aspentech handbook a technical aid for chemical engineering process design students

Aspen Tech Handbook: A Technical Aide for

Chemical Engineering Process Design Students

Contents

For information on the contents of any section, read the opening paragraph of that section, or use the alphabetical index in the back of the handbook for easier searching.

Chapter

Title

Page

1

Computer Management

2

2 Multicomponent Flash Drums

6

3 Generating Graphs in Aspen

10

4 Modeling Distillation Columns

13

5 Modeling Reactors

20

6 Using RateFrac for Rate-Based Simulations

22

7 Putting Everything Together

26

8 Using Aspen Icarus for your Cost Estimate

34

9 Using Aspen Pinch For Your Heat Integration

39

Appendix A

Thermodynamic Property Tree

47

Appendix B References

48

Appendix C Index

49

Note: The makers of the AspenTech, Inc.s Aspen Plus simulation engine have no affiliation with this tutorial. This tutorial was produced for educational purposes and any distribution of this material without the expressed written consent of Cornell University is strictly prohibited. Possible side effects of using this software include: blurred vision, self-induced hair loss, hours of frustration, and the realization that you probably never need to use activity coefficients again. Please consult your physician before operating the simulation engine.

Preface to the Manual

Over the last several years, we have introduced modern Chemical Engineering software to the Unit Operations and Design Courses. Even more recently, we have introduced AspenPlus simulation to the Separations Course and we hope this "downward" integration will continue. The value to the senior courses has been enormous and many graduates have expressed that knowing something about these computer techniques has been helpful in their jobs. Unfortunately, as the programs get more able to predict real processes and equipment, they become more complicated. The "Help" manuals are not easy (although they are complete) and step-by-step tutorials are non-existent. Keep in mind that we are being provided with several hundred thousand dollars worth of software and licenses for a tiny fraction of the normal price. While Aspen does provide technical service, we cannot expect them to answer our specific questions overnight. In the spring of '01, Brock Tuczynski was a TA for Separations and wrote a very nice tutorial for getting started with Aspen. I had written several sets of instructions for solving various problems with Aspen and Icarus and, over the years, we had assembled lots of specific but unorganized answers to questions. Over the summer of '03, Josh Banke and Meghan Cuddihy put together this tutorial. They edited Brock's original work, tied together all we had and wrote many new sections. They have tried to make this into a comprehensive user's guide to the Aspen and Icarus software and have added quite a bit of material about the computer lab organization and setup that I hope you will find useful. Good luck as you become more proficient with the simulations and remember that these are difficult and complicated programs to use. Patience is important. One last piece of advice: the fundamentals of Chemical Engineering are the most important tools you have. I have seen many, many flash drums that were not adiabatic, columns with a reflux ratio below the minimum( try a McCabe-Thiele Diagram for starters), and overall material balances that were not right. Start every problem with a sketch, a material balance and a heat balance and you will spend a lot less time trying to get your process to converge. Ken Ackley

Chapter 1: Computer Management

The purpose of this chapter is mainly to demonstrate how to setup a shared folder on the computer lab network, how to optionally set up one from your home, as well as to discuss some of the more pertinent issues regarding managing and securing the integrity of your computer that are useful to know. Windows XP File Sharing Windows XP lets you share a computer's disks and folders with other computers on the network, using a method called Simple File Sharing. If a disk or folder is shared, everyone with whom you give permissions to the file may access it from the network. This can be useful for several reasons: a shared folder will make it easy for all members of the group to access any saved Process Design or UO files; it obviates a need for writing files to discs or other data transfer media; and it can ward off any answer predators, who might want access to your Aspen or Icarus files. Probably the simplest method for going about this is to share and set permissions on a folder in your account on the Instruct Domain. Here is a brief explanation of how to do that. o Note: The computers in the computer lab in Olin Hall should have Windows XP

Professional, if not the latest operating system, installed and running, so we will try to base our example on that assumption. In our example, we've used Windows Explorer to browse to the directory of the My Documents folder, which should be located on the desktop. o In the right-hand pane, right-click, select New and then Folder, and enter the name

Shared Folder (or whatever you feel comfortable naming it). o Note: For maximum compatibility with all versions of Windows, use 1-12 characters.

o Now, you should specify sharing options for the folder you have created. To do this, right-click the folder and select Sharing and Security.

o On the Sharing tab, select Share this folder and enter a share name. You can add a comment that describes the share on other computers on the same network.

o Leave the User limit alone. On XP professional, the maximum limit is 10. o You should probably set some password permissions, so that only you or your group

members can access it. To do this: o Click Permissions . Notice that, by default, the group Everyone has Full Control . This

means that all users can read, write, and even delete files. That's not what we want at all! o Click Add, and then choose Object Types. Un-check Built-in security principles and

Groups, because we only want to see Users. Click OK.

o Choose Advanced, and click Find Now. Click on the users who should have access to this share. This should include your group partners and you. You can repeat this to add additional users. When you are done, click OK.

o You should be back at the Permissions List. By default, the newly-added users have read-only access. You may want them to have read/write access, meaning each user who logs in can alter files as well as read them, so tick the Change box. You should repeat this for each user as you see fit.

A powerful alternative is to set up a share at home, if you dont want to worry about being in the lab to access the shared folder. This way, you and your partners can telecommute and can exchange ideas and work without being physically present. You would just need to drop your work on the shared folder and it would be accessible by all members of your group. But this method has some issues with compatibility that need to be taken into account first.

Important! Making Sure You Can Set Up a Shared Folder at Home

The process for setting up a shared folder that is accessible from another connection is not necessarily easy. There are a few requirements that need to be met first.

Before setting up a shared folder that can be easily accessed from home, either you need to be on the Resnet Domain, or you should have a direct connection to the internet with what is called a static IP given to you by your internet service provider. Being on Resnet automatically allows you to access your computer easily from home, but if you live off campus, and want to set up a share that you can access from outside home, you will need to make sure you have a static IP. Your IP address is essentially the identifying address that your computer uses to interact with the internet, and, as the name suggests, a static IP address is one that doesnt change. If you are not sure whether you have a static IP or its counterpart, a dynamic DHCP-based address, there is a way to check.

o First you need to know what your IP is, if you dont already know it. There are a number of ways to go about this. One way is to hit the Start button, then click the Run button. Type in cmd and hit enter. This should bring up what is called a command prompt or a dos prompt.

o At the prompt, type in ipconfig, and the result should be a listing of numbers, but the one you will need to actually take note of will be the number labeled by IP Address. Keep this prompt open. To make IP address changes from within a script, you can use Win2K's multipurpose Net Shell (Netsh) command. This command provides several functions that relate to viewing and changing IP addressing on a Win2K system. For example, to switch from DHCP to static IP, enter the following command:

netsh interface ip set address "[connection name]" static [ip address] [netmask] [gateway] [metric]

o Netmask is the subnet mask (e.g. 255.255.255.0) associated with the IP address, gateway is the default gateway on the command prompt, and metric is the numerical difference between the IP and the default gateway. For example, to change to the static IP address 192.168.0.4, mask 255.255.255.0, and gateway 192.168.0.1 (metric 3), you would type:

netsh interface ip set address "local area connection" static 192.168.0.4 255.255.255.0 192.168.0.1 3 o If you still have access to the internet, then your ISP supplied you with a static IP but if you no longer have access to the internet, then you

dont have a static IP. To switch back to DHCP-based addressing, you need to merely enter into the command prompt: netsh interface ip set address "" dhcp

If you do not have Resnet or a static IP it isnt the end of the world. You have 3-4 members per group, so there is a good chance that someone in the group has a direct internet connection with a static IP. But lets be optimistic, and assume your apartment has viable access. Windows XP Home Editions Simple File Sharing Windows XP lets you share a computer's disks and folders with other computers on the network, using a method called Simple File Sharing, and it really is simple. If a disk or folder is shared, everyone on the network can access it. There are no user permissions and no passwords. Because sharing in this way is so wide open, Windows XP tries to protect you from some potential security risks. Just a small note: Windows XP Professional has a much more powerful way to control the sharing of files, so if you have it or have access

to it, I recommend it over the Home Edition. Its a little more complicated, but you will worry less about who is rummaging through your computer and who isnt. However, since your computer more likely has the home edition installed on it, here is a short how-to for setting up a shared folder with XP Home. o Right click the disk or folder that you want to share and select Sharing and Security. The disk or folder that you share, along with all of

the folders that it contains, will be accessible by other network users.

