Problems Using Aspen Plus

TABLE 6.7. Conditions in Distillation Columns in Gas Plant Separation 

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

Results for the gas plant are given in Tables 6.6 and 6.7. Which separations are the most expensive If you were the designer, where would you want to spend your time  

Carbon dioxide from a fermentation process contains 1 mol percent ethyl alcohol. The alcohol needs to be removed by contact with water at 35°C and 1 atm. The gas flow rate is 400 lb mol/h and the water stream is 620 lb mol/h and contains 0.02 mol percent alcohol. Determine the compositions out of the absorption column if you model it with 10 stages. 

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When you use Aspen Plus, the parameters are stored in a database, and the calculations are pre-programmed. Your main concern is to use the graphical user interface (GUI) correctly. Aspen Plus is extremely powerful and is needed for other classes of problems. 

This chapter looks first at equations governing an isothermal flash, and then shows how you can predict the thermodynamic quantities you need to solve the isothermal flash problem. The problems are all sets of algebraic equations, and you can solve these problems using Excel and MATLAB . The chapter then addresses more complicated vapor-liquid separations, but now using Aspen Plus because of its large database. 

You can use Aspen Plus to solve this problem using the DSTWU block, which stands for DiSTillation-Winn-Underwood. 

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. 

Simulate the benzene process (Problem 5.1) using Aspen Plus. Take the feed at room temperature and 1 atm. Compress it to 35 atm. Preheat the feed to the reactor to 550 C and cool the effluent. Model the reactors as RStoic reactors, and keep the hydrogen/me thane separations as simple splitters model the other separations using distillation towers. You will have to decide on the number of stages and reflux ratio, and using DSTWU first might be useful. 

Simulate the vinyl chloride process (Problem 5.4) using Aspen Plus. Take the feed at room temperature and 20 psia. Operate the direct chlorination reactor at 65°C and 560 kPa. A distillation column removes the trichloroethane and the rest of the stream is sent to the furnace. Heat the stream to 1500 F so pyrolysis takes place. Cool the effluent from the furnace, and recycle the vapor (mostly HCl). Send the hquid (vinyl chloride and ethylenedichloride) to a distillation column for separation. 

If you run the problem, make a change in a parameter, and run the problem again, Aspen Plus will use the first solution as the starting guess for the second problem. If you do not want this to happen, choose Run/Reinitialize or the reinitialize button mJ, and the starting values are put back to the default values (flow rates are usually zero). You can also do the iterative calculations one unit at a time by choosing the Run/Step menu or Ctrl + F5 or the open triangle. 

Monochlorobenzene separation process. This process was introduced in Section 4.4, with simulation results using ASPEN PLUS provided on the CD-ROM (ASPEN PLUS — Principles of Flowsheet Simulation Interpretation of Input and Output —> Sample Problem). Beginning with the file, MCB.bkp, the equipment sizes, purchase costs, and installation costs are estimated using Aspen IPE. 

After the simulation file is augmented, the revised simulation is run and the results are sent to Aspen IPE. Note that the ASPEN PLUS and HYSYS.Plant simulators contain menu entries to direct the results to Aspen IPE. For details, the reader is referred to course notes prepared at the University of Pennsylvania (Nathanson and Seider, 2003), which are provided in the file. Aspen IPE Course Notes.pdf, on this CD-ROM. This section presents estimates of equipment sizes and purchase and installation costs using Aspen IPE for two examples involving (1) the depropanizer distillation tower presented on the CD-ROM (either HYSYS —> Separations —> Distillation or ASPEN PLUS Separations Distillation), and (2) the monochlorobenzene (MCB) separation process introduced in Section 4.4, with simulation results using ASPEN PLUS provided on the CD-ROM (ASPEN Principles of Flowsheet Simulation —> Interpretation of Input and Output —> Sample Problem). Just the key specifications and results are presented here. The details of using Aspen IPE for these two examples are presented in the file. Aspen IPE Course Notes.pdf... 

