Technical comparison of ammonium sulfate processes and equipment in coking plants

Ammonium sulfate production process 1. Process flow The spray saturator is divided into an upper section and a lower section. The upper section is the absorption chamber and the lower section is the crystallization chamber. The gas from the desulfurization process is preheated to 60-70°C or higher by the gas preheater in order to maintain the water balance in the saturator. After the gas is preheated, it enters the upper section of the spray-type saturator and is divided into two streams to flow annularly along the annular chamber in the horizontal direction of the saturator. Each stream of gas is sprayed through several nozzles with circulating mother liquor containing 3.5% to 4% free acid to absorb ammonia in the gas. Then the two streams of gas merge into one stream and enter the back chamber of the saturator. The mother liquor from the small mother liquor circulation pump (also called a secondary spray pump) is used for secondary spraying to further remove ammonia in the gas. The gas then enters the acid remover in the saturator in a tangential direction to remove the acid mist droplets entrained in the gas. It leaves the saturator through the upper center outlet pipe and then passes through the mist catcher to capture trace amounts of acid mist in the gas before entering the final cooling benzene washing section. The ammonia content in the gas after the spray saturator is generally less than 0.05g/m3. The upper and lower sections of the saturator are connected by a downcomer. The mother liquor after spraying and absorbing ammonia flows from the falling liquid concept to the bottom of the crystallization chamber, where the crystallization nuclei are pushed upward by the saturated mother liquor, and the mother liquor is continuously stirred to grow the sulfate anhydride crystal nuclei and cause particle classification. Use a crystallization pump to send the slurry at the bottom to the crystallization tank. The mother liquor containing small particles rises to the upper part of the crystallization chamber. The mother liquor circulation pump extracts the mother liquor from the upper part of the crystallization chamber and sends it to two sets of spray boxes in the upper section of the saturator for cyclic spraying, so that the mother liquor continuously circulates between the upper section and the lower section. The upper section of the saturator is equipped with a full flow pipe to maintain the liquid level and seal the gas so that the gas cannot enter the lower section. The full flow pipe inserted into the overflow tank 7 also seals the gas so that the gas cannot escape. The mother liquor overflowing from the full flow port of the saturator flows into the liquid seal tank in the overflow tank, then overflows to the full flow tank, and then is pumped with a small mother liquor to the back room of the saturator for spraying. When flushing and adding acid, the mother liquor passes through the overflow tank to the mother liquor storage tank, and then is pumped to the saturator with a small mother liquor. In addition, the mother liquor storage tank can also be used to store the mother liquor during maintenance of the saturator. The slurry in the crystallization tank is left to stratify, and the crystals at the bottom are discharged into the centrifuge. The separated and washed anhydrous sulfate crystals are sent to the vibrating fluidized bed dryer by a belt conveyor, and dried with air heated by an air hot air blower, and then cooled by cold air before entering the anhydrous sulfate storage hopper. Then weigh, package and send to finished product warehouse. The mother liquor filtered out by the centrifuge flows back to the lower section of the saturator together with the mother liquor flowing out of the crystallization tank. The tail gas of dry sulfate anhydride is discharged to the atmosphere from the exhaust fan after passing through the cyclone dust collector. In order to ensure a certain acidity of the circulating mother liquor, concentrated sulfuric acid with a mass fraction of 90% to 93% is added to the inlet pipe or full flow pipe of the continuous mother liquor circulation pump to maintain the normal acidity of the mother liquor. The sulfuric acid sent from the oil depot is sent to the sulfuric acid storage tank, and then pumped out by the sulfuric acid pump to the high sulfuric acid tank, and then flows to the full flow tank. The spray-type saturator is used to produce sulfuric acid anhydride. The spray-type saturator is made of stainless steel. The equipment has a long service life. It integrates pickling absorption, crystallization, acid removal, and evaporation. It has the characteristics of small gas system resistance, large crystal particles, an average diameter of 0.7mm, good sulfate anhydride quality, short process flow, and easy operation. This process is often used in newly constructed and renovated coking plants to recover ammonia in coal gas. 2. Control of factors affecting the crystal particle size of ammonium sulfate. For the recovery of ammonia in coking gas in coking plants, the saturator method is generally used to produce ammonium sulfate. Since the crystallization process of ammonium sulfate is affected by many factors, if certain factors are not properly controlled during production, it will cause the product particles to become small, the moisture and acidity to exceed standards, and affect the product quality. Therefore, it has become an important topic to find out the key factors affecting the crystal size of ammonium sulfate and to seek the best operating methods. 