I. Scenario

Suppose you want to investigate how the dynamics of the liquid level of a cylindrical tank is affected:

a) by using different diameters of the tank - design parameters
b) by changing the valve opening - manipulated variable
c) by changes in inlet flow rate - disturbance effects

II. Process Model

Use NOTEPAD (or other text editors) to create a file named TANK.T (in your own subdirectory) and input the following text:

CONTINUOUS SYSTEM TANK
STATE h
DER dh
" System Equations
dh = ( fin - fout ) / A "ft/sec
A = pi*(d^2)/4 "ft^2
fout = u*kv*(h^0.5) "ft^3/sec
fin = 1.0 "ft^3/sec
pi = 3.1416
" Parameters
u : 0.5
kv : 2.5 " (ft^3/sec)
d : 1.0 " ft
" Initial Values
h : 0.0 " ft
END

Remarks:

1. The model file name needs an extension .T ( for text ). ( Since Notepad in Windows 95 is likely to append the file name automatically by the extension .txt, you need to override this by enclosing the filename you are saving by double quotes, e.g. "tank.t")
2. The structure of the file includes the heading: CONTINUOUS SYSTEM TANK in which TANK is the name of the model file but without the .T extension. This heading has to be the first line of the file.
3. After the heading, the declaration statements: STATE h and DER dh, declares the variable h as the variable which is going to change with time, while the other statement declares dh as the time derivative variable of h.
4. Comment lines included in the file are marked by the double quotation mark.(It is good practice to include some useful description, such as dimensions, after the statements.)
5. The equal sign signifies that the line is treated as an equation, while the colon signifies that that particular variable will be treated as constant which can be changed during run time as will be shown later.
6. The last line of the model file has to be the END statement
7. One feature of simulators like SIMNON is that except for the heading, declaration and END statements, the lines in between do not have to be in any particular order.
8. SIMNON does not distinguish between capitalized and small letters, thus temp and TEMP will be treated as the same variable.
9. As an alternative to using NOTEPAD, you can use the editor that comes with SIMNON. First, run SIMNON and the choose File|New menu item, and input the file name (no extension). When the editor appears, comment out the lines which contain INPUT, OUTPUT and TIME by preceding these lines by a double quote character. After the file has been edited, select File|Save and the file will be saved with the extension .T automatically applied.

1. Click on the SIMNON icon on the desktop or select from Start|Programs|Simnon menu to enter the program.
2. Click once more on the grey box and a command window should appear. (Should the command window disappear, just select Window|Command Dialog from the menu. )
3. Select File|Open menu item and open the file just created.
4. In the command window, compile the model file by typing:
SYST TANK
(without the extension .T). If an error appears, go to the model/editor window and correct the syntax error and compile again until no error message appears.
5. Declare the variables you wish to store during simulation by typing:
STORE h
6. Begin the simulation by typing:
SIMU 0 20
which states that you are going to simulate from time=0 to time=20.
7. Plot the results, type:
ASHOW h
8. Print the plot by selecting File|Print in the menu bar (or press the corresponding icon). Make sure the plot window is active.
9. To print to a file, you can use the HCOPY command, for example
HCOPY EPS TANK
will produce a file TANK.EPS which can then be imported to other word processors.

1. You can zoom out by double-clicking the left button while the cursor is in the plot window. You can zoom in by dragging a window inside the plot. (Caution: after zooming in, the plots are clipped depending on the window size.)
2. You could also specify the range of the horizontal axis
AXES H 0 5
or the range of the vertical axis
AXES V 0 0.7
3. To read the graph you could type the command
GIN
which means "graph input" and the position of the cursor based on the graph will be shown in the window title bar. Click the left mouse button to end this command.
4. You can put a label for the y-axis at the top of your plot using the TEXT command,
TEXT 'h ( ft )'
5. You can also put text inside the plots by a sequence of commands
GIN
MARK A XS. YS.
MARK "This is a sample text
where the text after the comment will be put in the position where the cursor was positioned last during the GIN command. Note that the periods after XS and YS are necessary. Also, the cursor is only allowed inside the plot ranges.

1. Go back to the original plot by typing: ASHOW h
2. To obtain the current value of diameter d, type
DISP d
( To display the current values of all variables and parameters, just type DISP by itself )
3. The value should show up in the bigger box in the command window. ( Currently, it should be d=1.0)
4. Now change the value of d using the PAR command then simulate again
PAR d:1.5
SIMU 0 20
(Note the use of colon instead of equal sign.)
5. Instead of using ASHOW, this time use SHOW
SHOW h
(The command ASHOW will always erase the previous plot before generating a new one. The command SHOW will plot on top of the previous plot.) It is a good practice to label each curve with the corresponding case using the GIN and MARK command sequence described above.

Exercise: How much time is needed for the level to rise to 0.5 ft when the diameter, d=1.0, d=1.5 and d=2.0 ? Does the diameter affect the steady state height ?

Exercise: How much time is needed for the level to rise to 0.5 ft with d=1.0, but now using u=0.25, u=0.5 and u=0.75 ? How is the steady state height affected ?

6. You could also change the initial condition of h by using INIT instead of PAR:
INIT h: 0.2

VII. Exit the program

Tomas B. Co
Associate Professor
Department of Chemical Engineering
Michigan Technological University
1400 Townsend Avenue
Houghton, MI 49931-1295

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