One question per day: A brief analysis of distillation unit startups and shutdowns

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This post was last edited by Xiao Yu on September 24, 2017, at 10:27. There are rewards for correct answers; there are also rewards for participating in the discussions. Let’s learn from the best practices in the “Brothers” section. Everyone can improve through these Q&A sessions and become more mature through posting replies. Experience-sharing question: A brief analysis of the process during a distillation unit shutdown. One night, at midnight, in a certain month and year, the feed pump J403A in the distillation pressure tower stopped working, and pump B failed to start for some reason; the on-duty control operator and shift leader immediately ordered the shutdown of the distillation unit. (At this point, it is sufficient to stop distillation and production, putting each column into full reflux mode; there is no need for a complete shutdown.) By the time they arrived at the workshop, distillation had already come to a complete stop. The J403B pump still could not be started, and the temperatures in all the distillation columns dropped rapidly; the temperature of the main column was only around 50 degrees. Since it was not possible to maintain the column pressure, nitrogen was even introduced to maintain positive pressure. It is obvious that the sudden cessation of feed caused the liquid level in the main column to drop, which prevented the reflux pump from being stopped. Without stopping the reflux pump, the circulation water in the reflux cooler continued to flow, resulting in too much steam being removed until its supply was cut off. This led to the main column being cooled down and losing pressure, forcing the use of nitrogen to maintain positive pressure. (At this point, there is no inflow or outflow in the distillation process; it is necessary to significantly reduce the steam supply, adjust the reflux rate, and adjust the circulating water volume. It is indeed difficult to determine the appropriate parameters for large-scale installations. However, as long as the distillation process does not experience overpressure or loss of pressure, it’s fine. The fact that they preferred to fill the system with nitrogen rather than stop the reflux pump suggests that the control system failed to recognize the problem at that time.) As soon as the reflux pump stops, the solids and liquid stored within the main tower release heat; nitrogen charging must be halted immediately, otherwise the safety valve might activate. Full recirculation – adjusting the circulating water volume to be the same as the return flow volume – is equally crucial. ) Arrived at the team site at 00:30; all condition checks and adjustments were completed by 1:00. The J403B pump was inspected on site, and preparations were made to restart it. After another thorough flushing and venting process, Pump B was successfully started. The likely reason was improper operation by the personnel on site, who failed to fully open the slightly open outlet promptly after the pump started, resulting in flow protection that caused repeated shutdowns of the pump. The reason for the shutdown of Pump A remains unknown; however, there was indeed sticking during rotation, so power was cut off, fluid was drained to relieve pressure, and work was handed over to a mechanic for repair. After the feed pump of the pressurized tower was successfully started, steam was reintroduced to the reboiler, and the system quickly entered a stable reflux phase; the reflux and circulating water were adjusted in an orderly manner, and distillation operations were initiated. Two hours later, the product from the pressurized tower met the required standards, and it was sent to the purified methanol storage tank. Four hours later, the product from the atmospheric pressure tower also met the standards and was sent to the same storage tank (if nitrogen hadn’t been used, the product from the atmospheric pressure tower should have met the standards after three hours). Distillation operations were fully restored, and a recovery tower was added midway through the process. By early morning, the methanol produced by the pressurized tower met U.S. standards, with an ethanol content of less than 10 ppm. Everything progressed at an astonishingly fast pace, and at 8:00 a.m., the shift was completed perfectly. (A special thank you goes to the analysts on the night shift for going to the extra effort to take additional samples for me.) The system load is 55%, the distillation load has increased to around 70%, feed is provided indirectly, and the output is approximately 38 T/min. The unit features three-column distillation, with the columns being packed towers. There are two operators on site at all times, who work methodically and at a steady pace.