Relationship between reboiler temperature and reboiler liquid level

As we all know, the bottom of the tower temperature is the saturation temperature at the pressure present in that tower bottom. But more precisely, it should be the saturation temperature at the outlet pressure of the heater, which is the vapor pressure at the bottom of the tower plus the static pressure at the liquid level in that bottom section. Since the liquid level in the tower bottom was reduced, the static pressure also decreased, and as a result, the temperature at the tower bottom dropped as well. Since the liquid level in the tower bottom is required to fluctuate within a very narrow range, the absolute value of the decrease is very small.

Reply to 2# DXJ122: Some people believe that by lowering the liquid level in the tower bottom, the total amount of low-temperature material that needs to be heated decreases, so the temperature at the tower bottom should rise

Some people believe that by lowering the liquid level in the tower bottom, the total amount of low-temperature material that needs to be heated decreases, so the temperature at the tower bottom should rise; then a specialized instrument can be used to measure this

The last edit to this post was made by DXJ122 on 2012-1-5 at 16:38. Reply to 3# TDI5: 1. The heater is used to vaporize the liquid in the tank, serving as a reboiler for the distillation tower; not all of the liquid is vaporized – the vaporization rate is generally around 20%. Therefore, the outlet temperature of the heater corresponds to the saturated temperature at that pressure. 2. Moreover, even if the flow rate changes, as long as the pressure remains constant, this saturation temperature stays the same, regardless of the flow rate. To give a simple, though perhaps not entirely appropriate, example: if you heat up a small cup of water and a pot of water at the same time, both will reach a boiling temperature of 100 degrees (at 760 mmHg). It can’t be said that less water means a lower temperature, right? That’s the principle behind it. The difference between the two is that the latter requires more heat, but at the same pressure, the boiling point of water remains unchanged. 3. If water is boiled at 760 mmHg (along the coast) and at 630 mmHg (on the northwestern plateau), their boiling points will be different. 4. Another operating condition that is often encountered in production is operation of the tower at low load, with the liquid level kept constant. The circulation rate is reduced, but the temperature of the reactor liquid should remain unchanged; otherwise, the liquid in the reactor will not meet the required standards. Of course, the heat supply from the heating medium is also reduced under such conditions.

This post was last edited and replied to by TDI5 on 2012-1-5 at 17:49. Reply 5# DXJ122: Hello, if a decrease in liquid level causes the temperature at the bottom of the tower to rise, then the evaporation rate should also increase, and the pressure in the tank should rise slightly as well. Regarding point 4 you mentioned, during low-load operation, it should be a situation where the feed rate to the tower is low; in such cases, the circulation rate of the forced circulation pump at the bottom of the tower should not be affected by the load and remain unchanged. Personally, I also agree with the view that the temperature in the reactor tank should decrease slightly when the liquid level is low. When the liquid level is low, the back pressure on the circulation pump decreases slightly, which results in a lower circulation rate of the pump. With a lower circulation rate, the heat transfer coefficient in the reboiler drops, and therefore the temperature in the reactor tank becomes slightly lower. However, they cannot provide any compelling argument in support of the view that the temperature is high after lowering the liquid level.

Reply to 6# TDI5: Low-load operation refers to a situation where the feed rate to the tower is low. I agree with you that \"in such cases, the flow rate of the forced circulation pump at the bottom of the tower should be independent of the load and remain unchanged.\" However, the amount of vaporization at low load decreases accordingly, and this happens automatically because there isn’t as much material to be processed as under normal conditions. I mainly want to point out that at low load levels, the temperature of the reactor should be the same as under normal conditions (ignoring other minor factors), which shows that there is no relationship between the reactor temperature and the amount of material present.

After the liquid level drops, the heat load at the bottom of the tower remains unchanged, while the amount of vaporization increases. If the amount of product taken from the top of the tower stays the same, the tower pressure rises. The temperature and pressure at the bottom of the tower are in equilibrium; therefore, the temperature increases

The principle is simple: the boiling point of a liquid is directly proportional to the pressure at the surface of the liquid; in other words, as pressure decreases, the boiling point also decreases. The liquid level in the bottom of the tower decreases, the static pressure drops, and the boiling point falls; therefore, the temperature in the tower also decreases.

Reply to 7# DXJ122: I believe that at low load, the evaporation rate inside the tower is low, which reduces the pressure within the tower; as a result, the temperature of the reactor can be lowered appropriately.

Reply to 10# 41644056: Please note that the original poster is asking what will happen to the temperature at the bottom of the tower if the liquid level there is reduced, while keeping the feed rate constant ” The feed rate remains unchanged, which means the load hasn’t been reduced, right? The feed rate and liquid level height are relatively independent variables. As for the situation you mentioned where the load is indeed low, I agree with your view.