Lecture Note - Column

EKC 451 Process Design and Analysis

Dr. Ing NoorAshrina A. Hamid

DSTWU – A Shortcut Distillation Model in Aspen Plus®

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Objectives •

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Understand Understand the capabilities (and limitations) of DSTWU. Know the methods implemented in DSTWU and the assumptions used in these methods. Learn how to configure a DSTWU block and review results.

Dr.-Ing. Dr .-Ing. NoorAshrina A Hamid

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Objectives •

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Understand Understand the capabilities (and limitations) of DSTWU. Know the methods implemented in DSTWU and the assumptions used in these methods. Learn how to configure a DSTWU block and review results.


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Background •

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When we are given a task to conceptually design a column to separate a stream into products streams, the following following questions pop up in our mind: –

How many stages do we need for the column?

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How big should the reflux ratio ratio be?

For mixtures where the following following assumptions are good approximations, approximations, we can use the DSTWU block to get a rough answer for those questions quickly. –

Constant relative volatilities.

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Constant Constant molar overflows. Dr.-Ing. Dr .-Ing. NoorAshrina A Hamid

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DSTWU – A Shortcut Distillation Model in Aspen Plus® •

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The DSTWU block implements the following methods: –

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Winn method to estimate the minimum number of stages. Underwood to calculate the minimum reflux ratio. ratio. Gilliland to relate the actual number of stages and reflux ratio.

These methods are based on the following two assumptions: –


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Constant Constant molar overflows.

Therefore, Therefore, the DSTWU block should not be used for mixtures that significantly deviate from the above two assumptions. Dr.-Ing. Dr .-Ing. NoorAshrina A Hamid

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Exercise •

Use DSTWU to calculate the following for the mixture of cis-nbutene/trans-n-butene butene/trans-n-butene with a mole composition of 0.5/0.5. The trans-nbutene recovery recovery is 0.99. –

Minimum reflux ratio.

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Minimum number of stages.

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Actual reflux ratio. ratio.

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Actual number of stages.


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First we will create a simulation for the separation of n-butane and cis-2-butene using the DSTWU model. 1. Start a new simulation using the Blank Simulation template in Aspen Plus. 2. The Components | Specifications | Selection sheet is displayed. Enter N-BUTANE in the Component ID column. Note that Component name and Alias are automatically filled for this component. Enter BUTENE as Component ID for the second component. Since BUTENE cannot uniquely identify a component, enter CIS-2-BUTENE as the Component name for the second component. Dr.-Ing. NoorAshrina A Hamid

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3.

Define methods. Go to the Methods | Specifications | Global sheet. Select PENG-ROB for Base method.

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Go to the simulation environment and place a DSTWU block onto the Main Flowsheet. The DSTWU model is located under the Columns tab in the Model Palette. Connect the inlet and outlet ports with Material streams and rename them accordingly.

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Specify feed streams. Double click on stream FEED or go to the Streams | FEED | Input | Mixed sheet. Select Vapor Fraction and Pressure for Flash Type. Enter 1 for Pressure and 0.5 for Vapor fraction. In the Composition frame, enter 50 for both N-BUTANE and BUTENE. Dr.-Ing. NoorAshrina A Hamid

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6. Specify column operating conditions. Double click the column block on the Main Flowsheet or go to the Blocks | DSTWU | Input | Specifications sheet. In the Column specifications frame, select the Reflux ratio option and enter 2 for Reflux ratio. In the Pressure frame, enter 1 for both Condenser and Reboiler . In the Key component recoveries frame, select N-BUTANE for Comp and enter 0.99 for Recov for Light key. Select BUTENE for Comp and enter 0.01 for Recov for Heavy key.

Dr.-Ing. NoorAshrina A Hamid

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7. Run the simulation by pressing the F5 key. After running the simulation, you should receive an error for the DSTWU block stating that the specified reflux ratio is less than the minimum reflux ratio. DSTWU also took corrective action and calculated the actual reflux ratio based on the minimum reflux ratio. Therefore, the results are valid despite the error message. Go to the Blocks | DSTWU | Results | Summary sheet. Dr.-Ing. NoorAshrina A Hamid

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8. This table provides very useful design information including Minimum reflux ratio, Minimum number of stages , and estimated duties. This table also provides an Actual reflux ratio and an Actual number of stages. These values will serve as good design estimates for this separation. 9. Now, save the simulation. On the ribbon, click File and then click Save As to save the simulation as Dist-006_DSTWU-C4C4.bkp. Close the Aspen Plus simulation window.


