Plu Flow Reactor Aspen

Rice University - CENG 403 - PF Reactor Ex - Aspen

http://www.owlnet.rice.edu/~chbe403/pfasp.html

This simulation uses AspenPlus to model the plug flow reactor design created in the Matlab program plugr1, plugr1, which simulates a plug flow reactor. Although a detailed description of building an Aspen model may be found e lsewhere, lsewhere, this t his section briefly covers building a model of a reactor in Aspen. The only icon needed for this setup is a plug flow reactor icon (RPLUG). Your setup should look something like this after placing the icon with a fe ed stream and a product stream.

After the flowsheet is complete, it is time to specify the model. On the "Setup.Main" page, we let Aspen know that we would like to view the products as both mole flows and mole fractions. The "Components" window for this setup should look like this.

For the property set, choose ideal since the Matlab model of this reactor is based on ideal thermodynamics. When specifying the feed fee d stream to the t he reactor, react or, fill in your your window such that it resembles this one. Keep in mind mind that these are the same conditions that may be found in the feed stream of the Matlab plug flow model.



The next page of importance to appear is the "Rplug.Main" "Rplug.Main" page. Note the section on this page entitled "Reac tions." Since Since we are going to specify the react ions and the kinetics kinetics of the reactions rea ctions that are going to occur in the reactor, we need to create a database on these reactions reac tions before we complete the "Rplug.Main" "Rplug.Main" page. The first thing we have to t o do, though, is determine the kinetics of the system. There are three reactions that are going going to occur within within the reactor. reac tor. 1. 2. 3.

CH4 + H2O --> 3H2 + CO CO + H2O --> H2 + CO2 H2 + CO2 --> CO + H2O

Note that the second two e quations are really the forward and reverse reactions reac tions of the same system. To model model a reversibl re versiblee reaction in Aspen it must must be treated treat ed as two reactions react ions--forward --forward and reverse. Using kinetics, the following two rate equations may be written. Keep in mind that the first equation applies to the forward-only forward-only methane reaction, rea ction, while while the second e quation incorporates incorporates both the t he forward and re verse reactions react ions of the water-gas shift reaction. r1 = DBED*km*e r2 = DBED*A*e

(-Ea1/(R*T))

*P*ym/(1+Kh*P*yh)

(-Ea2/(R*T))

*yc*yw - DBED*A*e

(-Ea2/(R*T))

*yd*yh/keq(2)

Information about the variables may be found in the following table: Variable

Abbreviation Value

Units

Bed Density

DBED

1200

kg/m

Pre-exponential Rate Constant

km

5.517e6

mol/kg/s/atm

Activation Energy of Reaction 1 Ea1

1.849e8

J/mol

Gas Constant

R

8.314

J/mol/K

Pressure

P

30.0

atm

3



-1

Absorption Parameter

Kh

4.053

atm

Mole fraction of CH 4

ym

Varies

Unitless

Mole fraction of H 2

yh

Varies

Unitless

Mole fraction of CO

yc

Varies

Unitless

Mole fraction of H 2O

yw

Varies

Unitless

Mole fraction of CO 2

yd

Varies

Unitless

Pre-exponential Constant

A

4.95e8

mol/kg/s

1.163e5

J/mol

Activation Energy of Reactions 2 Ea2 Equilibrium Constant

1

keq(2)

e

-4.946 + 4897/T

Unitless (T in K)

1

To determine the value of keq, the Matlab program kequil was used.

Consider Consider the first reaction. The form of the rate equation required by Aspen is a powerlaw equation, which has this form. n

r1 = k1*T *e

(-E1/(R*T))

*Cm

where Cm is the molar concentration of methane ( P*ym/R/T). This This relation is valid valid provided provided that the correct correc t value of n is chosen and the gas behaves ideally. ideally. In the Matlab Mat lab example, the value of the mole mole fraction frac tion of hydrogen hydrogen varies from 0.30 to about 0.38. This causes the value of (1 + Kh*P*yh) to vary from 36.5 to 46.2. Using Using an average of 41.4, we c an compare both versions of the r1 equation to obtain r1 = DBED*km*e (-Ea1/(R*T))*P*ym/41.4 = k1*T n*e(-E1/(R*T)*P*ym/R/T Therefore, to make these two equations equal to one another, 1. n = 1 2. E1 = Ea1 3. k1 = DBED*km*R/41.4 Once this has been det determin ermined, ed, we need to make sure tha t the correct units are being employed. employed. When using powerlaw powerlaw equations in Aspen, Aspen, the units used for the equation as a whole whole are based on the units used in the concentration variables. 3

One of the choices Aspen has is molarity, which allows us to use kgmol/m . To get k1 in the right units, we must do the following. 3

First, First, convert R (the Gas Constant) from J/mol/K J/mol/K to m *atm/kgmol/K. We do this because of the units Aspen requires for kinetic data.:

J L*atm L*a tm m^3 mol atm*m^3 at m*m^3 8.314 ----- * 0.00987 ----- * 0.001 --- * 1000 ----- = 8.21e-2 ------mol*K J L kgmol kgmol*K With this complete, we may now compute k1: kg mol m^3*atm kgmol 1 k1 = 1200 --- * 5.517e6 -------- * 8.21e-2 8.21e-2 ------- * -------- * ---m^3 kg*s*atm kg*s*at m kgmol*K 1000 mol 41.4 1 = 13100 -----



s*K Now that we have determi dete rmined ned the reaction rea ction rate equation for the first reaction, we need to do the same for the second and third reactions. The second reaction is the forward reaction of the water-gas shift. r2f = DBED*A*e

(-Ea2/(R*T))

n

*yc*yw = k2f*T *e

(-E2f*(R/T))

*(P*yc/(R*T)*(P*yw/(R*T))

Keep in mind that we are performing a procedure on this equation similar to those performed on the above equation. To make the two equal to one another, 1. n = 2 2. E2f = Ea2 3. k2f = DBED*A*(R/P)

2

Just as we did above, we must must convert k2f to the proper units.