o Note: If you're sharing an entire disk, Windows XP gives a warning. Whats implied in the warning is that it's better to share a specific folder, since only that folder (and its subfolders) will be accessible by others, and the rest of the disk will be inaccessible. Click where indicated if you want to go ahead and share the entire disk. This screen doesn't appear if you're sharing a folder.

o The first time that you set up sharing, Windows XP displays a warning, urging you to use the Network Setup Wizard for safety. Click where indicated to either run the Wizard or to share files without running the wizard. o Note: The Wizard automatically enables the Internet Connection Firewall (ICF) to prevent other Internet users from accessing your

shared disks and folders. Enabling ICF is a good idea if you connect directly to the Internet through a dial-up, DSL, or cable modem. But it's a terrible idea if you connect to the Internet through your LAN, using a software or hardware router, since it will block File and Printer Sharing.

o XP should display another warning. If you want ICF enabled, select Use the wizard to enable file sharing. Otherwise, select Just enable file sharing.

o Having successfully run the Wizard's obstacle course, you may now specify a Share name, which users on other networked computers will

use to access this disk or folder. o By default, users on other computers have full access: they can read, write, and delete shared files. If you only want them to be able to read

files, un-check Allow network users to change my files. o Warning: If a user has full access, deleting a file doesn't put it in the Recycle Bin. Once it's deleted, it's gone for good. Also, if you

dont have an Antivirus program, there is a good chance to pick up a virus accidentally from one of the computers in the lab. We strongly urge you to never run Aspen without an updated anti-virus agent, and, as a corollary, update your virus definitions frequently.

o To access your shared folder, you have a couple of options. You can either memorize your IP address, typing \\ followed by the IP into the address bar in Internet Explorer, or you can map it as a network drive. To map a network drive, open My Computer, click Tools, and select Map Network Drive .

Specify an unused drive letter and enter the network path for the share (ie. In the format of \\Computer Name\ Folder Name or \\IP Address). Once it has a drive letter, you access it just like a disk on the same computer. o Check Reconnect at logon, so that the mapping will happen automatically every time you start your computer. Otherwise, you'll have to

map it manually every time. Click Finish . The mapped drive is connected and appears in a new window. It should also be available in My Computer as a drive.

One Method for Protecting Your Shared Files Hiding Them Most likely, the safety afforded by Simple File Sharing is not enough for you. Its not incredibly safe for both your computer and your files, as people would be able to add files to your shared folder willy-nilly. One answer is to create a hidden share by adding a dollar sign ('$') to the end of the share name. A hidden share doesn't appear in My Network Places or Network Neighborhood on any of the networked computers. Only people who know the share name should be able to access it. o To create a hidden share, right click the disk or folder and select Sharing and

Security. Specify a share name that ends with a dollar sign. Once again, use 1-12 characters (1-11 before the dollar sign).

o Here we labeled the share Design$, but if you think anyone you dont want to give access is clever enough to guess a name like Design$, use a more secure name, like a combination of letters and numbers.

o To access your folder, either map the hidden folder as a network drive like before, or type in your IP address, followed by the name of the share. (For example, \\192.168.0.4\Design$).

One last point that we can never stress enough is to make sure that, if you share a folder at home, you constantly protect your computer from viruses and worms. If you already have an anti-virus program, such as Symantec Anti-Virus, this should be quite sufficient for protective means. However if you dont have it, you can download it from the CIT website: http://www.cit.cornell.edu/services/nav/. You only need to enter your netid and your password. Make sure you update your virus definitions every two or three weeks.

Chapter 2: Aspen Plus Simulation Engine

Multi-Component Flash Drums

The purpose of this chapter will be to (re-)introduce you to the vile world of Aspen. It first goes step -by-step through the basics of Aspen, then specifies an important piece of equipment you should need to know how to model in Aspen, and covers how to perform a sensitivity analysis. The sensitivity analysis is instrumental in observing how process performance changes with varied operating conditions. Problem Statement: A flash chamber operating 80oC (176oF) and 500kPa (72.5 psia) is separating 1000 kg/hr (2204 lb/hr) of a feed that is 10 mol% Ethane, 5% Propane, 15% n-Butane, 10% n-Pentane, 12% iso-Pentane, 8% n-Hexane, 30% Heptane and 10% Nonane. What are the product compositions and flow rates? Feed conditions are the same as that of the flash chamber. (Example taken from http://chemeng.stanford.edu/~charles/cheme120/Lectures/Lecture6-BubbleDew.ppt)

Solution Methodology: The Basics (This section is a step-by-step process for those who have never used Aspen):

Starting Up: First we shall discuss how to open Aspen. o Click on the button, then click on and

. o This will bring up the Aspen startup screen. If this is the first simulation you have ever made, choose to create a Template , click OK, and

then select . The purpose of this is to have all default settings in English Units. Click OK.

Setting up your flowsheet: First you need to select the piece(s) of equipment you desire. o To get to flash drums click on the Separators tab on the bottom of the screen. Then click the down arrow next to the picture labeled

Flash2. Some choices of drums come up; select the top right by clicking on it. o Note: When you hold the cursor over each directory like Flash 2 or Flash 3, an explanation appears under it. All the selections in a

given directory are exactly the same program and only LOOK different so that your flow diagram can look the way you want it. o Now click the mouse in the middle of the screen, and the drum should appear. Right clicking stops you from placing drums on the screen.

o To name the piece of equipment, left click on each piece(s) of equipment, right click and then select Rename Block . An easier alternative is to hold CTRL and type M. Type in an 8 character name for your unit, such as FLASHDRM for example. o Note: If you dont want names to be shown on the flowsheet, click on the name, hold CTRL and type H.

o Next you will need to connect streams, which is done by first clicking on the Material Streams tab at the bottom right of the screen. For the feed, click and hold away from the feed side of the drum and pull the mouse to connect with the Red feed arrow of the drum (its on the left side).

o For the exiting streams, click and hold the arrow on the top of the drum and pull the stream off the drum and release. Follow the same procedure for the red arrow on the bottom of the drum. Name streams using the same method as for naming the drum. o Note: Holding the mouse over the colored arrows, blue or red, will tell you the purpose of that stream (feed, tops, bottoms, etc) For your reference, our flowsheet is shown in the diagram above.

o Click the (Next) button in the middle of the top of the screen. Click OK to proceed to the next step

Setting Properties o To add a title, type it in the Title block that appears then click . o Note: Its often more useful to have results with components in the form of mole fractions instead of mole flow rates. To do this, under

the Setup folder, simply go to Report Options, then the Stream tab, and choose Mole under Fraction Basis. o Now you need to add all of your components. Simply type in the name if you know it , or you can search for it using the Find button and

typing in either a component formula or name. Unfortunately, you might have a little trouble finding iso-Pentane, since Aspen has its own conventions for carbon chains with branching. To locate it, use Find, and type in butane, or search using its formula. The component with a name of 2(or 3)-Methyl-Butane fits the description of iso-Pentane, so press the Add button to add it to the list of components. Continue adding all components in our column and press Close . o Note: You can rename the components by changing their Component ID , and choosing Rename at the screen that pops up. When you

are done, click .

Choosing an Appropriate Thermodynamic Property: o Now, you need to choose an appropriate thermodynamic property for the specific separation. Doing this can often be confusing, but dont

stress over it. Located in Appendix A is the very handy Thermodynamic Decision Tree. It gives you numerous options for deciding on a calculation method. Just start from the first branching (polar vs. nonpolar) and continue along it. o Note: For most problems that you face in your college career, you will rarely have to worry about your system having an electrolyte or

not having available interaction parameters. It is probably best to just assume that Aspen has the interaction parameters, unless you are dealing with a very complex system or one that has little research associated with it .

Our system has a number of nonpolar components, so start from there, and continue on. o Note: Here are some tips from Aspen on choosing a Thermodynamic system when you really dont know what to use:

1. If you have a mixture of liquids that separate into two or three layers or if the liquids are non-ideal use an activity-based system. NRTL-RK, according to Aspen, is the one to pick if you have no real data to check your selection with.

2. If there is a dissolved gas in the liquid phase (as from sparging a gas into your reactor, etc) designate that gas as a Henrys Component.

3. For all other systems, as a selection of last resort, use RK-SOAVE. This is an equation of state model. For this sp ecific operation, I chose to use PENG-ROB, because it fits our thermodynamic path. PENG-ROB is usually a good choice if you have a system with a several hydrocarbons. With water in your system, things can get complicated. If water will separate out as a third immiscible phase, dont worry about it; but, if water is miscible (ie. water and ethanol), you will need to use a polar calculation method, such as NRTL-RK. o Important Note: The only real way to tell whether you have selected the proper system is to compare a T-x-y or x-y diagram generated

by Aspen with REAL DATA. We cannot stress this point enough. If you have a relatively simple system, or one that has been researched thoroughly, you can probably get real data from numerous sources. This includes one of your professors, the infamous Perrys Chemical Engineers Handbook, other library resources, etc.

