Technical renovation of the evaporation condenser in urea plants

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Our company’s urea plant (1,740 t/d) uses the CO2 stripping process, and its urea production volume has now exceeded the designed capacity. In the original design of the urea plant, the cooling water used for the first-stage evaporation condenser (11702–C) and the second-stage evaporation condenser (11703–C) was supplied in series, with primary water at a temperature of 16°C being used as the cooling medium. The cooling water first enters the secondary evaporation condenser for cooling, and then proceeds to the primary evaporation condenser. The amount of water used for cooling is 850 m3/h; the cooled primary water (700 m3/h) is fed into the urea cycle water return network as make-up water for the circulation water system. Since not all of the water is utilized in one go, the excess water has to be discharged, resulting in a waste of water resources. To this end, our factory has decided to carry out water-saving technological upgrades by switching from using raw water to recycled water as the cooling medium, in order to reduce raw water consumption, lower wastewater discharge, and save operating costs. 1. Contents of technical renovation: Taking into account the actual conditions of our factory, this renovation utilizes advanced process technologies and new types of equipment designs, making full use of existing installations and utility facilities to develop a reasonable design plan. During the transformation process, every effort should be made to minimize or avoid any disruption to the normal operation of the existing production facilities, in order to save on investments, accelerate the construction schedule, and improve the economic efficiency of the enterprise. The specific modification details are as follows: (1) Eliminate the use of fresh water, and replace it entirely with recycled water for cooling the first and second stage evaporation condensers. Since the temperature of the circulating water is higher than that of the primary water, the area of the first and second evaporation condensers is insufficient, requiring modification; moreover, due to the large amount of circulating water used, the cooling water pipeline system also needs to be altered. (2) The original design capacity of the urea plant was 1,740 t/d. Taking into account the improvements to the current process flow and the enhancement of equipment capacity, the production load after the capacity expansion is set at 115% of the original design capacity, resulting in an output of 2,001 t of urea per day. Regarding the temperature conditions of the circulating water, considering that temperatures may rise to 30–32°C in summer, the inlet temperature of the circulating water is set at 32°C, while the outlet temperature is set at 42°C. The inlet pressure of the circulating water is 0.5 MPa (absolute), and the outlet pressure is 0.3 MPa (absolute). (3) After switching to circulating water, the temperature is higher, the temperature difference decreases, and the consumption of circulating water increases. At a production load of 100%, the consumption of fresh water (t=16°C) is 850 t/h; after switching to recycled water (t=32°C), the water consumption rises to 2185 t/h. When the production load is 115%, the circulating water consumption is 2,650 t/h (the maximum value). (4) After switching to circulating water, temperature differences affected the heat transfer efficiency; the original heat exchange area was insufficient. Calculations showed that the heat exchange area of the first-stage evaporation condenser should be increased to 1,047 m2, and that of the second-stage evaporation condenser to 1,021 m2 – both values being 1.5 times the existing areas. (5) After switching to recycled water, the original process flow remains unchanged. Since recycled cooling water is used in place of fresh water, the temperature of the cooling water changes; specifically, the designed temperature for the recycled water entering the second-stage evaporation condenser is 32°C, while the exit temperature is 35.5°C. This water then enters the first-stage evaporation condenser, where the exit temperature of the water from that stage is kept below 42°C. 2 Selection of energy-efficient equipment: The primary evaporation condenser was originally a typical fixed-tube-sheet type heat exchanger, with a diameter of φ950mm, heat exchange tubes of φ25mm×2mm, and a heat exchange area of A＝693.6m2. The design pressure for the shell side is 0.35 MPa/total vacuum, and the design temperature is 150°C. The design pressure of the pipe train is 0.61 MPa, and the design temperature is 70°C. Due to the large size of the gas inlet pipe on the shell side, guide cylinders and expansion sections are installed at the gas inlet to avoid affecting gas distribution and the exhaust pipes. Additionally, a grid is used for shell-side support to reduce pressure drop and improve heat transfer. Materials: The shell-side cylinder is made of 00Cr19Ni10, the tube-side cylinder is made of 16MnR, and the heat exchange tubes are also made of 00Cr19Ni10. The heat exchange area of the two-stage evaporation condenser is 678.8 m2, while all other parameters are the same as those of the single-stage evaporation condenser. This renovation uses an efficient and energy-saving heat exchange device – a bellows-type heat exchanger. This heat exchanger is an upgraded version of heat exchange equipment designed taking into account the characteristics of the petrochemical industry; it has been widely used in industries such as petroleum, chemicals, metallurgy, and power generation, with its technical specifications being 2 to 3 times higher than those of other types of heat exchangers. Its advantages are as follows: (1) High heat transfer efficiency. Due to the use of specially designed and manufactured variable-cross-section corrugated tubes as heat transfer elements, various media can induce turbulence at normal flow rates (0.5–1.0 m/s), disrupting the boundary layer and thereby increasing the heat transfer coefficient. At the same time, the variable-cross-section bellows features high pressure resistance and thin wall thickness, which reduces thermal resistance and further improves heat transfer efficiency; its heat exchange capacity is 4 times higher than that of a plain tube, resulting in significant energy-saving effects. (2) No scaling, low risk of clogging. Due to the turbulent flow of the fluid and its ability to automatically compensate for thermal expansion, the corrugated tube heat exchanger possesses self-cleaning capabilities; it does not suffer from scaling and can clean itself, which reduces the risk of clogging and saves significant amounts on cleaning costs. (3) Strong corrosion resistance and long service life: By using corrosion-resistant materials for the heat transfer elements, the equipment’s corrosion resistance is significantly improved, and its service life is about 3 times longer than that of traditional heat transfer elements. (4) Small size and compact structure: Under the same load conditions, the volume and floor area of the heat exchanger are significantly reduced, which can lower the costs associated with civil construction. (5) Low equipment resistance: The heat transfer elements of this heat exchanger have a large inner diameter, do not suffer from scaling, and are not prone to blockage, which reduces the power consumption of the system and lowers the production costs of the installation. (6) Low thermal stress: Due to the ability of bell-type heat exchangers to automatically compensate for thermal expansion, the thermal stress is low, thereby ensuring that the entire equipment operates in a safe and stable condition. (7) Ministry of Investment: In addition to features such as long service life, low space requirement, and low maintenance costs, bell-type heat exchangers can reduce the initial investment cost by 20%. 3 Conclusion: By upgrading the evaporation condenser of the urea plant and replacing primary water with recycled cooling water, 1.188 million tons of primary water can be saved each year, reducing water consumption and increasing the company’s profitability. After the technical upgrades, the first and second stage evaporation condensers are capable of handling a plant load of 115%, laying the foundation for increasing fertilizer production.