How should the internal and external recirculation within the actual tower top be set during simulation?
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I would like to ask simulated friends here: in actual industrial applications, there is generally a distinction between internal and external reflux at the top of the tower. Internal reflux occurs when the vapor at the top of the tower condenses before entering the condenser, and then returns to the first tray in the tower. External reflux happens when the vapor at the top of the tower passes through the condenser and the reflux drum, and then returns to the first tray in the tower via a pump or due to gravity. However, during simulation, Pro/II generally uses external reflux as a basis and does not take into account internal reflux; as a result, the simulated reflux rate differs from the actual reflux rate in the tower, which in turn affects the vapor-liquid load within the tower. I would like to ask everyone here: when encountering such problems, how should one make the settings so as to get a result that is closer to the actual reflux?
I think only the part of gas-phase extraction undergoes condensation, and its mechanism is the same as that of internal reflux. The original poster can give it a try.
Internal reflux is the reflux that occurs due to heat loss in the tower equipment, causing the vapor phase inside the tower to condense on the tower walls. The magnitude of the internal recirculation is related to the insulation of the tower, its size, as well as the system temperature and ambient temperature – in other words, it is related to heat loss. Design generally does not take internal recirculation into account. Internal reflux has a slight impact on the vapor-liquid load within the tower, as only data on external reflux is available during operation, and it is difficult to determine the amount of internal reflux. The key question is: what is the typical amount of internal recirculation? In fact, when the insulation of the tower is designed in accordance with standards, the internal recirculation is very small and can be ignored. If you really want to simulate internal recirculation, then install a cooler on each tray within the tower’s modules to simulate the gradual heat loss.
I didn’t quite understand what the original poster meant; after reading the answer on floor 3, I got it. Thank you.
After thinking about it again, I have come up with an estimation method for internal recirculation as well as the general design approach. First, the heat loss over the distance from the mass transfer zone to the condenser is calculated (using data such as the temperature difference inside and outside the tower or pipes, pipe length, inner and outer diameters, and material properties; using specialized pipe design software would make this process easier). Once the heat loss is known, it is possible to determine the actual conditions upon reaching the heat exchanger, thereby determining how much condensation will occur.
There is also internal recirculation within the mass transfer unit, as heat is lost across the entire tower wall; however, these effects can be ignored in empirical design.
Thank you all for participating, and a special thank you to the friends on the third floor – I learned a lot!
It seems that the backflow within the mass transfer unit is somewhat different from that at the top of the tower; the former is involved in gas-liquid equilibrium, and this occurs to some extent in both packed towers and tray towers. Gas-liquid equilibrium also exists in the latter case, but it has a greater impact on the operation of the equipment.
There might be a slight difference. However, the heat loss at the top of the tower is considered to consist of two parts. The first part runs from the first plate at the top (or the uppermost end of the packing) to the outlet of the vertical section of the vapor pipe; in this section, the condensate resulting from heat loss returns directly into the tower, and this is regarded as internal reflux. This section is involved in the vapor-liquid mass transfer process. The other section runs from the start of the horizontal part of the vapor pipe (assuming it slopes in the direction of the condenser; otherwise, it falls under the previous section) to the inlet of the condenser. The liquid formed in this section flows into the condenser and the reflux tank, and this should be considered external reflux. The part you mentioned that does not participate in gas-liquid mass transfer is this very section. The condensate on the tower wall is entirely internal recirculation; of course, it also participates in vapor-liquid mass transfer. In practice, the external reflux value is adjusted to the design value, so that the amount of material entering the condenser is at the designed level. However, due to heat losses, the actual vapor-liquid load in the tower must be greater than the designed load; more material needs to be evaporated to compensate for these heat losses, otherwise it will not be possible to maintain the desired amount of material reaching the condenser. This is why internal reflux affects the vapor-liquid load in the tower.