In Aspen simulation for distillation, increasing the feed rate (along with the recycle rate) does not meet the separation requirements
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Using Aspen to simulate distillation allows the process to meet the design requirements; however, when a recycle stream is introduced (resulting in an increased feed rate), the separation requirements cannot be satisfied (excessive product taken from the top of the tower increases the concentration of heavy components there). When designing a distillation column, the reflux ratio and D/F ratios are used. I hope an expert can give some advice; thank you.
If, after increasing the circulation rate, the composition of the circulation stream differs from that of the original feed, it is necessary to adjust D/F in order to achieve an optimal separation ratio, and also to adjust RR in order to reach a certain purity level.
Is D/F adjusted manually? Since D is constantly changing, it’s difficult to determine D/F. Also, the ethanol in the tower bottom cannot be completely vaporized; increasing the extraction rate will result in the ERROR RADFRAC NOT CONVERGED IN 200 OUTSIDE LOOP ITERATIONS message
In distillation column simulation, the most fundamental conservation law is mass conservation. In: Light separation of fresh material + light separation of circulating volume = Out: D*xD obtained from the top of the tower. There’s more entering now, and if purity xD is to be maintained, the amount harvested must be increased. I generally don’t set D/F; I only set the recovery rate, which is determined based on the total amount of light fractions in all feed streams. If the purity cannot be achieved even after setting up simulation, it is either because the reflux ratio is too low or the number of theoretical plates is insufficient.
One of the feed streams to the distillation column is ethanol, water, decanol, undecanol, tetradecane, and pentadecane. The goal is to have all of the ethanol evaporated from the top of the tower; the simulated results show that the ethanol concentration at the tower top reaches 95%+, but there is still some ethanol left in the bottom of the tower that has not been evaporated. Increasing the D/F ratio, the number of trays, or the reflux ratio does not help to resolve this situation. How should this be adjusted?
Based on experience, there are multiple azeotropes in this system: 1. The lowest azeotrope is for ethanol/water at 95/5 wt%； 2. There is a heterogeneous azeotrope between water and decanol. If you simulate in this way without first analyzing the distillation sequence, you certainly won’t get the desired result. It is not surprising that the minimum azeotrope ethanol/water appears at the tower top.
Actually, the purpose of that distillation tower is to vaporize all of the ethanol; the concentration doesn’t matter.
If that is the goal, it should be easier to evaporate the ethanol.
Could an expert help take a look? My feed composition consists of ethanol, decanol, undecanol, tetradecane, pentadecane, water, and octadecane. The mass fractions are as follows: 0.721752313651753, 0.0442076564776895, 0.0442055086490175, 1.37575414578302E-07, 4.83385915049734E-08, 0.175851656768755, and 0.0139826785387782. The mass flow rate is 1767.31767722422. The goal is to have all of the ethanol evaporated at the top of the tower; the presence of water is acceptable, but not in large quantities. The remaining components exit from the bottom of the tower.
That's all. Extract a little more ； If that doesn’t work, increase the reflux ratio further ； If that still doesn’t work, increase the number of theoretical plates and adjust the position of the feed plate accordingly.