A newbie is here to ask a few questions
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This post was last edited by lflsedin on 2015-8-4 21:01. First of all, the design pressure for the high-flash point unit in our factory is 0.9 MPa. However, during production it was found that almost no flash vapor existed after the high-flash point condenser. In one accident, I noticed that when the pressure dropped to 0.6 MPa, stable flash vapor began to be produced after the high-flash point condenser, and its volume also reached the designed value. Therefore, I would like to ask the experts here: Is it acceptable to operate the unit at a pressure of 0.6 MPa? If not, what are the potential hazards? Second: Due to design reasons in our plant, the high-flash point is located at the top of the framework, while the true flash point is at the bottom. The pipeline that carries fluid from the carbon scrubber to the high-flash point is not only long but also has several bends; as a result, this pipeline often gets blocked. Once, it became blocked right after the plant started operating, and it took 3 days to clear it before the plant could resume operation. It’s just my personal opinion, but the amount of black gas emitted by the carbon scrubbing tower is around 17 m3/h, which is not much. The pipes and angle valves used to send this gas to the true flash unit are located at relatively low positions, with few bends. I wonder whether it’s possible to send this black gas to the true flash unit during normal operation at a pressure of 6.2 MPa after boosting the pressure. If it’s not possible, what could be the consequences? Thank you all in advance
The first question: I’m not aware of your specific process flow, so it’s difficult to say for sure. I can only base my answer on my own experience; please bear with me if that’s not sufficient. First, you mentioned that when the high-flash pressure is maintained at 0.9 MPa, almost no flash vapor is produced. In my opinion, this is related to the temperature and flow rate of the black water from your gasification furnace. If the temperature is low or the flow rate is low, it will certainly result in less flash vapor. It’s normal for the amount of flash vapor to increase when the pressure is reduced to 0.6 MPa, as water that couldn’t be vaporized before now becomes vaporized, thus increasing the amount of steam produced. As for the second point, it’s best not to discharge the water from the washing tower at 6 MPa into the true flash vessel, as its high temperature causes intense vaporization upon entry, which can easily disrupt the vacuum level in that vessel and hinder stable operation. Additionally, the water from the washing tower is generally not designed to be discharged to the true flash vessel; instead, it is the wastewater from the vaporization furnace that is discharged to the true flash vessel during the initial startup phase when there is no pressure (pressure below 1 MPa). Once the pressure in the vaporization furnace exceeds 1 MPa, discharge is directed to the high flash vessel. Since the pressure difference between the wash tower and the true flash vessel is very small, it is not possible to drain water to the true flash vessel; therefore, this pipeline is generally not designed. I don’t know why you designed this pipeline.
This issue is quite complex, and the answer may not be comprehensive. Regarding the first question: when the high flash point design pressure is 0.9 MPa and it is reduced to 0.6 MPa, several factors can affect operations. You should consider whether a pressure reduction is necessary based on your company’s process flow. These factors include: 1) The reduced pressure increases the pressure difference between the gasification furnace and the high flash point, leading to severe wear on the angle valves or tees; 2) After the pressure is reduced, a large amount of heat is carried away by the flashed vapor, resulting in inadequate recovery of waste heat. This leads to a decrease in the water-to-gas ratio at the gasification outlet, and such a decrease has significant implications for subsequent processes. The second issue is that when the high flash point is set high, the black water from the wash tower often gets clogged in the high flash line. This problem can be resolved through two methods: 1) During each maintenance session, the scale and debris accumulated at the bottom of the wash tower must be thoroughly cleaned; 2) After shutting down the system, it is not advisable to rush to switch to the black water flow – instead, the system should be kept under pressure while in circulation, with an increased circulation rate and greater discharge of black water, so as to thoroughly clean the system. Both of these methods can address the problem of blockages in the black water pipelines of the wash tower. It is not possible to directly use the true flash point for this purpose, as first, the residual heat cannot be fully recovered, and second, a large amount of non-condensable gas is emitted during true flashing. Additionally, the pressure difference between the two systems is too large, which increases the requirements for the equipment.
The low-pressure flash temperature is low, the amount of steam is high, and the heat content is high, so the heat exchanger cannot be fully cooled. Elbows are prone to clogging during the pressure increase process; it is necessary to ensure thorough cleaning during maintenance. After feeding material in, make sure that the scrubber discharges sufficient volume of fluid. When shutting off water flow, ensure that the pressure in the scrubber is much higher than the high flash point pressure. After shutting off water flow, increase the pressure promptly while simultaneously carrying out vaporization and flashing. Before stopping the operation and shutting off water flow, ensure that there is a sufficient pressure difference between the scrubber and the high flash point system. . .
The design pressure for high flash is 0.9 MPa. This value is determined based on the temperature and flow rate of the water discharged from the vaporization furnace, which allows the system’s heat load to be determined; thus, the pressure can be fixed fairly easily. Reducing the pressure lowers the boiling point of water, resulting in the evaporation of large amounts of water and dissolved gases. However, high-flash gas is used for heat exchange with high-pressure gray water; it is necessary to determine whether this amount of heat is sufficient. If it is, it is recommended to reduce the pressure, as this will lower the temperature and thereby reduce the stress on the subsequent heat exchangers. Second, the blockage in the drainage pipeline of the carbon scrubber tower: to address this fundamentally, the first thing to consider is the condition of the pipeline itself, namely whether it has been properly flushed, and the second thing is the cause of the blockage. Could low pressure be causing poor drainage and blockages? Is it still clogged by dirt particles? It is recommended to discharge a large volume of water when the tangential flash point is high, in order to increase the pressure before integrating it into the system.
I am extremely grateful for the responses from all of you; they have helped clarify and broaden my thinking. I will study hard and come back to ask more questions later