Implementation in Aspen Dynamics

Vapor Flows. Pressures and pressure drops in the various sections of the steady model can be specified, and no valves are required between the fictitious vessels. However, 

Aspen Dynamics requires that valves be inserted between the vessels, and these valves must have some pressure drop. To compensate for this additional pressure drop, three fictitious compressors are installed on the vapor fines coming out the top of the stripper and the tops of the two sides of the wall. Three pressure controllers are installed to hold the back pressure in each of these three vessels by manipulating the work to the compressors. At the top of the rectifier, pressure is controlled in the conventional way by manipulating condenser heat removal. 

After the compressor in the stripper vapor line, the vapor line splits into two lines, one going to the prefractionator and the other going to the sidestream side of the wall. Control valves were inserted in both lines. A ratio control system is used to keep the vapor split constant. The total vapor from the stripper is determined by the compressor (to hold pressure in the stripper) and is measured. This flow signal is sent to a multiplier, whose other input is the desired ratio of vapor to the prefractionator to total vapor (the vapor split ratio). The output of the multiplier is the set point signal of a flow controller that controls the flow of vapor to the prefractionator by changing the position of the control valve in the line. 

Liquid Fiows. The liquid levels in the base of all three fictitious columns must be controlled. Fictitious pumps and control valves are installed at the base of each column. The base level in the stripper is controlled in the conventional way by manipulating bottoms flow rate. The liquid level in the base of each of the absorber columns (the prefractionator and sidestream side of the waU) are controlled by their corresponding control valves. 

The liquid level in the base of the rectifier corresponds physically to the total liquid trap-out tray. A pump and two parallel lines with control valves in each are installed. Since the flow rate to the sidestream side of the wall is the larger of the two, the level on the trap-out tray is controlled by manipulating the control valve in the liquid line to that side of the wall. A ratio scheme then adjusts the other control valve to maintain the desired liquid split. The liquid flow rate to the sidestream section is measured, and this signal is sent to a multiplier whose other input is adjusted to give the desired liquid split. The output of the multiplier is the set point signal to a flow controller that manipulates the valve in the liquid line to the prefractionator to achieve the specified flow rate. Note that this ratio is changed by the composition controller in the prefractionator. 

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Te mentioned control loops have been implemented in Aspen Dynamics (Fig. 13.37) that includes the control of the heat integration loop around the reactor (Fig. 13. 27) and of the separation system (Fig. 13.9). The following scenario was used to evaluate the performance of the control system initially, the production rate is set to 120 kmol/h, after 2 hours, increased to 150 kmol/h, later, reduced in two steps to 90 kmol/h. 

The control of units may follow the standard control structures applicable for standalone units. The HDA plant has been decomposed in several parts for an easier control implementation in Aspen Dynamics, as follows ... 

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... 

Ramp functions can be implemented in Aspen Dynamics by setting up a task that changes a variable over a period of time. The variable must be a fixed variable (in the notation of Aspen Dynamics). We want to ramp the set point of the feed flow controller, so the controller must be placed on cascade and the variable that is ramped is FC. SPRemote. Figure 15.6 gives the two Aspen Dynamics tasks used to ramp up and then ramp down the flow controller set point. Both tasks are edited, compiled, and activated. 

So the basic conventional control structure selected has reflux-to-feed ratio. This is implemented in Aspen Dynamics using a multiplier block (R/F) with one input being the molar flow rate of feed and the other input the specified reflux-to-feed molar ratio. Since Aspen Dynamics has the rather odd limitation of only being able to directly specify the mass flow rate of the reflux, a flow controller must be installed whose process variable signal is the reflux molar flow rate, and whose output signal is the reflux mass flow rate. This flow controller is put onto cascade with its set point signal coming from the R/F multiplier. 

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. 

In this chapter, we illustrate how external reset feedback can be implemented in Aspen Dynamics and demonstrate the improved dynamic performance using two process examples. 

Advanced control stmetores can be easily implemented in Aspen Dynamics. In this section we illustrate two of the more important methods. A simple distillation column is used to illustrate the installation of ratio elements (multipliers) and the use of cascade control. 

The dynamic simulation file prepared in Aspen Plus is exported in Aspen Dynamics [10]. We select the flow-driven simulation mode. Aspen Dynamics files have already implemented the basic control loops for levels and pressures. Units with fast dynamics, such as the evaporator or some heat exchangers, may be handled as steady state. The implementation of control loops for the key operational units, chemical reactor and distillation columns, take into account some specific issues from the plantwide perspective, which are developed in detail in Luyben et al. [8]. 

