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ChE 441 Washington State University

Process Control Voiland School of Chemical Engineering and Bioengineering

Fall, 2011 Richard L. Zollars

Solving Differential Equations

Many problems in chemical engineering are naturally expressed as differential

equations. For example, in any rate driven process (heat transfer rate, mass transfer rate,reaction rate) the rate may be expressed as a change per unit time, i.e., as a time

derivative. Similarly properties may not change in time but in space. As a fluid flows

down a pipe through a furnace its temperature increases. Thus the temperature of thefluid changes with axial position in the pipe. A proper mathematical description would

thus involve a differential of the temperature with axial position, i.e. a dT/dz term.

Whenever one (or more terms) in an equation is described by a differential the resulting

equation will be a differential equation.

While it would be nice if we could solve every differential equation we encounter(as you did in Math 315) more often than not analytic solutions to many of thedifferential equations we face in chemical engineering either do not exist or are too

complex to derive. Thus we rely heavily on numerical techniques to solve the differential

equations we face. At present there are a wide variety of computer packages that includedifferential equation solvers. In general these packages contain all of the input/output

and integration algorithms. All that you have to do, as a user, is to specify the equations

you want solved. In what follows two different packages will be examined for use as

differential equation solvers.

The Problem To Be Solved

For the software packages to be demonstrated we will solve the same problem,

which is given below. Consider a chemical reactor initially full of the compound A. A

undergoes a series reaction first to compound B then to compound C as shown below

CBAkk 21

The following two differential equations can be written describing the change in the

concentration ofA and B with time

BA

B

A

A

CkCkdt

Cd

Ckdt

Cd

21

1

subject to the initial conditions

00

02

tatCC

tatCC

BoB

AoA

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The values of the two rate constants are k1 = 0.5 min-1

and k2 = 1.5 min-1

. We wish toobtain the values of the concentrations of A, B and C as a function of time for times up to

10 min. To obtain the concentration of C we could write a third differential equation

002 tatCCwithCkdtCd CoCB

C

or use a material balance to get

BABACoBoAoC CCCCCCCC 2

We now will solve this problem using each of the packages starting with MathCAD.

MathCAD

The following is copied directly from a MathCAD program for the solution of theproblem stated above.

This is the solution to the example problem in MathCAD. We will solve the problem using the two

differential equations given and the material balance for compound C. The first step is to define a

vector that contains the initial values of the variables (CA and CB). Thus

y2

0

We now need to define a vector that tells the integrator how to compute the derivatives for CA and

CB. This is given by

D t y( )0.5 y0

0.5 y0 1.5y1

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In the definition of D the first argument is the independent variable (in our case t since both

differential equations are time derivatives), while the second argument is a vector containing alldependent variables (CA and CB in our problem).

Note that in this expression the variable y is itself a vector (a two component vector where the firstcomponent is the value of CA and the second is C B). Remember that in MathCAD the subscripts in

vectors start with 0 not 1. Thus y0 refers to CA and y1 refers to CB.

We are now ready to integrate as we have both defined the initial values and the manner in whichthe derivatives are to be computed. MathCAD offers us a number of different integrators; odesolve,

rkfixed. Bulstoer, and Rkadapt plus some other more specialized options. odesolv solves only a

single differential equation and will not be presented in this description. rkfixed used a fourth-orderRunge-Kutta integration algorithm with a fixed integration step size to solve sets of ODE's.

Bulstoeruses the Bulirsch-Stoer integration method that is slightly more accurate than theRunge-Kutta technique. Rkadapt uses a variable step size integration algorithm that adjusts

according to how fast the solution is changing. The argument used to invoke any of these three isthe same - only the name of the algorithm changes. You may take a look at the Help feature in

MathCAD (under differential equations) for MathCAD's on-line help.

We'll use the Rkadapt algorithm for this example. The statement needed to start the integration

algorithm looks like

Rkadapt(y,x1,x2,npoints,D)

where y is the vector of initial values, x1and x2 are the endpoints of the integration interval, npoints

is the number of points (in addition to the initial value) we want calculated, and D is the vector thatdefines how the first derivatives are to be evaluated. The initial values of y must be defined at the x1

value.

