What is internal recirculation?

Internal reflux is likely related to the equipment and the process; in one distillation unit I encountered, the heat exchanger was located right at the top of the tower, and the reflux served to cool the material by utilizing the cooling effect of that heat exchanger. That’s more or less what it is!

There have been discussions on the forum; please refer to: http://bbs.hcbbs.com/viewthread.php?tid=372038&highlight=%C4%DA%BB%D8%C1%F7

Internal reflux in distillation generally refers to the reflux on the tray, which is composed of the liquid formed by the condensation of the descending liquid and the rising gas； External reflux involves drawing out a portion of the liquid from the tower, cooling it, and then returning it to the tower. External reflux: It refers to the situation where the material leaves the top of the tower through external pipes, flow meters, etc., and then returns back into the distillation tower. Internal reflux: It is a situation where the material does not leave the top of the tower; after condensing there, it flows back directly into the distillation tower. External return flow can be measured and diverted. It is difficult to measure internal reflux; it’s hard to determine precisely the ratio between fractionation and reflux. External reflux is forced reflux: after the vapor at the top of the tower condenses, it enters the reflux tank, from where it is pumped back into the tower via a reflux pump, with the flow rate being adjusted using a self-regulating valve and a flow meter. Internal reflux is self-reflux; after the vapor at the top of the tower condenses, it returns directly to the tower. During operation, attention must be paid to the output volume, i.e., the temperature at the top of the tower, in order to prevent the product from being of substandard quality. Due to the corrosive nature of some distillation materials, glass towers or towers lined with enamel are preferred for reflux applications. But it is indeed difficult to adjust and control the reflux ratio! External reflux is quite common; most distillation processes make use of external reflux! In general design, external recirculation should be used as much as possible! 1. External recirculation is beneficial for controlling process flow and temperature, but it incurs high operating costs and fails to utilize the potential energy of the liquid, resulting in high expenses. If the top condenser fails to meet the condensation requirements, a forced condensation reflux system can be added to enhance the operation of the distillation column. Additionally, when making investments, it is also necessary to compare the relative size of operational costs and investment costs for infrastructure. 2. If the requirements for the amount of reflux liquid are not high, or in other words, if there is a large operating range for the reflux ratio, then internal reflux can be used ； 3. If an “on-line flow measurement instrument” can be installed or developed along the backflow path, backflow is possible. Typical distillation uses external reflux! In general design, external recirculation should be used as much as possible! In distillation, a full-flow configuration with a condenser installed at the top of the tower is used, with the product being taken off at the distributor. But in actual industrial distillation, external reflux is commonly used! External recirculation is adopted in the design, mainly for reasons of controllability. Internal recirculation can save thermal energy while reducing equipment costs. The recirculation ratio is controlled by cooling water, which does not allow for an accurate determination of this ratio; it is suitable for applications with large processing volumes and modest requirements. The above is taken from http://bbs.hcbbs.com/thread-36273-1-1.html

Internal reflux: It refers to the situation where the material does not leave the tower; after condensing at the top of the tower, it returns directly back into the distillation tower.

The recirculation inside the tower occurs only through packing or trays

The material does not leave the top of the tower; after condensing there, it is directly returned to the distillation tower.

According to the third edition of \"Petroleum Refining Engineering\" by Lin Shixiong, internal reflux refers to the liquid-phase load between two trays in a distillation column used for separating complex petroleum mixtures (excluding trays where side streams are taken off); the gas-liquid phase loads in such cases 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 recirculation). In other words, the temperature of the liquid phase falling at plate n is lower than that at plate n+1; as a result, it vaporizes at 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 top product, as well as all the stripping steam and internal reflux below plate n+1. This internal return flow can be calculated through heat balance by applying a certain isolation 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 hydraulic calculations of the tower trays.

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 returns to 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 common in practice, while internal reflux is used mainly in smaller towers. Its disadvantage is that the reflux ratio is not easy to determine – only the amount of product taken out is known, while it’s difficult to figure out the amount of reflux. Last edited by www_jack on 2009-4-10 10:14.]

Internal reflux: This means that the material does not leave the tower; after being condensed internally at the top of the tower, it is directly returned to the distillation tower. It is generally used in batch operations, packed towers, small-scale production, and manufacturing with a variety of products.

Internal reflux: The gaseous material inside the distillation tower rises to the top of the tower, where it is cooled by a condenser and turned into a liquid state before flowing back to the bottom of the tower. The distillation tower and the condenser are integrated devices. External reflux: The distillation tower and the condenser are separate units connected by pipes. The gaseous material is drawn from the top of the tower and sent to a condenser where it is condensed into a liquid state, after which it is pumped back into the tower. Internal recirculation devices are commonly used in small systems, requiring less investment, but the recirculation volume is not easily observable. External return flow devices are often used in large systems where the investment is high, but the amount of return flow is easy to monitor directly.