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1. Problems with the Pro2 flash tank: Original question: A stream containing 70% ethanol by weight and 30% water by weight, with F=1000 kg/hr, P=0.8 MPa, and T=100 ℃, is mixed with a stream containing 60% n-hexane and 40% ethanol by weight, with F=500 kg/hr, P=0.6 MPa, and T=70 ℃, and both streams are subjected to adiabatic flashing in a flash tank until the pressure reaches 0.11 MPa. The liquid droplet entrainment rate of the light liquid phase in the vapor phase is 5%, while that of the heavy liquid phase is 1%. Find the temperature, mass flow rate, and composition of the vapor, liquid, and liquid-liquid phases leaving the flasher. For the selected flash drum with three-phase separation and NRTL thermodynamic model, why is there no flow at the lowest outlet? According to the regulations, NRTL can only be used in calculations of two-phase flows; you cannot select VLLE. However, guided by practical experience, for example when the aqueous phase is very small, you can ignore its presence, that is, not take it into account. In cases like yours where there is a large amount of aqueous phase, if you want to perform VLLE calculations, it is necessary to select a dehydrated stream; the SRKKD method is recommended for this purpose. This method (SRKKD) is a modification of the Soave-Redlich-Kwong method designed to improve the prediction of vapor-liquid-liquid phase equilibria in hydrocarbon systems containing water. The SRKKD method is also applicable to water-hydrocarbon systems. 2. PROII is showing the following error; could someone please tell me what the cause is? Original question: PROII is showing the following error. I would appreciate it if someone experienced this could tell me what the cause is 2 error(s), 0 warning(s), and 0 message(s) detected: *** error *** Invalid specification in unit 1, 't1' - the “primary” section refers to a non-e*stent stream''. *** error *** Invalid specification in unit 1, 't1' - the “primary” section refers to a non-e*stent stream''. The first segment in your unit 1 refers to a stream that does not exist, which is what causes the error. Check to see if you might have deleted a certain logistics record, or if a control indicator is linked to a non-existent logistics record. You asked about the control issues related to batch distillation in another section; are you using this software to simulate batch distillation? When will it be put into use? 3. The difficulty in converging of proII. Original message: It keeps showing that COLUMN calculations have been terminated. The value of Alpha (the damping factor) was BELOW 0.01 for 4 consecutive iterations after 2 new matrix calculations. Even a low value for Alpha isn’t acceptable! All process conditions are known. What could be the possible reasons? In addition to the thermodynamic models. You might have two mistakes: 1 is that the number of iterations in the loop is too low; increasing it won’t do any harm. 2 is the value of the damping coefficient alpha; it seems to have been reversed, and it must not be less than 0.01. The situation with dry plates sometimes does not cause interruptions. 4. Issues with proII: invalid stream. Original issue: I encountered the following message: *** error *** Invalid specification in unit 19, ‘p402’ – the “primary” section refers to an invalid stream’. But after checking, I didn’t find any invalid streams; I’m not sure what the problem is! You have selected a stream that should not be used as a reference. Or choose the correct one or cancel. 5. Using PRO/II and Excel to determine the optimal reflux ratio. Original question: Develop a program to determine the optimal reflux ratio using Visual Basic for Applications (VBA) that comes with Excel. This program utilizes PRO/II to carry out process simulations and hydraulic calculations for different numbers of trays; the resulting data are then imported into Excel to calculate and compare the costs associated with equipment and operation, thereby determining the optimal reflux ratio. Calculations on the examples show that the method yields reliable results, and its widespread application will help improve design quality and efficiency. Who knows this calculation formula... I’ll share my own opinion; I’m not sure if it’s accurate, just for everyone’s reference. First, we need to understand why it is important to find the optimal reflux ratio for a distillation column It’s like finding an accurate reflux rate to control the quality of the product at the top of the tower in actual tower operations. If that’s not the case, then who would want to do such a thing? Don’t you think? But I advise everyone not to do this; such a proposition is only useful in professional papers. Why is it said this way? Because in actual tower top product quality control algorithms, artificial external data is introduced to simplify the calculations. In other words, online analytical instruments are used to analyze the components at the top of the tower in real time, and the data is collected by the computing program in real time to serve as a basis for correcting the behavior of the program. Without this, relying solely on the software to perform infinite-level regression calculations not only leads to deviations from the correct results, but I guess this approach is only