Let's now integrate this problem and store the results in an array called Z. The following commandcompletes the integration

Z Rkadapt y 0 10 100 D( )

where I have asked that the integration be reported at 100 points in addition to the initial value, thusa total of 101 values for t, CA and CB. The array Z is thus a (101 x 3) array where the three rows

correspond to t, CA and CB. We now need to compute CC. This can be done as shown below.

i 0 100

Z i 3( ) 2 Z i 1( ) Z i 2( )

We could have the array printed out but this would consume a lot of paper and large sets of

tabular data are not that understandable. Instead, let's plot the results.

0 5 100

1

22

0

Z i 1( )

Z i 2( )

Z i 3( )

100 Z i 0( )

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This plot would work so long as you remember that Z(i,0) is t, Z(i,1) is CA, Z(i,2) is CB

and Z(i,3) is CC. You can get the plot to look like the one below by doing the following.

Generate the plot as above. Now double click on the plot itself to get the chart formatting

menu. Select Traces from the menu. You'll see a table with various options. At the

bottom are the current selections. In the box that says trace 1 type in CA. Then selecttrace 2 from the table and type in CB in the box at the bottom. Similarly for trace 3 as

CC. In the boxes below the table click off the Hide Legend selection and click on the

Hide Arguments selection. Now go to the Labels menu and type in the labels you want toappear on the plot (t for the x axis and CA, CB, CC for the y axis. You may (and should)

also add a title. You now may exit the formatting menu by placing the cursor at some

other point on the MathCAD sheet and clicking. You will notice that whenever you have

selected a plot, i.e., it is surrounded by the blue border, both your labels and the actualarguments (the Z's) will be displayed. When the plot is not highlighted only the labels

will be displayed if you've set the options as stated above.

0 5 100

1

2

CACB

CC

Example Problem

t

CA,CB,CC

Finally, suppose you do want to see printed values but not all of them that are needed to produce a

smooth plot. Let's say you only want to see the values at every minute, including the original

values - a total of 11 values. You could do this by doing the following

ii 0 10

j 0 3

ZZ ii j( ) Z 10 ii j( )

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ZZ

0 1 2 3

0

1

2

3

4

5

6

7

8

9

10

0 2 0 0

1 1.213 0.383 0.404

2 0.736 0.318 0.946

3 0.446 0.212 1.342

4 0.271 0.133 1.596

5 0.164 0.082 1.754

6 0.1 0.05 1.851

7 0.06 0.03 1.909

8 0.037 0.018 1.945

9 0.022 0.011 1.967

10 0.013 6.73810 -3 1.98

Matlab

There are a couple of things to remember about Matlab that will make its useeasier. It was developed to work with vectors and matrices. Thus many of its operations(subroutines, etc.) are more easily used if your problem is expressed in a vector/matrix

format. When you open Matlab there will be heading then a >> symbol. This indicates

that Matlab is waiting for you to tell it to do something. If you want to assign a value to avariable (say let x = 3) type this in and hit the return key. The result you will get is

shown below.

This is a Classroom License for instructional use only.

Research and commercial use is prohibited.

>> x=3

x =

3

>>

Note that below the line containing x = 3 Matlab has returned x = then on the next line 3.

This is just Matlab echoing back what you just commanded it to do. This can get tedious.

To avoid getting the echos simply end each line with a semicolon (;) as shown below

>> x = 3;

>>

Since the variable used above (x) has only a single value it can be entered in the usual

fashion. Suppose you wished to define a row vector (1 column, n rows). This is thesame as a 1xn matrix. To enter values into a matrix in Matlab enter the name of the

matrix (lets use y) and equal sign then a square bracket ([). The square bracket lets

Matlab know that what will follow is a matrix (array). Separate entries in a row by

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spaces; separate rows by using a semicolon (;). When youre done entering values close

the expression with a closing square bracket (]). So if y were a 1 x 2 row vector (1 row,two columns) you could enter the values as shown below

>> y = [1 2];

Note the semicolon at the end of the line that prevented an echo. To make sure that y

really does contain the proper values all you have to do is just to type y then enter to get