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10. We will now create a simulation for the separation of ethanol and water using the DSTWU model. Start another new simulation using the Blank Simulation template in Aspen Plus. 11. The Components | Specifications | Selection sheet is displayed. Enter WATER and ETHANOL in the Component ID column. 12. Define methods. Go to the Methods | Specifications | Global sheet. Select ALL for Method filter. Select UNIQ-RK for Base method. 13. Press the Next Input button on the Quick Access Toolbar (or press the F4 key on your keyboard). Binary parameters are automatically populated on the Methods | Parameters | Binary Interaction | UNIQ-1 | Input sheet. Dr.-Ing. NoorAshrina A Hamid

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14. Go to the Simulation environment and place a DSTWU block onto the Main Flowsheet. The DSTWU model is located under the Columns tab in the Model Palette. Connect the inlet and outlet ports with Material streams and name them accordingly. 15. Specify the feed stream. Go to the Streams | FEED | Input | Mixed sheet. Select Vapor Fraction and Pressure for Flash Type. Enter 1 for Pressure and 0.5 for Vapor fraction. In the Composition frame, enter 50 for both WATER and ETHANOL.


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16. Specify column operating conditions. Go to the Blocks | DSTWU | Input | Specifications sheet. In the Column specifications frame, select the Reflux ratio option and enter 2 for Reflux ratio. In the Pressure frame, enter 1 for both Condenser and Reboiler . In the Key component recoveries frame, select ETHANOL for Comp and enter 0.99 for Recov for Light key. Select WATER for Comp and enter 0.01 for Recov for Heavy key.


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17. Run the simulation by pressing the F5 key. There are error messages during simulation and the calculated minimum number of stages is a negative number. Therefore, the results cannot be used. We know that water and ethanol form an azeotrope and thus the assumption of constant relative volatility is not valid for this system.


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DSTWU – A Shortcut Distillation Model in Aspen Plus® •

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For mixtures where the following two assumptions are good approximations, the shortcut methods implemented in DSTWU are very useful and efficient to get us started.

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Constant relative volatilities

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Constant molar overflow

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For systems that significantly deviate from those two assumptions (e.g., ethanol-water), these shortcut methods are not useful and we should not rely on results from these shortcut methods. For such cases, we need to use more rigorous models (e.g., RadFrac).


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Distl – A Shortcut Distillation Model in Aspen Plus® •

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Objectives –

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Understand the capabilities (and limitations) of Distl. Know the method implemented in Distl and associated assumptions. Learn how to configure a Distl block and review results. Learn how to configure a RadFrac block. Learn how to use Design-Spec/Vary in RadFrac


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Background –

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For mixtures where the following assumptions are good approximations, we can use the Distl block to calculate compositions of product streams after we obtain the number of stages and reflux ratio required. •


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Constant molar overflows.

The accuracy of the results from a Distl block is very limited. More rigorous model should be used for much better accuracy. Of course, results from DSTWU and Distl can be used as a good starting point for rigorous models such as RadFrac. Dr.-Ing. NoorAshrina A Hamid

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Distl implements Edmister’s method to calculate column compositions for a given number of stages and reflux ratio. The Edmister’s method is also based on the assumptions of constant relative volatilities and constant molar overflows. Thus, Distl should not be used for mixtures that significantly deviate from these assumptions. Results from shortcut models such as Distil typically contain very noticeable inaccuracy. A rigorous model (e.g., RadFrac) should be used to generate more accurate results. In RadFrac, reflux ratio is typically varied to achieve a purity target. Dr.-Ing. NoorAshrina A Hamid

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For the mixture of cis-n-butene/trans-n-butene with a mole composition of 0.5/0.5, use Distl to calculate the compositions of the two product streams. The results from the DSTWU block in the exercise for example “Dist-001_DSTWU” are used as the inputs. Then, use RadFrac to obtain the required reflux ratio to achieve a purity of 0.99 (on the mole basis) for the distillate. Use these input to run the RadFrac to achieve purity of 0.99 Number of stages: 83 Feed stage: 46 Reflux ratio: 26.18 Distillate to feed mole ratio: 0.5 Condenser & Reboiler pressure: 1 bar Dr.-Ing. NoorAshrina A Hamid

