I’m encountering issues with multi-stage thermally coupled towers in production; could someone expert please explain this?
Thread Content
In the application of multi-stage heat-coupled towers, a vapor balance line for pressure equilibrium connects the reflux drum of the lower stage tower to the reboiler of the upper stage tower, as the vapor phase at the top of the lower stage tower passes through it. Taking the diagram as an example, it can be understood that during operation of T2, the gas phase passes through the reboiler of T1 for heat exchange, and then enters the reflux tank via the circulating water cooler of T2. Under ideal conditions, the gas equilibrium line provides stable pressure to the reflux tank, helping to maintain system pressure balance. However, in the case of forced reflux, the height of the reflux tank is lower than that of the tower top; therefore, it is necessary to control the gas equilibrium valve so that the pressure at P2 is slightly lower than that at P1. Otherwise, the gas returning through the gas equilibrium line, having not passed through the cooler, will be at a higher temperature, which can affect the temperature of the liquid phase in the reflux tank, increase waste of the refrigerant medium, and raise the risk of cavitation in the reflux pump. At the same time, the pressure relief control at point P3 is regulated based on changes in the top pressure P1; since some of the dissolved hydrogen in the material is released during the heating process and is discharged along with the pressure release from the top of the tower. The gas phase at the top of tower T2 enters the reflux tank through the gas-phase equilibrium line, and it is also discharged in large quantities as part of this pressure release, resulting in waste of material. I hope an expert can explain whether my analysis is correct, what the role of this gas-phase equilibrium is, under what circumstances adjustments are needed during operation, and how to make those adjustments
Your analysis is basically correct. In a multi-stage heat coupling tower, the gas-phase equilibrium lines play a role in balancing the system pressure and ensuring stable operation. Now I will explain your question in detail. 1. Function of the vapor equilibrium line: The vapor equilibrium line connects the reflux tanks at the upper and lower levels; by balancing the pressure of the vapor phase, it ensures that the pressure remains stable during system operation. If there is no vapor equilibrium line, pressure differences will cause fluctuations in the liquid level inside the reflux tank, thereby affecting the operation of the entire system. 2. Situations requiring adjustment of gas-phase equilibrium: In actual operation, there may be situations that necessitate adjusting the gas-phase equilibrium. For example, when the operating conditions of the lower tower T2 change, resulting in variations in the amount of vapor generated or in the pressure, it may be necessary to adjust the vapor equilibrium valve in order to balance the vapor pressure and maintain stable operation of the system. 3. How to adjust the gas phase equilibrium: The gas phase equilibrium can be adjusted by controlling the gas phase equilibrium valve. Depending on the required adjustments, the opening degree of the balance valve can be changed appropriately to adjust the pressure in the upper tower, keeping it slightly lower than the pressure in the lower tower. In this way, the liquid level in the reflux tank can be maintained stable, preventing the gas phase from entering the reflux tank without passing through the cooler; this reduces waste of the refrigerant medium as well as the risk of cavitation in the reflux pump. Furthermore, for the make-up and relief pressure control at point P3, it can indeed be controlled based on changes in the top pressure P1. This prevents pressure release caused by the release of dissolved hydrogen in the material, thereby reducing waste of the material. In short, the role of vapor-phase equilibrium is to maintain pressure balance in the system and ensure its stable operation. In actual operation, adjustments need to be made according to the circumstances; by controlling the vapor equilibrium valve, the pressure difference between the upper and lower towers can be adjusted to achieve an ideal balance. .
1. The latent heat released during the phase change of the vapor at the top of T2 is related to the pressure on the shell side of the reboiler; therefore, if the pressure in the reflux tank is too low, the vapor at the top of T2 releases only its sensible heat on the shell side of the reboiler, while the latent heat cannot be released due to the low pressure. As a result, the efficiency of heat utilization is low, so it is necessary to increase the pressure in the reflux tank； 2. Some inert gas may accumulate at the top of T2, and this pipe can be used to release that inert gas ；