Corrosion and control of two-stage evaporation heater

In the urea production process, in order to process the urea aqueous solution (temperature 90-120°C, concentration about 68%-75%) after synthesis and separation of unreacted materials into solid granular urea, a two-stage vacuum evaporation process is generally used to evaporate most of the water in the urea solution to obtain a urea melt with a concentration of 98%-99.8%, which is sent to a granulation tower for granulation. The flow diagram is shown in Figure 1. http://www.nmtech.com.cn/jishuwang/upload/0603281539005652.jpg The main task of the evaporation process is to remove the water in the urea aqueous solution through vacuum evaporation to obtain a urea melt with a moisture content of <0.5% (wet basis), a biuret content of <1% (wet basis), and other impurity content less than the specified index, and send it to the granulation tower for granulation to obtain qualified solid granular urea. In order to obtain granular urea that meets the conditions, the analysis and selection of process conditions are generally carried out using the urea-water equilibrium phase diagram. Urea aqueous solution has the characteristics of double boiling points. In order to avoid the operating curve entering the area between the first and second boiling points, the evaporation process conditions are generally selected as:: flash tank: Pressure 32kPa (absolute), temperature 100℃ ； A period of evaporation: Pressure 31kPa (absolute), temperature 130℃ ； Second stage evaporation: The pressure is 6~9kPa (absolute), and the temperature is 136~142℃. During the operation of the evaporation equipment, the second-stage evaporation heating separator, especially the pipeline between the second-stage evaporation heater outlet and the second-stage cyclone separator inlet, is prone to corrosion and leakage. Air is sucked into the system, making it difficult to control the second-stage vacuum degree within the specification, affecting the quality of the finished product, and in severe cases causing the separator to become unstable and scrapped. We analyzed the corrosion problem of the second-stage evaporation heating separator. 1 Reasons for corrosion of the second-stage evaporation and heating separator 1.1 Parts of the second-stage evaporation and heating separator that are corroded The parts of the second-stage evaporation and heating separator that are most prone to corrosion are in order:: The upper tube of the rising film heater, the heater outlet elbow, the rectangular reducer tube, the lower liquid pipe of the separator, etc. The schematic diagram of the corrosion points of the second-stage evaporation heating separator is shown in Figure 2. http://www.nmtech.com.cn/jishuwang/upload/0603281540163730.jpg 1.2 The material selection and processing of the two-stage evaporation heating separator consider that the process media are mainly Ur(L), H2O, Bi, Am, NH3, CO2, etc. In terms of equipment material selection, 304L stainless steel is generally used based on the operating temperature and operating pressure. Sub-arc welding technology is used for processing and welding to reduce the input of line energy and reduce the sensitization of welded joints. ； Use 25-22-2 welding materials to obtain ideal corrosion resistance. 1.3 The working medium and process conditions of each part of the two-stage evaporation heating separator are shown in Table 1. http://www.nmtech.com.cn/jishuwang/upload/0603281541069100.jpg Note: The content of other components in the process medium, the Ur content incorporated into the liquid phase, and the H2O content incorporated into the gas phase. The other components of the liquid phase are mainly Bi and Am ； Other components in the gas phase are mainly NH3, CO2, etc. 1.4 The main reasons for the corrosion of the second-stage evaporator separator (1) A form of damage caused by the abrasion and corrosion of the abrasive metal surface by the fluid medium at the same time. In the upper part of the heater tube, the velocity of the fluid medium in the tube is very high, close to turbulent flow, and turbulent corrosion occurs. Within this distance, the corrosion rate of the tube is much greater than that of other parts. Since the second-stage evaporation heater uses a rapid rising film heater, among various corrosion factors, abrasion should be an important factor. According to the material balance, a simple analysis was conducted on the changes in the velocity of the material flowing through the tubes in the heater. The results are as follows: ①Material form changes. In the tubes at the bottom of the heater, the temperature is lower, the urea solution is liquid, and the speed is slower. ； In the middle and lower part of the tube, the temperature is higher, and the solution part bubbles and boils faster. ； In the middle and upper part of the tube, the fluid is in a constant-temperature boiling state. The high-speed flowing air column in the center of the tube drives the liquid film on the inner wall of the tube to rise rapidly, and the fluid flows very fast. ②Calculated based