Brief description of the process flows in various units for coking gas purification -- Ammonium sulfate unit (production of ammonium sulfate using a spray-type saturator)

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Ammonium sulfate production section (ammonium sulfate produced in a spray-type saturator) – Why is it necessary to remove ammonia from the gas? Ammonia is formed when the coal pyrolysis temperature is above 500°C; about 20–25% of the nitrogen in the gas produced by high-temperature coking is converted into ammonia. The ammonia content in the raw gas ranges from 8–11 g/m3 (1.0–1.5% on a volume basis). 8–16% of the ammonia present in the gas dissolves in the condensate during gas cooling. Most of the ammonia remaining in the gas is absorbed by the condensate water, and when this water is sprayed for cooling in the cooling tower, it is desorbed back into the atmosphere, causing pollution ； Ammonia in the gas combines with hydrogen cyanide to form highly soluble complexes, thereby exacerbating corrosion. Furthermore, ammonia in gas produces toxic and corrosive nitrogen oxides when burned ； Ammonia, in the recovery of crude benzene, enables oil and water to form a stable emulsion, hindering their separation. All of the above pose difficulties for modern coking production; for this reason, the ammonia content in the gas is not allowed to exceed 0.03 g/m3. Therefore, it is necessary to recover ammonia from the gas. At present, most coking plants in China use sulfuric acid to absorb ammonia from the gas and produce ammonium sulfate, which is then used as a chemical fertilizer. 1. Process flow: The gas supplied by the gas blower enters the saturator via a gas preheater. In the upper section of the saturator, the gas enters the annular chamber in two streams and is sprayed with circulating mother liquor; the ammonia present in it is absorbed by the sulfuric acid in the mother liquor. Afterwards, the gas combines into one stream and enters the rear chamber, where it is sprayed once more with mother liquor before passing into the cyclone acid removal device in the saturator, so as to separate the acid mist carried by the gas. Finally, the gas is sent to the final cooling and benzene washing unit. The mother liquor located in the upper part of the lower section of the saturator is continuously pumped out by a mother liquor circulation pump and sent to the annular chamber for spraying. The recycled mother liquor, having absorbed ammonia, flows downward through the central downcomer to the bottom of the lower section of the saturator; there, nuclei move upward through the saturated medium, causing the crystals to grow and leading to particle size separation. Use a crystallization pump to transfer the slurry at its bottom to the crystallization tank. The mother liquor that overflows from the full-flow port of the saturator flows to the mother liquor storage tank through a full-flow channel; this full-flow channel is equipped with a liquid seal tank, and the mother liquor at the bottom of the full-flow channel is pumped by a small mother liquor pump to be sprayed in the rear chamber of the saturator. The mother liquor from the mother liquor storage tank is pumped by a small mother liquor pump to the full-flow trough. In addition, the mother liquor storage tank can also be used to store mother liquor during the maintenance of the saturator. The slurry from the crystallization tank is discharged to a centrifuge; the separated ammonium sulfate is conveyed to a vibrating fluidized bed dryer, where it is dried using air heated by a hot air blower. After being cooled by cold air, it enters an ammonium sulfate storage bin, after which it is weighed, packaged, and sent to the finished products warehouse. The filtered mother liquor, along with the mother liquor flowing out fully from the crystallization tank, flows back by gravity to the lower section of the saturator. The exhaust gas after drying ammonium sulfate undergoes two stages of dust removal before being released into the atmosphere. First, most of the dust contained in the exhaust gas is removed using two sets of dry cyclone dust collectors. The exhaust gas is then pumped to an exhaust gas cleaning tower by an exhaust gas fan, where it is continuously sprayed using a pump in the tower to further remove any remaining dust. Finally, the liquid droplets present in the exhaust gas are removed by a mist catcher before the gas is released into the atmosphere. The sulfuric acid from the oil depot is sent to the sulfuric acid elevated tank, and then flows by gravity to the full-flow tank. 2. Process characteristics
