Aspen Plus decanters

In Chapters, the steady-state design of a heterogeneous azeotropic distillation process for the dehydration of ethanol using benzene as a light entrainer was studied. The process consisted of two distillation columns, one decanter and two recycle streams. One of the recycle streams was successfully closed, but the second would not converge using steady-state Aspen Plus. 

An alternative simulation was developed using the Flash3 model, as shown in Figure 8.40b after exporting and installing a control structure. In Aspen Plus, a vapor line with a valve is added. The decanter is specified to be adiabatic and at a fixed pressure (0.6 atm). The temperature specified in the upstream condenser HX2 is adjusted to 320 K to give a very small vapor flow rate (3% of the feed). After the file is exported, a pressure controller is inserted on the decanter, but it is put on manual and the vapor valve is closed. Now decanter pressure varies with temperature and composition. Its steady-state value is 0.366 atm with the decanter temperature controller set point set at 313 K so that a direct comparison with the previous case can be studied. 

Set the reflux ratios for both columns to 0.5. We don t really want to reflux from the condensers, but want to take our reflux from the decanter however, if you try setting the reflux ratio to 0.0 Aspen Plus will not run. Reduce both reflux ratios in a few steps down to about 0.025. These ratios are small enough that the results will be very close to using only reflux from the decanter phases. If you don t do this in steps, the condenser will dry up and the run will have errors. Pamper Aspen Plus, and make it happy After you finish the lab, you might try reinitializing and see what happens if you set reflux ratio to 0.025. Results when the column dried up are not useable. 

In Part 3 of this book an extrainer is added to the system so that liquid-liquid sphtting can appear in the top decanter and also maybe in the top few stages of the azeotropic column. The LLE behavior in the decanter, or the VLLE behavior in the top stages of flie azeotropic column, can be predicted by Aspen Plus. The system of separating an isopropanol-water mixture using cyclohexane as the entrainer will be used as an example to demonstrate the way to generate a LLE envelope in Aspen Plus. 

While we are on the subject of decanters, it is a good time to discuss how to simulate a decanter using Aspen software. There are two models that can be used. These are found under the page tab of Separators on the Aspen Plus window. One is labeled Decanter. The other is labeled FlashS. They have distinct differences that need to be understood. 

One important feature of the flowsheet shown in Figure 7.5 is a vapor stream leaving the decanter. As discussed in Section 4.1.3, there are two liquid-liquid decanter models available in Aspen Plus and Aspen Dynamics. The first is DECANTER, which has only two liquid streams leaving the vessel. The second is FLASH3, which has two liquid streams and a vapor stream. Both of the models worked for this system in the steady-state simulations conducted in Aspen Plus. However, in the dynamic simulations conducted in Aspen Dynamics, we found that the FLASH3 model worked well for the butanol-water system, but the DECANTER model predicted that the organic level steadily increased and could not be controlled. The reason for this problem is unclear. 

The Aspen Plus steady-state simulation in the last section is exported to the dynamic simulation of Aspen Dynamics. The tray sizing option in Aspen Plus is utilized to calculate the column diameter to be 0.3259 m and the tray spacing is 0.6096 m. Other equipment sizing recommended by Luyben is used here. The volume of the reboiler is sized to give 10 min holdup with 50% liquid level. The decanter is sized to be bigger to allow for two liquid phases to separate. The holdup time of 20 min is used in the dynamic simulation. Pressure-driven simulation in Aspen Dynamics is used with the top pressure of the azeotropic column controlled at 1.1 atm to allow for some pressure drop in the condenser and decanter to give the decanter at atmospheric pressure. The pressure drop inside the colunm is automatically calculated in Aspen Dynamics. Since the tray pressures in the colunms are different than the constant atmospheric pressure assumption used in steady-state simulation, the established base-case condition in Aspen Dynamics will be slightly different than Table 9.11. The final base-case steady-state condition used for control study can be seen in Table 9.15. 

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