Aspen Plus simulation

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. 

Several authors have already developed methodologies for the simulation of hybrid distillation-pervaporation processes. Short-cut methods were developed by Moganti et al. [95] and Stephan et al. [96]. Due to simplifications such as the use of constant relative volatility, one-phase sidestreams, perfect mixing on feed and permeate sides of the membrane, and simple membrane transport models, the results obtained should only be considered qualitative in nature. Verhoef et al. [97] used a quantitative approach for simulation, based on simplified calculations in Aspen Plus/Excel VBA. Hommerich and Rautenbach [98] describe the design and optimization of combined pervaporation-distillation processes, incorporating a user-written routine for pervaporation into the Aspen Plus simulation software. This is an improvement over most approaches with respect to accuracy, although the membrane model itself is still quite... 

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. 

Figure 1.7. Gibbs energy of mixing for 1-propanol(1)-water(2) by the Aspen Plus simulator using the NRTL model.

At atmospheric pressure, the n-butanol-water system exhibits a minimum boiling azeotrope and partial miscibility, and hence a binary heterogeneous azeotrope. Figure 1.8 shows the Tyx and Pyx phase diagrams for l-propanol(l)-water(2) azeotropic mixture obtained from the Aspen Plus simulator using the NRTL activity coefficient model. 

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. 

The Aspen Plus simulator with a suitable thermodynamic model generates the stage-exergy loss profiles of distillation columns from the exergy balance of stage i... 

The converged mass and heat balances and the exergy loss profiles produced by the Aspen Plus simulator can help in assessing the thermodynamic performance of distillation columns. The exergy values are estimated from the enthalpy and entropy of the streams generated by the simulator. In the following examples, the assessment studies illustrate the use of exergy in the separation sections of a methanol production plant, a 15-component two-column... 

Table 4.25 Column configuration for a five-component distillation used in the Aspen Plus simulator with the Peng-Robinson model qc= 3395.3367 kW qR = 3432.2069 kW Number of stages = 14 Location of feed stage = 7 Reflux ratio = 8.87.

Use the Aspen Plus simulator with the following input summary to estimate the thermodynamic efficiency of the distillation column ... 

Hydraulic analysis of the Aspen Plus simulator produces thermodynamic ideal minimum flow and actual flow curves for rigorous distillation column simulations. These types of calculations are performed for RADFRAC columns. Using the input summary given in problem 4.48 construct the stage-flow curves. Assess the thermodynamic performance of the column. 

For more sophisticated spreadsheet models, Aspen Plus allows the user to link a spreadsheet to a simulation via a user model known as a USER block. The designer can create a new spreadsheet or customize an existing spreadsheet to interact with an Aspen Plus simulation. The USER block is much easier to manipulate when handling large amounts of input and output data, such as streams with many components or unit operations that involve multiple streams. The procedure for setting up a USER MS Excel model is more complex than using a calculator block but avoids having to... 

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

Fig. 3.43. User interface of the integrator for Comos PT PFDs and Aspen Plus simulation models (in German)...

As shown in Figure 4.B.2, the Aspen Plus simulation file is opened first. In VBA, Dim statement is used for declaration of variables, whereas Set statement is used for creating new objects. Then, values of feed flow rate, feed heater outlet temperature, reflux ratio for column 1 and feed stage for column 2 and number of stages for column 3 are transferred from cells C3 to C7 in Excel worksheet named DV to Aspen Plus , to Aspen Plus... 

Aspen Plus simulation is now activated from VBA. After the convergence of process simulation, values of energy input for feed heater, columns 2 and 3 reboiler duty, and flow rates of all three product streams are transferred from the simulator to cells C3 to C8 in Excel worksheet named Data from Aspen Plus . VBA code for these variables and/or data transfer is given in Figure 4.B.2... 

Place Excel file with the created macro and Aspen Plus simulation file in the same directory/folder. Update the directory address and simulation file, as required, in the VBA code (see Eigure 4.B.2). In Excel, the control button can be used to mn the simulation for this, create a control button by going through Excel - Developer -Insert - Form Control, and then assign the new macro (that is, Distillation Synthesis) to the control button by a right click on it... 

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