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Aspen Plus Tutorial 1:Vinyl Chloride Monomer Production Process Process Discription Vinyl chloride monomer (VCM) is produced through a high-pressure, noncatalytic process involving the pyrolysis of 1,2-dichloroethane (EDC), according to the following reactions: CH2CI-CH2CI (EDC)
HCl + CHC1=CH2 (VCM)
The process flow diagram is shown in Figure 1. The cracking of EDC occurs at 900 oF and 300 psia in a direct-fired furnace. The pure EDC feed enters the furnace at 60oF and 390 psia, with a rate of 2000 lbmol/hr. Byproducts of pyrolysis consist principally of acetylene and chloroprene through the following reactions: EDC
C2H2 + HCl
VCM + C2H2
C4H5Cl
Overall EDC conversion is maintained at 55%, with a selectivity of 98% for VCM production and 2% for acetylene production. 75% of the acetylene produced is converted to chloroprene. The hot gases from the furnace are quenched to 10 degrees below saturation, prior to fractionation. Two distillation columns are used for the purification of the VCM product. In the first column, anhydrous HCI is removed overhead and sent to the oxychlorination unit. In the second column, VCM product is delivered overhead, and the bottoms stream containing unreacted EDC is recycled back to the cracker. The recycle EDC stream is treated, in a black box separator, to remove chloroprene, which could hinder pyrolysis and fractionation. Specifications for the two columns are: HCl Column
Theoreticalstages Condenser pressure Reboilerpressure HClinbottoms VCM recovery in bottoms
15 367 psia 372psia 50ppm(weight) 99.9 percent
VCM Column Theoretical stages Condenser pressure Reboiler pressure VCM recovery in overheads EDC recovery in bottoms
10 115 psia 118 psia 99.9 percent 99.9 percent
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Figure 1: Process Flow Sheet
Logon to ASPEN PLUS. You will start with a blank flowsheet.
Creating a Simulation Flowsheet This section of the tutorial takes you through the problem definition steps in creating the simulation flowsheet shown in Figure 1. Aspen Plus automatically numbers the blocks and streams you create, so you do not have to enter IDs for these objects if you do not want to. You can change this default and enter your own prefix. In this tutorial, you will let Aspen number the streams automatically, using the prefix “ S-“, and you will enter your own IDs for the blocks. To indicate that you want to enter your own IDs for blocks: On the menu bar, click on “Tools|Options”. The Options dialog window will be displayed; click on the tab named “Flowsheet”. Uncheck the box to the left of “Automatically assign stream name with prefix”. Make sure the option labeled “Automatically assign stream name with prefix” is selected (there should be a check in the box to the left of this label). In the text box to the left of the label enter S- . Click OK to accept the changes. Save your work by choosing “File|Save” from the program menu. You can name the file whatever you choose. Lets call it “Aspen Tutorial 1 – Vinyl Chloride”. Remember to SAVE YOU WORK
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OFTEN! This point cannot be stressed enough. If something goes wrong and ASPEN PLUS
crashes (it does pretty often, you could loose a lot of work. Its also a good idea to make periodic backups of your files each week or so or at major milestones. Note: backup files (*.bkp) take up much less space than the normal “Aspen PLUS Documents” files (*.awp). The Model Library at the bottom of the program window is for selecting the ASPEN PLUS unit operation models for the blocks in your simulation flowsheet. This menu is organized as follows: ASPEN PLUS Model Sets
Mixers and Splitters Separators Heat Exchangers Columns Reactors Pressure Changers Manipulators Solids User Models Each of these groups has process block that can be used in your flowsheet. Many of the blocks have other symbols that can be used on your flowsheet to make it more descriptive. Such blocks will have a small down arrow next to the symbol. Use this to select other icons for this block. Note that the symbol changes are completely aesthetic and there is no change to the way Aspen will interpret the block. If you want more information about a model before selecting it, you can use the Help menu from the menu bar. Click on the index tab, and type “heater” to get help on the heater block. Select double click “Heater Model”,mark” then double click Guide Volume could also click on theon“Arrow question button on the“Heater toolbar (User and then click on the 1)”. item You you want. The first block in your flowsheet is a stoichiometric reactor. Click on the “Reactors” tab to display the reactor models provided by the model manager. Click on the “RStoic” button. Move the mouse pointer to the middle of the screen, but do not click yet. Notice that when the mouse pointer is on the Model Library it looks like the normal arrow, signifying Select mode. When the pointer is on the workspace, it looks like crosshairs, signifying Insert model. If you would like to cancel your selection of the RStoic model without inserting it onto the flowsheet then press the arrow button at the upper left corner of the model library or press ESC. Move the Rstoic mouse block. pointer to the left of center of the flowsheet, and click the left mouse button to insert the Aspen Plus displays a dialog box asking for a block ID, or name, since you turned off the automatic block ID numbering option.