1. Principle of ammonium sulfate crystallization The crystallization of ammonium sulfate is a reaction process, which mainly consists of several stages: reaction, formation of supersaturated solution, generation of crystal nuclei and growth of crystals. As the reaction proceeds, a supersaturated solution forms. When a certain degree of supersaturation is reached, solid phase microscopic grains are precipitated. This is the formation process of crystal nuclei, also known as primary nucleation, followed by the growth of crystal nuclei, also known as the growth process of crystals. At the same time, due to the flow of crystal liquid, friction and collision between crystals and between crystals and equipment, and the erosion of the crystal surface by the liquid, new crystal nuclei are generated, which is called secondary nucleation. Usually the formation of crystal nuclei and the growth of crystals proceed simultaneously. During the crystallization process, both the formation and growth of crystal nuclei consume the solute in the solution, both driven by a certain degree of supersaturation. Each crystal is grown from a crystal nucleus. Under certain conditions, if the crystal nucleation rate is greater, the amount of crystal nuclei generated will be greater. The limited solute in the solution must supply a large number of crystal nuclei for growth at the same time, and the growth rate of the crystal nuclei will be slower, resulting in a large number of fine crystals. ； On the contrary, the smaller the amount of crystal nuclei generated, the larger the crystal grain size will grow. It can be seen that the generation rate of crystal nuclei and the growth rate of crystal nuclei are in a trade-off relationship. If these two rates can be controlled, the particle size of the crystal can be controlled. In addition, crystallization conditions also have a great impact on the particle size of the product, such as temperature, stirring, acidity, impurities, etc., which all affect the crystallization process in certain ways. 2. Factors affecting the crystal particle size of ammonium sulfate. According to the analysis of crystallization principles, the factors affecting the crystal particle size of ammonium sulfate can be summarized as follows.: (1) The working temperature of the saturator (2) The degree of stirring of the mother liquor (3) The acidity of the mother liquor and the acid addition system (4) The crystal ratio of the mother liquor (5) The impurities in the mother liquor. For certain process conditions in production, the factors that have a greater impact are often those that change frequently or are sensitive to changes in quantity, and the degree of control is increased due to the lag in the resulting results. By analyzing the above factors, it can be found that the working temperature of the saturator and the degree of stirring of the mother liquor do not change much, and can be said to be approximately constant. ； The acidity and crystal ratio of the mother liquor change periodically with time. Frequent changes and improper control will have a great impact on the crystal particle size. ； The impact of impurities in the mother liquor is relatively sensitive to changes in quantity. Generally speaking, it is very contingency. However, once it occurs, it will have a great impact on production. Therefore, the acidity, crystal ratio, and impurity content of the mother liquor are the focus of production control. 3. Control of influencing factors 1. Mother liquor acidity The influence of mother liquor acidity on ammonium sulfate crystallization is mainly reflected in two aspects.: One is the impact of acidity on the shape of crystals, and the other is that frequent changes in acidity destroy the normal growth conditions of crystals. Under certain conditions, as the acidity of the mother liquor increases, the metastable zone of the mother liquor decreases, and the crystal form of ammonium sulfate changes from polyhedral particles to slender and fragile hexagonal prisms or even needles. At the same time, the viscosity of the mother liquor increases, and the diffusion resistance of ammonium sulfate molecules increases, hindering the normal growth of crystals. ； But it won’t work if it’s too low. Although ammonium sulfate crystallizes at pH 5 to 6 Larger round crystals are generated in the weakly acidic