Click when you are done.

o Enter the stream data and conditions for the feed stream (Temperature, pressure, flowrate, and composition). For having the composition in terms of mole fractions, you must select Mole-Frac from the menu on the right. Click .

o Set the Temperature and Pressure of your Flash Drum, and again click on . Click OK to run the simulation, sit back and let the machine do the work.

o To view your results click on the button towards the upper right of the Control Panel that should now be up. o Click on and then you can scroll down to see the results. o Note: Any time you change conditions and want to run again you must Reinitialize the process before running. This is done by clicking

the button that looks like . o In order to print your results, you can print your data table summary onto the flowsheet by clicking on the Stream Table button. o Assuming you have input the data correctly, you should have results similar to the data shown on the next page:

These results are pretty close to what you would obtain using a Depriester Chart, so lets move on. Performing a sensitivity analysis: If you wanted to determine how a component (say Ethane) in one of your output streams (lets use the Vapor stream) is affected by the temperature, you could perform a sensitivity analysis showing how the flash changes while varying temperature.

Methodology: o Click on the Data Browser button : o Open the Model Analysis Tools folder. Then click on Sensitivity. o Create a new sensitivity object by clicking on the button.

Name it something you will recognize or just keep the default S-1 name given to you.

o Then, under the define tab, create a new Flowsheet Variable by clicking on the New button. Create a name and specify what kind of variable it will be. Here we named it ETHVAP. o Note: Think of the flowsheet variable as that which is watched as

Aspen performs its calculations, so it would likely be either mole fraction of Ethane in the liquid or vapor streams.

o This might be a little confusing at first, so, as an example, look at our example in the diagram to the right:

o Under the Vary tab, you are now asked to input conditions for the Manipulated Variable, which is the item Aspen changes while it performs its iterative calculations. This should be what you want to increment so that you obtain a change in the flowsheet variable, which will be the block variables temperature or pressure.

o Choose a range of values over which the temperature will be changing. If you are performing an simulation where you want to show all the results in a range, merely choosing a range both above and below the initial guess, then narrowing the range down can be quite useful. I recommend using 100 to 300 as the temperature range to look at, and incrementing by 1. Again, if you are a little unsure of what this screen should look like, to the right is an example.

o Under the Tabulate tab, type in a column number, and enter the name of the Flowsheet Variable that you wish to evaluate. o Click the Next button to run the sensitivity simulation. o To view the results click on the Check Results button on the control panel, once the calculations have been completed. o Click on the Model Analysis Tools folder, then Sensitivity, and S-1 (or whatever you named the object) and then you can scroll down to

see the results. If you want to graph the results, it is recommended that you highlight the data, copy (CTRL-C) the data then paste (CTRL-V) it into Excel or another database program. You should be able to graph it quite easily from that point on. o Note: To highlight a whole column of data, merely left-click on the columns label box. This should select all data below it. If you want

to highlight all columns in one click, just click the blank box in the top left corner of the Summary table. This is usually located next to the Status column.

As always make sure your answer makes sense. If your distillation column consists of 7 trays, and operates between -50oF at the top and 700oF at the bottom, you probably should take a closer look at your column. This ends the first chapter on using Aspen. From here on, the manual will no longer be written in a step-by-step -by-step fashion, mostly because we feel you do not need to be led by the hand through the entire process. If there are potentially confusing instances, we will try to accommodate by including a visual aide. However, the main purpose for this manual is more geared to hopefully providing useful hints on managing the rather confusing simulation tool we refer to as Aspen.

Chapter 3: Aspen Plus Simulation Engine Generating Graphs in Aspen

Now might be a good time to demonstrate how to use Aspen to create and edit pure component, binary and tertiary system property graphs. Its a pretty useful tool to use if you need to give your manager hard-to-find physical property data for your system, or if you want to evaluate your systems temperature and composition data before you perform a simulation. Pure Component Systems: Problem Statement: Assume you are running an experiment using a simple pure substance (>99.9%), such as methanol (g) at 68oF, and you require a few bits of information. You have found out through your various reference sources or your employer that, at atmospheric pressure, methanol boils at 65oC (149oF), so you will be required to heat your stream of methanol (obviously without exposure to air). However, your experiment requires you to gather some information, for purposes of data verification, about methanol over the change in temperature from 68oF to 165oF. This includes documenting the constant volume heat capacity, the change in Gibbs Free Energy and Enthalpy, and density over the specified range; as such, you will need to use Aspen to graph these physical properties of methanol. Solution Methodology:

First you need to input the components and specify which thermodynamic system you are going to use. o Much like in the previous chapter, open either a new template or blank document in Aspen, and then add a piece of equipment,

including the streams. o Open the data browser, click on the Components folder, and then go to the Specifications tab. Add all component(s) to the system. In our

case, this is merely methanol. o Choose the thermodynamic system you will be working with by using the Thermodynamic Decision Tree , located in Appendix A. To

input and edit your choice, click on the Properties folder. Then, click on the Specifications tab, and choose the system from those listed under Base Method. Here I chose UNIQUAC, but you could also choose NRTL, or WILSON. UNIQUAC usually provides a reasonable amount of accuracy, but the only way to be completely sure of a correct choice is comparison to real data.

Now we need to actually make the graphs. o Click on the Tools button located on the toolbar at the top of the screen. Click on Analysis and choose the option of Property. Then

choose Pure. o When the Pure Component Properties Analysis Screen pops up, you need to choose the property you wish to analyze. From the problem

statement, you are asked to plot constant volume heat capacity, t he change in Gibbs Free Energy and Enthalpy, and density over the given range. The screen might look something like this:

Hit the Go button to plot the results of your analysis. With the plot done, you now have a couple of options. o If you are satisfied with your plot, you can perform another plot

analysis using a different property, or you could edit the way the graph looks by playing around with the Plot Wizard.

Optional: If you want to edit the plot: o Close the plot, and click on the Plot Wizard button on the analysis

results screen. Click Next, then choose the plot type (there is probably only 1 or 2 options) and go to the next screen.

o Select the data you wish to plot and click the > button to add data from the left column, < to remove data from the right column.

o On this screen you are also given the option to change the units that are displayed.

o On the next screen you can alter the title of the graph, and you can alter the title of the axes (Usually you would like to have the axis display the units, so do not remove %Unit%).

o You can also change the type of gridlines that are present on the graph by changing the Grid Type or Line Type selections. When you are finished, click the Finish button. Our plot is shown in the diagram below.

o With the editing done, you can print the new plot, and repeat the process to create plots for the remaining properties asked for.

o Note: With each type of Aspen graph, you will be presented more editing options when you right-click on the diagram. You can Zoom in or Zoom out, change the color scheme, etc. However, since these options are mostly for aesthetic purposes, I havent covered them.

Binary Systems: Problem Statement: Now assume that you are running a similar experiment, however you now have a mixture of Methanol, and n-Butane (50-50 mixture), and you wish to obtain an xy, Txy, and KVL diagram of your system. Solution Methodology: o First you need to add n-Butane to your components list, and make any necessary changes to your thermodynamic system, if any.

Remember this is done by going to the Properties folder, then hitting Specifications, and changing your Base Method. o Note: Whenever you make a modification to the current system, make sure you make any pertinent changes, especially if you are adding

a complex component, or if your system is no longer at a low pressure, etc. Some other possible modifications could include adding Henrys components, electrolytes, petroleum free-water method calculations, etc if you were dealing with the appropriate binary mixtures.

o Click on Tools, then click on Analysis and Property. Under Property you should have the choice of Pure and Binary. Here instead choose Binary.

D

o When the Binary Analysis screen pops up, go the Analysis Type option, and you should have the choices of Txy, Pxy, and Gibbs Free Energy of Mixing. Choose Txy.

o Make sure you are operating at Atmospheric Pressure , and that your property options (Base Method, Henrys Components, etc), range and number of points/increments are how you like it.

o When you are done click Go. o The plot should pop up, and it should look like the diagram to the

right: o You can either edit the plot or go on to the next chart. In any case,

go to the Binary Analysis Results window created along with the plot, and choose Plot Wizard.

o If you want to edit the current plot, you would select the Txy type of plot. You would now have the same options for editing a plot as you would in a pure component situation. After making your diagram look the way you want it to, plot the new graph.

o The other option is to choose either the xy or KVL type of diagram. When you choose either of these, you enter the plot -editing pages, which have already been discussed. When you are done editing, you can plot each graph.