on the Url50t shift output, when the urea aqueous solution flows through the two-stage evaporation heater tubes, the speed of each part under different pressures (tube specification φ25×2000, 85 tubes in total) is shown in Table 2. http://www.nmtech.com.cn/jishuwang/upload/0603281542224027.jpg Note: The calculation conditions are: The material Ur in each tube: 0.061 274 5kg/s ； H2O(g)M: 0.00292kg/s ； Tube cross-sectional area A: 0.000 314m2, consider the fluid flow as a steady flow. The lower part of the tube is in the liquid phase. Ignoring the influence of pressure on the density of the liquid phase and the change in the volume of the liquid phase, the calculation results can illustrate the problem. More accurate results can be obtained using the process simulation software Pro-Ⅱ (teaching version). (2) Scouring and Corrosion The high-speed liquid has a mechanical scouring effect on elbows, tees, etc., which will destroy the passivation film on the surface of stainless steel or titanium and prevent its re-passivation, so the corrosion rate will increase significantly. It can be seen from Table 2 that within the normal operating pressure range, the flow rate of the fluid is particularly large, especially for the upper part of the tubes, the effect of erosion and corrosion is more obvious. Since the rising film heater is a tubular heater, the heater can be reverse-installed in a planned manner according to the usage and cycle of the heater to extend the service life of the heater. (3) Corrosion caused by operational control factors. It mainly means that changes in temperature and pressure cause the operating curve to enter between K1-K2 of the Ur-H2O equilibrium phase diagram (see Figure 3), resulting in crystallization. http://www.nmtech.com.cn/jishuwang/upload/0603281543195641.jpg Note: W1 to W2 is the process of sudden temperature drop ； W2 to W3 are the vacuum breaking process ； W3 to W1 are the process of adjusting and restoring the system process status. W1 is the process point of normal control. There are many factors that can cause the temperature of the evaporation system to change during production. However, the main factors that have a greater impact on the second-stage evaporation are large changes in production load and pressure changes in low-pressure steam. Both of these factors will cause a sudden drop in the temperature of the second section. When the temperature loop of the automatic control system adopts simple control, the temperature fluctuations are more obvious. When the liquid phase outlet temperature of the second-stage evaporation heating separator suddenly drops, the process medium system enters the supersaturated zone and crystallizes. In order to avoid further deterioration of the working conditions, according to the operating procedures, the operating pressure is generally increased for a short period of time, that is, the vacuum degree of the system is reduced to promptly exit the system from the supersaturated zone. It can be seen from the equilibrium phase diagram that if the pressure is increased in a short time, since the operating point moves to the right (temperature decreases) and upward (pressure increases), the water content in the system increases and the urea content decreases, which will cause the quality of the product to decrease. There is always a certain time lag in this manual operation process. Before new equilibrium conditions are formed, the operating point will enter the supersaturated zone along the isobaric line, leaving some solid urea in the solution. Due to timely adjustment and control, the system exits the supersaturated zone in a short period of time, slowly adjusts, and returns to normal. During this adjustment process, partially crystallized urea, driven by high-speed fluid, will cause severe abrasion and erosion corrosion to the secondary steam heater, rectangular pipe and cyclone separator lower liquid pipe. The operation of breaking the vacuum and restoring normal control in the urea-water (pressure-temperature-concentration) equilibrium phase diagram is shown in Figure 3. 2 Main optimization and transformation measures 2.1 Equipment (1) Equipment selection. Considering that although 304L stainless steel is relatively corrosion-resistant in the urea production system, since the operating temperature of the secondary steam heater is (140±2)°C, and the flow rate of the fluid in the upper tube of the heater is basically at a high subsonic speed, the fluid corrodes 304L stainless steel very violently, so it is not suitable to choose 304L stainless steel as the heater manufacturing material. After comprehensively considering various factors, we commissioned the Ti-series heater produced by Shandong Linyi Chemical Machinery Factory to be produced. It has been in use for more than two years and there has been no corrosion or leakage in the heater. (2) Manufacturing and installation. Equipment manufacturing and installation units are required to use welding materials in strict accordance with the requirements and use sub-arc welding technology for welding and repair welding of stainless steel to reduce the input of line energy and reduce the sensitization of welded joints. 