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Distl Method We

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1. Create a new simulation in Aspen Plus using the Blank Simulation template. 2. The Components | Specification | Selection sheet is displayed. Enter N-BUT-01 and CIS-2-01 for Component ID. In the Component name column, enter N-BUTANE and CIS-2BUTENE. 3. Define methods. Go to the Methods | Specifications | Global sheet and select PENG-ROB for Base method. 4. Create flowsheet. Go to the simulation environment and place a Distl block on the Main Flowsheet. The Distl block is located under the Columns tab in the Model Palette. Connect the inlet and outlet ports with material streams and rename them accordingly. Dr.-Ing. NoorAshrina A Hamid

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Distl Method We

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5. Define feed stream. Double click on stream FEED or go to the Streams | FEED | Input | Mixed sheet. Select Vapor Fraction and Pressure for Flash Type. Enter 1 for Pressure and 0.5 for Vapor fraction. In the Composition frame, enter 50 for both N-BUT-01 and CIS-2-01. 6. Define block operating conditions. Double click on the column block or navigate to the Blocks | DISTL | Input | Specifications sheet. Enter 83 for Number of stages , 46 for Feed stage, 26.18 for Reflux ratio and 0.5 for Distillate to feed mole ratio. In the Pressure specifications frame, enter 1 for both Condenser and Reboiler . 7. Go to the Setup | Report Options | Stream sheet. In the Fraction basis frame, check the Mole check box. Dr.-Ing. NoorAshrina A Hamid

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Distl We

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8. Press the F5 key to run the simulation. Go to the Blocks | DISTL | Results | Summary sheet to check the results. 9. Check stream results. Go to the Blocks | DISTL | Stream Results | Material sheet. You will see that the distillate and bottoms streams are close to but not quite reaching the desired purity of 99 mol%. Now, we can move to rigorous distillation. 10. Next we will create a simulation for this same separation using the RadFrac block. Create a new simulation in Aspen Plus using Blank Simulation template. 11. Define Components and Methods the same as in the previous simulation file. Dr.-Ing. NoorAshrina A Hamid

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Radfrac We

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12. Create flowsheet. Go to the simulation environment and place a RadFrac block onto the Main flowsheet. The RadFrac block is located under the Columns tab of the Model Palette. Connect the inlet and outlet ports with Material streams and rename them accordingly. Note that stream DIST is connected to the Liquid Distillate port (not the Vapor Distillate ). 13. Define feed streams. Double click on stream FEED or navigate to the Streams | FEED | Input | Mixed sheet. Enter the same information for this stream as in the previous simulation


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Radfrac We

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14. Specify column operating conditions. Double click on the column block or navigate to the Blocks | RADFRAC | Specifications | Setup | Configuration sheet. We use the same specifications for this column as those previously used for the Distl block. Enter 83 for Number of stages. Select Total for Condenser. Enter 50 for Distillate rate and 26.18 for Reflux ratio. 15. Go to the Blocks | RADFRAC | Specifications | Setup | Streams sheet. Enter 46 in the Stage column for FEED. 16. Go to the Blocks | RADFRAC | Specifications | Setup | Pressure sheet. Enter 1 for Stage 1 / Condenser pressure.


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Radfrac We

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17. Create design spec to achieve purity target by varying reflux ratio. In the navigation pane, select the Blocks | RADFRAC | Specifications | Design Specifications node. The object manager for Design Specs is displayed. Click the New… button to create a new Design Specs called 1. 18. Go to the Blocks | RADFRAC | Specifications | Design Specifications | 1 | Specifications sheet. Select Mole purity for Type and enter 0.99 for Target. 19. Go to the Blocks | RADFRAC | Specifications | Design Specifications | 1 | Components sheet. In the Components frame, move N-BUT-01 to the Selected components list. In the Base components frame, move all components to the Selected components list. Dr.-Ing. NoorAshrina A Hamid

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Radfrac We


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Radfrac We

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20. Go to the Blocks | RADFRAC | Specifications | Design Specifications | 1 | Feed/Product Streams sheet. In the Product streams frame, move DIST to the Selected stream list as shown below.


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Radfrac We

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21. Now, we create a corresponding Vary section. In the navigation pane, select the Blocks | RADFRAC | Vary node. The object manager for Vary is displayed. Click the New… button to create a new Vary called 1.