Tertiary Systems: Problem Statement: Now assume that you are running a similar experiment, however, this time, some of the methanol in your system is actually ethanol (30-20 and 50 butane mixture), and you need to generate a ternary diagram of the system. Solution Methodology: o Again, add the new component to your system and make the necessary changes to your thermodynamic system, if any. o Click on Tools, then on Analysis and then Property. Under Property you should have the choice of Pure, Binary and Residue with the

addition of a third component. Here instead choose Residue. o When the Residue Analysis screen pops up, check to make sure that all of the options chosen are correct. Make changes as needed and

select the number of curves you wish to generate. When you are done click Go. o The plot should pop up, and, depending on the number of curves you chose, it should look something like this:

I think the diagram looks fine the way it is. There isnt much in the way of editing options, other than the ability to add a legend, and modify the axis labels, so your graphing task should be complete.

Chapter 4: Aspen Plus Simulation Engine Modeling Distillation Columns

Distillation columns will be the major units of your overall process flowsheet. Setting up a column is very similar to that of flash drums, but optimizing becomes much more complex. To guide you through some of the more useful simulation techniques, well walk you through a quick example. Problem Statement: Its your first assignment at your new job at the AMC Refinery, and youve been assigned to design a series of columns to remove propane. Propane tends to build up in reactor units downstream, and it is also saleable. For these reasons, you will need to first design a column to remove ethane, so that the saleable propane only contains 1% ethane. Next, your depropanizer column needs to remove 99% of the propane. You are given the following information about your stream, which enters your process at 100oF and 250 psia:

Component lb/hr lbmol/hr Mole % Ethane 1,804 60 5% Propane 12,214 277 25% i-Butane 16,100 277 25% n-Butane 28,887 497 44% i-Pentane 938 13 1%

Total: 59,943 1,124 100 Solution Methodology:

The following instructions will guide you through the early steps of designing your deethanizer. o Add a column by selecting the Column tab at the bottom of the screen. RadFrac is the most common method we use to model our

columns, so click on the arrow next to RadFrac and select your favorite picture of a distillation column. Your selection really doesnt affect the way Aspen does its calculations. o However, note: If your column does not have a reboiler or condenser, pick the picture that most closely represents your column.

o Place the column on your flow sheet and name it. Now add your material streams and name them. Feed, Tops and Bottoms or some variant of them usually works. Remember if you are ever unsure of what a stream on the block (your column) represents, hold the mouse over the red or blue arrow and Aspen will tell you.

o Click Next and then click OK to go to the Data Browser. o Another hint: By now, you should realize that whenever tabs display , it means you need to enter more information before

moving onto the next step. Once those nasty red symbols turn into encouraging blue checks , you can press to proceed to the next step. If you have entered all information and need to go back to something else, feel free to pick the section you desire on the left bar of the screen. You can also navigate backwards and forwards by clicking on the appropriate arrow: .

o Supply a title if you wish. o Add your two components, remembering that if you

cant type in the name of the component, you can search for it using the find options. As always, the next step is to choose a thermodynamic system from the Thermodynamic System Tree . We are going to use PENG-ROB for our example, and choose it under Property method, as shown at right.

o Next, in the Streams Data Browser, enter the conditions and flow rates of your feed stream (Remember: Make sure that all of the components add up or else you will get horrendous errors later on! We suggest using mole fraction because it is easier to make sure that the total is what it should be, since it should always be 1.)

o In the Blocks Data Browser, choose a number of stages, condenser, distillate rate , and reflux ratio. o Note: The condenser counts as the first stage,

and the reboiler counts as the last stage. o For a good starting point on column properties, use the FACT Method. The FACT Method will estimate the number of trays and reflux

ratio, based on the relative volatilities of components you want to separate, providing an excellent starting point. To refresh your memory, the number of actual distillation trays is:

Product Purity 1 +/- 0.5 +/- 0.15 R/D 1 2.5 6

o Using FACT to start from, the relative volatility, a, estimated by the vapor pressure ratio of ethane and propane, is approximately 4. This

indicates that our reflux ratio should be around 1. Next, we recommend using Figure 19-19 in the GPSA Handbook to get a more precise starting set of conditions, which, for a deethanizer, is 30 stages, a reflux ratio of 1.5. We also chose a distillate rate of 60 lbmol/h, since we are rejecting approximately that amount of ethane.

o Now specify where the feed(s) enter the column. If you are feeding to the bottom of a column (below the lowest tray) you must change the Stage Convention from Above-Stage (the default) to On-Stage . Well choose to feed at tray 15.

o Specify the column and/or condenser pressure . Setting only the condenser pressure is fine. o Note: Make sure the units you are using are correct. Most of the time you will be dealing with absolute pressure, so if you tell the

column that it is operating at 10 inches of water it will treat the system as a vacuum. In other words just make sure you are using absolute pressure (unless the units on the right say psig or barg).

o The condenser pressure dictates the column pressure, as well as the temperature of the condenser. You typically want to choose a condenser pressure to be the vapor pressure of the component you want to remove at around 20o greater than your cooling temperature.

o Looking at Figure 23-20 in the GPSA Handbook, the vapor pressure of ethane at 120oF is over 1000 psia! If we were to set the condenser pressure that high, too much of the heavier components would vaporize. The separation we are making is mostly between ethane and propane. The vapor pressure of propane at 120oF is around 250 psia. Therefore we want to pick a pressure that is reasonably in-between these two pressures.

Well try running at 425 psia, and now the column should be ready to Run. Our results are pictured to the right: o Our separation converges, always a good starting point.

We are removing 35% of ethane, but only losing 3% of

propane out the tops. But lets see if a couple changes can improve this separation to our target point. o We can try changing several things to improve our column: reflux ratio, distillate rate, number of trays, and feed tray.

Optimizing reflux ratio: o A good place to start our analysis is with the reflux ratio. We want to find the smallest rate that satisfies our design spec of 99% recovery

of propane in the tops. Well do this by performing a Sensitivity Analysis . o Remember that this is done by first selecting the Model Analysis Tools folder, then selecting the Sensitivity Analysis folder and pressing

New to define a new variable. Its fine to call this S-1. o Next, define the flowsheet variable of interest; again, this is the variable which you are observing while another variable is varied. Well

define two: ETH as the molar flow rate of ethane in the tops, and PROP, the amount of propane in the tops (just to keep track). o A Variable Definition window should pop up that looks like this:

o Again, if the variable of interest is a block variable (temp, pressure, heat duties) select Blocks on the left and then choose the Type, Block, and Variable. For Stream, blocks follow the same procedure but select Streams on the left and then select the Type, Stream, and Component.

o Similarly, in the Vary tab, choose your manipulated variable. Select what you wish to vary by selecting the variable from the pull-down menus on the left (molar reflux ratio is MOLE-RR).

o Now either create a list of values for your variable or pick a range and a number of points or increments. Well vary the reflux ratio from 1 to 10, incrementing by 0.25.

o If you wish to have more than one variable select the Variable Number pull-down menu and select New. Repeat the process above until you have all of your variables. o Note: The more variables or points you have significantly increases the amount of time it takes to run the simulation so choose wisely.

o In the Tabulate menu, for each column write a number and then type in the name of your variable. o Reminder: Instead of typing in the name, you can select

the variable from a list. First right click in the variable space and then select Variable List. Double click on the variable you want to tabulate then close the Defined Variable List window. Right click again in the variable space and paste your variable name.

o Make sure you have reinitialized by clicking then click to run your simulation.

o To view the results of the sensitivity analysis, in the results browser menu select Model Analysis Tools, Sensitivity, S-1 (if this is what you named it), and finally .

o A table of your results should now appear. You can either scroll down the list in Aspen or select and copy the desired columns and view them in Excel. The latter is useful if you wish to plot your results for visual comparison.

Shown to the right is a plot of the % recovery of Ethane and Propane in the tops vs. the Reflux Ratio:

% Component Recovery vs Reflux Ratio

0102030405060708090

100

0 2 4 6 8 10 12Reflux Ratio

% R

eco

very

EthanePropane

o It looks like we might have a tough time recovering a lot of ethane with a low reflux ratio. o Note: we have to reject at least 80% of the ethane to meet our 1% ethane in propane spec. The graph indicates that above 8 there isnt

much change in ethane recovery when increasing the reflux ratio. Well set it at 8 for now and see if after we change some other specs we can lower it a bit.

