Thread Content
Our plant’s ammonia synthesis capacity has now reached 130,000 tons per year (of which 15,000 tons are methanol, 150,000 tons are urea, and 80,000 tons are ammonium carbonate). It also produces high-purity methanol, formaldehyde, morpholine, medical oxygen, dissolved acetylene, and chemical machinery; it is a comprehensive chemical enterprise that manufactures more than a dozen different chemical products. Due to the lack of a major overhaul for 5 years, the operation of the entire plant is very inefficient, as evidenced by the high steam consumption in the conversion system and high resistance (0.25 MPa). To change this situation, the focus of this major overhaul is to carry out technical upgrades to the conversion system. 1 Process modification and explanation: The original transformation process used a medium-high-low sequence, while after the modification it became a medium-medium-low-low sequence. Specifically, the semi-water gas coming from the second stage of the compressor (at a pressure of 0.75 MPa) is cooled to below 40°C before entering the coke filter, and then it goes to the bottom of the saturation tower. The temperature of the gas exiting the top of the saturated tower is 123–127°C, with a vapor-to-gas ratio of 0.35–0.40. This gas enters the tubes at the bottom of the main heat exchanger, where it exchanges heat with the shift gas; as a result, its temperature rises to 310–330°C. After exiting the top of the main heat exchanger, it proceeds to the upper section of the intermediate reformer. Steam is supplied at the bottom of the main heat exchanger; the temperatures in the upper and lower sections of the medium temperature section are controlled at (450±10)°C and (460±10)°C respectively, with adjustment via a cold shock valve bypass. The exhaust gas enters the upper part of the main heat exchanger, exchanges heat with semi-water gas through the tube banks, and then proceeds to the one-water stage. There, it is cooled to 180–210°C by hot water through the tube banks before entering the upper section of the low-temperature reformer. The exit temperature of the upper section of the low-pressure shift reactor is approximately 250–280°C; the shifted gas is cooled to 180°C using a two-stage cooling system before entering the lower section of the reactor, where the exit temperature is about 210°C. After passing through the first water addition unit, it enters the hot water tower; the temperature of the shifted gas exiting the hot water tower is around 75–85°C. It then proceeds to the second and third water addition units, where the shifted gas is cooled to about 40°C, and after separation it goes on to the shift removal unit. Water flow path: The temperature of the water at the outlet of the hot water tower is around 100°C. After being pressurized by a hot water pump, it passes through the first water addition unit, the second water adjustment unit, and the first water adjustment unit successively, before entering the saturation tower. At this point, the temperature of the hot water is between 135 and 145°C. The water supply for the system comes from the condensate of urea vapor. 2 Main equipment and inlet/outlet pipe diameters The main equipment and inlet/outlet pipe diameters after the technical renovation are shown in Table 1. http://www.nmtech.com.cn/jishuwang/upload/0605081605133338.jpg 3 Catalyst loading scheme The catalyst loading scheme is shown in Table 2. http://www.nmtech.com.cn/jishuwang/upload/0605081606114794.jpg The heating and reduction scheme for the medium-temperature catalyst (B117) is shown in Table 3. http://www.nmtech.com.cn/jishuwang/upload/0605081607004068.jpg Notes: (1) The higher the air velocity, the better; the lower the system pressure, the better. The vent valve should be fully open. Generally, the air velocity should be between 200 and 300 h—1 (under standard conditions). (2) During the 200℃ constant temperature phase, the minimum temperature of the catalyst should be raised to at least 20℃ above the steam dew point temperature. (3) In the case of retrofitting existing equipment (changing the third section of the original medium-voltage furnace to a connection with the upper section head of the low-voltage furnace), attention should be paid to system resistance to prevent damage to the head during temperature increase. 5 Low-variation vulcanization scheme: The vulcanization scheme is shown in Table 4. http://www.nmtech.com.cn/jishuwang/upload/0605081607596083.jpg 6 Operating indicators: The operating indicators are shown in Table 5. http://www.nmtech.com.cn/jishuwang/upload/0605081608457957.jpg 7 Transformation results: After 2 months of operation, it has been shown that the transformation of the medium, low, and very low process levels was highly successful, achieving the intended goals with excellent results. (1) The resistance of the transformation system decreased from 0.25 MPa before the modification to 0.03 MPa ； (2) Ammonia steam consumption decreased from 500 kS to 190 kg ； (3) The ammonia production per compressor unit in this system has increased from 9.1 t/d to 10.5 t/d ； (4) The system has a good thermal efficiency, with a significant reduction in the amount of steam required.