Flash3 decanter model

Figure 8.41c gives results when the Flash3 decanter model is used with a direct-acting aqueous-level controller. A comparison of the upper left graphs in Figure 8.41b Decanter...

Figure 8.40 (a) Control structure with decanter model, (b) Control structure with Flash3 model, (c) Controller faceplates with Flash3 model. 

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 decanter model using FLASH3 requires two specifications. The first is selected as pressure and the second is specified to be a very small vapor fraction (0.0001). During the dynamic simulations, the control valve on the vapor stream is completely shut by putting the decanter pressure controller on manual and setting its output signal at 0%. 

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. 

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