Distillation problem solving
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As shown in the figure: it is the pressure behind the steam control valve of this distillation tower. Is this pressure related to the level of the liquid in the bottom of the distillation tower? Is it the level of the liquid in the tower bottom that causes these fluctuations in pressure? Is it still related to the changes in the pressure at the bottom of the distillation tower?
Put it this way: the tower bottom is a heat exchange device; the liquid inside it serves as the liquid to be heated, while steam acts as the heating medium to warm this liquid, thereby providing heat and a certain level of pressure to the tower. As a heating medium, steam does not come into direct contact with the liquid in the bottom of the tower; its pressure depends mainly on the pressure in the subsequent liquid storage tank, and is related to the level of the liquid in the tower bottom, though the relationship is minimal. The effect of the tower bottom liquid level on the pressure inside the tower cannot be simply understood as following a definite trend; it can only be said that, within normal liquid level ranges, there is a certain relationship.
But today, the situation was such that the liquid level in the separator tank was controlled automatically; despite the liquid level in the separator tank remaining unchanged, the liquid level at the bottom of the distillation tower decreased. However, the pressure behind the steam control valve increased. Once the liquid level at the bottom of the distillation tower returned to its normal level, this pressure was restored
To be honest, your process is too vague. From the perspective of process principles, the liquid should flow through the shell side of that reboiler, while the gaseous medium in the condenser is the liquid at the bottom of the tower flowing downward. ，，，，。 I’m not sure what process your workflow is intended for; it would be better to refine it further to facilitate discussion.
The liquid level at the bottom of the tower is low → the effective heat exchange area is reduced → both the temperature and pressure inside the tower decrease → this can be addressed by increasing the amount of heating steam → the control valve is opened wider → resulting in an increase in pressure behind the valve. The above represents the logical reasoning derived from the conditions you described; what you have here is a process in which one balance is disrupted, giving way to another balance. PS: It is recommended to take the time to thoroughly study the basic theoretical knowledge; the three types of balance are widely used in work, and they also serve as one of the bases for our daily analysis and operations.
Once the liquid level at the bottom of the distillation tower decreases, I believe it is not necessary to maintain the original steam flow rate, as the heat transfer amount will decrease and less steam will be required; as a result, the control valve will adjust downward. However, during the process of the liquid level dropping in the tower bottom and while the system needs time to respond, it is normal for the pressure behind the control valve to increase, which is equivalent to a pressure buildup
Let’s discuss it; I’m also a bit confused
Steam condensate separated by the water separator? ?
Personally, I believe the pressure in the steam pipeline network remains constant. The steam pressure behind the valve is lower than the pressure in the pipeline network, and this pressure drops further due to steam condensation. The greater the amount of condensation, the lower the pressure; when the amount of condensation is small, the pressure rises. Thus, this pressure is related to the liquid level in the bottom of the tower. When the liquid level there is low, the heat load on the reboiler needs to be reduced in order to raise the liquid level; when the amount of steam that condenses is small, the pressure downstream of the valve increases.
This is a thermosiphonic reboiler. The bottom liquid level of the tower affects the reboiler heat load. The heat load affects the condensation volume on the steam side of the reboiler. The amount of condensation affects the pressure downstream of the valve. For example, when the liquid level in the tower bottom decreases, the amount of cold material in the self-circulation on the cold side of the reboiler reduces, which leads to a decrease in the steam required for heating; as a result, less steam condenses and the pressure behind the valve increases. By controlling the regulating valve to reduce its opening, the steam supply is decreased, which in turn reduces the amount of material evaporating in the bottom of the tower; as a result, a new balance is established among liquid level, pressure, and the opening of the steam regulating valve.