Plu Flow Reactor Aspen

This simulation uses AspenPlus to model the plug flow reactor design created in the Matlab program plugr1, whichsimulates a plug flow reactor. Although a detailed description of building an Aspen model may be found elsewhere, thissection 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 afterplacing the icon with a feed 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 wewould like to view the products as both mole flows and mole fractions. The "Components" window for this setup shouldlook like this.

For the property set, choose ideal since the Matlab model of this reactor is based on ideal thermodynamics. When specifyingthe feed stream to the reactor, fill in your window such that it resembles this one. Keep in mind that these are the sameconditions that may be found in the feed stream of the Matlab plug flow model.

Rice University - CENG 403 - PF Reactor Ex - Aspen http://www.owlnet.rice.edu/~chbe403/pfasp.html

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The next page of importance to appear is the "Rplug.Main" page. Note the section on this page entitled "Reactions." Sincewe are going to specify the reactions and the kinetics of the reactions that are going to occur in the reactor, we need tocreate a database on these reactions before we complete the "Rplug.Main" page. The first thing we have to do, though, isdetermine the kinetics of the system.

There are three reactions that are going to occur within the reactor.

CH4 + H2O --> 3H2 + CO1.CO + H2O --> H2 + CO22.H2 + CO2 --> CO + H2O3.

Note that the second two equations are really the forward and reverse reactions of the same system. To model a reversiblereaction in Aspen it must be treated as two reactions--forward and reverse.

Using kinetics, the following two rate equations may be written. Keep in mind that the first equation applies to theforward-only methane reaction, while the second equation incorporates both the forward and reverse reactions of thewater-gas shift reaction.

r1 = DBED*km*e(-Ea1/(R*T))*P*ym/(1+Kh*P*yh)

r2 = DBED*A*e(-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/m3

Pre-exponential Rate Constant km 5.517e6 mol/kg/s/atmActivation Energy of Reaction 1 Ea1 1.849e8 J/molGas Constant R 8.314 J/mol/KPressure P 30.0 atm


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Absorption Parameter Kh 4.053 atm-1

Mole fraction of CH4 ym Varies UnitlessMole fraction of H2 yh Varies UnitlessMole fraction of CO yc Varies UnitlessMole fraction of H2O yw Varies UnitlessMole fraction of CO2 yd Varies UnitlessPre-exponential Constant A 4.95e8 mol/kg/sActivation Energy of Reactions 2 Ea2 1.163e5 J/mol

Equilibrium Constant1 keq(2) e-4.946 + 4897/T Unitless (T in K)

1To determine the value of keq, the Matlab program kequil was used.

Consider the first reaction.

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

r1 = k1*Tn*e(-E1/(R*T))*Cm

where Cm is the molar concentration of methane (P*ym/R/T). This relation is valid provided that the correct value of n ischosen and the gas behaves ideally.

In the Matlab example, the value of the mole fraction of 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 an average of 41.4, we can compare both versions of the r1 equation toobtain

r1 = DBED*km*e(-Ea1/(R*T))*P*ym/41.4 = k1*Tn*e(-E1/(R*T)*P*ym/R/T

Therefore, to make these two equations equal to one another,

n = 11.E1 = Ea12.k1 = DBED*km*R/41.43.

Once this has been determined, we need to make sure that the correct units are being employed. When using powerlawequations in Aspen, the units used for the equation as a whole are based on the units used in the concentration variables.One of the choices Aspen has is molarity, which allows us to use kgmol/m3. To get k1 in the right units, we must do thefollowing.