For the distillation columns, linear model-order reduction will be used. The linear model is obtained in Aspen Dynamics. Some modifications to the previous study have been done to the linear models, in order to have the reboiler duty and the reflux ratio as input or output variables of the linear models. This is needed to have access to those variables in the reduced model, for the purpose of the dynamic optimization. A balanced realization of the linear models is performed in Matlab. The obtained balanced models are then redueed. The redueed models of the distillation columns are further implemented in gProms. When all the reduced models of the individual units are available, these models are further connected in order to obtain the full reduced model of the alkylation plant. The outeome of the model reduction procedure is presented in Table 1, together with some performances of the reduced model. 

In this chapter, we present a numerical example to illustrate quantitatively the performance of pressure-compensated temperature control. In addition, a simple but accurate method for finding temperature/pressure/composition relationships is described, and the techniques for implementing pressure compensation in Aspen Dynamics are presented. 

The improvement in control by the use of pressure compensation has been quantitatively demonstrated. The implementation of this type of structure in Aspen Dynamics has been outlined. A simple procedure for deriving the relationships between temperature, pressure, and composition has been illustrated. Pressure compensation should be considered in distillation columns where pressure changes at the control tray are significant. 

The implementation of external reset feedback in Aspen Dynamics has been presented. The available Aspen Dynamics control blocks can be configured to simulate external reset feedback. Getting the simulation to run requires that the lag elements used for integral action must first be initialized with fixed signals and then connected to the rest of the blocks. 

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. 

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. 

Subsequently, we used Aspen Dynamics for time-domain simulations. A basic control system was implemented with the sole purpose of stabilizing the (open-loop unstable) column dynamics. Specifically, the liquid levels in the reboiler and condenser are controlled using, respectively, the bottoms product flow rate and the distillate flow rate and two proportional controllers, while the total pressure in the column is controlled with the condenser heat duty and a PI controller (Figure 7.4). A controller for product purity was not implemented. 

Finite difference methods are implemented in gPROMS (2001). Aspen Dynamics uses the third approach. Care should be paid to a convenient description of a PFR. A number of ten CSTR s is sufficient, but when the temperature variation is highly nonlinear, the use of several PFR reactors in series is recommended. 

The dynamic simulation model has been adapted to meet the constraints of a large scale problem and of the equation solving mode of Aspen Dynamics. The final model contained more than 6000 equations. Since the change in material balance (inventory) takes place at long time scales, some substantial simplifications of the local control of units can be considered. Finally, the plantwide control problem is reduced to analyse a 3x7 system, where three outputs (concentration of impurities li, I2, and I3) should be controlled with three among five inputs (D2, SS2, Q2, D4, and Q4), in the presence of two disturbances (Fdce, X ). Because of decentralised control, at most three SISO controllers should be physically implemented. 

The obvious way to implement this ratio is to simply use a multiplier block whose first input signal is feed flow rate, whose second input is the desire R/F ratio and whose output set the reflux flow rate. However, Aspen Dynamics has the strange limitation that the reflux flow rate sent to the column block is a mass flow rate. However, the R/F ratio determined in the feed composition sensitivity analysis is a molar flow rate ratio. 

Figure 8.49 shows the Aspen Dynamics process flow diagram that implements the control structure. The controller faceplates are also shown. Notice that the output signal of the reflux-drum level controller LC12 is the reboiler heat input (in cal/s). 

Each loop in the plantwide control structure is listed and described below. Figure 14.18 shows the Aspen Dynamics implementation of the control structure. Figure 14.19 shows the controller faceplates. 

First, we will show how external reset feedback is implemented in the widely used commercial dynamic simulator Aspen Dynamics. Developing effective control structures for processes often require the use of override controllers to handle operating up against constraints. Unfortunately, Aspen Dynamics does not have a module for an external reset feedback controller. The following section shows how one can be implemented using the available control element blocks and points out some of the problems in getting the simulation to initialize and run. 

Once these issues have been resolved in Aspen Plus, the procedure is to export the Aspen Plus file into Aspen Dynamics. Controllers must then be added to implement the... 

Since the pressure in the high-pressure column changes with operating conditions, a pressme-compensated temperature control structure is required to maintain product specification in this column. The detailed steps in implementing the various flowsheets and control structures in Aspen Plus and Aspen Dynamics have been discussed. 

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