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Radfrac We

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22. Go to the Blocks | RADFRAC | Specifications | Vary | 1 | Specifications sheet. Select Reflux ratio for Type. Enter 1 for Lower bound and 200 for Upper bound. 23. Press the F5 key to run simulation and it completes without any error or warning. Go to the Blocks | RADFRAC | Profiles | Compositions sheet. Select Liquid for View. Note that the purity of the liquid from the top stage is 0.99 in mole fraction, which is consistent with the target we entered earlier. 24. Go to the Blocks | RADFRAC | Results | Summary sheet to check results. The reflux ratio required to produce a distillate stream with 99% mole purity of n-butane is 33.1252. Dr.-Ing. NoorAshrina A Hamid

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Reactive Distillation Column We

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Learn how to configure a reactive distillation column. Learn a few tips to get a RadFrac block to converge, which is very useful for dealing with strongly nonlinear and/or reactive mixtures. Learn how to deal with mixtures involving electrolytes.


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Chemical absorption is typically more effective than physical absorption. Chemical absorption is typically achieved in reactive distillation columns. Capture of CO2 with amines has been a very active topic in both the academic world and the industrial world. Absorption of CO2 with amines is often achieved in reactive distillation columns.


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Electrolyte wizard can be used to generate the chemistry for the mixture involving electrolytes.

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Define reactions for distillation columns.

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Reference the defined reactions in RadFrac.

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Smaller holdup means less reaction, which typically means easier convergence. So, when it is difficult to get a reactive column to converge, start with a small holdup and then gradually increase the holdup to the target value. Generating estimates and switching to the Newton’s method typically make RadFrac numerically more robust. Dr.-Ing. NoorAshrina A Hamid

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Example Reactive Distillation Column We

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Global warming and greenhouse gas emissions have been gaining more and more attention in the world. As a result, CO2 capture has been a hot topic in both the academic world and in industry. This example shows how to use Aspen Plus to simulate the process of CO2 capture using MDEA. Start a new simulation using the Blank Simulation template in Aspen Plus. The Components | Specification | Selection sheet is displayed. Enter WATER, CO2, N2, and MDEA for Component ID. Enter C5H13NO2 in the Alias column for MDEA.


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Use the Elec Wizard to define the electrolytes in the system. Click the Elec Wizard button found on the Components | Specifications | Selection sheet. On the window that pops up, select unsymmetric for reference state for ionic components. Click the Next button.


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On the following window, select all available components, and check the box to include water dissociation reaction. Click the Next button.


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1. The next window will allow you to remove any undesired reactions or species, and to choose the property method. In this case we will not make any changes, click the Next button. 2. Another window will appear asking to select the electrolyte simulation approach. True component approach is the default option. Confirm that this option is selected and click the Next button. 3. Finally, a summary window will appear. Click the Finish button on the Electrolyte Wizard window.


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4. You will notice that all components are defined now.


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5. Confirm Henry components. Go to the Components | Henry Comps | Global | Selection sheet, or press the Next button (the F4 key) to get there.


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6. Confirm methods and parameters. Use the Next button (the F4 key) to confirm Base method on the Methods | Specifications | Global sheet.


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7. Press the F4 key three times (one at a time) to confirm Binary Interaction parameters.


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8. Press the F4 key twice (one at a time) to view Electrolyte Pairs.


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9. Create flowsheet. Go to the simulation environment by clicking the Simulation bar at the bottom left corner of the screen. 10. Place a RadFrac block onto the Main flowsheet and connect two feed streams, a vapor distillate stream, and a liquid bottoms stream to the column inlet and outlet ports. Name the streams MDEA, FEED, CLEANGAS, and RICH-SOL as shown below. Also, rename the RadFrac block ‘COL-MAIN’.


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11. Specify feed streams. Double click on the FEED stream or use the navigation pane to go to the Streams | FEED | Input | Mixed sheet. Specify the Temperature, Pressure, Total flow rate, and Composition of the FEED stream as shown below. 12. Define MDEA stream. Navigate to the Streams | MDEA | Input | Mixed sheet. Specify Temperature, Pressure, Total flow rate, Composition, and Solvent 13. Go to the Flash Options sheet. Specify the Valid phases as Liquid-Only. This option means that, when specifying the temperature, pressure, and concentration of a stream, the stream will be only liquid.


