RadFrac model

TABLE 12.1 Comparison of Divided-Wall and Conventional Flowsheet Optimum Designs for BTX System 

---

Figure 12.1 Divided-wall column flowsheet using RadFrac models.

In designing a conventional single column, the RadFrac model in Aspen Plus is used. To use a. MultiFrac model with a Peflyuk configuration, select the appropriate icon and drop it onto the process flow diagram as shown in Figure 12.2. Then feed, distillate, and bottoms material streams are added as shown in Figure 12.3. The model block is labeled DWC. 

The MultiFrac model is somewhat easier to set up and converge than the RadFrac model discussed in the next section. However, the MultiFrac model cannot be exported... 

The steady-state RadFrac model in Aspen Plus consisted of four-column sections one stripper, two parallel absorbers, and a rectifier. In reality, there is only one column, but these four fictitious vessels are used in the simulation to model the real physical equipment Before exporting the file into Aspen Dynamics, a number of important changes had to be made in order to obtain a pressure-driven dynamic simulation. Figure 12.21a gives the Aspen Dynamics process flow diagram with aU the real and fictitious elements shown. The lower part of Figure 12.21b shows the controller faceplates. Note that the two controllers with remote set points (RCl and RC2) are on cascade. 

The basic Aspen RadFrac model incorporates implicitly a condenser and a reboiler. The process flow diagram of this base model is shown in Figure 13.2. The dynamics of the system depend on the column diameter, the tray weir height, and the holdups in the column base (reboiler) and reflux drum (condenser). 

RADFRAC MODEL WITH EXPLICIT HEAT-EXCHANGER DYNAMICS... 

The standard basic RadFrac model in Aspen simulations does not accurately predict the rapid pressure changes during emergency situations because the default heat-exchanger models do not account for heat-exchanger dynamics (condenser and reboiler). Simulations can be developed that include external heat exchangers whose dynamics can be incorporated with the model of the column vessel. 

In the original simulation, the stripper was modeled using a normal RadFrac column with a partial condenser and a total reboUer. Serious simulation issues arose when the stripper was exported to Aspen Dynamics. When a normal RadFrac model with condenser and reboiler was used (see Fig. 14.3a), the file could not be initialized in Aspen Dynamics. 

The material in this chapter is based on articles by Subawalla and Fair, AlArfaj and Luyben, and Luyben. Rigorous steady-state simulation of the process is performed using Aspen Plus. All columns use rigorous RadFrac models. Details of how to use Aspen Plus for simulating conventional as well as reactive distillation columns are provided in work by Luyben. ... 

The calculations were made with the RATEFRAC program and comparisons were made with the compamon RADFRAC program, which utilizes the inside-out method for an eqiiilibniim-based model. 

The simulation is performed with the Radfrac module in ASPEN Plus and the NRTL liquid-activity model [20]. Tracing the RCM (Figure 3.15) shows that acetone and chloroform are unstable nodes, toluene is a stable node and the maximum-boiling azeotrope acetone-chloroform is a saddle. The distillation boundary shows a strong curvature. 

Rmin and the corresponding number of trays calculated ( 2N J. The shortcut models were replaced by rigorous RADFRAC units, where the reflux and distillate feed ratio were adjusted by means of design specifications, in order to meet the desired separation. The trays were sized using Aspen s facilities. Finally, the dimensions of the reflux drum and column sump were found based on a residence time of 5 min and aspect ratio H D = 2 1. Table 9.7 presents the results of distillation column sizing. 

This chapter introduced you to the many thermodynamic models available in Aspen Plus. The equations for short-cut distillation were summarized, and the Aspen Plus was used to solve a variety of distillation problems, with either short-cut methods (DSTWU) or plate-to-plate methods (RadFrac). You also learned how to solve gas absorption problems using Aspen Plus. 

Model column B2 in Figure 6.8 using a detailed model, RadFrac. (Hint Use the results from Problem 6.1 to choose key operating parameters.) Prepare a report comparing the output when using DSTWU and RadFrac to model the distillation column. What information was needed for each How do the results compare ... 

Model example 1 on pp. 13-36 in Perry s Chemical Engineering Handbook (Seader et al., 1997) using (1) DSTWU (2) RadFrac. 

Use RadFrac with the reflux rate set by the output from the DSTWU model. 

This unit can simulate any type of separation processes, as distillation, absorption, stripping, or extraction columns, modelled as cascade of counter-current equilibrium stages. The model Radfrac in Aspen Plus is particular powerful. It is first built on the inside-out algorithm that increased dramatically the robustness in simulating distillation-based operations (Boston, 1980). Columns with multiple feeds, side streams products, stage heaters or coolers, can be treated, as illustrated in Fig. 3.13. The following capabilities are generally available ... 

In this section, equipment sizes and costs are estimated for the monochlorobenzene (MCB) separation process, which is discussed in Section 4.4 of the textbook and in the multimedia portion of the CD-ROM (ASPEN Principles of Flowsheet Simulation Interpretation of Input and Output —> Sample Problem) that contains these course notes. Beginning with the file, MCB.bkp, which is available on the CD-ROM, additional mixture properties are added and the DISTL subroutine, used to model the D1 distillation column, is replaced with the RADFRAC subroutine. The reflux ratio computed using the RADFRAC subroutine is 3.35, as compared with 4.29 computed using the approximate DISTL subroutine. Also, the stream flow rates differ slightly (< 1%). Both of the files, MCB-IPE.bkp and MCB-IPE.rep, are on the CD-ROM. 

Process simulation units (that is, blocks, modules, or subroutines) are mapped into more descriptive models of process equipment (e.g., mapping a HEATX simulation unit into a floating-head, shell-and-tube heat exchanger mapping a RADFRAC simulation unit into a tray tower, complete with reboiler, condenser, reflux accumulator, etc.) and associated plant bulks, which include installation items, such as piping, instrumentation, insulation, paint, etc. 

A light-hydrocarbon mixture is to be separated by distillation, as shown in Figure 9.29, into ethane-rich and propane-rich fractions. Based on the specifications given and use of the Soave-Redlich-Kwong equation for thermodynamic properties, use ASPEN PLUS with the RADFRAC distillation model to simulate the column operation. Using the results of the simulation, with Tq = 80°F, a condenser refrigerant temperature of 0°F, and a reboiler steam temperature of 250°F, calculate the... 

The valves and pumps are standard equipment, but the column is different than the typical RadFrac used when specific chemical components are used. A petroleum fractionator is selected from the model library menu on the bottom of the Aspen Plus window by clicking Columns and then PetroFrac. Figure 11.17 shows the palette of possible configurations. We choose fourth from the left on the top row, which is a rectifier with a furnace. 

There are two approaches to using Aspen Plus for the simulation of a DWC. A model can be developed using four RadFrac vessels a rectifier with only a condenser, two absorbers with neither a condenser nor a reboiler, and a stripper with only a reboiler. Interconnecting vapor and liquid streams and splitters are used to model the streams feeding and leaving all vessels. Alternatively, a model can be developed using a MultiFrac model that inherently contains all the vessels and connections. In the following sections, we discuss the use of both approaches. 

Steady-state simulations are performed in Aspen Plus using Choa-Seader physical properties. Dynamic simulations are performed in Aspen Dynamic using the rigorous RadFrac distillation column model. 

In this chapter, the detailed step-by-step instructions to build a steady-state model are given with an extractive distillation column example. The unit operation blocks such as RadFrac,... 

The rigorous distillation column in Aspen is Radfrac column in the model library. The following data obtained from the short cut column is used to provide the data required to the rigorous column ... 

---