Production consequences of a low feed position in a plate distillation column
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During the design of the plate distillation tower, the feed concentration was low; however, in actual operation, this concentration became 2.5 times higher than originally planned. However, the original 3 interfaces for tower feeding cannot be changed, which results in the feeding position being much lower than the appropriate one. Please participate in the analysis and provide explanations regarding aspects such as tray efficiency, energy consumption, and production capacity. And how to improve it. Thank you!
Due to the low feed position, the stripping section is too short, resulting in a short gas-liquid contact time and poor mass transfer efficiency. To improve mass transfer and reduce the loss of light components in the bottom liquid of the tower, increase the steam supply to the reboiler. As a result, the liquid-to-gas ratio decreases, and the temperature and pressure at the bottom of the tower increase. The temperature difference between the trays in the entire stripping section is small, resulting in poor mass transfer efficiency in this section. Moreover, the low feed temperature causes a large portion of the gas stream that rises after mass transfer beneath the feed plate to condense. Further increase the reboiler steam consumption. The overall tower pressure drop increases, and the gas velocity is high, which can lead to an increase in the amount of mist entrained.
The temperature difference between adjacent trays in the stripping section is small; can this be interpreted as the composition of the binary system being similar across these trays, resulting in an insignificant separation effect? Due to the low feed temperature and the high concentration of light components in the liquid, the temperature difference between the highest tray in the stripping section, namely the feed tray, and the trays below it is large. In the distillation section, the temperature difference across the first few trays is relatively large. Due to the low feed position, the rectification section is too long, resulting in a very small temperature difference across the last ten trays. Can it be understood that this tower has high separation efficiency only in the beginning of the distillation section, while the separation efficiency is low in the stripping section and the upper part of the distillation section?
1. Hello, OP! Welcome to exchanges in the Ta District! 2. First, I would like to confirm whether you are referring to the product at the top of the tower or the product at the bottom of the tower? Based on your description, it seems that the feed position is too low, resulting in an excessive amount of light components and a high concentration at the bottom? So, is the bottom part the product? 3. If light components enter the bottom of the tower, the separation efficiency can certainly be improved by increasing the amount of heating steam. Of course, an increase in the steam volume leads to an increased gas-liquid load, so it is necessary to check whether this load is approaching the flooding limit.
Thank you to the moderator for their support! The product is obtained from the top of the tower, while water is obtained from the bottom of the tower. Because the stripping section is shorter, the concentration of the top product in the bottom liquid of the tower tends to be high. The numbers from flow meters and similar devices are inaccurate and have no value as a reference. Currently, it is simply determined by observing the temperature changes within the tower; if the temperature rises significantly, it is considered that foam entrainment is causing flooding. The existing treatment method is also too simple: the product obtained from the top of the tower is poured into the tank for defective products, and the steam flow rate to the reboiler is reduced.
3. If light components enter the bottom of the tower, the separation efficiency can certainly be improved by increasing the amount of heating steam. Of course, an increase in the steam volume leads to an increased gas-liquid load, so it is necessary to check whether this load is approaching the flooding limit. May I ask the moderator, how can one determine in practice whether an operation falls within the allowable range? How to determine whether it is approaching or exceeding the flooding load line? How then to adjust it? Thank you!
If a modification is to be made, the tower equipment simulation should be redone first. Or ask experienced engineers to see if there are any practical approaches, such as drawing a side stream from a lower position in the tower bottom to the middle or upper part of the tower; the light components with high concentration in the tower bottom must be recovered (this is purely my personal guess) ； The existing treatment method is also too simple: the product obtained from the top of the tower is poured into the tank for defective products, and the steam flow rate to the reboiler is reduced. What is the effect of this? Is the product at the top of the tower also substandard?
"The existing treatment method is also too simple: the product obtained from the top of the tower is poured into the tank for defective products, and the steam flow rate to the reboiler is reduced. What is the effect of this? Is the product at the top of the tower also substandard? " A temperature of 7 floors being high means that the temperature at various points throughout the tower is above the set value. In such cases, it is likely that the amount of steam used in the reboiler is too large, resulting in a fast upward flow of gas. This causes droplets from the lower layers of the tower to be carried up to the upper layers, disrupting the gas-liquid equilibrium there and reducing the efficiency of those layers. As a result, more heavy components end up in the product at the top of the tower. So the product at the top of the tower is substandard.
A too low feed position mainly affects the following: 1. The yield of light components decreases, and the concentration of heavy components becomes unsatisfactory； 2. The load on the reboiler at the bottom of the tower increases, while the reflux flow at the top of the tower can be reduced. 3. The temperature at the top of the tower is low ； Essentially, it means that the number of separation stages in the distillation section above the feed section increases, while the number of separation stages in the stripping section below the feed section is insufficient. In summary, an inappropriate feed position ultimately leads to incomplete separation of the components at the top and bottom of the tower, resulting in an excessive amount of light components at the bottom!
From the perspective of distillation principles, an inappropriate feed position increases the number of theoretical plates required to achieve the desired separation accuracy; however, since the tower is already fixed, the actual number of theoretical plates remains unchanged. This results in an insufficient number of theoretical stages being required, with the ultimate consequence being a decrease in the concentration or recovery rate of the light components and an increase in the concentration of the heavy components. It cannot meet the separation requirements.
So, how is the tower operating now? Are the products at the bottom of the tower of satisfactory quality?