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14. Define reactions. Go to the Reactions folder in the simulation navigation pane. Click the New button and select REAC-DIST as the type. 15. For this process we have 5 individual reactions to input into the R-1 reaction group. Under the newly created reaction group (R-1) click the New button to enter a new reaction. Select Kinetic/Equilibrium/Conversion as the reaction type. 16. For the first reaction, select MDEA+ and WATER in the Reactants frame and MDEA and H3O+ in the Products frame. Enter a stoichiometric Coefficient of 1 for each component and select Equilibrium as the Reaction type. Note that a minus sign is automatically added for numbers in the Coefficient column in the Reactants frame. Dr.-Ing. NoorAshrina A Hamid

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17. Enter each reaction using this same procedure. The reactions are listed below.


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18. Make sure to input the correct stoichiometric coefficients and to select the correct reaction type for each reaction. Also note that for kinetic reactions, an exponent must be defined for the reactants . In reactions 4 and 5 the Exponent is 1 for all reactants. Leave the products exponent blank. Once the stoichiometry is defined for each reaction, the kinetic and equilibrium parameters must be specified. Go to the Reactions | R-1 | Kinetic sheet. 19. For reaction 4, enter a value of 4.3e+13 for k and a value of 13300 cal/mol for E. 20. For reaction 5, enter a value of 3.75e+14 for k and a value of 25300 cal/mol for E. Dr.-Ing. NoorAshrina A Hamid

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21. For reactions 1, 2, and 3 we must enter equilibrium parameters. Go to the Reactions | R-1 | Equilibrium sheet. For each reaction select Compute Keq from built-in expression and then enter the values for A, B, C, and D.


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Reaction #2


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Reaction #3


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22. Specify column operating conditions. Navigate to the Blocks | COL-MAIN | Specifications | Setup | Configuration sheet. Enter 30 for Number of stages, None for Condenser and None for Reboiler . 23. Go to the Streams sheet. For stream MDEA, enter On-Stage for Convention and 1 for Stage. For stream FEED, enter OnStage for Convention and 30 for Stage.


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24. Go to the Pressure sheet and specify 10 bar in the Top stage/Condenser pressure frame. 25. Specify reactions. Go to the Blocks | COL-MAIN | Specifications | Reactions | Specifications sheet. Select R-1 as the Reaction ID, enter stage 1 for Starting stage and 30 for Ending stage. 26. Go to the Blocks | COL-MAIN | Specifications | Reactions | Holdups sheet. Enter 1 for Starting stage, 30 for Ending stage, and 160 L for Liquid holdup.


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27. Go to the Setup | Calculation Options | Reactions sheet. In Activity coefficient basis for Henry components frame, select Aqueous as shown below


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28. Go to the Blocks | COL-MAIN | Specifications | Convergence | Convergence | Basic sheet. Change Maximum iterations to 100. 29. Now, we need to get the flowsheet to converge. Go back to the Blocks | COL-MAIN | Specifications | Reactions | Holdups sheet. Note that Liquid holdup should eventually be 160. Change it to 16. Then, press the F5 key to run the simulation. The simulation should complete without any error or warning. 30. Go to the Blocks | COL-MAIN | Reactions | Holdups sheet. Change Liquid holdup to 50. Then, press the F5 key to run the simulation. The simulation should complete without any error or warning. Dr.-Ing. NoorAshrina A Hamid

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31. Change Liquid holdup to 100. Then, press the F5 key to run the simulation. The simulation should complete without any error or warning. 32. Change Liquid holdup to 160. Then, press the F5 key to run the simulation. The simulation should complete without any error or warning. 33. Go to the Blocks | COL-MAIN | Specifications | Setup | Configuration sheet. Select Custom for Convergence. 34. Go to the Blocks | COL-MAIN | Convergence | Estimates | Temperature sheet. Click the Generate Estimates… button. In the popup dialog box, select options to generate the most estimates as shown below. Then click Generate button and wait for estimate generation to complete. 35. Go to the Blocks | COL-MAIN | Convergence | Convergence | Basic sheet. Change Algorithm to Newton. Dr.-Ing. NoorAshrina A Hamid

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36. In the Advanced sheet, select Dogleg strategy for StableMeth. 37. Click the button in the Home | Run group of the ribbon to reinitialize simulation. Run the simulation again to ensure it still converges. 38. Check results. Go to the Results Summary | Streams | Material sheet. You can see that the stream CLEANGAS contains significantly less CO 2 than stream FEED.


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Lecture Note - Column

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