DWSIM-3.4/C2/Rigorous-Distillation/English
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Welcome to the spoken tutorial on Simulating a Rigorous distillation column in DWSIM.
My name is Kannan Moudgalya. |
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In this tutorial, we will carry out a rigorous simulation of a Distillation column.
We will learn to specify column pressure profile. We will see where to specify tray efficiencies. We will check whether the product compositions are as desired. We will also learn how to view the column profiles. |
| Systems requirement slide | To record this tutorial, I am using DWSIM 3.4 |
| Prerequisite Slide | To practice this tutorial you should know
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Our website spoken tutorial dot org gives details of the prerequisite tutorials.
You can access these tutorials and also all the associated files from this site. |
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This slide shows the problem solved in one of the prerequisite tutorials.
This was solved using shortcut distillation. Let us open the corresponding file in DWSIM. |
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I have already opened DWSIM.
I have already opened the file shortcut dash end dot dwxml This file is available to you for download from our website spoken tutorial dot org. Let me save this as rigorous. You can see that the file name has changed now to rigorous. |
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Click on the Configure simulation button.
Below the Thermodynamics tab, locate the Options menu. Let me click it. In the white space above it, you get an option called Units System. Let me click it. Locate Pressure on the right hand column. Change its units to atmosphere. Similarly, change the units of Delta_P also to atmosphere. Change also the units of Molar flow rate to kilo moles per hour. Click on Back to simulation. |
| Let us switch to the slides.
The file we opened just now in DWSIM, solves the problem in this slide. The solution is given in the next slide. This problem was solved in the spoken tutorial on shortcut distillation. These values form the basis for the rigorous distillation column problem. You may want to write down these values in a piece of paper. We will be using these values shortly. | |
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Let us begin by replacing the Shortcut column with a rigorous distillation column.
Let me go back to the simulation. Press right click on the Shortcut column, and delete it. Answer yes to the prompt. |
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Locate the Distillation column in the Object palette.
Click on it and drop it in the place of shortcut distillation column. You may have to adjust its position. Click the column and select it. |
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Go to Selected Object window.
Under the Properties tab, locate the Connections menu. It is the third item. In this, we can see Edit Connections. Click on it. Immediately after clicking it, a button appears at the right extreme. A button with three dots. Click on it. Now there is a pop-up window. Under the Feeds menu, click on the + button, which is the Add button. Under To Stage column, the option is Condenser by default. Here we have to specify the stage at which feed enters. Click on the arrow next to the Condenser. In my version of DWSIM, I had to click a second time. We can see the list of stages. Select the stage where we want the feed to enter the Distillation column. Here we select Stage_6. Now we will select the material stream. Click on the down arrow below the Stream menu. You may have to click this also twice. Click on Feed. We have done the pairing that the Feed should go to Stage_6. The actual connection will be done later in the flowsheet. |
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This is as per the solution we got in the shortcut distillation column.
Let us see it in the slide, where we see the optimal feed location to be six. Let us go back to DWSIM. |
| In a similar way, you have to pair the product streams.
Pair the Condenser to Distillate. Pair the Reboiler to Bottoms. | |
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Now we will connect heat duties to reboiler and condenser.
Pair C-Duty with Condenser and R-Duty with Reboiler. We will now convert all the pairing into connections. Click on any one of Feed or Condenser or Reboiler. This completes the connections indicated by pairing. We can move this and check. You can see the connections have been made. Let me close this. Let me shift some streams to make it look more beautiful. |
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Click the Column and select it.
Under Properties tab, locate the Column Properties section. It is the first option. This section is used to specify the various attributes of a Distillation Column. In this section, the first option is Condenser Pressure. By default, it is 1 atmosphere. We will leave it as it is. Next is the Reboiler Pressure. I will change the value to 1.1 atmosphere. Using this, I will show how to establish a linear profile in the column. |
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Locate the Number of Stages option next.
