Equilibria Using Aspen Plus

You can also use the process simulator Aspen Plus to solve chemical reaction equil-brium problems. It has a huge advantage over Excel and MATLAB Aspen Plus contains the Gibbs free energies of many chemicals, and it can calculate them as a function of temperature. Thus, the data-gathering aspect of the problem is handled for you. Your job is to compare the results and the predicted A -values with experimental information. 

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With all these choices, and limited knowledge of your system, you will likely want to use the recommended options and make predictions of vapor-liquid equilibrium using Aspen Plus in order to compare those predictions with experimental data. Chapter 3 presented an example of such a comparison for the ethanol-water system. 

Start with an equimolar amount of ethylene and hydrogen chloride and determine the equilibrium composition at 350°F and 250 psig using Aspen Plus. 

Ethylene is made by heating ethane at a high temperature. (1) Determine the equilibrium composition of ethane, ethylene, and hydrogen at 1100 K and 1 atm using Aspen Plus ... 

The composition out of the reformer is based mainly on chemical equilibrium (Marsh et al., 1994, p. 165). Determine the equilibrium composition under the following conditions using Aspen Plus. (1) 750°C, 14 bar, steam/hydrogen = 3 (2) 650 C, 14 bar, steam/hydrogen = 2. 

The vapor-liquid equilibrium for the very nonideal systems studied in this chapter may not be fit well with any of the correlations in Aspen Plus if the parameters embedded in Aspen Plus are used. An alternative is to use Aspen Plus to fit the parameter values to give the best fit to VLE data. This procedure is explained in Appendix B at the end of the book. 

Using Aspen Plus the process flow sheet and stream table compositions are shown in Figure 8.25, as a base model NRTL was used. The extraction column consists of three equilibrium stages. 

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... 

The steps in setting up the Aspen Plus simulation are outlined below. The rigorous RCSTR model is used, which requires specifying reactions and kinetic parameters. An alternative, which is useful in some systems with reversible reactions, is the RGIBBS reactor module. Kinetic parameters are not required. Chemical equilibrium compositions are calculated for given feed and reactor temperature and pressure. If the forward and reverse reactions are known to be fast, so that the reactor effluent is at equilibrium conditions, the RGIBBS reactor provides a simple way to model a reactor. In Chapter 3 we will illustrate how this type of reactor can incorporate some approximate dynamics for developing control systems. 

The study of chemical equilibrium can detect thermodynamic constraints on the achievable conversion and selectivity. In this section we make use of the Gibbs free-energy minimization method available in Aspen Plus [9], We assume that both cyclohexanone and cyclohexanol are products. The curves in Figure 5.2 show the evolution of the phenol equilibrium conversion, yield and selectivity with the ratio hydrogen/phenol at temperatures of 180, 200, 220 °C and a pressure of 3 bar. 

In the following, the strategy presented before will this time be applied for developing a process for the esterification of lauric acid with methanol. All the thermodynamic data for pure components and binary mixtures are available in Aspen Plus. A residue curve map of the reactive mixture at equilibrium can be computed as described in Appendix A. A useful representation can be done in reduced coordinates defined by Xx = water + add and X2 = add + ester. The diagram displayed... 

Next, we specify the x, between 0 and 1, and estimate the total pressure P and yx from Eq. (1.193) to prepare the total pressure and equilibrium compositions shown in Table 1.10. In Figure 1.9, we can compare both the Tyx and Pyx diagrams obtained from Raoult s law and the NRTL model using the Aspen Plus simulator. As we see, ideal behavior does not represent the actual behavior of the acetone-water mixture, and hence we should take into account the nonideal behavior of the liquid phase by using an activity coefficient model. 

In Aspen Plus, solid components are identified as different types. Pure materials with measurable properties such as molecular weight, vapor pressure, and critical temperature and pressure are known as conventional solids and are present in the MIXED substream with other pure components. They can participate in any of the phase or reaction equilibria specified in any unit operation. If the solid phase participates only in reaction equilibrium but not in phase equilibrium (for example, when the solubility in the fluid phase is known to be very low), then it is called a conventional inert solid and is listed in a substream CISOLID. If a solid is not involved in either phase or reaction equilibrium, then it is a nonconventional solid and is assigned to substream NC. Nonconventional solids are defined by attributes rather than molecular properties and can be used for coal, cells, catalysts, bacteria, wood pulp, and other multicomponent solid materials. 

As in Example 4, the EXTRACT block in the Aspen Plus process simulation program (version 12.1) is used to model this problem, but any of a number of process simulation programs such as mentioned earlier may be used for this purpose. The first task is to obtain an accurate fit of the liquid-liquid equilibrium (LLE) data with an appropriate model, realizing that liquid-liquid extraction simulations are very sensitive to the quality of the LLE data fit. The NRTL liquid activity-coefficient model [Eq. (15-27)] is utilized for this purpose since it can represent a wide range of LLE systems accurately. The regression of the NRTL binary interaction parameters is performed with the Aspen Plus Data Regression System (DRS) to ensure that the resulting parameters are consistent with the form of the NRTL model equations used within Aspen Plus. 

