Figure 11.26 Initial control scheme.
(LCW1) that pulls off free water from the reflux drum. The other level controller (LC12) manipulates reflux flowrate to hold the liquid level of the organic phase in the reflux drum. Since the reflux ratio is only 0.344 in this column, we change the control structure to hold reflux drum level with the NAPHTHA flowrate (valve V12). Pressure controller PC1 manipulates the valve V14 in the vapor line.
Flow controllers are added to the two crude feeds and the stripping steam. A base-level controller is added that manipulates bottoms flowrate. A temperature controller is added that holds the furnace outlet temperature by manipulating furnace heat input.
The only remaining loop is a controller that manipulates the flowrate of the NAPHTHA to maintain a 95% point at 375°F (191 °C in the metric units used in Aspen Dynamics). The default properties available in Aspen Dynamics for any stream do not include boiling point information. To obtain these data, we must turn on the "Stream Sensor." Select the NAPHTHA stream, right-click, select Forms, and select Configure Sensor. The view shown at the left in Figure 11.27 shows the window that opens on which we click the Sensor On and Calculate Phase Properties boxes, specify the Valid Phases to be Liquid-Only, and select the additional properties that we wish to have available. In our case the ASTM D86 Temperature is the item of interest, so it is moved to the right column under Selected Properties. The number "95" is entered in the Liquid Volume % Distilled box. Now this property is reported in the stream results for the NAPHTHA stream (see the right side of Fig. 11.27). This property can also be selected as an input to a controller ("95BPt"), which will manipulate reflux flowrate to maintain the desired ASTM 95% boiling point of the naphtha.
Figure 11.28 shows the control structure. All level controllers are proportional with gains of 2. The furnace temperature loop has a deadtime of 3 min instead of the normal 1-min deadtime used in temperature loops because the dynamics of a fired furnace are usually slower than those of a steam-heated reboiler. Relay-feedback testing and Tyreus-Luyben settings give controller tuning constants of KC = 0.465 and tj = 13 min (with a temperature transmitter range of 100-500°C and a maximum heat input of
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