Atmospheric distillation without cooling reflux
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Dear experts, recently I’ve seen that some new large-scale atmospheric and vacuum distillation units are designed without the use of constant top cooling reflux. This is different from what I learned in school, so I’m here to seek advice. I believe there must be advantages to this approach, such as a reduction in the pressure drop within the towers; however, this means that the reflux ratio is zero, which should have an impact on the quality of the product at atmospheric pressure. I’m not sure either if the analysis is correct; I’d like to hear everyone’s opinions. Why was it designed this way? What is the purpose behind it?
Large ones? Is it set inside? Also, it seems like you didn’t explain clearly what kind of device it is; it would be best to provide an image.
Cold reflux at the tower top is one of the common types of reflux; in addition, there are also secondary cold reflux at the tower top and circulating reflux at the tower top. I’m not sure whether what you’re seeing is a secondary cold reflux at the top of the tower or a circulating reflux there Secondary condensation reflux at the tower top is commonly used in large-scale plants; it reduces the heat exchange area, but the process is complex. The tower top recycle reflux absorbs heat in a sensible heat manner, with no phase change and no reflux steam; this reduces the pressure drop in the pipelines, increases the vacuum level of the vacuum distillation tower, and improves the utilization efficiency of the heat source.
Going back to the upstairs discussion: the newly built 10 million-ton atmospheric and vacuum distillation unit in Dalian uses a non-atmospheric top cooling cycle instead, with an atmospheric top cycle being employed for heat extraction. There are no images; I only saw their papers.
If the top circulation reflux can meet the requirements for the temperature at the top of the stabilizer, then a cold reflux is not necessary; however, from a design perspective, such a tower should have several more trays at its top than a tower that uses cold reflux, in order to increase the internal reflux. I believe the purpose of doing this is to reduce the cooling load at the top of the tower, as the heat generated there generally cannot be recovered and must be cooled through air cooling or water cooling. Moreover, the heat in this area consists not only of heat due to temperature differences but also of heat absorbed during phase changes. Therefore, in the past, the overhead cooling systems in refineries generally had a high load, resulting in significant consumption of cooling water. Without cold reflux at the tower top, this section will not be cooled repeatedly, resulting in significant energy savings! The above views are merely personal speculations and lack any factual basis. . That’s really lazy computing!
What the friend on the 5th floor said makes sense; I think it should be energy-saving and efficient.
I have a question: Is no cold reflux used at the top of the atmospheric tower, and is the gasoline obtained from there simply cooled before being sent out? This seems to be able to reduce the gas-liquid load on each tray above the first side line at the top of the tower; could it also help to reduce the tower diameter? I hope experts can continue to give guidance.
It would be enough to take a look at the design drawings and examine the design data on them; that should help to understand things.
It’s designed this way presumably to save energy, but the gasoline outlet at the top of the tower still needs to be cooled to a certain temperature; in other words, cooling capacity is still required. So how can energy be saved? I don’t understand it – let’s wait for someone more knowledgeable to explain it.
Most of the usable heat in the overhead gas can be recovered by heat exchange with crude oil. On the other hand, eliminating the cold reflux at the top of the tower requires adding several more trays. Even though heat is extracted through the top circulation, the high temperature of the liquid returning to the tower still affects the precision of fractionation of the product at the top. I can’t see what the advantages of doing this are; those who have more information on this topic are welcome to share it.
Although there is no separate recirculation system, there are still ways to extract heat; it’s just a different approach to achieve the same goal
The units at Dalian Xitai use normal top reflux; generally, cold top reflux is employed, and just like in vacuum distillation columns, top circulation reflux is also used. All these methods are available, and the choice depends on the specific circumstances
Has the top reflux been removed, and is top circulation used to extract heat? The two methods are different, but they serve the same purpose. I’m not sure how to ensure that the temperature is within the required range at the device used for transporting the oil from the top of the tower. Generally, the temperature of the oil when it enters this device is around 50 degrees; therefore, the temperature of the distillate exported from the top of the tower should be lower than that. I’d like to hear from those who are more experienced on this matter