ASPEN Tutorial #3ChE 473K Fall 2017

Simplified Vinyl-Acetate Process: Introduction to Reaction Kinetics

In this handout, the process being discussed is the production of vinyl-acetate from ethylene and acetic acid. In this exercise, we are going to see how to set up a plug-flow reactor and input the terms for the Langmuir-Hinshelwood kinetics. For the purposes of this simulation, we will only consider the primary reaction in the vinyl-acetate production process.

H2C=CH2 + CH3COOH + ½O2 → H2C=CHOOC-CH3 + H2O

In Aspen, there are 4 options for kinetic based reactions: Langmuir-Hinshelwood-Hougen-Watson (LHHW), Power Law, Reactive Distillation, or User Defined. Descriptions for the Power Law model and the Reactive Distillation can be found in the Help file. The basic LHHW model can be broken down into 3 basic terms:

Rate1=[ KineticFactor ] [ Driving Force ]

[ Adsorption Term ]

The rate equation for reaction 1 is:

Rate1=[1. 1379∗10−12e

−7301. 3RT ] [PO2PC 2H 4P AA ]

[ 0. 50+8 . 00∗10−5PO2+4 . 95∗10−4P AA+7 .92∗10−8PO2PAA ]Note: E = -7301.3 cal/mol

PART I

The first thing we will do is to operate an isothermal, isobaric plug-flow reactor.

1. First, build the flow sheet as shown in Figure 1. We will be using the RPLUG reactor, and we only need a feed stream and a product stream.

Figure 1: Flow sheet diagram

2. After the flow sheet is connected, Select METCBAR as the units of measurement. Also, under Report Options, make sure the Mole Fraction box is checked on the Stream tab.

3. Click the NEXT button and add the components for the simulation: ETHYLENE, ACETI-01, OXYGEN, VINYL-01, and WATER.

4. Next, click the NEXT button, and select RK-Soave for the global property method.

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5. After clicking the NEXT button again, we are ready to enter feed information. The feed conditions are 200°C and 100 psi. The total flow rate is 100 kmol/hr, and is composed of 40 mole % ethylene, 40 mole % acetic acid, and 20 mole % oxygen.

6. Clicking the NEXT button will open the block specifications box. For the Reactor type, choose “Reactor with specified temperature.” Then check the box next to “Constant at Inlet Temperature.” The specifications should look like Figure 2.

Figure 2: Reactor Specifications

7. Next, go to the Configuration tab. This is where the specifications for the reactor’s volume are made. For right now, we are just going to specify a total volume of the reactor: diameter 1.5 m, and the length as 4.5 m.

8. Now go to the Pressure Tab. Set the process stream to 100 psi. Remember we are assuming no pressure drop across the reactor.

9. Now notice that the Reactions tab is still red. We need to enter the kinetic information before we can complete the necessary specifications on this tab. Look at the navigation pane, and go to the Reactions folder. There are 2 choices here, Chemistry and Reactions. The Chemistry folder is used for equilibrium reactions, and the Reactions folder is used for rate-based reactions. Open the Reactions folder and click on NEW. Under Select type, choose LHHW. The screen should look like Figure 3.

Figure 3: Selection of Reaction Kinetics

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10. Click the ok button in the Create new ID window and a new window opens entitled “Stoichiometry”. Click the new button to add Reaction 1. Enter the stoichiometric coefficients in the window using the appropriate reactants and products defined by the problem statement. Go to the Kinetics Tab.

11. This is where we enter the coefficients for the LHHW kinetics. First, set the Reaction Phase to Vapor. Enter the values for k and E. Leave the box for To blank. The data should be entered just like in Figure 4.

Figure 4: Entering values for the Kinetic factor.

12. Click on the Driving Force button. Set the concentration basis to Partial Pressure. There are 2 Terms that must be specified in this section. Select Term 1 in the drop down box. For the concentration exponents, enter values of 1 for the reactants, and 0 for the products. Additionally, enter values of 0 for the coefficients at the bottom of the window. The window should look like Figure 5.

Figure 5: Entering Values for the Driving Force Term 1

13. Now select Term 2 from the drop down menu. Enter values of 0 for all the concentration exponents and the driving force coefficients. After doing this, the blue checkmark should appear.

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14. Now click on the Adsorption button. Enter the adsorption expression exponent as 1. Looking at the rate equation, we see we have 4 terms in the Adsorption Term. Enter the values for the concentration exponents and adsorption coefficients as seen in Figure 6. Click on the NEXT button.

Note: For LHHW, adsorption equilibrium constants in ASPEN are in the form:

ln (K i )=Ai+BiT

+Ci∗ln (T )+D i∗T

Figure 6: Entering the Values for the Adsorption Terms

15. Next, open the block specifications sheet for the reactor (Block 1) and open the Reactions Tab. Place R-1 in the box for the Selected Reaction Sets (Figure 7).

Figure 7: Completion of the Reactor Specifications

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16. Now run the simulation. If we look at the stream summary under results, we see that the conversion of ethylene to vinyl-acetate for this simulation is 70%. Under the Summary for the Reactor, there is a window called Profiles. Go to Profiles and Select View: Molar Composition. Make a plot of mole fraction vs. length of reactor for ethylene, vinyl-acetate, and oxygen. The results should be like Figure 8.

Comparison of the Concentration Profiles as a Function of Reactor Length (D=1.5m, T=200C, P=100psi.)

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0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

Length (m)

Mol

e Fr

actio

n

Ethylene

Vinyl-Acetate

O2

Figure 8Questions

1. What is the enthalpy of the inlet and outlet streams? What is the heat duty?2. Is the reaction exothermic or endothermic?3. What is the residence time ?

PART II.

For this section, we are going to add heat exchange to the plug-flow reactor. We will assume that we will produce steam at 110°C in the shell side of the reactor. This means that we can make an assumption that the process fluid is operating isothermally at this temperature. For our first estimates, we will use an average overall heat transfer coefficient of 1140 W/m2-K, which is an average value for industrial reboilers. Remember that when doing these calculations, this is an iterative process. The case we will use in this simulation would not be a final design case, but would be an example of a case that would be feasible to make.

1. First, we are going to operate the reactor at 200°C, and 500 psi. Change the inlet feed pressure to 500 psi.

2. Next, go to the reactor setup sheet. Under the tab marked pressure, change the pressure to 500 psi.

3. Next, go to the reactor specifications tab. In the drop-down menu, select the option “Reactor with constant thermal fluid temperature.”

4. Under the specify heat transfer parameters tab, enter the values for U and Coolant temperature. The screen should look like Figure 9.

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Figure 9: Entering the Heat Transfer Specifications

5. Finally, go to the configuration tab. We are going to use 172 3” OD tubes that are 3 meters long. Enter the data like in Figure 10.

Figure 10: Entering the Reactor Configuration Data

6. Now run the simulation. View the reactor’s temperature profile, and you will see that the driving force T is only about 3.5°C. This is on the close side, but still possible. You can see this in Figure 11. Clearly this design needs further optimization.

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Figure 11: Reactor Temperature Profile

ASPEN Assignment #3

a) What is the enthalpy of the inlet and outlet streams?b) What is the heat duty?c) What is the residence time?d) What are the minimum and maximum temperature for the reactor?e) What is the new conversion of ethylene to vinyl acetate?f) Generate a plot of mole fraction of ethylene, vinyl acetate and oxygen as a function of reactor length.

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