2.2 Operation control is mainly based on the double boiling point characteristics of urea solution and the process characteristics of two-stage vacuum evaporation, and is optimized from both automatic control and manual operation. (1) Automatic control system ① Stabilizes and adjusts the low-pressure steam pressure used in the evaporation system to reduce the impact of large fluctuations in steam pressure on the evaporation production system. ②Adjust the evaporation temperature control parameters of the urea DCS system to improve the sensitivity of the evaporation temperature control system and ensure the relative stability of the second-stage evaporation temperature. (2) Strengthen theoretical and practical training for operation control personnel. Operation control personnel are required to master the production principles of the urea evaporation process and understand the basic knowledge of phase diagrams. The urea-water equilibrium phase diagram can be used to guide production operations. In particular, it is necessary to understand the process of the system entering and exiting the supersaturated zone after changing the temperature and pressure in normal production. During production control, attention should be paid to changes in liquid level and temperature in the two-stage tower of the circulation system, and the operation of the first-stage evaporation should be adjusted in a timely manner to avoid the front system having a greater impact on the second-stage evaporation. The purpose is to improve the consciousness of controllers to strictly implement the "Urea Workshop Evaporation Process Operating Procedures", to minimize and avoid the system entering the supersaturated zone, to reduce the abrasion of the system during this process, and to ensure the stability and uniformity of product quality. 3. Effect of optimization and transformation (see Table 3) http://www.nmtech.com.cn/jishuwang/upload/0603281544163682.jpg Note: Since the heater has been used continuously for more than two years after being changed to Ti material, and the heater tube has not been leaked or repaired, the heater service life should be longer. 4 Conclusion The corrosion resistance of chemical production equipment has always been the primary factor in equipment selection. During the use of equipment, it is necessary to promptly find out the causes of equipment corrosion and solve them according to changes in usage conditions. This is an important guarantee measure for safe, stable, high-yield, high-quality, and low-consumption urea production.

1 Urea secondary evaporation heater and its corrosion? 1.1 Equipment overview? The secondary evaporation heater (referred to as the secondary evaporation heater) is the key equipment of the urea evaporation system. Its function is to further evaporate and concentrate the urine with a concentration of 95% in one stage to 99.7%, meeting the requirements of the urea granulation process. It is a typical shell-and-tube heat exchanger. Its technical characteristics are shown in Table 1. The upper and lower tube plates of the heater are made of 1Cr18Ni9Ti and have a thickness of 26mm. ； The tube material is 0Cr18Ni12Mo2Ti, the specification is φ25×2mm, and the length is 2.5m, a total of 99 tubes. ； The tube sheets and tubes are strength welded, and the welding rod grade is A207 ； Shell material is Q235-A, thickness δ=6mm ； The material of the shrimp back bend outlet pipe is 1Cr18Ni9Ti, and the specification is φ377×5mm. ； The shell steam inlet pipe is 250mm away from the upper tube plate. Table 1 Technical characteristics of secondary steam heater http://www.nmtech.com.cn/jishuwang/upload/0602161054522543.jpg? During operation, urine with a concentration of 95% and a temperature of about 120°C enters the tube from the bottom of the heater. Under boiling point conditions, it is heated by the steam outside the tube (normal steam is between 220 and 250°C to ensure a sufficient heat transfer temperature difference), causing the water in the urine to quickly vaporize. Under the pumping of the rising steam, the urine rises rapidly as a thin film on the inner wall of the tube. The urine sprayed out at high speed at the top of the pipe enters the secondary steam separator through the shrimp back-bent outlet pipe (hereinafter referred to as the outlet pipe) for vapor-liquid separation to obtain urine with a concentration of 99.7%. 