Type the ID CRACK and press Enter or click on OK. Aspen Plus display a RSTOIC block with the ID CRACK. The second block in your simulation flowsheet (see Figure 1) is a reactor effluent quench that uses
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the ASPEN PLUS model HEATER. On the Model Library, select Heat Exchangers then click on Heater. Insert the Heater block to the right of the RSTOIC block. When you insert blocks in the flowsheet, be sure to leave about an inch and a half of space between blocks for the connecting streams and their labels. Type the ID QUENCH in the dialog box when asked, and press Enter. The next block the fhowsheet is the HCL distillation column, which will be modeled by a shortcut distillation column model, DSTWU.
On the Model Library, select Columns then click on DSTWU. Insert the DSTWU block to the right of the HEATER. Type the ID COL1 in the dialog box when asked, and press enter. The last block in the flowsheet is VCM column. Initially it will be modeled using DSTWU and later a rigorous model will be used. Insert the DSTWU block to the right of the COL1 Type the ID COL2 in the dialog box when asked, and press enter. If you accidentally placed an icon in the wrong location, or just want to realign a few things, you can move the icon by using the arrow keys or by dragging it with the mouse. Use the arrow keys for small adjustments. Use the mouse to make larger changes in position. If you click on an ID, you can move it instead of a block or stream using the same methods. This is a good time to save your work again Click on the Material STREAMS button located at the bottom of the flowsheet next to the models. Now when you move your mouse into the flowsheet area red and blue arrows will appear on all of the model blocks located on the flowsheet. Red arrows are required ports and blue arrows are optional ports. The arrows appear around the block on ports that are available. By pointing on an arrow information about the port is given. Do this for each port to see the type of information provided. The leftmost arrow on the CRACK block is marked “Feed (Required, 1 or more)”, indicating that this port is required and that it can have one or more inlets connected to it. Click the left mouse button on this (Feeds) arrow. Move the cursor to the empty space left of the CRACK block and press the left mouse button. A feed line labeled S-1 should appear. Click on the product arrow located on the right side of the CRACK block. Arrows appear on all the possible connecting ports. Click on the arrow on the QUENCH block. The stream ID ASPEN PLUS generated is S-2. Next click on the Product arrow for the QUENCH block. Arrows appear on the possible Feed ports. Click on the Feed port stream on the COL1 block. ASPEN PLUS generates stream ID S-3. Next click on the Distilate arrow for the COL1 block. Move the cursor above the block and press the left mouse button. A stream labeled S-4 will be created.
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Click on Bottoms arrow on COL1. Connect to Feed on COL2, ASPEN PLUS generates stream ID S-5. Next click on the Distillate arrow for the COL2 block. Move the cursor above the block and press the left mouse button. A stream labeled S-6 will be created. Next click on the Bottoms arrow for the COL2 block. Move the cursor below the block and press the left mouse button. A stream labeled S-7 will be created. Click on the Feeds arrow for the Rstoic block. Create a second feed line labeled S-8. Before 1. you continue with this tutorial, be sure that your simulation flowsheet looks essentially like Figure Now is a good time to save.
Configuring the Input Specifications Your ASPEN PLUS run now contains all the information about the simulation flowsheet. Next you will enter the remaining data required to complete the problem specification, using forms. This includes components, properties, the feed stream, and block operating parameters. At this point you have the choice of letting the ASPEN PLUS expert system take you through all the steps needed to complete the problem specifications, or going through the steps in an order you choose. When you are working from a form, you access the expert system by using the Next function (the N button). This button leads you through the process of including all of the necessary information in a worksheet (See Table 1). Using the expert system is convenient, so you will use the expert system in this tutorial. You click the next button when The sheet you are on is incomplete.
The sheet you are on is complete. You have selected an object that is complete.
You have selected an object that is incomplete.
Then Displays a message listing the input you must provide to complete the sheet. Takes you to next required input sheet for the current object. Takes you to next object or the next step in making a run.
Takes you to the next sheet you must complete.
Table 1: Result of pressing next button.