Optimizing distillate rate: o Increasing the distillate rate is not helping us very much for

every increase, we are letting at least as much propane into the tops as ethane.

o We should keep the distillate rate at just over 60, and see what else we can change to improve our process. With the distillate rate at 61 lbmol/hr, 96% of ethane is rejected out the tops.

o Lastly, well try to find the best number of trays and feed tray.

This is done by running a design spec on the feed tray for several different sized columns. In doing so, we found that increasing to 35 total trays and feeding on tray 10 gave the best separation (results below), and also allowed us to decrease our reflux ratio to 5.75:

o The ethane rejected is around 98%, and we are only losing 1% of the

propane Not bad! Since our overall goal is propane recovery downstream, and the ethane in the bottoms is around 0.4% of the propane in the bottoms, we should be safe with these numbers, but

o Note also that the tops temperature is 52oF, which will require very expensive refrigeration. To increase the tops temperature, we can adjust the column pressure, reflux ratio, and distillate rate.

o First, we ran a design spec on the reflux ratio to obtain an ethane molar flow rate in the bottoms of 2.7 lbmol/hr (approximately 1% of propane). This gave us a reflux ratio of 2.2.

o Though we rejected our desired amount of ethane, the tops temperature was still too low. To try and increase the temperature, we increased the condenser pressure to 460 psi. This caused more ethane to leave through the bottoms.

o Lastly, we incrementally increased the distillate rate until the amount of ethane in the bottoms was nominal. The more total moles you let out the tops, the more ethane will be rejected through the tops. The tops temperature ended up at 113.5oF:

o Originally, we used the FACT Method to calculate our reflux ratio

to be around 1, but we ended up with a reflux ratio of 2.2. The vapor pressures of ethane and propane are so different that a ends up being huge, which means that it should be an easy separation with a low reflux ratio. Instead, were seeing a hard separation, and needing to use a higher reflux ratio. This is probably because there is so little ethane in the feed that the separation is actually more like a stripping column than a distillation column.

o The message from this is that FACT is a good place to start from, but you will still need to be aware of the specific circumstances of what you are trying to model.

% Component Recovery In The Tops vs Distillate Rate

0102030405060708090

100

58 60 62 64 66 68 70 72 74 76

Distillate Rate (lbmol/hr)

% R

eco

very

Ethane

Propane

pr

o Next, we will design a depropanizer to remove the saleable propane from the deethanizers bottoms stream. The composition of the bottoms stream is:

Component lb/hr lbmol/hr Mole %

Ethane 34 1.1 0.1 Propane 12,078 273.9 25.7 i-Butane 16,093 277 26 n-Butane 28,887 497 46.8 i-Pentane 938 13 1.4

Total: 58,030 1,062 100

o We will design the column in the same way as we did before, in a separate file, and later we will join them together.

o We will start with 35 trays, and feed on tray 15. Well set the reflux ratio to 3, the distillate rate to 275, and the condenser pressure to 200 psia, which is the approximate vapor pressure of propane at 120oF. The results from this initial simulation are:

o These results are very good for an initial simulation. Were

recovering 94.5% of the propane, and the tops are 94% pure. Lets see if we can improve the purity of the tops to less than 1% impure.

Since the overheads of this column will be a lot greater than the overheads of the deethanizer, there shouldnt be any problems with a low reflux ratio. Lets try to find RD. Finding RD:

In the beginning stages of process design, you are not expected to know anything definitive about number of trays, reflux ratio, heat duties, etc. You can tentatively set RD to RMIN + 10%. Well try playing with some Design Specs to find RD. o The minimum reflux ratio can be estimated by increasing the number of trays dramatically, and changing the feed stage to get the best

separation. Note the reflux ratio required for the best separation, and repeat until increasing trays does not change the reflux ratio. This will be your RMIN.

o To perform this analysis, go to Design Spec under Flowsheeting Options in the Data Browser.

o Well define two specs, setting the mass flow rate of propane in the tops and bottoms to our desired separation. o Well then vary the reflux ratio (our example will choose 1 to 10) and the distillate rate (well try 274 to 350 lbmol/hr), and increase the

number of trays until the reflux ratio satisfying our specs does not get any smaller. Our best results are shown in the Results Summary to the right:

o With 600 trays, feeding on stage 100, our reflux ratio is 3. RD is then approximately 3.3. o Plugging in 3.3 as the reflux ratio, our stream results are (right): o These results are excellent 98.9% recovery of propane, and 97.8% purity. We can try improving these results by performing a Sensitivity

Analysis on distillate rate, feed tray, and total number of trays. o After checking out feed tray and number of trays, we get the best results with 35 trays, feeding on tray 11. o Lastly, we will try doing a Design Spec, to see if we could improve our results. Our two specs will be 99% recovery of propane in the tops,

and 99% molar purity of the tops. Well vary our distillate rate and reflux ratio from 273 to 350, and 1 to 10, respectively. Here are the results:

With a distillate rate of 274 lbmol/hr and a reflux ratio of 4.2, we have obtained our design specs! Connecting Columns:

o Now that weve optimized our two columns, where the feed of the 2nd is the product of the 1st, it is very easy to connect them. All you have to do is select the picture of one column, copy it, and paste it onto the other flowsheet:

o Next, right-click on one stream, well Reconnect Destination of BOTDEETH to DEPROP. Arrows should show up on various locations

where you can reconnect the stream to. We will reconnect it to the feed of DEPROP, the depropanizer. We can then delete PROPFEED:

o You may then reinitialize and run the simulation; Aspen may ask you for a new feed stage for the second column. Enter the same stage you

had been operating at previously. In addition, you might need to add a heat exchanger or pump in case what you took from the first column is too hot or cold for what you designed for the feed of the second column or the pressure of the second feed doesnt match the pressure of the first bottoms product. Here are the results of the combined columns:

o The two columns now produce saleable propane with 99% purity.

We can finally call this chapter to a fond end. We realize this is your first time attempt to model distillation columns. It's quite a lot of information to take in, but once you have read through and perhaps tried running this example yourself, you can officially call yourself an Aspen Guru.

Chapter 5: Aspen Plus Simulation Engine Modeling Reactors

Eventually, all components of your design process will need to be modeled in Aspen so that a cost estimate in Icarus can be evaluated; this includes your reactor, too. In many cases you will not be able to model your reactor at all, but it still needs to be represented in Aspen. The simplest way of representing a reactor is by using RStoic type reactor. It is a reactor that separates components and performs reactions based on chemical stoichiometry. The main purpose for this chapter, therefore, will be to give you an introduction to the black boxes of Aspen. These are for stating material x goes into your device, and x (or y, in the case of reactors) comes out, and for having your equipment (simple reactors, mixers, splitters, and various solid separators) represented in Aspen. Each can usually be adequately modeled in Aspen, but for your design needs, you dont need to worry about it, nor should you immediately have enough information to perform such a task. For right now, lets work with a reactor. RStoic Type Reactor You would usually use RStoic if the kinetics of the reaction are unknown. The only real requirements are that you know the stoichiometry of the reaction and the reactions percent molar conversion. RStoic can model reactions occurring simultaneously or sequentially and can determine selectivity and perform heat of reaction calculations. However, it is always recommended that hand-calculations of these are done before modeling, since there is no real basis for comparison. Lets start with an example. Lets assume you were in one of the Tuesday ethanol plant design groups and you needed to model your reactor. The typical ethanol reactor operates according to the reaction:

)2(CO OH)CH2(CH OHC 2236126 + Assuming that you found in several journal papers or patents with verified documentation that the typical molar extent of reaction for an ethanol plant was 99.5% you should be able to model this reaction in Aspen. (I am making this number up for this example. If you are in a Tuesday group, you will need to find this number or more pertinent information yourself) Solution Methodology: o Open Aspen, select the Reactors tab and place an RStoic reactor on the flowsheet. Connect all required streams (there should be 2), and

open the data browser.

o Add a title if you like, change the report options (Setup Report Options Stream Tab) so that you have both molar flow and mass

flow reported, and then add your components (there are 3: Ethanol , CO2 and Dextrose). If you were dealing with a whole system, there would obviously be more components but these are the only ones pertinent to our little example.

o Select your base thermodynamic method. I chose UNIQUAC, but it isnt really too important for this example. o Specify the conditions of the feed stream. In this case we can assume that the feed is at normal conditions (68oF, 1 atm), and that the flow

rate is 150 lbmol/hr. Under the composition, there is only one component in the feed (Dextrose), so enter it as a 100%. o Next, you should be in the block options. Specify the temperature and pressure at which the reactor should be operating. You may again

find, in a paper or journal, a typical temperature that the microorganism facilitating in the reaction operates best around is usually 95-98oF; In that case, lets use 98oF as our reactor temperature. For the sake of having a number, lets use 2 atm for the pressure.

o You next need to specify the reaction. You create a reaction by clicking on the New button in the Reactions tab; if the reactor had multiple reactions occurring within, you create an item for each reaction. If the reactions occur in series you would need to check the box below the New button.

o You need to now edit the stoichiometry of each reaction that is involved, and this means selecting each reactant or product, and supplying a reaction coefficient. Products have a positive coefficient, but reactants will be negative since they are being used up. (ie: dextrose = -1, ethanol = 2).

o The final step is to specify the Molar Extent of Reaction or Fractional Conversion. Either works. The screen could look something like the diagram to the right:

o Run the simulation.

