Flowsheet equations

Aspen Dynamics has the capability of using flowsheet equations for specifying a desired relationship. We illustrate this by setting up an equation that defines the PV signal to a temperature controller as T(6). The reactor block is COOLANT, so the T(6) temperature is BLOCKS( COOLANT ).T(6). 

The control structure is changed to control the temperature at 2 m from the inlet of the reactor T(6). Figure 6.62 shows the flowsheet equation Text Editor with the input to the deadtime block set equal to the temperature in the reactor block COCUR at lump 6. The input to the deadtime block is changed to Free. 

Figure 6.71 Flowsheet equation and freeing input to deadtime.

These are implemented in Aspen Dynamics by using Flowsheet Equations. Figure 8.28a shows the window that opens when Flowsheet is clicked in the Exploring-Simulation window. The parallel bars labeled Flowsheet are clicked and the Constraint-Flowsheet window shown in Figure 8.28b opens. The two required equations are entered using... 

Figure 11.50 Flowsheet equation for controlling wash flow rate.

This, of course, is not what we want since the pressure (and temperature) must vary to change the heat-transfer rate and the steam flow rate. It is necessary to use Flowsheet Equations in Aspen Dynamics to change the specification to have an exit hot stream with a vapor fraction of zero. Figure 13.5 shows the equation used. It is also necessary to change the pressure of the hot exit stream fxom fixed to free so that the system is not over specified. 

The equations derived above are implemented in Aspen Dynamics using Flowsheet Equations. Figure 16.7 shows the syntax required to use the measured pressure and temperature on Stage 55 to estimate the C4 composition on Stage 55. This calculated variable is the input signal to the deadtime block. The control signal line from the column icon to the deadtime block is deleted on the process flowsheet diagram. 

Figure 16.7 Flowsheet equations for pressure-compensated temperatures.

The dynamic controllabilities of the three designs considered above are compared in this section. Implementing heat integration in Aspen Dynamics requires the use of Flowsheet Equations. First we look at the case with complete heat integration. 

Achieving dynamic simulations that rigorously capmre the neat heat integration require the use of Flowsheet Equations in Aspen Dynamics. Two conditions must exist at each point in time during the dynamic simulation. First, the heat transfer in the condenser/reboiler must be equal to the product of the area, the overall heat-transfer coefficient, and the current temperature difference between the reflux drum of the high-pressure column and the base of the low-pressure column. These two temperatures both change dynamically as compositions and pressures vary. The pressure in the high-pressure column is not controlled but floats. 

The next step is to set up the flowsheet equations. As shown in Figure 6.10a, the Flowsheet item is selected in the Exploring Simulation window, and Flowsheet in the lower window (two parallel blue bars) is double clicked. A text editor window opens (Fig. 6.1 Of ) on which the appropriate equations are entered. 

Figure 6.10 (a) Setting up flowsheet equations, (b) Equations for condenser/reboiler coupling and... 

Figure 6.21 Flowsheet equations with auxiliary reboiler.

Both the temperature on Stage 9 and the column pressure are used to calculate Tpc, which is the process variable signal fed to the TC2 temperature controller. In Aspen Dynamics, this is easily achieved by using flowsheet equations, as shown in Figure 6.27. The last equation calculates the signal fed to the deadtime element in the TC2 loop. Figure 6.28 gives the control structure. 

Figure 6.27 Flowsheet equations with pressure-compensated temperature.

The energy requirement in the low-pressure column is 14.79 MW, so the auxiliary reboiler must provide 14.79-5.86 = 8.93 MW (32GJ/h, as shown in the TCI faceplate in Fig. 6.30). Flowsheet equations similar to those described in the THF-water system are needed in this system. The pressure-compensated temperature measurement uses the temperature (411.5 K) and pressure (10.354 atm) on Stage 53. 

Implementing heat integration in Aspen Dynamics requires the used of Flowsheet Equations, as discussed in detail in Ghapter 6. There are two conditions that must be satisfied at each point in time during the dynamic simulation. 

These conditions correspond to the first and second laws of thermodynamics. Figure 11.12 shows the flowsheet equations for the extractive heat-integrated system. The two conditions listed above are specified in the top two equations in the Text Editor window. 

Figure 11.12 Aspen Dynamics flowsheet equations for heat-integrated extractive process and pressure-compensated temperature.

The deadtime element dead2 has no input shown in Figure 11.13, but its input signal is the pressure-compensated temperamre signal as dehned in the flowsheet equation given in Figure 11.12. 

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