To solve equations of state, you must solve algebraic equations as described in this chapter. Later chapters cover other topics governed by algebraic equations, such as phase equilibrium, chemical reaction equilibrium, and processes with recycle streams. This chapter introduces the ideal gas equation of state, then describes how computer programs such as Excel , MATLAB , and Aspen Plus use modified equations of state to easily and accurately solve problems involving gaseous mixtures. 

In this chapter, you have derived the equations governing phase equilibrium and seen how the key parameters can be estimated using thermodynamics. You have solved the resulting problems using Excel, MATLAB, and Aspen Plus. You also learned to prepare a T-xy diagram as a way of testing the thermodynamic model chosen to represent the phenomenon. 

Vapor-Liquid Equilibrium Data Collection (Gmehling et al., 1980). In this DECHEMA data bank, which is available both in more than 20 volumes and electronically, the data from a large fraction of the articles can be found easily. In addition, each set of data has been regressed to determine interaction coefficients for the binary pairs to be used to estimate liquid-phase activity coefficients for the NRTL, UNIQUAC, Wilson, etc., equations. This database is also accessible by process simulators. For example, with an appropriate license agreement, data for use in ASPEN PLUS can be retrieved from the DECHEMA database over the Internet. For nonideal mixtures, the extensive compilation of Gmehling (1994) of azeotropic data is very useful. 

One of the most important issues involved in distillation calculations is the selection of an appropriate physical property method that will accurately describe the phase equilibrium of the chemical component system. The Aspen Plus library has a large number of alternative methods. Some of the most commonly used methods are Chao-Seader, van Laar, Wilson, Unifac, and NRTL. 

Figure 9.13 gives a ternary diagram for the isopentane-methanol-TAME system at 4 bar. The phase equilibrium of this system is complex because of the existence of azeotropes. The UNIFAC physical property package in Aspen Plus is used to model the VLB in all units except the methanol/water column where the van Laar equations are used because of their ability to accurately match the experimental data. 

Note that Aspen Plus gives a huge amount of results. Spend some time exploring these. Write down the values for vapor and liquid mole flow rates and drum temperature. Also look at the phase equilibrium and record the x and y values or print the xy graph. Of course, all these numbers are wrong, since we used the wrong VLE model. 

This problem was also run on the Aspen Plus process simulator (see Problem 4.G1 and chapter appendix). Aspen Plus does not assume CMO and with an appropriate vapor-liquid equilibrium (VLE) correlation (the nonrandom two-liquid model was used) should be more accurate than the McCabe-Thiele diagram, which assumes CMO. With 5 equilibrium stages and feed on stage 4 (the optimum location), = 0.9335 and Xg = 0.08365, which doesn t meet the specifications. With 6 equilibrium stages and feed on stage 5 (the optimum), Xq = 0.9646 and Xg = 0.0768, which is slightly better than the specifications. The differences in the McCabe-Thiele and process simulation results are due to the error involved in assuming CMO and, to a lesser extent, differences in equilibrium. 

Proof The mixed feed to the column is 55% methyl butyrate, 20% toluene and 25% methanol. Flow rate is 200 kmol/h, and it was assumed to be a saturated liquid. The system in Figure 11-10a was simulated on Aspen Plus using NRTL for equilibrium. For the feasibility study pure methyl butyrate (instead of the recycle stream) and fresh feed were mixed together and input on the same stage of the first column. After some trial-and-error, the following results were obtained. 

Aspen Plus allows you to have sections with equilibrium calculations as long as at least one section is done rate-based). Clicking on this box activates the menus below. Use the default values for Calculation Parameters. The Mixed flow model (called Mixed-Mixed in the report) assumes that vapor and liquid are well mixed so that the bulk properties are the same as the exit properties. This model is appropriate for trays (not packing) and was used in Section 16.6 to derive Eq. (16-77) for binary distillation. The effect of flow model will be looked at in item 10. Select film for both Liquid and Vapor in the Film Resistance section, and select No for both nonideality corrections. 

FIGURE 1.10 Phase equilibrium diagrams for acetone(l)-water(2) mixture (a) Tyxat 101.3 kPa, (b) Pyx at 50 °C both estimated from the Raoult s law (c) and (d) from the NRTL model using the Aspen Plus simulator. 

The VLE date were obtained by Aspen Plus using the Wilson model, with the separator module is Flash2. The temperature and pressure of the simulation were just the same as the vapor-liquid equilibrium experiment. Thus, The Simulation of material s VLE date and relative volatility were shown in Table 3. 

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