>> y

y =

1 2

>>

To enter a column vector (n rows, 1 column) just separate the entries by semicolons

within the square brackets. So for a 2 x 1 column vector z you would get

>> z = [3;4]

z =

34

>>

Note that by not putting the semicolon at the end of the line I did get the echo of thecommand that was typed, verifying that z is a 2 x 1 column vector. To get an n x n

matrix (array) simply combine the two procedures. So for w (a 2 x 2 ) array you would

get

>> w = [ 5 6;7 8]

w =

5 6

7 8

>>

Multiplying matrices and vector then is easily done, e.g., multiplying y by z gives

>> y*z

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ans =

11

or multiplying z by y

>> z*y

ans =

3 6

4 8

>>

Finding an inverse of a matrix is also simple

>> w^-1

ans =

-4.0000 3.00003.5000 -2.5000

>>

How do you use Matlab to solve ODEs? Like MathCAD, Matlab has a numberof integrators that can be used to solve ODEs. The one that Matlab recommends is

ode45, which is also a Runge-Kutta type integrator. Setting-up to run the integrator in

Matlab is a bit more complex. When you first start-up Matlab you are in the MatlabCommand Window. From this window you can execute many of the functions in Matlab.

However, for the integrator to work you need to create an m-file that will contain the

definitions for all of the derivatives. To do this click on File, then click on New, then

select M-File. This will open a new editing window for you. To solve the exampleproblem from above I created the following m-file.

f uncti on dC = exampl e( t , C)dC=zeros( 2, 1) ; %cr eat es a 2x1 vect ordC( 1) =- 0. 5*C( 1) ; %def i nes t he der i vat i ve f or dCA/ dt

dC( 2) =0. 5*C( 1) - 1. 5*C( 2) ; %def i nes t he der i vat i ve f or dCB/ dt

The vector dC will contain the values for dCA/dt and dCB/dt. The vector C will contain

CA and CB. Remember that the semicolon at the end of each line prevents Matlab fromdisplaying the results for that line on the screen and the % sign allows you to enter

comments. When you have finished creating your m-file for calculating the needed

differentials you then need to save this file. When you click on Save you will be askedwhere to save the m-file. Unless you want to go into Matlab and redefine the default path

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values generated to get a good looking plot than you want to see in tabular form. I could

have gone back and tried to manipulate C (to eliminate some values) but since it is soeasy to invoke the integrator I simply did the integration again but this time with a larger

increment in t. Remember that you can use the key to scroll through previous

commands. Thus I went back through the commands until I found the original call for

the integrator, then changed the range to [0:1:10] and recomputed. Again I scrolledthrough the prior commands to find the line where CC was computed. Finally typing

[t,C] and hitting enter gives the printed results I wanted.

After hitting the plot command the new window that opens contains the following plot.

0 1 2 3 4 5 6 7 8 9 100

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

Along the menu line at the top is one that has an arrow leaning to the left. Click on this

to enable plot editing. If you now double click somewhere on the figure youll get a

menu window that allows you to enter a title and label the axes. If you click on one of

the lines you can modify the appearance of the line. Finally, if you click on Tools on thetop menu you can select an option for Show Legend. If you double click on the text in

the legend you then can add the text you wish. You also can drag the legend box around

on the figure to get it to the place you want. You should be able to generate a figure thatlooks like the figure on the following page.

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0 1 2 3 4 5 6 7 8 9 100

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

t

CA,C

B,C

C

Example Problem

CA

CB

CC

Simulink (included in Matlab)

Matlab includes an option called SimuLink that also allows the solution of

ODEs. You can initiate Simulink by clicking on the icon in the menu bar

or by typing simulink at the command line (the line with the >> symbol). This will open

up a new window, the Simulink Library Browser as shown below.

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In the upper left hand corner of the Library Browser click on the new file symbol to

obtain a work space for you model as shown below.

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In the menu in the Library Browser window select Continuous. From the symbols that

appear left-click and hold on the symbol for State-Space and drag it to the work spaceto obtain the following

Double-clicking on the symbol in the workspace gives

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Remember that Matlab was designed to work with vectors and matrices. Thus the model

form

uDxCy

uBxAdt

xd

expectsA,B, C, andD to be n x n matrices andx,y, and u to be n x 1 column vectors.

