ASPEN Properties Plus

The boiling points of the pure components at atmospheric pressure are as follows ethyl acetate (ETAC) 77.2 C ethanol (ETOH) 78.3 C water (H20) 100.0 C acetic acid (HAG) 118.0 G. There are three binary azeotropes and one ternary azeotrope summarized in Table 10.2, with respective boiling points at atmospheric pressure. The normal boiling points for the pure components as well as the compositions of the azeotropes are obtained from ASPEN Properties Plus using UNIQUAG and show satisfactory agreement with the data available elsewhere [105]. 

The Aspen Properties implementation of the NRLT-SAC method is available as a template. aprbkp file to license holders of Aspen Properties or Aspen Plus release 12.1 or above, by contacting Aspen s support centre or regional sales offices. The template is distributed with an Excel interface to simplify the data regression process and is suitable for non-expert users of Aspen Properties. Numerous Excel templates are available for data analysis and design calculations, based on the NRTL-SAC model. 

As mentioned earlier, some process models are not available in the simulators such as Aspen Plus. In such situations, ACM can be used to implement models available in the literature or newly developed models. Subsequently, these ACM models can be included in Aspen Plus and/or Aspen HYSYS for use like built-in models in any process. The above model for gas separation using membrane (Section 4.2.3) can be implemented and solved in ACM see Appendix 4A for more details on the ACM model for no permeate mixing membrane module. In order to implement the membrane model in ACM, all chemicals are defined from the component list in the Aspen Properties User Interface program, and then... 

Although the MDEA/piperazine process can be modelled in a very similar fashion to MDEA-only (Section 2.3) using the ElecNRTL physical property approach in Aspen Plus, the ions of piperazine and their electrolyte reactions in Eqs. (14)-(17) are not contained in the Aspen Properties database. Therefore, the electrolyte wizard cannot be used to add the equations and their components, and instead they must be added in manually. Once the components have been added, the electrolyte reactions can then be manually added in the Chemistry section (be sure to include it in the same chemistry specification which also includes the MDEA electrolyte reactions). Note that newer versions of Aspen Plus now include a simple example for using this setup in the Examples folder (select ElecNRTL Rate Based PZ+MDEA Model.bkp). It is usually easier to start with this file and modify it for your own purposes than it is to enter the data manually. 

At the core of many of these algorithms for solvent substitution is a method for predicting the properties of proposed molecules, given only the molecular structure. Much work has been done in this area alone, and several programs have been developed to guide this process. Some of these programs are listed in table 9.1. Additionally, process simulation software such as Aspen Plus contain several different approaches for the prediction of properties from molecular structure. 

The reactor pressure is calculated from the temperature and the liquid composition. Vapor pressure constants for aniline and CHA and a Henry s law constant for hydrogen were calculated from data obtained from Aspen Plus using the Chao-Seader physical property package... 

Figure 6.34 shows the Aspen Plus flowsheet with these two adiabatic reactors installed. The empty reactor is 10 m in length. The catalyst-filled reactor is 20 m in length. The reactor effluents for the two cases are identical. Control valves are installed on the gas feedline and the gas reactor effluent line. Figure 6.35 shows the Catalyst page tab window under Setup for the reactor with catalyst. The catalyst properties are specified. 

Although ASPEN-Plus is widely used to simulate petrochemical processes, its uses for modeling biomass processes are limited owing to the limited availability of physical properties that best describe biomass components such as cellulose, xylan, and lignin. For example, Lynd et al. (1) used conventional methods to calculate the economic viability of a biom-ass-to-ethanol process. However, with the development by the National Renewable Energy Laboratory (NREL) of an ASPEN-Plus physical property database for biofuels components, modified versions of ASPEN-Plus software can now be used to model biomass processes (2). Wooley et al. (3) used ASPEN-Plus simulation software to calculate equipment and energy costs for an entire biomass-to-ethanol process that made use of dilute-H2S04 acid pretreatment. 

Physical property data for many of the key components used in the simulation for the ethanol-from-lignocellulose process are not available in the standard ASPEN-Plus property databases (11). Indeed, many of the properties necessary to successfully simulate this process are not available in the standard biomass literature. The physical properties required by ASPEN-Plus are calculated from fundamental properties such as liquid, vapor, and solid enthalpies and density. In general, because of the need to distill ethanol and to handle dissolved gases, the standard nonrandom two-liquid (NRTL) or renon route is used. This route, which includes the NRTL liquid activity coefficient model, Henry s law for the dissolved gases, and Redlich-Kwong-Soave equation of state for the vapor phase, is used to calculate properties for components in the liquid and vapor phases. It also uses the ideal gas at 25°C as the standard reference state, thus requiring the heat of formation at these conditions. 

