The pressure-swapped distillation cycle for the aspen azeotrope system does not converge
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As in the pressure-swapped distillation process, the azeotrope from the second column is recycled to the first column. Material balance calculations for the two columns were carried out beforehand; the amount and composition of the azeotrope recycled from the second column to the first column were determined and entered into the RE2 stream. The calculations worked fine without recycling, but it was not possible to adjust them properly even after introducing recycling. Changing the convergence method, splitting the flow streams, or varying the number of cycles did not help either. Logically, there shouldn’t be much information available regarding the quantity and composition of D5 and RE2, right? I seek advice from experts; thank you very much in advance. Include both cyclic and acyclic processes, simulated using 8.0.
The regulations are too strict; they specify the discharge volume at the bottom of the tower, leaving no room for adjustment. If the discharge is set based on the top of the second tower, then there will be no problems
This was calculated by looking at Sun Lanyi’s book and performing manual material balance calculations for the bottom of the two towers; I’ll give it a try by changing the outlet point to the top of the second tower.
I took a look; the value of the top of the second tower that I calculated manually through material balance was used as the initial value for the RE2 stream.
Indeed, the first tower requires that neither the bottom nor the top be used, while the second tower allows the use of the top only; moreover, the amount at the top is the same as that in RE2, and then the circulation works properly. Thank you so much; I’ll optimize it further.
I’m seeking help from experts. I’ve managed to establish circulation, but as a result, the azeotopic composition and azeotopic temperature at the tops of both columns have changed compared to those specified in the original design. As a consequence, the product at the bottom of the columns is not as pure as it was before circulation, and I’m unable to optimize the situation. Why do this azeotrope composition and temperature change?
It’s because you’re not at an azeotrope condition; with the introduction of a cycle, the number of theoretical plates or the reflux ratio becomes insufficient. You can treat the azeotrope as a pure substance, which makes it easier to understand. The material exiting the top of the tower now isn’t pure (probably due to an insufficient reflux ratio or number of plates), which leads to temperature changes. The azeotic composition remains the same, but the material is no longer pure (it contains some other components)
This post was last edited by dxyan on 2018-7-25 11:25. Thank you! My feed composition is not an azeotropic feed to begin with. Indeed, the purity decreases when a recycle tower is added; when performing mass balance calculations earlier, it was done assuming the presence of recycling. Moreover, the simulation without cycling is included, and RE2 in front of the mixer also takes cycling into account. RE2 and D2 should be similar; why isn’t it working as soon as they’re connected? I calculated it, and indeed the relative azeotopic composition of the two components at the top of the two towers remains unchanged; it’s just that the purity decreases, which causes the mole fractions to change. So how to solve the problem of purity at the bottom of the two towers? Can this adjust the reflux ratio and the number of plates? It seems that making adjustments doesn’t have much impact, as the values for D and W were already determined through previous material balance calculations.
The two W values can be determined through material balance calculations, but D for Tower 2 clearly cannot be calculated in this way; the circulation rate is directly related to the separation purity
Theoretically, for a binary minimum azeotrope, there are countless tower top compositions and recycle rates that can satisfy the purity requirements of the product at the bottom of the tower. You can use the TXY diagrams at different pressures to estimate the deviation of each tower top composition from the azeotrophic composition; by performing simple calculations, you can determine the value of the circulation rate, after which further adjustments and optimizations can be made. The circulation rate is an important optimization parameter, which is directly determined by the compositions at the tops of the two towers (theoretically, these compositions are azeotropic; in practice, they deviate from the azeotic points at the two pressures). ), the greater the difference between the two sets, the smaller the circulation volume.
I determined the separation purity myself before performing the material balance calculation; during the material balance, a total material balance is done for the two columns, and then another material balance is performed for the last column, allowing D and W to be calculated