You can also find the status of the flowsheet by looking in the lower righthand corner of the window. If the flowsheet is not complete or more input is needed, the information will be shown in red. See Figure 1 as an example At this time press the Next button. Aspen will as you if you would like to display the next input form, click on OK. ASPEN PLUS displays the data browser window. The data browser window consists of folders containing input fields. Folders containing all of the necessary information have a blue check mark on them. Folders requiring additional information have a half filled red circle. You can navigate through the folders as you would in Microsoft Explorer. ASPEN PLUS should open the component specifications form, indicating that this is the next form
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that it has encountered that requires additional information. Before we enter the component information we would like to enter a name for the simulation. Click on the Setup folder. This should display several icons one of which is the Specifications form. Click on the Specifications form within the Setup folder and the Setup-Specifications form should appear with the Global tab highlighted (see Figure 2). We would like to enter a title for the Aspen simulation. The title will appear at the top of each page of your ASPEN PLUS report. Enter “Vinyl Chloride Monomer Process” as the title. From this menu you can also change many of the general Aspen parameter such as the unit of measurement. By selecting the Description tab button you could also enter a description of the process if you so desired.
Figure 2: Global Tab of the Setup Specification form. One can very quickly see that Aspen offers many features for the user, many of which we will not discuss in this tutorial. You should explore some of these features during the course of the semester. Many of these features are very powerful, and can make simulating any give process much easier. We would like to return to the Components-Specifications form. We have two choices for doing this. First of all we could select the folder using the directory tree located on the leftmost portion of
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the screen, or we can use the Next button. Components Folder
At this time you should be looking at the Components-Specifications form with the Selection tab highlighted. We are about to enter the components to be used with this simulation. Aspen stores the physical property data for thousands of chemicals, but only recalls the information that you specify in this form. For this reason it is important to specify every chemical that you plan on using in the simulation. You will also give each chemical an arbitrary Component ID. This is the name that each chemical will be referred to by Aspen. Before you Click start toonenter the chemicals in the youselected must activate that are appropriate databases. the Databanks tabused button andsimulation the currently databanks displayed along with all of the options for databanks to be used. For this simulation the following databanks should be in the “Selected databanks” column: Selected Databanks
PURE10 AQUEOUS SOLIDS INORGANIC If one of these is not in the selected databanks field, then select it in the “Available databanks” field and press the > button to move it into the “Selected databanks” field. Reselect the Selection tab. Move the highlight to the first Comp ID field using the mouse. The first component is Hydrogen chloride. On the first Comp ID field, type HCL and press enter. Both the data bank Alias (the formula) and the data bank Component Name appear on the form. ASPEN PLUS automatically searches all the data banks you have specified to find an alias or component name that matches your component ID. If ASPEN PLUS does not find an exact match, you must enter either the alias or the component name if you want to retrieve data for the component from data banks. You could also use the FIND button to locate the component you wish to add. The folder status indicator to the left of the form has changed to a blue checkmark, since you have specified a component for your simulation. If you were to use Next now, ASPEN PLUS would take you to the next required step. However, your problem requires additional components. On the second Comp ID field below HCL, type VCM and press enter. No Formula appears for VCM. Because VCM is simply your abbreviation for Vinyl chloride monomer, there is no Formula or Component Name match in the data bank; so you must enter one of them. You know that the formula for Vinyl chloride is C2H3Cl, so on the Formula field, enter C2H3CL. ASPEN PLUS will then find the data for the component. On the next Comp ID field, enter the component ID EDC. Move the highlight to the Component Name field and enter 1,2-DICHLOROETHANE. On the next Comp ID field in the space below EDC, enter the final component C2H2. The Formula C2H2 and the rest of the line is filled in automatically. This completes specifications on the Components form. You can return to this form at any time to
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add or delete components, or to change data bank aliases or component names. Save, and then hit the NEXT button. Properties Folder
An ASPEN PLUS simulation requires physical properties, such as enthalpy and density, of streams and other mixtures. A wide variety of methods are available in ASPEN PLUS for calculating these properties. You specify the methods to be used by selecting property method. A property method defines the methods and models used to calculate each thermodynamic and transport property in a simulation. Use the “Base Methods” box to display property methods available (click the button on the field). Move the highlight to SYSOP0. The prompt area below displays a description of SYSOP0. If you want a more detailed description of the option set, use Help (type F1 after you select the method). Select SYSOP0. The status indicator for the folder changes to a blue checkmark, indicating that the rest of the fields are optional; in this tutorial you will skip over them. Save and use NEXT to display the next form requiring input. When prompted make sure the “Go to Next required input step” is selected and click on OK. Streams Folder
Aspen Plus displays the Stream S-1 input form.You must enter stream data for each process feed stream in a simulation. The blank Stream field on the form on your screen indicates a process feed stream. The units set for the stream input data is also shown. On the Temperature field, enter 60. The temperature for this problem is in degrees Fahrenheit, so you can leave the default F. On the Pressure field, enter 390 psi. On the value field for EDC enter 2000 lbmol/hr. This completes the information for stream S-1 (See Figure 2), you will notice that the folder now has a blue check. Save and click the next button and the folder for stream S-8 should appear.
