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Running A Column Simulation

 We are now ready to begin performing column calculations since all the variables have been specified.

Figure 3

Running the Column Simulator

We are now ready to run the column simulator to obtain all the dependent variables such as the overhead and bottom distillate conditions, the condenser load and so on. 

Double click on the column to enter the environment in which the calculations will be performed. When you enter the window you will find a screen that looks like Figure 4. In order for HYSYS to perform calculations the Degree of Freedom box in Figure 4 must be zero. If it is –1, the problem is over specified, in our case it is not because we have fully defined all the independent variables. The independent variables must be activated for the simulator to be run. In the specifications section are listed all of the possible independent variables. 

In order to select the independent variable that we want to use for the run, we have to check the active box. This is shown in Figure 4. Make sure that all the independent variables we have selected are checked. In our case, the reflux ratio should be the only one checked. Any other box such as the Distillate Rate should be deactivated because we did not choose it as our independent variable. 

Once the boxes of the variables are not selected as independent variables, the Degree of Freedom box becomes zero and the column simulator is automatically solved. You will obtain a "Converged" icon in the bottom indicating that the entire column simulator is solved.
 

Results

To review your results you can either exit Figure 4 by clicking on CLOSE and double clicking on the individual streams to see the results or click on WORK SHEET shown in Figure 4 to get the spreadsheet with the combined results. We obtain a spreadsheet as shown by Figure 5. From this spreadsheet, we can see that the overhead flowrate is 79.3273 lb/hr and the bottom distillate flowrate is 83.6727lb/hr. By going out of the screen and double clicking on the individual streams we notice that the overhead concentrations for ethanol is 64.9% and for the bottom distillate is 35.9% (mass fractions).

Additional Steady State Cases

Having worked through the set up process, we will now do some cases to illustrate how HYSYS can be used to evaluate different conditions as specified by the user.

Case 1:

Let us say that we would like to know what amount of steam (the reboiler load in BTU/hr) would be needed to obtain an overhead ethanol concentration of 55% using the same conditions as we used in the setup. To solve that problem, follow these steps:

1. Click on SPECS as shown in Figure 5
2. Once there, you will need to import the variable you will want to specify which is the overhead ethanol concentration. To do so, click on ADD, you will obtain a window like the one shown in Figure 6. Select COMPONENT RATIO then click on ADD SPEC(s). A Comp Ratio Spec window will appear. In this window you will need to specify what you want the concentration to be (0.55). In the Name box you can leave it as is or put any name you want. In the Stage box, you need to specify which column stage you want to have the concentration to be 0.55. To do so, click on "<<Stage>>" then on the arrow in the top right corner of the screen. There among all the choices offered, pick CONDENSER, that is the overhead concentration. In the next box, Flow basis, select what basis you want. Since we are specifying mass fraction, click on the arrow and choose MASS FRACTION. The overhead is liquid so we will leave the Phase box as is. In Spec Value, enter 0.55, the value we would like the overhead ethanol concentration to be. Then in the Components box select ethanol by clicking on "<<Components >>" and then clicking at the top arrow. Once all this is done, click on CLOSE and the concentration will be added in the Column Specification section.
3. The next step is to delete the reboiler duty we had input earlier to find the concentration.  To do so, click on CLOSE to go back to the graphic environment then double click on the reboiler duty stream (in our case called Qr).  Once you get the Energy Stream window, click on the Heat Flow box then press DELETE in the keyboard, then exit the window.
    Now go back in the monitor screen by double clicking on the distillation column.  When in the column window environment, click on MONITOR at the bottom of the screen to go to the Monitor window.  Once there you will notice that the degree of freedom value is 1.  The problem is under specified because you have not yet activated the concentration specification.  Scroll down the specification column and click on the Activate box for the Comp Fraction.  The degree of freedom will be zero and the column will automatically calculate the variables.  Once the “Converged” signal appears at the bottom of the window, you can go back at the reboiler duty stream to get the new calculated value for the heat duty, which is in this case: 206,730 BTU/hr.

Figure 6

 

Case 2:

In the second case let us say that we would like to know the reflux ratio that would give an ethanol concentration of 80% with a reboiler duty of 300,000 BTU/hr using the same feed specifications as in the setup discussion. To solve this problem, do the following:

1. First, in the Monitor window, deactivate the Reflux Ratio box. Then go under the Specified Value column click on the current value of the reflux ratio. Then press DELETE on the keyboard to empty the box. Then go in the reboiler duty stream and input 320,000 BTU/hr. Go back to the monitor screen by double clicking on the distillation column.
2. The next step is to input the concentration of ethanol desired. Go again under the Specified Value column and go to the Comp Fraction box, click on it. Go in the bar at the top of the screen and input 0.8. Then activate the box by clicking the activation box under the Active column. HYSYS will automatically solve the system. Once the "Converged" signal appears at the bottom of the window, click on UPDATE INACTIVE then scroll up to see the reflux ratio necessary to have 80 % ethanol at 320,000 BTU/hr. The reflux ratio needed is: 17.78

Dynamic Models

The steady state model is useful for modeling operations after they have leveled off and have attained to steady state conditions.  Also of interest is how the column reaches steady state and what happens when set points on the column control system are changed.  The modeling of a column attaining steady state is discussed next.