1) It employs a spray-type saturator that integrates acid washing, acid removal, and crystallization. The equipment is compact in size, features high deammoniation efficiency, produces large-sized ammonium sulfate particles, has a simple process flow, and represents an advanced and reliable technology. 2) The spray saturator system has low resistance, resulting in low energy consumption for the blower. 3) The equipment and pipelines in the ammonium sulfate mother liquor system are made of ultra-low carbon stainless steel, which ensures a long service life, permits the plant to operate continuously and stably over time, and reduces maintenance costs. 4) Vibrating fluidized bed is used for the drying of ammonium sulfate, which provides good drying results and is easy to operate and maintain. 5) The dried exhaust gas undergoes two-stage dust removal using dry and wet methods, resulting in high dust removal efficiency with no dust carried in the exhaust gas. 3. Key technical performance indicators: Ammonia content in the gas after passing through the saturator ≤ 0.05 g/m3; temperature of the gas after passing through the saturator: ~55°C; moisture content in ammonium sulfate after drying ≤ 0.3%; resistance of the saturator ≤ 2 kPa. 4. Main environmental protection measures: 1) The mother liquor is discharged into an underground drainage tank and then returned to the system, without being released outside. 2) The dried ammonium sulfate exhaust gas is subjected to two-stage dust removal, dry and wet, ensuring no dust entrainment. 5. Working principle of the main equipment: The spray-type saturator – The gas passes through electrostatic tar collectors and desulfurization units, and is then pressurized by a blower before entering the gas preheater, where it is preheated to 60–70°C. This is done to evaporate the moisture present in the saturator, thereby preventing dilution of the mother liquor. The gas emerges from the mother liquor layer by bubbling through the bubbling umbrella via the central gas pipe of the saturator; the ammonia in it is absorbed by sulfuric acid to form ammonium sulfate. After separating the carried droplets in the acid extractor, it proceeds to the crude benzene recovery section. The ammonia content in the gas after the saturator is generally required to be less than 0.03 g/m3. In the saturator, the mother liquor enters the full-flow tank through a water seal tube, and from there it is pumped back to the bottom of the saturator, thus forming a mother liquor circulation system that creates an upward flow of mother liquor within the vessel. Ammonium sulfate crystals settle at the bottom of the cone in the saturator; a pump is used to send the slurry back to the crystallization tank, where ammonium sulfate crystals precipitate from the slurry. Once the crystallization tank is filled with liquid, it sends the liquid back to the saturator. The ammonium sulfate crystallization slurry has its crystals separated in a centrifuge; these crystals contain 1%–2% moisture. After dehydration in a dryer, they are sent to the warehouse. 6. Main influencing factor: Mother liquor acidity: The acidity of the mother liquor in the ammonia absorption equipment primarily affects the particle size of the ammonium sulfate crystals. As the acidity of the mother liquor increases, the average crystal size decreases. Additionally, with increasing acidity, the viscosity of the mother liquor rises, which increases the diffusion resistance of ammonium sulfate molecules and hinders the normal growth of crystals. Mother liquor temperature: The mother liquor temperature affects the growth rate of crystals. Generally, the growth rate of crystals increases with rising temperature of the mother liquor. Moreover, since the average growth rate of each crystal facet is higher than the growth rate along the length direction of the crystal, increasing the temperature helps to reduce the length-to-width ratio, thereby resulting in better-quality crystals. However, the temperature should not be too high; otherwise, it may lead to local supersaturation, prompting the formation of numerous crystal nuclei. Generally, the temperature of the mother liquor is controlled at 50–55°C. Mother liquor circulation: The purpose of mother liquor circulation is to ensure thorough stirring of the mother liquor within the vessel, thereby increasing the mass transfer rate. At the same time, efforts should be made to keep the acidity and temperature of the mother liquor inside the vessel uniform, which is beneficial for the growth of crystal nuclei. Additionally, there are the effects of crystal ratio and impurities: The crystal ratio refers to the percentage of the volume of ammonium sulfate crystals suspended in the mother liquor relative to the total volume of the mother liquor. Impurities in the mother liquor inhibit crystal growth. Final cooling and benzene washing section