Optional : Finding the Heat of Reaction o You might need to specify the heat of reaction so that the heat duty can be accurately transferred to Aspen Pinch or Icarus. o The best way of doing this is either to have Aspen calculate the value (by going to the Heat of Reaction tab, selecting Calculate heat

of reaction and input the reference component), or to perform the calculations by hand and enter the resulting value by selecting Specify heat of reaction and inputting it.

Here is a little rundown of the requirements for each type of reactor. For Design class you probably should just stick to either RStoic or REquil, and realize that no professor in Design class will seriously expect you to completely model all aspects of your reactor in ASPEN. Reactor Conditions/Restrictions Output (Tab) RStoic o Reaction stoichiometry known;

o Reaction kinetics are not important or known; o Conversion is known.

o Mass & energy balances (balance); o Compositions of outlet stream (phase equilibrium); o Heats of reaction (reactions); o Product selectivities (selectivities).

RYield o Unknown stoichiometry or kinetics; o Yield distributions known.

o Mass & energy balances (balance); o Compositions of outlet stream (phase equilibrium); o Outlet component weight distribution factors (weight distribution); o Pseudocomponent results (pseudocomponent breakdown).

REquil o Stoichiometry known; o Some or all reactions reach equilibrium; o OK for one- or two-phase reactors.

o Mass & energy balances (balance); o Equilibrium constants for reactions (Keq).

RCSTR o Reaction kinetics known; o Contents of the reactor have same properties as outlet

stream (Assumes perfect mixing); o Specify temperature or heat duty, calculates the other.

o Mass & energy balances (balance).

Plug o Reaction kinetics known; o Cooling stream (cocurrent and countercurrent) optional; o Rate-based kinetics only.

o Mass & energy balances (balance); o Polymer distributions (distributions).

RBatch o Reaction kinetics known; o Rate-based kinetics only; o Specify stop criteria.

o Mass & energy balances (balance); o Polymer distributions (distributions).

Chapter 6: Aspen Plus Simulation Engine Using RateFrac for Rate-Based Simulations

The purpose of this chapter is to get you acquainted with a new and powerful Aspen tool for systems such with rate-based models such as extractive and azeotropic distillation columns, absorbers and strippers, called RateFrac. We will run through a fairly thorough example, and we will try to give some useful hints for designing your absorber or stripper. RateFrac is a rate-based nonequilibrium model for simulating many types of multistage vapor-liquid operations and simulates actual tray and packed columns, rather than the idealized representation of equilibrium stages. This is a relatively new item in your Aspen arsenal, so understanding how to fully use it can be complicated, but it shouldnt be completely unfamiliar territory. Problem Statement: Lets assume first that we have a packed tower absorber and acetone in a stream of air is being absorbed by water. The acetone composition in the inlet air entering at the bottom of the column is 3 mol%, and in the outlet gas leaving the top of the column it should be at most 0.5 mol%. The total entering gas flow rate is 15,000 mol/h, and the pure water inlet flow rate is 45,000 mol/h. You will need to find the outlet acetone composition in the water. The column specifications are as described below:

Temperature and Pressure: 68oF, 14.7 psia Diameter: 1.5 ft Height: 7 ft

Solution Methodology:

First we need to set up our example simulation. o First of all, open a blank simulation in Aspen. Under the Columns subdirectory, place a RATEFRAC column on the flowsheet. Pressing

the arrow next to the RateFrac block will give you a list of icons you can use to represent the column. Choose any icon, but it is recommended that you choose a picture that most accurately represents your desired piece of equipment.

o Next attach all required Material Streams: this includes 2 inlet streams, one tops stream and one bottoms stream. If you have a more complicated system you could have to add side product, free water distillate, or pseudo streams. o Note: You are able to move the head of the stream arrow by clicking on it, and dragging it along the edge of the piece of equipment to

another location. o Once the flow sheet connectivity is complete, click Next.

o Under the Setup folder, you can give your model a title, but more importantly you can change the output options for your results to a mole fraction basis. o Note: Remember this is done by going to the Setup folder Report Options folder Stream tab and de-checking mole in Flow

Basis and checking it in Fraction Basis. Once you are done, click the Next button.

o Enter your components. There should be 3 in this example: Air, Acetone, and Water. Usually you will need to Find your components in Aspens directory, but you can merely enter these names and the correct item should appear. Click Next when you are done.

o Under the Base Method option, enter the Thermodynamic Method you will be using to evaluate the properties of your components. Again, if you are unsure at any time what your property method is, dont hesitate to use the Thermodynamic Method Tree located in Appendix A. In our example, we will be using NRTL. Click Next. If the Binary Interaction screen appears, just ignore it, click Next again and choose to Go to Next required input step.

o On the stream property sheets for each inlet streams, fill in the required fields according to the problem statement, including temperature, pressure, flow rate, and composition. While inputting your values, remember that the molar flow rate is in metric units, and not the English Unit lbmol . Also, in order to enter your compositions in terms of mole fractions, you will need to change the composition basis from Mole-Flow to Mole-Frac. o Note: In the inlet vapor stream sheet, if you were given vapor fraction and one other property, instead of both temperature and pressure,

you can enter the percent vapor fraction under the missing property.

Now, we need to specify information for our Absorber. This includes specifying numbers of segments, defining packing types and sizes, and entering column dimensions. o Once you are done, click Next and the input sheet for your Block, the absorption column, should appear. Here you will need to enter the

number of segments. o Note: Aspen, for the purpose of easing calculations in evaluating heat and mass transfer rates between contact phases, divides columns

into segments; in a packed bed column, these get translated into segments of packing, or a series of trays in a trayed column. More segments usually mean more accuracy, but usually you should stick to one segment per foot or two of packing.

In our example, we will use 10 segments, but feel free to use more if it will be more accurate. o You will also need to select the Condenser and Reboiler type. However, since we are modeling an absorber, select the None option for

both the condenser and reboiler, and click Next. o Note: Because of this, the Operating Specifications at the bottom of the screen should have both condenser and reboiler duty set at 0,

which is what we want. o Next, you will need to enter the pressure, which, as stated in the problem, is 14.7 psia for the first segment. The first segment is the first

segment from the top, so first change the view to Top/Bottom if it isnt already chosen, and enter our pressure as the Top Segment Pressure . When you are done, click Next.

o You should be at the Tray Specs object manager screen. Since we are using a packed tower instead of a trayed tower, you can skip this step by simply going to the folder in the data browser. Now, click the New button, and enter the starting segment number (ie. 1) and click OK.

o Input the number of the last segment, which in our example is 10, under Ending Segment. Since the packing type was not specified in the problem, you are free to pick any type. We have chosen 1 in. Plastic Pall Rings, but if you wish to have a basis for comparison, there is a listing of information on various packing types in Figures 19-23 and 19-24 of the GPSA Handbook.

o Next, we need to input an initial packing height that will be used to calculate our gas and liquid concentrations. This is given in the problem statement as being 7 ft, so when you are done, click Next.

o On the next screen, enter the diameter given in the problem statement of 1.5 ft and click Next. o Note: If we were given the percent flooding instead of the column diameter in

the problem statement, we could have Aspen calculate the diameter for us by selecting Use calculated diameter. The only requirements are inputting the percent flooding, the number of the bottom segment, and an initial estimate for the diameter.

o On the Material Streams screen, you need to specify where the segments the feed and outlet streams are located. Since this is an absorber, the feed and outlet streams all leave either the top (Segment 1) of the column or the bottom (Segment 11) of it, with the gas entering from the bottom and the liquid from the top. o Note: Notice that the number 11 was stated for the bottom segment. This is

because the default convention for stream location is Above Segment. You can make it so that the bottom segment is 10 by changing the convention to On Segment as shown to the left.