For the purpose of conceptual design of the bioethanol plant, Aspen Plus will be used as the flowsheet simulator. However, most of the key components involved in the process are not defined in the standard Aspen Plus property databases, and therefore their physical property data are not available. The National Renewable Energy Laboratory (NREL) has developed a database that includes a complete set of properties for the currently identifiable compounds in the ethanol process [28]. 

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. 

The overall mass balance was developed with the use of ASPEN PLUS and CHEMCAD-III software programs utilizing the Soave-Redlich-Kwong equation of state relation. This equation of state provides a good match between simulated properties and actual properties reported in the literature. 

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. 

Many solids-handling operations have an effect on the particle size distribution (PSD) of the solid phase. The particle size distribution can also be an important product property. Aspen Plus allows the user to enter a particle size distribution as an attribute of a solid substream. In UniSim Design, the particle size distribution is entered on the PSD Property tab, which appears under worksheet on the stream editor window for any stream that contains a pure or hypothetical solid component. Unit operations such as yield-shift reactor, crusher, screen, cyclone, electrostatic precipitator, and crystallizer can then be set up to modify the particle size distribution, typically by using a conversion function or a particle capture efficiency in each size range. 

Results are obtain by Aspen Plus simulations using the NRTL property model with the NRTL parameters taken from Aspen Plus . 

Programs such as Excel and MATLAB allow us to easily solve for the specific volumes. However, one advantage of process simulators like Aspen Plus is that the physical properties of many components are saved in a database that users can access. In fact, users do not need to look up the numbers because Aspen Plus will do that when it needs them. The next section illustrates how to use each of these programs to solve equations of state. 

You can also find the specific volume using Aspen Plus. One feature of Aspen Plus allows users to find properties of a pure substance. Given here are commands that will enable you to find the specific volume of n-butane at the stated conditions. You may need to review Appendix C, too, which has more detail about Aspen Plus. 

Figure 2.4. Aspen Plus window for property method.

Figure 2.6. Aspen Plus window showing pure component property analysis.

Step 5 In the list at the left, choose Component/Specifications and identify the components as you did in Chapter 2 (see Figure 2.3). Type the name or formula of the chemicals. If Aspen Plus does not recognize your chemical, a window appears that allows you to search again, and it will suggest a number of possibilities. When the components are completely specified, you should have an entry for every chemical in the column labeled Component name. The first column is what you are naming the chemicals, but the third column is what Aspen Plus uses when it obtains physical properties. If that column is blank, the program will not work. 

For this example, you choose the hrst column in the process shown in Figure 5.4. In Aspen Plus you use the module DSTWU for the shortcut method you also use RK-Soave as the physical property method, because it is a good one for hydrocarbons. The feed is 100 lb mol/h propane, 300 lb mol/h /-butane, and the other chemicals as listed in Table 6.1, at 138 psia and 75°F. The column operates at 138 psia with a reflux ratio of 10 (a wild guess initially, confirmed because the column worked). Remember, the minimum number of stages goes together with infinite reflux, so if your column does not work, increase the reflux ratio. 

The next simulation is for the same column, but using the RadFrac block in Aspen Plus. The feed is the same, the pressure is 138 psia, and the Refinery/Chao-Seader property method is used. This example uses 26 stages, and you run Aspen Plus to see what the split is. (Notice that you cannot easily set the split and find the number of stages or reflux ratio needed to achieve it.) Set the reflux ratio to 3.44 and enter the feed on the thirteenth stage. 

Choose the thermodynamic property method as Refinery/Chao-Seader. Choose the pressures of the columns to agree with with the gas plant in Figure 5.4. Each column has 26 stages, and the temperatures shown in Table 6.6 are the result of thermodynamics predicted with Aspen Plus. They are very close to those listed in Figure 5.4. 

The problems solved in Chapters 5 and 6 are simple problems with many numerical parameters specified. You may have wondered where those numbers came from. In a real case, of course, you will have to make design choices and discover their impact. In chemical engineering, as in real life, these choices have consequences. Thus, you must make mass and energy balances that take into account the thermodynamics of chemical reaction equilibria and vapor-liquid equilibria as well as heat transfer, mass transfer, and fluid flow. To do this properly requires lots of data, and the process simulators provide excellent databases. Chapters 2-4 discussed some of the ways in which thermodynamic properties are calculated. This chapter uses Aspen Plus exclusively. You will have to make choices of thermodynamic models and operating parameters, but this will help you learn the field of chemical engineering. When you complete this chapter, you may not be a certified expert in using Aspen Plus , but you will be capable of actually simulating a process that could make money. 

You use Property Method to tell Aspen Plus what thermodynamic model you want to use. 

---