Aspen Dynamics simulation

It might be wise to point out that there can be other problems with a cooled reactor that will influence the selection of reactor type. An adiabatic reactor with a bed of catalyst is certainly mechanically straightforward to construct and maintain. Catalyst is easily loaded or discharged. A cooled reactor with multiple parallel tubes is mechanically more complex. Loading and emptying the tubes of catalyst can be difficult. 

The issue of flow maldistribution (unequal flows through the parallel flow paths) can be important in some reactors. 

The steady-state designs of four types of tubular reactors were illustrated in Chapter 5. Now we want to explore the dynamics and control of each of these reactors. 

---

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... 

The flow rate of the medium is manipulated in the Aspen Dynamic simulations, not Qc or (2r directly. However, this model contains no d5mamics. The holdup of medium in the heat exchanger is not considered. Medium flow rate changes produce instantaneous changes in temperature driving forces and subsequent heat-transfer rates. 

If the condenser is cooled by a vaporizing liquid (boiler feed water in high-temperature columns or liquid reMgerant in low-temperature columns), the difference in temperature between the specified temperature of the vaporizing liquid and the temperature of the process in the condenser is used to calculate the UA and the flow rate of the coolant from the known condenser duty Qc- The flow rate of the coolant is manipulated in the Aspen Dynamics simulations, not Qc directly. 

An Aspen Flash model is used for the reflux drum with pressme set at 1 bar and design specification of a vapor fraction of 10 , which makes the drum essentially adiabatic. A small vapor flow rate is necessary so that the control valve in this vent line can be sized. In the Aspen Dynamics simulation, the valve is completely closed. The liquid holdup in the drum is set to give 5 min at 50% full (diameter 3 m and length 6 m). 

Figure 16.4 shows the Aspen Dynamics flowsheet with controllers installed. A steam-to-feed ratio is used with the ratio changed by the temperature controller. The need for this ratio to improve load performance is illustrated in Figure 16.5. A 50% increase in feed flow rate is the disturbance. The solid lines show responses without the QR/F ratio. There are very large drops in Stage 55 temperature that result in large transient increases in the C4 impurity in the bottoms (xB). The units of the multiplier must be metric in the Aspen Dynamics simulation (GJ/kmol).

Relay-Feedback Testing. A relay-feedback test is an easy and accurate method for experimentally determining important dynamic parameters of a process. The test can be used on a real process or on a simulation. It is widely used in both industrial and academic applications. Relay-feedback tests can be easily performed in Aspen Dynamics simulations. 

In the Aspen Dynamics simulation, a valve must be installed between the reactor and the column. In reality, no valve is necessary if the reactor pressure can be the same as the... 

---