o You have entered all the information Aspen needs, so Run the simulation by clicking Next and hitting OK at the Required Input Complete screen.

o You can access your results by clicking on the button and choosing the Streams folder. They should look something like the picture shown on the next page:

Unfortunately, our resulting acetone composition in the exiting gas stream doesnt meet the requirement set by the problem statement of 0.5 mol%. There are a couple of things you can do to correct this. Change a non-numeric estimate or a variable Create a Design Spec Changing a Non-Numeric Estimate or a Variable The first option is probably the easiest to do if you have a result that is very close to what you want and you werent looking for an answer with a high degree of error tolerance. Though, as a warning, this does not always work to benefit you, so try it only when your results are very close to the spec. If this is not the case, do not attempt at all, since this will be very tedious and potentially unreliable. In our case, this would be a good option, since we have at least one non-numeric variable, that being our type of packing. Any one of the options associated with it could be altered to theoretically give us a better answer; this includes size, type, and material. One possible solution is to change the Pall Rings from Plastic to Metal material. This results in an acetone composition of 0.4 mol% which satisfies the problem statement. But, for the sake of getting more accurate results, lets try a Design Spec. Creating a Design Spec Creating a design spec for a RateFrac model is practically the same as creating one for a distillation column, so this should be a refresher course for those with distillation experience, or those who have already read Chapter 4. Our wish here is to vary the packing height in the column in order to obtain the desired mole fraction of acetone in the gas outlet stream. o Reopen the data browser and choose the Flowsheeting Options folder and then open the Design Specs folder. o Create a new design spec by clicking the New button on Object Manager Screen that pops up. Click OK to keep the name of DS-1, or

change it if you are the discerning type. o Now we have to create a flowsheet variable, which will be the mole fraction of acetone leaving the column in the gas outlet stream. So,

click on New, and name it something easily remembered, like ACETON and select OK. On the Variable Definition form, select the category All (or Streams), and fill out the required fields under Reference and click Next.

o Under the Spec tab, enter the name of the variable just created, and enter under Target the value of the variable, which is 0.005. Finally, set the error Tolerance , which is the allowable difference between the spec and the target, and it is necessary to stop iteration. Here, we set the tolerance to 0.0001, though you dont have to adhere to that number.

o Under the Vary tab, you will need to specify t he manipulated variable, which, in our case, is the height of packing. Choose Block-Var, then name of your block ABSORBER, and the type of variable that Aspen is changing. In this case, since we are varying the total height of the packing, select HTPACK as the variable. o Note: Dont get discouraged while looking through this giant list of strangely labeled manipulated variables. Remember that, if you are

confused, you can highlight the variable and look at the dialogue box at the bottom of the browser to obtain a description of what each represents.

o In the ID1 field enter the column number, which should be 1, and in ID2 enter the starting segment number, which should also be 1. o Next, choose a range for the upper and lower limits for your manipulated variable. A reasonable set of values for Aspen to iterate between

is 30 (ft) as your upper limit and 1 (ft) as your lower limit. o Reinitialize and Run your simulation. You can view your results by clicking on Results Summary, then choosing the Streams folder to

obtain the resulting stream compositions, or you can click on the Convergence folder to obtain your new packing height. This information should be listed under Variable Value as having a value of 8.06 ft of packing material if everything was done per our instructions. Notice that this is not the actual height of the absorber. You would need to account for the area above and below the equipment.

Optional: Modeling Reactions Within Your Absorber

Some design groups might have system that incorporates a stripping or absorbing column with one or more reactions occurring inside the model. If you have time and feel the need to have your process as detailed as possible, you might want to model the reactions occurring within your piece of equipment. Reactions are introduced to Aspen in a similar manner as they are in Reactors (Chapter 7). o After you have finished entering your packing material specifications, click on the Reactions sub-directory located below it. If you are

unable to find this, look at the picture below. o Enter the starting segment and the ending segment, then, since we havent entered the reaction yet, create a new Reaction ID by choosing

New from the pull down menu. Again, you can give the reaction a name, but its default should be R-1.

o Under the Reactions main directory, click on the newly created R-1 reaction. You will need to specify the stoichiometry of the reaction. So, simply click on New, and select the reaction type that fits your reaction. o Note: Usually you will be dealing with an equilibrium or conversion reaction, but a salt precipitation is not out of the question. In that

specific case, select the precipitating salt from the list, then hit OK. o Now you need to fill out the stoichiometry form. Kinetic/Equilibrium/Conversion type reactions should yield virtually the exact same

form as the one in the previous chapter, but if you are dealing with a Salt Precipitation, you merely need to specify the two reacting agents, and their stoichiometric coefficients, that precipitate out the salt.

o With your Reaction sheet filled out, merely Run your simulation, and Aspen should model all reactions, as well as create your column. This concludes our discussion on RateFrac columns. We hope that it has been helpful in modeling any absorber or stripper that you have in your design process, or, say, if you are furiously writing up your Stripping column report for UO Lab one or two days before it is due because you were unable to figure out how to get this part to run. In such situations, keep telling yourself Its not me, its ASPENs fault. That always made my group feel a whole lot better.

Chapter 7: Aspen Plus Simulation Engine Putting the Pieces Together

This will be our final chapter dealing solely with Aspen Plus, and as such, we have decided to give an example that tries to pull together most of the work and information gleaned from the previous chapters into one. We will try to show you how to model an entire process, which you could hopefully translate to your own work. Remember that your system might be a little more rigorous than ours, or not have the exact same pieces of equipment, but it isnt impossible to model it if you take what you have learned here and try to adapt it to your own specifications. Problem Statement: In the process pictured below, we have a stream of natural gas at 400 psig and 110oF. This stream contains numerous saleable propane, butane, and C5+ (pentanes and above), as well as a large percentage of methane, all of which could be sold at a higher price in its purified liquid state. To extract these, we first cool our feed to -20oF by heat exchanging with our cold residue gas, and then with propane refrigerant, thereby causing condensation of the heavier hydrocarbons. The feed is then introduced to the bottom of an absorber, where chilled lean oil (nonane) is introduced at the top. The lean oil will absorb the heavier hydrocarbons from the gas. The methane-rich Residue Gas can then be resold at a higher price than purchased. The lean oil and hydrocarbons leave the bottom of the absorber, and are introduced to the top of a stripping column, where some propane and lighter hydrocarbons are removed by a vapor stream created by a reboiler. The stripper tops are recycled back and mixed with the feed, whereas the stripper bottoms are then fed into a depropanizer, where saleable propane is removed in the tops. The bottoms are then sent to a debutanizer, where saleable butane is removed. We could, later on in our process, remove any excess nonane from our process with another distillation column and recycle it back to the absorber, but lets just stick with what we have now. Our goal in this example is to recover more than 85% of the propane in the feed. Its product stream can contain at most 1 mol% ethane.

Solution Methodology: You may realize that you probably dont have enough information to completely design this system. The absorption/stripping column can be pretty challenging, because, aside from not knowing the column dimensions, you also do not know the flowrates of the absorbing oil or the distillate rate in the stripping column. One of the best suggestions we can give is to use the GPSA Handbook, and find initial estimates for your specific column. Listed below are some good estimates for the first two columns.

Absorption and Stripping Column Specifications: (Partially suggested by the GPSA Handbook Fig. 19-20) Absorber Height: 23 ft. Stripper Height: 15 ft. Diameter: 36 in. Diameter: 18 in. Packing: 2 in. Plastic Pall Rings Packing: 1 in Plastic Pall Rings

Setting Up Your Absorber:

First, lets setup our flowsheet. Our recommendation is to input one piece of equipment at a time, instead of putting in every piece all at once. It is easier to try to figure out why your column dries up with one column than with 4. Without going into details because this should be somewhat obvious, column optimization is also much easier separately. So, we are only initially going to deal with our first main column, the absorber, and not worry about the recycle or the stripper. However, since there is a recycle, it wouldnt hurt to add a stream mixer before the feed, as shown below in our flowsheet. Trust me, it will save you time in the long run, when you have actually added the recycle stream.

