A petroleum fractionator, such as a pipestill or a cat fractionator, is almost overwhelmingly complex. In addition to the main column, there are strippers that have vapor and liquid streams going back to and coming from the main column. There are a very large number of control loops to set up. Let us enumerate the loops that we will set up, considering both the preflash column and the pipestill:
1. Temperature loops—temperatures of both furnaces (two loops)
2. Flow loops—two crude feeds, steam to two column bases, steam to three strippers, and stage 19 liquid in the pipestill (eight loops)
3. Pressure loops—condensers in both columns and three strippers (five loops)
4. Level loops—base level of two columns, water level in two reflux drums, organic level in two reflux drums, and base levels in three stripper bases (nine loops)
5. ASTM boiling points—95% boiling point of light naphtha, 95% boiling point of heavy naphtha, 5% boiling point of diesel, 95% boiling point of diesel (four loops)
There are 28 controllers to set up on these two columns!
The equipment associated with the preflash column has already been sized. The diameters of the pipestill column and the three strippers are sized using the Tray Sizing feature of Aspen Plus for each vessel. The results are
Vessel Diameter (ft)
Stripper S-1 5.0
Stripper S-2 4.7
Stripper S-3 3.4
The reflux drum, the column base, and the bases of the three strippers are sized to give 5 min of holdup at a 50% level.
The file is pressure-checked and exported into Aspen Dynamics. When it opens, using version 12 of Aspen Dynamics, there is an error message that the system is overspecified. This problem was corrected by Aspen Support using a "hot fix."3
The initial control structure set up by Aspen Dynamics is shown in Figure 11.45. The flowsheet is quite congested with many process lines, control lines, and equipment. It takes a fair amount of artwork to rearrange the drawing to make it readable.
The two condenser pressure controllers, two organic-phase level controllers, and the water-phase level controller in the preflash column have been installed. In addition, pressure controllers on the three strippers are set up. Note that the vapor flows from the strippers back to the main column are manipulated. No control valve is shown in the vapor line, indicating that a "flow-driven" assumption is made in this flow. Also note that the three stripper pressure signals all appear to come from the first stripper. These will be moved in the final flowsheet to start from the appropriate stripper for each pressure controller. This is done by clicking the control signal line, clicking the blue arrow at the point of origin, and dragging it to the correct location when the arrow turns red. The original flowsheet showed the three pressure controller output signals all going to the vapor line of the top stripper. These lines have been relocated to show them going to the correct vapor line of each stripper.
Installing the level controller to hold the levels in the base of each column and the two levels in each reflux drum is straightforward. Note that the organic level is controlled by manipulating the reflux flowrate in both columns because the reflux ratios are large. When selecting the PV signal for the levels in the reflux drums, the organic phase is Level 1 and the water phase is Level 2. Proportional controllers with gains of 2 are used on all levels. The two furnace temperature controllers are installed in the conventional way. Deadtimes of 1 min are used in these loops, and temperature transmitter ranges are 100-500OC. Relay-feedback testing and Tyreus-Luyben tuning give controller gains of 0.6 and integral times of 4 min in both controllers.
Flow controllers are installed on the steam to the base of the two columns. These flow-rates are ratioed to the feedflows to the respective column by using multipliers. The molar steam to feed ratio in the preflash column is 125.9/2722 = 0.04625. The total crude feed is
3The extensive help of Arabella Geser (Global Customer Support & Training, Aspen Tech Europe) in getting the pipestill dynamic simulation to run is gratefully acknowledged. The new version of Aspen 2004 has incorporated this correction.
used (after the summer). The molar steam to feed ratio in the pipestill is 302.1/1654 = 0.1827. The two steam flow controllers are "on cascade" with their setpoints coming from multipliers set up with the appropriate constant and with the appropriate flow signal input.
Setting up the stripper base-level controllers requires a little graphical skill. All the input and output arrows appear on the top stripper and must be moved to the correct location. For example, Figure 11.46 shows an arrow pointing to the liquid line between the main column and the top stripper. This is the correct location for the Stripper Draw (S-1) when the output signal for the stripper S-1 level controller is being set up. For the other strippers, the arrow must be moved to the correct location. Figure 11.47 shows the selection of the manipulated variable (level controller OP signal). In Aspen Dynamics, the OP signal is called the Control Variable instead of the less confusing terminology of calling it the manipulated variable. In this book the "controlled" variable is the PV signal.
Specifying the PV signal to the stripper S-1 level controller is shown in Figure 11.48. The stripper has four stages, so the level on stage 4 is selected. Each of the three stripper level controllers is set up in the same way. The diagram is quite congested. Figure 11.49 gives an enlarged view of the stripper section of the flowsheet showing the three pressure and three level controllers with the PV and OP signals coming from the correct locations on the appropriate stripper vessels. Note that the level controllers are "reverse"-acting since they control level by changing the flows of material into the strippers.
A flow controller is installed to control the "overflash" flow of liquid below the AGO drawoff tray. This is achieved by manipulating the control valve V22 in the AGO line. Remember that the liquid drawn from the column to the stripper controls the liquid level in the base of the stripper. Manipulating the AGO flow changes the liquid drawoff rate and therefore the amount of liquid that is not drawn off and, as a result, flows down the column. The FCwash flow controller must be "direct"-acting; specifically, if there is too much stage 19 liquid, the AGO flow should be increased.
The internal liquid and vapor flowrates are not listed in the possible output variables to be controlled, so a "flowsheet equation" is used. Figure 11.50 shows the equation used. The mass flowrate of liquid leaving Stage 19 ("Fml_out") of the main column ("CDU") is defined as the PV signal to a flow controller ("FCwash"). When this equation is compiled, the red light at the bottom of the window indicates that the system is overspecified. This is corrected by changing the PV variable in the FCwash controller from "fixed" to "free." The appropriate ranges of the variables are inserted in the controller. The steady-state flowrate of stage 19 liquid is 48,800 kg/h.
The final controllers to install are the ASTM boiling point controllers. The appropriate boiling points are selected as discussed earlier in this chapter by using the Configure Sensor feature for each stream. The light-naphtha flow in the preflash column is manipulated to control its 95% boiling point at 190.6°C. The heavy-naphtha flow in the pipestill is manipulated to control its 95% boiling point at 190.6°C. The diesel flow is manipulated to control its 95% boiling point at 338°C. All of these controllers are reverse-acting.
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