Steady-State Aspen Plus Simulation

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At this point all the units in the flowsheet are installed and converged. The last issue is to converge the recycle stream. The initial guessed values are adjusted to be close to the calculated values of flow and composition leaving the split S1. When these two streams are fairly close, the source of the recycle stream is defined as the split SI and the recycle stream is defined as a Tear stream. The flowsheet did not converge when the default convergence method 

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The stream conditions shown in Figure 14.1 are from the dynamic simulation of the process at steady-state conditions with the recycle of solvent loop closed. This loop did not converge in the steady-state Aspen Plus simulation. Other simulation issues are discussed in the next section. 

Steady State from Dynamic Model The simulation is run out to a steady-state condition with the adjustable temperatures set to the same values as used in the steady-state Aspen Plus. These temperatures are the exit temperature from the first reactor (277°C) and the exit temperature from the heater HX3 (150°C). The power to the recycle... 

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 EWB column was simulated repeatedly using ASPEN PLUS (version 9.3) and CHEMCAD (version 3) with the same set of specifications as above and the same UNIQUAC interaction parameters (from Bekiaris et al., 1996). All runs converged to steady state II (low purity steady state) and not to steady state I. The single steady state located by simulators proves that wrong designs could be developed if the user is not aware of the presence of MSS and/or does not have the facility to obtain the same. Table... 

The previous chapter discussed the methods and techniques for using Aspen Plus simulation software to develop and optimize steady-state designs for azeotropic distillation systems. Once the steady-state design is complete, the dynamic controllability of the process should be explored. Only looking at the steady state does not tell you whether the process is operable. Dynamic simulations and the development of an effective control stmcture are vital parts of process development. 

Most of the treatments in the above books are qualitative and concepmal in namre, emphasizing VLLE issues and alternative configurations. Few of these books present in-depth rigorous designs that achieve optimum economic criteria. None of these books deal with the control and operation of azeotropic distillation systems. Detailed discussions of these two areas are the main contribution of this book. Rigorous steady-state and dynamic simulation tools (Aspen Plus and Aspen Dynamics) are used for design calculations and rigorous dynamic simulations. 

Steady-state analysis indicates that the type I and type III systems are more economical than the type II system. This chapter explores the dynamic controllability of these three flowsheets. Of more importance, we want to devise a systematic approach to the control of these three types of reactive distillations. AU of the results are obtained from steady-state and dynamic simulations using Aspen Plus and Aspen Dynamics. 

Aspen Plus Steady-state process simulation www.aspentec.com... 

To study different operating conditions in the pilot plant, a steady-state process simulator was used. Process simulators solve material- and energy-balance, but they do not generally integrate the equations of motion. The commercially-available program, Aspen Plus Tm, was used in this example. Other steady-state process simulators could be used as well. To describe the C02-solvent system, the predictive PSRK model [11,12], which was found suitable to treat this mixture, was applied. To obtain more reliable information, a model with parameters regressed from experimental data is required. 

S 2] With the steady-state process simulator Aspen Plus , thermodynamic models for the sulfur-iodine cycle given in [132] are combined with chemistry models which describe the dissociation and precipitation reactions. 

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 dynamic simulation file prepared in Aspen Plus is exported in Aspen Dynamics [10]. We select the flow-driven simulation mode. Aspen Dynamics files have already implemented the basic control loops for levels and pressures. Units with fast dynamics, such as the evaporator or some heat exchangers, may be handled as steady state. The implementation of control loops for the key operational units, chemical reactor and distillation columns, take into account some specific issues from the plantwide perspective, which are developed in detail in Luyben et al. [8]. 

Aspen Plus Steady State Simulation, Aspen Technology Inc., Cambridge, MA, 2000. 

