Aspen Plus Steady-State Design

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

Several important types of reactions are considered in the following sections. The equations describing each of these systems are developed. The steady-state design of CSTRs with these reactions are discussed, using Matlab programs for hypothetical chemical examples and the commercial software Aspen Plus for a real chemical example. 

In the steady-state design using Aspen Plus, two Design Spec/Vary functions are typically used to manipulate the flow rates of reflux and distillate to achieve the specified purities (or impurities) of the two products. These calculations are done at the design feed composition and the design feed flow rate. 

StBady-State Design. The feed conditions, the pressure, the total stages, and the feed stage are specified in the steady-state design using Aspen Plus (version 7.3). Two Aspen Design Spec/Vary functions are set up to achieve the desired product specifications of 0.001 mol fraction water in the liquid distillate and 0.001 mol fraction methanol in the bottoms. 

In Chapters, the steady-state design of a heterogeneous azeotropic distillation process for the dehydration of ethanol using benzene as a light entrainer was studied. The process consisted of two distillation columns, one decanter and two recycle streams. One of the recycle streams was successfully closed, but the second would not converge using steady-state Aspen Plus. 

In this chapter, we discuss both the steady-state design and the dynamic control of divided-wall columns. Aspen simulation tools are used. The industrially important ternary separation of benzene, toluene, and o-xylene (BTX) is used as anumerical example. The normal boiling points of these three components are 353, 385, and 419 K, respectively, so the separation is a fairly easy one with relative volatilities aa/ax/ax of about 1. I2.2I. The feed conditions are a flow rate of 3600 kmol/h, a composition of 30/30/40 mol% B/T/X, and a temperature of 358 K. Chao-Seader physical properties are used in the Aspen simulations. Product purities are 99mol%. All simulations use rigorous distillation column models in Aspen Plus. 

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. 

Modeling in multifunctional operations can also be categorized as steady-state, dynamic and data-driven modeling. The steady-state models based on equilibrium stage concept are employed by commercial software such as ASPEN PLUS for process design purposes. The CFD modeling is useful in evaluating the intrinsic mechanisms of mass, momentum, heat transfer and reaction kinetics with the help of rate based models. Very few studies have been reported on the development of dynamic models for reactive and hybrid separations, with the exception of reactive distillation [30] and reactive... 

Aspen Plus is used for the steady-state designs of the real chemical systems. Convergence problems can occur because of the difficulty of trying to solve the large set of very nonlinear simultaneous algebraic equations. Another problem is that the current version of... 

In the MTBE case in which equilibrium can be assumed, this problem would seem to be of no consequence. In the steady-state design using Aspen Plus, the chemical equilibrium model can be used. However, a serious limitation arises when one attempts to export the file... 

In Chapter 9 we explored the steady-state designs of both the MTBE and the ETBE reactive distillation columns using Aspen Plus. In this chapter we export the files into Aspen Dynamics as pressure-driven dynamic simulations and then look at dynamics and control. The control structures evaluated on both systems are based on those developed in Chapter 12 for ternary systems with inerts. 

A steady-state model was developed in Aspen Plus. Knowledge gained from the reactor/separation/recycle study proved very helpful in choosing the correct design... 

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. 

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. 

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