Aspen Dynamics Simulation of CSTRs

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. 

All of the dynamic simulations discussed in this book use pressure-driven flows. The alternative of using a flow-driven simulation is more simple, but not at all realistic of the actual situation in a real physical process. The plumbing in the real process has to be set up so that water flows downhill. Pumps, compressors, and valves must be used in the appropriate locations to make the hydraulics of the system operate. If valves are not designed with sufficient pressure drop under steady-state conditions, they may not be able to provide the required increase in flow even when wide open. So valve saturation must be included in the rigorous nonlinear dynamic simulation. It is much better to simulate a realistic system by using a pressure-driven simulation. 

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Convergence User Subroutine Dynamic Block Options ResulCs 

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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. 

This chapter deals with basic fundamentals of novel reactor technology and some of green reactor design softwares and their applications. Basic understanding of flow pattern in stirred-tank reactor by computational fluid dynamics and simulation of CSTR model by using ASPEN Plus were mainly presented in this chapter. 

A case of practical interest is a chemical reactor coupled with a separation section, from which the unconverted reactants are recovered and recycled. Let s consider the simplest situation, an irreversible reaction A—>B taking place in a CSTR coupled to a distillation column (Fig. 13.5). Here we present results obtained by steady state and dynamic simulation with ASPEN Plus and ASPEN Dynamics. The reader is encouraged to reproduce this example with his/her favourite simulator. The species A and B may be defined as standard components with adapted properties. In this case, we may take as basis the properties of n-propanol and iso-propanol, and assume ideal phase equilibrium. The relative volatility B/A increases at lower pressures, being approximately 1.8 at 0.5 atm. We consider the following data nominal throughput of 100 kmol/hr of pure A, reactor volume 2620 1, and reaction constant =10 s". For stand-alone operation the reaction time and conversion are r= 0.106 hr and = 0.36. 

The reactor simulation with Aspen Dynamics sets some modelling constraints. In dynamic mode a PFR is transformed in a series of CSTR s. In order to get a reasonable model size, the number of elements is limited at 10 to 20. Therefore, we consider two PFR s in series. Because the temperature variation is the most important in the first zone, the first reactor has 100m and 20 elements. In the second zone the variations are almost linear, so that 10 elements for 900 m are sufficient. 

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