1.2 Equipment corrosion status? According to statistics, from 1995 to 2002, Hunan Jinxin Chemical Company used and replaced 7 of the above-mentioned secondary steam heaters in 7 years. The longest replacement interval was 1 year and 7 months, and the shortest was only 1 year. At the same time, replace 5 heater outlet pipes. The reason for the replacement was equipment corrosion, which caused serious leakage and affected the production of the urea system. The basic conditions of secondary steam heater corrosion are as follows. (1) Corrosion of the welds between the upper tube plate and the tube rows. It can be seen from the maintenance records that the reason for each replacement of the secondary steam heater is the corrosion and leakage of pores and micro-cracks in the weld area connecting the upper tube plate and the tube row, and each piece of equipment has been repaired many times before annual overhaul. The weld leakage of the three heaters replaced before 1998 was only partial pipe weld leakage. However, the situation of the four heaters replaced after 1998 was more serious. Almost every repair found 15% to 20% or even 30% weld leakage. When using condensate to check for leaks, the leak appears like a fountain, the surface of the weld is rough, and there are serious pitting corrosion, pinholes, and cracks. ? (2) Corrosion of upper tube sheet. The material of the tube sheet in contact with the urine medium is 1Cr18Ni9Ti, which has a large amount of corrosion thinning. For example, the heater that was overhauled and replaced in 1998 was overhauled and replaced in 1999. During the thickness measurement and inspection of the upper tube plate, it was found that the corrosion rate was as high as 2mm/a. ? (3) Corrosion of urine outlet pipe. From 1995 to 2000, the material of the three outlet pipes replaced was 1Cr18Ni9Ti, δ=9mm. After thickness measurement inspection, the corrosion rate was also surprising. For example, when the outlet pipe used in 1997 was replaced and measured in 2000, the thinnest part was 4.2mm and the corrosion rate was as high as 1.79mm/a. However, in the "Nitrogen Fertilizer Process Design Manual (Urea Volume)", the recommended corrosion rate for the second-stage evaporator is only 0.15mm/a. There is obviously a big difference between the two. In addition, the leakage of the four welds of the outlet pipe was also serious, and holes often appeared in the welds. Checking the replaced outlet pipe, the inner surface of the outlet pipe was silvery gray and very bright due to urine erosion. The inner surface of the weld was poorly formed, with obvious slag inclusions and incomplete welding. ? (4) Pipe corrosion at the steam inlet pipe. During the maintenance, it was also discovered that some tubes on the side near the steam inlet within 200mm from the upper tube plate were thinned and cracked. For example, on September 18, 2001, the moisture content of the finished urea was too high. When the system was stopped for a short time to check for leaks, the secondary steam heater was inspected and it was found that two tubes were broken, causing a large amount of water vapor to escape into the urine. This situation occurred a total of 4 times. ? Since the above corrosion leakage often occurs after each heater is replaced and has been running for up to 7 months, and the shortest maintenance period is only one month, they not only cause the second-stage vacuum degree to drop, but also the moisture in the urea product to increase significantly (for example, in August 2000, due to leakage in the second steam heater, the first-grade urea product rate was only 76%), and each short stop processing takes about 3 to 5 hours, reducing urea production by 50 to 85 tons. Frequent corrosion and leakage have caused large economic losses. It is estimated that the annual economic loss caused by the leakage of the secondary steam heater is 150,000 yuan. If the annual cost of replacing equipment is added, the economic loss is as high as 300,000 yuan. ? 2 Analysis of causes of corrosion ? 2.1 Improper material selection, tube sheet and outlet pipe are not corrosion-resistant? In the original design, the tube material of the parts of the secondary steam heater that is in contact with the highly corrosive medium of urine is 316 steel, that is, 0Cr18Ni12Mo2Ti. Based on the economic considerations of the equipment, the material of the upper and lower tube sheets and outlet pipes of the equipment are all 1Cr18Ni9Ti. The Mo and Ni in 316 steel (0Cr18Ni12Mo2Ti) can improve the corrosion resistance of the steel. The steel contains 1.8% to 2.5% Mo, which can improve the corrosion resistance of the steel to urine media. * * Improvement, especially the resistance to pitting corrosion and crevice corrosion, is particularly prominent in reducing media such as urine. There is literature proving that adding 2% Mo to stainless steel can reduce the sensitization tendency of steel and resist corrosion caused by oxygen-deficient urine. The Ni content of 316 steel is 3% to 5% higher than that of 18-8 steel. Such high Ni can make the steel become unidirectional austenite, preventing the multi-phase structure from producing too many galvanic cells and reducing its stability. ； At the same time, it can reduce the solubility of carbon in austenite and improve the ability to resist intergranular corrosion. Therefore, 316 steel is a more suitable material to resist the highly corrosive medium of urea. Compared with 316 steel, 1Cr18Ni9Ti has the disadvantages of Mo and Ni, and its carbon content is higher. Under long-term high temperature conditions, it is easy to produce intergranular corrosion, resulting in serious corrosion of the tube sheet and its welds, as well as the urine outlet pipe. Therefore, it is not suitable to use 1Cr18Ni9Ti steel with poor corrosion resistance for the tube sheet and outlet pipe. ? 