Next, you need to setup the specifications for your column in the Data Browser (If you are confused on how to do this, or are unsure, see Chapter 7 for details on setting up your absorber column). o Enter all of the components into the component setup page, realizing that, if you are entering iso-Hexane you would need to Find using

Methyl -Pentane. Make sure to also add n-Nonane . o Enter the Thermodynamic Property Method you will be using. o Note: As well as the recommended Thermodynamic Property Tree, an index of most common abbreviations for the models is given in

the online help. However, the list is not complete. A complete list of abbreviations is given in the online manuals, accessible by going to Start -> Programs -> AspenTech AES 11.1 -> Aspen AES 11.1 Documentation. Look for the pdf file Physical Property Methods and Models. This manual will have details of the parameter names and the models.

o Enter your stream specifications. Remember that both nonane and feed streams will be at 400 psig and -20oF. We dont know what our flowrate of nonane will be, but we can take a fairly educated initial guess by using the suggested L/G ratio in the GPSA Handbook Fig. 19-19, and scaling it up for this process. A good starting guess for the mass flowrate of the nonane would be 1,600 lb/hr (this is a number we arbitrarily chose), but, as this number will be overridden later on, try not to marry yourself to any initial guess made. o Note: If you are unsure of the volumetric flowrate of your gas, you can find a pretty reliable online weight, flowrate and volume

converter for crude oil, its products, and various hydrocarbons at http://www.processassociates.com/process/basics/oil_vw.htm, or one of many other online unit conversion websites. You could alternatively use Perrys Chemical Handbook, but using a conversion calculator, such as the one on the website, will save you precious time.

o Now you need to enter the number of segments and the conditions for your Reboiler and Condenser (you have neither, so choose None for both). More segments usually translate into more precision, but make your preliminary guess. We are using 10 as our number. o Important Note: From the problem statement, we are missing two important pieces of information: the column pressure, and the

flowrate of our absorbing oil, Nonane. Since we are trying to optimize our system, while remaining within the specifications set by the problem, we will first need to choose one of these two variables to keep constant, while we optimize the column via the other variable. In our example, we chose to set the pressure to be our constant, and, later on, tried to find the best flowrate of nonane to obtain a desired percentage of our component(s) retained.

o So, first you will need to enter the pressure at the top segment, which can often be tricky. If you use a pressure higher than that of your feed streams, your segments could dry up because there isnt enough driving force into the column, which will give you severe errors, or stop calculations entirely. o Note: Since this wasnt really stated in the problem, we are going to set some goals for the amount of some components in our packed

columns. That said, we will arbitrarily try to achieve a minimum of 95% recovery of propane in the absorber, and between 85 90% in the stripper, so that when it gets to the DePropanizer, it has more than enough room to achieve the purity specification, and still maintain 85% recovery.

o Shown below is a diagram of how some important components will be retained in our absorbing oil when we vary the pressure of the

column (We created this using a Sensitivity Analysis after completing our first run, so dont worry about where this came from right now). o Note: I have only shown the effects on Methane, Ethane and Propane, since we are attempting to retain most of our propane, while

getting rid of most of our lighter hydrocarbons. Our heavier hydrocarbons, you will find later on, should have an even better retention rate than propane (near 99%), so, for now, we will mainly be concerned with these three components.

% Component Retained vs. Column Pressure

0%10%20%30%40%50%60%70%80%90%

100%

0 100 200 300 400

Column Pressure (psig)

% C

om

po

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MethaneEthane

Propane

For now, lets assume that the added benefits to our separation gained from using a lower pressure wont change too dramatically upon changing the flowrate of our absorbing oil. You may notice that, with a column pressure greater than 80 psig, there is no appreciable loss in propane, however a significant amount of our lower weight hydrocarbons are removed. For our current flowrate of nonane (1,600 lb/hr), at 80 psig, we have removed 95.3% of the methane and 48.5% of the ethane, while still retaining 99.4% of our propane. This is a pretty good number to use.

o Next, you need to enter your Packing Specifications, Column Diameter, and Feed Locations (remember to use On Segment convention).

o Now enter any specifications for the mixer. This includes the stream pressure and temperature conditions. After you are done, Run your simulation. Your results should look something like those shown below.

We have successfully removed a fair amount of the methane and nitrogen, but some of the other components that we wanted to remove (ethane and carbon dioxide) are still present in relatively large amounts.

We will minimize this by now running a sensitivity analysis on the flow rate of our absorbing oil, but you could take a possibly more difficult path by creating a design spec and solving for the flow rate of a specific component. The latter is potentially more complicated because you have the risk of going crazy from running your design spec and finding it doesnt work, or it doesnt get the separation you want, etc. You may take either path, however, we specifically chose to use a sensitivity analysis on the flowrate of nonane because we can see easily see how many components change with varying the nonane flowrate. (For more information on performing a Sensitivity Analysis, see Chapter 2) o Firstly, and always remember to do this before you run your simulation, Reinitialize. You will find out that if you dont reinitialize you

will get fatal errors, and Aspen will terminate its calculations. You probably wont realize this at first, but if change your input and then run your fairly complicated simulation, Aspen will freak out! You will, at the very least, see warnings everywhere.

o Next, go to your Model Analysis Tools folder and then Sensitivity Analysis . Under the Define tab, create a flowsheet variable and input the information for the first component you want to look at; here, you will want to find the Mole Flow of Methane in TOSTRIP. When you are done, repeat this for the other two components we were looking at earlier (Ethane and Propane).

o Under the Vary tab, you want to input the information for the Mass flowrate of Nonane. I would recommend varying the flowrate from 500-4,000, incrementing by 100. Finally under the Tabulate tab, enter the name of each variable and the order you want their spreadsheet column to appear as. Run the simulation. Our results are shown in the plot to below.

% Component Retained vs Nonane Flowrate

0%10%20%30%40%50%60%70%80%90%

100%

500 1000 1500 2000 2500 3000Flowrate of Nonane (lb/hr)

% C

ompo

nent

Ret

aine

d

Methane

EthanePropane

o By using a flowrate of Nonane of 1,000 lb/hr, we will retain 95.2% of our propane, 36.6% of the ethane, and 3.4% of the methane. Using this new flowrate, run the simulation once more as a check.

We have managed to optimize this column just by varying the pressure of the column and the flowrate of the absorber oil. If you want to try varying another aspect of this column (number of segments, packing type, temperature or pressure of the feed, etc) go for it. You might very well achieve a better separation. Using the new chosen flowrate, our results are as follows:

With your work on the absorber ostensibly completed, you will need to add your second piece of equipment, the stripping column. However, since you need the feed going to both columns at 400 psi, you will require the addition of a pump as well. So, go back to your flowsheet, and attach a Pump (found under Pressure Changers item directory) to the absorber bottoms, then add another RateFrac column to the stream coming from the pump. The diagram on the next page is a visual description. o Note: In the final design, it is probably a good idea to add a pressure changer to the Residual Gas Stream, which exits at 80 psig. Re-

pressurizing your residual gas (or adding a multi-stage mixed-propellant cascade refrigeration unit) will cause it to condense, thus making your methane saleable as well as yielding a 600-fold decrease in volume and making it easy to transport .

o Reopen the browser and add the setup information to the Pump block. This will really only include the discharge pressure of 400 psig. o Now you need to set up the stripping column. This should be very similar to the manner in which you set up the absorber, except that you

now have to include a Reboiler when you get to the Setup page. This also means that, if you have 10 segments, the Ending Segment is now 9. o Note: Instead of using 10 segments, I recommend you try using 5

7. I speak from experience when I say that you will probably receive errors otherwise. I am not trying to dissuade you from using 10, merely saying that it happened when I ran my column.

o You have to assign it an Operating Specification , such as distillate flow rate, boilup ratio, reboiler duty, etc, so that it can minimize the number of variables. I personally think that using the Distillate Flow Rate makes it easier to find out later on how large your recycle stream has to be in order to achieve your specifications. A good initial estimate for your distillate flow rate should be around the size of the molar flow rate of the amount of impurities you are trying to remove (Methane, Ethane, CO2, N2), so try using a molar distillate rate of 1.75 lbmol/hr.

o Again, here we dont know the column pressure. Shown to the right is a sensitivity analysis plot for % of component retained in the bottoms vs. column pressure. According to the chart, the optimum pressure to operate at would be approximately 30 psi, since the minimum ethane composition occurs there.

o Reinitialize, and run your simulation. Hopefully your results match ours below.

% Component Retained vs Column Pressure for Initial Distillate Rate of 1.75

0%10%

20%30%40%

50%60%70%

80%90%

100%

0 100 200 300 400Column Pressure (psi)

% C

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EthanePropaneIso-Butane

While we do manage to remove a lot of the ethane bottoms stream, we also remove more propane than our spec allows. For right now, this is fine, since we will be recycling the stripper vapor back to the feed. So, now connect the STRIPVAP stream to the mixer, reinitialize and run the simulation. The results you obtain are ok, but still do not manage to meet the specifications that we were hoping for. To find the best separation possible, try running a

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