Throughout this book, we have seen that when more than one species is involved in a process or when energy balances are required, several balance equations must be derived and solved simultaneously. For steady-state systems the equations are algebraic, but when the systems are transient, simultaneous differential equations must be solved. For the simplest systems, analytical solutions may be obtained by hand, but more commonly numerical solutions are required. Software packages that solve general systems of ordinary differential equations— such as Mathematica , Maple , Matlab , TK-Solver , Polymath , and EZ-Solve —are readily obtained for most computers. Other software packages have been designed specifically to simulate transient chemical processes. Some of these dynamic process simulators run in conjunction with the steady-state flowsheet simulators mentioned in Chapter 10 (e.g.. SPEEDUP, which runs with Aspen Plus, and a dynamic component of HYSYS ) and so have access to physical property databases and thermodynamic correlations. 

It may be concluded that Sequential-Modular approach keeps a dominant position in steady state simulation. The Equation-Oriented approach has proved its potential in dynamic simulation, and real time optimisation. The solution for the future generations of flowsheeting software seems to be a fusion of these strategies. The release 11.1 of Aspen Plus (2002) incorporates for the first time EO features in the environment of a SM simulator. 

Sequential-Modular approach is mostly used in steady state flowsheeting, among we may cite as major products Aspen Plus, ChemCad, Hysys, ProII, Prosim, and Winsim (see Table 2.2 for information). However, there are some dynamic simulators built on this architecture, the most popular being Hysys. 

Aspen Plus steady state simulation environment with comprehensive database and thermodynamic modelling feasibility studies of new designs, analysis of complex plants with recycles, optimisation. 

We start by examining the feasibility of alternative feeding policies by steady state simulation in Aspen Plus. Figure 13.12 depicts the flowsheet. The key units are the reactor and the distillation column. We chose a PFR model, with 100% per-pass conversion of A. The reactant B is converted exactly in the same proportion as the amount of fresh A, but the excess is recovered by distillation and recycled. Further, we consider a distillation column with 10 stages and feed in the middle. 

In this chapter, the principles behind the use of several widely used flowsheet simulators are introduced. For processes in the steady state, these include ASPEN PLUS, HYSYS.Plant, CHEMCAD, and PRO/n. For batch processes, these include BATCH PLUS and SUPER-PRO DESIGNER. 

Be able to prepare a steady-state simulation using ASPEN PLUS and HYSYS.Plant and be familiar with the capabilities of CHEMCAD and PRO/II. 

Have completed several exercises involving steady-state simulation using one of the four simulators, ASPEN PLUS, HYSYS.Plant, CHEMCAD, and PRO/II, and involv-ing batch process simulation using one of the two simulators, BATCH PLUS and SUPERPRO DESIGNER. 

In this chapter, the methods for shortcut C R analysis, using the results of steady-state simulations, have been described. The methods require the use of software for the solution of material and energy balances in process flowsheets (e.g., ASPEN PLUS, HYSYS.Plant) and for controllability and resiliency analysis (i.e., MATLAB). The reader is now prepared to tackle small- to medium-scale problems, and in particular should... 

Process simulators, steady state, dynamic, and batch, are used throughout the textbook (ASPEN PLUS, HYSYS.Plant, CHEMCAD, PRO/II, BATCH PLUS, and SUPERPRO DESIGNER). This permits access to large physical property, equipment, and cost databases... 

The steady-state simulation of distillation columns in Aspen Plus discussed in previous sections took a rating approach to the problem. Specific values for the total number of trays and the feed-tray location were selected, and the required reflux ratio and reboiler duty were determined for this specific configuration, subjected to attaining the desired product specifications. Then, economics must be used to find what the optimum tray configuration is. 

It is important to note that all of these methods use only steady-state information, so steady-state process simulators such as Aspen Plus can be easily used to perform the calculations. The methods all require that various variables are held constant, while other variables change. For example, two product compositions can be held constant, or a tray temperature and reflux flow rate may be held constant. The Design Spec/Vary feature in Aspen Plus is used to achieve the fixing of the desired independent variables and the calculation of all the remaining dependent variables. 

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