2.2 The working environment is harsh, and the welds between the upper tube plate and the tubes are severely corroded? The most common problem of the secondary steam heater is the leakage of the welds between the upper tube plate and the tubes. In addition to factors such as materials, poor working conditions are also a very important factor. Due to the particularity of the welding structure between the tubes and the tube sheet, and the excessive penetration of the tube sheet mouth during welding, the urine flowing up the pipe wall at high speed suddenly spreads at the outlet, easily causing erosion and corrosion to the edge weld joints. When the welding process of the upper tube plate and the tubes is not well controlled and the temperature difference stress is large, micro-cracks are easily generated on the surface of the weld, and the urine oxygen content in the secondary steam heater itself is low (about 0.45×10-6). When the oxygen trapped in the cracks is exhausted and fresh oxygen is not replenished, crevice corrosion is easy to occur. The temperature difference in the upper part of the tube is large, and the steam inlet is at the upper part of the tube. The water in the urine near the top of the tube will boil and turn into steam. It will cause severe vapor-liquid turbulence at the outlet of the tube along with the urine along the tube wall. Moreover, the edge distance between the tubes and tubes is very close (only 7mm), and the ammonia liquid vapor flow between each other is also prone to turbulence. At this time, interference and impact phenomena will occur in the stagnation area of ​​the upper tubesheet weld. In addition, the temperature at the tube sheet is relatively high (200-300°C) and the oxygen content in the medium is low, causing the welds between the upper tube sheet and the tubes to suffer impact corrosion in a high-temperature, oxygen-deficient environment. ? In contrast, due to the small heat transfer temperature difference of the condensate at the lower tube plate, the above corrosion phenomenon does not exist. ? 2.3 The production load is high and the outlet pipe is severely eroded and corroded? The design capacity of our company's urea plant is 110kt/a. Since the late 1990s, production has exceeded design capacity every year. In 2000, the system produced a total of 137,145t of urea, with a maximum daily output of 512t and an average daily output of 405t. Due to the long-term high-load operation of the equipment, large input and output, and high evaporation gas flow rate, the outlet pipe and weld seam were severely eroded and corroded. In addition, the material was 1Cr18Ni9Ti, which had low corrosion resistance, resulting in a sharp reduction in the wall thickness of the outlet pipe and frequent weld leaks. 2.4 Poor manufacturing quality and poor welding quality? The four circumferential welds of the shrimp back bend of the outlet pipe are cut by gouging during production, and the heat affected zone is large. In addition, the welding gap during assembly welding will inevitably have uneven sizes and misaligned edges. In addition, the welder's lack of responsibility during welding may easily lead to slag inclusion and incomplete penetration, resulting in weld leakage during use. When welding tubes and tube sheets, the welding process is not strictly controlled, the line energy input is too large, and the welding quality is poor. Defects such as pores, slag inclusions, and undercuts that are not fused during the welding process may accelerate local corrosion. In addition, due to improper selection of welding materials, the corrosion resistance of A207 electrode in certain environments is poor. ? 2.5 Improper position of steam inlet? The rupture of a few tubes within 250mm below the upper tube plate is also caused by various factors. In addition to erosion corrosion and poor welding quality (the metallurgical quality of individual tubes is poor and the impurity content is high), another important reason is that the surface temperature is higher, the urine concentration is the highest, and the gas phase partial pressure is the lowest, which causes the wall thickness near the outlet of individual tubes to become thinner and cause rupture. The original design steam inlet is only 250mm away from the upper tube plate, and the steam temperature is generally above 200°C. Data shows that the operating temperature of 316 steel is not allowed to be higher than 195°C. It should not exceed 186°C under low oxygen conditions, especially at 195 to 200°C, the corrosion rate increases three times that of normal temperature. It can be seen that tube rupture and tube-sheet weld leakage are related to improper selection of steam inlet position and excessive operating temperature. ? 