used in nuclear explosion simulations. When I was studying *device simulation optimization in the United States, Americans didn’t do it that way. If simulation is carried out using purely software-based methods, the process would likely be as follows (taking PRO_II as an example): after establishing an accurate model for calculation, a CASE STUDY is created within the module, online analyses are set up (you can create countless of these), and then the data is fed back into the ASPEN model until it runs successfully. Then enable the dynamic model (like Invensys Simsci Dynsim from Purple Moon on this forum), and find the optimal reflux ratio in a dynamic setting. The process is the same with ASPEN as well. Do you still want to do it? So how is device optimization done in reality? For distillation towers, this can now be achieved at the DCS level thanks to powerful CPUs. DCS controls the reflux flow rate to adjust the temperature and quality at the tower top; the data from the on-line analysis instruments at the tower top are used as the quality control parameters for the product taken from there, which in turn serve as the set value SP for the super-sequential control of the reflux flow rate at the tower top. The method of using pure Excel came about 2 or 3 years after 1989; before that, people used Lotus123 to perform simulation calculations for devices. It’s something from the great-grandparent’s generation. 6. How to analyze PRO_II errors? In an example in the attachment, there was no error message from the program during solving, but regression was not possible. I took a look. The program displays the error “no feature exsist -5357”. There are mainly two mistakes. One issue is that the algorithm cannot choose “RATEFRAC”, as RATEFRAC belongs to the Rate-based distillation routines; these routines calculate the actual mass transfer that occurs during each stage, thereby eliminating the need to consider component efficiencies. The non-equilibrium stage model used in RATEFRAC routines relies on fundamental principles of heat and mass transfer to model the distillation process. The algorithm can be modified to use an inside-out approach or some other method instead. It’s just that RATEFRAC can’t be used. If ASPEN is chosen, it should work fine. Secondly, in the performance specifications, the bottom flow rate is over 3300, while the feed flow rate is only around 380, which is clearly incorrect. After the modifications, it could run right away; the results came out and they looked great. In an example of the attachment, there were no error messages regarding the program status during solving, but regression was not possible. I took a look. The program displays the error “no feature exsist -5357”. There are mainly two mistakes. One issue is that the algorithm cannot choose “RATEFRAC”, as RATEFRAC belongs to the Rate-based distillation routines; these routines calculate the actual mass transfer that occurs during each stage, thereby eliminating the need to consider component efficiencies. The non-equilibrium stage model used in RATEFRAC routines relies on fundamental principles of heat and mass transfer to model the distillation process. The algorithm can be modified to use an inside-out approach or some other method instead. It’s just that RATEFRAC can’t be used. If ASPEN is chosen, it should work fine. Secondly, in the performance specifications, the bottom flow rate is over 3300, while the feed flow rate is only around 380, which is clearly incorrect. After the modifications, it could run right away; the results came out and they looked great. This is a case of me helping a friend from the Baisi Forum; it has now been brought to our Haichuan for everyone’s reference. Students who are interested can come and take a look. This is not my original work; please, the moderator, do not give it extra points. Perhaps everyone has better ideas, let’s improve together. Remove the .txt extension after downloading, then unzip it. Unmodified case PRO_II.rar.txt (2007-08-12 20:09:08, Size: 90.6 KB, Downloads: 14) 7. Why is the temperature change greater at the top of the tower? Original question: Why is the temperature change greater at the top of the tower? There is a condenser at the top of the tower; the first diagram shows full cooling, while the second one shows partial cooling. Your question isn’t actually related to software – it’s a matter of basic knowledge. For example, the components at the top of the tower are CH4 and C2; if complete condensation occurs, then all components above CH4 must also condense, and the temperature must be below the bubble point of CH4. Meanwhile, the temperature on the second tray is definitely higher than the bubble point of C2, otherwise C2 would not rise to the top condenser. Just think about how large the temperature difference is If it is fractional condensation, then assuming that the top of the tower requires C2 to condense while substances below CH4 do not condense, and C2 is used as reflux, the temperature at the top of the tower will differ significantly from that in the case of complete condensation. Similarly, what would be the difference in temperature at the second tray compared to the bubble point of C2? Therefore, to conduct good simulations, it is essential to focus on both theory and practice, and to excel in both areas.