Yin Hua Du Yahui Zhao Bing Liu Zhiling Abstract: Observing the change of the gas phase outlet temperature of the dryer in the melamine plant, the operating condition of the centrifugal drying system can be known in time, and the problem can be detected early and properly treated, which is beneficial to protect the centrifugal drying system from safe and stable long-term operation. Hair Removal Machine,Hair Removal Device,Best Hair Removal Device,Laser Hair Removal Device Shenzhen Jie Zhong Lian Investment Co., Ltd. , https://www.meizons.com
(Henan Coal Chemical Industry Group Co., Ltd. Zhongyuan Dahua Branch, Fuyang, Henan 457004)
Keywords: dryer gas phase outlet temperature change effect
CLC number: TQ051·8 Document code: B Article ID: 1009-9859(2009)01-0017-02
1 Overview
The melamine plant of Zhongyuan Dahua Branch of Henan Coal Chemical Industry Group Co., Ltd. adopts the technical design of Italian Eurotech. One of the most important large-scale devices in the plant is the DELMONEGO company in Italy. It is a tumble dryer, which is equipped with 3 rings of 81 axial steam pipes along the inner wall of the drum. The wet melamine filter cake separated by the centrifuge is dried by a dryer by a combined drying method of convection and conduction. It becomes a qualified product with a water content of less than 0.1%. In the whole drying process, the gas phase outlet temperature of the dryer not only indicates whether the water content (mass fraction, the same below) in the melamine product is qualified, but also timely and accurately reflects the important equipment such as centrifuges and dryers. Is the operation normal? Observing the change of the gas phase outlet temperature of the dryer is conducive to timely finding the problem, properly handling it, and minimizing the loss.
2 Process
The wet melamine content separated by the centrifuge has about 10% moisture, and the wet melamine enters the screw conveyor from the lower hopper of the centrifuge, and then enters the dryer for drying. The heat required for the dryer is provided in two parts: 1 by low pressure steam (0.66 MPa, 165 °C) through a steam tube to melamine; 2 by 155 °C hot air directly in contact with melamine. The hot air is in countercurrent contact with the material, and a bag filter is arranged at the gas phase outlet of the dryer to recover the melamine dust from the hot air. There are three temperature measuring points at different positions of the gas phase outlet line of the dryer: point a is set on the gas phase outlet line of the dryer; point b is set on the inlet line of the bag filter; c point is set in the bag type On the outlet line of the filter. The process flow of the melamine device dryer is shown in Figure 1.
3 Effect of dryer gas phase outlet temperature
The normal operating temperature of point a of the melamine plant centrifugal drying system is 90~100 °C, the lowest can not be lower than 80 °C, and the highest can not be higher than 130 °C. Within this range, the temperature at point a is determined according to the load of the centrifuge, and the smaller the load, the higher the temperature at point a. The temperature at points b and c is closely related to the temperature change at point a. If the temperature at point a is normal, then point b should be about 10 °C higher than the temperature at point c.
3·1 dryer gas phase outlet temperature is high
In the case of normal load of the melamine device, if the gas phase outlet temperature of the dryer is higher than 120 ° C and rises rapidly, there are generally three cases: 1 there is accumulation in the hopper under the centrifuge; 2 bag filter is blanked There is accumulation in the pipe; the filter bag of the 3-bag filter is broken.
No matter which of the above occurs, it should go to the site to check and confirm, and also refer to whether the current output is significantly lower than the average. If it is confirmed that the above accident occurred, the centrifuge should be reduced in load according to the severity of the accident.
The temperature at point a can reflect the blockage of the centrifugal drying section faster and more accurately than the output. This is because: 1 Since the point a is located on the inlet gas phase line of the dryer, once the material enters the dryer, the temperature at point a will change significantly (this is due to the heat exchange between the incoming wet material and the countercurrent hot air). The results); 2 According to the empirical formula, it can be estimated that the residence time of melamine in the dryer is 51min, the situation of the blocking material in the dryer is not easy to be found in time.
In production, the sooner an accident is discovered, the faster it is processed, and the smaller the loss is, so the change in the gas phase outlet temperature of the dryer can reflect the operation of the equipment in time. The capacity of the melamine plant is 1.667 t/h. If it is not discovered and treated in time after the accident, the average hourly product of 1.667 t is retained in the centrifugal drying section, which eventually causes the centrifuge to be overloaded and easily blocked. In the case of normal system load, if the temperature of the gas outlet of the dryer is too high, the output is small and the material is not blocked, it is suspected that the filter bag of the filter is damaged, thereby further checking the pollution of the machine sealing water.
3·2 dryer gas phase outlet temperature is low
The point a leaving the dryer is 70 ° C. Since the 15 kg/h backflush air added by the bag filter is cold air, the local temperature on the filter bag is lower than the display temperature. In March 2001, the temperature of point a was operated at 82 °C for two or three days. The bag filter quickly became clogged. After opening, it was found that the filter bag partially had a length of 600 mm, indicating that the air temperature had reached the dew point. Therefore, it is necessary to ensure that the temperature at the gas outlet of the dryer is above 90 °C. If the temperature is lower than 80 °C, the system will not be treated, and the water content of the melamine product will be greatly increased.
On the one hand it will affect the quality of the product, on the other hand it will also have serious consequences of sticking and blocking due to the high humidity of the material. The reason for the low temperature of the gas phase outlet of the dryer is multi-faceted. Since the first drop of the gas phase outlet temperature of the dryer was discovered on March 1, 2001, the system experienced multiple temperature drops due to various reasons, such as: 2003 On March 17th, the spiral blade of the screw conveyor of the dryer inlet was damaged, resulting in poor feeding and blocking the inlet of the dryer. On June 12, 2003, the ammonia fan was flooded, causing the outlet fan to be pressed; Washing tower packing plugging (by-products from system wastewater); On April 7, 2005, due to the replacement of the centrifuge jaws, the humidity of the filter cake increased, and the materials were often blocked at the inlet of the dryer.
In the case that the production load of the melamine unit is constant, if the gas phase outlet temperature of the dryer is low, the following aspects should be noted.
(1) Check whether the pressure of the steam system and steam pipe network is normal;
(2) Check the operation of the centrifuge, refer to the water content of the wet cake at the lower hopper of the centrifuge (≤15%). If the water content is too high, or the centrifuge equipment itself has a problem or the process is overloaded. Exceeding the design capability of the equipment, the load reduction operation is required at this time;
(3) Check the operation of the ammonia scrubber;
(4) Check if the amount and temperature of hot air are normal;
(5) Check the operation of the bag filter (if there is crusting on the filter bag, the outlet pressure will be low);
(6) Check the operation of the dryer: whether the soft joint is damaged, causing excessive cold air to enter; the opening of the bypass valve of hot air is too large, the amount of hot air entering the dryer is not enough; the load of the dryer is too large ; dryer equipment problems.
4 Conclusion
After nearly 10 years of continuous exploration and summarization, the influence law of the vapor phase outlet temperature of the melamine dryer on the centrifugal drying system was obtained. Through careful operation, multiple system parking accidents are avoided. It not only ensures the stable operation of the centrifugal drying system, but also avoids huge economic losses and environmental pollution.
references
1 Material Accounting and Calorie Balance of Gao Xiu Xue·Triamine Dryer · Big Nitrogen Fertilizer, 2002, 4(25)
2 Editor-in-Chief of Jin Guojun·Dryer Equipment Design·Shanghai: Shanghai Science and Technology Press, 1983