Why, when the wind pressure increases, cannot the fluid overflow from the lower side, but instead overflows from the higher side? For details, see below (attached figure).
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The diagram below shows a simplified schematic of the wastewater oxidation process. The diameters of the wastewater outlet and the overflow port are the same; during normal operation, the wastewater flows from the outlet into the buffer tank. However, as the pressure of the oxidizing air gradually increases, the balance is disrupted; the level of the wastewater rises, and the wastewater cannot flow out through the outlet into the buffer tank (the level in the buffer tank drops to zero). Instead, it overflows from the wastewater overflow port, and in severe cases, this can lead to the wastewater spilling over the top of the tank. May I ask: Is the cause of this phenomenon related to the fluidization state? For example: as the fluid is blown up by the airflow, the gap between the fluids increases and its density decreases; when this exceeds a certain critical value, the resistance at the outlet increases (such as aerodynamic resistance), preventing it from overflowing to the buffer tank at that outlet and causing it to overflow instead from the overflow port on the upper layer. I hope the experts here can give me some advice! It is best to have theoretical or data support, and animations would be even better.
This post was last edited by qugd on 2020-12-25 at 14:50. The original poster had better review the principles of chemical engineering again; it’s a very simple issue related to pressure or differential pressure comparison, and different differential pressures or pressures can be used to control or regulate the flow of liquid out of the overflow port or wastewater outlet. I redrew the diagram and marked the pressure or differential pressure; the wind pressure was not marked, so it is considered to be the static pressure of the gas blown into the oxidation tank. During normal operation, the overflow at the wastewater outlet is controlled by the liquid level of wastewater in the buffer tank. By sending the wastewater outside, a lower liquid level can be maintained in the buffer tank, ensuring that the wastewater from the oxidation tank can flow smoothly out through the outlet and into the buffer tank. When the pressure of the oxidizing air increases, oxidation operations take place inside the oxidation tank. To ensure effective oxidation, the buffer tank may (and this is indeed necessary in actual operation) stop receiving wastewater, which causes the liquid level in the buffer tank to rise and its pressure to increase. This prevents wastewater from the oxidation tank from flowing into the buffer tank, thereby causing the liquid level in the oxidation tank to rise as well. When it rises to the overflow port of the oxidation tank, the wastewater can then overflow out of it. This design is intended to prevent roof caving and tank surging during processing. During the operation you mentioned, the liquid level in the buffer tank dropped to zero. It is likely that the buffer tank is a pressurized, sealed container; otherwise, the wastewater would not flow automatically into the buffer tank. If the tank is not pressurized, there is no additional pressure difference when the liquid level is at zero, so abnormal overflow would not occur. The overflow opening above the wastewater outlet is designed to ensure that, during normal operation and when the wastewater outlet is closed (or no liquid is being discharged), the liquid level rises to a certain height as a precaution to prevent the oxidation tank from overflowing. In terms of control logic, the difference between the overflow level and the outlet level in the oxidation tank is necessarily related to the liquid level pressure in the buffer tank; if gas is introduced into the oxidation tank, the static pressure of this incoming gas also plays a role in the calculation relating these two values. If the overflow port of the oxidation tank cannot discharge liquid properly, it indicates that a valve in the overflow pipeline is closed or the pipeline is blocked. If the pipeline is designed to open to atmospheric pressure, there should be no reason why it cannot overflow on its own. Since I’m not a professional in chemical engineering, my understanding and description may not be entirely accurate, but I believe the basic principles are as I’ve explained them.
It seems there isn’t a very good solution for this issue. The oxidation process requires thorough contact between the vapor and the liquid, but at the same time, separation of the vapor from the liquid is necessary – these two requirements are contradictory. The simplest approach is to increase the height of the oxidation tank, as well as the diameter of the outlet and overflow ports; an upward-facing branch pipe can then be added outside the tank to serve as an air release outlet. This branch pipe should narrow down to its normal size after passing through that point. The difficulty here lies in determining the appropriate height for the tank, which requires experience; otherwise, it has to be decided based on guesswork. A better solution is to install mechanical defoamers at the top of the oxidation tank, taking into account factors such as the density of the mixture and the residence time, in order to remove bubbles mechanically. The gas rises through the central channels, while the liquid is forced against the walls of the tank and flows out.
When the oxidation air flow increases, a two-phase flow of gas and liquid forms inside the tank, which causes air to build up in the pipeline leading from the oxidation tank to the buffer tank, preventing normal flow. To address this issue, either a separation tank can be added to the pipeline or the liquid can be introduced at the top of the buffer tank
Hahaha! The wastewater is continuously discharged; even when the level in the buffer tank is empty, the medium in the oxidation tank does not flow out through the outlet – instead, it overflows from the overflow port. As for the pressure difference issue you mentioned, I have calculated it, and actually there is no such issue. Actually, we later considered the issue of the state of the medium in the oxidation tank – a fluidized state; that is, if the gas flow velocity is too high, it lifts the liquid or creates air pockets. Recently, we climbed to the top of the oxidation tank to record its condition, which confirmed this as well.
Initially, the issue of air resistance was considered; the buffer tank itself was also a tank at atmospheric pressure, with pipes connecting it to the atmosphere having a diameter of 50. Later, opening the manhole on top of the oxidation tank did not solve the problem either. Actually, it’s the high gas velocity in the oxidation tank that lifts the liquid; it’s a problem caused by too high gas velocity. The problem has been solved now.
It was already raised by one meter on the basis of the original design; the company requires energy savings and reduced consumption. The fans have 10% to 30% air leakage, and this problem arose in order to channel all that leaked air back in. The issue of increasing the tank height you mentioned is also a possible solution; however, since raising the tank height requires consulting a design firm and involves high costs, this option was not adopted. When designing the outlet of the oxidation tank, exhaust holes were already provided to prevent air blockage. Now that all the air is combined together, the gas-phase load increases; this issue has been resolved by adding a line from the overflow line of the oxidation tank to the buffer tank, ensuring that the production requirements are still met.
It was already raised by one meter on the basis of the original design; the company requires energy savings and reduced consumption. The fans have 10% to 30% air leakage, and this problem arose in order to channel all that leaked air back in. The issue of increasing the tank height you mentioned is also a possible solution; however, since raising the tank height requires consulting a design firm and involves high costs, this option was not adopted. When designing the outlet of the oxidation tank, exhaust holes were already provided to prevent air blockage. Now that all the air is combined together, the gas-phase load increases; this issue has been resolved by adding a line from the overflow line of the oxidation tank to the buffer tank, ensuring that the production requirements are still met.
It was already raised by one meter on the basis of the original design; the company requires energy savings and reduced consumption. The fans have 10% to 30% air leakage, and this problem arose in order to channel all that leaked air back in. The issue of increasing the tank height you mentioned is also a possible solution; however, since raising the tank height requires consulting a design firm and involves high costs, this option was not adopted. When designing the outlet of the oxidation tank, exhaust holes were already provided to prevent air blockage. Now that all the air is combined together, the gas-phase load increases; this issue has been resolved by adding a line from the overflow line of the oxidation tank to the buffer tank, ensuring that the production requirements are still met.