Introduction

The objective of this tutorial is to demonstrate how to model reactions using reaction kinetics. In this tutorial,\ we will model the conversion of sulphur dioxide to sulphur trioxide using reaction rates or kinetics.

During this tutorial you will practice the following functions within Cycad Process:

  • Drawing the flowsheet
  • Setting up the mass balance
  • Adding the reaction kinetics
  • Running the model
  • Examining the results.

In this tutorial you will go through the following six steps:

  1. Draw the flowsheet
  2. Set up the mass balance
  3. Add reactions and reaction kinetics
  4. Set the energy balance properties
  5. Run the model and examine the results
  6. Suggestions for extending the model

Step 1: Draw the flowsheet

The first step is to draw the flowsheet, as shown in Figure 1.

Use Tanks to represent the catalyst beds. Set the Tank drawing property “DrawAsABlock” to true.

 Tut13 image7

Figure 1. The initial flowsheet for Tutorial 16.

For each catalyst bed, change the Fill Properties to Hatch and the HatchType to “Cross”.

Changing the Fill Properties is shown in Figure 2.

 Tut13 image8

Figure 2. Changing the Fill Properties for each catalyst bed.

 


The drawing after this stage should look like that shown in Figure 3.

 Tut13 image9

Figure 3. The flowsheet with the catalyst beds hatched in.

Finally, add another tank and set its “Active” Property to False.

Set the Agitator Property to None, the Lid Property to Rounded and the Bottom property to Rounded. This will serve as the Reactor Shell.

Position the reaction shell so that the diagram looks like that shown in Figure 4.

Tut13 image10

Figure 4. The final flowsheet for Tutorial 13.


Step 2: Set up the mass balance

Add the components shown in Table 1.

Table 1. Feed flowrates to the reactor.

Stream Tag

     

100

Stream Description

     

FEED

Stream Units

   

t/hr

 
         

SO2

(g)

sulphur dioxide

t/hr

1.00

SO3

(g)

sulphur trioxide

t/hr

0.00

O2

(g)

oxygen

t/hr

0.55

N2

(g)

nitrogen

t/hr

3.45

H2O

(g)

water

t/hr

0.00

H2O

(l)

water

t/hr

0.00

         

Total Mass

   

t/hr

5.00

The compositions for the feed shown be those shown in Table 2.

Table 2. Feed compositions to the reactor.

Component

Flowrate

 

Concentration

 

SO2 (g)

t/hr

1.00

% vol

10.00

SO3 (g)

t/hr

0.00

% vol

0.00

O2 (g)

t/hr

0.55

% vol

11.00

N2 (g)

t/hr

3.45

% vol

79.00

H2O (g)

t/hr

0.00

% vol

0.00

H2O (l)

t/hr

0.00

g/L

0.00

 


Step 3: Add the reactions to the catalyst beds

Select each catalyst bed and navigate to the “Reactions” property in the Property Viewer. Select the ellipsis button to launch the Reaction editor.

Add the following reaction:

SO2 (g) + 0.5 O2 (g) -> SO3 (g)

This reaction has the following rate equation:

rate = 100.0*[SO2(g)]*[O2(g)]^0.5 – 0.5*[SO3(g)]

To add this rate, select “Irreversible” in the Reversible column, and HomogeneousGas in the CalculationMethod column.

Type the rate expression in the Expression column.

The required selection is shown in Figure 5.

 Tut13 image11

Figure 5. The Reaction Grid Form.

Navigate to the “Volume” property of each of the catalyst beds, and make the Volumes 10, 40 and 60 m3, respectively.

(Because the reaction is a gas phase reaction, the CalculationMethod in the Reaction Grid Form should be HomogeneousGas, and the GasVolume should be greater than zero.)


Step 4: Set the energy balance properties

For each of the catalyst beds, set the “EnergyBalance” property to “NonIsothermal”.

Also set the pressure to 120 kPaG for each of the beds.

For each of the heat exchangers, set the exit temperatures as shown in Table 3. This is done by setting the temperature of the Heat exchanger which can be found under the operating conditions in the property viewer

Table 3. Exit temperatures from each of the heat exchangers.

Exchangers

Description

Exit Temperature

HX-101

Feed Heater

350°C

HX-102

Interstage cooler

427°C

HX-103

Interstage cooler

437°C

HX-104

Product cooler

200°C

Set the feed stream to a temperature of 37°C.

 


Step 5: Run the model and examine the results

Run the model by pressing the Run Project button in the Process Toolbar.

The results should be those shown in Table 4.

Table 4. Results for the SO3 Converter.

Stream

SO2

(t/hr)

Temperature (°C)

100

1.000

37

101

1.000

350

102

0.1571

630.3

103

0.1571

427

104

0.0193

473.2

105

0.0193

437

106

0.0134

439

107

0.0134

200

 


Step 6: Suggestions for extending the model

Try the following:
1. Changing the volume of the catalyst beds
2. Changing the rate constants in the rate expression