2.6 Poor process operation? The production is unstable, the output is unbalanced, the load is high and low, the system process operation indicators fluctuate greatly, and the equipment cannot be used in a balanced manner, making the corrosion rate polarized and difficult to control, which is also one of the factors causing equipment corrosion. ? 3 Preventive measures to control corrosion ? 3.1 Use urine corrosion-resistant materials? In order to improve the resistance of the secondary steam heater to urea medium corrosion, all parts and components in contact with the highly corrosive medium of urine are made of 316 steel. At the same time, in order to better improve its resistance to intergranular corrosion, 316L steel with lower carbon content, namely 00Cr18Ni12Mo2Ti, is used. so, * * Improves the corrosion resistance of equipment. ? 3.2 Reasonable structural design? (1) Change the outlet pipe form from shrimp back elbow pipe to blind plate tee pipe (see Figure 1). This can not only facilitate leak detection, but also make the four shrimp back bend ring welds that are difficult to weld into one weld, and the plate thickness is changed from 5mm to 8mm. On the premise of meeting the process conditions, the structure is simplified and the corrosion resistance is improved. http://www.nmtech.com.cn/jishuwang/upload/0602161055395273.jpg Figure 1 Improvement of urine outlet tube structure (2) Increase the extension length of the tube. In the original design, the length of the tubes protruding from the tube plate is 2mm according to GB151-89 "Steel Shell and Tube Heat Exchanger". This structure has extremely weak resistance to erosion corrosion, impact corrosion and crevice corrosion of the welds in this specific environment. For this reason, the extension length of the column tube is modified to: The upper tube plate is 25mm and the lower tube plate is 10mm. This special structural design can * * Reduce or weaken defects caused by erosion, impact and crevice corrosion. ? (3) The steam inlet position moves downward. The tube section and the upper tube plate at the shell side steam inlet have the largest heat load. They are two important parts where tube rupture occurs and the upper tube plate weld leaks. The corrosion reactions in these two parts are the most severe. Changing the position of the steam inlet can evenly distribute the heat load throughout the entire tube side, thereby eliminating or mitigating local overheating. At present, there is still a lack of sufficient experimental research on the exact position of the steam inlet. According to experience, the inlet position is at the lower part of the pipe, and is preferably 0.5m higher than the condensate outlet. For this reason, in the updated design, the distance between the steam inlet pipe and the upper tube plate was changed from 250mm to 950mm, and a porous plate distributor was added at the steam inlet to reduce the erosion of the steam on the tubes. ? 3.3 Elaborate production and strict control of welding quality? Make the secondary steam heater strictly according to the drawing requirements, select the appropriate welding process, and implement strict supervision and inspection management during production. The highly corrosion-resistant ultra-low carbon welding rod BM310M-L imported from the Netherlands is used to replace the welding material A207 for the export tee pipe welding seam and the welding of the tube plate and the tube row. It places higher requirements on the quality of the groove. The heat input is strictly controlled during welding and the welding is performed carefully to improve the welding quality. ? 3.4 Strict quality control of process operations? (1) Strictly control various process indicators. On the premise of meeting the process indicators, the shell side steam pressure should be reduced as much as possible. ? (2) Strive to achieve balanced production, control daily output at around 390t, and minimize high-load production time to reduce the impact of erosion corrosion and other corrosion. ? (3) Fully ensure the oxygen supply of the system to increase the content of urine entering the secondary steam heater. Make the tube sheet welds have higher corrosion resistance. ? 4 Conclusion? Corrosion of urea secondary steam heater is a common problem in brother units of medium nitrogen fertilizers. The urea secondary steam heater designed, manufactured and installed according to the corrosion control countermeasures proposed in this article has passed the system operation test for more than three years, and the anti-corrosion effect is very significant.: There has been no leakage in the upper tube plate and tube welds and the outlet tee welds so far. ； There is no obvious thinning phenomenon in the rows, tube sheets and outlet tubes. ； There is no tube rupture in the lower part of the upper tube plate. ； The service life of this type of equipment is expected to be more than 5 years. The effective control of the corrosion of the urea secondary steam heater creates important conditions for ensuring the quality of urea products and long-term operation of the urea system. It has been highly recognized by users and provides a reference for the manufacturers of China Nitrogen Brothers.