medium, but the absorption efficiency of ammonia is reduced, and the saturator is easily blocked. Especially when the mother liquor is not stirred sufficiently or the acidity fluctuates, a neutral or alkaline zone may appear locally in the mother liquor. Impurities such as iron and other metal ions and ammonium generate colloidal hydroxide and cover the ammonium sulfate crystals, making it difficult for the crystal to grow and complicating the crystallization process. Moreover, when the acidity of the mother liquor is lower than 3.5%, foam is easily generated due to the decrease in density of the mother liquor, which worsens the operation of the saturator. In order to avoid these effects, crystallization must be carried out in an acidic medium. During normal production, the acidity of the mother liquor should be maintained at 4% to 6%. The influence of acidity on crystal size is also reflected in the fact that when acid is added to the system regularly, the acidity of the mother liquor increases significantly, causing the crystal seeds in the mother liquor to disappear and destroying the normal growth conditions of the crystal. During recrystallization, primary nucleation occurs at a higher degree of saturation, causing an increase in fine crystals in the mother liquor. Therefore, in order to produce large particles of crystalline thiamine, the number of large acid additions should be reduced. For example, the acid addition should be changed from once per shift to once every 1 to 2 days, and the stable operation time of the saturation period should be extended as much as possible. 2. Mother liquid crystal ratio The ratio of the volume of thiamine crystals contained in the mother liquid to the total volume of the mother liquid and crystals is called the crystal ratio. Controlling the crystal ratio in the saturator is an important measure to control the size of thiamine crystals. It can be known from the crystallization principle that if the nucleation rate and the growth rate of crystal nuclei can be controlled, the particle size of the crystal can be controlled. However, these two rates are extremely difficult to control in production. Whether it is the explosive primary nucleation or the secondary nucleation caused by friction and collision, it is difficult to control. The generated nuclei are always excessive, that is, the nucleation rate is too high. The growth rate of crystal nuclei is very slow compared to the nucleation rate. When other conditions remain unchanged, it mainly depends on the supersaturation of the mother liquor. Increasing the supersaturation can accelerate the growth of crystals. The degree of supersaturation depends on the number of crystal nuclei in the mother liquor at a certain temperature. When there are sufficient crystal nuclei in the mother liquor, the newly generated thiamine solute is completely used for the growth of crystals, the supersaturation tends to be stable, and the crystals are in a stable growth environment. In production, the method of controlling the crystal ratio is used to control the degree of supersaturation and achieve the purpose of controlling the crystal particle size. The size of the crystal ratio directly affects the particle size of the crystal. When the crystal ratio is too large, the chances of friction and collision increase, and the large crystal particles are broken, which increases the amount of secondary nucleation, slows down the crystal growth rate, reduces the crystal particle size, and increases the stirring resistance of the mother liquor, resulting in poor stirring. It also reduces the volume required for the reaction between ammonia and sulfuric acid, which is not conducive to the absorption of ammonia and can easily aggravate clogging. ； If the crystal ratio is too small, there may be less crystal nuclei, which will increase the degree of supersaturation and produce a large amount of primary nucleation, which will reduce the crystal particle size. If the crystal ratio is too small, the number of removals will increase, the growth time of the crystal will be shortened, and the crystal particle size will also be reduced. Therefore, a certain crystal ratio must be controlled in the mother liquor to obtain large particles of thiamine. The control principle of crystal ratio should be: Avoid primary nucleation, properly control secondary nucleation, and try to extend the crystal growth time. To control the crystal ratio, in addition to controlling the crystal ratio in quantity, we must also observe the shape and color of the crystals, predict the crystallization situation in the saturator, and make judgments and adjustments based on the actual crystal particle size. If it is found that the crystals in the crystal liquid are small and the crystal grains taken out are also small, it should be considered whether the acidity is too high, or whether the crystal ratio is inappropriate or caused by excessive nucleation. ； If the crystals are not only fine but also colored, the influence of impurities should be considered. In normal production, the crystal ratio should be controlled to a minimum of not less than 10%, and it is appropriate to take it out when it reaches 30%. 