This block is called a state-space model since the dependent variables (CA, CB and CC inour case) are considered to be states for our system. The vectorx is the column vector

containing the values of the states for our system, i.e.,

C

B

A

C

C

C

x

Thusx(1) = CA,x(2) = CB andx(3) = CC. Our original set of equations then can berewritten as

)2(5.1)3(

)2(5.1)1(5.0)2(

)1(5.0)1(

xdt

xd

xxdt

xd

xdt

xd

with the initial condition

0

0

2

u

since the vectoru contains the input conditions (initial values). Rewriting the set of

differential equations in vector/matrix format gives

xAxdt

xd

05.10

05.15.0005.0

Where a single underscore represents a vector and a double underscore represents a

matrix. We now can enter the data in the window for the State-Space model

remembering to start the definition of any array with a square bracket ([), separate

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elements in any row by spaces and between columns using a semicolon (;) and end the

definition of the array with a closing square bracket (]).

In Simulink the vectory is the output vector. In the format used in Simulink this is theresult of the integration of the ODEs plus and input values. For this problem we want

only the results of the integration (Later we will be using deviation variables in many

problems where the numerical value we want is the result of integrating the deviationvariable plus its steady-state value). This means that the matrix Cthat multipliesxshould be the identity matrix, so that the multiplication does not change any values. Thus

the definition ofCin the State-Space block is that of the identity matrix (1s on the main

diagonal and zeros in all other positions). The two matricesB andD were both zeromatrices (all elements were 0) since the differential equations and final values did not

include any additive terms. Once you have entered all the values click on Apply, then

OK. Before solving this problem we want someway to see the results. Go to theLibrary Browser and click on Commonly Used Blocks. Find Scope and click and

drag the symbol into the workspace. On the righthand side of the State-Space block there

is a connector (the < on the side); on the lefthand side of the Scope that is another

connector (the > on the side). Click on the connector on the State-Space block the draguntil you get the double cross on the connector on the scope. This should give you an

arrow connecting the State-Space to the Scope as shown below.

Now go to the menu bar at the top of the workspace, click on Simulation, then

click on Start in the dropdown menu. Note that in the upper right hand corner there are

two blocks, one saying 10 and the other Normal. The 10 indicates that the simulationwill stop when t = 10. Normal refers to the simulation mode (integrator). These can be

changed to the values you desire (in this case we wanted t = 10 to be the final value so no

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changes are needed). Nothing seems to have changed but if you double click on the

Scope you will get a plot of the results of the integration, shown in the next figure.

Rather than graphical output you may want tabular results. To do this go to the Sinkslabel in the Library Browser then click on the To Workspace block and drag it into the

workspace. Double click on the Workspace block and under the variable name entery

and under the Save Format block enter Array. You will also want to connect this to theState-Space block but there are no remaining connectors. Place the cursor on the line

between the State-Space block and the Scope. Now hold down the Control key and the

left mouse button and drag to the connector on the Workspace block. This should giveyou the following. Now click on Simulation and Start again. Now go to the Matlab main

window and enter y to get the next figure.

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Note that we have obtained the values for all of the concentrations but no values for time.To get time go to the Sources block in the Library Browser, find Clock, click and drag

the clock into the Simulink workspace. Since we want these values to be available in the

tabular output we also need to connect this to a Workspace block. Go to Sinks, click anddrag another workspace block into the Simulink workspace. Click on the connector for

Clock and drag the cursor to the connector on the Workspace block. Double click on the

Workspace block and entertime the variable name and Array under Save Format. Your

screen should now look like the following.

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Now click on Simulation and Start. Now go back to the main Matlab screen and enter[time,y]. This will form a new array with the values from time in the first column and the

values fromy (CA, CB, and CC) in the following columns. You should see the following

output.

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Note, however, that the time increments are not uniform. This is a result of the

Decimation and Sample Time setting in the two Workspace blocks. The Decimationvalue indicates that a value will be printed every nth sample. The default value of 1

outputs the values at every sample time. However, the default integrator in the State-

Space model has a variable step size. The time steps initially are small since the states

are changing rapidly. The time steps get larger towards the end of the simulation sincevalues do not change as rapidly. The default Sample Time of -1 indicates that it accepts

the sample time from the driving block (State-Space in this case). To get equally spaced

time increments change the value of the Sample time to 0.5 in BOTH workspace blocksthen hit Simulation and Start to get the following.

With the data in the main Matlab workspace we could now use the plot commands as we

did in the earlier section on solving odes in Matlab.