Aspen Plus Simulation of CSTRs

The reactions considered in previous sections have involved hypothetical components A, B, C, and I) for which arbitrary physical properties and kinetics could be selected to illustrate various phenomena. Simple Matlab programs can be easily generated for these systems. 

The specific chemistry used to illustrate the use of Aspen Plus is the reaction of ethylene (E) with benzene (B) to form the desired product ethylbenzene (EB). There is a consecutive reaction that produces an undesirable product diethylbenzene (DEB). A third reaction combines benzene and diethyl benzene to form ethylbenzene  

The reactions occur in the liquid phase and are assumed to be irreversible. The reaction rates of the three reactions are assumed to be those given here  

The units of R are kmol s-1 m 3. Concentrations have units of kmol/m3. Activation energies have units of J/kmol. Temperature is in degrees Kelvin. 

There are two feedstreams to the reactor 0.2 kmol/s of pure ethylene and 0.4 kmol/s of benzene. The excess of benzene is used to keep the ethylene concentration low so that the formation of DEB is suppressed. 

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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. 

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. 

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. 

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