Discuss how to determine the optimal feed position for the tower?

This is how I did it. Take a distillation tower as an example. 1. First, we clarify the separation purpose of the distillation tower. In other words, our goal is to determine what mass fraction of the key components should be present at the top of the tower, as well as what mass fraction should be present at the bottom of the tower; in other words, it is necessary first to ensure that separation is achieved. Of course, sometimes when there are azeotropic components present, it is not possible to set overly strict requirements on the mass fraction of the component to be separated, as such requirements cannot be met. 2. During simulation, we can achieve the separation goal mentioned above through the tower design specs. At this point, you can choose the feed to your tray arbitrarily (I usually use the operating variable bottom rate), as long as the run proceeds normally. 3. Use sensitivity analysis tools to conduct two sets of analyses. One is the change in reflux ratio, and the other is the change in feed to the tray; the resulting values are the heat load on the condenser and the reboiler, which are key factors determining the operating costs of the tower. 4. The above covers selecting an appropriate reflux ratio and an appropriate tray feed. Another point is that the selection of the BOTTOM rate must take into account the choice of the bottom pump (in case it is needed). Also, the reflux ratio has the greatest impact on the heat load at the bottom of the tower; it is possible to first select an appropriate reflux ratio and then choose an appropriate feed rate for the tower plates. I generally aim to minimize the load. Also, the actual operating conditions need to be taken into account. For example, in the distillation process in a tower, there is a certain component that must not reach the bottom of the tower, as severe leakage would result in waste; in such cases, adjustments are necessary. In summary, the selection of the feed for the tower, the choice of the reflux ratio, and the determination of the degree of separation of components are all determined based on a balance between the separation objectives and operating costs; economic analysis is very important. The operating costs of the tower mainly include expenses related to cooling water, heat, and electricity. The TAC analysis conducted by National Taiwan University is very detailed. A bit wordy.

Reply to 2# llbear: I have a question. What determines the setting of the operating variables in step 2 of your design? For example, if it is specified that 90% of the output from the bottom of the tower should be a, is this value determined by dividing the mass of a by 90%? I’m not sure if my understanding is correct

The operating variable is a range value. This ensures that the operating variable stays within this range, and that a certain value of this operating variable within that range enables the achievement of the values specified in your design requirements (for example, 90% output as specified). As for how to choose the operating variables, run the program a few times and check the results, and you’ll have an idea.

Reply to 4# llbear: I saw in a dissertation that the feed location is determined in this way; I’m not sure if it’s reasonable. Could you please advise? “Under full reflux, the liquid ethanol concentration on the third tray of the crude distillation column is closest to the raw ethanol concentration of 10% (M), so the feed should be introduced at the 3rd tray.” ”

Reply to 6# thebest1984: This is based on the principle that the composition of the feed should be as similar as possible to that of the material coming out at the feed inlet location in the tower. The goal is to disrupt the material balance inside the tower as little as possible. But usually, we determine the feeding position by following the steps provided by the colleague on the second floor.