I have a question regarding a kettle-type reboiler
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This post was last edited by Qingse Nianhua on 2018-11-28 at 09:26. Dear fellow sailors, I would like to ask what the definition of the vaporization rate for a kettle-type reboiler is. I often hear that the vaporization rate in such reboilers should be kept around 80%, but I’m not quite sure about this. After all, shouldn’t the material exiting from the top of the kettle-type reboiler and returning to the distillation tower be in a fully gaseous state? Shouldn’t the vaporization rate be 100%? Or rather, if this batch gasification rate refers to the gas-phase output/input, then shouldn’t this gasification rate already be determined by the reflux ratio at the top of the tower (since what is returned is in gas phase)? With the gasification rate already fixed by the reflux ratio, what is there to control?
Trust your own judgment; there are just some people who like to act mysterious. Could he be referring to the ratio of the total feed that gets vaporized? It is unknown.
The liquid phase enters the reboiler, where it vaporizes; the gas phase accounts for 80% of the feed
The last edit to this post was made by zhxbkofkyo on 2018-11-28 at 13:49. Well, even according to this definition, the vaporization rate is controlled by the reflux ratio and the output volume; a kettle-type reboiler cannot adjust the vaporization rate. For example, in my distillation column, the reflux ratio is very low (let’s assume it’s 0). If the ratio of the output from the top of the column to that from the bottom is 1, then isn’t the vaporization rate already determined at that point? =50%, so how can a kettle-type reboiler achieve an evaporation rate of 80%?
The amount of vaporization is determined by the heat load of the reboiler; the amount of heat generated determines the amount of vaporization that occurs. The circulation within the entire tower relies on the vaporization that takes place at the bottom of the tower. If there isn’t enough vaporization, then the amount of gas at the top of the tower will be reduced, and the desired reflux ratio cannot be achieved. As for the vaporization rate, it refers to the ratio of the amount of vaporization to the feed to the reboiler. When the heat supply to the reboiler remains constant, batch reboilers typically have forced circulation pumps, and the vaporization rate can be adjusted using the pump that circulates fluid at the bottom of the tower. Therefore, it is also adjustable, which means it has a high degree of operational flexibility as is commonly said. This is just my personal opinion; please feel free to correct me
According to you, will the forced pump cause a flow of liquid phase to return to the tower bottom? This allows the gasification rate to be adjusted
Reorganize your thinking: if you consider the reboiler as a theoretical plate, it becomes easier to understand. On this theoretical plate, there is a pure liquid phase, which corresponds to the feed rate to the reboiler; the gas phase that returns to the previous tray represents the amount of vaporization that occurs. In this way, the gasification rate becomes easy to understand. Different reboilers simply have different structures, but the principle remains the same. Some can be controlled, while others cannot.
I understand what you’re saying, but I’m not quite clear on the concept of keeping it at 80%. For example, in a distillation column, the amount of material leaving the top of the column and the amount leaving the bottom are determined during design. Based on the required purity level, the reflux ratio is also fixed, which in turn determines the heat load on the reboiler. Once the heat load is fixed, it means that the amount of steam sent back to the bottom of the column is also fixed, as is the amount of material leaving the bottom of the column. If there’s no reflux stream, doesn’t that mean the vaporization rate is also fixed? How can the vaporization rate be adjusted then? Upon adjustment, the reflux ratio and the amount of product at the top of the tower will inevitably change, deviating from the design values. If the designed gasification rate is over 70%, adjusting it to 80 may result in only limited changes in the aforementioned values; but what if the designed gasification rate is 50%?
The concept of an 80% vaporization rate in the reboiler should be bottom product flow/(feed flow + reflux flow) = 20%. The specific adjustments are made based on the heat load and the variables in the aforementioned formula. This is just my humble opinion; please feel free to give your feedback.
Unless there is a change in the column load, the amount of vapor at the bottom of the column must be maintained. The vaporization rate of many reboilers cannot be adjusted manually; this is the case for heat-siphon reboilers, as the amount of vapor is determined by the heat load, while the reboiler feed is ensured by a driving force, allowing the column and reboiler to reach an equilibrium on their own. At this point, the vaporization rate of the reboiler cannot be adjusted. However, if there is a forced circulation pump or a kettle-type reboiler with level control, the gasification rate can be adjusted by changing the reboiler feed rate. But at this point, it is necessary to consider the worst-case operating conditions when designing the reboiler, as the size and structure of the reboiler will vary depending on the circulation rate. These are just my personal opinions; I can’t guarantee they’re entirely correct.