the inlet are most seriously corroded. The corrosion products are the most, the thermal stability is the worst, and the porosity is the highest.
3) Suggestions are made to prevent scaling, blockage and pitting corrosion in the inner wall of tube bundles of tubular graphite heat exchangers from the following aspects: firstly, phosphoric acid which meets the production requirements is selected, i.e. controlling the impurity content of phosphoric acid ((32%
Author：Jiang Fengyi，Tian Mengkui，Hao Litong，Shao Zhuhua，Yang Ying，Luo Xiangli
