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Analysis of Color Changes in AN Sulfuric Acid Production 2017-11-21 Chemical Encyclopedia 999 Information. Color changes can occur in AN sulfuric acid production for various reasons; addressing such color changes requires comprehensive analysis and the implementation of effective measures. Our factory uses a spray-type saturator in the process for producing sulfuric acid AN. Since its commissioning, our factory has observed that sulfuric acid AN has taken on various colors such as black and gray, resulting in the production of products that do not meet quality standards. Some experience was gained through the process of identifying and solving problems. Under normal conditions, pure anhydrous sulfuric acid appears as colorless, rhombic crystals with a density of 1766 kg/m3. With a molecular weight of 132.15, high-quality sulfuric acid contains ≥21% nitrogen; due to the presence of impurities, it often takes on various colors such as gray or dark black. The crystals are mostly needle-shaped, flaky, or powdery; the formed particles are very small, and the nitrogen content decreases. There are many factors that affect the quality of sulfuric acid AN, such as sulfuric acid concentration, mother liquor temperature, saturator resistance, and so on. Sulfuric acid AN takes on a black or gray color. The main reasons for this darkening are as follows: there is a malfunction in the electrostatic tar catcher, resulting in poor tar removal efficiency ； The raw ammonia contains a high amount of tar, which causes the tar to enter the ammonium sulfate system along with the ammonia gas during the ammonia vaporization process ； The quality of the sulfuric acid used is poor ； The mother liquor tank was emptied, and the acid tar floating on the surface was pumped into the saturator. Problems identified and measures taken: (1) Under normal circumstances, this is due to a malfunction in the electrostatic tar catcher in the drum cooling section, resulting in an excessive level of tar in that electrostatic tar catcher. At this point, obvious impurities can be observed in the sulfuric acid an particles. Due to the excessive tar content, large amounts of tar entered the sulfuric acid AN mother liquor, where it reacted with the acids present in the mother liquor to form large quantities of acidic tar. As a result of the circulation caused by the large mother liquor pump, this acidic tar mixed with the sulfuric acid AN mother liquor, causing its color to turn black. The darkening of the sulfuric acid AN mother liquor is usually caused by the presence of tar. The measures taken by our factory at that time were mainly: to restore normal operation of the electrostatic precipitator as soon as possible, to increase the voltage applied to it appropriately, and to raise the frequency of flushing it, changing it from once a month to once every two weeks ； Increase the frequency of removing acid tar from the full-flow tank, from once every hour to once every half hour. (2) There is a significant amount of tar in the residual ammonia tank; this tar enters the ammonia vaporization tower along with the ammonia. The ammonia gas that reaches the saturator contains more impurities and water vapor, which combine with the crystals, causing the color of sulfuric acid to turn gray or black. The measures taken by our plant are to clean the water seals in the ammonia pipelines from the ammonia vaporization tower to the saturator, thereby reducing the amount of condensate (such as tar and other impurities) that enters the saturator ； Increase the heat exchange area of the condenser at the top of the ammonia vaporization tower, so as to promptly condense the impurities carried in the ammonia gas and reduce the amount of such impurities in it ； Control the oil content in the residual ammonia water resulting from ammonia evaporation; intensify the process of removing tar from this residual ammonia water, doing so every 2 hours, in order to minimize the amount of tar present in it. (3) The main impurities in the mother liquor inside the saturator include iron salts, aluminum salts, thiocyanates, various arsenides, and various organic compounds such as phenols, pyridines, naphthalenes, and various components of tar. These impurities in the mother liquor are generally harmful to crystal growth; the ions of the impurity salts adsorb onto the crystal surface, covering the active areas there and slowing down the rate of crystal growth. Sometimes, the selective adsorption of impurities on certain crystal surfaces restricts crystal growth, affecting the crystal particles and their color. This is also one of the reasons why the particles of colored sulfuric acid AN are smaller. To ensure the purity of sulfuric acid AN, when using sulfuric acid, our factory gives priority to manufacturers with a high cost-performance ratio among the raw material suppliers, as well as to customers with whom we have long-term partnerships ； Unqualified sulfuric acid tests upon arrival at the plant prohibit unloading. (4) Perform a thorough acid wash every three months (keep the acidity at 15% and then carry out washing to ensure thorough cleaning of the saturator), in order to maintain smooth operation of the equipment. (3) Shorten the cleaning cycle of the mother liquor tank from once a year to once every six months, thereby reducing the amount of crystals deposited at the bottom of the tank. Color changes are quite common during the production of ammonium sulfate. To prevent the production of ammonium sulfate products with unacceptable colors, it is necessary to monitor the process continuously and make adjustments as needed, selecting different measures to address various color changes that occur.