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Batch Distillation With Total Reflux

In the batch distillation case with total reflux, the reboiler pot is is initially charged with a certain amount of liquid.  But, during the run there is no feed and there are no products removed (no distillate or bottom products). In this case we have an unsteady-state problem instead of the steady-state case we modeled and analyzed in Modeling Continuous Steady State Distillation.

HYSYS can perform unsteady-state (dynamic) model when properly instructed to do so. The initial charge of liquid will be heated and evaporation begins. The column will be run until it reaches the desired condition (in our case a desired amount of distillate concentration). Then the distillate will be drawn off.  

Before we begin the set up of the process we need to establish the total reflux ratio case operating conditions.

Operating Conditions

We will illustrate this approach in the form of a study case itself. Once the reboiler starts providing heat, we would like to reflux the distillation column until we obtain a concentration of 80% ethanol by providing 120,000 BTU/hr to the reboiler, and then we will draw distillate off at the top. HYSYS will tell us the profile of the batch distillation model and how long it will take to do so. We will use the same tower with 24 plates. 

The feed stream will be fed to the reboiler and will be 0 lb/hr since we do not have any feed during the run but just an initial charge. We will assume that 200 lb of a mixture of 50% ethanol and isopropanol are initially charged to the reboiler. (Note that in the example that follows we will assume the total capacity of the reboiler to be 400 lb for simplicity. Therefore the tank will be half full).

Setting Up Your Simulation Environment

You will set your column the same way you did for the continuous distillation case:

1. Select a property package (PRSV) and a set of components (ethanol and 2-propanol)
2. Install a distillation unit as the one in the continuous case with 24 plates. Select TOTAL REFLUX for the condenser. The feed F (make it enter the reboiler), the distillate D and the bottoms B, the condenser and reboiler duties Qc and Qr are to be input. The condenser and reboiler pressures are both 1 atm. Leave the temperature in the third page blank as usual and do not specify anything else.
3. In the feed stream enter the following specifications shown on the table below. (When you go under COMP to enter the composition, click on EDIT then select MASS FRACTION then enter 0.5 for both ethanol and 2-propanol)

Installing Controllers

Four controllers are required for the dynamic control of the batch distillation operation. The first controller, we will call it Duty Control in our example, maintains the reboiler heat flow by adjusting the reboiler duty. The next controller, in our example LIC-100, adjusts the Reflux stream flowrate to control the condenser liquid level at 50% so that reflux can occur until the desired concentration is reached. This external condenser level controller is used to override the condenser’s local control. The two other controllers are flow controllers attached to the valves that empty the condenser and reboiler respectively. The first one, in our example FIC-100 controls the flowrate of the distillate stream and thus, is used to draw the distillate product from the condenser. When a batch has been emptied, this controller is used to remove the liquid from the condenser. The other flow controller, in our example FIC-101 is used to empty the contents of the reboiler. To install the controllers, do the following:

1. Click on COLUMN ENVIRONMENT, inside the distillation column window as shown in Figure 4, to go in the column environment. Close the window and you will find an environment like Figure 7 below. To install the controllers, click on this logoand click on the screen anywhere next to the reboiler and you are ready to install the first controller.
2. Double click on the controller logo and you will find a screen that looks like Figure 8. Click on SELECT PV. Another window will show up. Under Object select Qr and under Variables select Heat Flow then click on OK.
3. Then click on SELECT OP, under Object select Qr then OK.
4. Press VIEW CONTROL VALVE button and you will obtain a window. In the Min. Available box of that window enter 0 BTU/hr and in the Max. Available, enter 600,000 BTU/hr (it is an arbitrary maximum value). Leave the Duty Source section on DIRECT Q then close the window.
5. Click on PARAMETERS as shown on Figure 7. On the PV Minimum box, enter 0 BTU/hr and on the PV Maximum box enter 300,000 BTU/hr as the range of heat duty allowed in the laboratory. Leave the Control Action box on "Reverse". On the Controller Mode section, select MANUAL. Leave the OP box on 0%. Leave the Execution section on INTERNAL.

 

6. Click on Tuning as shown in Figure 8. On the Kp box input 0.5, in the Ti box input 0.5 and leave the Td box "<empty>". These are respectively the proportional and integration parameters that will be used to control the batch operation. When this is done, you are ready to install the three other controllers.

   
    
    

   
   Below are controller face plates that show the input necessary for each controller. If instructions are not specified, do not alter data, leave as is.
    
    

Size And Initialize the Model

Once you are done installing the controllers, the next step is to size and initialize the distillation unit. To do so, do the following:

1. First get off the controller screens and go onto the column screen. Double click on the condenser. You will be given the condenser screen. Select the DYNAMIC window at the bottom of the screen as shown by Figure 8. Once there, you are ready to enter the specifications for the condenser. In the Vessel Volume box, enter 0.2 m³, in the Liquid Level SP, enter 50 %, then select DRY STARTUP. Close the window.
    
2. Then go in the reboiler and repeat the same thing. In the Vessel Volume box, enter the total volume of the reboiler, which is about 180 Liters (or 300lb of mixture). Since in our example we will start with 150 lb of mixture, enter 50% in the Liquid Level SP section. Select INIT FROM USER , as you do that the button will appear bold. Click on it and a screen will appear where to input the initial composition of each component of the mixture, which is 50% of ethanol and 2-propanol (If HYSYS asked you to input in mole fractions it is 0.566 for ethanol and 0.434 for 2-propanol). Once done, close all the windows.

1. The next step is to double click on the column itself. Once there, go in the DYNAMICS section. There, in the HOLDUP section, select DRY STARTUP. Then enter the column diameter, which is about 1 foot. When this is done, you are ready to move to the next step, which is to install the Strip Chart to record the dynamic behavior of the column parameters.

