ASPEN DYNAMICS

This structure assures correct pressure-flow solutions and, thus keeps the model consistent. Software packages, such as ASPEN Dynamics , will ensure this correct coupling. In general, two flow calculating devices cannot be connected directly, but must have a pressure (typically a volume) element in between. Two flow devices can be connected if a single equation can be written that describes the pressure drop over the connected section. For instance, some programs allow two pipe models to be connected. 

In Chapter 3, when we discuss the dynamic simulation of this CSTR using Aspen Dynamics, we will return to the problem and be more specific about the details of realistic heat transfer issues in a CSTR. 

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 message shown in Figure 3.60 provides some information about the exported file and indicates the exported file is ready to run in Aspen Dynamics. A simple way to... 

TRANSLATE USING COMPILED AST PROCEDURES IN C Piogfdm Ffles AspenTech Aspen Dynamics 2004.1 VAst DIRECTORY. 

WARNING Reaction Paragraph name R-1 is not valid in Aspen Dynamics It has been renamed to R 1. 

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. 

Results of running the simulation in Aspen Dynamics are shown in Figure 3.108 for a step change in feed composition. The compositions of the exit stream show a first-order lag response. 

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. 

Effect Of Number Of Lumps If the number of plotting points in Aspen Plus is set at 10 (the default), the resulting exit temperature from the reactors under steady-state conditions in Aspen Dynamics is 578 K. Remember that it should be 583 K from the rigorous integration of the ordinary differential equations describing the steady-state tubular reactor that are used in Aspen Plus. Changing the number of points to 20 produces an exit temperature of 580 K. Changing the number of points to 50 produces an exit temperature of 582 K, which is very close to the correct value. Therefore a 50-lump model should be used. 

Figure 6.37 Aspen dynamics flowsheet with default controllers.

However, some numerical difficulties are sometimes encountered in running the simulation in Aspen Dynamics when a large number of lumps are used. The 50-lump case runs very slowly or not at all in the adiabatic reactor cases. In this situation the number of lumps is reduced to get reasonable computing times. 

The reactor tubes are filled with catalyst, so the number of tubes is increased from the 250 used in Chapter 5 for the empty reactor case to 500 tubes. The tubes are 0.1 m in diameter and 10 m in length. The coolant temperature is 400 K at steady state. A 30-lump model is used and runs with no difficulty in Aspen Dynamics. 

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). 

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