ASPEN PLUS™ Example Problems

This section is devoted to example reaction problems. The problems were taken from the 4th Edition of Elements of Chemical Engineering by H. Scott Fogler. Both problems come from Example 8-5, the first is an adiabatic reactor and the second is a PFR with constant cooling temperature. Please note, it is assumed that the user knows how to create a flowsheet and enter process conditions, since these examples explain only the values to enter for each input window.

Example 8-5 Adiabatic Production of Acetic Anhydride

Jeffreys, in a treatment of the design of an acetic anhydride manufacturing facility, states that one of the key steps is the vapor-phase cracking of acetone to ketene and methane:

CH₃COCH₃ CH₂CO + CH₄

He states further that this reaction is first-order with respect to acetone and that the specific reaction rate can be expressed by

where k is in reciprocal seconds and T is in Kelvin. In this design, it is desired to feed 8000 kg of acetone per hour to a tubular reactor. If the reactor is adiabatic, the feed pure acetone, the inlet temperature 1035K, and the pressure 1 62 kPa (1.6 atm), a tubular reactor of what volume is required for 20% conversion?

Creating the Flowsheet

The flowsheet consists of one inlet stream, a PFR, and one product stream. It should look like this:

Create a flowsheet like this in ASPEN PLUS^TM. If you do not know how, see Example 4-3. When the flowsheet is complete, Required Input Incomplete should appear in the lower right corner of the screen. Click the Next button. Click OK when prompted to Enter Required Data.

Entering Process Conditions

This section will explain what values to type in for each input window. If you do not know how to enter values, change units, or navigate through the input windows, see Example 4-3.

Setup

1. Title: Enter any title you wish.

1. Under Component Name type the following in a column: ACETONE, KETENE, METHANE
2. Under Comp ID type in any id names for the above components: A, K, C1
3. Click Next

Properties

1. From the Property method pull-down menu select SYSOP0
2. Click Next.
3. Click OK when prompted about continuing entering stream input.

Stream

1. Description: Enter any description of stream 1.
2. Temp: 1035 K (change units if necessary)
3. Pres: 1.6 atm (change units if necessary)
4. Composition: Change to MASS-FLOW KG/HR
5. For a mass flowrate of A (acetone) type 8000. Leave ketene and methane at zero (no mass flow in reactant stream).
6. Total: Change to MASS-FLOW and enter 8000 KG/HR
7. Click Next.

Blocks

1. Description: Enter any description for PFR, perhaps Adiabatic PFR.
2. Type: ADIABATIC
3. Length: Need to guess a length, 3 METERS is a good starting point.
4. Diam: Need to guess a diameter, 1 METER is a good starting point.
5. Click Next
6. After you create the reaction set, add it in the Reactions tab

Reactions - Stoichiometry

1. Select New...
2. Reaction Name: enter a name for the reaction set
3. Select Type: LHHW
4. Select OK
5. In Stoichiometry select: New
6. Under Rectants: select acetone from the components pull-down menu and set the coefficient to -1
7. Under Products: select ketene and methane and set both coefficients to 1
8. Click Next.

Reactions - Kinetic

1. Select the reaction from the pulldown menu
2. Change Reacting Phase to Vapor
3. Enter 1.125 for k
4. Enter 1000 K for To
5. Enter the activation energy E of the Arrhenius equation, 67999 CAL/MOL. This value was solved for using equation E8-5.1:
   k (s^-1) = 8.2x1014exp(-32,444/T)
   (Note that R is missing in the denominator.)

   Activation Energy = E = (32,444)(R)

   E = (32,444)(1.987 cal/mol K) = 67999 cal/mol

Reactions - Kinetic - Driving Force

1. Under Reactants: set acetone exponent to 1
2. Under Products: set ketene and methane exponent to 0
3. Set constants to 0
4. Select Term 2 from the pull-down menu
5. Set all Exponents and constants B, C, and D to 0
6. Set constant A to -1000000
7. Click Next.

