8.3.1 Development of Control Structure
The column shell, reflux drum, and column base are sized for each column, and the file is exported into Aspen Dynamics. The flowsheet opens with two default pressure controllers manipulating condenser duties. Four level controllers are installed. Reflux drum levels are controlled by manipulating the distillate flowrates. Base levels are controlled by manipulating the bottoms flowrates. A temperature controller "TC2" is installed on the high-pressure column C2, which controls stage 24 at 417.08 K by manipulating reboiler heat input Qr2.
The major issue is to make the reboiler heat input QR1 to the low-pressure column C1 equal to the heat removal rate QC2 in the condenser of the high-pressure column C2. The latter is being calculated by the pressure controller in the high-pressure column.
Aspen Dynamics has the capability of using "flowsheet equations" to solve this "neat" operation problem. In the Exploring window, select Flowsheet. The window below Exploring is labeled Contents of Flowsheet as shown in Figure 8.15.
In this window there are two parallel blue bars. Double-clicking opens a window called Text Editor—Editing Flowsheet, on which equations can be written. As shown in Figure 8.16, the heat duty in the low-pressure column C1 is equated to the negative of the heat duty in the high-pressure column C2. The syntax must be precise.
Reboiler heat input in C1: BlocksC'Ci' ').QReb Condenser heat removal in C2: Blocks(,,C2' ').Condenser(1).QR
Note also the minus sign. The equation must end with a semicolon. Right-click the Text Editor window and select Compile. If your equations are correct, a message appears on the Simulation Messages window stating that no compilation errors occurred.
Note the little red block at the bottom of Figure 8.16. The simulation is overspecified by one variable, so the reboiler heat input in column C1 must be changed from Fixed to Free. This is done by clicking on the C1 icon, right-clicking, selecting Forms, and clicking All Variables. As shown in Figure 8.17, scroll down until QReb is found. Click the arrow and select Free. The red block turns green, and the simulation is ready to run.
It is important to clearly understand what the setup is. As the simulation runs along in time, the pressure controller PC2 in the high-pressure column C2 is controlling pressure by manipulating QC2. The flowsheet equations set the reboiler input QR1 in the low-pressure column C1 equal to the negative of QC2.
The temperature controller TC2 in the high-pressure column is tuned by inserting a one-minute deadtime and relay-feedback testing. The tuning constants are KC = 1.15 and t/ = 10.6 min. A temperature transmitter range of 350-450 K is used. The action of this temperature controller is Reverse.
But we need some way to control the temperature in the low-pressure column C1. One way to do this is to adjust the feed F1 to the low-pressure column. A temperature controller is installed that maintains the temperature on stage 26 at 356.5 K by manipulating the control valve V1. This controller is set up to be Direct-acting. If the temperature is increasing, there is too little feed for the given reboiler heat input, so we should feed more to the column. Remember, we cannot adjust the reboiler heat input in this column. This is whatever is dictated by column C2.
The final element of the control scheme involves setting the total feedflow to the unit. This is done by installing a flow controller that looks at the total fresh feed and manipulates the valve in the feedline to the high-pressure column. Figure 8.18 shows this control structure.
Obviously these two temperature controllers are interacting. An increase in temperature in the high-pressure column will reduce reboiler heat input in column C2. This will reduce the condenser duty in column C2, which means a reduction in the heat input to column C1. This will decrease the temperatures in column C1, and the temperature controller TC1 will reduce the feed to column C1. However, now the total flow controller will increase the feed to column C2, which will tend to drop the temperature in column C2.
One way to take this interaction into account is to first tune TC2 with TC1 on manual. Then TC2 is placed on automatic and a relay-feedback test is run on TC1. With a
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