Measures to improve the quality of modified asphalt

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Measures to Improve the Quality of Modified Asphalt Abstract: To address issues such as coking in the reaction vessels and low levels of quinoline-insoluble substances in the production of modified asphalt at Jiugang Coking Plant, measures were taken including the reuse of flash vapor oil and the adoption of a low-temperature, long-process technology in the reaction vessels, which effectively solved the problem of coking and blockage in these vessels. Keywords modified asphalt ； Quality ； Modified asphalt is a high-value product obtained through high-temperature thermal polymerization of coal tar pitch; it is used as a binder for carbon products such as graphite electrodes used in electric arc steelmaking, as well as electrode plates and electrode pastes used in electrolytic aluminum production. Since its production accounts for about 50% of the total coal tar output, its volume and quality have a decisive impact on the overall efficiency of coal tar processing. The production of modified asphalt at Jiugang uses the thermal polymerization method. Medium-temperature asphalt undergoes thermal polymerization in series-connected atmospheric-pressure reactors; after polymerization and condensation reactions occur, the low-boiling components in the asphalt are flashed off. As the softening point of the asphalt increases, the β-resin content, which is an indicator of the asphalt’s adhesive properties, also rises, resulting in modified asphalt. The modified asphalt flows by gravity into the intermediate tank, and then is pumped to the elevated tank using an submersible pump, where it is cooled and shaped by a chain plate machine. 1 Main existing problems: 1.1 Severe coking and blockage of equipment – There is a serious problem of coking in the reaction vessels during the production process; this often leads to blockages in the angle valves at the bottom of the vessels as well as in the pipelines, resulting in increased system resistance and poor flow within the reaction vessels. During production operations, asphalt has on multiple occasions entered the condenser through the self-venting oil pipes, causing the condenser to become clogged and unusable, as well as blocking the angle valves at the bottom of the tank; this prevented the asphalt from being discharged from the tank, forcing manual cleaning of the tank. Especially after 20 days of operation, coking and blockage become particularly severe. 1.2 The low quality grade of the product: To reduce high-temperature coking, measures were taken to lower the heating temperature; however, the modified asphalt obtained had low levels of toluene-insoluble and quinoline-insoluble substances, which meant it could only meet the standards for grade two according to national regulations. Its quality did not satisfy the requirements of customers. 2. Cause analysis: Upon inspection, it was found that the coking on the reactor wall was 10–15 mm thick; especially in Reactor No. 2 where the temperature was higher, the coking thickness at the bottom of the reactor reached 20 mm. The reasons for this can be attributed to several factors. First, during the production process, the light components of asphalt evaporate, resulting in an increase in the high-carbon components and thus an increase in viscosity. This leads to a reduction in the fluidity of asphalt, and it tends to coking at high temperatures. Secondly, the system temperature is too high. Since the content of quinoline-insoluble substances QI in the asphalt produced by our factory is low, the softening point of the asphalt is below 115°C; in modified asphalt, the QI content does not reach 6%. Therefore, it is necessary to keep the softening point above 120°C in order for the QI content to meet the required 6% level. The heating temperature in each reactor is high, with the temperature at the bottom of the reactor reaching 410–420°C, which leads to high-temperature coking. Third, at the beginning of operation, the asphalt at the bottom of the tank does not flow, and the failure to start the mixer in a timely manner leads to coking at the bottom of the tank, with high temperature being the main factor causing this issue. The low quality grade of the product is due to the relatively low levels of BI% and QI% in the coal tar used as raw material in our factory; in other domestic and international producers of modified asphalt, the BI% level in coal tar is around 7%, while the QI% level is around 3%. A statistical analysis of the comprehensive analysis of coal tar in our plant over the past two years has shown that the BI% content in the tar produced by our plant ranges from 1.5 to 2.0%, while the QI% content ranges from 1.0 to 2.0%. This forces our factory to produce modified asphalt with a quality that only reaches grade two. 3 Main measures: There is an inherent relationship among the quality parameters of coal tar pitch; the higher the softening point of the pitch, the higher the values of BI% and QI% ； By adding appropriate additives to asphalt with a high softening point, its softening point can be significantly reduced, while the BI% and QI% values remain largely unchanged. Based on these properties of coal tar pitch, and taking into account the actual conditions of our factory, the following improvement measures were proposed and implemented: 1. Raise the softening point of the pitch so that its toluene-insoluble content exceeds 28%, which is the requirement for first-class quality, and its quinoline-insoluble content exceeds 8%, also in line with the requirements for first-class quality ； Then, an appropriate flash oil is added to lower its softening point to no more than 115°C, as required for first-grade products. Thereby ensuring that all the quality and technical parameters of the modified asphalt meet first-class standards. .2) Add one more reactor to establish a low-temperature, long-process production system with four reactors in series. At the same time, the kettle temperature at the beginning of operation was kept below 150°C, and the startup time of the mixer was adjusted from 1 hour after feeding to 30 minutes. 3) To address the coking issue and extend the production cycle, 4 reactors are used for simultaneous and balanced heating, thereby reducing the heating load on each reactor ； 4) Change adding flash oil inside the reaction kettle to adding it in the asphalt intermediate tank. The original setup, which used a single 10KW pump to feed the flash oil, has been changed so that part of the oil coming from the condenser goes into the flash oil tank, while the other part flows directly into the intermediate tank. 4 Implementation results: The coking problem in the reaction kettle has been significantly improved, and the production cycle of the plant has been extended by more than twice. Table 1: System operation parameters and product quality before and after improvement. After switching to using flash vapor oil in the intermediate tank, production increased steadily, with the quality of the product reaching over 98%. One pump with a capacity of 10 KW was eliminated, and there were no more issues resulting from high moisture levels that could affect normal operation of the reactor. Table 1 System operation parameters and product quality before and after improvement Item Before Improvement After Improvement Heating furnace chamber temperature, °C 700–750 550–600 Temperature at the bottom of the reaction vessel, °C 410–420 390–395 Softening point of modified asphalt, °C 110–120 108–115 β-resin, % 18–20 20–22 Toluene-insoluble matter (TI), % 26–29 28–32 Quinoline-insoluble matter (QI), % 6–8 8–10