【Weekly Topic】The Impact of Non-condensable Gas at the Top of the Tower
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This post was last edited by Guan Gongyu on 2016-4-4 at 14:04: bg9.png【Is there any impact if there is non-condensable gas at the top of the tower? 】 1. Does the top temperature increase or decrease? 2. Does the top pressure increase, decrease, or remain uncertain? 3. What is the temperature difference between the inlet and outlet of the tower top condenser? And most importantly, does the condenser outlet temperature increase or decrease? ============================= “Does it be “increase,” “decrease,” or “uncertain” depend on the type of non-condensable gas? Is it related to the operating pressure of the tower? (Especially during the material feeding stage at the beginning of operation.) ) Everyone is welcome to watch and discuss. Extra 5-10 points for in-depth discussions: 1. [Petroleum and Chemical Industry Events] Photo and text documentation of the process from plant construction to commissioning – Registration post: http://bbs.hcbbs.com/thread-1577026-1-1.html (Source: Haichuan Chemical Industry Forum) 2. “My Plant, My Home” photo competition – Registration post for the “Stories of Spring” event: http://bbs.hcbbs.com/thread-1577027-1-1.html (Source: Haichuan Chemical Industry Forum) 3. [Petroleum and Chemical Industry Events] Discussions on skill competitions in various positions: http://bbs.hcbbs.com/thread-1575812-1-1.html (Source: Haichuan Chemical Industry Forum) 4. [Petroleum and Chemical Industry Section] Compilation of posts on professional “basic knowledge” – Supported by the main forum; amazing prizes await you: http://bbs.hcbbs.com/thread-1557057-1-1.html (Source: Haichuan Chemical Industry Forum) 5. [Petroleum and Chemical Industry Section] Proposal regarding the use of prefixes for topics: http://bbs.hcbbs.com/thread-1573958-1-1.html (Source: Haichuan Chemical Industry Forum) @ylb913 @liuquan1100
There is non-condensable gas at the top of the tower; as a result, the temperature at the top rises, the pressure increases, the temperature difference in the condenser increases, and the outlet temperature goes up as well.
Even when there is non-condensable gas at the top of the tower, it still depends on the operating conditions; with non-condensable gas present, the pressure at the top of the tower will definitely rise； At this point, if the heating at the bottom of the tower (via steam or heat transfer oil) remains unchanged and the reflux rate stays the same, then as the pressure at the top of the tower increases, the temperature at the top should decrease, because the heavier components cannot reach the top of the tower; as a result, the components at the bottom of the tower contain lighter components (which are not of acceptable quality) ； If the heating at the bottom of the tower is increased to ensure the quality of the heavier components at that level, the temperature at the top of the tower will rise (in this case, the reflux rate should be increased to ensure that the components at the top meet the required standards) ； Due to the presence of non-condensable gases at the tower top, the heat transfer coefficient of the condenser decreases; therefore, the temperature difference in the tower top condenser should decrease! The condenser outlet temperature varies depending on the heating at the bottom of the tower and the reflux operation: as the temperature at the top of the tower decreases, the outlet temperature also decreases ； As the tower top temperature rises, the outlet temperature rises as well.
The pumping capacity of a steam ejector increases proportionally to the non-condensable gas, at constant suction temperature. Therefore, reducing non-condensable gas is an important measure to reduce the pumping volume and improve vacuum level. Main factors affecting the amount of non-condensable gas at the top of the vacuum distillation tower and adjustment methods: (1) Temperature of the vacuum furnace. The higher the temperature in the vacuum furnace, the greater the degree of cracking of the oil. The outlet temperature of the vacuum furnace must be controlled: it should not exceed 398°C when producing lubricating oils, and not exceed 400–420°C when producing cracking feedstocks. At the same time, a higher flow rate of oil and gas is used in the high-temperature furnace tubes to reduce the residence time and thereby minimize oil cracking. (2) The temperature of the vacuum residue and its residence time in the vacuum distillation column. The residue at the bottom of the vacuum distillation tower is the heaviest material; if it remains at high temperatures for too long, reactions such as decomposition and condensation occur more significantly, resulting in an increased amount of non-condensable gas and a decrease in the vacuum level of the tower. Therefore, it is necessary to prevent the residue oil from reaching too high a temperature and having an excessively long residence time. (3) Air leakage volume. For existing operating devices, the amount of air that leaks in remains constant. Because as long as the sealing of the pressure relief system equipment and pipelines is in good condition, the amount of air that leaks in is very small and remains relatively constant, it has little impact on the vacuum level at the top of the tower
One can refer to the top of the upper column in air separation units; since the pressure there is the lowest, the temperature is also the lowest
Non-condensable gas control has always been a complex issue; especially at the beginning of operation, there is definitely a large amount of such gas, so it is necessary to vent it through flares or other means
1. Top temperature decreases 2. Top pressure increases 3. Temperature difference between the inlet and outlet of the tower top condenser increases 4. Outlet temperature of the condenser decreases
4. The condenser outlet temperature decreases. Why doesn’t it increase? The presence of non-condensable gases results in poor cooling efficiency of the heat exchanger, hence the temperature of the cooled material remains high. So the temperature difference decreases? ?
At the beginning of operation, nitrogen is generally used for pressure maintenance; the gas that needs to be released cannot be condensed. The process of emission is actually quite challenging. If a liquid level cannot be established in the reflux tank, it is usually due to too much non-condensable gas. So the question is: how to discharge it? How do you determine that the emissions are appropriate? ?