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Figure 3: Stream 2-1 Specification Form Use the following stream data for the S-8 Form •
On the Temp Field, enter 60 F.
•
On the Pressure Field, enter 390 psia.
On the Value Field for EDC enter 1600 lbmol/hr.
Save again and click Next. Blocks Folder
The Blocks-COL1 form will appear with the specifications tab highlighted. We will enter the following information into the appropriate blocks. The significance of these blocks will not be explained now. That will all be done later on in the semester.
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•
On the Number of Stages Field, enter 15
•
On the Pressure Condensor Field, enter 367.
•
On the Pressure Reboiler Field, enter 372.
•
On the Light key Comp Field, enter HCL (you can use List and then select HCL)
•
On the Light Key Recov Field, enter 0.999
•
On the Heavy key Comp Field, enter VCM.
On the Heavy key Recov Field, enter 0.001.
The form should look like Figure 4. Save and use the next button to get to the next form.
Figure 4: COL1 Specification form
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The system takes you to the Blocks-COL2 form. Enter the following values: •
On the Number of Stages Field, enter 10.
•
On the Pressure Condensor Field, enter 115.
•
On the Pressure Reboiler Field, enter 118.
•
On the Light key Comp Field, enter VCM
•
On the Light Key Recov Field, enter 0.999
•
On the Heavy key Comp Field, enter EDC.
On the Heavy key Recov Field, enter 0.001.
Save and click the next button. This will take you to the Blocks-Crack Specifications form. Under operating conditions enter the following values: •
On Pressure Field, enter 300 psi.
On Temperature Field, enter 900 F.
Save and click the next button. This takes you to the Blocks-Crack Specifications. Click the New… at the bottom of the form. This brings up the Edit Stoichiometry window. Set the first Reactants Component field to EDC. Then enter the coefficient of –1. Set the first Products Component field to HCl. Then enter the coefficient of 1. Set the first Products Component field to VCM. Then enter the coefficient of 1. In the Products Generation section of the form, select Fractional conversion and set a value of .55 of the component EDC. The final result should look like Figure 5.
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Figure 5: Edit Stoichiometry dialog box for reaction 1 Save and click the next button. A summary of the reaction you just input will appear. Save and click next again. The Blocks-QUENCH Specifications form should appear. Use your mouse to select Degrees of Subcooling where it says Temperature. Then enter a value of 10 F. On the Pressure field, enter –5 psi. The negative signifies this is a pressure drop specification. A positive number would be a pressure specification. Save and click the next button. All the required specifications for this problem are complete so ASPEN PLUS will ask if you want to run your simulation now. You do not want to, so select cancel. Now you can select optional input, such as a flowsheet design specification. You could go to any of the forms and make any changes. If you were ready to run and you remembered to save your flowsheet, you would then press OK to run. You can use the data browser to look at view and modify any of the input values at any time. Running The Simulation And Displaying The Results
The next step is to run the ASPEN PLUS simulation. Note that the status indicator in the lower right portion of the screen reads Required Input Complete. Save and click Next to start the simulation. Click OK when it asks if you would like to run the
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simulation. Be sure to save before you Run. Aspen is most likely to crash when you run and to savegaurd all of your hard work SAVE. A trace of the run progress is written to the screen in the control panel window. When the run is complete, ASPEN PLUS displays Results Available in the lower left hand corner of the window. Click the check results button (it looks like a blue folder with a blue check through it). A series of folders will be displayed: Streams, Blocks and Results Summary. You can brows through the results by clicking on any one of the various folders. These results can be copied and saved to other programs such as Microsoft Excel or Word or printed directly. You can also export a formatted report using the File|Export command. Choose the file type to be Aspen Plus Report (*.rpt). For your assignment please breifly describe in a paragraph the flow rates of your product streams and sizing/temperature/heating and cooling duty estimates for the two distillation columns. Also include a copy of the report file. Please email the assignment to
[email protected]. Don’t forget to save the simulation.
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