Running the Simulation and Interpreting the Results

Click Next again until you are prompted to run the simulation. Click OK. When the simulation is complete, click next and choose to Display Run-Status results form. If you do not know how to interpret the results window, see Example 4-3. Otherwise, check the conversion (X = moles reacted/moles fed). Does X = 20%? If X < 20%, you must increase the length of the PFR. If X > 20%, you must decrease the length of the PFR.

In this case where length = 3 m, diam = 1m, the conversion was greater than 20%. Therefore, you need to go back to the PFR and input a smaller length. You must access the Rplug.Main window to do this. If you do not know how to reenter inputs, see Example 4-3.

This time, try a length of 2.5 m while holding the diameter constant at 1 m. When you rerun the simulation, you will find that X = 20%! Finishing up the example, the volume of the PFR with these dimensions is V = 1.96 m³.

Reference: G. V. Jeffreys, A Problem in Chemical Engineering Design: The Manufacture of Acetic Anhydride, 2nd ed. (London: Institution of Chemical Engineers, 1964).

Example 8-5 Operation of a PFR with Heat Exchanger

We again consider the vapor-phase cracking of acetone used in Example 8-5:

CH₃COCH₃ CH₂CO + CH₄

The reactor is to be jacketed so that a high-temperature gas stream can supply the energy necessary for this endothermic reaction (see Figure E8-5.1). Pure acetone enters the reactor at a temperature of 1035K and the temperature of the external gas in the heat exchanger is constant at 1150K. The reactor consists of a bank of one thousand 1-in. schedule 40 tubes. The overall heat-transfer coefficient is 110 J/m²-s-K. Determine the temperature profile of the gas down the length of the reactor.

Figure E8-5.1

Creating the Flowsheet

Use the same flowsheet as the adiabatic example.

Entering Process Conditions

Follow instrctions for Setup, Components, Properties, Stream, Reactions - Stoichiometry, reactions - Kinetic, and reactions - Kinetic - Driving Force. The only chnage from the adiabatic example is in the Block input data.

Blocks

1. On the Specifications tab, change reactor type to: Reactor with constant coolant temperature.
2. Enter the heat transfer parameter U: 110 J/sec-sqm-K.
3. Enter the coolant temperature: 1150 K
4. Length: Need to guess a length, 3 METERS is a good starting point.
5. Diam: Need to guess a diameter, 1 METER is a good starting point.
6. Click Next.
7. If the reaction set has already been made add it in the Reactions tab

Run the simulation. Again, adjust the length until the conversion is X = 20%. In this example, the proper length was 1.9 m with a diameter of 1 m. Thus the volume was V = 1.49 m³.

Temperature Profiles down the Length of the PFR

To see the temperature profile down the length of the PFR, do the following:

1. In the left explorer window, double click on blocks and then select B1
2. From the main file menu, select Plot and then Plot Wizard
3. In the window that appears, hit next, and then select Temperature. Hit Next
4. Find the "Which X-axis variable would you like to plut?" Mark the reactor Length choice. Change any other options (such as units) and when you are done select Finish. You will see the plot of temperature as a function of reactor length.

You should see a plot of the temperature profile that looks like this:

1. Introduction

2. Accessing ASPEN PLUS^TM

3. Creating a Reaction Engineering Process Model
   1. Building a Process Flowsheet
   2. Entering Process Conditions
4. Running the Process Model
   1. Interpreting the Results
   2. Changing Process Conditions and Rerunning the Model
5. Example Problems
   1. 8-5: Adiabatic Production of Acetic Anhydride
   2. 8-5: Operation of a PFR with Heat Exchanger
6. Other Need-to-Knows
   1. Saving your Process Model
   2. Printing your Process Model
   3. Changing Names of Streams and Unit Operations
   4. Changing Units of Parameters
   5. Exiting ASPEN PLUS^TM
7. Credits

Back to ASPEN PLUS^TM Main Page