First, convert R (the Gas Constant) from J/mol/K to m3*atm/kgmol/K. We do this because of the units Aspen requires forkinetic data.:

J L*atm m^3 mol atm*m^38.314 ----- * 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 1k1 = 1200 --- * 5.517e6 -------- * 8.21e-2 ------- * -------- * ---- m^3 kg*s*atm kgmol*K 1000 mol 41.4

1 = 13100 -----


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s*K

Now that we have determined the reaction rate equation for the first reaction, we need to do the same for the second andthird reactions. The second reaction is the forward reaction of the water-gas shift.

r2f = DBED*A*e(-Ea2/(R*T))*yc*yw = k2f*Tn*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. Tomake the two equal to one another,

n = 21.E2f = Ea22.k2f = DBED*A*(R/P)23.

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

kg mol kgmol [ m^3*atm 1 ]2

k2f = 1200 --- * 4.95e8 ---- * -------- *[8.21e-2 ------- * ------] m^3 kg*s 1000 mol [ kgmol*K 30 atm] m3

= 4450 ---------- kgmol*K2*s

Finally, we carry out the same operation on the reverse reaction.

r2b = DBED*A*e(-Ea2/(R*T))*yd*yh/e(-4.946+4897/T) =

k2b*Tn*e(-E2b*(R/T))*(P*yd/(R*T)*(P*yh/(R*T))

Again, to make the above statement true,

n = 21.E2b = Ea2 + 4897*R2.k2b = DBED*A*e4.946*(R/P)2 = k2f * e4.9463.

Converting k2b into the correct units is merely a matter of multiplying k2f by e4.946.

m^3 m3

k2b = 4450 ----------- * e4.946 = 626000 ---------- kgmol*K^2*s kgmol*K2*s

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


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Fro this example we will choose to simulate an adiabatic reactor.

The next thing to do is enter the reaction data. Double click on the "Reactions" folder in the input section of the Flow Sheetwindow. The menu now present should have "Chemistry" and "Reactions." Click on "Reactions" to bring up a windowwhere you can enter information about the reactions. Click on the "New" button in that window.

By selecting "New" we are telling Aspen that we want to define a set of reactions that is going to take place in one of theunits in the model. From the "Object Type" menu that just appeared, select "Powerlaw." Next, name your reaction set in the"Create" window that just appeared. This example will use the default name, "R-1." We can then edit the reaction windowfor the first reaction to make it look like.

This page represents the stoiciometry for the reaction

CH4 + H2O --> 3H2 + CO

The numbers in the "Coefficient" column represent the stoiciometry of the reactions. The "Exponent" column contains thepower law exponents in the equations. For example, in the following equation:

r1 = k1*Tn*e(-E1/(R*T))*Cm

only methane appears to affect the rate; therefore, it gets a "1" in the "Exponent" column while all other compounds get azero.

Use of the "New" button at the bottom of that window allows us to enter similar data for the second reaction:


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This represents the reaction:

CO + H2O --> H2 + CO2

Finally, here is the 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 able to see the reactions that you have specified. Here is what you get afterdoing that following the specification of the 3rd reaction:


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Now that this is complete, we need to enter the kinetics data for the three reactions. To do this, click on the "Next" buttonshown 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 nextreaction:

Repeating for the third reaction:


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This concludes (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 reactor diameter in the Matlab example. In the Matlab example, theporosity (phi) is 0.48. To account for the porosity, we use the following relation:

pi*(Deff)2 phi*pi*D2

--------- = ---------- 4 4

or

Deff = sqrt(phi) * D = sqrt(0.48)*3.19 m = 2.21 m

The next window allows us to specify that we want to include the "R-1" reaction set. It should look like this after we finish.


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

Select "Cancel" 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, first "View" the Control Panel and execute with the "Run" buttonunder the "Run" menu.. When Aspen has finished the calculations, pull up the results under the Data menu: ResultsSummary: Streams to see:


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You can use the scroll bar to see even more information about the "FEED" and "PRODUCT" streams. You can also changeto 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:


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These results compare well to those found using Matlab.

To allow you to see how the results compare, the following graph compares the mole fraction of water in both systems. Thedotted blue line represents the Matlab data while the solid red line corresponds to the Aspen data.


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Plu Flow Reactor Aspen