Aspen Dynamics Control

Control with Only Bypass The important control loop in this process is the temperature controller that manipulates the bypass flow to control the temperature of the mixed hot and cold streams. The controller is direct acting (an increase in temperature opens the bypass valve). A 1-min deadtime is inserted in the loop, and a relay-feedback test is run that gives Tyreus-Luyben settings Kc = 0.48 and T/ = 4.0 min. The temperature transmitter span is 350-450 K. 

If the reactions had higher activation energies and were more sensitive to temperature, the reaction rates could become so small with the low reactor temperatures that the process could quench (with nothing reacting and all temperatures dropping to the feed temperature). 

Control with Both Bypass and Furnace A second temperature controller is 

The reactor outlet temperature drops to 450 K instead of the 438 K that occurs without the furnace. Thus the chances of a reactor quench are reduced. 

This control structure is tested on the 10mol% chlorine feed composition case. The tuning constants do not chance much. For the TCmix temperature controller, Kc = 0.32 and T/ = 5.3 min. For the TCin temperature controller, Kc = 3.9 and 

---

Dynamic Performance. The three control structures are simulated in Aspen Dynamics, controllers are tuned, and feed flow rate disturbances are imposed on the system. At time equal to 0.2 h, the feed flow rate is increased from 100 to 120 lb mol/h. At time equal to 4 h, the feed is dropped to 80 lb mol/h. Finally, at time equal to 7 h, the feed is increased to 120 lb mol/h. These very large disturbances are handled with different degrees of effectiveness by the three control structures. 

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. 

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... 

Once the file has been exported into Aspen Dynamics, controllers are installed to achieve the desired control structure and dynamic simulations are mn to check the stability and performance of the control system. Various types of disturbances should be imposed on the system, such as throughput changes, feed composition changes, and changes in the set-points of the product-quality controllers (temperature and composition controllers). In this section we demonstrate, in a detailed step-by-step fashion, how these operations are performed in Aspen Dynamics. 

The Aspen Plus file is pressure checked and exported into Aspen Dynamics. Controllers are installed to achieve the following control structure (see Fig. 4.48). 

Figure 15.5 Aspen Dynamics control structure with flow control of C4.

The ethylbenzene CSTR considered in Chapter 2 (Section 2.8) is used in this section as an example to illustrate how dynamic controllability can be studied using Aspen Dynamics. In the numerical example the 100-m3 reactor operates at 430 K with two feedstreams 0.2 kmol/s of ethylene and 0.4 kmol/s of benzene. The vessel is jacket-cooled with a jacket heat transfer area of 100.5 m2 and a heat transfer rate of 13.46 x 106 W. As we will see in the discussion below, the steady-state simulator Aspen Plus does not consider heat transfer area or heat transfer coefficients, but simply calculates a required UA given the type of heat removal specified. 

The tuning of the temperature controller is achieved by mnning a relay-feedback test, which the recent versions of Aspen Dynamics has made quite easy to do. The button on the... 

It is important to remember that a deadtime or several lags must be inserted in most control loops in order to mn a relay-feedback test. To have an ultimate gain, the process must have a phase angle that drops below —180°. Many of the models in Aspen Dynamics have only a first-order transfer function between the controller variable and the manipulated variable. In the CSTR temperature controller example, the controlled variable is reactor temperature and the manipulated variable is medium temperature. The phase angle of a first-order process goes to only —90°, so there is no ultimate gain. The relay-feedback test will fail without the deadtime element inserted in the loop. 

Some rudimentary controllers can be used with the RBatch (see Fig. 4.31) reactor, but they are less realistic than those found in Aspen Dynamics. Lags and deadtimes cannot be... 

Tubular reactors can be simulated using Aspen Plus. Several configurations are available constant-temperature reactor, adiabatic reactor, reactor with constant coolant temperature, reactor with countercurrent flow of coolant, and reactor with co-current flow of coolant. The isothermal reactor cannot be exported into Aspen Dynamics because it is not possible to dynamically control the temperature at all axial positions. Therefore only the last four types will be discussed. 

The program in Aspen Plus is run and pressure-checked. It is then exported to Aspen Dynamics as a pressure-driven dynamic simulation as was done in Chapter 3 with CSTRs. The Aspen Dynamics file is opened, giving the window shown in Figure 6.37. The default control scheme has a pressure controller manipulating the valve in the reactor exit line. The simulation is run until all variables stop changing. 

Figure 6.37 Aspen dynamics flowsheet with default controllers.

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 dynamics and control of a number of tubular reactor systems have been studied in this chapter. Both adiabatic and cooled tubular reactors have been explored in both isolation and a plantwide environment. Ideal systems have been studied using Matlab programs. Real chemical systems have been studied using Aspen Dynamics. 

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. 

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]. 

When exporting the steady-state solution file for dynamic simulation. Aspen dynamics provides inventory (i.e., level and pressure) controllers. However, the tuning of P level controllers is too tight, and need to be changed to 1 %/%. 

Aspen Dynamics generates automatically a linear model by means of a Control Design Interface (CDI). The state space matrices A, B, C, D are saved as sparse matrices in ASCII files. These can be imported in MATLAB and used for further calculations. 

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 ... 

Aspen Dynamics provides implicit pressure Pl-conlrollers, and sets the range of manipulated and controlled variables to [0, twice the nominal value]. Controller gain of 20 %/% and integration time of 12 min are used. These values are suitable. Contraiy, the P level controller gain of 10 %/% seems too aggressive, and may be changed to 1 %/%. 

For unknown reasons. Aspen Dynamics 10.1 does not allow the purge flow rate to be manipulated. Hence, vapour flow rate is used for pressure control. However, the gas recycle flow rate (stream 11) is set to the nominal value of 1600 kmol/h. The maximum... 

Dynamic controllability analysis. Based on the non-linear plant model, a linear dynamic model is derived, either as a set of transfer functions (identification method), or as a state-space description (matrices A, B, C. D). The last alternative is offered in some advanced packages, as Aspen Dynamics , but the applicability to very large problems should be verified. Then a standard controllability analysis versus frequency can be performed. The main steps are ... 

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. 

As shown in the case studies, it is recommended that dynamic simulation be employed to verify the results obtained by C R analysis. This simulation is routinely performed using HYSYS.Plant and ASPEN DYNAMICS, as demonstrated in this chapter. The reader is referred to the book Plantwide Dynamic Simulators in Chemical Processing and Control (Luy-ben, 2002) for many additional examples in which dynamic simulation assists plantwide controllability analysis. 

It is important when specifying the control valves to select the correct valid phase. If the stream is aU liquid, select Liquid-Only in the Valid Phases under Flash options on the Operation page tab of the valve block. If the stream is all vapor, select Vapor-Only. Some valves have both phases (particularly when the inlet is liquid at its bubble point temperature and pressure, which means flashing occurs when the pressure decreases as the fluid flows through the valve) and Vapor-Liquid should be selected. Numerical problems can occur in Aspen Dynamics if these valid phases are not correct. 

The information from Aspen Plus is exported into Aspen Dynamics by generating two additional files. The first is a.filename.dynffile, which is used in Aspen Dynamics and is modified to incorporate controllers, plots and other features. The second file is a filenamedyn.appdf file that contains all the physical property information to be used in Aspen Dynamics. 

---