Does internal recirculation belong to hot recirculation or cold recirculation?

I think it’s thermal backflow, which can be seen from the control equations of automatic control

It is hot reflux; cold reflux becomes hot reflux after passing through the first tray.

Internal reflux is a different concept from cold reflux and hot reflux. Cold reflux refers to the situation where the product at the top of the tower, after being cooled, has a temperature lower than its bubble point and is in a supercooled state; a portion of this product is sent back to the top of the tower as reflux, and this is what is known as cold reflux. The reflux drawn out from the middle section of the tower is called hot reflux. Internal reflux refers to the liquid flow from top to bottom within the tower, including cold reflux at the tower top and hot reflux in the middle section.

Reflux is a method used in distillation columns to remove heat; the vapor at the top of the column condenses into a saturated liquid phase. The reflux drawn from the top in this case is known as hot reflux. As this hot reflux continues to cool down, its temperature drops below the bubble point temperature at that pressure, and it is then called cold reflux. A stream is drawn from the middle of the distillation section, cooled, and then returned to the upper layers or the lower layer as a heat recovery reflux for the middle section. Those who perform simulation calculations know that the liquid flow rate on each tray of the distillation column is much greater than the amount of liquid reflux from outside; likewise, the gas flow rate on the trays is higher than the flow rate of the gas exiting at the top of the column. This ensures material balance for the materials entering and leaving each tray. These internal recirculations in the gas or liquid phase flow continuously up and down within the tower, enhancing the gas-liquid contact on the tray surfaces. The amount of these internal recirculations is determined through heat balance calculations, and this phenomenon is caused by the difference in vaporization enthalpies of the light and heavy components in the tower. If the vaporization latent heats of the two are equal, the column operates at a constant molar flow rate, with the same gas and liquid flow rates in each tray.

I agree with what was said on floor 6 and disagree with the view on floor 4. Internal recirculation refers to the actual amount of fluid that recirculates after heat transfer occurs on the tray, and it should have nothing to do with the location from which the fluid is drawn off. It is generally believed that the subcooled liquid after passing through one tray stage is already close to its saturation temperature; therefore, internal recirculation should be considered a form of thermal recirculation. The above are merely subjective assumptions; no research has been conducted. Please point out any errors if there are any.

There are generally two ways of reflux: internal reflux and external reflux. The difference between the two lies in the arrangement of the top condenser; internal reflux is commonly used in smaller towers, where the liquid phase flows back into the tower after being condensed by the top condenser. In external reflux, the liquid phase enters the tower after being condensed. External reflux is more frequently seen in practice, while internal reflux is used mostly in smaller towers. A disadvantage of internal reflux is that the reflux ratio is not easy to determine – only the amount of product extracted is known, while it’s difficult to figure out the amount of reflux. Most distillation towers have intermediate circulation for reflux, and cooling coils are also installed between the tray levels to remove heat. It is due to the inadequate heat removal by the condenser at the top of the tower; the internal reflux consists of saturated liquid that vaporizes as it passes through the lower tray, and then gets condensed as it rises to the upper tray. The temperature difference between the two trays is determined by the effect of this internal reflux.

In a less strict sense, internal recirculation is hot recirculation, while external recirculation is cold recirculation

Hot reflux and cold reflux are classified based on whether the reflux temperature is above the boiling point. Internal reflux and external reflux refer to whether the reflux liquid is condensed inside the tower or through a condenser – isn’t that a strange way of phrasing it?

If we set aside the issues of controlling and stabilizing the reflux rate, and consider solely the separation process in the distillation column, is internal reflux more beneficial for separation or external reflux?

There’s a slight issue with the answer on the fourth floor; let’s discuss it: internal recirculation refers to thermal recirculation. There is also cold recirculation, where the recirculation temperature is below the boiling point, resulting in supercooled liquid. A phase change occurs after being returned. Circulation and reflux serve only to cool down; the phase remains unchanged after being pumped back.

Internal reflux should refer to the liquid phase load between two trays in a distillation column used for fractionating complex petroleum mixtures (excluding tray plates for side-stream extraction), whose gas-liquid phase loads do not satisfy the constant molar flow assumption stated in \"Principles of Chemical Engineering\". Due to the temperature distribution along the plate height in the distillation column, for two adjacent plates, the temperature of the upper plate (plate n) is lower, while the temperature of the lower plate (plate n+1) is higher. The vapor components rising from plate n+1 are at a higher temperature than the liquid components descending from plate n; as a result, a portion of them is condensed (this portion constitutes the internal reflux). In other words, the temperature of the liquid phase falling on plate n is lower than that on plate n+1; as a result, it vaporizes on plate n+1 and becomes part of the rising gas phase. Note that this rising gas phase includes all the side-stream extracts above plate n+1, the overhead product, as well as all the stripping steam and internal reflux below plate n+1. This internal reflux can be calculated through heat balance by applying a certain separation system to the tower; for details of the calculation process, refer to Chapter 7 of the third edition of Lin Shixiong’s \"Petroleum Refining Engineering\". The effect of internal recirculation is primarily determined through heat balance, providing the gas and liquid phase loads for subsequent hydrodynamic calculations of the tower trays. Reflux is a method used in distillation columns to remove heat; the vapor at the top of the column condenses into a saturated liquid phase. The reflux drawn from the top in this case is known as hot reflux. As this hot reflux continues to cool down, its temperature drops below the bubble point temperature at that pressure, and it is then called cold reflux. A stream is drawn from the middle of the distillation section, cooled, and then returned to the upper layers or the lower layer as a heat recovery reflux for the middle section. Those who perform simulation calculations know that the liquid flow rate on each tray of the distillation column is much greater than the amount of liquid reflux from outside; likewise, the gas flow rate on the trays is higher than the flow rate of the gas exiting at the top of the column. This ensures material balance for the materials entering and leaving each tray. These internal recirculations in the gas or liquid phase flow continuously up and down within the tower, enhancing the gas-liquid contact on the tray surfaces. The amount of these internal recirculations is determined through heat balance calculations, and this phenomenon is caused by the difference in vaporization enthalpies of the light and heavy components in the tower. If the vaporization latent heats of the two are equal, the column operates at a constant molar flow rate, with the same gas and liquid flow rates in each tray.

The discussion was intense; personally, I think it was thermal backflow

The discussion is intense; let’s get involved, hehe. Personally, I think it’s thermal backflow