You have to enter the total number of trays here. Enter 15 here. Because, this number includes the condenser also in DWSIM. We plan to use a total condenser. So, this number should be one more than the equilibrium stages given by shortcut method. We got the equilibrium stages to be 14 from the shortcut method. We can see it in the slide. Let us go back. The next option is Edit Stages. Click it. Press the button at the right extreme. It shows the condenser and reboiler pressures as 1 and 1.1 atmosphere. These are the values that we entered just now. New stages that have been added have been assigned zero pressure, which is incorrect. We would like the intermediate stage pressures to take interpolated values. Go to the left column and click the interpolation symbol at the bottom. Immediately, linearly interpolated values are assigned to every stage. One may also change the pressure in any of the stages one through thirteen. For example, I will click this pressure and change it to one atmosphere. Let me undo this by pressing the interpolate button again. This is a very useful and important method. Whenever the number of trays changes, you will have to press the interpolate button. If you forget this, there could be a lot of problems. For example, there could be negative flow rates. Remember to do this in Assignment 3 at the end of this tutorial. One may also change the efficiency in one or more stages, as shown here. Close this popup. |
| Highlight 1.47 in the slide
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Next we have the Condenser type.
For that, we have to go up. By default, it is total condenser. We will leave it as it is. Next we have Condenser Pressure drop in atmosphere. By default, it is 0. We will leave it as it is. Next we have Condenser Specifications. Under this menu, we have the type as Stream_Ratio by default. Below this, we can see the Value menu. Click on the field besides it. Enter the required Reflux ratio. Here we enter it as 2. Recall that the minimum reflux ratio from the shortcut distillation was 1.47. By multiplying 1.47 by 1.3 and rounding it off, we get 2. Next we have Reboiler Specifications. Under this menu, we have type as Product Molar Flow Rate by default. Change the units to kmol/hr, if necessary. We see the Value menu. Enter the required Molar flow rate. Here we enter it as 61.1. This is suggested by the shortcut solution. We can see it in the slide. |
| Now we will select the solution method.
Locate the Solving Method option from the Properties tab. It is item 7. Click on the field besides it. Click on the arrow on the right extreme. We can see a list of solving methods. Select WangHenke_BubblePoint. | |
| Now we will run the simulation | |
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To do this, go to the calculator options
Click on the Play button Now click on the Recalculate All button When the calculations are completed, click the product compositions. Choose a stream, for example distillate. You know how to check the product compositions from the molar compositions option. Next, click on the Distillation column. Under Properties tab, locate the Results menu. It is item 8. This shows all the required results like
To see the Column profiles, click on it. We can see the button appearing on the right extreme. Click on it. Now we can see the pop-up window. We can see the temperature and pressure profiles. Flows profiles. Component flows. Component fractions. If you want to know the exact numbers, use the Table tab to the right of the Graph tab. Let me close this. Let us save this file. |
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Let me summarise. |
| Summary slide | We learnt how to simulate a rigorous Distillation column
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| Let me give some assignments. | |
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Repeat the calculations for a constant column pressure of 1atm.
That is, with reboiler pressure = 1atm. Do you see major changes in the results? |
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Next, simulate the distillation column at a constant pressure of 1 atmosphere.
That is, the reboiler pressure also is at 1 atmosphere. If the reflux ratio is increased beyond 2, does the purity improve? What reflux ratio should you use, if the actual purity is as desired. In a future tutorial, we will show how sensitivity analysis can help do this easily. |
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In the next assignment, we simulate the column at a reflux ratio of 2.
Keep the column pressure constant at 1 atmosphere. Increase the total number of trays by 1. That is, from 15 to 16. As the number of trays has changed, you have to use the interpolate option. This was mentioned earlier also. With a larger number of trays, has the purity increased? |
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In the next assignment, we verify the following relationship:
Composition of vapour flow to the condenser = distillate product composition Explain why this equation has to be satisfied. |
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In the next assignment, check the consistency of the variables of the reboiler.
Use compositions, temperature and pressure at the reboiler for this purpose. Do this through an equivalent flash calculation. |
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In the last assignment, solve the problem by different Solving Methods.
Compare the answers. Compare the computation times. |
| We have come to the end of this tutorial | |
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