Installing the Strip Chart

To monitor the key process variables in dynamic mode, a strip chart will be installed.  

1. Open the data book by either pressing <Ctrl> <d> or by going under TOOLS and selecting DATA BOOK.  You will need to make available the variables that you will add to the strip chart.  On the Variable window, insert the following variables by pressing the INSERT button and making the appropriate selections from the Variable Navigator.  Every time you select an object click on ADD until you have selected all six objects shown in Table A.
2. Then select STRIP CHARTS at the bottom of the window.  Once there, you will see all the variables that you have just imported.  Now do the following

1. On the Strip Charts page, press the ADD button in the Available Strip Charts group. In the Individual Strip Chart Data Selection group, activate the Active check box for each variable in the list. Then press the STRIP CHART button. A colorful graph will be displayed in a new window. It is the chart that will show how your batch distillation occurs. Access the STRIP CHART1 SETUP view by double clicking the plot area of the Strip chart. Once there, make the following modifications to the ranges of the variables. These parameters are to label

The next step the will be to set the integration parameters.

Setting Integration Parameters

Since user interaction (i.e. opening/closing valves, specifying the target set points and ramp durations for controller set point changes) is requires during the procedure for dynamic operation, it may be necessary to change the default integrator parameters. If the integration of the model is proceeding too quickly, you may miss steps in the procedure while you are reading. One of the key factors in this situation is the speed at which your computer operates in dynamic modes.

For a Pentium 133 MHz processor, it was found that real time could approximately be simulated during dynamic operation by reducing the Minimum and Maximum integration Step Size values to 0.01 and 0.04 seconds respectively. This is a significant reduction for the default values of 0.5 and 5 seconds. Depending on the type of processor on which HYSYS is running, you may want to use values that are closer to the default step size values. To access the integrator press <Crtl> <i>.

Setting Up the Desktop

Return to Main Environment and clear the Desktop of all views with the exception of the Strip Chart. The next step is to display the faceplates where you will manipulate the simulation by tuning the different parameters. To do so, select TOOLS then FACE PLATES or press <Crtl> <f>.Select your column number in the Flowsheet window, then select all four faceplates and click on OPEN. To arrange the Desktop, select WINDOW then ARRANGE DESKTOP. The desktop is now ready for the dynamics run. The next step is to save your original case because running the model might me a little tedious at the first time. That way, if you save it you can always come back and modify your model.

Saving Your Model

At this point HYSYS offers two options to simulate the dynamic model: Script recording and Playback and Spreadsheets. We will use the first case because it is more interactive. The second case is automatic and eliminates the necessity of repetitious manual input steps. If you are interested you can always go to the main HYSYS tutorial to learn the Spreadsheet method.

The Script Recording and Playback Method

Each step made in dynamic mode will be recorded in a script file, which can be played back at a later time using the previously saved case (batch1.hsc). In this way, the script tool can be used as a training device for dynamic operation. To record the script, open the saved case (batch1.hsc) if it is not already open. Select TOOLS in the main menu then select SCRIPTMANAGER. From the script manager property view, select the NEW button, which accesses the NEWSCRIPT view. Provide a name and description for the script (in our case name: batch.scp, description: batch distillation dynamic). Then these 14 steps will guide you through the process of establishing your simulation profile. (Put in mind that there is a lot of manual steps in this method, i.e. opening and closing of valves and so on)

1. Press the RECORD button
2. Start the Integrator
3. On the Duty Control faceplate, press the TUNING button. The property view for the duty controller will appear, opened to the Tuning page.
4. In the Set Point Ramping Group, specify a Target SP of 120,000 BTU/hr and a Ramp Duration of 10 minutes. This will provide the heat to the reboiler to start the distillation.
5. Press the START RAMP button and close the Duty Control property view.
6. On the Strip Chart, monitor the distillate (D) mole fraction of ethanol. Once the product specification of 95% ethanol is reached, the distillate valve can be opened to remove product from the batch operation. In the OP cell of the Distillate flow control (FIC-100) faceplate, type "100" (meaning 100% open).
7. Continue to monitor the distillate mole fraction of ethanol. Once the value begins to drop, press the TUNING button on the FIC-100 faceplate. The product flowrate will be stopped by ramping the valve shut. In the Set Point Ramping Group, set he Target SP at 0 BTU/hr and the Ramp Duration at 2 minutes.
8. Press the START RAMP button and close the FIC-100 property view.
9. When the distillate valve is closed (FIC-100 faceplate shows OP = 0 %) you can stop the heat to the reboiler. Press the TUNING button on the Duty Control faceplate and provide a Target SP of 0 BTU/hr and a Ramp Duration of 5 minutes in the Set Point Ramping Group.
10. Press the START RAMP button and close the Duty Control property view.
11. After heat application to the reboiler has ceased (Duty Control faceplate shows OP box = 0 %), you can empty the condenser and the reboiler. To do so, in the OP cells of both FIC-100 and FIC-101, type "100" (meaning 100% open)
12. Monitor the levels on the Strip Chart. When both units are empty, stop the integrator by clicking on STOP.
13. Under the TOOLS option in the main menu, select SCRIPTMANAGER. At this point you have ran your column and all the specifications and solutions are available. You can go back to the main environment and obtain your results.

Results

A copy of the strip chart is displayed as Figure 9 below. It took the column about 250 minutes to reach the 80 % mass fraction concentration needed operating at 120, 000 BTU/hr. The solid black line indicates the concentration in mass fraction of the ethanol in the distillate.

Batch Distillation With Distillate Drawn Off

The batch distillation with distillate being drawn off is the same as the previous case but the only difference will to open the distillate valve.