Since process simulators are used extensively in commercial practice, I have continued to include process simulation examples and homework problems throughout the text. I now teach the required three-credit, junior-level separations course at Purdue as two lectures and a two-hour conputer lab every week. The computer lab includes a lab test to assess the ability of the students to use the simulator. Although 1 use Aspen Plus as the simulator, any process simulator can be used. Chapters 2. 6, 8,10,12, 13. and 16 include appendices that present instructions for operation of Aspen Plus. The appendices to Chapters 2. 4, 5,15, and 17 have Excel spreadsheets, some of which use Visual Basic programs. I chose to use spreadsheets instead of a higher-level mathematical program because spreadsheets are universally available. The appendix to Chapter 18 includes brief instructions for operation of the commercial Aspen Chromatography simulator—more detailed instruction sheets are available from the author wankat purdue.edu. 

E. Check. The results are checked throughout the trial-and-error procedure. Naturally, they depend upon the validity of data used for the enthalpies and Ks. At least the results appear to be self-consistent (that is, Z X = 1.0,1 y = 1.0) and are of the right order of magnitude. This problem was also solved using Aspen Plus with the Peng-Robinson equation for VLE (see Chapter 2 Appendix A). The results are x j = 0.0079, Xp = 0.5374, Xfj = 0.4547, L = 1107.8, and y j = 0.7424, yp = 0.2032, yjj = 0.0543, V = 392.2, and = 27.99°C. With the exception of the drum tenperature these results, which use different data, are close. 

Note The easiest way to solve this problem is to use Aspen Plus for part a (trial and error) and once you have solved part a, obtain the solution for part b. Although Aspen Plus does not do the drum sizing, it does calculate the physical properties needed for drum sizing. Obtain these and do the drum sizing with a hand calculatiom... 

Prerequisite This appendix assumes you are familiar with Appendix 2A fin Chapter 21. which included Labs 1 and 2, and that you are able to do basic steps with your simulator. If you need to, use the instructions in Lab 1 as a refresher on how to use Aspen Plus. If problems persist while trying to run the simulations, see Appendix A. Aspen Plus Separations Trouble Shooting Guide, at the end of the book. 

Two of the senior engineers are arguing over the proper column pressure to use. One claims that 1.0 atm is better and the other that 4.0 atm is better. They both agree that the NRTL VLE package is satisfactory. Use Aspen Plus to determine which pressure is better, and explain why it is better. There are at least two different approaches to solving this problem, and one is considerably less work than the other. 

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... 

To get an idea about the relative volatilities of components we proceed with a simple flash of the outlet reactor mixture at 33 °C and 9 bar. The selection of the thermodynamic method is important since the mixture contains both supercritical and condensable components, some highly polar. From the gas-separation viewpoint an equation of state with capabilities for polar species should be the first choice, as SR-Polar in Aspen Plus [16]. From the liquid-separation viewpoint liquid-activity models are recommended, such as Wilson, NRTL or Uniquac, with the Hayden O Connell option for handling the vapor-phase dimerization of the acetic acid [3]. Note that SR-Polar makes use of interaction parameters for C2H4, C2H6 and C02, but neglects the others, while the liquid-activity models account only for the interactions among vinyl acetate, acetic acid and water. To overcome this problem a mixed manner is selected, in which the condensable components are treated by a liquid-activity model and the gaseous species by the Henry law. 

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. 

The material balance from Problem 3-6 and either ASPEN PLUS or CHEMCAD-III computer software is used to develop the energy balance around each piece of equipment in the ethylene separation section. For example, around distillation column, C-601, the computer program establishes the heat content 6f streams 533, 602, and 603 above a selected datum plane. The distillation calculation indicates the flow rates of the oveiiiead and bottoms streams. The reflux and reboil then indicate the flow rates of the streams that are returned to the column and permits evaluation of the condenser and reboiler duties. In kW " this pan be expressed as... 

The Aspen Plus manual provides several useful recommendations for specifying optimization problems (Aspen Technology, 2001) ... 

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. 

You have solved a very simple problem to find the specific volume of a pure component or a mixture using three methods Excel, MATLAB, and Aspen Plus. Excel is readily available, and easy to use. MATLAB is a bit more difficult for beginners because it uses files, which require data transfer. It is extremely powerful, though, and is needed for other classes of problems. With both Excel and MATLAB, you must look up the critical temperature, critical pressure, and perhaps the acentric factor of each chemical. You then must carefully and laboriously check your equations, one by one. 

Aspen Plus allows you easily to solve this same problem using the Flash2 unit operation. 

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. 

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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