Dynamic simulation ASPEN DYNAMICS

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 %/%. 

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. 

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. 

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. 

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. 

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. 

To conclude our examples of Aspen Dynamics simulation of tubular reactor systems, we study a very important industrial process for the production of methanol from synthesis... 

---