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Increase the vacuum level further, to -96 kPa

This post was last edited by Dedication on 2021-4-7 10:13

It doesn’t matter what the pump head is like; it’s not located at the top of the tower. The tower itself has not yet reached its designed pressure level

Could the original poster share your PID diagram? Here, vacuum tower top of -93 kPa probably refers to gauge pressure, right? If so, the vacuum level is low enough.

Is there no temperature distribution inside the tower? What is the temperature of the feed?

Is there a large difference between the designed and actual values of your steam usage?

Looking at the data, it seems that the thermal siphon heat exchanger has no circulation (Analysis: 1. There is no pressure difference between the top and bottom of the tower, which indicates that there is no evaporation at the bottom of the tower)； 2. A decrease in the temperature at the bottom of the tower adjacent to the normally open steam trap indicates that heating has begun at that point. Solutions: 1. The steam temperature may be too high, causing overheating in the upper section of the heater and resulting in a gas blockage; reduce the steam temperature or wait until the heat exchanger is filled with material before introducing steam. 2. Increase the forced circulation pump at the bottom of the tower

No material can be vaporized; it’s clear that the tower isn’t receiving any heat supply. In other words, there is no circulation in the thermal siphon reboiler. Try raising the liquid level in the tower.

The temperature at the bottom of the tower is 140°C, with a vacuum level of -90 KPa. If these are the design parameters set by the designer, then the actual temperature measured on site, which is 152°C at the bottom of the tower, is clearly 12°C higher than the designed value. Meanwhile, the vacuum level at the top of the tower is -93 kPa, which is higher than the designed value, while the temperature there is 120°C, which is lower than the designed value. . Based on this phenomenon, it is very likely that your feed material deviates from the designed composition. The high vacuum level at the top of the tower can be explained by the temperature being lower than 130°C. However, the high vacuum level at the bottom of the tower suggests that the temperature should actually be lower; yet it is 12°C higher than the design value. This indicates that your feed contains an excessive amount of heavy components, and unfortunately these are components with a higher molecular weight and boiling point than C. . Of course, as the amount of such components increases, there will naturally be less of the light component A, and the separation process will become more difficult. At this point, the steam pressure at the bottom of the tower should be increased slightly to force an increase in the temperature difference and thus boost the evaporation rate. Will substances like C or D, which are heavier than C, experience coking? If that’s the case, it’s a bit sad. . . Based on the current observations, it seems that there is a shortage of Component A, and the increase in unknown recombinant components is causing greater difficulties in separation. It is recommended to conduct a full analysis of the feed. . . Be sure not to analyze only ABC; consider the possibility of other components such as DEF. . With a low flow rate, the backflow will be 1 m3/h more ； A decrease in feed volume accompanied by an increase in reflux indicates that the vaporization amount at the bottom of the tower is insufficient, as well as a lack of heat source. In such cases, it’s necessary to further reduce the feed volume or increase the steam supply. The design team has increased the area by 40 m2; however, if this increase involves only more tubes while the length of those tubes remains unchanged, then it won’t be very effective. There are specific techniques for increasing the heat exchange area, especially in the case of thermal siphon reboilers under high vacuum conditions. As mentioned earlier by a colleague, trying to raise the liquid level slightly might also help, but it’s likely to have little effect. Furthermore, didn’t you remove a reboiler once? Check if it’s clogged inside, and what the condition of the scale is like ? These are all tasks that can be done. . The biggest issue is likely in your feed composition; unknown high-boiling-point components may have been overlooked. . .

The normally open steam trap bypass is activated; at this point, steam flows through the bypass, thereby utilizing the sensible heat of the steam. As a result, there is insufficient heating, and the temperature at the bottom of the tower naturally drops. Based on what you’ve described, I think the main issue is insufficient heat extraction at the bottom of the tower. There are many reasons for this insufficient heat extraction: inadequate siphoning, too low a liquid level at the bottom of the tower, or an empty space there; as a result, the heat from the steam cannot be extracted, which leads to an abnormally high temperature at the bottom of the tower. Check whether the return line to the tower has a temperature indicator; if the temperature at the return point is much higher than that at the bottom of the tower, it means there is empty space at the bottom of the tower. Reduce the discharge volume from the bottom of the tower accordingly and adjust the liquid level there. If the heat from the vapor at the bottom of the tower cannot be removed, no reboiler, no matter how large, will be useful.