3. Impurities Impurities in the solution affect all stages of the crystallization process. The type and content of impurities in the ammonium sulfate mother liquor mainly depend on the technological process used, the quality of sulfuric acid, the quality of water and the anti-corrosion quality of the equipment. The soluble impurities contained in the mother liquor mainly include iron, aluminum, copper, lead, antimony, arsenic and other salts. Most of them come from sulfuric acid, equipment corrosion and industrial water. These ions are adsorbed on the surface of the ammonium sulfate crystal, covering the active area of ​​the crystal surface, causing the crystal to grow slowly. ； Sometimes due to the selective adsorption of impurities on certain crystal planes, small deformed particles are formed. Metal ions have a great influence on the growth of ammonium sulfate crystals, especially iron ions, which have the greatest impact. Even if the content is very small in the mother liquor, the crystal growth rate will be significantly reduced. For example, ferric iron ions will promote the expansion of the metastable zone and slow down the crystallization speed. When the content in the solution reaches 0.1%, it will cause the ammonium sulfate crystal to become longer, and at higher concentrations, it will lead to the formation of needle-like crystals. This crystal will be broken into large quantities during the production process, greatly reducing the particle size of the finished ammonium sulfate. In addition, insoluble impurities in the mother liquor, such as coal tar mist, sometimes form stable emulsions with the mother liquor and adhere to the crystal surface, hindering crystal growth. The impurities in the mother liquor not only affect the crystal form and crystal growth of ammonium sulfate, but also make the total crystal volume growth per unit time smaller than the amount of ammonium sulfate generated in the saturator, breaking the solid-liquid balance and increasing the supersaturation of the mother liquor. This not only reduces the crystal strength, but also forms a large number of needle-like crystal nuclei, which quickly fill the solution and disrupt normal operations. Therefore, effective measures must be taken in terms of processes, equipment, etc. to reduce the entry of impurities from the source.

Vibrating fluidized bed dryer manufacturer

The saturated concentration crystallization process is used before the thermoelectric desulfurization process. The original product contains about 5% water and a temperature of about 50 degrees. The average daily output is about 50 tons. The wet ammonium sulfate itself after separation by the centrifuge has a high moisture content. After the wet ammonium sulfate is cooled to normal temperature, it begins to clump and harden. Coupled with the old woven bags used for packaging, the ammonium sulfate can easily absorb moisture in the air. ； As the storage time increases, the hardening phenomenon will become more serious, affecting later use and failing to meet the needs of external sales. Picture In October 2019, a research team composed of Zhu Lixin, Dong Zengyi and Liu Hongwei went out to inspect the drying process of anhydrous sulfate, a by-product of ammonia desulfurization. After many discussions, reviews and demonstrations with relevant personnel of the production center. The final project adopts a vibrating fluidized bed drying scheme, without adding a new drying plant and using the original ammonium sulfate complex building for layout. ； The exhaust gas does not use a water film dust collector and is directly sent to the desulfurization tower to wash and discharge qualified emissions. ； The surrounding area of ​​the original wet ammonium sulfate transfer warehouse is used as a finished product warehouse for anhydrous sulfate. The project has started construction and equipment installation in June 2020. The workshop has established a special start-up team and compiled operating procedures and various plans. Through training and field exercises, the operators have mastered the process flow and operating methods. Single-unit commissioning will begin in December. The joint test run will begin in January 2021, and the feeding of materials will begin on January 29, and the feeding test will be successful once. The drying system added this time reduces the moisture content of ammonium sulfate from 5% to less than 1%, making it easier to store and sell ammonium sulfate for a long time. ； After drying, it is packed in tons, which reduces the labor intensity of workers and significantly improves the on-site environment. Through analysis reports and communication with marketing companies, it was determined that after drying, it is packaged in tons and can be sold as compound fertilizer raw materials.