2

kg mol kgmol [ m^3*atm 1 ] k2f = 1200 --- * 4.95e8 ---- * ---- ---- *[8.21e-2 ------- * ------] m^3 kg*s 1000 mol [ kgmol*K 30 atm] at m] 3

m = 4450 ---------2

kgmol*K *s Finally, Finally, we carry out the same operation on the re verse reaction. r2b = DBED*A*e n

k2b*T *e

(-Ea2/(R*T))

*yd*yh/e

(-4.946+4897/T)

=

(-E2b*(R/T))

*(P*yd/(R*T)*(P*yh/(R*T))

Again, Again, to make the above state ment true, 1. n = 2 2. E2b = Ea2 + 4897*R 3. k2b = DBED*A*e

4.946

2

*(R/P) = k2f * e

4.946

Converting k2b into the correct units is merely a matter of multiplying k2f by e m^3 k2b

4.946

.

3

m

= 4450 ----------- * e 4.946 = 626000 ---------kgmol*K^2*s

2

kgmol*K *s

Now that all of the cumbersome cumbersome calculations are complete, it is time time to enter the t he data into Aspen. If we hit the " Next" button, we will will be asked for the re actor type:



Fro this example we will choose to simulate simulate an adiabatic reactor. r eactor. The next thing to do is enter t he reaction reac tion data. Double click on the "Rea ctions" folder in the input section of the Flow Sheet window. The menu now present should have "Chemistry" and "Reactions." Click on "Reactions" to bring up a window where you can enter information about the reac tions. Click Click on the "New" button in that window.

By selecting "New" "New" we are telli te lling ng Aspen Aspen that we want to define a set of reactions react ions that is going going to take place in one of the units in the model. model. From the "Object Type" menu that just appeared, select " Powerlaw." Next, name your react ion set in the "Create" window window that just appeare d. This example example will will use the default defa ult name, "R-1." We can then edit the reaction re action window window for the first f irst reaction to make it look like. like.

This page page represents the stoiciometry stoiciometry for the reaction CH4 + H2O --> 3H2 + CO The numbers in in the "Coefficient" " Coefficient" column column represent the t he stoiciometry stoiciometry of the reactions. reac tions. The The "Exponent" "E xponent" column contains contains the power law exponents in the equations. For example, in the following equation: n

r1 = k1*T *e

(-E1/(R*T))

*Cm

only methane methane appears to affect the t he rate; therefore, it gets a "1" in the "Exponent" column while while all other compounds compounds get get a zero. Use Use of the "New" button at the t he bottom of that window window allows allows us to enter ent er similar similar data for the second reaction:



This represents the reaction: CO + H2O --> H2 + CO2

Finally, Finally, here is the window window for the third reaction.

The above window is for the reaction H2 + CO2 --> CO + H2O When you press the "Next" button, you will be be able to see the reactions reac tions that you have specified. Here is what what you get after doing that following the specification of the 3rd reaction:



Now that this is complete, complete, we need to e nter the kinetics data for the three reactions. To do this, this, click on the "Next" button shown in the above figure and edit the file to look like:

Changing to the second reaction in the small window that displays the reaction, allows us to set the next reaction:

Repeating for the third reaction: react ion:



This concludes concludes (finally!) (finally!) the section on entering the reaction data for the system. The next page to fill specifies the size of the reactor. This window will appear when you click on the "Next" button:

Notice that the reactor diameter is different from the react or diameter in the Matlab example. In the Matlab example, example, the porosity (phi) is 0.48. To account for the porosity, we use the following relation:

2

2

pi*(Deff ) phi*pi*D --------- = ---------4 4 or

Deff = sqrt(phi) * D = sqrt(0.48)*3.19 m = 2.21 m The next window allows allows us to specify that we want to include the " R-1" reaction reac tion set. It should look like like this after we finish. finish.



Note that the "Reactions" "Rea ctions" are now complete and clicking on the "Next" button brings up the welcome message: message:

Select Select "Cancel" "Cance l" before you run the model so that you may first save the file. It is best to save it in the back up mode. We are now ready to execute the system. To do this, this, first "View" "View" the Control Panel and exec ute with the "Run" button under the "Run" menu.. When Aspen has finished the calculations, pull up the results under the Data menu: Results Summary: Streams to see:



You can use the scroll bar to see even more information about the "FEED" and "PRODUCT" streams. You can also change to look at "Profiles" using the browser part of the window to show:

You can change the "View" to see changes in mol fractions of the compounds:



These results compare well to those found using Matlab. Matlab . To allow you to see how the results compare, the following graph compares the mole fraction of water in both systems. The dotted blue line represents the Matlab data while the solid red line corresponds to the Aspen data.

Plu Flow Reactor Aspen

Recommend Documents