Example 5- :
A feed stream to a process is to consist of 500 kg/h of propane at 30°C and 1 atm. In the reactor, propane is dehydrogenated by the following reaction:
The reaction conversion can be assumed to be 4% at 30°C and atmospheric pressure, with propane as the basis. The vapor product is then sent to a splitter which divides the product flow with an80/20 split. The bulk product is then recycled through the reactor to achieve a higher overall conversion. The process flow diagram can be seen in the diagram below. Find the overall conversion for the process.
Diagram 1: Dehydrogenation of propane process flow diagram.
Method of Solution:
Use the Recycle logical operator in HYSYSTM to feed 80% of the unreacted product back toreactor. Use the Spreadsheet logical operator to manually calculate the process conversion.
1) Open the AspenTM program: Start/Programs/AspenTech/Aspen Engineering Suite/Aspen Plus 2004.1/Aspen Plus User Interface
2) Select “Blank Simulation” and click OK.
3) If necessary, be sure that the program has a connection to an appropriate server and can access thesoftware license (Local PC).
4) Eventually, the Process Flowsheet Window should appear.
5) Add a flash 2 separator to the flowsheet and connect a material stream to the feed, vapor, and liquid of the unit.
6) Open the Data Browser by clicking on the [pic] icon on the toolbar. First, add the system components. Click on the Components section of the Data Browser.
7) On theSelection tab click Find. The Find window will appear. Type “propane” and click Find now. Select PROPANE and click Add. Add propene and hydrogen in the same fashion. If you add a component by mistake, close the Find window, right-click on the row to delete and select delete row from the drop down menu.
8) Next, specify the property package that Aspen PlusTM will use to definethe system. Open the Properties section by clicking on the folder. Click on Specifications.
9) On the Global tab, under Base method, select Peng-Robinson. Then click Parameters, Binary Interaction, and PRKBV-1
10) Now we will define the batch charge. Double click on Stream 1. The data browser should open. Set the Temperature to 30oC, the Pressure at 1 atm, and atotal flow of 500 kg/h. Change the flow units by using the drop-down menu in the box above the units. Change the Composition units to Mole-Frac and enter a value of 1 for propane.
17) Open a new case by either selecting File/New/Case from the top menu bar or clicking on the [pic] icon on the tool bar.
18) The Simulation Basis Manager willappear. Click on the Fluid Pkgs tab. Click Add. Since we are dealing with a relatively ideal system and hydrocarbons, let’s select the Peng-Robinson equation of state. Click on Peng-Robinson in the Property Package Selection list.
19) Now let’s add the process components. Click View. Type “propane” into the Match field, be sure that you highlight the appropriate component,and then click Add Pure or press enter to add it to the Selected Components list. Add propene and hydrogen in the same manner.
20) When all components have been added, close the Component View and Fluid Package windows.
21) Now, we will add the reaction. Click on the Reactions tab. The selected components can be seen in the Rxn Components section of the open window.[pic]
22) Click on Add Rxn. In the small Reactions pop-up window select Conversion and click Add Reaction. In the Conversion Reaction window that just appeared, add propane by clicking on the empty entry field marked **Add Comp** and then on the drop-down arrow. Select Propane.
23) Add Propene and Hydrogen in the same fashion. Under Stioch Coeff, enter -1, 1, and 1,...