Regarding the reboiler of the distillation column

It is necessary to have a temperature measurement point for the heating medium in the reboiler, as it is based on this temperature that the amount of steam to be added can be determined

Personally, I think a temperature measurement and control point should be added at the bottom of the tower, as it is ultimately the temperature at the bottom of the tower that needs to be adjusted.

It’s better to add it, but it’s not necessary; however, the temperature at the bottom of the tower and the temperature at the sensitive points of the tower must definitely be measured. From what the original poster said, it seems to be a siphon-type heat exchanger?

It should be sufficient to check the temperature at the bottom of the tower and the temperature at the sensitive points of the tower; it seems that none of our company’s units have such features installed.

Temperature can’t determine the amount of steam needed, right? The operating pressure (vapor pressure) remains constant, and the temperature at the bottom of the tower stays unchanged (provided the composition does not change). The amount of steam used should be determined based on the amount of vaporization.

There is no restriction on adding a temperature measurement point at the medium outlet of the reboiler, but do you want to control the temperature there? The important control temperature points in a distillation tower are either the sensitive plate or the top and bottom of the tower. Due to the properties of the material, the temperature indicated at the temperature measurement point at the outlet of the reboiler medium is unreliable.

I really haven’t seen any mention of temperature measurement at the reboiler outlet in the design manuals or PIDs.

All distillation columns I have seen have a reflux temperature, because in the course of operation, if there is a malfunction related to low temperatures, the reflux temperature can be used to maintain a constant steam flow temporarily

There is no need to measure the outlet temperature; the steam volume can be adjusted by using the temperature at the bottom of the tower.

The temperature measurement point at the steam outlet of the reboiler serves a specific purpose. A distillation column operates primarily based on the heat generated in its bottom section; therefore, a stable supply of heat from this bottom section is the primary prerequisite for maintaining stable operation of the distillation column. Under normal operating conditions, the temperature at the steam outlet of the reboiler (hereinafter referred to as the outlet temperature) remains essentially constant. However, in abnormal situations, changes in this temperature can play a crucial role. 1. When the liquid level at the bottom of the tower is normal, a downward trend in the outlet temperature indicates that either the feed rate or the reflux rate to the tower is too high, disrupting the tower’s thermal balance. Immediate adjustments are necessary to prevent the tower temperature from dropping too low ； If the feed rate remains unchanged, then the pressure and flow rate of the heating steam must change; attention should be paid to adjusting the heat supply. 2. When the liquid level is full, the outlet temperature remains unchanged, indicating that no backflow of liquid from the bottom of the tower has occurred; increasing the heat supply can restore the normal liquid level. If the outlet temperature drops sharply, it signifies that backflow has taken place, and it is necessary to drain the liquid immediately to lower the liquid level at the bottom of the tower. 3. The change in outlet temperature is much more sensitive than the change in the temperature at the bottom of the tower. If attempts are made to regulate using the temperature at the bottom of the tower, the entire distillation tower can become unstable if not handled carefully. By monitoring changes in the outlet temperature, unstable factors can be identified in a timely manner, allowing for prompt adjustment.

The reboiling mode requires adjustment... The falling film mode determines the opening degree based on the equilibrium temperature of the tower.

Let me tell you the most advanced method for measuring steam volume: with the liquid composition remaining unchanged, no matter how much steam is added, its boiling point stays the same; what changes is only the amount of vaporization. Therefore, it is absolutely impossible to determine the amount of steam by measuring temperature, nor can the amount of heating steam be controlled based on the temperature at the bottom of the tower. The best approach is to measure the pressure difference between the bottom of the tower and the middle or top of the tower, and use this value to control the amount of heating steam. A low pressure difference indicates low evaporation rate, while a high pressure difference indicates high evaporation rate. In other words, if there is a calculated value for the pressure difference based on the design for liquid flooding in the tower, it is possible to control the maximum evaporation rate in actual production according to the prevailing conditions (which corresponds to the amount of heating steam used in the reboiler).

So, it can be added, but it’s not necessary. :)

No addition is needed; the steam flow rate is controlled based on changes in the liquid level in the tower bottom.

Thank you for the information; it’s useful to me

It should be added that, based on our existing distillation columns, there is still a difference between the temperature at the reboiler outlet and the temperature at the bottom of the column. There are many factors that affect the temperature at the bottom of the column, and changes in this temperature do not accurately reflect changes in the heating steam.

Don’t you think your opinions are mostly wrong? This post was last edited by weiqj on 2008-1-30 11:42]

The most convenient and reliable way to determine the heating steam volume for a reboiler is to look at the reflux flow rate plus the product flow rate (if it is at liquid phase temperature). Temperature is used to determine the content of the material; it is not possible to determine the amount of steam being used for heating. To give a simple example, turn off the outlet at the top of the tower and then open the steam valve fully. With the tower pressure remaining essentially unchanged, observe whether the temperature at the bottom of the tower rises or falls.

Is that so? I don’t know where the mistake is; I would appreciate it if someone experienced could give me some advice. I use these principles to guide production, and at present, the steam consumption for distillation is very low, ranking among the top three in the country. The specific steam consumption figure cannot be disclosed, but it is definitely much lower than 0.9.

The temperature inside the tower is different from that in the reboiler; it is a relationship between average and instantaneous values. Only then can the temperature of the reboiler truly reflect the amount of steam required under pressure.